No full-text available
To read the full-text of this research,
you can request a copy directly from the author.
Tissue Engineering Strategies in Dental Implantology
Abstract
Tissue engineering and regenerative medicine strategies have gained increased attention in dental implantology, since dental implantology faces new challenges in clinical dentistry. Whereas conventional implant placement and implant loading protocols under ideal anatomical and biological conditions seem to be clinically solved, engineering and regenerative strategies are aimed at allowing implant placement under compromised conditions (lack of bone or mucosa tissue, impaired regenerative capacity, need for early loading protocols). To understand the new approaches in engineering and regenerative medicine therapies, it is important to recapitulate the history and basics of implant dentistry in order assess the clinical impact of new approaches.
... However, conditions as smoking, diabetes, radiotherapy, and postmenopausal estrogen therapy may compromise bone quantity and quality leading to scenarios in which bone healing may become challenging [68]. Through developing biomaterials and strategies that may increase bone cell adhesion, modulate cell signaling, deliver growth factors and promote osteoblast differentiation followed by matrix deposition and mineralization [69,70], it might be possible to place implants even under less than ideal anatomical or biological circumstances [71]. ...
Unlabelled:
Tissue engineering is an interdisciplinary field that combines the principles of engineering, material and biological sciences toward the development of therapeutic strategies and biological substitutes that restore, maintain, replace or improve biological functions. The association of biomaterials, stem cells, growth and differentiation factors has yielded the development of new treatment opportunities in most of the biomedical areas, including Dentistry. The objective of this paper is to present the principles underlying tissue engineering and the current scenario, the challenges and the perspectives of this area in Dentistry.
Significance:
The growth of tissue engineering as a research field has provided a novel set of therapeutic strategies for biomedical applications. Indeed, tissue engineering may lead to new strategies for the clinical management of patients with dental and craniofacial needs in the future.
Craniofacial and dental structures are complex tissues that perform vital functions. Defects and deformities of this region can severely affect an individual's quality of life, leading to both significant physical impairments as well as psychological sequelae. Current standard-of-care reconstructive procedures have significant limitations. Recent advances in bioprinting and nanotechnology have shown promise for the improvement of patient outcomes. Currently, research into craniofacial and dental regenerative medicine therapies have utilized growth factors; however, the use of resorbable scaffolds and autologous or banked cells have not yet become standard of care. There is a revolution occurring in developing 3D printable materials that aims to cover the range of material properties needed for patient-specific implants to either resorb at an appropriate time or perform mechanically like the tissues to which they are attached. As advances are made in bone, retina, teeth, and other areas of craniofacial and dental tissue engineering, they are likely to be incorporated into standard-of-care therapies.
Craniofacial and dental structures are complex tissues that perform vital functions. Defects and deformities of this region can severely affect an individual’s quality of life, leading to both significant physical impairments as well as psychological sequelae. Current standard-of-care reconstructive procedures have significant limitations. Recent advances in bioprinting and nanotechnology have shown promise for the improvement of patient outcomes. Currently, research into craniofacial and dental regenerative medicine therapies have utilized growth factors; however, the use of resorbable scaffolds and autologous or banked cells have not yet become standard of care. There is a revolution occurring in developing 3D printable materials that aims to cover the range of material properties needed for patient-specific implants to either resorb at an appropriate time or perform mechanically like the tissues to which they are attached. As advances are made in bone, retina, teeth, and other areas of craniofacial and dental tissue engineering, they are likely to be incorporated into standard-of-care therapies.
Bone remodeling around three different endosseous dental implant designs placed in dog mandibles was studied using radiography during lengthy periods of function and by histology after animal sacrifice. The three designs investigated were (a) threaded (c.p. titanium), (b) fully porous-coated (titanium alloy), and (c) partially porous-coated (titanium alloy). The implants were kept in function for either 32 weeks (fully porous-coated) or 73 to 77 weeks (partially porous-coated and threaded). The studies indicated that some crestal bone loss occurred for both the threaded and partially porous-coated implants while no significant bone loss was seen with fully porous-coated implants in the absence of plaque-associated infection. It is suggested that these observed differences are a result of the different stress states that develop in bone surrounding the three designs underlying the importance of implant design on bone remodeling.
We report here the results of a histological assessment of the initial healing response following implantation into the dog mandible of a porous-surfaced, titanium alloy endosseous dental implant. Two implants were placed in edentulous areas on each side of the mandible of each dog and covered with a full-thickness mucoperiosteal flap. The implant sites on one side of the mandible were allowed to head for four weeks, while those on the other side were allowed to head for eight weeks before the animals were killed. Histological specimens were obtained and assessed both qualitatively and by computer-assisted morphometry. All but one of the 24 implants were well-tolerated and healed with a variable ingrowth of bone into the porous-surface geometry. The histomorphometric measurements revealed that bone ingrowth had reached a plateau by four weeks of initial healing.
A total of 2895 threaded, cylindrical titanium implants have been inserted into the mandible or the maxilla and 124 similar implants have been installed in the tibial, temporal or iliac bones in man for various bone restorative procedures. The titanium screws were implanted without the use of cement, using a meticulous technique aiming at osseointegration--a direct contact between living bone and implant. Thirty-eight stable and integrated screws were removed for various reasons from 18 patients. The interface zone between bone and implant was investigated using X-rays, SEM, TEM and histology. The SEM study showed a very close spatial relationship between titanium and bone. The pattern of the anchorage of collagen filaments to titanium appeared to be similar to that of Sharpey's fibres to bone. No wear products were seen in the bone or soft tissues in spite of implant loading times up to 90 months. The soft tissues were also closely adhered to the titanium implant, thereby forming a biological seal, preventing microorganism infiltration along the implant. The implants in many cases had been allowed to permanently penetrate the gingiva and skin. This caused no adverse tissue effects. An intact bone-implant interface was analyzed by TEM, revealing a direct bone-to-implant interface contact also at the electron microscopic level, thereby suggesting the possibility of a direct chemical bonding between bone and titanium. It is concluded that the technique of osseointegration is a reliable type of cement-free bone anchorage for permanent prosthetic tissue substitutes. At present, this technique is being tried in clinical joint reconstruction. In order to achieve and to maintain such a direct contact between living bone and implant, threaded, unalloyed titanium screws of defined finish and geometry were inserted using a delicate surgical technique and were allowed to heal in situ, without loading, for a period of at least 3--4 months.
