A cadaveric biomechanical study comparing the ease of femoral nail insertion: 1.0- vs 1.5-m bow designs.

INTRODUCTION: Anatomic fit of intramedullary nails was suggested by previous studies to improve significantly when the nail radius of curvature (ROC) is closer to the average femoral anatomy. However, no attempt has been made to investigate the impact of different ROC designs on the nail insertion process. Therefore, this biomechanical study quantitatively compared the ease of insertion between femoral intramedullary nails with a 1.0-m and a 1.5-m bow radius.

MATERIALS AND METHODS: Long TFN-ADVANCED™ (TFNA, 1.0 m ROC) and Proximal Femoral Nail Antirotation nails (PFNA, 1.5 m ROC) were implanted pairwise into seven paired cadaver femora. All bones were reamed 1.5 mm larger than the nail diameter. Using a material testing machine, intramedullary nailing was then performed stepwise with 20-mm steps and a 10-mm/s insertion rate, and force was measured. The nail deformation caused by the insertion was assessed through 3D computer models built from pre- and post-nailing CT scans. The ease of insertion between TFNA and PFNA nails was quantified in terms of insertion force, insertion energy and nail deformation.

RESULTS: There was no significant difference in the peak force generated during nailing between TFNA and PFNA nails (P = 0.731). However, the force measured at the end of insertion (P = 0.002) was significantly smaller in TFNA nails compared to PFNA nails. After implantation, TFNA nails showed significantly smaller deformation when compared to PFNA nails (P = 0.005, both ends aligned). Furthermore, less energy was required to insert TFNA nails; however, the difference was not significant (P = 0.25).

CONCLUSIONS: Compared to PFNA nails, a significant decrease in insertion force and nail deformation was found at the end of insertion for TFNA nails. Results suggest that TFNA having a 1.0-m ROC is easier to insert for the set of femora used in this study compared to PFNA with a 1.5-m ROC.