The hybrid humerus reverse shoulder arthroplasty: a prospective study of bone grafting and metaphyseal fixation with undersized stem in the assessment of proximal humeral stress shielding and associated clinical outcome

A significant proportion of revisions after reverse shoulder arthroplasty (RSA) is attributed to the humeral component. The purpose of this study is to evaluate the radiographic and clinical outcomes of the hybrid humerus technique for RSA using a Grammont-style humeral prosthesis in an onlay fashion with metaphyseal bone impaction grafting technique and undersized stem to avoid humeral stress shielding, notching, and loosening. This is a prospective case series of patients who underwent RSA using the hybrid humerus technique with a minimum 2-year follow-up. Key steps of this technique include the use of undersized Grammont-style stem, impaction bone grafting of the proximal 5cm of the humerus, and adjusting the height and offset of the stem depending on the patient stature, desired lateralization and distalization, and joint and soft tissue tension. Radiographic assessments were performed immediately after surgery, and at one and two years after surgery. These included assessment of metaphyseal and diaphyseal filling ratio, cortical narrowing, radiolucent lines, cortical lucencies, spot welding, scapular notching, and stem alignment. Preoperative and 2-year postoperative clinical assessments included American Shoulder and Elbow Surgeons (ASES) score, Constant-Murley Score (CMS), University of California Los Angeles (UCLA) score, visual analogue scale (VAS) for pain, and active range of motion. Correlation between the filling ratios and clinical outcomes were also evaluated. Sixty-one patients were included in the study. The average metaphyseal and diaphyseal filling ratio on the postoperative x-ray was 0.66 and 0.54, and 0.67 and 0.54 at two years, respectively. Stress shielding was graded as none in 24 (40.7%), mild in 33 (55.9%), and moderate in 2 (3.4%). No stem had a change in position of more than 5 degrees. At two years follow-up, no humeral implant loosening was noted, with only 2 (3.4%) of the stems at risk of loosening. Thirty-nine (66.1%) had no notching, 14 (23.7%) were graded as mild, and 6 (10.2%) had moderate signs of notching. All clinical assessments significantly improved at two years (p<0.001), with a weak negative correlation between VAS and metaphyseal filling ratio (r=-.268, p=0.036) but none between diaphyseal filling ratio and clinical outcomes. The hybrid humerus technique of metaphyseal bone grafting with a low filling ratio stem presents a promising solution for reducing humeral complications in RSA. This technique demonstrates a low incidence of stress shielding and loosening, with excellent clinical outcomes at two years.