Background
Reverse shoulder arthroplasty (RSA) is widely used for cuff tear arthropathy and degenerative shoulder disease, where surgical accuracy is a key determinant of outcomes. Although patient-specific instrumentation (PSI) has been introduced to improve precision, most systems focus on the glenoid component. With the increasing use of shorter and smaller humeral stems, accurate humeral preparation has become more critical because these stems are more susceptible to varus or valgus malalignment. Inaccurate humeral retroversion or stem alignment may compromise implant stability and biomechanics. We developed a novel humeral PSI system designed to improve the accuracy of humeral head osteotomy, canal reaming, and stem alignment. This study aimed to evaluate the radiologic accuracy and feasibility of this humeral PSI system compared with conventional techniques.
Materials and Methods
This retrospective comparative study reviewed patients who underwent RSA between March 2021 and December 2025. Patients treated using the humeral PSI system after its introduction in May 2023 were compared with those who underwent conventional surgery. Inclusion criteria were age >50 years, diagnosis of cuff tear arthropathy or primary glenohumeral osteoarthritis, and use of a single implant system. Patients with prior ipsilateral shoulder surgery or RSA for proximal humerus fracture were excluded.
The humeral PSI system included guides for humeral head osteotomy and canal reaming, as well as a dedicated broach insertion guide designed to restrict varus or valgus deviation. All guides were created using CT-based three-dimensional planning and manufactured with patient-specific resin components and reusable metallic interfaces. All procedures were performed by a single fellowship-trained surgeon. Primary outcomes were accuracy of humeral retroversion reproduction and humeral stem alignment. Measurements were independently performed by two observers. Statistical analyses included independent t-tests, Fisher exact tests, and intraclass correlation coefficients.
Results
Forty-four patients were included, with 22 patients in each group. All cases used short humeral stems with a standardized stem size. The PSI group demonstrated significantly improved accuracy of humeral retroversion. The mean absolute difference between planned and postoperative retroversion was 2.4° ± 2.0° in the PSI group versus 6.7° ± 3.0° in the non-PSI group. Acceptable retroversion accuracy was achieved in all patients in the PSI group, compared with 8 patients in the non-PSI group.
Humeral stem alignment was also more accurate in the PSI group, with a mean deviation from the planned humeral axis of 1.1° ± 1.0°, compared with 3.5° ± 2.0° in the non-PSI group. Neutral stem alignment was achieved in most patients in both groups.
Conclusions
The novel humeral PSI system significantly improved radiologic accuracy of humeral retroversion and stem alignment in RSA. The dedicated broach insertion guide appears effective in preventing varus or valgus malalignment, particularly relevant with short humeral stems. Despite limitations related to its retrospective design and sample size, this study provides early clinical evidence supporting the utility of humeral PSI in RSA. Further prospective randomized studies are needed to evaluate long-term clinical outcomes.
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