Screw-blade fixation systems in Pauwels three femoral neck fractures: a biomechanical evaluation.

Abstract

To reduce mechanical complications after osteosynthesis of femoral neck fractures, improved fixation techniques have been developed including blade or screw-anchor devices. This biomechanical study compares different fixation systems used for treatment of unstable femoral neck fractures with evaluation of failure mode, load to failure, stiffness, femoral head rotation, femoral neck shortening and femoral head migration. Standardized Pauwels type 3 fractures (AO/OTA 31-B2) with comminution were created in 18 biomechanical sawbones using a custom-made sawguide. Fractures were stabilized using either SHS-Screw, SHS-Blade or Rotationally Stable Screw-Anchor (RoSA). Femurs were positioned in 25 degrees adduction and ten degrees posterior flexion and were cyclically loaded with an axial sinusoidal loading pattern of 0.5Hz, starting with 300N, with an increase by 300N every 2000cycles until bone-implant failure occurred. Mean failure load for the Screw-Anchor fixation (RoSA) was 5100N (IQR 750N), 3900N (IQR 75N) for SHS-Blade and 3000N (IQR 675N; p=0.002) for SHS-Screw. For SHS-Screw and SHS-Blade we observed fracture displacement with consecutive fracture collapse as the main reason for failure, whereas RoSA mainly showed a cut-out under high loadings. Mean stiffness at 1800N was 826 (IQR 431) N/mm for SHS-Screw, 1328 (IQR 441) N/mm for SHS-Blade and 1953 (IQR 617) N/mm for RoSA (p=0.003). With a load of 1800N (SHS-Screw 12° vs. SHS-Blade 7° vs. RoSA 2°; p=0.003) and with 2700N (24° vs. 15° vs. 3°; p=0.002) the RoSA implants demonstrated a higher rotational stability and had the lowest femoral neck shortening (p=0.002), compared with the SHS groups. At the 2700N load point, RoSA systems showed a lower axial (p=0.019) and cranial (p=0.031) femoral head migration compared to the SHS-Screw. In our study, the new Screw-Anchor fixation (RoSA) was superior to the comparable SHS implants regarding rotational stability and femoral neck shortening. Failure load, stiffness, femoral head migration, and resistance to fracture displacement were in RoSA implants higher than in SHS-Screws, but without significance in comparison to SHS-Blades.