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Professional issue
What is ‘manipulation’? A reappraisal
David W. Evans
a
,
*
, Nicholas Lucas
b
a
Research Centre, British School of Osteopathy, London SE1 1JE, UK
b
Sydney School of Public Health, University of Sydney, Australia
article info
Article history:
Received 27 November 2008
Received in revised form
3 November 2009
Accepted 21 December 2009
Keywords:
Joint manipulation
Spinal manipulation
Taxonomy
Definition
abstract
Due primarily to its colloquial function, ‘manipulation’ is a poor term for distinguishing one healthcare
intervention from another. With reports continuing to associate serious adverse events with manipulation,
particularly relating to its use in the cervical spine, it is essential that the term be used appropriately and
in accordance with a valid definition. The purpose of this paper is to identify empirically-derived features
that we propose to be necessary and collectively sufficient for the formation of a valid definition for
manipulation. A final definition is not offered. However, arguments for and against the inclusion of
features are presented. Importantly, these features are explicitly divided into two categories: the ‘action’
(that which the practitioner does to the recipient) and the ‘mechanical response’ (that which occurs
within the recipient). The proposed features are: 1) A force is applied to the recipient; 2) The line of
action of this force is perpendicular to the articular surface of the affected joint; 3) The applied force
creates motion at a joint; 4) This joint motion includes articular surface separation; 5) Cavitation occurs
within the affected joint.
Crown Copyright Ó2009 Published by Elsevier Ltd. All rights reserved.
1. Introduction
Scientific enquiry often requires researchers to consider the
foundations upon which important clinical and academic
assumptions have been built. For the professions that use manual
therapy, few foundations lie as deep as definitions of the very
interventions that distinguish manual therapy from other areas of
healthcare. It is difficult for practitioners to make rational decisions
about the use of an intervention when that intervention is poorly
defined or not mechanistically understood. Indeed, the ramifica-
tions of this uncertainty may be more far reaching than judgements
made by individual clinicians.
For example, definitions of healthcare interventions may be
used by purchasers to make inferences about the potential efficacy,
safety and appropriateness of that intervention, when applied to
populations (e.g. Shekelle et al., 1991; Coulter et al., 1996; Gatter-
man et al., 2001). Given that clinical trials have so far provided few
clear answers to inform the choice of one physical treatment over
another, particularly in relation to musculoskeletal problems (Kel-
ler et al., 2007; van der Velde et al., 2008), the perceived charac-
teristics of an intervention are likely to be used to provide clinical
guidance. In addition, with reports continuing to associate serious
adverse events with manipulation (e.g. Ernst, 2007), particularly
relating to its use in the cervical spine, the term should be used
appropriately and in accordance with a valid definition.
Manipulation is one intervention for which a satisfactory defi-
nition is lacking. Due primarily to its colloquial function, ‘manipu-
lation’ is a poor term for distinguishing one physical treatment from
another. Indeed, so vague is the term that when used in scientific
journals, supplementary details are often required to differentiate
‘real’ manipulation from its manual therapy counterparts (e.g.
Keller et al., 2002; Harvey et al., 2003; Skyba et al., 2003; Colloca
et al., 2004, 2006; Song et al., 2006). Oversights of this kind may be
avoided if what is currently termed ‘manipulation’ were accurately
defined.
The purpose of this paper is to present features proposed to be
necessary components of a valid definition of manipulation. A final
definition of manipulation is not offered, but arguments for and
against the inclusion of these empirically-derived features are
presented as a first step in this direction.
2. Defining manipulation
Prior to contemplating a definition of manipulation, it is
necessary to consider how a definition should be formed. Estab-
lished criteria for a definition are presented in Table 1 and are
compared to those criteria that meet the requirements for
adescription. A useful definition of manipulation should encom-
pass all characteristics that empirical research has shown to be
universally valid in all parts of the body, yet exclude any
*Corresponding author. Tel.: þ44 7853914487.
E-mail address: dwe@spinalmanipulation.org.uk (D.W. Evans).