Mechanotransduction plays a crucial role in the physiology of many tissues including bone. Mechanical loading can inhibit bone resorption and increase bone formation in vivo. In bone, the process of mechanotransduction can be divided into four distinct steps: (1) mechanocoupling, (2) biochemical coupling, (3) transmission of signal, and (4) effector cell response. In mechanocoupling, mechanical loads in vivo cause deformations in bone that stretch bone cells within and lining the bone matrix and create fluid movement within the canaliculae of bone. Dynamic loading, which is associated with extracellular fluid flow and the creation of streaming potentials within bone, is most effective for stimulating new bone formation in vivo. Bone cells in vitro are stimulated to produce second messengers when exposed to fluid flow or mechanical stretch. In biochemical coupling, the possible mechanisms for the coupling of cell-level mechanical signals into intracellular biochemical signals include force transduction through the integrin-cytoskeleton-nuclear matrix structure, stretch-activated cation channels within the cell membrane, G protein-dependent pathways, and linkage between the cytoskeleton and the phospholipase C or phospholipase A pathways. The tight interaction of each of these pathways would suggest that the entire cell is a mechanosensor and there are many different pathways available for the transduction of a mechanical signal. In the transmission of signal, osteoblasts, osteocytes, and bone lining cells may act as sensors of mechanical signals and may communicate the signal through cell processes connected by gap junctions. These cells also produce paracrine factors that may signal osteoprogenitors to differentiate into osteoblasts and attach to the bone surface. Insulin-like growth factors and prostaglandins are possible candidates for intermediaries in signal transduction. In the effector cell response, the effects of mechanical loading are dependent upon the magnitude, duration, and rate of the applied load. Longer duration, lower amplitude loading has the same effect on bone formation as loads with short duration and high amplitude. Loading must be cyclic to stimulate new bone formation. Aging greatly reduces the osteogenic effects of mechanical loading in vivo. Also, some hormones may interact with local mechanical signals to change the sensitivity of the sensor or effector cells to mechanical load.
It has been suggested that the long-term success of blade implants could be improved by placing the implants using a two-stage surgical protocol to allow the implant to develop a direct bony interface instead of a fibrous tissue interface. This study compared the implant interfaces of delayed and immediately loaded blade implants in nonhuman primates. A second objective was to compare the effects of hydroxylapatite coating on delayed and immediate loading applications. The delayed loaded uncoated titanium and hydroxylapatite-coated blade implants did osseointegrate. This study also suggests that hydroxylapatite-coated blade implants may form a direct bony interface even when they are loaded immediately after implantation, provided that they are splinted to a firm natural tooth.
Concerning the biomechanical function of the different hard tissues, it is important to analyze the gross arrangement of the mineralized collagen fiber bundles. While the characteristic pattern of parallel-arranged fiber bundles has been described in the lamellae of the osteons, the collagen fibers in dentine form a network. We shall concentrate on the small dimensions of the collagen fibers with reference to the way in which the interior and the surface regions of the fibers are mineralized. At this point it should be also mentioned that normally the whole collagen fibers in the different hard tissues become mineralized. However, when the collagen fibers are parallely arranged in thicker bundles, as in lamellar bone, cementum, and turkey tibia tendon, interior regions might exist that are less mineralized, probably because of a restricted influx of Ca and phosphate ions. Further, the minieralization of the surface region of the collagen fibers is connected with bound noncollagenous proteins.
A study was conducted to investigate the hypothesis that mechanical loading of implants and the consequent stress and strain fields influence bone modeling and remodeling at the bone-implant interface. Two implants ad modum Brånemark were placed in each of 20 canine tibiae, allowed to heal for I year, and then subjected to a controlled loading protocol. Implants in the left limb were loaded in axial tension with a triangular waveform (300 N maximum, 10 N minimum, 330 N/s) for 500 cycles per day for 5 consecutive days; implants in the right limb served as unloaded controls. Twelve weeks after loading, polished undecalcified thick sections were examined with light and scanning electron microscopy to provide bone modeling and remodeling data, including the surface area of periosteal and endocortical modeling, the percentage of mineralized tissue in the bone threads, and the frequency of occurrence of preloading and postloading fluorochrome bone labels. Also, a three-dimensional finite element model was developed to investigate the strain state in the bone near loaded implants. The morphometric data were statistically analyzed in terms of individual load-control pairs and showed the following trends for loaded implants: (1) a net bone loss near the coronal portion of the implant, (2) a smaller percentage of mineralized tissue in the cortex, and (3) a decreased frequency of occurrence of postloading fluorochrome bone label in the cortex adjacent to the implant. The finite element model indicated regions of high strain on the periosteal surface adjacent to the loaded implants. The results support the premise that the bone loss observed around the neck of the loaded implants at 12 weeks postloading was a consequence of bone modeling and remodeling secondary to bone microdamage caused by the loading protocol. This scenario, as well as certain other features of the bone response at the interfaces, can be interpreted in light of existing bone modeling and remodeling theories that relate bone activities and mechanical loading. When considering any load-bearing implant in bone, it is important to understand not only the mechanics of stress transfer at the interface, but also the biological response of the interfacial tissues to these stresses. The ability to maintain a healthy bone-implant interface is widely believed to be critical to the survival of the implant. JOMI on CD-ROM, 1994 Mar (345-360): Mechanical Loading of Brånemark Implants Af… Copyrights © 1997 Quinte… 1 Although success rates for most dental implant designs are good, implant failure is often characterized by minor to massive bone loss, implant mobility, and the inability of the implant to perform its intended function.2 The importance of mechanical factors in dental implant failure cannot be overlooked given the increased incidence of component fracture,3,4 coronal bone resorption,5 and fixture loss6 in situations of compromised prosthetic reconstruction and long-term loading. The exact nature of the mechanical stimuli resulting in bone destruction or deposition and the biological response to such stimuli has yet to be established. The objective of this study was to investigate interfacial bone response to implant loading in a controlled model system and to examine the response of the interfacial tissues in terms of general bone modeling and remodeling concepts. The bone biology literature has used the terms bone modeling and bone remodeling nearly interchangeably to describe almost all types of bone growth and turnover. Recent publications7,8 have distinguished between the two terms and have given separate definitions, which will be adopted here. Bone modeling has been defined as the deposition or removal of bone from bone surfaces. Modeling produces an overall change in the size or shape of the bone via activation followed by either formation or resorption. Bone remodeling, however, involves activation followed by a sequence of bone resorption and formation that results in the formation of a secondary osteon within the existing bone tissue. A remodeling cycle rarely alters gross bone architecture or size.
