1874-2106/19 Send Orders for Reprints to email@example.com
DOI: 10.2174/1874210601913010274, 2019, 13, 274-287
The Open Dentistry Journal
Content list available at: https://opendentistryjournal.com
Modes of Action and Clinical Efficacy of Particulate Hydroxyapatite in
Preventive Oral Health Care − State of the Art
Joachim Enax1, Helge-Otto Fabritius2,3, Kathia Fabritius-Vilpoux3, Bennett T. Amaechi4 and Frederic Meyer1,*
1Department of Research, Dr. Kurt Wolff GmbH & Co. KG, Johanneswerkstr. 34-36, 33611 Bielefeld, Germany
2Bionics and Materials Development, Hamm-Lippstadt University of Applied Sciences, Marker Allee 76-78, 59063 Hamm, Germany
3Microstructure Physics and Alloy Design, Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Str. 1, 40237 Duesseldorf, Germany
4Department of Comprehensive Dentistry, University of Texas Health Science Center at San Antonio, 703 Floyd Curl Drive, San Antonio, Texas
Particulate Hydroxyapatite (HAP; Ca5(PO4)3(OH)) is being increasingly used as multifunctional active ingredient in oral care. Due to its high
similarity to human enamel crystallites, it is considered as a biomimetic agent.
The aim of this narrative review is to identify the modes of action of HAP in preventive oral health care based on published studies. The outcomes
are expected to improve the understanding of the effects of HAP in the oral cavity and to provide a knowledge base for future research in the field
of biomimetic oral care.
The data analyzed and discussed are primarily based on selected published scientific studies and reviews from in vivo, in situ, and in vitro studies
on HAP in the field of preventive oral health care. The databases Cochrane Library, EBSCO, PubMed and SciFinder were used for literature
We identified different modes of action of HAP in the oral cavity. They are mainly based on (I) Physical principles (e.g. attachment of HAP-
particles to the tooth surface and cleaning properties), (II) Bio-chemical principles (e.g. source of calcium and phosphate ions under acidic
conditions and formation of an interface between HAP-particles and the enamel), and (III) Biological principles (e.g. HAP-particles interacting
Although more mechanistic studies are needed, published data show that HAP has multiple modes of action in the oral cavity. Since the effects
address a wide range of oral health problems, HAP is a biomimetic agent with a broad range of applications in preventive oral health care.
Keywords: Teeth, Hydroxyapatite, Biomimetics, Caries, Periodontitis, Toothpaste.
Article History Received: April 18, 2019 Revised: June 03, 2019 Accepted: July 11, 2019
The mineral phase of human tooth enamel and dentin
consists of Hydroxyapatite (HAP; Ca5(PO4)3(OH)), a crystal-
* Address correspondence to this author at the Department of Research, Dr. Kurt
Wolff GmbH & Co. KG, Johanneswerkstr. 34-36, 33611 Bielefeld, Germany;
Tel: +49 521 8808-6061; Fax: +49 521 8808-626064;
line calcium phosphate (Fig. 1). Enamel has an average HAP
content of 97% while dentin is far less mineralized with an
average HAP content of 70% [1 - 6]. From an evolutionary
point of view, tooth enamel consisting of HAP is an ancestral
character present in the large majority of vertebrate groups.
The only notable exception are sharks and a small number of
bony fishes whose teeth are covered with enamel-like
enameloid consisting of fluoroapatite (Ca5(PO4)3F) [4, 7, 8].
Under healthy conditions, human tooth enamel is already fully
Modes of Action of Hydroxyapatite The Open Dentistry Journal, 2019, Volume 13 275
mineralized when a tooth erupts . The mineralization of
enamel is a highly complex process, where ameloblasts, the
enamel-building cells, interact with calcium and phosphate
ions building the HAP-crystals and finally the enamel prisms
[9 - 11]. Enamel is a tissue that cannot be rebuilt naturally .
Teeth are subjected to constant mechanical and chemical wear
often accompanied by various clinical conditions during their
service. Both phenomena are promoted to different extents by
locally varying dietary habits [12, 13]. In consequence, almost
the whole world population is affected by negative enamel and
dentin conditions with the major factors being excessive
erosion and caries [12, 14].
Fig. (1). Schematic depiction of the hexagonal HAP crystal structure
(Ca5(PO4)3(OH)). Legend: Yellow=OH-; red=Ca2+, blue/white=PO4
modified after Lu et al. .
1.2. Hydroxyapatite in Oral Care
Contemporary everyday oral care strategies focus on the
protection and preservation of enamel and dentin, mainly by
using fluorides [15 - 17]. In the field of clinical dentistry, tooth
repair and replacement are relying on ceramics, composites,
polymers, and other materials [18, 19]. Despite a constant
increase both in the treatment as well as financial efforts, the
problems with dental health still prevail [20, 21]. Therefore, the
demand for alternative and complementary treatment strategies
is on the rise and researchers are increasingly investigating new
approaches [3, 6, 22 - 30]. One strategy focuses on the analysis
of bioinspired concepts that rely on mimicking the natural
constituents of human teeth like crystalline calcium phosphate
minerals [3, 6, 22 - 26, 31]. Among the group of these calcium
phosphates, HAP is the most extensively investigated in the
field of oral care e.g. [1, 3, 6, 26, 27, 32]. Dorozhkin & Epple
 and Enax & Epple , for example, describe the synthesis
routes and characterization of various non-biogenic apatites. In
addition to synthetic HAP, HAP from natural sources (e.g.
bovine bone as well as fish bone and scales) can be used for
medical applications .
Several studies show the abilities of HAP to remineralize
early caries lesions [3, 6, 24, 26, 34, 35]. The effectiveness of
HAP as active ingredient is not limited by the quantity of
calcium and phosphate present in saliva, since HAP can
intrinsically act as source for these ions. This as well as its high
chemical and structural similarity to natural enamel crystallites
[1, 3, 24, 36, 37] qualifies HAP as a promising preventive oral
health care agent.
The similarity to human enamel and dentin crystallites is
the reason why particulate HAP is already in use as a
biomimetic agent in oral care formulations [3, 6, 22, 31, 32, 38,
39]. Several clinical trials have demonstrated the effectiveness
of HAP-toothpastes regarding plaque reduction, improvement
of periodontal health, and caries prophylaxis among caries-risk
individuals [25, 26, 34, 40, 41]. Hu et al. recently showed in a
meta-analysis the clinical evidence of HAP with reducing
dentin hypersensitivity . Compositional and structural
accordance with tooth minerals confers an excellent biocom-
patibility to HAP [1, 24, 42, 43]. Consequently, the applicable
dosage of HAP is not quantitatively restricted, and no
fluorosis-risk is present [31, 44, 45]. Therefore, HAP as active
ingredient may be especially suitable for individuals where
high fluoride exposure is not recommended such as children
(risk of swallowing), pregnant women, and individuals suf -
fering from hyposalivation (i.e. lack of calcium and phosphate
ions for remineralization) [31, 35, 46]. Another important
factor is the ability of HAP-particles to interact with enamel
and dentin surfaces (Fig. 2) [23, 24, 47, 48].
Despite this wide range of applications in preventive oral
health care, the exact modes of action of HAP in some areas
remain quite elusive. References indicate that key effects are
related to the ability of HAP to bio-chemically interact with
tooth tissue as well as intrinsic and extrinsic substances present
in the oral cavity, such as salivary components or micro-
organisms. Other effects seem to be more related to the
particulate delivery form. In either case, understanding HAP’s
mode of action is very important to exploit the full potential of
HAP to improve its efficacy in biomimetic oral care formu-
lations and to enable effective evidence-based oral hygiene
recommendations for patients.
1.3. Aims and Literature Search Strategy
To date, a review that correlates the various described
effects of HAP on different oral/tooth tissues (enamel, dentin,
and gingiva) and oral diseases with the underlying modes of
action has, to our knowledge, not been presented yet.
Therefore, the aim of this narrative review is to analyze
published scientific studies related to the effects of HAP in the
clinical management of different oral and dental diseases.
Based on the numerous studies recently reviewed by Meyer et
al. and Enax & Epple [3, 6] as well as a further comprehensive
literature search, the authors develop hypotheses about the
modes of action of HAP used in oral care formulations with a
focus on the prevention of caries, periodontitis, dental erosion,
and dentin hypersensitivity. Cochrane Library, EBSCO,
PubMed, and SciFinder were screened for relevant studies. Full
text search terms were: hydroxyapatite AND (in vitro study OR
in situ study OR in vivo study OR clinical study AND bacteria
OR plaque OR biofilm OR caries OR periodontitis OR
remineralization) AND (toothpaste OR dentifrice OR mouth
rinse OR mouthwash) (Fig. 3).
276 The Open Dentistry Journal, 2019, Volume 13 Enax et al.
Fig. (2). Scanning electron micrographs showing the attachment of synthetic HAP particles to the tooth surface in vitro (the hydroxyapatite phase of
the particles was confirmed by X-ray powder diffraction). (A) Overview of attached synthetic HAP particles (arrows) as used in various oral care
applications on clean bovine enamel substrate. (B) Synthetic HAP particles (arrows) attached to the surface of bovine teeth including enamel lesions
and pellicle. (C) High magnification at the dentin-enamel junction shows a mineral–mineral interphase between HAP crystallites from synthetic
particles and enamel. (D) Synthetic HAP particles attached to polished dentin surface including open tubules where particles (arrows) can be observed
inside the open tubules. For more details see Fabritius-Vilpoux et al. .
