Correlation Analysis Between PFNA Insertion Point and Neck-Shaft Angle in Intertrochanteric Fractures, and Selection of PFNA (240mm) Under Varying Radius of Femoral Curvature

Background The Intramedullary fixation of unstable intertrochanteric fractures is usually performed using the proximal femoral nail anti-rotation (PFNA), but the problems of optimal insertion point of intramedullary nail and anterior cortical impingement have not been solved.Meanwhile, the variation of the femoral neck-shaft angle (NSA) may also result in the deviation of the PFNA's insertion point. The purpose of this study to determine the ideal intramedullary nail placement position in the treatment of intertrochanteric fractures through the three-dimensional (3D) models, and to investigate the relationship between the differences in the RFC and the selection of intramedullary nail length when the ideal insertion position is established. Methods We retrospectively collected the femur computed tomography (CT) scans of 200 adults, and the DICOM-format CT scan images of each patient were imported into Mimics software to creat virtual femoral models. By inserting the PFNA into the femoral model, we recorded the length of each femur model and the neck-shaft angle (NSA); the vertical distance between each bony landmark and the optimal insertion point were measured in the projected plane;the distance of the insertion point to the apex of the greater trochanter (L2); the distance of the insertion point to the vastus lateralis muscle ridge (L3); the distance of the insertion point to the insertion of piriformis tendon (L4); the vertical distance between the optimal insertion point and the apex of the trochanter in the sagittal plane (L5) and the vertical distance in the coronal plane (L6); the radius of femoral curvature (RFC). Results The mean distance of the insertion point to the apex of the greater trochanter (L2) was 8.85±2.49mm; the mean distance of the insertion point to the vastus lateralis muscle ridge (L3) was 21.88±3.57mm; the mean distance of the insertion point to the insertion of piriformis muscle (L4) was 16.38±3.33mm. The optimal insertion point for the intramedullary nail is primarily located in the anterolateral region of the apex of the femoral greater trochanter.the mean of the length of each femur model (L1) was 390.37±30.47mm; the mean neck-shaft angle (NSA) was 128.01±7.59°; the mean radius of femoral curvature (RFC) was 841.88±125.64mm;the mean vertical distance between the optimal insertion point and the apex of the trochanter in the sagittal plane (L5) was 5.51±2.05mm. For the data captured above, L1, NSA, RFC, L5 of the intersex difference was significant (P<0.05). There was a significant positive correlation (r ² =0.4851,p<0.0001) between L5 and the NSA in males;there was a significant positive correlation(r ² =0.5267, p<0.0001) between L5 and the NSA in females. The mean RFC of medullary cavities compatible and incompatible for the PFNA (intramedullary nail lengths of 240 mm) in males were 990.68±75.76mm, 817.37±79.57mm and in females were 1032.92±105.53mm, 770.44±88.69mm. For the data captured above,the effect of the RFC on whether the PFNA (intramedullary nail lengths of 240 mm) could match the medullary cavity in females and males was statistically significant. Meanwhile,the mean RFC of medullary cavities compatible for the PFNA (intramedullary nail lengths of 240 mm) was statistically significant between males and females (P<0.05). Conclusions This study provide theoretical and data support for the optimal insertion point of the PFNA and the feasibility of inserting the longer PFNA in different femurs by the 3D simulation. Preoperative three-dimensional reconstruction was utilized to develop individualized treatment plans,and data from 3D model need to be combined with clinical experience for more safely inserting intramedullary nail into the medullary cavity. Further biomechanical tests and clinical studies are needed to verify its effects.