An In Vitro Study of the Role of Implant Positioning on Ulnohumeral Articular Contact in Distal Humeral Hemiarthroplasty.

PURPOSE: To investigate the effect of implant positioning on ulnohumeral contact using patient-specific distal humeral (DH) implants.

METHODS: Seven reverse-engineered DH implants were manufactured based on computed tomography scans of their osseous geometry. Native ulnae were paired with corresponding native humeri and custom DH implants in a loading apparatus. The ulna was set at 90° of flexion and the humeral component (either native bone or reverse-engineered implant) was positioned from 5° varus to 5° valgus in 2.5° increments under a 100-N compressive load. Contact with the ulna was measured with both the native distal humerus and the reverse-engineered DH implant at all varus-valgus (VV) angles, using a joint casting method. Contact patches were digitized and analyzed in 4 ulnar quadrants. Output variables were contact area and contact pattern.

RESULTS: Mean contact area of the native articulation was significantly greater than with the distal humeral hemiarthroplasty (DHH) implants across all VV positions. Within the native condition, contact area did not significantly change owing to VV angulation. Within the DHH condition, contact area also did not significantly change owing to VV angulation. Conversely, in the DHH condition, contact pattern did significantly change. Medial ulnar contact pattern was significantly affected by VV angulation. Lateral ulnar contact was variably affected, but generally decreased as well.

CONCLUSIONS: Ulnar contact patterns were changed as a result of VV implant positioning using reverse-engineered DH implants, most notably on the medial aspect of the joint. Implant positioning plays a crucial role in producing contact patterns more like those observed in the native joint.

CLINICAL RELEVANCE: Recent clinical evidence reports nonsymmetrical ulnar wear after DHH. This work suggests that implant positioning is likely a contributing factor and that more exact implant positioning may lead to better clinical outcomes.