Summary

The risk of hinge fracture between single-planar (SP) and biplanar (BP) medial closing wedge distal femoral osteotomy (MCWDFO) were compared using finite element analysis with four models (SP or BP MCWDFO models with supra-condylar or condylar hinge). The risk of hinge fracture was equally lower when the hinge was placed within the condylar area in both SP and BP models.

Abstract

Introduction

Hinge fracture is a frequent complication of medial closing wedge distal femoral osteotomy (MCWDFO). We have previously reported that the condylar hinge position would reduce the risk of hinge fracture in single-plane (SP) MCWDFO. Meanwhile, bi-plane (BP) technique is widely used as a surgical technique for MCWDFO. However, whether there is a difference in risk of hinge fracture between the single- and bi-planar techniques has not yet been examined. This study aims to investigate the risk of hinge fracture between single-planar and biplanar MCWDFO with different hinge positions using finite element analysis (FEA). The hypothesis was that the risk of hinge fracture would be reduced by setting the appropriate hinge position regardless of osteotomy techniques.

Methods

Three-dimensional knee model was created from preoperative computed-tomography (CT) images of 11 knees of 10 patients who underwent MCWDFO for the valgus knee: Virtual MCWDFO was performed in each knee with four difference conditions: SP osteotomy with condylar hinge (SP-C) and supra-condylar hinge (BP-S), BP osteotomy with condylar hinge (BP-C) and supra-condylar hinge (BP-S). The condylar and supra-condylar hinge positions were set to be 5 mm distal and proximal to the proximal margin of the lateral distal femur, respectively. The correction angle was set at 5 degrees. The anterior flange thickness of the bi-plane osteotomy was 25% of the overall hinge thickness. As the boundary condition, the most proximal parts of the models were fixed. The appropriate displacements at the femoral medial condyle were set to close the wedge completely. Young's modules were calculated using their CT values based on the data proposed by Keyak et al. Poisson's ratio was set at 0.40. The 1.5 mm tetrahedral mesh was selected and the 0.4mm shell was pasted surface. The number and distribution of failure elements in the hinge area were compared after closing the gap.

Results

The number of failure elements in SP-C, SP-S, BP-C, and BP-S were 67.7 ± 45.9, 206.3 ± 86.0, 70.6 ± 59.6, and 177.9 ± 59.3, respectively. The number of failure elements in the models with supra-condylar hinge (SP-S and BP-S) were significantly larger than those in the models with condylar hinge (SP-C and BP-C) (SP-S or BP-S vs SP-C and vs BP- C, all P < 0.05). There was no difference in the number of failure elements between SP-C and BP-C and between SP-S and BP-S.

Discussion

The FEA showed that the number of failure elements in the both SP and BP models with the condylar hinge was less than those with the supra-condylar hinge. Meanwhile, there was no difference in the number of failure element between SP osteotomy and BP osteotomy, suggesting that the risk of hinge fractures is largely dependent on the hinge position and can be reduced equally in both osteotomy techniques by placing the hinge in the condylar area .

Conclusion

In MCWDFO, the risk of hinge fracture can be reduced by setting the hinge position within the lateral condylar area regardless of single-plane or bi-plane techniques.