Hemiarthroplasty of the elbow: the effect of implant size on kinematics and stability.

Background: Distal humeral hemiarthroplasty is a treatment option for distal humeral fractures, nonunions, and avascular necrosis. The biomechanical effects, however, have not been reported. The purpose of this in vitro study was to quantify the effects of hemiarthroplasty and implant size on elbow joint kinematics.

Methods: Eight fresh-frozen cadaveric arms were mounted in an in vitro motion simulator. An electromagnetic tracking system quantified elbow kinematics. A custom distal humeral stem was implanted by use of navigation, and 3 humeral articular spools were evaluated: optimally sized, undersized, and oversized. Statistical analysis was performed with repeated-measures analysis of variance.

Results: Distal humeral hemiarthroplasty altered elbow kinematics, regardless of implant size. In the valgus position, the optimally sized implant resulted in a mean increase in valgus angulation of 3° ± 1° (P = .003) as compared with the osteotomy control. In the varus position, the optimal and undersized implants both resulted in significant increases in varus angulation: 3° ± 1° (P = .01) and 3° ± 1° (P = .001), respectively. The undersized implant had the greatest alteration in kinematics, whereas the oversized implant best reproduced native elbow kinematics.

Conclusion: This study showed a small but significant alteration in elbow joint kinematics with placement of a distal humeral hemiarthroplasty implant, regardless of implant size. This could be due to errors in implant positioning and/or differences in the shape of the humeral implant relative to the native elbow. These changes in joint tracking may cause abnormal articular contact and loading, which may result in pain and cartilage degeneration over time.