Causes and Effects of Variation in Wrist Range of Motion: Investigating the Kinematics of the Proximal Carpal Row
Kinematics , Carpal Bones , Scaphoid , Lunate , Triquetrum
Due to the complex nature of the wrist, we still do not fully understand how the healthy wrist functions. There are few muscle insertions at the wrist, so bone motion is induced by passive ligaments and bone to bone contact, making carpal bone kinematics challenging to understand and model. This challenge is magnified by subject-specific variation in ligament properties and bone shape. Previous work has linked variation in carpal bone motion to changes in wrist flexibility, but these investigations have focused on the radial side of the wrist. Thus, many potential causes of variation in carpal motion patterns have yet to be thoroughly explored. To further understand the causes and ramifications of variation in normal carpal kinematics, we performed two studies using a database of 3D in vivo carpal bone postures. For the first study, we investigated variation in carpal bone motion on the ulnar side of the wrist. Contrary to theories embedded in the literature, we found that the triquetrum and lunate did not move in synchrony across a variety of functional wrist movements. We also identified a relationship between motion of the triquetrum and wrist laxity during wrist movement from 20° of radial deviation to 20° of ulnar deviation. During this movement, the component of flexion-extension in the motion of the triquetrum increased with increased wrist flexibility. In our second study, we investigated the influence of scaphoid shape on wrist flexibility. We found that the first two principal components of scaphoid shape were correlated with wrist flexion and extension range of motion, respectively. We also found evidence that scaphoid posture may be independent of scaphoid shape. Scaphoid posture affected the amount of scaphoid rotation during wrist flexion and extension. Finally, we discuss how we can apply our findings to the development and evaluation iof surgical techniques through two ongoing projects: simulating wrist motion in cadaveric specimens, and investigating how scaphoid shape impacts screw placement following scaphoid fracture. The findings from this work provide insights about the intricacies of carpal bone motion, a step towards explaining differential responses to injury and treatments with the potential to inform targeted, patient-specific interventions.