Biomechanical Comparison Between Fixation Techniques for First Metatarsophalangeal Joint Arthrodesis

Research Type: Level 2 - Prospective comparative study, Meta-analysis of Level 2 studies or Level 1 studies with inconsistent results Introduction/Purpose: Hallux Rigidus and Hallux Valgus can cause long-term morbidity and reduce mobility. First metatarsophalangeal joint (MTP) arthrodesis with a plate and screw construct has shown union rates over 90%, pain relief, and improved function. However, factors like bone resorption or stress relaxation can cause gapping at the fusion site, increasing the risk of nonunion. To address this, a nickel-titanium alloy (Nitinol) was developed, offering dynamic compression due to its superior elasticity and shape memory. The primary aim of this study was to analyze biomechanical outcomes between a traditional plate and screw construct, a 4-prong nitinol staple and screw construct, and a nitinol hybrid screw construct. We hypothesize that the nitinol constructs will demonstrate noninferior biomechanical outcomes compared to the plate and screw construct group. Methods: Twelve pairs of fresh-frozen cadaveric metatarsophalangeal joints were organized into three groups according to a balanced incomplete block design: 1) a traditional plate and cross screw (PS) construct, 2) a 4-prong nitinol staple and cross screw (NSS) construct, and 3) a nitinol hybrid screw (NHS) construct. The PS construct included a pre-contoured 1st MTP plate with a cannulated headed compression cross screw and washer; the NSS construct included a 4-prong nitinol staple with a cannulated headed compression cross screw and nitinol disc; and the NHS construct included a novel hybrid nitinol staple-compression staple with a cannulated headed compression cross screw and a nitinol disc. The plantar surface of the phalanx was cyclically compressed on a testing machine over 100 cycles at 1 Hz from 20 N to 90 N followed by failure testing. A camera recorded gapping throughout testing. Failure load, failure mode, deflection, and stiffness were recorded. Results: A graphical display of all results can be found in Figure 1. While the NSS and NHS constructs were not significantly different from each other across all metrics, they exhibited significantly higher ultimate failure load (PS: 113 ± 61 N; NSS: 196 ± 101 N, NHS: 161 ± 45 N), higher stiffness (PS: 15 ± 8 N/mm; NSS: 33 ± 15 N/mm, NHS: 29 ± 12 N/mm), lower deflection after cyclic loading (PS: 10 ± 6 mm; NSS: 6 ± 3 mm, NHS: 7 ± 3 N/mm), and lower gapping at failure (PS: 3 ± 1 mm; NSS: 1 ± 1 mm, NHS: 2 ± 1 N/mm) than the standard PS construct. No difference was found in deflection at failure between the nitinol repair constructs. Conclusion: This study demonstrates that two distinct nitinol constructs for first MTP arthrodesis have a higher ultimate failure load, higher stiffness, and reduced gapping when compared to a traditional plate and cross screw construct. This study supports a growing body of evidence that nitinol has unique properties that are optimal for joint stabilization in the setting of arthrodesis and reinforces that some constructs offer improved load to failure and displacement after cyclic loading. Figure 1. Failure load (top left), stiffness (top right), deflection after cyclic loading (bottom left), and gapping at failure (bottom right) outcomes for the PS, NSS, and NHS constructs. Significance was α = 0.05.