BACKGROUND:
The literature is sparse comparing humeral implants in reverse shoulder arthroplasty (RSA). With minimal supporting literature, there has been a trend away from standard length cemented humeral implants to press-fit stems, and recently to shorter stem implants. It is known that early implant migration, within the first two years postoperatively, is predictive of later implant loosening and possible revision surgery. Therefore, the purpose of this study was to compare clinical outcomes and implant migration between cemented standard length humeral stems and press-fit short stems using model-based radiostereometric analysis.

METHODS:
After a power analysis, forty-one shoulders were prospectively randomized to receive either a cemented standard or a press-fit short humeral stem for primary reverse shoulder arthroplasty between July 2017 and June 2019. Following surgery, participants were imaged with stereo x-rays acquired at 6 weeks (baseline), 3 months, 6 months, 1 year, and 2 years. Migration of the humeral stem at each time point was compared to baseline, with differences in migration between cohorts assessed using a mixed-effects model with Bonferroni's test for multiple comparisons. Patient reported outcome measures (Subjective Shoulder Value, the American Shoulder and Elbow Surgeons shoulder score, the Simple Shoulder Test, the Disabilities of the Arm, Shoulder and Hand score, and the Constant Score) were also compared.

RESULTS:
At six months (P = 0.025), one year (P = 0.004), and two years postoperatively (P = 0.001), press-fit short stems migrated significantly more than cemented stems along the superior-inferior translation axis, and at two years for total translation (P = 0.003). Mean total translation (± SD) at two years was 0.4 ± 0.2 mm, and 1.0 ± 1.1 mm for cemented and press-fit cohorts, respectively. Mean migration between the one-year and two-year time points was minimal for both stem fixation groups along all axes (less than 0.1 mm and 0.6°). There was no difference in active range of motion, pain, or in validated outcome measures between cohorts at two years (P ≥ 0.170).

CONCLUSION:
This randomized clinical trial shows that press-fit short humeral stems subside substantially more than standard length cemented stems, but ultimately achieve stability from one through two years. Conversely, no significant differences were observed in clinical outcomes between cohorts.





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