Biomechanical analysis of biodegradable magnesium, zinc, and polylactide pins for fixation of radial head fractures.

Background: Biodegradable implants have raised constant interest for fixation of displaced radial head fractures due to avoiding implant removal and minimizing cartilage damage. Polylactide pins (PP) are frequently used in clinical practice, but inferior mechanical properties showed higher rates of secondary dislocation compared to metal implants. Magnesium pins (MP) provide superior stability but exhibit inconsistent resorption and relevant hydrogen gas formation. Recently, zinc pins (ZP) have emerged as a promising alternative, offering comparable mechanical strength with favourable biocompatibility. Since these implants have not been tested for specific fracture fixation, this study aims to evaluate their applicability in a validated Mason type II radial head fracture model.

Methods: Standardized Mason type II fractures were conducted in biomechanically validated composite radii, and fixed by using either two 2.0 mm MPs, ZPs, or PPs. Biomechanical testing included 10 cycles of transverse loading, 1,000 cycles of axial loading (15-50 N at 0.1 Hz), and load-to-failure testing (2 N/sec). Stability was assessed by stiffness (kN/mm) under axial and transverse loading, fracture displacement (mm) after 1,000 cycles, and failure load (N) at dislocation ≥ 2 mm.

Results: MPs demonstrated the highest primary stability, followed by ZPs and PPs under both transverse (PP: 0.36 ± 0.08 kN/mm vs. MP: 1.30 ± 0.31 kN/mm, p < .001; vs. ZP: 0.87 ± 0.33 kN/mm, p = .012) and axial loading (PP: 0.43 ± 0.10 kN/mm vs. MP: 1.25 ± 0.31 kN/mm, p < .001; vs. ZP: 0.77 ± 0.18 kN/mm, p = .035). Fracture displacement after 1,000 cycles was lower with MPs and ZPs than PPs (PP: 0.038 ± 0.009 mm vs. MP: 0.013 ± 0.003 mm, p < .001; vs. ZP: 0.022 ± 0.007 mm, p = .003). MPs (282 ± 26 N) showed the highest load-to-failure at 2 mm dislocation, followed by ZPs (261 ± 38 N) and PPs (215 ± 53 N) (PP vs. MP: p = .032; PP vs. ZP: p = .164; MP vs. ZP p = .650).

Conclusion: In this biomechanical model of Mason type II radial head fractures, biodegradable magnesium and zinc pins demonstrated superior primary stability and load-bearing capacity compared to polylactide implants. MP showed the highest stiffness and lowest fracture displacement, while ZP achieved comparable performance in fracture stabilization. These findings suggest that zinc-based implants could offer a clinically valuable alternative for radial head fracture fixation, potentially reducing complications seen with the other implants. Further in-vivo, cadaveric, and clinical studies are necessary to confirm long-term outcomes and biological integration.

Level of evidence: Basic Science Study.