The fit of implant supported fixed prostheses is said to be of clinical concern because of the rigid fixation of an oral implant in its surrounding bone. The influence of the torque sequence of the set screws during fixation of implant supported fixed full prostheses on the final pre‐load was investigated in vitro. No significant effect of the torque sequence of the set screws on the final pre‐load was observed. The main objective of this study was to quantify and qualify the pre‐load in vivo on implants supporting a fixed full prosthesis. This was performed when the prostheses were supported by all five or six implants and was repeated when the prostheses were supported by only four and three implants. A total of 13 patients with a fixed full implant supported prosthesis were selected. The existing abutments were changed for strain gauged abutments. After tightening the set screws with a torque of 10 N cm, the pre‐load conditions were registered. The average (SEM) axial forces and bending moments in case of five or six, four and three supporting implants were 323 N (43 N), 346 N (59 N), 307 N (60 N) 21 N cm (3 N cm) and 21 N cm (2 N cm), 23 N cm (5 N cm), respectively. In addition, the pre‐load was registered after fixation of a machined gold cylinder, as delivered by the manufacturer, on each of the supporting implants, representing the ‘optimal fit’ situation. The corresponding average (SEM) axial forces and bending moments in case of five or six, four and three supporting implants were 426 N (36 N), 405 N (40 N), 413 N (46 N) and 8 N cm (1 N cm), 8 N cm (1 N cm), 8 N cm (1 N cm), respectively. The induced axial forces after tightening the prostheses were significantly lower then after tightening the gold cylinder in case of five or six supporting implants (P P Document Type: Research Article DOI: http://dx.doi.org/10.1111/j.1365-2842.2001.00665.x Affiliations: 1: Department of Prosthetic Dentistry, BIOMAT Research Group, K.U.Leuven, U.Z. St. Raphaël, Kapucijnenvoer 33, B-3000 Leuven, Belgium 2: Department of Mechanics, Division of Biomechanics and Engineering Design, K.U.Leuven, Celestijnenlaan 200 A, B-3001 Heverlee, Belgium 3: Department of Electrotechnics, ESAT-MICAS, K.U.Leuven, Kardinaal Mercierlaan 94, B-3001 Heverlee, Belgium Publication date: March 1, 2001 (document).ready(function() { var shortdescription = (".originaldescription").text().replace(/\\&/g, '&').replace(/\\, '<').replace(/\\>/g, '>').replace(/\\t/g, ' ').replace(/\\n/g, ''); if (shortdescription.length > 350){ shortdescription = "" + shortdescription.substring(0,250) + "... more"; } (".shortdescription a").click(function() { (".originaldescription").slideDown(); return false; }); }); Related content In this: publication By this: publisher In this Subject: Dentistry By this author: Duyck, J. ; Van Oosterwyck, H. ; Vander Sloten, J. ; De Cooman, M. ; Puers, R. ; Naert, I. GA_googleFillSlot("Horizontal_banner_bottom");
Background: Examination of the tissue surrounding retrieved implants involve routine investigations on cut and ground sections. Undecalcified sections with implants in situ are histologically stained followed by qualitative and quantitative observations of the tissue response to the implants by light microscopy.
Purpose: A novel technique that allows for the accurate definition and quantification of enzymes involved in bone formation (alkaline phosphatase) and resorption (acid phosphatase) in the tissue is presented.
Materials and Methods: Commercially pure titanium and titanium alloy (Ti6A14V) implants were retrieved after 6 and 12 weeks of healing in rabbit bone. In addition, 4-week specimens from commercially pure titanium bone harvest chambers placed in rabbit bone were used. Undecalcified cut and ground sections were produced and evaluated with enzyme and immunohistochemical staining techniques.
Results: The titanium implants retrieved after 6 weeks of insertion in rabbit bone revealed a higher activity of both alkaline phosphatase and acid phosphatase activity compared to the implants followed for 12 months. The former samples revealed ongoing bone-tissue remodeling in the interface, whereas the latter ones showed steady-state bone conditions. Applying the new technique allowed for investigation of various bone proteins present in the tissue that had formed inside titanium canals of harvest chambers at various times of follow-up.
Conclusions: The combination of routine histologic stainings with enzyme and immunohistochemical technique of cut and ground specimens is a valuable tool in the investigations of retrieved implants from humans and animals. This novel technique now may be used to describe the state of bone regeneration in the interface zone associated with implant research.
Bone tissue repeatedly formed in titanium 6–aluminum 4–vanadium rabbit bone harvest implants was collected in vivo at various times between 12 days and 5 weeks. Qualitative and quantitative examinations on undecalcified thin sections were performed in the light microscope. The amount of bone tissue was calculated on routinely stained sections. Alkaline (ALP) and acid phosphatase (ACP) enzyme activities were investigated. We also performed immunohistological detection of bone matrix proteins. Increasing bone density as well as an increasing mineralization of the tissue was observed in the biopsies with increasing time. The ALP and ACP activities were similar at short times (12 days and 2 weeks). With increasing time the ALP activity was stronger than that of ACP. The results from the immunohistochemical detection of osteonectin, osteopontin, bone sialoprotein, and collagen I and II demonstrated changes in the tissue differentiation with time. The tissue formation in the canal became more mature with time of ingrowth, as observed with the various techniques used in this study. Owing to these methodical developments, undecalcified ground sections may be used for detailed analysis of various phases of tissue formation in close proximity to implants. © 2000 John Wiley & Sons, Inc. J Biomed Mater Res, 51, 280–291, 2000.
Finite element models were created to study the stress and strain distribution around a solitary BAnemark implant. The influence of a number of clinically relevant parameters was examined: bone-implant interface (fixed bond versus frictionless free contact), bone elastic properties, unicortical versus bicortical implant fixation and the presence of a lamina dura. Bone loading patterns in the vicinity of the implant seem to be very sensitive to these parameters. Hence they should be integrated correctly in numerical models of in vivo behaviour of oral implants. This necessitates the creation of patient-dependent finite element models.
Experimental evidence indicates that the surface geometry of bone-interfacing implants influences the nature and rate of tissues formed around implants. In a previously reported animal model study, we showed that non-functional, press-fitted porous-surfaced implants placed in rabbit femoral condyle sites osseointegrated more rapidly than plasma-sprayed implants. We hypothesized that the accelerated osseointegration observed with the porous-surfaced design was the result of this design providing a local mechanical environment that was more favourable for bone formation. In the present study, we tested this hypothesis using finite element analysis and homogenization methods to predict the local strains in the pre-mineralized tissues formed around porous-surfaced and plasma-sprayed implants. We found that, for loading perpendicular to the implant interface, the porous surface structure provided a large region that experienced low distortional and volumetric strains, whereas the plasma-sprayed implant provided little local strain protection to the healing tissue. The strain protected region, which was within the pores of the sintered porous surface layer, corresponded to the region where the difference in the amount of mineralization between the two implant designs was the greatest. Low distortional and volumetric strains are believed to favour osteogenesis, and therefore the model results provide initial support for the hypothesis that the porous-surfaced geometry provides a local mechanical environment that favours more rapid bone formation in certain situations. © 2001 Orthopaedic Research Society. Published by Elsevier Science Ltd. All rights reserved.
