Passive (no muscle contraction) joint laxity is dependent on the conformity of the articular geometry, the properties of the ligaments, menisci (at the knee), and the capsule at the joint. Any changes in the mechanical properties of these structures, whether they occur with aging, disease, or following a joint injury, will have an effect on this mechanical environment. For instance, joint laxity appears to be critically important for the health of a joint (Sharma et al., 1999). Joint laxity can produce large displacements of the articular surfaces, alter congruity and contact sites, and increase local shear and compressive stresses. Joint laxity depends on several factors including the mechanical properties of ligaments and muscle strength and coordination (see Figure 77.4).
The knee, a common site of osteoarthritis, provides a good illustration of the primary components of the joint that maintain joint stability and thus influence joint laxity. The major ligaments stabilizing the knee in translation and rotation are the anterior (ACL) and posterior (PCL) cruciate ligaments. The stiffness and elastic modulus of these ligaments decrease with age (Noyes and Grood, 1976; Woo et al., 1991). The major changes in muscle with age include reduced muscle activation (Stackhouse et al., 2001; Stevens et al., 2003), cross-sectional area (Frontera et al., 2000; Jubrias et al., 1997; Kent-Braun and Alexander, 1999), force per cross-sectional area (Jubrias et al., 1997), and maximum muscle strength (Frontera et al., 2000; Jubrias et al., 1997; Lindle et al., 1997). These age-associated changes in the properties of ligaments and muscle can cause increased joint laxity with age.
Active muscle contraction plays an important role in the dynamic stability of the joint during functional activities such as walking. As illustrated in Figure 77.4, the lateral stability (resistance to lateral joint opening) is dependent on the tension in passive soft tissue (lateral collateral ligament) and active muscle force producing a moment (tendency of a force to produce a rotation) resisting the extrinsic moment tending to adduct the
Ligament 4 Force
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