Variability in ultimate humeral height of an inlay humeral stem does not impact outcomes following reverse shoulder arthroplasty

Background

In reverse shoulder arthroplasty, humeral components can be classified as either inlay or onlay implants. However, factors like the depth of seating and use of spacers can cause an inlay component to vary in height, functioning as an onlay component. It is unclear whether the positioning of an inlay humeral component as inlay or onlay position influences clinical outcomes. The aim of this study was to analyze the impact of humeral component position on clinical outcomes following reverse total shoulder arthroplasty.

Methods

This was a multicenter retrospective study analyzing radiographic and clinical data from reverse shoulder arthroplasties performed with the same 135° humeral component. Postoperative radiographs were examined to quantify the lateralization and distalization of the humeral component. The distance from the anatomical neck of the humerus to the glenosphere was measured to categorize the implantation as either inlay or onlay. Clinical data, including patient-reported outcomes and range of motion, were compared between groups. Linear regression was used to assess the association of angular measures and clinical outcomes during follow-up.

Results

A total of 194 patients with a mean age of 69.3 years (42-90) were included. Postoperative humeral position was classified as a true inlay in 25.3% and some degree of onlay in 74.7%. The modified distalization shoulder angle was greater in the inlay group, whereas the distance to the glenosphere was greater in the onlay group. Baseline patient-reported outcomes were similar, apart from a higher internal rotation at 90° in the onlay group. At 2-year follow-up, only the Western Ontario Osteoarthritis of the Shoulder index showed a statistically significant difference, with a higher score for the inlay group (88.9 ± 17 vs. 82.8 ± 18.1; P = .04). Linear regression analysis showed an association between the distance to the glenosphere and 0° and 90° external rotation, as well as the spinal level internal rotation. The complication rate was similar between groups.

Conclusion

Clinical outcomes following reverse shoulder arthroplasty with a 135° neck shaft angle designed inlay humeral implant are favorable, with either inlay or onlay implantation. Clinical outcomes were excellent, with minimal differences based on final humeral positioning. The inlay stem offers the advantage to be placed as either inlay or onlay, based on the surgeon’s preference or the patient’s needs. Patients’ characteristics and surgeon preference should be considered when deciding which configuration to use.