Fig. (3). Literature search strategy for this review was adopted from two recent papers by Enax and Epple  and Meyer et al. . Additionally,
Cochrane Library, EBSCO, PubMed, and SciFinder were screened for further relevant studies using the search terms shown above.
Modes of Action of Hydroxyapatite The Open Dentistry Journal, 2019, Volume 13 277
We correlate the objectives of the studies with proposed
mechanisms of HAP in preventive oral health care (e.g. pre-
vention of caries, periodontitis, dental erosion, dentin hyper-
sensitivity) and conclude presumable modes of action based on
physical, bio-chemical, and biological principles as well as
combinations of these.
2. SHORT OVERVIEW OF PREVALENT ORAL
DISEASES THAT ARE AFFECTED BY ORAL
The oral cavity is a highly complex environment that can
be affected by a variety of diseases, some of which are highly
prevalent among our society. The most predominant of which
are dental caries, periodontitis, erosion, and dentin hyper-
sensitivity. All of these have in common that they can be
prevented up to a certain level without clinical intervention,
namely by oral hygiene habits, reduced sugar (and acid) diet,
smoking cessation, and stress-free life style.
Caries is a disease that affects nearly every individual at
least once in their life time and has the highest prevalence
among all diseases that are known worldwide . Even
though its prevalence has declined in some parts of the world
over the last years, it is still prevalent in all age groups and its
treatment remains the highest unmet need in the world [20, 31,
50, 51]. Untreated caries affects more than 2.4 billion people
around the world . Caries is a biofilm-driven disease where
the biofilm switches from homeostasis to dysbiosis .
Microorganisms that form biofilms on tooth surfaces meta-
bolize sugars as energy-source, producing organic acids,
mostly lactic acid, as end-products [53, 54]. By locally
lowering the pH within the biofilm on the tooth surface, these
acids are able to demineralize the underlying tooth tissue [1,
55]. This process is slowed down to individually different
extents by the natural remineralization process, the rate and the
extent of which depend on the availability of free calcium and
phosphates from saliva and extrinsic sources. Local dis-
solution/recrystallization processes of the enamel surface on
the microscopic scale caused by local concentration gradients
may be an additional reason for superficial remineralization
Periodontitis is an inflammatory disease affecting the
whole periodontium. In most cases periodontitis will start with
gum inflammation and bleeding, also known as gingivitis .
Periodontitis can affect up to 90% of the population, while the
severe form of this disease can be detected within up to 20% of
some populations [50, 56]. Periodontal disease is one of the
most prevalent diseases in the world (rank 11 in 2016) [21, 49,
57]. The incidence of periodontitis increases with the age, as
teeth nowadays remain longer in the oral cavity .
Periodontal disease is caused by a dysbiotic state of the dental
plaque and can be triggered by a suppressed immune response
or genetic factors [58 - 60]. Traditional agents for prevention of
this inflammatory disease focus on antimicrobial effects
[61, 62]. Frequently used oral care products are, for example,
based on chlorhexidine as well as stannous and zinc salts [30,
62, 63]. All of these antimicrobial agents might lead to a
dysbiosis of the oral microbiota. In contrast to that, the modern
approach in preventing periodontitis is focused on keeping the
ecological balance of the microbiota: Not killing but
controlling the harmful microorganisms [61, 64].
Dental erosion, otherwise known as erosive tooth wear, is a
dissolution primarily of the enamel caused by acids of non-
bacterial origin, both from extrinsic or intrinsic sources such as
seen with patients suffering from gastroesophageal disease or
bulimia [14, 65 - 67]. Erosion is an increasing challenge for
dentists, oral care practitioners and manufacturers of oral care
products [14, 65]. The prevalence known from studies varies
between 6 and 100% depending on the age group and the
geographical region . The highest prevalence was found
with children aged 9 to 17 years . Within adults, the
prevalence ranges between 4 and 83% . Erosive tooth wear
is found to be mainly a consequence of modern dietary habits
with fruits, juices, lemonades, and other acidic aliments .
2.4. Dentin Hypersensitivity
Patients often report dentin hypersensitivity to their
dentists . Prevalence ranges from 3 to 98% within several
studies . Depending on the age group, women are slightly
more affected than men and most of the people at the age
between 30 and 40 years suffer from dentin hypersensitivity
. Buccal surfaces are mostly affected. Reasons might be
gingival recession or abrasion at the cemento-enamel junction
exposing the dentin into the oral cavity . Patients describe
dentin hypersensitivity as a short, sharp pain . The pain
will occur when the nerves in the pulp are stimulated via the
liquid-filled tubules in exposed dentin when triggered by
stimuli, typically thermal, evaporative, tactile, osmotic or
chemical that cannot be ascribed to another defect or disease
. In general, two different concepts can be described to
treat dentin hypersensitivity with oral care agents: Desen-
sitization of the pulp / nerve and occlusion of dentinal tubules
[32, 38, 70]. Occlusion might last longer due to physical
mechanisms, as ions for desensitization (i.e. potassium) are
only momentary pain-relieving.
3. HAP IN ORAL CARE
Studies show the efficacy of particulate synthetic HAP in
prevention of caries [26, 34], remineralization of early stages
of dentin and enamel caries [35, 71, 72], prevention of
periodontitis [23, 25, 47], reduction of gingival bleeding [25,
41], prevention of acid erosion [48, 73, 74], and reduction of
dentin hypersensitivity [27, 75 - 78]. The fact that HAP is
effective within multiple clinical indications in a number of
different preventive oral care applications strongly indicates
that this biomimetic calcium phosphate mineral has different
relevant modes of action. These are either related to inherent
physico-chemical properties of HAP or the chosen adminis-
tration form. To develop formulations that are even more
efficacious in combating caries and other oral diseases, the
understanding of these modes of action is a very important key.
Following this, the effects as demonstrated by the results of the
scientific studies are discussed with respect to presumable
modes of action of HAP.
278 The Open Dentistry Journal, 2019, Volume 13 Enax et al.
3.1. Studies and Mechanisms of HAP in Preventing Caries
(1) The first clinical trial using HAP in the field of caries
prophylaxis was published in 1989 . Two different schools
with at least 200 fourth-graders were included in the study. The
study duration was three years. One group used a (fluoride-
free) toothpaste containing 5% HAP, and the other group used
a placebo-toothpaste without any active ingredients. Every
year, the dmft/DMFT was acquired. While on the level of the
primary dentition only minor effects could be observed, the
caries reduction on the level of newly-erupted teeth was
statistically significant in the HAP-group compared to placebo
(mean caries inhibition: about 45%) .
(2) These above results were confirmed by a randomized
clinical trial (non-inferiority-trial) from Schlagenhauf et al.
. This study was performed at five German university-
hospitals using high-caries risk-patients undergoing ortho-
dontic treatment. Since it is well known that these patients
develop caries within four weeks [80, 81], the study was
conducted over a period of six months and the 147 patients
were randomly distributed using either a (fluoride-free)
toothpaste containing 10% microcrystalline HAP, and a tooth-
paste containing amine fluoride/stannous(II)fluoride (1400
ppm fluoride). Caries incidence was measured using ICDAS-
scores 1 and 2. Out of the 133 participants overall who finished
the study per protocol, the HAP-group was not inferior to the
(3) Lelli et al. used extracted teeth from an in vivo study
(so called ex-in vivo study) to investigate whether a calcium-
layer can be observed after using a HAP-toothpaste. Due to
orthodontic or prosthetic reasons, teeth had to be extracted.
Participants had either brushed their teeth with a fluoride-
toothpaste or with a HAP-toothpaste for at least 8 weeks. The
extracted teeth were analyzed using scanning electron
microscopy (SEM) to visualize a possibly formed superficial
layer and energy dispersive X-ray spectroscopy (EDX) to
analyze its elemental composition. While in the fluoride-group
no additional layer could be observed, the use of HAP-
toothpaste leads to the formation of a calcium phosphate-
containing layer on top of the natural enamel .
(4) Najibfard et al. conducted a double-blind in situ study
with thirty participants using enamel slabs with and without
artificial carious lesions . Using a crossover-design
(washout-phase in between the phases was seven days),
participants brushed for at least 28 days with a fluoride-
toothpaste (1100 ppm fluoride) or a HAP-toothpaste (5% HAP
and 10% HAP, respectively). The slabs were analyzed using
microradiography to quantify mineral loss and lesion depth.
The results showed that HAP-toothpaste prevented the enamel
from demineralization. Additionally, remineralization of
enamel was not different between the toothpastes containing
fluoride (1100 ppm fluoride), and HAP-toothpaste (5% and
10% HAP) .
(5) A clinical trial compared three different mouth rinses
containing either HAP, CHX or fluoride with respect to plaque
accumulation, gingival indices, and remineralization properties
. 81 children used the respective mouth rinse twice a day
under parental supervision for two weeks. The children were
examined after 1, 2, 4, and 6 weeks. With respect to caries
(remineralization), HAP performed equally compared to
fluoride and was also effective in reducing dental plaque and
improving gingival health .
(6), (7) Two in situ studies investigated the bacterial
adhesion to enamel surfaces after rinsing with an (6) aqueous
dispersion containing HAP-particles  and (7) a mouth rinse
containing HAP-particles . Participants wore enamel slabs
and rinsed with the respective dispersions. As positive control,
CHX-containing mouth rinse was used, and negative control
was also studied. Enamel slabs were carried in the oral cavity
for six to twelve hours and then analyzed using bacterial
staining and microscopy-techniques. Additionally, in vitro
experiments regarding antimicrobial effects were also carried
out. HAP does not kill the bacteria but leads to a decrease of
bacterial attachment on enamel surfaces. The reduction of
initial bacterial colonization to enamel surfaces in situ was
comparable with the reduction of the antimicrobial agent
chlorhexidine [23, 47].
