K.RASAPPAN1, D.HUANG2, A.K.S.YEW3, S.M.CHOU2, E.B.K.KWEK4
National University Hospital1, Nanyang Technological University2, Singapore General Hospital3, Woodlands Health4
Intertrochanteric hip fractures are prevalent in our population, for which the Synthes® PFNA cephalomedullary nail is a commonly used fixation device. A distinctive but understudied mode of failure of this implant is the ‘cut-in’, where there is axial migration of the blade. This study aims to develop a reproducible biomechanical model to induce this clinical phenomenon, as well as to determine clinically relevant factors that prevent this mode of failure.
Groups of Sawbones® osteoporotic femur models with induced similar intertrochanteric fractures had PFNA nails implanted and were tested according to intramedullary canal diameter (16, 18mm), PFNA nail diameter (9, 12mm) and proximal nail fit (snug, loose). They were subject to cyclic bidirectional loading to simulate physiological gait. A novel load applicator was 3D-printed to engage the femur head. The number of cycles to ‘cut-through’ failure for each group was determined.
12mm diameter nails (2256±375) failed earlier than the 9mm nails (9938±2107) (p=0.016); 18mm canal diameter femurs failed earlier (7532±439) than 16mm femurs (9938±2107) (p=0.003); Snug proximal nail fit models were more resistant to failure (7532±439) compared to loose fit models (1216±634) (p<0.001).
Nail toggling, contributed by a larger canal and looser proximal fit may predispose to ‘cut-ins’. A larger diameter nail also culminated in earlier failure, which may be explained by its increased shaft stiffness inducing decreased bending during axial loading and thus reciprocally channelling the force towards the blade-head interface. We recommend surgeons to use proximally tight fitting and smaller diameter nails during PFNA surgery to mitigate ‘cut-ins’.