How to get better TAD? Relationship between anteversion angle of nail and position of femoral neck guide pin during nailing of intertrochanteric fractures

Patients with IFFs who underwent closed reduction and internal fixation with femoral intramedullary nails under two-dimensional (2D) fluoroscopy in the Zhongshan Hospital, Fudan University, the First Affiliated Hospital of Dalian Medical University and Shanghai Pudong New District Zhoupu Hospital between March 2010 and October 2015, were retrospectively enrolled. Inclusion criteria were: (1) age > 65 years; (2) the time interval from the injury to admission < 24 h; (3) intraoperative fracture end was well reset and the anatomical repositioning was generally achieved; (4) the position of the guide pin in the femoral neck was adjusted by changing the entry point via axial rotation of the main intramedullary nail; (5) the imaging data were completed; and (6) unilateral closed fracture. Exclusion criteria included: (1) concurrent femoral fractures at other parts of the same side; (2) long-term wheelchair use before injury; (3) pathological IFFs; (4) old IFFs; (5) without fixed address or some subjective reasons not want to participate in the trials; and (6) incomplete follow-up information. This study was approved by the ethics committee of our institution.

All surgery in three centers was performed by the same senior surgeon under general, continuous epidural or nerve block anesthesia. The main intramedullary nail was placed into the proper location in the femoral shaft according to the standard surgical procedures: First, the guide pin was placed into the femoral neck, and non-ideal location of the guide pin was seen under fluoroscopy at the anteroposterior and lateral positions during the operation; Then, the main nail was rotated along its longitudinal axis to adjust the entry point of guide pin followed by replacement of the guide pin and observation under fluoroscopy. The length of the main intramedullary nail and mode of distal locking were determined according to the patients’ condition. The location of the femoral neck guide pin was adjusted by axially rotating the main femoral intramedullary nail to alter the entry point and ensure the TAD not exceeding 25 mm [9‐13]. The TAD was measured by three individual observers through C-arm x-ray intra-operatively. The rotation of the nail will not stop until guide pin is on an ideal position. So, the nail should be rotated slowly and steadily with the assistance of intra-operative c-arm X-ray to decide the rotation of the nail. Meanwhile, the rotation angle of the nail was recorded based on the location change in pre- and post- rotation of T-shaped wrench. PSA and anteversion angle before (∠α1, ∠β1) and after (∠α2, ∠β2) the adjustment of the guide pin was respectively measured for all the included patients using professional drawing software on X-ray (Fig. 1). In order to reach a good reduction for the proximal femur, the intact opposite proximal femur was also x-rayed preoperatively to provide a reference for reduction procedure during the surgery. The angle of the nail was chosen for fracture fixation based on neck-shaft angle from the opposite femur.

At 24 h after operation, patients were encouraged to walk with the aid of double crutches or walker. Partial weight-bearing was also allowed on the affected limb. Rivaroxaban was orally administered for 7 weeks to prevent thrombogenesis. The operative time (min), overall fluoroscopy time (min), blood loss during surgery, amount of transfused blood (U), length of hospital stay (d) and postoperative complications were recorded. The radiographs of the hip joints (anteroposterior position, bilateral; lateral position, the affected) were made to evaluate the fracture reduction at 24–48 h after operation. The quality of the fracture reduction was graded as poor (> 10^° varus/valgus), acceptable (5^° – 10^° varus/valgus) or good (< 5^° varus/valgus) [21]. Functional outcome was assessed postoperatively, at 6-week and 3-month follow-up on the basis of the Timed Up and Go (TUG) test [22] and the Harris hip score (HHS) [23].

Among the included patients, one patient was randomly selected to extract the raw computerized tomography (CT) data and establish the femoral and intramedullary nail geometric models. Data requirements included: (1) the patient underwent postoperative 64 slice spiral CT scan (GE company, USA), with the scan scope from the hip to the lower 1/3 of the femur; (2) the slice gap of each scan was 1 mm; (3) the density of pixel matrix was 512 × 512; and (4) 2 bytes were assigned for each pixel. The obtained CT images were stored in Dicom format. Patient’s data in Dicom format were imported into Mimics 10.01 (Materialize company, Belgium). Through image positioning, threshold setting, dynamic area growth and hole filling, the redundant data were removed and the 3D visualization model of the femur (Fig. 2a) was rebuilt. According to PFNA intramedullary nailing data provided by SYNTHES (Switzerland), the intramedullary nail and guide pin models (Fig. 2a) were established using Pro/E 3.0 software. Models were exported in a .lis format. The femoral shaft was bent to the anterolateral direction. Mimics 10.01 was used to draw the axis of the femoral marrow cavity, and the physical radian of the marrow cavity axis from the site of 2 cm under femoral intertrochanter to lower 1/3 of the femur was measured and presented as curvature radius (R) (Fig. 2b). Detail processes of operation simulation and mathematical analysis (Fig. 2c-e) were described in Supplementary file 1.

