Abstract
Objective
To compare the ability of a new composite bio-absorbable screw and two conventional metal screws to maintain fixation of scaphoid waist-fractures under dynamic loading conditions.
Methods
Fifteen porcine radial carpi, with morphology comparable to human scaphoids, were osteotomised at the waist. Specimens were randomised in three groups: Group I were fixed with a headed metal screw, group II with a headless tapered metal screw and group III with a bio-absorbable composite screw. Each specimen was oriented at 45° and cyclically loaded using four blocks of 1000 cycles, with peak loads of 40, 60 (normal load), 80 and 100 N (severe load) respectively. Permanent displacement and translation (step-off) at the fracture site was measured after each loading block from a standardised high-magnification photograph using image analysis software (Roman v1.70, Institute of Orthopaedics, Oswestry). Statistical analysis was by ANOVA and tolerance limits.
Results
No gross failure or fracture gap displacement occurred. Average translations (step-off) at the fracture site after 4000 cycles up to 100N were 0.05mm±0.02SD (headed metal), 0.14mm±0.14SD (headless metal) and 0.29mm±0.11SD (composite) and differed significantly (p<0.01). Using tolerance limits, the data allowed us to predict that, with 95% certainty, the maximum average translation (step-off) following severe loading in 95% of any sample fixed with a headed metal screw will be below 0.17mm, headless metal screw below 0.74mm, and composite screw below 0.76mm.
Conclusion
We observed only small average translations (step-off) for all three screws. Moreover, translations of more than 1mm that would predispose to non-union were highly unlikely for any of the screws, even after severe cyclic loading. We therefore conclude that a new bio-absorbable composite screw can serve as an alternative to conventional screws when fixing scaphoid fractures.