The local mechanical environment around bone-interfacing implants determines, in large part, whether bone formation leading to functional osseointegration will occur. Previous attempts to relate local peri-implant tissue strains to tissue formation have not accounted for implant surface geometry, which has been shown to influence early tissue healing in vivo. Furthermore, the process by which mechanically regulated peri-implant bone formation occurs has not been considered previously. In the current study, we used a unit cell approach and the finite element method to predict the local tissue strains around porous-surfaced and plasma-sprayed implants, and compared the predictions to patterns of bone formation reported in earlier in vivo experiments. Based on the finite element predictions, we determined that appositional bone formation occurred when the magnitudes of the strain components at the tissue–host bone interface were <8%. Localized, de novo bone formation occurred when the distortional tissue strains were less than approximately 3%. Based on these threshold tissue strains, we propose a mechanoregulatory model to relate local tissue strains to the process of peri-implant bone formation. The mechanoregulatory model is novel in that it predicts both appositional and localized bone formation and its predictions are dependent on implant surface geometry. The model provides initial criteria with which the osseointegration potential of bone-interfacing implants may be evaluated, particularly under conditions of immediate or early loading. © 2001 John Wiley & Sons, Inc. J Biomed Mater Res 55: 63–71, 2001
Background
The treatment of patients with early or immediately loaded dental implants has renewed interest in the behavior of osseointegration at the implant surface. Whereas it is generally accepted that peri-implant tissue formation and mineralization are dependent on the local mechanical environment in the interface zone, controversies exist concerning the impact of implant design on peri-implant bone formation. The aim of the present study was the in vivo evaluation of peri-implant bone formation by two different implant systems: cylindrical (ITI) versus conical (ILI).
Material and method
A total of 60 implants (30 ITI and 30 ILI) were placed in the cranial and caudal part of the tibia of eight Göttinger minipigs. Half of the minipigs were sacrificed at 7 days and 28 days of osseointegration. Implant-containing bone specimens were prepared for histological and ultrastructural investigations.
Results
Histological and scanning electron-microscopic investigations showed a direct contact of bone-like minerals over the whole implant surface from day 7 of implant/bone interaction. Whereas the ILI implant showed direct contact up to the top of the crestal bone, ITI implants demonstrated a crestally located narrow gap without ossification over the whole experimental period.
Conclusion
Our investigations support the hypothesis of an implant design-inherent emergence and maintenance of crestal bone.
It has been documented that the long-term clinical outcome of the Brånemark system is very favourable. However, failures do occur before and after loading. This study examined the differences in marginal bone loss between standard and self-tapping fixtures and attempted to explain excessive marginal bone loss or loss of osseointegration during the first 3 years of loading. Marginal bone loss (scored on long cone radiographs) and fixture failure rate were compared for different fixture designs. For standard fixtures, in comparison with self-tapping fixtures, the failure rate was clearly higher before as well as after loading. However, for successful fixtures no difference in marginal bone loss was observed. For the conical fixtures an increased marginal bone loss around the smooth part was observed. The effect of fixture overload, marginal bone height and loss of osseointegration was examined in 69 patients with 1 and 15 patients with 2 fixed full prostheses, and in 9 patients with an overdenture in the upper jaw. Excessive marginal bone loss (more than 1 mm) after the first year of loading and/or fixture loss correlated well with the presence of overload due to a lack of anterior contact, the presence of parafunctional activity and osseointegrated full fixed prostheses in both jaws.
Long-term stability of orthopaedic and dental implants depends on the integration of the artificial material into the surrounding bone tissue. The physical and chemical properties of implants, including those made of metals such as titanium, are thought to influence osseointegration. Despite the known importance of this interface, little is known about the factors that promote its formation. In this study, chick embryonic calvarial osteoblasts were cultured in vitro on smooth, rough-textured and porous-coated titanium surfaces and examined for morphology, biosynthesis of extracellular matrix and mineralization as a function of culture time. Scanning electron microscopy revealed that osteoblasts adhered securely to the titanium surface and frequently bridged the uneven surface by means of cellular processes. The osteoblast phenotype was retained in the cell cultures on titanium. In addition, the synthesis of extracellular matrix and subsequent mineralization were both substantially enhanced in the cultures on rough-textured and porous-coated titanium. These results strongly suggest that porous or rough titanium implant surfaces may act like "natural" substrata to permit microscopic tissue/cell ingrowth to improve clinical implant fixation.
It has been hypothesized that the histological pattern of fracture healing is controlled at least in part by the local mechanical strains in the interfragmentary region. To test this "interfragmentary strain hypothesis," we applied cyclic bending deformations to tibial osteotomies in 11 sheep. An instrumented flexible plate spanning a 1-mm osteotomy gap was deformed to create a gradient of tissue elongation from 10% under the plate to 100% at the opposite cortex. The cyclic deformations were applied three times per minute, 24 h per day, for 1-5 weeks. However, as a result of tissue differentiation, the bone-plate complex increased in stiffness with healing time, resulting in a marked reduction of the gap deformation at approximately 4 weeks. Fracture healing was evaluated using vascular injection of India ink and conventional histology. A nonlinear three-dimensional finite element model of the interfragmentary tissue at the initial stage of healing was used to predict the complex tissue strains. The ingrowth of vascularized soft tissue into the interfragmentary gap, as well as the subsequent differentiation of this tissue, occurred earlier and to a greater degree in regions of lower strain. In contrast, the proliferation of callus tissue was greatest at the periosteal and endosteal surfaces of the cortex opposite the plate. Direct comparison of the finite element predictions with the histology demonstrated that the spatial distribution of bone resorption at the fracture fragment ends directly corresponded to the locations of elevated tissue strain and stress. However, there was no consistent numerical relationship between the magnitude of these local peak strains and the corresponding volume of cortical bone resorption over the bone cross section.
Test implant plates surgically retrieved from distal femurs of dogs were studied by Raman spectroscopy in order to characterize the bone-implant interface. The implant surface consisted of phosphate mineral, plasma sprayed on a titanium substrate. On the basis of its spectroscopic signature, the phosphate mineral of bone and the implant surface formed a mixed phase in the interface.
This study reviews the long-term outcome of prostheses and fixtures (implants) in 759 totally edentulous jaws of 700 patients. A total of 4,636 standard fixtures were placed and followed according to the osseointegration method for a maximum of 24 years by the original team at the University of Göteborg. Standardized annual clinical and radiographic examinations were conducted as far as possible. A lifetable approach was applied for statistical analysis. Sufficient numbers of fixtures and prostheses for a detailed statistical analysis were present for observation times up to 15 years. More than 95% of maxillae had continuous prosthesis stability at 5 and 10 years, and at least 92% at 15 years. The figure for mandibles was 99% at all time intervals. Calculated from the time of fixture placement, the estimated survival rates for individual fixtures in the maxilla were 84%, 89%, and 92% at 5 years; 81% and 82% at 10 years; and 78% at 15 years. In the mandible they were 91%, 98%, and 99% at 5 years; 89% and 98% at 10 years; and 86% at 15 years. (The different percentages at 5 and 10 years refer to results for different routine groups of fixtures with 5 to 10, 10 to 15, and 1 to 5 years of observation time, respectively.) The results of this study concur with multicenter and earlier results for the osseointegration method.
