Review of “Acromial and scapular fractures after reverse shoulder arthroplasty: comparison of 3018 reverse total shoulders by inlay and onlay humeral component design”
Reverse total shoulder arthroplasty provides excellent pain relief and good function in most patients. However, acromial stress fractures remain among the most troublesome complications, as they can often permanently impair the function and pain relief provided by the arthroplasty. Avoiding these fractures has thus been a primary focus in recent research.
Implant-related Factors and Surgical Focus
Many patient factors are known to associate with acromial and scapular spine fractures and these are useful prognostically, but most are non-modifiable. Surgeons are thus most interested in implant-related factors. These include both the design of the implants and their placement relative to the bones. On the humeral side, the two major implant designs are inlay, in which the humeral tray is below the cut surface, and onlay, in which the humeral tray is above the cut surface. While each implant may be designed as an “inlay” or “onlay” ultimately the final position of the implant is also influenced by its application.
Why Inlay vs. Onlay Is Difficult to Classify
Thus, these definitions have always been fraught because (1) the cut is a surgically modifiable factor, (2) the metaphyseal geometry of the implant relative to the proximal humerus can influence the ability of the surgeon to “inlay” the component, and (3) the diaphyseal sizing of the implant relative to the proximal humerus can influence the ability of the surgeon to “inlay” the component, and (4) the quality of the bone within the proximal humerus can also dictate the proximodistal position of the implant. It remains controversial whether inlay or onlay humeral components have increased risk for acromial and scapular spine fractures.
About the Study
In late 2024, the Journal of Shoulder and Elbow Surgery published “Acromial and scapular fractures after reverse shoulder arthroplasty: comparison of 3018 reverse total shoulders by inlay and onlay humeral component design” by Marigi and colleagues. This retrospective study of a very large number of reverse total shoulder arthroplasties crosses multiple institutions, multiple surgeons, multiple operative indications, and includes nine implants. So while there is heterogeneity in the dataset, the results are likely widely generalizable.
Confirmed Findings from Prior Research
Some of the findings of this study mirror prior studies, such as demonstrating that patients who suffer acromial and scapular spine fractures after reverse total shoulder arthroplasty are more commonly female, reflecting the prevalence of post-menopausal osteoporosis in this populations. In addition, a history of a rotator cuff repair associated with acromial and scapular spine fractures. These findings align well with the findings from the American Shoulder and Elbow Surgeons Multicenter Cohort. These positive findings validate the methodology and dataset. They also demonstrate that the cohort was at least sufficiently powered to find associations of this magnitude.
Inlay vs. Onlay Outcomes
However, Marigi et al. also specifically examined for an association between inlay vs. onlay components and acromial and scapular spine fractures and were unable to find any association. Within the analysis, 31% of the fracture group had inlay components, while among the non-fracture group, 25% had inlay components. In other words, inlay components were more common in the fracture cohort, but this difference did not reach significance (p=0.08).
Study Limitations
The study has multiple limitations, including the short-term follow-up, the retrospective nature, the potential for selection bias due to loss to follow-up, the difficulties with accurately classifying implants as “inlay” or “onlay” described above, and an uneven distribution of patients between the inlay and onlay groups. Future, randomized clinical trials could thus demonstrate different findings, but such studies are unlikely to be performed as this is a rare complication and thus the sample size necessary to demonstrate a difference, should one exist, would be very large.
Conclusion
Thus, given the large size of the cohort included (3018 patients), the inclusion of multiple surgeons/institutions/implants, and the coherence of these findings with the American Shoulder and Elbow Surgeons Multicenter Study on the same subject, we can conclude that inlay vs. onlay is not strongly associated with acromial and scapular spine fractures.