Midcarpal Arthrodesis Biomechanics: Memory Staples versus Cannulated Screws.

BACKGROUND: Midcarpal arthrodesis is a treatment of choice in patients with midcarpal arthritis. Traditionally a four corner fusion has been favoured, however recent research has shown improved results when the triquetrum and scaphoid are excised. There is no clear evidence as to which remaining bones should be fused or which implants should be used. The purpose of this study is to compare the biomechanics of midcarpal arthrodesis after scaphoid and triquetrum excision, using memory staples or cannulated screws, in recognised construct patterns.

METHODS: 36 identical sets of carpal bones were 3D printed from acetyl butyl styrene. Midcarpal arthrodeses were performed in three configurations with shape memory alloy staples or headless compression screws. This gave 6 treatment groups; lunocapitate single staple or screw, lunocapitate with 2 staples or screws, three corner fusion with 2 staples or screws. Peak torque to distraction was measured and analysed.

RESULTS: The peak torque to distraction was significantly greater in almost all constructs utilizing screws compared to staples, with two lunocapitate screws having the highest peak torque at both 1 and 3 mm distraction with 244 Nmm and 749 Nmm respectively (p < 0.05).

CONCLUSIONS: Constructs utilizing screws have a peak torque to distraction significantly higher when compared to staples. Our recommendation when performing a midcarpal arthrodesis after scaphoid and trapezium excision is to fuse the midcarpal joint with 2 headless compression screws.