Mechanical evaluation of revision surgery for femoral shaft nonunion initially treated with intramedullary nailing: Exchange nailing versus augmentation plating.

INTRODUCTION: Exchange nailing (EN) or augmentation plating (AP) has been employed to treat nonunions after intramedullary nailing for femoral shaft fractures. Although instability is a factor in hypertrophic nonunion, mechanical evaluations have been limited because the contribution of the callus to fracture site stability varies with healing. Our previous study illustrated the potential for evaluation using a finite element analysis (FEA) that incorporates callus material properties. This study aimed to mechanically evaluate revision surgery for nonunions using FEA. MATERIALS AND METHODS: A quantitative computed tomography-based FEA was performed on virtual revision models of a patient with suspected nonunion after intramedullary nailing. In addition to the initial nailing model (IN) with an 11-mm diameter (D) and 360-mm length (L), four EN models with D12mm (EN1), D13mm (EN2), D12mm-L400mm (EN3), and D13mm-L400mm (EN4) nails and three AP models with 5- (AP1), 6- (AP2), and 7-hole (AP3) plates were created. As with bone, callus was assigned inhomogeneous material properties derived from density based on an empirical formula. The hip joint reaction force and muscle forces at maximum load during the gait cycle were applied. The volume ratio of the callus at the fracture site with a tensile failure risk of ≥1 (tensile failure ratio) and bone fragment movement were evaluated. RESULTS: The tensile failure ratio was 11.6 % (IN), 10.1 % (EN1), 6.3 % (EN2), 10.9 % (EN3), 6.2 % (EN4), 6.4 % (AP1), 7.2 % (AP2), and 7.7 % (AP3), respectively. The bone fragment movement showed an opening on the lateral side with the initial intramedullary nailing. However, both revision surgeries reduced the opening, leading to compression except in the EN1 model. The proximal bone fragments were internally rotated relative to the distal fragments, and the rotational instability was more suppressed in models with lower tensile failure ratio. CONCLUSIONS: For EN, the increase in diameter, not length, is important to suppress instability. AP reduces instability, comparable to a 2 mm increase in nail diameter, and screw fixation closer to the fracture site reduces instability. This study suggest that AP is mechanically equivalent to EN and could be an option for revision surgery for femoral shaft nonunions.

Evaluation of bone healing process after intramedullary nailing for femoral shaft fracture by quantitative computed tomography-based finite element analysis.

BACKGROUND: There is no proven method for quantitative evaluation of bone healing progress or decision to remove the nail after intramedullary nailing for femoral shaft fractures. Finite element analysis has become commonly utilized in bone analysis, but it may also be used to evaluate callus. The goal of this study was to use quantitative CT-based finite element analysis to assess the bone healing process and predict bone strength with the nail removed. METHODS: Quantitative CT-based finite element analysis was conducted on CT images from patients who had intramedullary nailing after a femoral shaft fracture at 6, 12, and 15 months postoperatively. The failure risk of the callus was evaluated with maximal load throughout the gait cycle. The tensile failure ratio was calculated using the volume ratio of the callus element with a tensile failure risk ≥100%. A virtual model with the nail removed was built for bone strength study, and the strength was calculated using the displacement-load curve. FINDINGS: The tensile failure ratio reduced with time, reaching 11.6%, 2.6%, and 0.5% at 6, 12, and 15 months postoperatively, respectively, consistent with bone healing inferred from imaging results. At 15 months, the bone strength at nail removal grew to 212, 2670, and 3385 N, surpassing the healthy side's 2766 N. INTERPRETATION: Quantitative CT-based finite element analysis enables mechanical assessment during the bone healing process and is expected to be applied to the selection of revision surgery. It is also applicable to the nail removal decision.