Contents lists available at ScienceDirect
Manual Therapy
journal homepage: www.elsevier.com/math
1356-689X/$ – see front matter Crown Copyright Ó2009 Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.math.2009.12.009
Manual Therapy 15 (2010) 286–291
characteristic shown to be surplus or redundant in any part of the
body.
Previous attempts at a definition of manipulation have appeared
in diverse sources of literature (representative examples are given
in Table 2), and reveal several notable features. Firstly, when
compared to the criteria in Table 1, it is clear that most of these
previous ‘definitions’ are actually descriptions. Furthermore, none
of these can qualify as definitive as there is variation, and discor-
dance, between them. Lastly, none is empirically-derived using the
existing basic science literature on manipulation; a process that has
the potential to identify characteristics that may distinguish
manipulation from other physical treatments.
One consistent attribute of previous ‘definitions’ is that they
relate to a physical intervention (oraction) that one person (usually
a practitioner) performs upon another (the recipient, who may be
a healthy subject or patient). General (colloquial) definitions of the
term manipulation focus entirely upon the action of the practi-
tioner, without conveying the potential importance of the events
that occur within the recipient. In comparison, many definitions in
a therapeutic context describe a proposed mechanical effect (or
response) within the recipient, which is caused by the action. This
mechanical response may be associated with distinct physiological,
neurological or psychological responses (Evans, 2002; Cramer
et al., 2006; Bolton and Budgell, 2006; Williams et al., 2007).
However, rather than including these secondary responses, which
have yet to be clearly delineated, we shall follow the convention of
prior definitions and limit our discussion to the action of the
practitioner and the passive mechanical response within the
recipient.
3. Features of manipulation
Several empirically-derived features are likely to be necessary to
define ‘manipulation’. A necessary feature should be applicable
irrespective of the body region in which manipulation is achieved.
We consider that, although each feature may not be unique to
manipulation, their combination will be. It is this combination that
will represent a framework to sufficiently define manipulation. We
have divided these features into two categories: (1) the ’action’
(that which the practitioner does to the recipient), and (2) the
accompanying ‘mechanical response’ (that which occurs within the
recipient). The merits of each identified feature are discussed
below.
3.1. Action (that which the practitioner ‘does’ to the patient)
3.1.1. A force is applied to the recipient
Manipulation involves a force being applied to the recipient.
Most commonly, this force is externally generated and is usually
applied to the recipient by physical contact at the skin surface
(Kawchuk et al., 1992; Herzog et al., 1993a, 2001; van Zoest and
Gosselin, 2003). The force may include reaction forces from furni-
ture, such as a plinth or chair (Kirstukas and Backman, 1999) and, in
some circumstances, gravitational force may be utilised. The
application of force is proposed to be a necessary feature for
a definition of manipulation.
3.1.2. The line of action of this force is perpendicular to the articular
surface of the affected joint
The earliest biomechanical studies to investigate what is now
termed ‘manipulation’ (Roston and Wheeler Haines, 1947; Uns-
worth et al., 1971) examined the phenomenon of ‘joint cracking’ in
metacarpophalangeal (MCP) joints. These studies investigated the
relationship between joint surface separation and the production of
a ‘crack’ sound (discussed in more detail later). Both studies used
a similar design to induce the cracking sound in that the surfaces of
MCP joints were separated using ‘traction force’, which was applied
along the length of the finger, perpendicular to the articular
surfaces. The results were equally simple: joint surface separation
beyond a certain magnitude created an obvious cracking sound and
an immediate increase in articular surface separation. For a short,
unspecified period this cracking noise could not be repeated; an
observation explained by reduced cohesion within synovial fluid
due to the presence of tiny bubbles (Unsworth et al., 1971;Mierau
et al., 1988;Evans, 2002). Similar findings were found in later
studies of MCP ‘joint cracking’ (Me
´al and Scott, 1986; Watson et al.,
1989).
Importantly, in every study in which the cracking sound in MCP
joints has been examined, the force has always been applied along
a line of action perpendicular to the articular surfaces of the
affected joint. Moreover, the motion produced by this force was
joint surface separation, without any obvious ‘gliding’ motion. As
synovial joint surfaces are designed to glide smoothly over one
another, the motion produced during this type of MCP joint
manipulation is hence distinguished from that produced during
typical ‘physiological’ motion.
