A Biomechanical Model for Mechanically Efficient Cavitation Production During Spinal Manipulation: Prethrust Position and the Neutral Zone

Typical force-time history of the perpendicular force exerted by a clinician on a patient during spinal manipulation. Adapted from Herzog.²⁵ Copyright 2000, Elsevier Inc.

Coordinate system located at the center of the vertebral body, showing the 6 possible movements in relation to the x-, y-, and z-axes. Reproduced from J Biomech 1976;9:185-92. Copyright 1976, Elsevier Science.

Load-deformation behavior of the spinal segment (FSU) highlighting the region known as the neutral zone. Reproduced from Curr Orthop 1994;8:100-5. Copyright 1994, Elsevier Science.

Schematic representation of the prevailing model of target joint motion during manipulation. The simple anatomy of the MCP joint is used to illustrate the arcuate motion predicted by the model. To preserve the symmetry of the model, frontal plane rotation (abduction-adduction) of the MCP joint is considered. The model predicts that the applied force creates a moment and subsequent rotation of the proximal phalanx in a single movement plane, about a stationary center on the metacarpal bone. During the prethrust phase, the joint rotates to the end of available passive movement until it reaches a “physiological barrier” provided by the viscoelastic SF. During the thrust phase, more force is applied, which if sufficient produces cavitation and “breaks” the physiologic barrier. The corresponding configurations of the MCP joint when at the ends of the range of motion (i and iii) and in the neutral position (ii) are displayed beneath.

Schematic representation of cavitation in a synovial joint produced by joint gapping. Frontal plane rotation (abduction-adduction) of the MCP joint is used to illustrate the model. Rotation occurs about a stationary center (C) on the metacarpal bone and point P is a constant location on the proximal phalanx that describes the motion of the joint. Configurations i and iii represent the joint when at the ends of the available range of motion and ii represents the resting, neutral position. In this model, the most efficient prethrust position to facilitate joint gapping is within the neutral zone region, because the comparative laxity of the joints restraining tissues allows the articular surfaces to separate with minimal loss of kinetic energy (iv). Once cavitation occurs, the available extra range of motion provided in the para-physiologic space will be perpendicular to the articular surfaces and limited only by the joints restraining tissues.