Comparative study on mechanical properties of different hinged external fixator configurations in the treatment of elbow stiffness

The most serious complication of elbow stiffness is iatrogenic radial nerve injury. A hinged external fixator combined with open arthrolysis is a common clinical operation. The purpose of this study was to compare the mechanical characteristics of different hinged external fixator configurations in the treatment of elbow stiffness by finite element method and to provide a basis for selecting the optimal design for clinical application. Five finite element models of the elbow joint combined with hinged external fixators were established. The angle between the humeral-ulnar axis, the system stiffness, the displacement distribution of pins, the peak contact pressure between cartilages, and the maximum equivalent stress of the external fixator components were compared under varus and valgus forces in the 0°, 45°, and 90° flexion, as well as under the dynamic torsion force in the neutral position. In torsion, the maximum stress values of the components of each configuration were always nearer or lower than those of configuration 1, and the pin stresses of the final four configurations were steadily reduced by about 20% compared with those of configuration 1. The displacement of pins in configurations 2 and 5 showed a trend of decreasing in the upper part and increasing in the lower part, while that of configurations 3 and 4 was opposite. In varus and valgus forces, with the commonly used configuration 1 as a reference, the results of configuration 2 were similar to the reference values. In the neutral position of the elbow joint, the system stiffness of configurations 3 and 4 with bending structural features decreased significantly, the contact pressure increased, and the maximum stress value of the components increased compared with configuration 1. The stability of the external fixator system of configuration 5 was slightly less than that of configuration 1 in the neutral position, but the simulation results were close to or even less than the reference values at 45° and 90° of flexion. In conclusion, the fifth hinged external fixator configuration had a certain amount of rotation flexibility based on maintaining the alignment of the elbow joint and was superior to the common configuration in anti-varus and anti-valgus forces.