Summary

This study found that at a two-year minimum follow up, the position of humeral tray as either inlay or onlay did not influence the clinical outcomes of function, range of motion, complications including baseplate loosening and acromial stress fracture, and scapula notching.

Abstract

Introduction

The inlay design of humeral tray in reverse shoulder arthroplasty (RSA) has been suggested to have the advantage of better humeral side fixation, but there are concerns of greater tuberosity fracture. In comparison, onlay humeral tray in RSA are suggested to have the advantages of impingement free range of motion and reduced scapular notching, but there are concerns of increased scapula stress fractures. The aim of this study was to compare the clinical results among patients with CTA undergoing RSA with two prostheses having lateralized glenosphere and 135° NSA, but which differed in the position of the humeral tray as either inlay or onlay design.

Methods

This was a retrospective study of prospectively obtained data from a single institutional database of shoulder division of a tertiary care center and was approved by our institutional review board. The database was searched for all patients who underwent primary RSA between 2009 to 2017 (N=511). To be included, patients with a diagnosis of and cuff tear arthropathy had to be treated with a RSA prostheses having a lateralized glenosphere and 135° NSA either with an inlay or onlay humeral tray design. 102 patients met the inclusion criteria and had a minimum of 2 years follow up (mean, 44, range 24-125 months). Of the included 102, 63 (62%) had implanted a RSA design with an inlay humeral tray (inlay group) and 39 (38%) had onlay tray (onlay group). All patients underwent a preoperative and postoperative evaluation including a physical examination for range of motion (ROM), radiographs, and multiple PROs (ASES, SST, and WOOS score). The clinical significance was evaluated using published minimal clinically important difference (MCID) values.

Results

Preoperatively there were no significant differences in the two groups demographically except for more proportion of females in the inlay group (75% vs 56%, P=0.04). The preoperative PROs and ROM were not statistically different between the inlay and onlay groups. The comparison of final follow up PROs and ROM including external rotation were not statistically or clinically significantly different between inlay or onlay groups.
There was no statistically significant difference between the inlay and onlay design for baseplate loosening (3% vs 5%, P=0.63) and revision (0% vs 5%, P=0.07). Of 3 patients in the onlay group who required a revision, the reason was baseplate failure in one patient, instability in another patient, and periprosthetic shoulder infection in the third patient. The rate of acromial stress fracture (3 vs 5%, P=0.63) and prosthesis dislocation (0 vs 2.5%, P=0.20) were also similar between inlay and onlay groups (Table 3). There was no difference between the onlay and inlay groups postoperatively for the rate scapula notching neither by incidence (21% vs 8%, P=0.08) nor by distribution.

Conclusion

This study found that at a two-year minimum follow up, the position of humeral tray as either inlay or onlay did not influence the clinical outcomes of function, range of motion, complications including baseplate loosening and acromial stress fracture, and scapula notching.