Fifty patients were included in this study (Table 1; Supplementary file 2), where 30 (60%) cases were male and 20 (40.0%) cases were female. Their age ranged from 60 to 85 years old, with an average of 76.04 ± 6.28 years old. All the patients suffered from IFFs with 28 (56.00%) occurred on the left side and 22 (44.00%) on the right side. Unstable trochanteric fractures were classified as AO – 31A2 in 46 (92.00%) cases and 31A3 in 4 (8.00%) cases. All of the fractures were fresh closed fractures and were treated with closed reduction and internal fixation with femoral intramedullary nails.

PSA (∠α) and anteversion angle (∠β) of the guide pin on intraoperative imaging were collected from these 50 clinical cases (Table 2) and then, Pearson correlation analysis was performed. In line with our occasionally seen results, there was indeed a significantly positive correlation between the PSA and the anteversion angle, with the correlation coefficient of 0.902 (p < 0.01). By altering the PSA and the anteversion angle (Figs. 3, 4, 5 and 6), TAD was adjusted to be less than 25 mm in all patients (20.52 ± 2.80 mm). Fracture reduction (Figs. 3, 4, 5 and 6) was considered good or acceptable in all 50 patients on post-operative radiographs (Table 2).

The mean operative time, fluoroscopy time and length of hospital stay were 65.82 ± 11.16 min, 2.03 ± 0.79 min and 6.66 ± 2.49 d. Thirty-one patients received postoperative blood transfusions, with the amount of transfused blood of 3.55 ± 1.95 U (Table 2). Complication occurred in 6 patients (10.90%), including urinary tract infection in 2 (4.00%), bronchopneumonia in 1 (2.00%), hypoglycemia in 1 (2.00%), delirium in 2 (4.00%), uroschesis in 1 (2.00%), lower limb vein thrombosis in 1 and vulvar ulcerations in 2 (4.00%) (Table 3). The mean TUG and HHS were 123.60 ± 12.78 s and 52.14 ± 13.78 postoperatively, 47.52 ± 27.22 s and 63.04 ± 10.72 at 6-week, 31.54 ± 20.95 s and 72.88 ± 8.79 at 3-month follow-up (Table 4).

The curvature radius of the medullary cavity was 90.02 cm. The virtual surgery was realistic with good 3D visual effects. The design of virtual surgery was in accordance with the actual intraoperative situation. The axis of the main intramedullary nail coincided with the axis of the upper femur. The depth of insertion was proper. The guide pin of tension screw was located at the axis of the femoral neck, and the TAD was 22 mm.

When the main intramedullary nail was externally and internally rotated of 20° at the standard location, the trajectory of the endpoint of the guide pin was curved. Under the fluoroscopy coordinate system O-XYZ, with the axial rotation angle of the main intramedullary nail (t) as the variable, the changes of the PSA at the anteroposterior position and anteversion angle of the guide pin at the lateral position were observed through calculation (Fig. 7). The PSA at the anteroposterior position and anteversion angle of the guide pin at the lateral position were both increased along with the increase in the t value when the main intramedullary nail was externally rotated (Fig. 7a); while they were decreased along with the decrease in the t value when the main intramedullary nail was internally rotated (Fig. 7b).

According to the correlation between the PSA and the anteversion angle of the guide pin, a personalized position map of the femoral neck guide pin was designed and developed (Fig. 7c). The abscissa was the axial rotation angle of the main intramedullary nail, while the ordinate values were respectively the PSA of guide pin at the anteroposterior position and the anteversion angle of the guide pin at the lateral position. t = 0 was defined as the ideal position of the guide pin in the femoral neck. Based on the figure, the PSA of guide pin at the anteroposterior position was 134°, while anteversion angle of the guide pin at the lateral position was 25° at the corresponding site.