6.9 Joint stability
- Thus far, we have discussed the resultant forces at joints without considering how those forces are passed
- Joint stability is a critical concept whereby the joint must be structurally sound to the applied loads
- Joint stability requires the joint to maintain functional position
throughout its range of motion
- with “normal” loads
- with “normal” contact forces
- The relationship between these loads and “normal” motion must be maintained or negative consequences can occur (locally and globally within the body)
6.9.1 Idealized stability in synovial joints
- Small changes in the magnitude or direction of the functional load do not lead to large changes in the position of the joint (or its contact points)
- Joint contact occurs between surfaces covered with articular cartilage
- Peripheral loading doesn’t occur
- Their exists a unique equilibrium position for each set of loads
6.9.2 Mechanisms for maintaining joint stability
- Contact at the articular surfaces (passive)
- ie hip socket provides significant contact surfaces for stability
- Healthy joints have near frictionless contact
- Curvature of the surfaces generally enhances the stability of the
joint
- Opportunity for lateral force components
- Muscle Action (active – voluntary)
- ie muscle contraction increase the contact forces adding stability
- Stretching of the ligaments and capsules (passive)
- Bicondylar joints provide two effective contact points, thus, they transmit moments
- The position of the joint contact can be another unknown in a
problem (under determined system)
- Muscle force and contact position are often interrelated… ie change the force and the contact position must change
- Muscles have a finite reaction time… thus they may not react quickly enough to counteract an unexpected set of forces
- Ligaments limit the range of motion/contact within a joint
- May apply limited forces for the normal range of motion
- A health balance exists between stability and laxity (limited range of motion vs dislocation)
- Trauma or disease can upset the balance
- Surgeons attempt to maintain a healthy balance of stability and laxity during surgical repair, must consider all three stabilizing mechanisms
6.9.3 Range of healthy joint contact forces
