Thomas Obermeyer, MD
The reverse shoulder prosthesis was introduced in France in the 1980s and was FDA-approved for use in the United States in 2004. Since that time, it has revolutionized the field of shoulder replacement. The reverse shoulder prosthesis has a proven track record of providing predictable and sustainable pain relief and functional improvement for patients with a host of shoulder pathologies, ranging from osteoarthritis to rotator cuff arthropathy. The stability and functional improvements afforded by the reverse shoulder prosthesis are achieved, in part, by transitioning the large outer deltoid muscle of the shoulder into a mechanically advantaged motor of the arm. As a result of the forces generated by the deltoid muscle, high loads are transmitted to the site of origin of that muscle on the scapula leading to fractures in a rare cohort of patients. These so-called acromial fractures are not widely studied due to their infrequency and the relative heterogeneity of reverse shoulder prosthesis designs on the market. Improvements in understanding acromial fractures is important as their occurrence can impair the clinical outcome of the reverse shoulder prosthesis. A recent study by Routman et al. is, to date, the largest cohort of patients reported, which sheds light on the patient risk factors and prosthesis-related factors leading to this unfortunate and uncommon complication of reverse shoulder replacement.
The study looked at 4,125 Equinoxe® reverse shoulder replacements. The rate of a radiographically evident acromial fracture was 1.48%, occurring at a mean of 17.7 months after surgery. The most common type of fracture occurred at the acromial base, which happened earlier after surgery than the less common acromial tip or scapular spine fractures. Fractures were significantly more frequent in females, those carrying a diagnosis of rheumatoid arthritis or rotator cuff arthropathy, and those with more baseplate screws (average of 4.1 in the fracture group). Unsurprisingly, the fracture group had a clinically important deterioration in outcomes compared with those without fracture.
There are several take-home points from this study that can assist surgeons in identifying and preventing acromial fractures so as to improve the clinical outcome of the prosthesis. The first take-home point is the patient risk factors, which, in this study, parallel my clinical experience with acromial fractures. I, personally, have not seen an acromial fracture in a male patient with an Equinoxe implant; although, I have seen these fractures in males referred with other prosthesis designs. Why these fractures occur more often in females is unknown, but it likely relates to the density and size of the acromial bone that may be more porotic and thin in females. Osteoporosis, while not evaluated in this study, likely contributes with the poor bone quality seen with rheumatoid arthritis, likely accounting for that diagnosis as a significant risk factor. The number of baseplate screws in the glenoid was also greater in the fracture group, which may not always necessarily be related to a stress riser at the tip of a screw but, in my estimation, is also a manifestation of the poor bone quality encountered during baseplate fixation, which correlates with compromised bone quality at the acromion. (I rarely use more than four screws except in patients with substantially poor bone quality.)
What is interesting in this study is the high rate of fracture diagnosed in individuals with a traumatic mechanism (23.3%), although there were a high number of patients with fractures thought to have been sustained during physical therapy. In my own practice, I have identified patients with an overly ambitious therapy regimen that seems to correlate with the development of acromial fracture. Identifying patients at risk for fracture, including those developing acromial tenderness, and modifying therapy programs and exercise regimens accordingly may help to prevent an onset of fracture in these patients.
The design factors that are unique to the Equinoxe implant are also worth discussing from this study. The Equinoxe implant produces a medial center of rotation (COR) just off the glenoid face and an onlay humeral design, which lateralizes the humerus, improving the moment arm, and therefore improving the efficiency of the deltoid. The concept of the deltoid moment arm is important, because a shorter moment arm requires a greater muscle force (and therefore acromial stress) to produce the same motion at the shoulder articulation. Shorter moment arms are seen in lateral COR designs such as the DJO or BIO-RSA, which may account for relatively higher rates of acromial fracture in those designs as high as 10%.1 The fracture rate at 1.48% in this study is quite low when comparing other prosthesis designs, even when comparing to onlay-humerus manufacturers such as Tornier (AscendTM), which produces a more inferior shift, instead of a lateral shift, of the humerus, therefore possibly accounting for higher rates of fracture with that design.2 Interestingly, in this study by Routman et al., a protective trend of lateralizing the humerus was seen in univariate, but not multivariate, analysis where the larger humeral tray/liner combined thickness of 0.8mm was seen in patients without fracture, but a smaller combined tray/liner thickness of 0.47mm was seen in patients with fracture.
Surgery-related factors including location, tilt, and size of the glenosphere, as well as location of the humeral osteotomy, were not evaluated in this study; although my suspicion is that these are all critical factors in maximizing the efficiency and function of the deltoid so as to prevent acromial fracture. Precision in component implantation is likely a factor associated with the low rate of fracture in this cohort, which can be optimized with careful preoperative planning and potentially intraoperative navigation. As noted in the radiographs cited in the study, judicious use of the superior baseplate screw is important, so as to prevent a stress riser at the scapular spine. In my own practice, when this screw is used, I ensure the screw purchases the robust more anterior coracoid base bone as opposed to the more posterior bone at the scapular spine. Checking the craniocaudal orientation of the baseplate to ensure it is not rotated posteriorly may also help to prevent generating a posterior stress riser from a screw tip.
In conclusion, acromial fractures after reverse shoulder replacements are an uncommon complication that can be mitigated by identifying patients at risk, optimizing surgical implantation, and modifying activity regimens of those patients developing acromial pain or tenderness. In this study, the Equinoxe shoulder prosthesis demonstrated that it is an industry leader with a low fracture rate of under 1.5%, resulting from its mechanical optimization of deltoid efficiency, thus minimizing acromial stress.
Thomas Obermeyer, MD, is a board-certified and fellowship-trained orthopaedic surgeon in Illinois, specializing in shoulder and elbow reconstruction and sports injuries. Dr. Obermeyer received his medical degree from Albany Medical College and completed his residency at Loyola University Medical Center in Chicago. He went on to complete a fellowship in shoulder and elbow at Mount Sinai Medical Center in New York City. Dr. Obermeyer is also an award-winning researcher and published author.
- Kang JR, et al. Primary reverse shoulder arthroplasty using contemporary implants is associated with very low reoperation rates. J Shoulder Elbow Surg. 2019 Jun;28(6S):S175-S180. doi: 10.1016/j.jse.2019.01.026. Epub 2019 Apr 20.
- Chalmers PN, et al. Comparative Utilization of Reverse and Anatomic Total Shoulder Arthroplasty: A Comprehensive Analysis of a High-volume Center. J Am Acad Orthop Surg. 2018 Dec 15;26(24):e504-e510. doi: 10.5435/JAAOS-D-17-00075.
- Reams RC, et al. A 10-year experience with reverse shoulder arthroplasty: are we operating earlier? J Shoulder Elbow Surg. 2020 Jul;29(7S):S126-S133. doi: 10.1016/j.jse.2020.04.040.
Kaveh Sajadi, MD, practices orthopaedics with Kentucky Bone and Joint Surgeons and is an instructor in the University of Kentucky’s residency program. He completed his residency at the Campbell Clinic and his fellowship at the NYU Langone Hospital for Joint Diseases. Dr. Sajadi is a frequent instructor at Exactech domestic and international shoulder courses.