A14 Joint Failure

Topic

In general, this joint relies on a ligament that acts as a passive restraint against excessive joint movement, thereby preventing the movement from exceeding the joint's normal range. Consequently, joint failure occurs whenever a ligamentous injury, meniscal damage, dislocation, or joint instability compromises this restraining function—either partially or completely—within the specific joint.

Ligamentous injury occurs precisely when the distractive force acting on the ligament exceeds its ultimate tensile strength; this follows the same principle whereby an applied load eventually surpasses the ultimate strength of a specific tissue. Such injuries are classified by increasing severity, ranging from a mild ligament strain to a complete rupture. If the distractive force on the ligament is greater, the resulting injury reaches a higher level of severity.

The joint becomes unstable following a ligamentous injury because the ruptured ligament fails to limit abnormal joint displacement, resulting in a previously nonexistent joint laxity. This laxity is measured by the extent of joint displacement under a standard applied force. If the ruptured ligament offers less resistance to abnormal displacement, the resulting joint laxity increases, thereby raising the risk of secondary injury to the unstable joint.

Dislocation of the joint occurs precisely when the joint contact force loses its normal orientation: the tangential component of this contact force overcomes both the passive restraint provided by the ligament and the active restraint provided by the muscles surrounding the joint. The fact that this tangential component of the joint contact force is even greater makes the eventual dislocation of the joint itself even more likely. Consequently, in this model—incorporating the specific ligament and passive constraint introduced at the outset—the joint contact pressure (described by the Hertz model for mechanical contact between curved surfaces) becomes concentrated on a much smaller area of ​​the joint following such injuries; this accelerates the degradation of the articular cartilage, a process governed by the Archard equation, which constitutes another model within this framework. Furthermore, the mechanical response of this ligamentous complex under an applied load follows a viscoelastic spring model; this model reflects—in principle—the same stiffness and energy loss per cycle established by another model in the set regarding degenerated tendons. Ultimately, the concentration of joint contact pressure on an even smaller area of ​​the injured joint causes the degradation of the articular cartilage to progress even more rapidly.

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