Several in vitro studies have investigated the remine-
ralization-potential of particulate HAP [71, 72, 82 - 84]. Most
of the evaluated studies used demineralized enamel or dentin,
and applied HAP-particles on these slabs [71, 83, 84]. Two
studies, however, used pH-cycling models [72, 82]. Out of
these, Esteves-Oliveira et al. did not detect a caries-preventive
effect of HAP in vitro , while de Carvalho et al. described
caries-inhibiting effects of HAP .
The effects described in the results of the analyzed studies
imply a number of different mechanisms where HAP plays a
major role. As caries is a biofilm-associated disease, this
biofilm should consequentially be reduced. HAP has already
been shown to reduce the bacterial biofilm on the teeth .
Bacteria need a rough surface for attachment .
HAP in oral care products might help smoothen the surface
through two different effects: Firstly, particulate HAP attaching
to the enamel surface may preferentially deposit in small
depressions such as scratches and superficial defects because it
can more firmly attach and may be harder to remove by either
abrasive food particles or brushing than on the smooth enamel
surface areas, thus leveling the surface on the microscopic
scale. Secondly, the remineralization effect and the resulting
bio-chemically induced formation of a protective layer may
also be more pronounced in surface depressions of the outer
enamel. Both effects lead to a smoother surface that con-
sequently decelerates the attachment of potentially harmful
Furthermore, bacteria can erratically attach to individual
HAP particles that are either loosely attached to the enamel
surface or float in the oral cavity, thus inducing bacteria co-
aggregation. In consequence, bacterial load will be reduced by
spitting out the residual HAP after thorough tooth brushing.
Despite all routine daily oral hygiene, bacteria can attach on
the teeth after a certain time. If a protective layer of HAP
particles is present and regularly sustained, this biofilm is
formed on its surface instead of the calcium phosphate of the
Modes of Action of Hydroxyapatite The Open Dentistry Journal, 2019, Volume 13 279
original enamel. When acids are produced within this biofilm,
this protective layer might be dissolved, and calcium and
phosphate ions will be released. This can have two main
effects: On the one hand, the biofilm formation might be
disrupted by influencing the energy metabolism of oral bacteria
. On the other hand, the excess of released calcium and
phosphate ions become available for other, different effects.
They might for instance help and promote remineralization of
demineralized tooth surfaces and support the natural
remineralization-mechanisms of saliva, i.e. shifting the
equilibrium from de- to remineralization. If residual plaque is
present (i.e. plaque that has not been removed by tooth-
brushing), HAP may also be incorporated into the biofilm as
demonstrated for other calcium phosphates before [6, 87].
When acids are produced by bacteria, the dissociation
products of HAP may act as buffering-solution. Particularly the
phosphate ions are able to neutralize acids to a certain level,
likewise the salivary phosphate-buffer. If present in excess, the
released calcium and phosphate ions can also switch the
solubility product balance back to remineralization.
In addition to the mechanisms described above, HAP can
remineralize early caries-lesions directly which was shown
under in situ conditions (Fig. 4) . From a histological point
of view, initial caries-lesions show larger distances between the
rod-shaped HAP crystallites of the enamel than healthy
portions. Small HAP particles from oral-care products might be
able to penetrate through these gaps between enamel-rods and
thus fill these initial lesions.
The HAP particles might bind to enamel/pellicle due to
electrochemical forces, because of partial electronegativity:
Zeta-potential of enamel and HAP-particles show a negative
net surface potential of -15 to -30 mV, and basic proteins (i.e.
from saliva/pellicle) a positive net surface potential of +20
to + 30 mV [88, 89]. HAP particles might also act as crystal
nuclei by attracting calcium and phosphate from saliva .
Local concentration gradients can induce the formation of
calcium phosphate through crystal growth on the enamel
Fig. (4). Microradiographic images of early caries (A) before and (B)
after in situ remineralization by use of a 5% HAP toothpaste. A
decrease in surface roughness is accompanied by a decrease of the
demineralized region below the surface indicated by the smaller dark
grey area. The images are taken with publisher’s permission from
Najibfard et al. .
Since HAP is a calcium phosphate source itself, the
described mechanisms do not only rely on ions provided with
the salivary flow. As soon as bacteria are able to attach to a
HAP-layer on tooth surfaces and acids are produced, calcium
and phosphate ions will be released which may have a positive
influence on the remineralization process (Fig. 5A-C).
HAP has a wide range of actions with respect to caries
prevention. The main advantage of HAP compared to other
oral care active ingredients is its high biocompatibility, i.e. no
potentially toxically side effects like fluorosis. In consequence
to this, there is no regulation in dosage [42, 43].
Fig. (5). Schematic overview showing the modes of action of HAP in the remineralization and protection of enamel. (A) HAP-particles from oral care
products are introduced into the oral cavity. (B) HAP particles from oral care formulations adhere to enamel surfaces as a protective layer [23, 48].
Bacterial adhesion is reduced. Free HAP-particles act as “magnet” for oral bacteria and are spit out. (C) Bacteria forming a biofilm on top of the
HAP-layer after a certain time will produce acids when sugar/starch is provided, and acids are produced. These acids might lead to dissolution of the
protective HAP-layer. The tooth will be protected, because of free calcium ions, and phosphate buffer mechanisms from the HAP-layer.
280 The Open Dentistry Journal, 2019, Volume 13 Enax et al.
3.2. Studies and Mechanisms of HAP in Preventing
Several clinical trials report an improvement of gingival
health after having used HAP-containing oral care products. (8)
Harks et al. tested in a randomized clinical trial the differences
in the plaque-formation-rate between a HAP-toothpaste and an
antibacterial amine fluoride/stannous (II) fluoride-toothpaste.
Sixty-seven patients with mild periodontitis completed the
whole double-blind 12 weeks-trial. Besides plaque-formation-
rate, several other clinical parameters were tested (i.e. bleeding
on probing). The two groups used their respective toothpaste
for 4 weeks without any therapy at home. After these 4 weeks
supragingival cleaning was performed and the toothpastes were
used for another 8 weeks. Plaque-formation rate did not change
within that time. However, gingival health improved within
both groups after 4 and 12 weeks. There were no differences
between the antibacterial toothpaste and the HAP-toothpaste
(9) Plaque samples from this study were also investigated
using 16S rRNA amplicons for next generation sequencing. No
differences of the plaque-composition could be observed
between these two groups . While mainly Fusobacterium
and Prevotella species were observed in the interproximal and
subgingival sites, buccal and lingual surfaces were dominated
by Streptococcus and Veillonella species. However, the use of
either antibacterial or anti-adhesive toothpastes did not change
the composition noticeably indicating the stability of the
ecological niche. Interestingly, gingival bleeding was reduced
in both groups showing an effect on periodontal health of both
(10) Additionally, Cosola et al. tested a mouth rinse
containing HAP (and zinc-PCA) with patients after surgical
procedures . Twenty-six patients were randomly allocated
to either CHX-group or HAP-group. All patients received oral
surgery. As postoperative treatment, patients rinsed with the
mouth rinse. After their removal the suture threads from both
groups, CHX and HAP (and zinc-PCA) were tested in vitro for
colony-forming units. Despite different treatment, threads from
both groups were found to have the same effect on bacterial
growth. Since prevention of periodontitis relies on controlling
bacterial growth, it is important to note that this study shows
HAP (with addition of zinc-PCA) to reduce bacterial growth on
suture threads. This goes in line with the in vivo (5) and in situ
(6), (7) studies presented before [23, 25, 41].
The results of these studies provide a number of useful
hints with regard to HAP’s possible involvement in the
prevention of periodontitis.
Dental plaque bacteria can interact with extrinsic HAP
when present in the oral cavity. Thus, early colonizers, such as
streptococci bind to free HAP derived from oral-care products
instead of the tooth surface. The HAP can be either free
particles in the oral cavity or particles bound to the tooth
surfaces thereby creating a protective film of alternative subs-
trate. As the early colonizers are needed for further biofilm-
formation, the biofilm will be consequentially reduced either
by directly spitting out of the free particles or by abrasion of
the HAP layer through oral hygiene related cleaning activity.
It is known that gingivitis and periodontitis are promoted
by a dysbiotic dental plaque if it is not removed. Hindering the
settlement of early colonizers and reducing growth of the
dental plaque can slow and potentially even prevent reaching a
The mode of action of HAP with respect to periodontitis is
indirect. HAP is not able to directly interact with the immune
system. Nevertheless, by reducing bacterial load and con-
trolling the biofilm-formation, periodontal pathogens (i.e.
Porphyromonas gingivalis and Tannerella forsythia) will not
be able to lead to a shift in the taxonomic composition of the
biofilm, as they are late colonizers and with that they are
dependent of the early colonizers and bridge microorganisms
(i.e. Fusobacterium nucleatum) [52, 62].
HAP also acts as an biomimetic cleaning agent (same
hardness as the tooth) in toothpaste formulations, directly
reducing dental plaque [32, 91].
The authors hypothesize that a HAP-layer applied to a
tooth surface directly after mechanical dental plaque removal
might lead to an aggravated biofilm formation . Formu-
lations with the addition of zinc salts or lactoferrin may boost
the effect of HAP, especially against already existing or
developing biofilms [30, 40, 92]. Zinc-substituted HAP, where
Zn2+ions replace a part of the Ca2+ ions [5, 48], might also be
dissolved in acidic environments, and the released zinc might
act as an antimicrobial agent. This might promote the positive
effects of HAP with respect to gingival health [25, 40, 47, 92].