A complexity of this feature is that most synovial joints are
curved rather than planar, and are not always congruent. Whereas
Table 1
A comparison of the criteria required for definitions and descriptions.
Definition Description
A statement expressing the essential
nature of something
Discourse intended to give a mental
image of something experimental
May be stipulated, or assigned
meaning
Must derive from observation
or experience
When applied to a class of
phenomena, must apply fully
to all members of the class
When applied to a class of phenomena,
may yield an aggregate set of features,
all of which need to apply to each
particular member of the class
From O’Connor et al., 1997.
Table 2
Previous definitions and descriptions of manipulation and spinal manipulation.
Definition/description (quotes) Source
General (colloquial)
To handle something, or move or work it with
the hands, especially in a skilful way
Chambers 21st Century
Dictionary, 2009
Therapeutic (general)
To apply therapeutic treatment with the hands
to (a part of the body)
Chambers 21st Century
Dictionary, 2009
The therapeutic application of manual force American Association of
Colleges of Osteopathic
Medicine, 2006
High velocity, low amplitude passive
movements that are applied directly
to the joint or through leverage
Chartered Society of
Physiotherapy, 2006
A manual procedure that involves a directed
thrust to move a joint past the physiological
range of motion, without exceeding
the anatomical limit
Gatterman and Hansen, 1994
Therapeutic (spinal)
Spinal manipulation is .the sudden
application of a force, whether by
manual or mechanical means, to any
part of a person’s body that affects a joint
or segment of the vertebral column’’
New South Wales
Department of Health, 2001
Spinal manipulation entails high velocity, low
amplitude manual thrusts to spinal
joints that extend slightly beyond
their physiological range of motion
Ernst, 2001
D.W. Evans, N. Lucas / Manual Therapy 15 (2010) 286–291 287
the line of action of the applied force may be perpendicular to one
point along the articular surface, this will not be the case with the
entire articular surface. Hence, the applied force may be more
accurately described as acting perpendicular to a plane that is
tangential to a point of contact between the articular surfaces of the
joint.
Although the relationship between forces applied to spinal
segments during manipulation procedures and the motion that
ensues is often assumed to be self-evident, the existence of
coupling patterns in spinal segments can preclude such certainty.
Limited kinematic data exist for spinal segmental and joint motions
during spinal manipulation procedures. However, the little data
that are available do appear to validate this proposed feature in the
spine (Evans, 2009).
Bereznick (2005) measured substantial force applied perpen-
dicular to the skin surface during side-posture lumbar manipula-
tion. Due to the negligible friction between the skin and the
underlying tissues (Bereznick et al., 2002), the line of action of the
majority of this force can be assumed to have been parallel to
the transverse plane of the recipient. Additionally, Cramer et al.
(2002) confirmed that the same side-posture lumbar spine
manipulation technique produces transverse rotation of lumbar
spinal segments. Since transverse rotation in the lumbar spine is
not obviously coupled with any other motion (Legaspi and Edmond,
2007), the applied force is again likely to act along the transverse
plane of the recipient. In turn, the approximately planar articular
surfaces of all lumbosacral posterior joints are perpendicular to the
transverse plane; typical lumbar zygapophysial joints (L1–L5) are
aligned close to the sagittal plane, whereas those of the lumbosa-
cral (L5–S1) joints are more frontally orientated (Giles, 1997; van
Schaik et al., 1997; Singer et al., 2004).
Evidence for a similar relationship exists in the thoracic spine.
Several studies (Herzog et al., 1993a, 2001; Ga
´l et al., 1995)have
shown that manipulation forces are applied in a posterior-anterior
direction, parallel with both sagittal and transverse planes, and
therefore perpendicular to the frontal plane. In contrast, the artic-
ular surfaces of typical thoracic zygapophysial joints (T4–10) are
known to be frontally orientated (Singer et al., 2004). Unfortu-
nately, there are limited kinematic data available for cervical spine
manipulation, but the small amount that does exist also provides
support for this proposed feature (Evans, 2009).
