Biomechanical Comparison of Compression Screws vs Nitinol Staples for Second Tarsometatarsal Arthrodesis

Research Type: Level 5 - Case report, Expert opinion, Personal observation Introduction/Purpose: Second tarsometatarsal (TMT-2) arthrodesis is commonly performed for midfoot arthritis, trauma, and deformity. It has been well established that adequate compression across a joint with bony apposition is important for union; however, the optimal fixation technique for arthrodesis of the TMT-2 joint has not been established. The aim of this study was to investigate under dynamic loading the biomechanical properties of standard cross screw fixation compared with low- profile nitinol staples in varying configurations, one vs. two staples in human cadaver models. Methods: Twenty-four human cadaver lower legs were randomized based on bone mineral density (BMD) into three groups for simulated TMT-2 arthrodesis: Group One (1) cross screw fixation with two 4.0 mm partially threaded lag screws; Group Two (2) one two-leg low profile nitinol staple; Group Three (3) two two-leg low profile nitinol staples placed orthogonally to each other. Each specimen was biomechanically tested simulating forefoot weight bearing on the toes and metatarsals. Testing was performed at room temperature. Initial stiffness was determined from the initial load from 20N to 150N. A stepwise increase in force was applied per 10 cycles until each specimen reached 700N or failure. Motion tracking capturing movement at the TMT-2 were used to determine gap amplitude, plantar gapping, gap angle unloaded, and the failure load. Results: Initial stiffness between the groups was higher in the staple constructs but was not associated with a significant difference between the three groups (21.38 N/mm ± 6.03N/mm vs. 18.09 N/mm ±5.43 N/mm, p=0.57). In contrast, the gap angle amplitude between the unloaded and loaded foot conditions within the 675N and 700N cycles was significantly less for the two staple groups compared to the cross-screw fixation (0.63º±0.46° vs. 2.3°±0.89 º, p=.007, 0.72º±0.57° vs. 2.42°±0.94 º, p=.0108). The staple group in general provided a stiffer construct with less motion at the joint but no significant difference was detected between the two staple groups and cross screw fixation technique for plantar gapping, gap angle unloaded, and the failure load (p>0.05). Conclusion: The low-profile nitinol staple constructs demonstrated increased stiffness, less motion, and less plantar gapping across the TMT-2 joint. Statistical significance was found in the upper cycles of gap angle amplitude. Two nitinol staples were comparable to one.