In addition to that, HAP might also interact with proteins
derived from microorganisms. Proteins are known to act as
virulence factors, i.e. arginine deiminase arc A from P.
gingivalis . HAP particles present in the vicinity of the
bacteria might bind these proteins . Being immobilized, the
virulence factors will not be able to interact with the human
Finally, HAP does not show unwanted side effects like dis-
coloration of teeth or irritation of taste and can be used on a
daily basis, in contrast to substances like chlorhexidine [23, 30,
Furthermore, HAP does not kill bacteria (in contrast to e.g.
stannous ions and chlorhexidine) but controls them by its
physical presence or by passively withdrawing potentially
harmful metabolic products from the oral cavity environment.
Modern approaches focus on controlling the oral bacteria rather
than killing them , since it is known that the use of
antibacterial agents might lead to resistance or tolerance of
bacteria against these agents. Biomimetic concepts have been
shown to be promising alternatives or supplements in promo-
ting periodontal health.
Modes of Action of Hydroxyapatite The Open Dentistry Journal, 2019, Volume 13 281
3.3. Studies and Mechanisms of HAP in Preventing Dental
Clinical studies in the field of erosion are rare due to a
feasible study population and an acceptable study duration for
the participants and ethical issues . Therefore, in situ and in
vitro studies using a sophisticated study design are much more
important. (3) An ex-in vivo study by Lelli et al. has shown the
formation of a (protective) calcium phosphate layer on top of
the teeth in the HAP-group . While some in vitro studies
were not able to show a positive effect of HAP-toothpastes on
erosion [96, 97], other studies were able to show protective
effects of HAP-toothpastes [24, 71, 73, 74, 98, 99].
(11) Fabritius-Vilpoux et al. tested in vitro, if HAP-
particles from an aqueous dispersion can attach to (me-
chanically and chemically eroded) enamel-surfaces .
Bovine enamel slabs were pre-treated with phosphoric acid.
Afterwards, these slabs were dipped into agitated aqueous
dispersions containing different HAP-concentrations. HAP
particle attachment and surface-coverage was determined using
SEM. Even a dispersion containing only 1% of HAP resulted
in an area coverage of 10% of the eroded enamel surface. A
dispersion with 10% HAP covered more than 30% of the
enamel after one-time application .
(12) Another in vitro study used also enamel slabs and
tested Vickers-hardness after erosive challenges and appli-
cation of toothpastes . Enamel slabs were erosively chal-
lenged four consecutive times (0, 8, 24, and 32 hrs.) for two
minutes. After each challenge, toothpaste was applied on the
surface. Vickers-hardness measurements revealed that acidic
attacks significantly reduce enamel surface hardness. However,
HAP-containing toothpastes caused significant re-hardening of
the surface, indicating the occurrence of remineralization .
(13) The same group also used human enamel slabs in
another study . Here, the slabs were pre-treated with the
respective toothpastes. Several fluoride-containing and
calcium-containing toothpastes were tested besides HAP-
toothpaste. These pre-treated slabs were then dipped into an
acidic soft-drink for a maximum of 32 min. As measurement
for erosive challenge, loss of weight of the slabs was
determined. The HAP-toothpaste showed significantly less loss
of mineral compared to the other products. This indicates a
protective effect of HAP against erosive challenges .
(14) Colombo et al. used a visual rating system imaged by
SEM to evaluate the enamel surface after application of a
HAP-toothpaste . Human incisor specimens were prepared
and underwent an erosive challenge. In four different groups
(positive control with no erosive treatment, negative control
without toothpaste-application, fluoride toothpaste, HAP-
toothpaste), different toothpastes were applied for 3 min. and
rinsed off with distilled water at 0, 8, 24, and 36 hrs time
intervals. In between the applications, specimens were stored in
artificial saliva. Using a systematic assessment-method by
SEM, enamel damage was recorded by four observers. The
HAP-toothpaste was the only tested toothpaste where mineral
deposition was observed after the erosive challenges. The
grade of damage was consistently lowest in the HAP-group
From to above studies, one can deduce the following
mechanisms: HAP forms, after application, a protective layer
on the tooth surface. The protective layer might act as expen-
dable shield that protects the tooth material from acidic attacks.
Consequently, the protective HAP layer will be dissolved,
releasing calcium- and phosphate ions that can also act as
Ca5(PO4)3(OH) + 4 H+ → 5 Ca2+ + 3 HPO4
2- + H2O
Furthermore, it is known that the content of calcium within
an erosive surrounding might lead to a shift of the equilibrium
from dissolution to homeostasis. When parts of the teeth are
eroded by acids, HAP is potentially able to remineralize
attacked surfaces. Dental erosion and dental caries are both
diseases caused by acids. However, the amount of acids and the
origin of the acids is different. Consequently, pathogenesis of
these two acid-driven diseases differs. Erosion is a fast process
compared to caries: Acids produced by bacteria slowly pene-
trate through the outer layers of enamel to create a subsurface
demineralization (caries), while extrinsic acids lead to a
dissolution of the enamel’s outer layers (erosion). Enamel
crystallites and prisms can be analyzed, when using SEM .
In erosion, HAP can cover the demineralized surfaces and
might also directly remineralize the underlying demineralized
tissue. In vitro experiments using clean enamel surfaces
indicate that HAP particles attach to the surface of eroded teeth
not only by electrochemical forces, but directly form solid
interfaces between HAP crystallites from the particles and the
enamel . Hornby et al. performed several experiments
where they used HAP in combination with fluoride treatment
. However, one experiment was performed using 45Ca-
labelled HAP to determine Ca-uptake from a HAP slurry after
an erosive attack. While the Ca2+-uptake from HAP with sound
surfaces was 1.05 mg/mm2, Ca2+-uptake with demineralized
enamel was three times higher (3.16 mg/mm2). The authors
concluded that this indicates an availability of Ca2+-ions from
HAP for remineralization .
In conclusion, HAP can form a protective layer on tooth
surfaces and remineralize eroded enamel and dentin [48, 71,
73, 74, 99]. By forming a “sacrificial layer”, acidic attacks will
not directly demineralize the teeth. Additionally, HAP leads to
a shift of the solubility equilibrium (Fig. 6). Regular use of
HAP is needed to cope with regular acidic challenges.
282 The Open Dentistry Journal, 2019, Volume 13 Enax et al.
Fig. (6). (A) shows a caries attack in the absence of HAP. The natural tooth will get demineralized at a pH around < 5.5. After a certain time, saliva
will clear the acids and salivary ions will lead to a (partial) remineralization of the enamel. (B) shows the same conditions, but in the presence of
extrinsic HAP (from toothpaste or mouth rinse), the dissolution of the natural enamel is decreased. Due to the presence of HAP, and a change of the
solubility equilibrium, the natural enamel will be protected [115, 116].
3.4. Studies and Mechanisms of HAP in Preventing Dentin
(15) Hu et al. conducted a systematic review and a meta-
analysis that proved the effect of HAP in relieving dentin
hypersensitivity . The authors searched five databases for
randomized controlled trials investigating dentin hyper-
sensitivity pain relief. Risk of bias was assessed following the
Cochrane guidelines. Confidence intervals and evidence were
also calculated in this study. Particulate HAP was evidentially
proven in relieving dentin hypersensitivity .
Amaechi et al. performed an (16)in situ study  and an
(17) in vivo study  in the field of dentin hypersensitivity
using HAP. (16) The in situ study tested the occlusion of dentin
tubules after application of either 10% or 15% HAP .
Overall, 20 participants per group were recruited to wear
human dentine blocks for at least 14 days. Additionally, a
toothpaste containing fluoride and another containing NovaMin
were also tested. Untreated blocks were used as control. After 7
and 14 days, respectively, dentine occlusion was visualized
using SEM. Both tested concentrations showed after 7 and 14
days a higher degree (up to 50%) of completely occluded
dentin tubules than the non-HAP toothpastes. In the 15% HAP-
group, the test surfaces were 100% covered with a precipitate
layer . (17) These results were confirmed by the clinical
trial from the same group where reduction of dentin
hypersensitivity was investigated .
(18) Huettemann and Doenges published already in the
year 1987 a double blind clinical trial using different
toothpastes with HAP particles of different diameters .
140 patients with dentine hypersensitivity were recruited.
Study duration was four weeks and sensitivity was clinically
tested using standardized tests (i.e. cold stimulus). The test
pastes containing HAP (diameter 2 µm and 6 µm) were
compared to a placebo. While in the placebo-group no
improvement could be measured, 90% of the HAP-group
reported an improvement of dentin hypersensitivity already
after 3-5 days. 50% of the HAP-group were pain-free within
the study-period (four weeks) . These results were
confirmed by several other clinical trials using HAP-containing
formulations for relieving dentin hypersensitivity [75, 76, 78,
(19) Hiller et al. tested the in vitro permeability of dentin
after application of particulate HAP . For this, they used
bovine dentin slabs where a HAP-toothpaste was applied.
Subsequently, the hydraulic conductance was tested and used
as a measure for the degree of occlusion of the dentin tubules.
Permeability was significantly reduced after application of all
tested toothpastes, including the HAP-application .
