Distal humerus cortical strains following total elbow arthroplasty

The aseptic loosening of total elbow replacements is a serious complication resulting in significant patient morbidity. It is thought to occur secondary to stress shielding of the distal humeral cortex by the stiff stem of the implant. Some total elbow prostheses incorporate an anterior flange intended to improve implant stability and peri-articular load transfer in an attempt to reduce this effect However, few studies have directly assessed the changes in cortical strains following total elbow arthroplasty or the biomechanical advantage of the anterior flange design. A regular and a long flange Coonrad-Morrey total elbow prosthesis were implanted into six Sawbone synthetic humeri. The constructs were subjected to physiological loads in axial compression (500 N), antero-posterior bending (50 N) and antero-posterior compression with condylar supports (300 N). Digital image correlation was used to measure the distal antero-lateral cortical strains and the results compared with those of whole Sawbones that had been tested in the same way. Significant stress shielding was demonstrated over the distal humeral cortex following prosthesis implantation during axial compression. In contrast, cortical strains increased following prosthesis implantation during antero-posterior compression with condylar supports. The increase in cortical strains following total elbow arthroplasty may help to maintain the integrity of the anterior cortex offering additional stability for implants with an anterior flange. These results are important for the development of future total elbow prosthesis designs and indicate that simulating the action of the forearm muscles is essential when evaluating changes in strain about the distal humerus in vitro.