A surgical technique for rehabilitation of severely resorbed edentulous maxillae using fixed prostheses or overdentures supported by osseointegrated fixtures in immediate autogenous corticocancellous bone grafts from the ilium is described. The results of the first 23 consecutively treated patients are reviewed. The mean observation time was 4.2 years (range 1 to 10 years). A total of 124 fixtures was originally placed into the grafts, supplemented with 16 fixtures inserted later into seven of the jaws. Throughout their observation period, 17 of the patients had continuously stable prostheses. The remaining five had overdentures, and one patient had resorted to a conventional complete denture. After 4 years, 12 of 16 patients had continuously stable prostheses. Corresponding values at 5 years were 7 of 8 patients. Calculated from the date of abutment connection, 82.1% and 81.6% of the original fixtures were clinically stable and radiographically osseointegrated after 4 and 5 years in function, respectively. From the date of fixture placement, the corresponding figures were 75.3% and 73.8%, respectively. The mean marginal bone loss after the first year of prosthesis function was 1.49 mm. The annual marginal bone loss thereafter was about 0.1 mm. The results indicate that this technique is worthwhile for patients with extreme maxillary atrophy and who cannot wear conventional complete dentures.
Peri-implant tissues of the single-crystal sapphire implant connected with neighbouring teeth by a metal bridge-work were examined clinically, radiographically, and histologically in ten monkeys. Professional tooth cleaning was performed during the study. At 3-12 months after insertion, most of the implants were firmly connected to the surrounding tissues and peri-implant gingiva was regarded as healthy, based on various periodontal parameter scores. Destructive changes of the peri-implant bone were not found radiographically. Histologically, peri-implant gingiva was revealed to show similar structure to that of the gingiva around natural teeth. Direct bone-implant interface was observed at 3 months after insertion, while a thin loose fibrous connective tissue layer was present between bone and implant at 6 and 12 months. Such different interrelationship between bone and implant might be attributable to the difference in distribution of functional stress.
A 91 per cent positive 5-9 year result has been reported when using titanium implants and gold bridges to restore edentulous jaws. About 400 consecutive patients have been operated. The reasons for the good results are believed to depend on the anchorage of the implants in the living bone without interposing soft tissue layers. Repeated X-rays ensuring a strict parallelism are used to indicate direct bone integration. Some implants had to be removed in spite of still being anchored in the bone. In these cases SEM and TEM provided direct evidence of an osseointegration.
The effect of surface roughness on osteoblast proliferation, differentiation, and protein synthesis was examined. Human osteoblast-like cells (MG63) were cultured on titanium (Ti) disks that had been prepared by one of five different treatment regimens. All disks were pretreated with hydrofluoric acid-nitric acid and washed (PT). PT disks were also: washed, and then electropolished (EP); fine sandblasted, etched with HCl and H2SO4, and washed (FA); coarse sandblasted, etched with HCl and H2SO4, and washed (CA); or Ti plasma-sprayed (TPS). Standard tissue culture plastic was used as a control. Surface topography and profile were evaluated by brightfield and darkfield microscopy, cold field emission scanning electron microscopy, and laser confocal microscopy, while chemical composition was mapped using energy dispersion X-ray analysis and elemental distribution determined using Auger electron spectroscopy. The effect of surface roughness on the cells was evaluated by measuring cell number, [³H]thymidine incorporation into DNA, alkaline phosphatase specific activity, [³H]uridine incorporation into RNA, [³H]proline incorporation into collagenase digestible protein (CDP) and noncollagenase-digestible protein (NCP), and [³⁵S]sulfate incorporation into proteoglycan.
Microstructures of the early external callus after diaphyseal fractures of human long bone were investigated by using scanning electron microscopy, X-ray diffraction, and transmission electron microscopy. It was found that the main structural framework of the human early callus consists of disordered, mineralized collagen fibrils with a small fraction of regions of ordered collagen fibrils. X-ray diffraction analyses show that hydroxyapatite containing some carbonate impurity has been the dominant crystalline phase in the human early callus. In addition, a small amount of brushite phase was detected. Selected area diffraction analyses indicated that hydroxyapatite microcrystals were embedded in microfibrils with a diameter of 4.5 nm and well-banded fibrils, whereas brushite particles of 15-20 nm in an irregular shape were located in the noncollagenous organic matter around the nonmineralized, ordered collagen fibrils. The spatial distribution of the brushite particles in the human early callus was for the first time determined. The brushite particles probably serve as the reservoir of calcium and phosphate ions for subsequent mineralized periods rather than the precursor of hydroxyapatite.
The bone-implant interface formed in a canine distal femur was examined by means of a Raman microprobe using an implant model designed to test calcium phosphate surface coatings. By using the 960 cm-1 band of calcium phosphate to characterize the interface and adjacent mineral, we obtained spatial and compositional information about the attachment of bone to the synthetic calcium phosphate coating on a titanium support. The interface between bone and the synthetic calcium phosphate is approximately 30-40 microns in width.
Matrix vesicles are extracellular 100-nanometer-diameter membrane-invested particles selectively located within the matrix of bone, cartilage, and predentin. They serve as the initial site of calcification in all skeletal tissues. Matrix vesicle biogenesis occurs by polarized budding and pinching off of vesicles from specific regions of the outer plasma membrane of chondrocytes, osteoblasts, and odontoblasts. Seeding of selected areas of matrix with matrix vesicles explains the localized distribution of subsequent zones of mineralization. Matrix vesicle biogenesis in the growth plate is linked to the chondrocyte cell cycle and reflects a stage in programmed cell death (apoptosis). Generation of initial hydroxyapatite mineral crystals occurs within the matrix vesicle membrane during Phase 1 of biologic mineralization. Phase 1 is controlled by phosphatases (including alkaline phosphatase) and Ca-binding molecules with which the matrix vesicles are well endowed. Phase 2 of biologic mineralization begins with breakdown of matrix vesicle membranes, exposing preformed hydroxyapatite to the extracellular fluid after which mineral crystal proliferation is governed by extracellular conditions. Phase 1 and Phase 2 of mineralization are under cellular control. Phase 1 is initiated by cells generating calcifiable matrix vesicles and releasing them into sites of intended calcification. Phase 2 is controlled by cells regulating extracellular ionic conditions and matrix composition.
We proposed that a similar mechanism for calcification exists for poly(ether)urethanes and glutaraldehyde stabilized tissue. The mechanism is based on the propensity of the polyether component of the materials to complex calcium and provide initiating sites for ultimate formation of calcific deposits. Data evaluating the role ether containing materials have on calcification demonstrate that the rate of mineralization of either tissue valves or polymer valves can be controlled by paying attention to the basic chemical mechanism of complexation occurring at the surface and within the bulk of the implant devices. The molecular models described above, point out that the driving force for complexation with either the polyethers of the polyurethane or the polyether of glutaraldehyde is very strong, therefore, controlling the driving force may lead to medical devices with longer term durability.