The mechanisms of HAP in preventing dentin hyper-
sensitivity can be summarized as follows: In exposed dentin
the dentinal tubules are open towards the oral cavity and to the
pulp. Thus, external stimuli can propagate through the dentinal
fluid directly to the nerve tissue in the pulp via the odon-
toblastic processes located in dentinal tubules. Since the
applied HAP-particles have a high polarity, they are able to
bind both to collagen and hydroxyapatite from dentin. Thus,
these particles will attach to dentinal surfaces and eventually
occlude exposed dentinal tubule openings if they are smaller
than the tubule diameter. The size of tubules/diameter close to
the tooth surface or the DEJ, respectively, is about 3.5 µm
. Most oral care products contain HAP-particles of sizes
between 0.1 and 10 µm . Particles need to have diameters
<5 µm to reliably occlude dentinal tubules . HAP particles
close the tubules by being pressed into the tubule openings
during for instance brushing. Within a certain time, HAP-
particles that have occluded tubules will bio-chemically bind to
the collagen-rich dentin and bio-chemically fuse with the inner
mineral lining of the tubule. Deposited HAP soon gets
mineralized by attracting calcium and phosphate ions from
saliva. Mineralization will lead to fewer open tubules, and
consequently less possibilities for external stimuli to induce
Modes of Action of Hydroxyapatite The Open Dentistry Journal, 2019, Volume 13 283
pain. When using HAP containing oral care products regularly,
dentinal tubules can be completely occluded due to the filling
effect building up over time and the concurrent formation of a
protective layer on the dentin. As positive side effect, dentin
might also be protected from acidic attacks by the mineralized
deposited HAP that can act as sacrificial layer.
In conclusion, HAP is known for decades to reduce dentin
hypersensitivity. Several studies have shown a significant
improvement of clinical parameters when using HAP-based
oral care products, which is confirmed by a recently published
3.5. Studies and Mechanisms of HAP in Promoting Tooth
Tooth whitening becomes more and more popular, because
the social demand for whiter and brighter smiles is increasing
. Beside in-office bleaching, several oral care products for
home use show whitening properties [105 - 108]. Most white-
ning toothpastes on the market are characterized by high
abrasiveness. Their effect relies on removing the outermost
stained layer of enamel which may lead to increased roughness
or other side effects, such as dentin hypersensitivity [91, 104,
105, 109]. In addition to an increased amount of abrasives,
toothpastes often use phosphate-systems for stain removal
[106, 107]. However, only extrinsic stain can be removed when
using whitening toothpastes. (20), (21), (22), (23), (24) There
are studies showing an effect of HAP with respect to tooth
whitening [110 - 114] and their results are promising. Basi-
cally, HAP acts as cleaning agent, but in contrast to other
abrasives used for tooth whitening products (i.e. perlite and
alumina) it has the same hardness as enamel . Con-
sequently, HAP does not lead to excessive enamel and dentin
abrasion. (20) Dabanoglu et al. showed whitening properties of
HAP. HAP-particles remained stable on the tooth surface after
application of hydrodynamic shear force . (21), (22), (23)
The whitening-effect can be attributed to an attachment of
HAP to the tooth surface, rather than a polishing process [111 -
113]. (23) The whitening effect of HAP could also originate
from physical properties related to its particulate structure
. (24) In vivo results showed whitening effects of HAP-
particles . These results were verified with in vitro testing:
Whitening properties of HAP are mainly based on diffuse
reflection leading to optical whitening effects .
Besides removing stain mechanically from tooth surfaces
during routine oral hygiene, HAP particles might also bind
proteins which often lead to discolorations. These would then
be removed together with the particles during the cleaning
process. The protective layer formed by HAP particles on tooth
surfaces appears inherently white in colour as long as it
remains firmly attached to the tooth surface. The size and
irregular orientation of the HAP crystallites constituting the
particles makes them ideal scatterers within the wavelength
range of visible light. The resulting reflection of light makes
the tooth surface appear even brighter white than natural,
untreated tooth surfaces, especially if they are naturally darker
tainted or contain incorporated amounts of extrinsic stains.
In conclusion, HAP particles are promising agents for
tooth whitening, as they do not lead to tooth abrasion. HAP
particles might remove staining proteins, cover tooth surfaces
and enhance the perceived whiteness by scattering and ref-
lection of light.
4. MODES OF ACTION OF HAP IN PREVENTIVE
ORAL HEALTH CARE: CONCLUSIONS AND
In contrast to other active ingredients in oral care, HAP
represents a multifunctional biomimetic agent whose different
effects in preventive oral health care are based both on its bio-
chemical activity and the ability to physically interact with the
oral cavity environment due to its particulate nature. The
modes of action of HAP-particles in preventive oral care
derived from data of the analyzed publications are summarized
in Table 1. They are mainly based on the following principles:
Table 1. Multifunctionality of HAP and correlation of the proposed modes of action with the described preventive properties
of HAP. The numbers indicated in the table refer to the respective sections mentioned in the text.
Effects Modes of Action of HAP Caries Periodontitis Erosion Dentin
(dentin and enamel)
Physical: Attachment and fusion of HAP-particles
with tooth tissue: (3), (6), (7), (8), (11), (14)
Bio-chemical: Local dissolution, ion donor,
crystallization nucleus: (1), (2), (4), (5), (12), (13)
X --- X X ---
Physical: Loose/loosely attached HAP-particles as
substrate substitute, ingestion and/or spit-out, abrasive
effects during application: (6), (7), (8), (10)
Bio-chemical: Binding affinity: (6), (7), (9)
Biological: Settlement/substrate affinity of different
microorganisms: (6), (7), (8)
X X --- --- (X)
Protective layer Physical: Attachment of HAP-particles, layer
formation: (3), (11), (21), (22), (23), (24)
Bio-chemical: Formation of interphase at interface
between HAP crystallites from tooth and synthetic
X X X X X
284 The Open Dentistry Journal, 2019, Volume 13 Enax et al.
Effects Modes of Action of HAP Caries Periodontitis Erosion Dentin
Occlusion of dentin tubules Physical: Introduction of HAP-particles into tubules,
layer formation: (15), (16), (17), (18), (19)
Bio-chemical: Local dissolution, ion donor,
crystallization nucleus, fusion: (19)
--- --- --- X ---
Calcium and phosphate-
Physical: Attachment of HAP-particles, layer
formation: (3), (11), (14)
Bio-chemical: Local dissolution through extrinsic
causes, ion donor: (2)
X --- X --- ---
Cleaning Physical: HAP-particles as cleaning agent X X --- --- X
Binding of proteins and
Physical: Loose/loosely attached HAP-particles as
substrate, ingestion and/or spit-out: (7)
Bio-chemical: Binding affinity: (7)
Biological: Settlement/substrate stimulus/repellent for
different microorganisms: (7), (8), (9), (10)
X X --- --- X
Compensatory substrate Bio-chemical: Shift of the ion-balance to a
supersaturated state of calcium and phosphate ions:
X --- X X ---
(I) Physical principles (e.g. attachment of HAP-particles to
the tooth surface and cleaning properties) [23, 24, 34, 48, 91].
(II) Bio-chemical principles (e.g. source of calcium and
phosphate ions  and formation of an interface between
HAP-particles and the enamel surface) .
(III) Biological principles (e.g. HAP-particles interact with
microorganisms [reduction of bacterial colonization to tooth
Several studies show the efficacy of HAP with respect to
prevention and remineralization of caries, biofilm control,
protection against erosion, and relief of dentin hypersensitivity
[3, 6, 23, 25, 26, 35, 36, 41, 48, 71, 73 - 75, 77, 102].
Furthermore, HAP particles within oral care products can be
used for tooth whitening [110, 114]. Due to its similarity to the
tooth mineral phase, HAP is considered a biocompatible and
biomimetic agent, and as such the applicable dosage is not
limited by health concerns as it is the case with fluoride in
fluorosis [42, 44]. Thus, oral care products with HAP can be
used for all age-groups, including children. From the studies
investigated and discussed it became evident that the effect of
HAP on each of the clinical conditions relies on more than one
mode of action. Therefore, HAP may experience future uses in
other fields too. Based on the current state of the art, it can be
concluded that HAP can be used as active biomimetic
ingredient in preventive oral care for several indications. For
future research, the physical, bio-chemical, and biological
mechanisms of action need to be investigated in much greater
detail using suitable in vitro and in vivo approaches, especially
the interactions of HAP particles with teeth under conditions
that mimic the situation in the oral cavity including pellicle,
biofilms, and the entire spectrum of clinical conditions. This
will enable to optimize the intrinsic modes of action and thus
the efficiency of HAP by tailoring formulations of applications
and the physical and chemical properties of the particles
In summary, HAP is a promising multifunctional bio-
mimetic active ingredient with verified efficacy for different
oral health concerns.
CONSENT FOR PUBLICATION
CONFLICT OF INTEREST
The authors declare no conflict of interest, financial or
The authors thank Dr. Med. Dent. Barbara Simader for
Dorozhkin SV, Epple M. Biological and medical significance of
calcium phosphates. Angew Chem Int Ed Engl 2002; 41(17): 3130-46.
NIE3130>3.0.CO;2-1] [PMID: 12207375]
LeGeros RZ. Apatites in biological systems. Prog Cryst Growth
Charact 1981; 4: 1-45.
Enax J, Epple M. Synthetic hydroxyapatite as a biomimetic oral care
agent. Oral Health Prev Dent 2018; 16(1): 7-19.
Lowenstam HA, Weiner S. On Biomineralization. Oxford University
Brown PW, Constantz B. Hydroxyapatite and related materials. Boca
Raton: CRC Press 1994.
