Biomechanical evaluation of biodegradable zinc and polylactide pins for radial head fracture fixation: a human...

Displaced or unstable radial head fractures are commonly treated with fragment-preserving fixation to restore joint congruity, maintain stability, and allow early mobilization. Biodegradable implants have become attractive alternatives to metallic implants. However, polymer-based pins may be limited by inferior mechanical strength and a higher risk of secondary loss of reduction. Recently, zinc pins have emerged as a potential alternative, but clinical data and biomechanical testing in human bone models are scarce. This study aimed to compare the biomechanical performance of zinc pins (ZP) and polylactide pins (PP) for the fixation of Mason type II radial head fractures using a human cadaver model.

Sixteen human cadaver radii from eight donors were used to create standardized Mason type II radial head fractures. Paired left and right radii from each donor were randomly assigned to fixation with either two 2.0 mm ZP or PP. After fracture fixation, specimens underwent cyclic transverse loading (10 cycles), followed by cyclic axial loading (1,000 cycles) between 15 and 50 N at 0.1 Hz, and final load-to-failure testing with continuously increasing axial load (2 N/s). Construct stiffness, micromotion at the fracture site, axial migration (implant loosening), as well as failure loads at 2 mm displacement and construct failure were recorded. A paired statistical analysis was performed to account for donor-specific variability.

Fixation with ZP demonstrated higher construct stiffness (0.77 ± 0.21 vs. 0.33 ± 0.05 kN/mm; p-adj. = 0.028) compared to PP constructs under axial load. Further, lower micromotion at the fracture site (0.05 ± 0.01 vs. 0.10 ± 0.02 mm; p-adj. = 0.042) and higher resistance to progressive loading with higher loads at construct failure were measured in ZP compared to PP (190 ± 44 vs. 116 ± 62 N; p-adj. = 0.042). However, no differences were observed between groups in axial migration (0.36 ± 0.08 mm vs. 0.36 ± 0.08 mm) during cyclic testing, indicating comparable resistance to implant loosening. Notably, construct stiffness under transverse loading and the loads required to reach a clinically relevant fracture displacement of 2 mm (161 ± 37 vs. 112 ± 60 N; p-adj. = 0.116) did not differ between ZP and PP constructs, suggesting adequate stability of both constructs within functional loading ranges.

In this cadaveric biomechanical study, zinc pins provided superior overall mechanical stability compared with polylactide pins for fixation of Mason type II radial head fractures, suggesting they are a promising alternative to the widely used polylactide pins. Importantly, polylactide pins also demonstrated sufficient primary stability without increased implant loosening or reduced resistance to clinically relevant fracture displacement. However, based on our data, zinc pins demonstrated favorable biomechanical performance in terms of construct stiffness and load to failure, offering a greater mechanical safety margin. Further clinical studies are required to evaluate in vivo performance and long-term outcomes of zinc-based biodegradable implants.