Biomechanical Analysis of a Novel Intercalary Prosthesis for Humeral Diaphyseal Segmental Defect Reconstruction

Abstract

Objective: To study the biomechanical properties of a novel modular intercalary prosthesis for humeral diaphyseal segmental defect reconstruction, to establish valid finite element humerus and prosthesis models, and to analyze the biomechanical differences in modular intercalary prostheses with or without plate fixation.

Methods: Three groups were set up to compare the performance of the prosthesis: intact humerus, humerus-prosthesis and humerus-prosthesis-plate. The models of the three groups were transferred to finite element software. Boundary conditions, material properties, and mesh generation were set up for both the prosthesis and the humerus. In addition, 100 N or 2 N.m torsion was loaded to the elbow joint surface with the glenohumeral joint surface fixed. Humeral finite element models were established according to CT scans of the cadaveric bone; reverse engineering software Geomagic was used in this procedure. Components of prosthetic models were established using 3-D modeling software Solidworks. To verify the finite element models, the in vitro tests were simulated using a mechanical testing machine (Bionix; MTS Systems Corporation, USA). Starting with a 50 N preload, the specimen was subjected to 5 times tensile (300 N) and torsional (5 N.m) strength; interval time was 30 min to allow full recovery for the next specimen load. Axial tensile and torsional loads were applied to the elbow joint surface to simulate lifting heavy objects or twisting something, with the glenohumeral joint surface fixed.

Results: Stress distribution on the humerus did not change its tendency notably after reconstruction by intercalary prosthesis whether with or without a plate. The special design which included a plate and prosthesis effectively diminished stress on the stem where aseptic loosening often takes place. Stress distribution major concentrate upon two stems without plate addition, maximum stress on proximal and distal stem respectively diminish 27.37% and 13.23% under tension, 10.66% and 11.16% under torsion after plate allied.

Conclusion: The novel intercalary prosthesis has excellent ability to reconstruct humeral diaphyseal defects. The accessory fixation system, which included a plate and prosthesis, improved the rigidity of anti-tension and anti-torsion, and diminished the risk of prosthetic loosening and dislocation. A finite element analysis is a kind of convenient and practicable method to be used as the confirmation of experimental biomechanics study.

Keywords: Biomechanics; Bone tumor; Finite element analysis; Intercalary prosthesis.

Figure 1

Specimen was fixed and loaded with tension or torsion. Test was divided into three groups: (A), intact humerus; (B) humerus–prosthesis; (C) humerus–prosthesis–plate. Same loading conditions were applied to three different groups.

Figure 2

Components of the intercalary humeral diaphyseal endoprosthesis. The proximal and distal stem are cemented into the bone canals and connected together by a bolt mechanism. The plate is dismountable and six self‐locking screws are used to connect the bone; two fasting bolts are used to connect the prosthesis.

Figure 3

Computational models: (A) intact humerus; (B) humerus–prosthesis; (C) humerus–prosthesis–plate. The models were divided into the three groups for mechanical testing.

Figure 4

Finite element model validation. The same boundary condition was set up for both finite element analysis (A) and mechanical test (B). To simulate the mechanical test as far as possible, the PMMA model was established for finite element analysis.

Figure 5

Stress distribution of humerus under tension; 200 N axial force was applied to the elbow joint surface with the glenohumeral joint fixed.

Figure 6

Stress distribution of prosthesis (with and without plate) under tension; front view of plate is shown at the right. Stress concentration was present to the junction of the plate and screws but far below its yield stress.

Figure 7

Stress distribution of humerus under torsion; 5 N.m torque force was applied to the elbow joint surface with the glenohumeral joint fixed.

Figure 8

Stress distribution of prosthesis (with and without plate) under torsion; front view of the plate is shown at the right. There is almost completely symmetrical stress distribution on the plate.