Manipulation can be distinguished from other manual therapy interventions such as joint mobilisation
by its bio mechanics,
both kinetics
and kinematics.
Kinetics
Until recently, force-time histories measured
during spinal manipulation
were described as consisting of three distinct phases: the preload (or
prethrust) phase, the thrust phase, and the resolution phase. Evans and Breen
added a fourth ‘orientation’ phase to describe the period during which the
patient is oriented
into the appropriate position in preparation for the prethrust phase.
When individual peripheral synovial joints
are manipulated, the distinct force-time phases that occur during spinal
manipulation are not as evident. In particular, the rapid rate of change of force that occurs
during the thrust phase when spinal joints are manipulated
is not always necessary. Most studies to have measured forces used to
manipulate peripheral joints, such as the metacarpophalangeal
(MCP) joints, show no more than gradually increasing load.
This is probably because there are many more tissues restraining a spinal motion segment
than an independent MCP joint.
Kinematics
The kinematics of a complete spinal motion segment
when one of its constituent spinal joints are manipulated
are much more complex than the kinematics that occur during manipulation of an
independent peripheral synovial joint. Even so, the motion
that occurs between the articular surfaces of any individual synovial joint
during manipulation should be very similar and is described below.
Early models
describing the kinematics of an individual target joint during the various
phases of manipulation (notably Sandoz 1976) were based on studies that
investigated joint cracking
in MCP
joints. The cracking was elicited by pulling the proximal phalanx away from the metacarpal bone (to separate, or
'gap' the articular surfaces of the MCP joint) with
gradually increasing force until a sharp resistance, caused by the cohesive
properties of synovial fluid,
was met and then broken. These studies were therefore never designed to form
models of therapeutic manipulation, and the models formed were erroneous in
that they described the target joint as being configured at the end range
of a rotation
movement, during the orientation phase. The model then predicted that this end
range position was maintained during the prethrust phase until the thrust phase
where it was moved beyond the 'physiologic barrier' created by synovial fluid
resistance; conveniently within the limits of anatomical integrity provided by
restraining tissues such as the joint capsule and ligaments. This model still
dominates the literature. However, after re-examining the original studies on
which the kinematic models of joint manipulation were based, Evans and Breen[2] argued that the optimal prethrust position is
actually the equivalent of the neutral zone of the individual joint, which is
the motion region of the joint where the passive osteoligamentous stability
mechanisms exert little or no influence. This new model predicted that the
physiologic barrier is only confronted when the articular surfaces of the joint
are separated (gapped, rather than the rolling or sliding
that usually occurs during physiological motion), and that it is more
mechanically efficient to do this when the joint is near to its neutral
configuration.
Cracking joints
Main article: Cracking joints
Joint manipulation is characteristically
associated with the production of an audible 'clicking' or 'popping' sound.
This sound is believed to be the result of a phenomenon known as cavitation occurring within the synovial fluid of the joint. When a
manipulation is performed, the applied force separates the articular surfaces
of a fully encapsulated synovial joint. This deforms
the joint capsule and intra-articular tissues, which in turn creates a
reduction in pressure within the joint cavity. In this low pressure environment,
some of the gases
that are dissolved in the synovial fluid (which are naturally
found in all bodily fluids) leave solution creating a bubble or cavity, which rapidly collapses upon itself,
resulting in a 'clicking' sound. The contents of this gas bubble are thought to
be mainly carbon dioxide.
The effects of this process will remain for a period of time termed the 'refractory period',
which can range from a few minutes to more than an hour, while it is slowly
reabsorbed back into the synovial fluid. There is some evidence that ligament laxity
around the target joint is associated with an increased probability of
cavitation.
Clinical effects and mechanisms of action
The clinical effects of joint manipulation have
been shown to include:
▪ Temporary
relief of musculoskeletal pain.
▪ Temporary
increase in passive range of motion (ROM).
▪ Physiological
effects upon the central nervous system.
▪ No
alteration of the position of the sacroiliac joint.
Common side effects of spinal manipulative
therapy (SMT) are characterised as mild to moderate and may include: local
discomfort, headache, tiredness, or radiating discomfort.
Shekelle (1994) summarised the published
theories for mechanism(s) of action for how joint manipulation may exert its
clinical effects as the following:
▪ Release
of entrapped synovial folds or plica
▪ Relaxation
of hypertonic muscle
▪ Disruption
of articular or particular adhesion
Unbuckling of motion segments that have
undergone disproportionate displacement