Critical events in the adaptation of osseous tissues to implant materials involve initial calcification of the newly synthesized bone. Previous studies indicated that bone-bonding but not nonbonding glass ceramics increase the matrix vesicle number, thereby compensating for delayed maturation of the extracellular organelles. The present study assessed whether this was also true for metal implants commonly used in orthopaedics and oral medicine. Bone-bonding titanium (Ti) or nonbonding stainless steel (SS) implants were placed in the right tibias of Sabra rats following ablation of the marrow. At 3, 6, 14, and 21 days postinjury, newly formed endosteal bone in the treated and contralateral limbs was removed and matrix vesicle-enriched membranes isolated. Alkaline phosphatase and phospholipase A2 specific activities and phosphatidylserine (PS) content were determined and compared with those of a nonsurgical control group. Results show that matrix vesicle alkaline phosphatase and phospholipase A2 activity and PS content was increased in the Ti-implanted limbs at 6 (peak), 14, and 21 days, although at levels less than observed in normal healing. Alkaline phosphatase activity remained elevated throughout the healing period. In contrast, these parameters were markedly inhibited in the SS-implanted limbs with respect to Ti or to normal healing. Both implants altered the systemic response associated with marrow ablation, but in an implant-specific manner. The results support the hypothesis that cells adjacent to bone-bonding materials can compensate for negative effects on primary mineralization during osteogenesis, whereas cells adjacent to nonbonding materials either do not compensate or are further depressed. The data support the use of the rat marrow ablation model as a tool for rapid, initial assessment of biomaterials in bone.
Bone is chemically built up as a mineralized matrix which comprises collagen and a small amount of noncollagenous proteins. This paper points out some useful methods to evaluate the bone composition. Demineralizing extraction of bone powder with EDTA allows the determination of matrix size and degree of extractability. These parameters vary with bone type, anatomical site of the bone, disease, species, and drug treatment. The study of bone particles in situ can be done by separation of bone powder according to their density. A shift of the bone particles to higher density fractions reflects an increased amount of older, more mineralized osteons in the bone with its consequences on the mechanical competence of the bone. Quantity and quality of bone matrix mineralization are related to bone cell activity which can be studied indirectly by further exploration of the composition of the bone matrix. Many noncollagenous proteins are buried in the extracellular bone matrix from where they can be released when bone is resorbed. These proteins can then act on bone cells in an autocrine or paracrine manner. Altered concentrations of noncollagenous proteins in bone matrix are described in three pathological conditions associated with changes in other bone properties: osteoarthritis, osteopenia, and osteogenesis imperfecta. The functional significance and origin of these changes will have to be subjected to further study.
The authors present an histologic study of bone peri-implant tissue responses to non-submerged unloaded and early-loaded titanium screw implants in monkeys. Titanium screws were placed in the maxillae of 3 Macacus rhesus monkeys. One crown for each of three implants was placed after 30 days under occlusal contact, while the other three implants were left unloaded (one pair per animal). Fifteen months later, the animals were killed, and block sections were obtained to be processed for histology. Thin ground sections were excised so that histomorphometric analyses could be performed. In all samples examined, a tight contact of new bone to the implant surfaces was observed. Moreover, around the implant necks of the early-loaded screws was observed an histologic pattern of lamellar, cortical bone, thicker than in unloaded implants. Comparison with peri-implant bone of adjacent teeth showed a similarity to the surfaces of loaded peri-implant bone.
Four major non‐collagenous bone proteins were localised by single and double immuno‐histochemistry during de novo mineralisation and bone resorption. Both osteopontin and bone sialoprotein were localised ahead of the mineralisation front, suggesting that both proteins are necessary for the initiation of bone mineralisation. This supports previous suggestions that bone sialoprotein acts as a crystal nucleator. The role of osteopontin is less certain, but might be related to ensuring that only the right type of crystal is formed. Osteocalcin and osteonectin were not present in areas of first crystal formation, but were present in the fully mineralised matrix. Their role may be to control the size and speed of crystal formation.
Osteopontin, bone sialoproteins and osteocalcin (but not osteonectin) were also present at reversal lines. Interpreting this localisation together with information from the literature, the following functions are suggested during resorption: Osteocalcin may act as a chemoattractant for osteoclasts, while both osteopontin and bone sialoprotein may facilitate the binding of osteoclasts via the arg‐gly‐asp motif.
Twenty-seven patients with highly active, refractory rheumatoid arthritis (RA) were treated with the new anti-rheumatic drug prospidine, in view of selecting the optimum pulse regimen and comparing its short-term use with methotrexate (MTX). Prospidine was administered intravenously 500 mg every 3-5 days in the hospital and then monthly. Fifteen patients received MTX (30 mg/week intravenously in hospital and then monthly. Fifteen patients received MTX (30 mg/week intravenously in hospital and then orally 7.5-15 mg/week). The randomisation code was 2:1. We assessed 7 clinical and 4 lab data. The clinical improvement was noticed statistically after 2-4 weeks in 85% prospidine-patients and sustained up to 6 months in 73% (cp. 40% and 57% by the MTX). Only in the prospidine patients were a significant reduction of the mean daily prednisolone dose and the levels of rheumatoid factor and immune complexes observed. Prospidine and MTX had a similar incidence of side effects (39% and 43%), but all drop-outs in prospidine pulse were due to lack of response (26%) and to initial intolerance (4%). Drop-outs in MTX pulse were connected both with drug toxicity (14%) and with lack of response (7%). Alternate prospidine pulse, as highly anti-inflammatory, rapidly acting and well-tolerated regimen, may be used in treating severe forms of RA.
Clinical and histologic evaluations of partially stabilized zirconia endosseous implants under unloaded and early loaded conditions in four beagle dogs were performed to examine the possibility of osseointegration of a newly developed one-stage zirconia implant during initial bone healing. No clear difference in clinical features was observed. Direct bone apposition to the implant was generally seen in both implants. However, loss of crestal bone height was quite evident around the loaded implants. These findings suggest that the initial unloaded condition is preferable to achieve osseointegration of one-stage zirconia implants.
The initial stages of bone healing with titanium alloy implants were compared clinically and histologically in three groups of beagle dogs: unloaded one-stage (group 1), loaded one-stage (group 2), and unloaded two-stage (group 3). No implant mobility was reported in any of the three groups. For the most part direct bone contact was achieved, but there was a significant difference in the histologic results of the interface among the animals in all three groups. Significant crestal bone loss was evident in group 2, which showed poor bone apposition to the bottom of the threads in the upper portion of the implant. New bone growth to these same areas of the threads was seen in the animals in groups 1 and 3, with the ratio of direct bone contact significantly higher for those in group 3. These differences could be attributed to the effect of early occlusal loading on the implant during initial bone healing.
