Biomechanical study of the femoral neck system in basicervical femoral neck fractures : a comparison with the dynamic hip screw

Abstract

Background: The purpose of the present study was to compare 1) the fixation stability of proximal fragments and 2) the mechanical characteristics in proximal femur models of basicervical femoral neck fracture fixed by the femoral neck system (FNS) versus the dynamic hip screw (DHS). Materials and Methods: Twenty composite femurs corresponding to the normal bone qaulity were used. A displaced basicervical femoral neck fracture was created using an engraving machine. These specimens were instrumented with FNS (n=10) and DHS (n=10). After cyclic loading, the migration of the proximal fragment according to 3 axes was assessed via stereophotogrammetry, and the migration of the screw tip within the femoral head was measured on a fluoroscopy. Thereafter, a vertical load was applied at a speed of 2 N/cycle until construct failure occurred. Construct failure was defined as fracture of the femoral neck, cut-out of screw, implant failure, displacement of the fragments more than 15 mm, and sudden decrease in load resistance observed at the load-displacement curve. Axial stiffness was measured in the vertical load cycle. Results: The mean axial stiffness was 234 ± 35 N/mm in the FNS group and 253 ± 42 N/mm in the DHS group, showing no significant difference (p=0.654). Mean values for varus collapse, y-axis rotation, and z-axis rotation after cycle load were 2.2 ± 0.5°, 6.5 ± 1.5°, and 2.5 ± 0.6°, respectively, in the FNS group and 2.5 ± 0.7°, 5.8 ± 2.1°, and 2.2 ± 0.9°, respectively, in the DHS group, showing no significant difference in rotational change of the proximal fragment (p=0.324, p=0.245, and p=0.312, respectively). The mean values of cranial and axial migration of screws within the femoral head were 1.5 ± 0.3 and 2.1 ± 0.2 mm, respectively, in the FNS group and 1.2 ± 0.3 and 2.4 ± 0.3 mm, respectively, in the DHS group. There were no significant differences in cranial and axial migration of screws between the groups (p=0.425 and p=0.625, respectively). The average failure load at vertical load was 1342 ± 201 N in the FNS group and 1450 ± 196 N in the DHS group, showing no significant difference (p=0.452). Conclusions: FNS fixation might provide biomechanical stability comparable to that of DHS for treating displaced basicervical femoral neck fractures.