3.1.3. The magnitude of this force increases to a peak over a finite
period of time
The available data demonstrate that the magnitude of the
applied force varies considerably between individuals, but consis-
tently increases from zero over a finite period of time until a peak
force is reached, after which the magnitude decreases once again to
zero, in a single, non-repeating cycle. The increase and decrease of
the force is not necessarily linear, sometimes taking the form of
several distinct phases of unequal duration that vary with the
location of the manipulated joint (Roston and Wheeler Haines,
1947; Unsworth et al.,1971; Watson et al.,1989; Hessell et al., 1990;
Kawchuk et al., 1992; Kawchuk and Herzog, 1993; Herzog et al.,
1993a; Herzog, 2000).
These observations suggest temporal limits for manipulation
forces, in contrast to other manual therapeutic interventions (e.g.
mobilisation), which may consist of periodical, repeating phases
(Lee et al., 2000). However, it is difficult to justify that such force-
time constraints are necessary for manipulation. It is feasible that
manipulation could still be achieved if the force-time characteris-
tics varied from that typically observed. Furthermore, other inter-
ventions may be modified to share such characteristics. Hence,
specifying the time frame over which force is applied is not
currently proposed as a necessary criterion to define manipulation.
3.2. Mechanical response (that which occurs within the recipient)
3.2.1. The applied force produces motion at a joint
The force applied to the recipient induces motion between the
articular surfaces of a joint. This is a fundamental feature of
manipulation and other manual therapy interventions (Lee et al.,
2000), and is frequently indicated in previous descriptions and
definitions (e.g. Table 2). We consider this criterion to be necessary.
While manipulation is often applied with the intent of
producing an effect at a specific joint (or joints), research has
demonstrated that some manipulation techniques are not suffi-
ciently accurate to always affect the chosen, ‘target’ joint (Ross
et al., 2004). It is therefore more precise to refer to the ‘affected’
joint rather than the ‘target’ joint.
3.2.2. This joint motion always includes articular surface separation
The applied force induces motion between the articular surfaces
of the affected joint, and when measured, articular surface sepa-
ration (gapping) has always been observed (Roston and Wheeler
Haines, 1947; Unsworth et al., 1971;Mierau et al., 1988;Watson
et al., 1989; Watson and Mollan, 1990; Cramer et al., 2002). We
propose that this is a necessary criterion for a definition of
manipulation as few, if any, other manual therapeutic interventions
appear to produce this type of joint motion.
3.2.3. The velocity of joint motion is variable
One of manipulation’s most common pseudonyms is the ‘high
velocity–low amplitude thrust’ – a composition of biomechanical
terms frequently appearing in prior ‘definitions’ (e.g. Table 2).
Velocity is the rate of change of displacement with respect to time.
High velocity joint motion may occur during everyday activities
(e.g. throwing, running or kicking), as well as during passive
manual or instrument-assisted procedures (e.g. manipulation and
mobilisation). Hence, the velocity of joint motion alone cannot
define manipulation. Moreover, several studies have shown that
manipulation may be achieved at relatively low velocity joint
motions (Unsworth et al., 1971; Me
´al and Scott, 1986; Watson et al.,
1989;Suter et al., 1994). Thus, given the current available data,
velocity is not considered a necessary criterion.
3.2.4. The sum displacement of the articulating bones is usually
zero
Importance has been attached to the amplitude of joint motion
achieved during physical interventions, and a ‘grading’ system has
been proposed (Maitland, 1966). However, the sum (resultant)
displacement or deformation of tissue does not appear to be
a necessary feature for the achievement of manipulation. Assuming
that tissues have not undergone damage through being deformed
beyond their elastic limit, are no longer under the action of any
external force, and are under constant environmental temperature
(Watson et al., 1989; Kernohan et al., 1990) and pressure (Semlak
and Ferguson, 1970), all studies that have measured bone
displacement before and after manipulation show no lasting
change, once elastic tissue deformation has been allowed to recover
(Unsworth et al., 1971;Mierau et al., 1988;Watson et al., 1989; Ga
´l
et al., 1994, 1995, 1997; Tullberg et al., 1998; Cramer et al., 20 02). As
such, the final resultant displacement of the articulating bones
following a manipulation is usually zero. This raises some concern
with use of the term ‘adjustment’, which conveys a notion of lasting
tissue displacement.
