Enhanced interfragmentary stability and improved clinical prognosis with use of the off-axis screw technique to treat vertical femoral neck fractures in nongeriatric patients

Abstract

Background: The optimal internal fixation strategy for vertical femoral neck fractures (VFNFs) in nongeriatric patients remains uncertain. Therefore, the purpose of this study was to compare the clinical prognoses and underlying mechanical characteristics of a novel off-axis screw technique with dynamic hip screws (DHSs) and three traditional parallel screws.

Methods: This study included a clinical investigation and a patient-specific finite element analysis (FEA). In the clinical investigation, VFNF patients were grouped by fixation type: (1) use of three parallel screws (G-TRI); (2) augmentation with an off-axis screw (G-ALP); and (3) DHS with an anti-rotational screw (G-DHS). Fixation failures (nonunion, femoral neck shortening (FNS), varus deformation, screw cut-out) and avascular necrosis (AVN) consequent to the three types of fixations were compared. In the FEA, twenty-four fixation models with the three fixation types were created based on the data of eight healthy volunteers. Models were assessed under walking conditions. Stiffness, interfragmentary motion (IFM), and implant stress were evaluated.

Results: In the clinical investigation, the fixation failure rate was significantly (p < 0.05) lower in G-ALP (18.5%) than in G-DHS (37.5%) and G-TRI (39.3%). No significant difference in AVN was observed among the three fixation groups. In the FEA, stiffness and implant stress in the G-DHS models were significantly (p < 0.05) higher, and the IFM of G-ALP was significantly (p < 0.05) lower among the groups.

Conclusions: Among fixation types for VFNFs, the off-axis screw technique exhibited better interfragmentary stability (lowest IFM) and a lower fixation failure rate (especially FNS). Analyzing interfragmentary stability in biomechanical experiments is more consistent with clinical prognosis than construct stability for VFNFs, suggesting that internal fixations should aim for this outcome.

Keywords: Biomechanical study; Clinical study; Cross-screw technique; Internal fixation; Vertical femoral neck fractures.

Fig. 1

Patient selection process

Fig. 2

The three groups of fixation models studied in the clinical and biomechanical investigations included the Alpha (G-ALP), Inverted Triangle (G-ITR), and Triangle (G-TRI) groups. Three parallel screws were all positioned dispersedly, at 2.5 mm to the cortex and 5 mm distal to the subchondral bone in the femoral head. The off-axis screw in G-ALP was implanted 5 mm proximal to the most prominent part of the great trochanter and targeted at the inferior femoral head-neck junction. DHS was implanted inferiorly. The anti-rotational screw was 1.5 cm parallel and superior to the nail

Fig. 3

The schematic diagram of finite element analysis. During the finite element analysis, thread-bone (yellow boxes) and thread-implant (yellow circle) interfaces were tied, while others were set to slide contact. All fixation models were constrained to within 80 mm distal from the lesser trochanter and subjected to 237.7% body weight loading along the femoral mechanical axis. A preload of 224 N for cannulated screws and 591 N for DHSs was applied ahead of the weight load

Fig. 4

Fixation failures and avascular necrosis in the three groups. The fixation-failure rate was significantly lower in G-ALP than in G-TRI and G-DHS. The avascular necrosis rate was also the lowest numerically in G-ALP but was not statistically significant. The height of each bar represents the total number of patients in each group, while the rust-colored section in each bar represents the proportion of patients with complications within the group, and the blue-colored section represents the proportion without complications. Red asterisks indicate significant differences

Fig. 5

3-D reconstructed model of hip and AP radiographs of fracture and treatment fixation in a 49-year-old male with a vertical femoral neck fracture. a, b Preoperative radiograph and reconstructed model showing the vertical femoral neck fracture. c AP radiograph showing the initial treatment with three parallel screws augmented with an off-axis screw. d AP radiograph of the same patient taken 36 months postoperatively, revealing bone union without any complications

Fig. 6

3-D reconstructed model of hip and AP radiographs of fracture, treatment fixation and revision implants in a 59-year-old woman with a vertical femoral neck fracture. a Reconstructed model (AP view) from preoperative CT images showing the location of the fracture. b AP radiograph showing initial treatment with a dynamic hip screw with an anti-rotational screw. c AP radiograph of the same patient taken 5 months after initial surgery, revealing severe screw withdrawal, femoral neck shortening, varus deformation, and delayed union. d AP radiograph of the same patient after revision operation with arthroplasty

Fig. 7

AP radiographs of a parallel screw fixation in a 58-year-old man. a Preoperative radiograph showing a vertical femoral neck fracture. b Postoperative radiograph showing treatment with three cannulated screws. c Radiograph taken 6 months postoperatively in the same patient, showing severe screw withdrawal, femoral neck shortening, and delayed union, as seen in the fracture line. Pre-OP preoperative, Post-OP postoperative, 6 m 6 months after surgery

Fig. 8

Results of patient-specific finite element analysis (FEA) in the biomechanical part of the study. a–c Comparison of mean stiffness (N/mm), interfragmentary motion (mm), and implant stress (MPa) for the three types of fixations. Error bars indicate the SD. d Von Mises distribution of the three fixation strategies

Fig. 9

Rank order of the construct stability, interfragmentary stability, and clinical prognosis (fixation failure) of the three fixation types. Construct stability, interfragmentary stability, and clinical prognosis were evaluated separately by stiffness, IFM, and fixation failure