A few biomechanical studies relevant to the TMC joint have been performed4,24-26, although much remains unclear about contact stress on the TMC joint under physiological loading because of difficulties in direct measurement of force distribution in the joints of living organisms. In this study, percentage of the high-density areas of the RD and UV regions in the articular surface of the trapezium were significantly higher than that of the UD region. The percentage of the high-density area of the UD region in the articular surface of the first metacarpal was significantly lower than those of the other three regions. From a previous report, incongruent joint contact caused by lateral pinch in the presence of metacarpal pronation led to localized stress peaks in the UV and RD regions of the trapezium8. Cartilage wear was not predisposed to occur in the UD region of the first metacarpal, even if TMC osteoarthritis progressed9. Furthermore, if TMC osteoarthritis progresses, degeneration develops in the RD and UV regions of the articular surface of the trapezium9. Our results support these previous reports. The percentage of the high-density area of the RD region and TI showed a significant positive correlation in the articular surface of the trapezium, and those of the UD and UV had significant negative correlations with TI in the articular surface of the first metacarpal, although no significant correlations were found between percentage of the high-density area and VT in both articular surfaces. These results indicate that the bony morphology of the trapezium strongly affects stress distributions of the TMC joint surface, and an articular surface of the trapezium with higher inclination toward the radial side would gather stress on the radial-dorsal side. Only one previous report has described stress distributions of the TMC joint measured by CT-OAM, which is relevant to pressure distributions on the first metacarpal caused by Bennett’s fracture27, although no previous studies have examined the normal TMC joint. We believe that this study is the first to clarify that stress distributions of the normal TMC joint are influenced by trapezium morphology.
Esplugas et al. showed that various ligaments around the TMC joint prevented translation of the first metacarpal toward the radial side. In particular, dorsal ligaments such as the posterior oblique, dorsoradial and dorsal central ligaments became engaged in preventing the first metacarpal from translating radial-dorsally24. Because the TMC joint has a structure prone to translation of the first metacarpal radial-dorsally in the absence of the stability provided by these ligaments, the first metacarpals of individuals having higher TI may be more likely to shift radial-dorsally as a result of degeneration or rupture of the ligaments around the TMC joint. The TMC joint has a structure in which the stress distribution is predisposed to gather on the RD side of the trapezium during pinch and opposition7,28. Moreover, the structure of the trapezium, in which the articular surface has a concave shape in coronal view, is more likely to gather stress on the RD side of the articular surface of the trapezium if the first metacarpal shifts radial-dorsally. From these factors, we hypothsized that stress distributions were higher in the RD region if the articular surface of the trapezium was more radially inclined (Fig. 4).
The stability of the TMC joint depends on muscular activity and ligament tension. In various studies, the anterior oblique ligament, the intermetacarpal ligament and the dorsoradial ligament have all been proposed as primary stabilizers of the TMC joint3-5,25,29-34. The anterior oblique ligament and dorsoradial ligament is considered to prevent translation of the first metacarpal to the dorsal side. Vincent et al. reported that laxity of the beak ligament induced radial translation of the first metacarpal, so the main contact area of the TMC joint shifted from the volar side to the dorsal side26. In terms of the etiology of TMC osteoarthritis, many investigators have theorized that ligamentous laxity of the TMC joint leads to an incongruous relationship between joint surfaces28,29,35. This incongruity is thought to lead to smaller contact areas and thus greater contact stresses in certain areas of the joint, leading to degeneration and osteoarthritis7,28. Matthew et al. suggested that there was significant cartilage wear on the RD quadrant of the trapezium in advanced-stage osteoarthritis9. Our results showed a larger high-density area in the RD region of trapeziums showing a higher TI. Thus, not just ligamentous laxity but also the bony morphology of the trapezium would be involved in TMC osteoarthritis. We believe that the bony morphology of trapeziums with a high TI would promote progression of TMC osteoarthritis.
In previous reports relevant to the relationship between bony morphology of the TMC joint and TMC osteoarthritis, cases of advanced TMC osteoarthritis tilted more toward the radial side of the trapezium and the dorsal side of the first metacarpal20,21. From these reports, the bony morphology of the TMC joint is suspected to be involved in the commitment to TMC osteoarthritis. However, these reports did not prove whether the bony morphology causes TMC osteoarthritis or bony morphological alterations occur as a result of the progression of TMC osteoarthritis. Bettinger et al. suggested that the bony morphology of trapeziums that tilted more radially tended to cause TMC osteoarthritis, based on the anteroposterior view of the radiograph20. We also think that differences in bony morphology of the trapezium are a cause of TMC osteoarthritis because of our results showing that stress distributions of the RD region on the articular surface of the trapezium were higher in cases having a higher TI. Miura et al. showed that average volar tilt in patients with TMC osteoarthritis increased significantly compared to those without TMC osteoarthritis and that dorsal translation of the first metacarpal was significantly higher in participants with TMC osteoarthritis21. However, our results showed that bony morphology of the first metacarpal was not involved in stress distributions on the TMC joint surface. The changes in the first metacarpal seemed to occur as a result of progression of TMC osteoarthritis, and the bony morphology of the trapezium might be more involved in TMC osteoarthritis than that of the first metacarpal. Hence, the bony morphology of the trapezium is suggested to strongly influence stress distributions in the TMC joint and that trapeziums showing a more radial tilt have increased stress distributions in the RD region of the trapezium, leading to TMC osteoarthritis. We believe that our results have the potential to elucidate osteoarthritic mechanisms of the TMC and that this characteristic morphology of the trapezium may provide a predictive factor for the occurrence of TMC osteoarthritis.
The present study showed several limitations. First, the sample number was small. We therefore think that investigation of further samples is required. Second, stresses in the articular surfaces of the trapezium and first metacarpal were measured indirectly, and the current results are based on the assessment of bone mineral density in both articular surfaces. Third, the background characteristics of participants were not constant, so stresses on the TMC joint underwent changes caused by employment, life environment, history of sports, and so on. Fourth, the current study did not investigate the effect of ligamentous laxity. Finally, this study focused only on the coronal plane of the trapezium and sagittal plane of the first metacarpal. In the future, we need to clarify the detailed mechanisms of the TMC joint by investigating relationships between 3-dimensional bony morphology and stress distributions.
In conclusion, the results derived from CT-OAM suggest that the stress distribution is highly concentrated in the RD region of the articular surface of trapeziums with more trapezial inclinations. For this reason, bony morphology of the trapezium may become an important factor in the diagnosis and treatment of TMC osteoarthritis. CT-OAM provides clinical information for analyzing loading conditions associated with the TMC joint. However, further study is required to clarify other pathological mechanisms involved in the relationship between bony morphology of the TMC joint surface and TMC osteoarthritis.