Tissue engineering in vitro and in vivo involves the interaction of cells with a material surface. The nature of the surface can directly influence cellular response, ultimately affecting the rate and quality of new tissue formation. Initial events at the surface include the orientated adsorption of molecules from the surrounding fluid, creating a conditioned interface to which the cell responds. The gross morphology, as well as the microtopography and chemistry of the surface, determine which molecules can adsorb and how cells will attach and align themselves. The focal attachments made by the cells with their substrate determine cell shape which, when transduced via the cytoskeleton to the nucleus, result in expression of specific phenotypes. Osteoblasts and chondrocytes are sensitive to subtle differences in surface roughness and surface chemistry. Studies comparing chondrocyte response to TiO2 of differing crystallinities show that cells can discriminate between surfaces at this level as well. Cellular response also depends on the local environmental and state of maturation of the responding cells. Optimizing surface structure for site-specific tissue engineering is one option; modifying surfaces with biologicals is another.
Bone-titanium contact was examined in young and mature rats on various days after insertion of pure titanium into the tibia. Under light microscopy, on the 14th day, lamellar mature bone was initially formed, and was seen to make direct contact with the titanium in both groups. In young rats on the 28th day, bone-titanium contact was greater than that in mature animals. On 1-micron sections, an amorphous zone 0.5-1.0 micron thick was found around the titanium, and a slender cell layer lay parallel to the implant, forming the superficial layer of the amorphous zone. Ultrastructurally, these slender cells were identified as osteoblastlike cells and made direct contact with the implant via a 20-50-nm thin amorphous zone. Below this cell layer, a collagen-containing, poorly mineralized zone was present and bordered by lamellar bone with a lamina limitans-like structure. However, this cell layer was absent in places, and therefore the thick amorphous zone without slender cell layer consisted ultrastructurally of a 20-50-nm thin amorphous zone and a poorly mineralized zone bordered by the lamellar bone. Sometimes this poorly mineralized zone was absent, and in such cases, the lamellar bone contacted the titanium by the thin amorphous zone formed on the lamina limitans-like structure. Thus, although bone was seen to make contact with the titanium implant, ultrastructurally a 20-50-nm thin amorphous zone, a slender cell layer, and/or a poorly mineralized zone were interposed between the bone and titanium.
Time-dependent distribution of extracellular proteins (albumin, fibrinogen, fibronectin, collagen-I and IgG) in the interface zone between implant and soft tissue has been investigated utilizing a recently developed method. Commercially pure (c.p.) titanium and polytetrafluoroethylene (PTFE) implants were inserted in the abdominal wall of rats for 1, 6 and 12 weeks followed by a mild fixation, cryoprotection, rapid freezing in LN2-cooled propane, cryosubstitution and low-temperature infiltration with UV curing of the methacrylate LR-Gold. Before sectioning, the bulk part of the titanium was removed by an electrolytical dissolution technique (electropolishing), while the PTFE implants were removed by a fracture technique. Employing a cryosubstitution method combined with postembedding immunohistochemistry, a light microscopic analysis was allowed. The selected proteins had an apparently varying distribution in the implant-close tissue and their distribution changed during the follow-up period. There was also a difference in the distribution pattern for each protein around titanium and PTFE implants. Insertion of the c.p. titanium implants elicited an inflammatory reaction in many respects similar to a normal wound healing response, while the PTFE implants caused a more pronounced, persistent inflammation.
An osseointegrated oral implant with surrounding bone was used for electron microscopical analyses of the implant-bone interface. The bulk metal was removed by sawing and grinding techniques, leaving only the plasma-sprayed titanium coating anchored in mineralized bone. Ultrathin sections were realized from these reduced interface areas and underwent ultrastructural and crystallographic assessments. The microscopical observations showed that ultramicrotomy was suitable for producing such interface sections. Two different, concomitant, interfacial structures were noticed. On the one hand it was possible to observe bone crystals directly apposed on the implant surface; on the other, a granular electron-dense substance was interposed between the plasma-sprayed coating and the bone. The applied technical approach allows one to study the osseointegration process, at high resolution levels, of intact interfaces from complete osseointegrated implants.
Implant failure as a consequence of prosthetic loading following clinical determination of successful stage I healing is poorly understood. A basic premise of accepted prosthetic protocol is passive connection of multiunit prostheses to the implant support. To better understand mechanical factors related to implant failure, this basic passivity premise was experimentally tested prior to study of functional loading research. The purpose of this preliminary study was to measure the bone response around implants placed in the mandible of baboons that supported prostheses exhibiting two levels of fit and not loaded occlusally. Screw-retained prostheses that exhibited a mean linear distortion of 38 microns and 345 microns made up the fit and misfit groups respectively. The results failed to distinguish a difference in bone response between the two levels of prosthetic fit. Although the finding can be argued as a sample size limitation, the data strongly suggest an opposite response than is clinically expected and, consequently, does not warrant the use of additional animals in this initial study. Because the design of this study does not mimic the clinical application of misfitting prostheses (where dynamic functional loads are superimposed with misfit loads), it cannot be inferred that, in clinical application, fit does not alter the osseointegrated interface. Ongoing investigation of failure due to nonpassive connections under dynamic loading conditions of mastication will help clarify the clinical significance of passivity.
The breakdown of bone around oral implants following excessive occlusal load or plaque accumulation was evaluated in monkeys. 5 screw type implants of pure titanium (Astra) were inserted in the mandible of 4 monkeys (Macaca Fascicularis). 2 implants were placed in each of the lateral segments and 1 in the frontal area. Each monkey was provided with 2 cemented splints covering the premolars and molars in the right and left side of the maxilla, respectively. 6 months after insertion of the fixtures, a fixed partial prosthesis was mounted on the 2 implants in one of the lateral segments. The prosthesis was in supra-occlusal contact with the antagonizing splint. Each prosthesis was replaced during the course of the experiment. The renewed prosthesis caused a lateral displacement of the mandible during occlusion, and therefore resulted in a lateral rather than axial excessive occlusal load. Implants retaining the prosthesis were brushed 1 x a week and subgingival cleaning was performed 1 x a month. The remaining implants were never cleaned and, additionally, a cotton cord was placed passively around each of these to promote plaque accumulation. 5 out of 8 implants with excessive occlusal load lost osseointegration (mobility and peri-implant radiolucency). The loss of osseointegration was observed 4.5 months to 15.5 months after the occlusal overload was commenced. None of the implants with plaque accumulation lost osseointegration, although an average loss of 1.8 mm in the radiographic bone level was assessed after 18 months.
