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Isolated Thumb Carpometacarpal Joint Dislocations With and Without Suture Augmentation: A Biomechanical Study
Devan O Higginbotham, MD, Heather Preston, MD, Hannah Joines, BS, Chaoyang Chen, MD/PhD and Andrew G Tsai, MD, Detroit Medical Center, Detroit, MI

Introduction: Isolated carpometacarpal (CMC) dislocations of the thumb are an infrequent injury. The optimal management of thumb CMC dislocations remains unclear, and the understanding of the biomechanics surrounding the injury are even more obscure. No study has recreated a CMC dislocation in the laboratory, though investigators have demonstrated the stabilizing role of certain ligaments surrounding the joint. We analyzed the biomechanics of the ligamentous complex of the CMC joint with dislocation as well as reconstruction with suture augmentation technique.
Materials & Methods: Biomechanical analysis was performed in 10 fresh frozen specimens after soft tissue dissection. The loading was applied to mimic a pure posteriorly-directed force or hyperflexion through the CMC joint. Load to failure/dislocation was performed at a rate of 1 mm/s via an Instron material testing machine until posterior CMC dislocation was achieved. The maximum load, load at clinical failure, stiffness and mode of failure were recorded. The native ligament was repaired and augmented with suture (InternalBrace), and the testing was repeated.
Results: Posteriorly directed force produced posterior CMC dislocations, while hyperflexion the through the CMC joint causes fracture/dislocations. The load to failure of the native CMC joint was -217.76 N (SD 66.03), and the stiffness of the ligamentous complex on average was 18.86 N/mm (SD 8.83 N/mm). The mean load to failure after repair with suture augmentation was 94.62 N (SD 39.77), with a stiffness 8.21 N/mm (SD 3.06) on average. With the native ligament, all failures were mid-substance. For the repaired specimens, one specimen failed with distal anchor pull-out and the remained failed with mid-substance stretch without frankly rupturing; there were no proximal anchor failures. The native ligament was noted to have greater stiffness (p = 0.002 using Paired T test) and greater load to failure (p = 0.0001 using Paired T test) than the repair with suture augmentation. The maximum displacement to failure of the native ligament was 14.5 mm compared to the repair with suture augmentation 11.9 mm (p = 0.068).
Conclusion: Isolated CMC dislocation was able to be achieved with a posteriorly-directed force rather than hyperflexion of the CMC joint. The ultimate failure load of the reconstructed ligaments was about half of that of the native ligaments. However, between the ligament reconstruction and the dynamic stabilizers, which were not studied, the procedure may provide enough stability in the post-repair period to reduce the need for k-wire fixation. Further research into this technique is warranted.


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