Carpal Kinematics In the Normal, Scapholunate Deficient and Surgically Reconstructed Wrist
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2020
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Melbourne Authors
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Biomedical Engineering
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PhD thesis
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Open Access
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© 2020 Xin Zhang
Abstract
Normal wrist function is ensured by a delicate interplay between bony architecture and soft-tissue constraints. Disruptions of this balance may alter carpal kinematics and joint contact, impairing the normal wrist physiology and biomechanics. The scapholunate interosseous ligament (SLIL), whose integrity is pivotal to normal scapholunate motion and that of the entire wrist, has been recognised as the most commonly injured carpal ligament. If left untreated, prolonged exposure to the altered joint motion may induce attrition of the secondary soft-tissue stabilisers, setting the stage for degenerative osteoarthritis. However, the exact role of these ligamentous stabilisers in maintaining normal scapholunate motion remains poorly understood. At present, many surgical procedures have been developed to treat scapholunate instability, but very few of them have been experimentally validated for their efficacy. Hence the aim of this investigation was twofold. Firstly, to evaluate the effects of different stages of scapholunate instability on wrist kinematics, and establish the relative contribution of various secondary scapholunate stabilisers to scapholunate motion and; secondly, to biomechanically evaluate the outcome of two reconstructive surgeries in treating scapholunate instability.
As these experiments were highly invasive, a novel cadaveric wrist testing platform (CWTP), consisting of a computer-controlled dynamic wrist cadaver simulation apparatus (DWSA) and a biplane X-ray fluoroscopy system, was developed. A comprehensive set of validation experiments were carried out to assess the performance of the CWTP in carrying out in-vitro wrist simulations. It was shown that the DWSA was capable of achieving highly accurate and repeatable wrist movements, with robust control performance maintained even in the presence of severe ligament injuries to the wrist. The measurement accuracy and precision of the custom-built biplane X-ray fluoroscopy system were also evaluated. And the results demonstrated that the system was capable of resolving rigid body motion to a sub-degree and sub-millimetre accuracy.
Dynamic wrist simulations, including flexion-extension (FEM), radial-ulnar deviation (RUD) and the dart-thrower's motion (DTM), were then performed in seven cadaveric wrist specimens. A four-stage ligament division protocol was developed based on a previous study that correlated specific anatomic lesions of the scapholunate supporting ligaments with different Geissler grades of scapholunate instability, as well as the widely-accepted injury mechanism of hyperextension, ulnar deviation and intercarpal supination. This included sequentially dividing the volar (v-) and the membranous (m-) SLIL in Stage I; the dorsal (d-) SLIL in Stage II; the radioscaphocapitate (RSC) and the long radiolunate (LRL) ligaments in Stage III; and the dorsal intercarpal (DIC) ligament, the scaphotrapeziotrapezoid (STT) ligament, and about 40% of the dorsal radiocarpal (DRC) ligament in Stage IV. Reconstructive surgeries were then performed with the goal of re-stabilising the severely disrupted scapholunate joint. Two different techniques, including a novel dorsal transarticular loop tenodesis (DTLT) procedure and the widely-used three-ligament tenodesis (3LT) procedure, were assessed for their efficacy in restoring the normal scapholunate biomechanics.
With incremental ligament division, the scaphoid became progressively more flexed, ulnarly deviated, pronated and radially translated. The largest and the second-largest changes in the scaphoid kinematics were observed after the Stage II and IV ligament divisions, respectively. The motion of the dissociated scaphoid is likely to be primarily dictated by the bony geometry. Dividing the v- and m-SLIL, or the RSC and LRL ligaments, however, did not induce appreciable alterations. The lunate was found to be progressively more extended, supinated, and translated towards the volar and ulnar aspect of the radius. The most considerable changes in the lunate kinematics were noted after the complete division of the SLIL in Stage II. Its dissociation from the scaphoid was immediately evident, as the lunate rotation in the sagittal plane was observed to be significantly attenuated. Dividing the volar extrinsic ligaments in Stage III caused the lunate to pronate instead of rotating into further supination, as well as to become ulnarly translated. In comparison, apart from causing the lunate to become statistically more supinated, dividing the dorsal carpal ligaments and the STT ligament in Stage IV induced no noticeable alterations in the lunate kinematics.
The DTLT resulted in a slightly insufficient reduction of the scaphoid in both the sagittal and transverse planes. And the procedure was found to induce a substantial amount of unphysiological volar translation of the scaphoid during ulnar deviation of the wrist. In contrast, the 3LT resulted in a much better correction of the scaphoid flexion; however, the scaphoid still appeared malrotated in pronation in the transverse plane. Furthermore, the pathological scaphoid supination was still present during wrist extension after the 3LT procedure, suggesting that the reconstruction was not capable in preventing the volar opening of the scapholunate interval. For the lunate, poor results were obtained after the 3LT, as the lunate remained collapsed in significant extension and supination. And since the tendon graft was tightened towards the ulnar side of the wrist, the 3LT not only did not correct the ulnar translation of the carpus, the condition was exacerbated by the procedure. The DTLT, on the other hand, led to a successful reduction of the dorsally collapsed lunate, which in turn restored its dynamic extension. The abnormal lunate supination was also corrected. However, cases of overcorrections were observed in these two planes.
The results suggested that apart from the SLIL, the dorsal carpal and the STT ligaments might have a vital contribution to the scaphoid stability, and the volar extrinsic ligaments are essential in preventing ulnar translation of the carpus. For the DTLT procedure, a complementary dorsal capsulodesis might be beneficial in correcting the residual scaphoid flexion. And the non-physiological volar translation of the scaphoid during wrist ulnar deviation might be due to the passive stretching of the radially situated ECR-B graft used by the procedure, pronating the scaphoid and lunate as a unit towards the volar aspect of the radius. For the 3LT procedure, its inefficacy in preventing the volar opening of the scapholunate interval might be attributed to the ulnar and dorsal directed pull of the tendon graft, which might not be adequate for counteracting the supination tendency experienced by the scaphoid during wrist extension. Therefore it may be beneficial for a volar reconstruction of the scapholunate joint to be carried out in conjunction with the 3LT procedure. Furthermore, given the unsuccessful reduction of the lunate after 3LT, a transosseous passage of the tendon graft might be considered, instead of simply attaching it to the dorsum of the lunate.
In summary, this study advances the understanding of normal and pathological wrist function, and provide a foundation for improved clinical practice in the surgical treatment of scapholunate injuries of the wrist.
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Carpal Kinematics, Wrist simulator, Wrist biomechanics, Dart-thrower’s motion, In-vitro wrist simulation, Biplane X-ray fluoroscopy, Scapholunate instability, Scapholunate ligament, Dorsal intercalated segment instability, Three-ligament tenodesis, Dorsal transarticular loop tenodesis, Radiocarpal contact