Short daily periods of controlled dynamic loading were applied in vivo through the flexed carpus and olecranon to the intact ulna of 240 g male Sprague-Dawley rats. This technique involved neither surgical preparation, nor direct loading of the periosteum at a site close to the region of the bone in which adaptive modeling was subsequently assessed. The animals used their limbs normally between loading episodes, thus approximating to the natural situation, in which short periods of exercise are generally superimposed on longer periods of less strenuous activity. The strain patterns associated with normal activities were established for the rat ulna from strain gauges implanted in vivo. Typical peak strain magnitudes during unrestricted locomotion varied between -0.0007 and -0.0012, with peak strain rates between 0.023 and -0.038 sec-1. Stride frequency was 1.5-4.2 Hz. The adaptive response to a single 10 min period of loading each day, causing peak dynamic strains of -0.002 (1200 cycles at 2 Hz, and a loading/unloading rate of +/-0.03 sec-1), involved modification of the normal growth related medial to lateral modeling drift, simultaneously reducing the rate of lateral periosteal bone deposition and medial bone resorption. This change to the normal modeling pattern reduced the total amount of new bone formation as well as the midshaft curvature of the ulna. At higher peak strain amplitudes (-0.004), adaptive straightening was accompanied by an increase in bone mass, achieved by an increase in the mineral apposition rate on the previously forming lateral face, and arrest of resorption on the medial ulna surface, with reversal to formation. These experiments show that the growing rat ulna underwent adaptive changes in both bone mass and architecture when short daily periods of axial loading, producing strains within the physiological range and with near normal strain distribution, were superimposed on the loading associated with normal activity. At moderate peak strain magnitude (-0.002), modification of drift produced a straighter bone, associated with a reduced periosteal bone formation. At higher strain magnitude (-0.004), adaptive modeling produced a straighter bone associated with increased periosteal bone formation.
One hundred and five 5-week-old male rats were used to study the influence of the stability of an osteotomy on longitudinal overgrowth of the bone. In 45 rats (Group 1) a transverse diaphyseal osteotomy of the right tibia was made; the left tibia was left untreated. In the second 45 rats (Group 2) the right tibia was osteotomized after applying an external fixator; the left tibia underwent a sham operation without osteotomy. Fifteen further rats (Group 3) were used as a nonoperated control group. After being measured at different periods up to 20 weeks, the osteotomized tibiae of the Group 1 (unstable) were 16-25% longer than those of their nontreated limbs (P < 0.001). The osteotomized tibiae of the Group 2 (stable) were 6-11% longer than those of the sham-operated opposite limbs (P < 0.001). The osteotomized and unstable tibiae were 9-17% longer than the osteotomized and stable tibiae (P < 0. 001) throughout the whole study. Although several factors have been considered to be responsible for longitudinal bone overgrowth after fracture in young animals, this work suggests that bone's stability plays a decisive role in the final outcome.
Small-angle x-ray scattering (SAXS) can provide information on mean size, predominant orientation and typical shape of mineral crystals in bone. In this paper, recent developments of this technique for application in bone research are reviewed. Then the structure of the collagen/mineral composite in bone, as determined by SAXS, is compared for a number of species. The thickness of the mineral crystals was found to increase with age up to a value around 3 to 4 nanometers in adult animals, depending on the species. The SAXS results also suggest the existence of needle-shaped mineral crystals in mouse or rat bone and more plate-shaped crystals in other tissues like adult human bone or mineralized turkey leg tendon.
This paper presents a general overview of calcification and involves aspects of the chemical, physical, and biological nature of mineral crystals in invertebrate and vertebrate tissues, with selected examples of the latter. Two broad areas are described: mineral structure and composition. Mineral formation is detailed in an incidental fashion. Both classical research and recent data appropriate to mineralization studies are noted in order to convey basic principles, as well as the sense and direction of current investigations on the mineral phases of calcified tissues. In this context, novel analytical and imaging techniques from a number of different laboratories lately have helped characterize crystal size, shape, and composition; mineral association with respect to collagen; atomic lattice structure of crystal surfaces; interrelationships between non-collagenous matrix components and mineral; and stereochemical organization of putative matrix nucleation sites. Together, this work has provided a more complete understanding of the mineral-matrix atomic, molecular, and macromolecular interactions that underlie the general mechanism of calcification in biological tissues.
THIS STUDY REPORTS ON THE histological findings of two immediately loaded titanium plasma-sprayed (TPS) implants, retrieved for a fracture of the abutment and for psychological reasons, after 8 and 9 months of loading, respectively. The microscopical analysis showed that mature, compact, cortical bone was present around both implants, with the bone implant contact percentage about 60 to 70%. No fibrous tissue or gaps were present at the interface. No resorption was present in the peri-implant bone. On both implants a few osteoblasts were found positive at the interface for alkaline phosphatase (ALP); while no cells positive for acid phosphatase (ACP) were present. Immediate loading can, perhaps, be used in very selected cases of good bone quality, with implants that have certain macro- (screw shape) and micro-interlocks (titanium plasma-sprayed surface) characteristics. Good results have been reported also for non-TPS surface (e.g., machined surface). More data about different designs (e.g., cylinders) or coatings (e.g., hydroxyapatite) are needed before any firm conclusions about immediate loading can be reached.
Rods of three bioactive materials, apatite/wollastonite glass ceramic (AW-GC), bioactive glass (BG), and dense slip-cast hydroxyapatite (HA), were implanted in the femora of 23 Wistar rats for periods of 1-4 weeks. The samples were harvested following vascular perfusion fixation and the femora freeze-fractured for scanning electron microscopy to expose the bone/implant interface. The focus of our observations was when new bone was forming on the implant surfaces irrespective of the implantation period. Scanning microscopy of the hydroxyapatite rods demonstrated that in areas where bone bonding had occurred, the implant surface was composed of globular accretions which fused to form a cement-like matrix to which collagen fibers were attached. Dissolution of individual grains of the implant surface created a roughened surface topography. Such features were not found in the transcortical portions of these implants. Similar globular accretions were also found on the surface of bulk AW-GC, although bone apposition was not disrupted by the critical point-drying procedure, and thus the interface was more difficult to image. Nevertheless, the collagen of the bony compartment interdigitated with an interfacial layer which was morphologically similar to that found on HA. The most surface reactive material, BG, demonstrated an interfacial structure where the surface reactive calcium phosphate layer was clearly distinguished from the underlying bulk implant material. However, this layer was separated from the overlying collagen-containing bony compartment by a second, thinner, calcified layer which corresponded to the cement line matrix into which the collagen fibers were inserted. Our results show that the new bone interface formed with these three bioactive materials is morphologically comparable to that of cement lines found naturally in bone-remodeling sites, and that this interfacial layer is formed on the chemically active surface of the biomaterial. The degree to which the cement line matrix interdigitated with the implant was a product of the reactivity of the implant surface.