Background: While the original Paul Grammont Inlay design had a 155° neck-shaft angle (NSA), developments in humeral stem designs have led to the emergence of the Onlay design with a more vertical angle. The purpose of the study is to compare three year clinical and radiological outcomes of two humeral stem designs, namely Inlay versus Onlay designs, with a 145° NSA and identical glenoid component.
Methods: In this multicentric retrospective study, 227 patients (141 Inlay versus 86 Onlay) that underwent primary reverse shoulder arthroplasty (RSA) between March 2019 and April 2020, were reviewed at a minimum follow-up of three years. Clinical evaluation included pain on visual analogue scale (VAS), active range of motion, subjective shoulder value (SSV), and Constant score. Radiological assessment included in situ stem inclination, cortical contact bone remodelling, and scapular notching.
Results: The two groups were comparable in terms of age, sex, diagnosis, and follow-up (mean follow-up, 3.3 ± 0.5 years). No significant differences were found for pain on VAS, SSV or Constant score. The Onlay group had significantly greater external rotation with 90° of abduction (ISA-Inlay, 52.2 ± 24.2; versus ISA-Onlay, 59.2 ± 25; p = 0.037), more valgus alignment (ISA-Inlay, -0.573; versus ISA-Onlay, -5.55; p < 0.001), and a higher rate of cortical contact (ISA-Inlay, 9%; versus ISA-Onlay, 39%; p < 0.001). No significant differences were found in terms of bone remodelling around the stem and scapular notching.
Conclusion: At a follow up of three years, both humeral stem designs resulted in comparable clinical and radiological outcomes, while the Onlay design seemed to improve external rotation without increasing the risk of bony complications. The choice of stem design should be motivated by patient specific functional needs and surgeon experience. However, the conclusions of the present study are limited to mid-term follow-up.
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