Summary

The appropriate hinge position for medial closing wedge distal femoral osteotomy (MCWDFO) to reduce the risk of hinge fracture was assessed using finite element (FE) models and biomechanical tests. The FE analysis and biomechanical tests suggested that a hinge point distal to the inflection point of the lateral femoral condyle may reduce the risk of hinge fracture.

Abstract

Introduction

Medial closing wedge distal femoral osteotomy (MCWDFO) is an effective treatment for valgus knee osteoarthritis while hinge fracture has been frequently reported. Ideally, the maximum principal strain in the hinge area should be as small as possible during closure of the gap to prevent fracture. However, optimal hinge position based on biomechanical background has not yet been fully examined.
Therefore, the aim of this study was to identify optimal hinge position using finite element (FE) models with biomechanical tests. Our hypothesis was that hinge position distal to the inflection point of the lateral condyle would be a favorable position with smaller maximal principal strain.

Methods

FE models of single-plane MCWDFO were created from computer-aided design (CAD) models with the following conditions. Condition 1: to examine the effects of wedge angle, wedge angle was changed as (A) 5 degrees (B) 7.5 degrees, and (C) 10 degrees while setting the hinge position 5 mm away from the lateral cortex at the level of the inflection-point of the lateral condyle. Condition 2: to examine the effects of the hinge proximal-distal position, the hinge position was changed as (D) 10 mm proximal to the inflection-point of the lateral condyle (E) at the level of the inflection-point of the lateral condyle , and (F) 5 mm distal to the inflection-point of the lateral condyle under the condition of 5-degrees wedge angle. A linear tetrahedral FE mesh was adopted and the homogeneous, isotropic, and linear elastic material properties were applied as follows: Young's modulus of 17 GPa and 155 MPa for cortical and trabecular bone, respectively. Poisson's ratio of 0.30 for both cortical and trabecular bones. To close the gap after removal of the wedge, displacement was applied to the medial femoral condyle. The maximum principal strains at the hinge areas were calculated and compared among the models.
To validate the FE analysis, biomechanical tests were also performed using composite replicate femurs. A static compression tester was used to apply loads with the same boundary conditions as Condition 2 in the FE models.

Results

The FE analysis showed that the maximum principal strains were smallest in the order of model (A) > (B) > (C) in Condition 1, and (F) > (E) > (D) in Condition 2. The highest maximum principal strain was observed in the area just proximal to the hinge point. In the biomechanical tests, hinge fractures occurred in model (D) and (E), while no hinge fracture occurred in model (F) and the gap closed completely. In FE models (D) and (E), hinge fractures occurred during gap closure and the fractures were located at the area where the highest maximal principal strain was observed.

Discussion

FE analysis based on maximum principal strain suggests that MCWDFO with a larger wedge angle has a greater risk of hinge fracture and the hinge position distal to the inflection point of the lateral condyle would be a favorable position in MCWDFO to reduce the risk of hinge fracture.