Impact of the nail insertion angle on the wedge effect in intramedullary nail fixation of intertrochanteric hip fractures.

Background: The wedge effect is known to be influenced by the insertion of the proximal femoral intramedullary nail through the fracture line and the large proximal diameter of the nail. However, the impact of the nail insertion angle (NIA) on the wedge effect remains unclear. This study aimed to investigate: (1) how to evaluate the NIA intraoperatively, (2) whether the NIA is associated with the wedge effect, (3) whether the NIA can serve as a reliable predictor of the wedge effect, (4) which factors affect the NIA, and (5) which surgical techniques can prevent the occurrence of the wedge effect associated with the NIA.

Hypothesis: We hypothesized that an excessive NIA is related to the wedge effect and that lateral deviation of the entry point is associated with an excessive NIA.

Patients and methods: Intraoperative fluoroscopy images of patients who underwent intramedullary nail fixation for intertrochanteric hip fractures between 2013 and 2023 were analyzed. NIA and insertion point distance (IPD) were measured on hip anteroposterior radiographs with the guidewire inserted. Femoral shaft lateralization (FSL) and neck-shaft angle (NSA) were measured on hip anteroposterior radiographs before and after nail insertion; differences in FSL and NSA were calculated. A negative difference in FSL combined with a positive difference in NSA indicated the occurrence of the wedge effect. Pearson's correlation test was used to determine relationships between continuous variables (NIA, FSL, NSA, and IPD). Binary logistic regression analyzed the association between NIA and the wedge effect. Receiver operating characteristic (ROC) curve analysis was used to determine the threshold value of NIA, with predictive performance assessed using the area under the ROC curve (AUC). Other potential factors influencing the wedge effect were also examined.

Results: A total of 408 patients were included. The mean NIA was 15.61 ± 4.49 °. Post-nail insertion, the average increase in FSL was 3.20 mm, and the average decrease in NSA was 1.90 °. Pearson's correlation test revealed that NIA was negatively correlated with the difference in FSL (R = 0.565, P < 0.001) and positively correlated with the difference in NSA (R = 0.509, P < 0.001). Binary logistic regression showed a significant correlation between NIA and the wedge effect (P < 0.001). ROC analysis indicated that the AUC for NIA was 0.813, with an optimal cutoff point of 14.85 °. IPD was positively correlated with NIA (R = 0.519, P < 0.001). Unstable fractures were associated with increased lateralization of the femoral shaft after nail insertion (P = 0.003).

Discussion: The NIA is positively correlated with the wedge effect in intramedullary nail fixation of intertrochanteric hip fractures. The wedge effect tends to occur when the NIA is >14.85 °, particularly in unstable fractures. Lateral deviation of the entry point is associated with an excessive NIA. Adducting the affected limb, moving the entry point slightly medial and using a medial pusher may help control the NIA to less than 14.85 ° to reduce the wedge effect.

Level of evidence: III.