This feature was considered useful as it distinguishes manipu-
lation from procedures to reduce a dislocation or realign fractured
bone. However, it is conceivable that a manipulation delivered with
excessive force may damage some of the joints restraining tissues,
and result in lasting tissue displacement or deformation. Moreover,
D.W. Evans, N. Lucas / Manual Therapy 15 (2010) 286–291288
a manipulation that induces tissue damage is still manipulation,
irrespective of an adverse outcome. Hence, the criterion for zero
tissue displacement seems unnecessary for the definition of
manipulation.
3.2.5. Cavitation occurs within the affected joint
Associated with joint surface separation is the elicitation of
a high frequency vibration that manifests as an audible ‘click’ or
‘crack’ sound (Roston and Wheeler Haines, 1947; Unsworth et al.,
1971; Watson et al., 1989). These vibrations are readily measured
using microphones or accelerometers, and have been investigated
in various joints across several studies (Me
´al and Scott, 1986;
Watson et al., 1989; Herzog et al., 1993b; Ga
´l et al., 1995; Reggars
and Pollard, 1995; Reggars, 1996a,b, 1999; Beffa and Mathews,
2004; Bolton et al., 2007).
The most likely and widely accepted explanation for this audible
sound during joint manipulation is a process known as cavitation,
occurring within the synovial fluid of the affected joint (Evans and
Breen, 2006). Cavitation is an engineering term used to describe the
formation and activity of bubbles (or cavities) within fluid, which
are formed when tension is applied to the fluid as a result of a local
reduction in pressure (Unsworth et al., 1971; Trevena, 1987; Young,
1999). Evidence for this explanation of the sound has come in
several forms.
There is face validity for cavitation as the explanatory mecha-
nism of ‘joint cracking’. The earliest scientific study of the
phenomenon identified articular surface separation as a key
component (Roston and Wheeler Haines, 1947). The characteristic
triphasic force–displacement graphs obtained during increasing
joint surface separation (Roston and Wheeler Haines, 1947; Uns-
worth et al., 1971; Watson et al., 1989), combined with the diver-
gent return pathway, are strongly suggestive of a rapid and
temporarily irreversible change in the cohesive properties of
synovial fluid, which was brought about by increased intra-artic-
ular volume and consequent decreased intra-articular pressure. In
synovial joints, the reduction in intra-articular pressure is likely
only achieved with a corresponding deformation of the joint
capsule (Brodeur, 1995), although this suggestion remains
speculative.
Radiographs have consistently demonstrated a radiolucent
region between the articular surfaces of the affected joint, imme-
diately following the elicitation of the sound, whilst these surfaces
remain separated (Fick, 1911; Dittmar, 1933; Nordheim, 1938; Fuiks
and Grayson, 1950; Unsworth et al., 1971; Watson and Mollan,
1990). No study has measured how long this state may persist by
continuously maintaining joint surface separation, although theo-
retically this could be indefinitely (Roston and Wheeler Haines,
1947 ).
Finally, several studies have shown that the sound cannot be
elicited more than once within a relatively short period of time
after the articular surfaces of the affected joint are allowed to return
to their resting configuration (Roston and Wheeler Haines, 1947;
Unsworth et al., 1971); a period that has been shown to extend as
long as 90 min following lumbar spine manipulation (Bereznick
et al., 2008). Furthermore, the location and quantity of these high
frequency vibrations recorded during manipulation procedures in
the spine is consistent with them originating from the synovial
zygapophysial joints (Ross et al., 2004; Bereznick et al., 2008).
