Abstract
Introduction:
Total shoulder arthroplasty (TSA) is the current standard treatment for severe osteoarthritis of the glenohumeral joint [1]. Often, severe arthritis is associated with abnormal glenoid version or excessive posterior wear [2]. Reaming to correct more than 15° of retroversion back to neutral is not ideal as it may remove an excessive amount of the outer cortical support and medialize the glenoid component [3]. Two recent glenoid components with posterior augments—wedged and stepped—have been designed to address excessive posterior wear and to allow glenoid component neutralization. Hypothetically, these augmented glenoid designs lessen the complications associated with using a standard glenoid component in cases of shoulder osteoarthritis with excessive posterior wear. We set out to determine which implant type (standard, stepped, or wedged) corrects retroversion while removing the least amount of bone in glenoids with posterior erosion.
Methods:
Serial shoulder CT scans were obtained from 121 patients before total shoulder arthroplasty. These were then classified using the Walch Classification. We produced 3D models of the scapula from CT scans for 10 subjects that were classified as B2 using the software MIMICS (Materialise, Belgium). Each of these 10 glenoid subjects were then virtually implanted with standard, stepped, and wedged glenoid components (Fig 1). The volume of surgical bone removed and maximum reaming depth were calculated for each design and for each subject. In addition, the area of the backside of the glenoid in contact with cancellous versus cortical bone was calculated for each glenoid design and for each subject (Fig 2). ANOVA testing was performed.
Results:
Arthritic bone loss in shoulder specimens was always posteroinferior, and the worn portion or neoglenoid made up an average of 68 ± 11% in the shoulder specimens. Mean surgical bone volume removed (2857 ± 1618 mm3) was least for the wedged component when compared to stepped (4307 ± 1485 mm3, p=.0003) and conventional (5385 ± 2348 mm3, p=.0003) designs. Maximum bone depth removed for the wedge (4.5 ± 2.3 mm) was less than the stepped (7.6 ± 1.4 mm, p=.00003) and conventional (9.7 ± 2.7 mm, p=.00001). The mean percentage of the implant's back surface supported by cancellous bone was 17.0% for the conventional, 6.1% for the stepped (p=.009), and 3.1% for the wedged (p=.0001).
Discussion:
Both wedged and stepped components were able to correct glenoid version to neutral and required less bone removal, required less reaming depth, and were supported by more cortical bone than the standard implant. The wedged component was significantly better in these three categories than the stepped implant. There may be a mismatch between the usual patterns of wear that occurs in B2 glenoids where neoglenoid comprises (68 ± 11%) vs. the stepped implant's 50%. A stepped implant that matches the usual B2 glenoid may correct version while removing less bone than the current design.
