Articular Morphology in Human Limb Synovial Joints: Variation, Covariation, and Application

Human limb joints are complex skeletal structures that must fulfill the roles of mobility, stability, and articular fit. These roles often place competing demands on joint morphology resulting in trade-offs (e.g., joints allowing greater mobility are usually less stable), but it is unknown how the tension among these antagonistic pressures patterns the morphological relationships within and among our joint surfaces. This dissertation is the first study to analyze intra- and inter-joint variation for all the major joints of the upper and lower limb in the modern human skeleton, and to explore potential applications of the patterns observed.

To address questions of variation, this dissertation takes a 3D geometric morphometric approach by analyzing landmarks placed on the shoulder, elbow, hip, and knee articulations for over 200 skeletons. Results establish a striking pattern of unequal morphological variance in convex vs. concave joint surfaces, potentially indicating differential regulation during development. Covariance analyses suggest that, despite accessory cartilages such as menisci and labra partially accommodating for articular fit, cartilage thickness is unrelated to the strength of correspondence between articulating surface shapes. Furthermore, analogous functional relationships between the humeral and femoral articular surfaces are a stronger source of covariance than direct articulation with neighboring morphologies. Variance-covariance patterns collectively hold clinical implications, as there appears to be opportunity for morphological “mismatch” between articulating surfaces, and in the case of some surfaces, stronger associations with articulations belonging to other joints than with their own complement. With these patterns in mind, the last portion of this dissertation focuses on the knee and determines that distal femur shape indeed is related to an individual’s knee replacement status, a pattern specifically driven by shape qualities of the medial femoral condyle.

These findings contribute to our understanding of morphological trait interdependence, which will hone future evolutionary models, and additionally provides a point of comparison for studies that often focus on linear articular dimensions. By determining the underlying articular shape variation within a single species, this study paves the way for future interspecific studies of joint morphology, development, and locomotion.