One may ask whether cavitation is a necessary feature of
manipulation? Physiological changes may take place during
‘manipulation’ in the absence of cavitation (e.g. electromyographic
signals). However, cavitation is associated with distinct osteoki-
nematics (Unsworth et al., 1971; Watson et al., 1989; Watson and
Mollan, 1990; Ga
´l et al., 1995; Cramer et al., 2002). In addition,
clinicians frequently regard cavitation as an indicator of success in
the technical delivery of a manipulation (Evans and Breen, 2006).
Conversely, some commentators consider cavitation to be an
unnecessary outcome of manipulation because research has yet to
demonstrate an association with clinical outcomes (Flynn et al.,
2003, 2006). Nevertheless, for the purpose of defining manipula-
tion, the clinical success, or otherwise, of the intervention is irrel-
evant. By corollary, the occurrence of surgery, acupuncture or any
other physical intervention would not be defined by a successful or
failed clinical outcome. Cavitation may also, on occasion, occur
spontaneously during everyday movements, or during extreme
joint motions that may damage a joint. Hence, the occurrence of
cavitation in isolation cannot constitute a definition of
manipulation.
We propose that cavitation is a necessary feature of manipula-
tion. However, we are aware that the inclusion of this criterion will
be controversial for the reasons given above. It is also reasonable to
argue that cavitation is not the intended outcome of other types of
manual therapeutic interventions. For example, traction of
peripheral joints has been shown to result in joint surface separa-
tion (Hsu et al., 2008). If such a procedure resulted in cavitation,
then this would, by definition, be a manipulation. By contrast,
traction of the lumbar spine does not result in zygapophysial joint
surface separation (Humke et al., 1996); a likely consequence of the
complex kinematics of spinal segments (Evans, 2009). Alterna-
tively, if all other proposed criteria were present, yet cavitation was
not achieved, this would not fulfil all necessary criteria of
a ‘manipulation’ so should not be referred to as such.
4. Summary
Of the features discussed above, those we propose to be
necessary for the achievement of manipulation are summarised in
Table 3. We have attempted to retain the minimum number of
features. Collectively, these features should sufficiently constitute
the required components of a valid definition. Used in isolation,
each of these features is insufficient to define manipulation; their
sufficiency is dependent upon their collective occurrence. This is
consistent with defining causal mechanisms as a set of factors that
are jointly sufficient to induce an outcome event (Rothman, 1976);
under the omission of just one factor, the outcome would be
different.
Fig. 1 demonstrates the relationship of the proposed necessary
features of manipulation compared to other manual therapy
interventions, illustrating their potential importance within
a wider empirically-derived taxonomy of manual therapy.
An important attribute of our proposed features is that they
are explicitly divided into two categories: the ‘action’ (that which
the practitioner does to the recipient) and the ’mechanical
response’ (that which occurs within the recipient). Interestingly,
Table 3
Proposed necessity of manipulation features.
Necessary
Action (that which the practitioner does to the recipient)
A force is applied to the recipient Yes
The line of action of this force is perpendicular to the
articular surface of the affected joint
Yes
The magnitude of this force increases to a peak
over a finite period of time
No
Mechanical response (that which occurs within the recipient)
The applied force creates motion at a joint Yes
This joint motion includes articular surface separation Yes
The velocity of joint motion is variable No
The sum displacement of the articulating bones is usually zero No
Cavitation occurs within the affected joint Yes
D.W. Evans, N. Lucas / Manual Therapy 15 (2010) 286–291 289
whilst all of the ‘action’ features are included at the discretion of
the practitioner (and if any are excluded, the minimally sufficient
criteria for ‘manipulation’ would not be met), there is a causative
chain in operation with the ‘response’ features; once all of the
‘action’ components have been achieved, the induction of some
joint motion is necessary for the occurrence of joint surface
separation, and in turn this is necessary for the occurrence of
cavitation (Fig. 1).
5. Conclusion
We have identified empirically-derived features of manipula-
tion that we propose to be necessary for a valid definition, and have
provided arguments for and against their inclusion in such a defi-
nition. In addition, we have specified that each feature must occur
in order that the required defining criteria for manipulation are met
and that it be clearly distinguished from other manual therapeutic
interventions within a wider empirically-derived taxonomy of
manual therapy.
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