The impact of capitellar arthroplasty on elbow contact mechanics: Implications for implant design.

BACKGROUND

Radiocapitellar arthroplasty is indicated for capitellar deficiency. Although current implants employ a spherical capitellar surface, the capitellum is elliptical. This has implications for congruency and wear in capitellar arthroplasty. Our objective was to evaluate the contact mechanics of radiocapitellar arthroplasty. We hypothesized that capitellar replacement would decrease joint contact area relative to the native articulation; and an anatomical implant design would replicate more normal contact morphology than a spherical implant.

METHODS

Eight paired humeri and radii were potted in a custom jig. A compressive load of 85N was applied with the articulation flexed at 45° in neutral forearm rotation. Joint casts were made and the articular contact area and shape were quantified. Anatomical and spherical capitellar implants were tested against the native radial head (hemiarthroplasty).

FINDINGS

The contact areas for the anatomical and spherical hemiarthroplasties were 59 and 51% of the native articulation (P<0.005), while the unicompartmental arthroplasties' contact areas were 84 and 89% (anatomical, spherical) of the native articulation (P<0.01). No implant was superior in recreating the native contact shape.

INTERPRETATION

Placement of any capitellar implant resulted in a large decrease in contact area when articulating with a native radial head. This suggests that the radial head cartilage would see a marked increase in contact pressure relative to the native articulation. The unicompartmental arthroplasties demonstrated an even larger reduction in contact area, raising concern about accelerated surface wear. Further investigation needs to correlate these contact mechanics to cartilage wear and implant longevity.