Meyer F, Amaechi BT, Fabritius HO, Enax J. Overview of calcium
phosphates used in biomimetic oral care. Open Dent J 2018; 12:
[http://dx.doi.org/10.2174/1874210601812010406] [PMID: 29988215]
Enax J, Prymak O, Raabe D, Epple M. Structure, composition, and
mechanical properties of shark teeth. J Struct Biol 2012; 178(3):
[http://dx.doi.org/10.1016/j.jsb.2012.03.012] [PMID: 22503701]
Enax J, Janus AM, Raabe D, Epple M, Fabritius HO. Ultrastructural
organization and micromechanical properties of shark tooth
enameloid. Acta Biomater 2014; 10(9): 3959-68.
[http://dx.doi.org/10.1016/j.actbio.2014.04.028] [PMID: 24797528]
Moll K-J, Moll M. Kurzlehrbuch Anatomie. Urban and Fischer 2000.
Moradian-Oldak J. Protein-mediated enamel mineralization. Front
Biosci 2012; 17: 1996-2023.
[http://dx.doi.org/10.2741/4034] [PMID: 22652761]
Moradian-Oldak J. The regeneration of tooth enamel. Dimens Dent
Hyg 2009; 7(8): 12-5.
(Table ) contd .....
Modes of Action of Hydroxyapatite The Open Dentistry Journal, 2019, Volume 13 285
Fejerskov O, Kidd E. Dental caries: The disease and its clinical
management. Wiley 2009.
Petersen PE, Bourgeois D, Ogawa H, Estupinan-Day S, Ndiaye C. The
global burden of oral diseases and risks to oral health. Bull World
Health Organ 2005; 83(9): 661-9.
Lussi A, Ganss C. Erosive Tooth Wear: From Diagnosis to Therapy. S.
Karger AG 2014.
Lussi A, Hellwig E, Klimek J. Fluorides - mode of action and
recommendations for use. Schweiz Monatsschr Zahnmed 2012;
ten Cate JM. Contemporary perspective on the use of fluoride products
in caries prevention. Br Dent J 2013; 214(4): 161-7.
[http://dx.doi.org/10.1038/sj.bdj.2013.162] [PMID: 23429124]
Walsh T, Worthington HV, Glenny AM, Appelbe P, Marinho VC, Shi
X. Fluoride toothpastes of different concentrations for preventing
dental caries in children and adolescents. Cochrane Database Syst Rev
2010; (1): CD007868
Craig RG, Welker D, Rothaut J, et al. Dental materials. Wiley Online
Lübke A, Enax J, Wey K, Fabritius HO, Raabe D, Epple M.
Composites of fluoroapatite and methylmethacrylate-based polymers
(PMMA) for biomimetic tooth replacement. Bioinspir Biomim 2016;
[http://dx.doi.org/10.1088/1748-3190/11/3/035001] [PMID: 27159
Kassebaum NJ, Bernabé E, Dahiya M, Bhandari B, Murray CJ,
Marcenes W. Global burden of untreated caries: A systematic review
and metaregression. J Dent Res 2015; 94(5): 650-8.
[http://dx.doi.org/10.1177/0022034515573272] [PMID: 25740856]
Kassebaum NJ, Smith AGC, Bernabé E, et al. Global, regional, and
national prevalence, incidence, and disability-adjusted life years for
oral conditions for 195 countries, 1990-2015: A systematic analysis for
the global burden of diseases, injuries, and risk factors. J Dent Res
2017; 96(4): 380-7.
[http://dx.doi.org/10.1177/0022034517693566] [PMID: 28792274]
Hannig M, Hannig C. Nanomaterials in preventive dentistry. Nat
Nanotechnol 2010; 5(8): 565-9.
[http://dx.doi.org/10.1038/nnano.2010.83] [PMID: 20581832]
Kensche A, Holder C, Basche S, Tahan N, Hannig C, Hannig M.
Efficacy of a mouthrinse based on hydroxyapatite to reduce initial
bacterial colonisation in situ. Arch Oral Biol 2017; 80: 18-26.
[http://dx.doi.org/10.1016/j.archoralbio.2017.03.013] [PMID: 2836
Fabritius-Vilpoux K, Enax J, Herbig M, Raabe D, Fabritius H-O.
Quantitative affinity parameters of synthetic hydroxyapatite and
enamel surfaces in vitro. Bioinspir Biomim Nan 2019; 8(2): 141-53.
Harks I, Jockel-Schneider Y, Schlagenhauf U, et al. Impact of the
daily use of a microcrystal hydroxyapatite dentifrice on de novo
plaque formation and clinical/microbiological parameters of perio-
dontal health. A randomized trial. PLoS One 2016; 11(7): e0160142.
[http://dx.doi.org/10.1371/journal.pone.0160142] [PMID: 27467683]
Schlagenhauf U, Kunzelmann KH, Hannig C, et al. Impact of a non-
fluoridated microcrystalline hydroxyapatite dentifrice on enamel caries
progression in highly caries-susceptible orthodontic patients: A
randomized, controlled 6-month trial. J Investig Clin Dent 2019;
[http://dx.doi.org/10.1111/jicd.12399] [PMID: 30701704]
Hu M-L, Zheng G, Zhang Y-D, Yan X, Li X-C, Lin H. Effect of
desensitizing toothpastes on dentine hypersensitivity: A systematic
review and meta-analysis. J Dent 2018; 75: 12-21.
[http://dx.doi.org/10.1016/j.jdent.2018.05.012] [PMID: 29787782]
Amaechi BT, Mathews SM, Ramalingam K, Mensinkai PK.
Evaluation of nanohydroxyapatite-containing toothpaste for occluding
dentin tubules. Am J Dent 2015; 28(1): 33-9.
Amaechi BT, Lemke KC, Saha S, Gelfond J. Clinical efficacy in
relieving dentin hypersensitivity of nanohydroxyapatite-containing
cream: A randomized controlled trial. Open Dent J 2018; 12: 572-85.
[http://dx.doi.org/10.2174/1874210601812010572] [PMID: 30288181]
Meyer F, Enax J. Hydroxyapatite in oral biofilm management. Eur J
Dent (in press)
Meyer F, Enax J. Early childhood caries: Epidemiology, aetiology, and
prevention. Int J Dent 2018; 2018: 1415873.
[http://dx.doi.org/10.1155/2018/1415873] [PMID: 29951094]
Loveren Cv. Toothpastes. Monogr Oral Sci 2013; 23: 61-74.
Granito RN, Muniz Renno AC, Yamamura H, de Almeida MC, Menin
Ruiz PL, Ribeiro DA. Hydroxyapatite from fish for bone tissue
engineering: A promising approach. Int J Mol Cell Med 2018; 7(2):
Kani K, Kani M, Isozaki A, Shintani H, Ohashi T, Tokumoto T. Effect
of apatite-containing dentifrices on dental caries in school children. J
Dent Health 1989; 19: 104-9.
Najibfard K, Ramalingam K, Chedjieu I, Amaechi BT.
Remineralization of early caries by a nano-hydroxyapatite dentifrice. J
Clin Dent 2011; 22(5): 139-43.
Roveri N, Battistella E, Foltran I, Foresti E, Iafisco M, Lelli M, et al.
Synthetic biomimetic carbonate-hydroxyapatite nanocrystals for
enamel remineralization. Adv Mat Res 2008; 47-50: 821-4.
Lu X, Zhang H, Guo Y, Wang Y, Ge X, Leng Y, et al. Hexagonal
hydroxyapatite formation on TiO2 nanotubes under urea modulation.
CrystEngComm 2011; 13: 3741-9.
Gillam DG. Advances in diagnosis, management, and treatment. In:
Dentine hypersensitivity. Springer 2015; pp. 63-70.
Daculsi G, Kerebel B. High-resolution electron microscope study of
human enamel crystallites: size, shape, and growth. J Ultrastruct Res
1978; 65(2): 163-72.
[http://dx.doi.org/10.1016/S0022-5320(78)90053-9] [PMID: 731784]
Cosola S, Marconcini S, Giammarinaro E, Marchisio O, Lelli M,
Roveri N, et al. Antimicrobial efficacy of mouthwashes containing
zinc-substituted nanohydroxyapatite and zinc L-pyrrolidone
carboxylate on suture threads after surgical procedures. J Oral Sci
Rehabil 2017; 3(4): 24-30.
Hegazy SA, Salama IR. Antiplaque and remineralizing effects of
Biorepair mouthwash: A comparative clinical trial. Pediatr Dent J
2016; 26: 89-94.
Epple M. Review of potential health risks associated with nanoscopic
calcium phosphate. Acta Biomater 2018; 77: 1-14.
[http://dx.doi.org/10.1016/j.actbio.2018.07.036] [PMID: 30031162]
Ramis J, Coelho C, Córdoba A, Quadros P, Monjo M. Safety
assessment of nano-hydroxyapatite as an oral care ingredient
according to the EU cosmetics regulation. Cosmetics 2018; 5(3): 53.
Limeback H, Robinson C. Fluoride therapy Comprehensive preventive
dentistry. John Wiley & Sons 2012; Chap 16: pp. 251-82.
Limeback H. Comprehensive preventive dentistry. John Wiley &
Sons, Ltd. 2012.
Bashash M, Thomas D, Hu H, et al. Prenatal fluoride exposure and
cognitive outcomes in children at 4 and 6-12 years of age in Mexico.
Environ Health Perspect 2017; 125(9): 097017.
[http://dx.doi.org/10.1289/EHP655] [PMID: 28937959]
Hannig C, Basche S, Burghardt T, Al-Ahmad A, Hannig M. Influence
of a mouthwash containing hydroxyapatite microclusters on bacterial
adherence in situ. Clin Oral Investig 2013; 17(3): 805-14.
