Cartilage will structurally adapt to joint loading (Arokoski et al., 2000). As previously noted, cartilage adaptation to loading has been well studied from an experimental and theoretical viewpoint. However, in vivo assessment of cartilage adaptation to joint loading has been limited by the availability of methods to evaluate in vivo changes in cartilage. MRI-based techniques are useful for evaluating in vivo quantitative morphological characteristics of articular cartilage. Typically, three-dimensional models of articular cartilage are assembled from a set of two-dimensional images requiring segmentation of the cartilage. Methods for cartilage segmentation have been extensively studied (Pham et al., 2000; Stammberger et al., 1999).
MR images can be used to examine the thickness variation of cartilage at the knee relative to the type of loads that occur during dynamic activities such as walking. For example, the highest loads at the knee occur at heel strike with the knee near full extension (Schipplein and Andriacchi, 1991). Typically, the thickest regions of the femoral and tibial load-bearing articular cartilage are aligned when the knee is at full extension, suggesting an adaptation to the high loads at heel strike on mating surfaces of the tibiofemoral articulation (see Figure 77.5).
There are several typical qualitative features (Andriacchi et al., 2004) in the variation of tibial and femoral cartilage thickness common to most healthy knees (see Figure 77.5). The tibial cartilage was thicker in the posterior weight-bearing region in the lateral compartment. The femoral condyle cartilage was thicker in the posterior region of the lateral femoral condyle. The tibial cartilage was thicker in the anterior weight-bearing region in the medial compartment. The femoral condyle cartilage was thicker in the anterior region of the medial
femoral condyle. The thickness variation on the tibia mirrored the thickness variation on the femoral condyles in each compartment. For example, thicker posterior cartilage on the lateral femoral condyle was mirrored by thicker cartilage in the posterior region of the tibial cartilage in the lateral compartment.
In vivo Function, Osteoarthritis, and Aging
As noted earlier, the mechanical environment of the cartilage is influenced by the intrinsic anatomy of the joint. The extrinsic loading (e.g., weight and inertia) during dynamic activities such as walking can also have a profound influence on the health of articular cartilage. For example, it has been shown (Schipplein and Andriacchi, 1991) that individual variations in the way people walk can influence the distribution of load between the medial and lateral compartments of the knee. Specifically the adduction moment at the knee during walking (adduction moment—the tendency of the external forces acting on the lower limb to produce adduction of the knee) causes higher forces to be transmitted to the medial side of the knee (see Figure 77.6).
It has been shown (Hurwitz et al., 1998) that the knee adduction moment during gait is positively correlated with the distribution of medial and lateral bone mineral content and is the best single predictor of the medial-to-lateral ratio of proximal bone mineral content. Thus, the higher the knee adduction moment, the greater the load on the medial plateau relative to that of the lateral plateau and the higher the bone mineral content in the proximal tibia under the medial plateau as compared to that under the lateral plateau. Consequently, variation in walking mechanics can influence the load sustained by cartilage in vivo and has the potential to influence the health and pathomechanics of articular cartilage.
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