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Biomechanical Study of Proximal Hamate Autograft in Scaphoid Nonunion
Catphuong L Vu, MD, MPH, Jerry I Huang, MD and Scott Telfer, Eng.D., University of Washington, Seattle, WA

Introduction
Treatment of proximal scaphoid fractures remains a difficult challenge with a high risk of nonunions and avascular necrosis. Ipsilateral proximal hamate has been described as a viable autograft option for osteochondral reconstruction of the proximal scaphoid. Our study evaluated the changes in the contact area and contact pressure of the radioscaphoid joint after proximal hamate autograft reconstruction.
Methods
TekscanTM pressure sensors (Tekscan Inc., Boston, MA) were placed in the radiocarpal joint in six fresh frozen cadaveric forearms. The test specimen was mounted securely to a fixed test bed through screws in the radius and ulna. The wrist and finger flexor and extensor tendons were connected to a rigid bracket via spring force gauges and were loaded through ten cycles with combined load of 150N in neutral, 45 flexion/extension positions. Proximal poles of the scaphoid and proximal hamate were excised 10 mm distal to the articular surface. Proximal hamate autograft was affixed to the scaphoid with 0.045 k-wire fixation. Pressure data was sampled at 100Hz during the loading cycles. Contact pressures and contact areas at scaphoid facet were averaged across loading cycles for each position. A mixed effect regression model was used to compare the different conditions.
Results
In neutral wrist orientation, contact pressure increased in the proximal hamate reconstructed wrist compared to native radioscaphoid joint (1.9MPa vs 1.1 MPa, p=0.14) while contact area decreased (54.7 mm2 vs. 91.1 mm2, p=0.09) with no statistical significant difference. Similarly, with wrist flexion and extension, contact pressures and contact areas changed but these were not statistically significant. Contact pressure increased after reconstruction (flexion: 1.0 MPa to 1.7 MPa, p=0.21; extension: 0.7 MPa to 1.3, p=0.06;) while contact area decreased (flexion: 69.5 mm2 to 54.4 mm2, p= 0.15; extension: 86.5 mm2 to 48.1 mm2, p=0.14).
Conclusion
Proximal hamate osteochondral autograft has been shown to have good anthropomorphic fit for proximal scaphoid reconstruction from previous studies. Our results showed a trend towards increased contact pressure and decreased contact area after hamate reconstruction. However changes were small and not statistically different from native pressures. The proximal hamate likely has a more pointed geometry, compared to the proximal scaphoid, leading to a smaller contact area and increased contact pressures. Our biomechanical study suggests hamate autograft may present a viable reconstruction for the proximal pole of the scaphoid without significant change in contact pressure that would alter wrist biomechanics and lead to increased risk of future radioscaphoid arthritis.


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