Biomechanics of joint manipulation

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.

▪   Shortened time to recover from acute back sprains (Rand).

▪   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