The biomechanical interaction between ligaments and joint shape in the thumb
Creators: name: Rusli, Wan Mohd Radzi Bin
Publisher: name: Imperial College London
Year: 2019

description: The first carpometacarpal (CMC) joint is one of the most common sites for hand osteoarthritis (OA). Hand OA causes pain and increases one’s dependencies in performing activities of daily living. To better target and treat the effects of OA, the anatomical and biomechanical features of the healthy joint must be understood. Thus, the aims of the thesis were to establish the morphological variation of the first CMC joint in a healthy population; to quantify the stability of the first CMC joint with intact and disrupted ligaments; and to determine the role of the bony features of the joint in maintaining stability. To examine morphological variability, a novel multi-object statistical shape model was created by incorporating a feature selection process prior to coarse-to-fine non-rigid registration and projection pursuit principal component analysis (ppPCA). The performance of the proposed non-rigid registration strategy was assessed using 50 three-dimensional models of the first CMC joint. The root mean square error (RMSE) and Hausdorff distance (HD) of the resultant objects were quantified. The proposed coarse-to-fine non-rigid registration performed better than a non-rigid coherent point drift (CPD) approach, as the proposed method produced lower values of RMSE and HD (p &lt; 0.05). With the same outlier ratio, the proposed non-rigid registration strategy required less computational time (11.8 minutes) compared to non-rigid CPD (81.6 minutes). Following the development of the multi-object statistical shape model, a morphological analysis was done on the first-two principal components (PCs), which represented 15% of the total variation in the population of the study. Four features were measured on the shape model, namely articular tilt and torsion angles of the first metacarpal and length and width of the first metacarpal facet of the trapezium. The first metacarpal articular tilt angle varied between -6.3° and 12.3° and was strongly associated with the first PC (R^2 = 0.99, p = 0.001). The variability of both articular tilt (R^2= 0.829) and torsion angles of the first metacarpal (R^2= 0.91) were strongly associated with the second PC and varied from 1.0° to 6.4° and 0.2° to 14.2°, respectively. The metacarpal facet width varied between 10.3 mm and 11.1 mm and was strongly associated with the first PC (R^2 = 0.84). Strongly associated with the second PC (R^2 = 0.97), the metacarpal facet length varied between 12.3 mm and 17.3 mm. The variation in the first metacarpal articular tilt angle was positively correlated with the variation in the facet width (r = 0.90, p = 0.005). In the second PC, the first metacarpal torsion angle was negatively correlated with the facet length (r = -0.92, p = 0.003), while the first metacarpal articular tilt angle and the facet length were positively correlated (r = 0.94, p = 0.001). To examine the role of the ligaments in maintaining stability of the first CMC joint, a sequential ligament sectioning study of sixteen specimens was performed. While a small compressive force was maintained, loads were applied to displace each specimen in four directions – volar, dorsal, radial and ulnar. Translation of the specimen in both dorsal-volar and radial-ulnar directions was measured. Initially, the tests were conducted with the specimen intact. These tests were then repeated following each of four ligaments being sectioned in the following order: anterior oblique ligament (AOL), ulnar collateral ligament (UCL), intermetacarpal ligament (IML) and dorsal radial ligament (DRL). When displaced in the dorsal direction, the first CMC joint experienced dorsal subluxation when the IML was transected (p = 0.046) and dorsal (p = 0.046) and ulnar (p = 0.046) subluxation after the DRL was transected. When displaced in the radial direction, the first CMC joint experienced radial subluxation after transection of IML (p = 0.003) and radial subluxation (p = 0.012) with volar subluxation (p = 0.046) following DRL transection. A morphology-function relationship of the first CMC joint was derived through combining the results of the statistical shape model and the in vitro experiment. The four anatomical features quantified in the first study were measured on the three-dimensional models of the sixteen specimens. In the first two PCs, the effect of the first metacarpal articular tilt angle and first metacarpal torsion angle variations was observed when the first CMC joint experienced external loads in the volar, dorsal, radial and ulnar directions. The variations in width and length of the trapezium’s articulating surface with the first metacarpal were observed to affect the joint translations when the joint subjected to external load in ulnar direction as described by the third PC. The implementation of morphology-function model allows to establish relationship between the variations in anatomical features of the first CMC joint and the ligaments surrounding the joint. The anatomical features of the first metacarpal were determined to affect the translations of the first CMC joint in all loading conditions. The shape-function analysis shows that articular tilt and torsion angles of the first metacarpal and facet width affect the stability of the first CMC joint, especially when the ligaments surrounding the joint are disrupted. These findings have implications for the early detection of first CMC joint OA, as the morphological features of both first metacarpal and trapezium could be used to predict the stability of the first CMC joint. In future, a morphology-function relationship focussing on OA patients needs to be done to verify this. In addition, a population-based model that incorporates both healthy and pathological data could help clinicians in detecting and treating first CMC joint OA earlier or more effectively.