Assessment of the Anatomic Neck as an Accurate Landmark for Humeral Head Resurfacing Implant Height Placement.

INTRODUCTION: Humeral head resurfacing has been described as a more anatomic replacement alternative for proximal humerus arthroplasties when compared to conventional stemmed implants. However, not all studies show that humeral head resurfacing is better at reproducing the proximal humeral anatomy with overstuffing of the joint being a common complication. The purpose of this study was to assess the use of the anatomic neck as a landmark for proper placement of humeral head replacements.

METHODS: Sixty-six cadaveric shoulder CT scans were reconstructed using Mimics to create 3D models of the humerus. After 3D reconstruction, each bone model was analyzed in Rapidform to establish the anatomic neck plane, the humeral head average radius of curvature, and anatomic center of rotation (CoR) using a best fit sphere over the articular surface. Humeral head resurfacing implants (Equinoxe®, Exactech, Inc.) were assembled onto the 3D humeral models, selected by matching the closest implant size available with the anatomic radius of curvature. Implants were constrained to match the anatomic neck angle and version and were spaced 2 mm away from the anatomic neck. The 3D distances between the anatomic center of rotation and the implant CoR and the implanted head thickness deviations were measured using Unigraphics to observe anatomic reproduction with the resurfacing implants.

RESULTS: When placing all resurfacing implants 2 mm from the anatomic neck, the average implant CoR offset from the anatomic CoR was determined to be 1.03 mm ± 0.75 mm. The average implant Humeral Head Thickness (HHT) deviation from the anatomic HHT was determined to be -0.36 mm ± 0.84 mm. There were no significant differences in CoR offset or HHT offset between implant sizes used.

DISCUSSION: Newer generation resurfacing implant designs allow for more anatomic reproduction of the humeral articular surface. Depth of reaming and resurfacing implant height placement are potential pitfalls in humeral head replacement and have been shown to have negative effects on reproducing the CoR and HHT. Using the anatomic neck as a landmark for the depth of reaming has been observed to closely reproduce anatomic HHT and CoR. Further work aims to validate the use of the anatomic neck as a consistent landmark in cadaveric studies and to investigate if these findings are clinically relevant.