[http://dx.doi.org/10.1007/s00784-012-0781-6] [PMID: 22782257]
Lelli M, Putignano A, Marchetti M, et al. Remineralization and repair
of enamel surface by biomimetic Zn-carbonate hydroxyapatite
containing toothpaste: A comparative in vivo study. Front Physiol
2014; 5: 333.
[http://dx.doi.org/10.3389/fphys.2014.00333] [PMID: 25249980]
Vos T, Abajobir AA, Abate KH, Abbafati C, Abbas KM, Abd-Allah F,
et al. GBD 2016 Disease and Injury Incidence and Prevalence
Collaborators. Global, regional, and national incidence, prevalence,
and years lived with disability for 328 diseases and injuries for 195
countries, 1990-2016: A systematic analysis for the Global Burden of
Disease Study 2016. Lancet 2017; 390(10100): 1211-59.
[http://dx.doi.org/10.1016/S0140-6736(17)32154-2] [PMID: 2891
286 The Open Dentistry Journal, 2019, Volume 13 Enax et al.
Frencken JE, Sharma P, Stenhouse L, Green D, Laverty D, Dietrich T.
Global epidemiology of dental caries and severe periodontitis - A
comprehensive review. J Clin Periodontol 2017; 44(Suppl. 18): S94-
[http://dx.doi.org/10.1111/jcpe.12677] [PMID: 28266116]
Meyer F, Karch A, Schlinkmann KM, et al. Sociodemographic
determinants of spatial disparities in early childhood caries: An
ecological analysis in Braunschweig, Germany. Community Dent Oral
Epidemiol 2017; 45(5): 442-8.
[http://dx.doi.org/10.1111/cdoe.12308] [PMID: 28547864]
Kilian M, Chapple ILC, Hannig M, et al. The oral microbiome - An
update for oral healthcare professionals. Br Dent J 2016; 221(10):
[http://dx.doi.org/10.1038/sj.bdj.2016.865] [PMID: 27857087]
Takahashi N, Nyvad B. The role of bacteria in the caries process:
ecological perspectives. J Dent Res 2011; 90(3): 294-303.
[http://dx.doi.org/10.1177/0022034510379602] [PMID: 20924061]
Sanz M, Beighton D, Curtis MA, et al. Role of microbial biofilms in
the maintenance of oral health and in the development of dental caries
and periodontal diseases. Consensus report of group 1 of the Joint
EFP/ORCA workshop on the boundaries between caries and
periodontal disease. J Clin Periodontol 2017; 44(Suppl. 18): S5-S11.
[http://dx.doi.org/10.1111/jcpe.12682] [PMID: 28266109]
Lingström P, van Ruyven FO, van Houte J, Kent R. The pH of dental
plaque in its relation to early enamel caries and dental plaque flora in
humans. J Dent Res 2000; 79(2): 770-7.
[http://dx.doi.org/10.1177/00220345000790021101] [PMID: 10728
Pihlstrom BL, Michalowicz BS, Johnson NW. Periodontal diseases.
Lancet 2005; 366(9499): 1809-20.
[http://dx.doi.org/10.1016/S0140-6736(05)67728-8] [PMID: 16298
Tonetti MS, Jepsen S, Jin L, Otomo-Corgel J. Impact of the global
burden of periodontal diseases on health, nutrition and wellbeing of
mankind: A call for global action. J Clin Periodontol 2017; 44(5):
[http://dx.doi.org/10.1111/jcpe.12732] [PMID: 28419559]
Meuric V, Le Gall-David S, Boyer E, et al. Signature of microbial
dysbiosis in periodontitis. Appl Environ Microbiol 2017; 83(14):
[http://dx.doi.org/10.1128/AEM.00462-17] [PMID: 28476771]
Deng Z-L, Szafrański SP, Jarek M, Bhuju S, Wagner-Döbler I.
Dysbiosis in chronic periodontitis: Key microbial players and
interactions with the human host. Sci Rep 2017; 7(1): 3703.
[http://dx.doi.org/10.1038/s41598-017-03804-8] [PMID: 28623321]
Cardoso EM, Reis C, Manzanares-Céspedes MC. Chronic
periodontitis, inflammatory cytokines, and interrelationship with other
chronic diseases. Postgrad Med 2018; 130(1): 98-104.
[http://dx.doi.org/10.1080/00325481.2018.1396876] [PMID: 2906
Marsh PD, Head DA, Devine DA. Ecological approaches to oral
biofilms: Control without killing. Caries Res 2015; 49 (Suppl.1):
[http://dx.doi.org/10.1159/000377732] [PMID: 25871418]
Meyer F, Enax J. Die Mundhöhle als Ökosystem. Biol Unserer Zeit
2018; 48(1): 62-8.
Marsh PD. Contemporary perspective on plaque control. Br Dent J
2012; 212(12): 601-6.
[http://dx.doi.org/10.1038/sj.bdj.2012.524] [PMID: 22722123]
Marsh PD. In sickness and in health-what does the oral microbiome
mean to us? An ecological perspective. Adv Dent Res 2018; 29(1):
[http://dx.doi.org/10.1177/0022034517735295] [PMID: 29355410]
Amaechi BT. Dental Erosion and Its Clinical Management. Springer
Ramachandran A, Raja Khan SI, Vaitheeswaran N. Incidence and
pattern of dental erosion in gastroesophageal reflux disease patients. J
Pharm Bioallied Sci 2017; 9 (Suppl.1): S138-41.
[http://dx.doi.org/10.4103/jpbs.JPBS_125_17] [PMID: 29284953]
Bretz WA. Oral profiles of bulimic women: Diagnosis and
management. What is the evidence? J Evid Based Dent Pract 2002;
[http://dx.doi.org/10.1016/S1532-3382(02)70078-X] [PMID: 2228
Splieth CH, Tachou A. Epidemiology of dentin hypersensitivity. Clin
Oral Investig 2013; 17(Suppl. 1): S3-8.
[http://dx.doi.org/10.1007/s00784-012-0889-8] [PMID: 23224064]
Addy M, West NX. The role of toothpaste in the aetiology and
treatment of dentine hypersensitivity. Monogr Oral Sci 2013; 23:
[http://dx.doi.org/10.1159/000350477] [PMID: 23817061]
van Loveren C, Schmidlin PR, Martens LC, Amaechi BT. Dentin
hypersensitivity management. Clin Dent Rev 2018; 2(1): 6.
Tschoppe P, Zandim DL, Martus P, Kielbassa AM. Enamel and
dentine remineralization by nano-hydroxyapatite toothpastes. J Dent
2011; 39(6): 430-7.
[http://dx.doi.org/10.1016/j.jdent.2011.03.008] [PMID: 21504777]
de Carvalho FG, Vieira BR, Santos RL, Carlo HL, Lopes PQ, de Lima
BA. In vitro effects of nano-hydroxyapatite paste on initial enamel
carious lesions Pediatr Dent 2014; 36(3): 85-9.
Colombo M, Beltrami R, Rattalino D, Mirando M, Chiesa M, Poggio
C. Protective effects of a zinc-hydroxyapatite toothpaste on enamel
erosion: SEM study. Ann Stomatol (Roma) 2017; 7(3): 38-45.
Poggio C, Gulino C, Mirando M, Colombo M, Pietrocola G. Protective
effect of zinc-hydroxyapatite toothpastes on enamel erosion: An in
vitro study. J Clin Exp Dent 2017; 9(1): e118-22.
Orsini G, Procaccini M, Manzoli L, Giuliodori F, Lorenzini A,
Putignano A. A double-blind randomized-controlled trial comparing
the desensitizing efficacy of a new dentifrice containing carbonate
/hydroxyapatite nanocrystals and a sodium fluoride /potassium nitrate
dentifrice. J Clin Periodontol 2010; 37(6): 510-7.
[http://dx.doi.org/10.1111/j.1600-051X.2010.01558.x] [PMID: 20507
Orsini G, Procaccini M, Manzoli L, et al. A 3-day randomized clinical
trial to investigate the desensitizing properties of three dentifrices. J
Periodontol 2013; 84(11): e65-73.
[http://dx.doi.org/10.1902/jop.2013.120697] [PMID: 23489232]
Hiller K-A, Buchalla W, Grillmeier I, Neubauer C, Schmalz G. In
vitro effects of hydroxyapatite containing toothpastes on dentin
permeability after multiple applications and ageing Sci Rep 2018;
[http://dx.doi.org/10.1038/s41598-018-22764-1] [PMID: 29559639]
Vano M, Derch G, Barone A, Covani U. Effectiveness of nano-
hydroxyapatite toothpaste in reducing dentin hypersensitivity: A
double-blind randomized controlled trial. Quint int 2014; 45: 703-11.
Schlagenhauf U, Kunzelmann K-H, Hannig C, et al. Impact of a non-
fluoridated microcrystalline hydroxyapatite dentifrice on enamel caries
progression in highly caries-susceptible orthodontic patients: A
randomized, controlled 6-month trial. J Investig Clin Dent 2019;
[http://dx.doi.org/10.1111/jicd.12399] [PMID: 30701704]
Enaia M, Bock N, Ruf S. White-spot lesions during multibracket
appliance treatment: A challenge for clinical excellence. Am J Orthod
Dentofacial Orthop 2011; 140(1): e17-24.
