The results suggest that the cartilage thickness distribution of the distal femoral cartilage was influenced by the knee flexion angle during walking only in the medial compartment. The medial and lateral compartments in the knee have different contact situations according to the shape of the contacting surfaces (Koo and Andriacchi, 2007
). The tibiofemoral contact surfaces in the medial compartment are more conforming (convex-concave), so the contact location is sensitive to small differences in knee joint kinematics as described in . Thus the relatively large variation of the contact locations in the medial condyle could influence the variation of the centroids of thick cartilage.
Fig 7 Movement of tibiofemoral contact points during walking; (a) the medial compartment has larger movement of the contact points than (b) the lateral compartment due to the contacting surface shapes and knee kinematics during walking. Arrows represent the (more ...)
Conversely, the contact regions in the lateral compartment may not vary as much as in the medial compartment because of the shape of the contact surfaces and the knee joint kinematics. The lateral compartment femoral and tibial surfaces are both convex in shape (Koo and Andriacchi, 2007
) so the contact locations are less sensitive to knee joint kinematics than in the medial compartment as described in . Also the center of rotation of the knee in the transverse plane is predominantly in the lateral compartment of the knee during walking (Koo and Andriacchi, 2008
) and produces less contact movement on the lateral side during walking and greater movement in the medial compartment during walking. These kinematical conditions during waking are consistent with the relatively small variation of the centroids of thick femoral cartilage in the lateral compartment as compared to the medial side.
The variation of centroids of thick cartilage region in the tibial cartilage was relatively small for both medial and lateral plateaus (Koo et al., 2005
). In our previous study, the average contact location during walking on the tibial cartilage was associated with regional cartilage thickness in anterior cruciate ligament injured subjects (Andriacchi et al., 2009
). The cartilage thickness on the tibial plateaus does not change sharply so it is possible that the location of thickest cartilage on the tibia may not represent the functional cartilage adaptation as well as the regional cartilage thickness variation in the femur.
The finding in this study that cartilage thickness adapts to the walking kinematics is consistent with other studies that report that physical activity was positively associated with cartilage volume in a cross-sectional study (Jones et al., 2000
) and a longitudinal study (Jones et al., 2003
) for healthy children, implying that loading and number of cycles influences cartilage thickness. Similarly Hudelmaier et al. (Hudelmaier et al., 2003
) showed that muscle cross-sectional areas were positively associated with knee articular cartilage volumes in healthy subjects. The fact that Eckstein et al. (Eckstein et al., 1998
) reported that height, weight and BMI were not significantly correlated with cartilage volume could be related to the fact that activity level was not considered.
Li et al. showed that generally the thick cartilage regions on the femur matched those on the tibia for the tibiofemoral positions taken from the supine position MRI, concluding that “the cartilage was thicker in regions where cartilage-to-cartilage contact was present” (Li et al., 2005
). While the study found the associations of thick cartilage locations between the femoral and tibial cartilage, they could not explain the variation of the location depending on the individual knee function.
Taken together, these observations suggest that regional cartilage thickness variations are influenced by both loading and the number of loading cycles. Thus, the mechanics of walking is an important consideration in the analysis of the morphological variations of articular cartilage since it is the dominant cyclic activity of daily living. This observation is also consistent with a previous study that reported that the medial and lateral cartilage thickness variations in the knee are influenced by the peak knee adduction moment during normal walking for healthy and osteoarthritic subjects (Andriacchi et al., 2004a
, Andriacchi et al., 2009
). It was also reported that the secondary motions, AP translation and IE rotation, in the knee during walking influenced the regional thinning patterns in the subjects with anterior cruciate ligament injuries (Andriacchi et al., 2009
While the study focused on a healthy population and tested 17 subjects, the statistical result should be accepted carefully. The correlation coefficient for the medial femoral cartilage (R2=0.41) is in the range that can be well accepted in human research, especially with the data from two different modalities – gait and imaging, but this still requires a larger population test to confirm the conclusion statistically.
Results from the current study help explain the mechanism leading to a higher incidence of medial rather than lateral compartment osteoarthritis in the knee in two high risk groups – ACL injured and obese groups. Findings from this study suggest that abnormal knee kinematics at heel strike during walking for ACL injured subjects (Daniel et al., 1994
), and knee hyperextension at heel strike during walking for obese subjects (DeVita and Hortobágyi, 2003, Spyropoulos et al., 1991
) may influence the tibiofemoral contact regions more in the medial than lateral compartment. The study may contribute to understand the kinematic pathways of knee osteoarthritis suggested in our previous studies (Andriacchi et al., 2004a
, Chaudhari et al., 2008
, Koo et al., 2007
Understanding the factors that influence cartilage morphology is important not only to help maintain thicker cartilage, but also to understand the etiology and pathomechanics of osteoarthritis. Biomechanical factors such as kinetic and kinematic conditions during walking should be carefully considered for the treatment of cartilage disease in weight bearing joints.