[http://dx.doi.org/10.1016/j.ajodo.2010.12.016] [PMID: 21724067]
O'Reilly MM, Featherstone JDB. Demineralization and
remineralization around orthodontic appliances: An in vivo study
American journal of orthodontics and dentofacial orthopedics: Official
publication of the American Association of Orthodontists, its
constituent societies, and the American Board of Orthodontics 1987;
Esteves-Oliveira M, Meyer-Lueckel H, Wierichs RJ, Santos NM,
Rodrigues JA. Caries-preventive effect of anti-erosive and nano-
hydroxyapatite-containing toothpastes in vitro. Clin Oral Investig
2016; 21(1): 291-300.
Huang SB, Gao SS, Yu HY. Effect of nano-hydroxyapatite
concentration on remineralization of initial enamel lesion in vitro
Biomed Mater 2009; 4: 034104/1-/6.
Gjorgievska ES, Nicholson JW, Slipper IJ, Stevanovic MM.
Remineralization of demineralized enamel by toothpastes: A scanning
electron microscopy, energy dispersive X-ray analysis, and three-
dimensional stereo-micrographic study. Microsc Microanal 2013;
[http://dx.doi.org/10.1017/S1431927613000391] [PMID: 23659606]
Hannig C, Hannig M. The oral cavity-A key system to understand
substratum-dependent bioadhesion on solid surfaces in man. Clin Oral
Investig 2009; 13(2): 123-39.
[http://dx.doi.org/10.1007/s00784-008-0243-3] [PMID: 19137331]
Modes of Action of Hydroxyapatite The Open Dentistry Journal, 2019, Volume 13 287
Astasov-Frauenhoffer M, Varenganayil MM, Decho AW, Waltimo T,
Braissant O. Exopolysaccharides regulate calcium flow in cariogenic
biofilms. PLoS One 2017; 12(10): e0186256.
[http://dx.doi.org/10.1371/journal.pone.0186256] [PMID: 29023506]
Vogel GL, Zhang Z, Carey CM, Ly A, Chow LC, Proskin HM.
Composition of plaque and saliva following a sucrose challenge and
use of an alpha-tricalcium-phosphate-containing chewing gum. J Dent
Res 1998; 77(3): 518-24.
[http://dx.doi.org/10.1177/00220345980770031101] [PMID: 9496925]
Young A, Smistad G, Karlsen J, Rölla G, Rykke M. Zeta potentials of
human enamel and hydroxyapatite as measured by the Coulter DELSA
440. Adv Dent Res 1997; 11(4): 560-5.
[http://dx.doi.org/10.1177/08959374970110042501] [PMID: 9470517]
Reynolds EC, Wong A. Effect of adsorbed protein on hydroxyapatite
zeta potential and Streptococcus mutans adherence. Infect Immun
1983; 39(3): 1285-90.
Hagenfeld D, Prior K, Harks I, et al. No differences in microbiome
changes between anti-adhesive and antibacterial ingredients in
toothpastes during periodontal therapy. J Periodontal Res 2019; 54(4):
[http://dx.doi.org/10.1111/jre.12645] [PMID: 30851050]
Enax J, Epple M. Die Charakterisierung von Putzkörpern in
Zahnpasten. Dtsch Zahnarztl Z 2018; 73: 116-24.
Nocerino N, Fulgione A, Iannaccone M, et al. Biological activity of
lactoferrin-functionalized biomimetic hydroxyapatite nanocrystals. Int
J Nanomedicine 2014; 9: 1175-84.
Gorbunoff MJ, Timasheff SN. The interaction of proteins with
hydroxyapatite. III. Mechanism. Anal Biochem 1984; 136(2): 440-5.
[http://dx.doi.org/10.1016/0003-2697(84)90241-0] [PMID: 6721144]
Adams SE, Arnold D, Murphy B, et al. A randomised clinical study to
determine the effect of a toothpaste containing enzymes and proteins
on plaque oral microbiome ecology. Sci Rep 2017; 7: 43344.
[http://dx.doi.org/10.1038/srep43344] [PMID: 28240240]
Huysmans MCDNJM, Chew HP, Ellwood RP. Clinical Studies of
Dental Erosion and Erosive Wear. Caries Res 2011; 45 (suppl1): 60-8.
Aykut-Yetkiner A, Attin T, Wiegand A. Prevention of dentine erosion
by brushing with anti-erosive toothpastes. J Dent 2014; 42(7): 856-61.
[http://dx.doi.org/10.1016/j.jdent.2014.03.011] [PMID: 24704085]
Ganss C, Lussi A, Grunau O, Klimek J, Schlueter N. Conventional and
anti-erosion fluoride toothpastes: Effect on enamel erosion and
erosion-abrasion. Caries Res 2011; 45(6): 581-9.
[http://dx.doi.org/10.1159/000334318] [PMID: 22156703]
Poggio C, Lombardini M, Vigorelli P, Colombo M, Chiesa M. The
role of different toothpastes on preventing dentin erosion: An SEM
and AFM study®. Scanning 2014; 36(3): 301-10.
[http://dx.doi.org/10.1002/sca.21105] [PMID: 23784952]
Poggio C, Gulino C, Mirando M, Colombo M, Pietrocola G.
Preventive effects of different protective agents on dentin erosion: An
in vitro investigation. J Clin Exp Dent 2017; 9(1): e7-e12.
Hornby K, Evans M, Long M, Joiner A, Laucello M, Salvaderi A.
Enamel benefits of a new hydroxyapatite containing fluoride
toothpaste Int Dent J 2009; 59(6S1): 325-1.
Huettemann RW, Doenges H. Untersuchungen zur Therapie
überempfindlicher Zahnhälse mit Hydroxylapatit. Dtsch Zahnarztl Z
1987; 42: 486-8.
Vano M, Derchi G, Barone A, Pinna R, Usai P, Covani U. Reducing
dentine hypersensitivity with nano-hydroxyapatite toothpaste: A
double-blind randomized controlled trial. Clin Oral Investig 2017.
Lenzi TL, Guglielmi CdeA, Arana-Chavez VE, Raggio DP. Tubule
density and diameter in coronal dentin from primary and permanent
human teeth. Microsc Microanal 2013; 19(6): 1445-9.
[http://dx.doi.org/10.1017/S1431927613012725] [PMID: 23947480]
Christensen GJ. Are snow-white teeth really so desirable? J Am Dent
Assoc 2005; 136(7): 933-5.
[http://dx.doi.org/10.14219/jada.archive.2005.0295] [PMID: 16060
Carey CM. Tooth whitening: What we now know. J Evid Based Dent
Pract 2014; 14(Suppl.): 70-6.
[http://dx.doi.org/10.1016/j.jebdp.2014.02.006] [PMID: 24929591]
Soeteman GD, Valkenburg C, Van der Weijden GA, Van Loveren C,
Bakker E, Slot DE. Whitening dentifrice and tooth surface
discoloration-A systematic review and meta-analysis. Int J Dent Hyg
2018; 16(1): 24-35.
[http://dx.doi.org/10.1111/idh.12289] [PMID: 28573755]
Schemehorn BR, Moore MH, Putt MS. Abrasion, polishing, and stain
removal characteristics of various commercial dentifrices in vitro J
Clin Dent 2011; 22(1): 11-8.
Joiner A. Whitening toothpastes: A review of the literature. J Dent
2010; 38(Suppl. 2): e17-24.
[http://dx.doi.org/10.1016/j.jdent.2010.05.017] [PMID: 20562012]
da Silva EM. Maia JNdSMD, Mitraud CG, Russo JdES, Poskus LT,
Guimarães JGA. Can whitening toothpastes maintain the optical
stability of enamel over time? J Appl Oral Sci 2018; 26e20160460
Dabanoglu A, Wood C, García-Godoy F, Kunzelmann KH. Whitening
effect and morphological evaluation of hydroxyapatite materials. Am J
Dent 2009; 22(1): 23-9.
Jin J, Xu X, Lai G, Kunzelmann KH. Efficacy of tooth whitening with
different calcium phosphate-based formulations. Eur J Oral Sci 2013;
[http://dx.doi.org/10.1111/eos.12063] [PMID: 23841792]
Kim BI, Jeong SH, Jang SO, Kim KN, Kwon HK, Park YD. Tooth
whitening effect of toothpastes containing nano-hydroxyapatite. Key
Eng Mater 2006; 309-311: 541-4.
Niwa M, Sato T, Li W, Aoki H, Aoki H, Daisaku T. Polishing and
whitening properties of toothpaste containing hydroxyapatite. J Mater
Sci Mater Med 2001; 12(3): 277-81.
[http://dx.doi.org/10.1023/A:1008927502523] [PMID: 15348313]
Bommer C, Flessa HP, Xu X, Kunzelmann KH. Hydroxyapatite and
self-assembling peptide matrix for non-oxidizing tooth whitening. J
Clin Dent 2018; 29(2): 57-63.
Min JH, Kwon HK, Kim BI. The addition of nano-sized
hydroxyapatite to a sports drink to inhibit dental erosion: In vitro study
using bovine enamel J Dent 2011; 39(9): 629-35.
[http://dx.doi.org/10.1016/j.jdent.2011.07.001] [PMID: 21763390]
Min JH, Kwon HK, Kim BI. Prevention of dental erosion of a sports
drink by nano-sized hydroxyapatite in situ study Int J Paediatr Dent
2015; 25(1): 61-9.
[http://dx.doi.org/10.1111/ipd.12101] [PMID: 24628844]
© 2019 Enax et al.
This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 International Public License (CC-BY 4.0), a copy of which is
available at: https://creativecommons.org/licenses/by/4.0/legalcode. This license permits unrestricted use, distribution, and reproduction in any medium, provided the
original author and source are credited.