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Stress Fractures After rTSA. Not Every Onlay is Created Equal.

Ian Byram, MD

Read complete study: Increased scapular spine fractures after reverse shoulder arthroplasty with a humeral onlay short stem: an analysis of 485 consecutive cases

Since its FDA clearance in 2003, reverse shoulder arthroplasty (RSA) has become increasingly utilized for the treatment of arthritis, fractures, and rotator cuff failures in the United States. Initial reports demonstrated a high rate of complications, including instability, impingement, scapular notching, and subsequent implant failure.Many implant designs have been successfully modified to minimize these problems. While scapular notching seen with the initial Grammont style prosthesis has decreased with modern lateralized implant designs, scapular spine fractures have become more prevalent.

The authors of this study analyzed a series of RSA cases utilizing a short stem, 145° neck-shaft inclination prosthesis hoping to shed light on implant features that may lead to scapular spine fractures. In a retrospective review of 485 consecutive arthroplasties, they reported 21 (4.3%) scapular spine fractures that were diagnosed at a mean of 8.6 months after surgery. Comparing those patients with matched controls, patients with fractures demonstrated significantly worse Constant scores and forward flexion. They concluded that utilization of a lateralized “onlay” convertible humeral design creates increased stress on the deltoid and acromion, resulting in an undesirable rate of scapular spine and acromial fractures.

Upon closer examination of the cohort, however, the bony glenoid lateralization (BIO-RSA) technique was utilized in 12 of the 21 patients with this unique complication – resulting in lateralization of both the glenosphere and the humeral shaft.  While they admit that the coexistence of humeral and glenosphere lateralization may increase deltoid stress on the scapular spine and acromion, they seem to attribute this stress largely to the humeral onlay design of the implant.  However, we must recognize, that the thicker polyethylene liner of the Tornier Ascend™ Flex prosthesis used in this series also inferiorly shifts the humerus approximately 4.5mm further than the Exactech Equinoxe® system, which has a similar 145° neck-shaft angle.2 We will need further clinical studies to assess the impact of this on overall clinical outcome.

While they admit that the coexistence of humeral and glenosphere lateralization may increase deltoid stress on the scapular spine and acromion, they seem to attribute this stress largely to the humeral onlay design of the implant.

Biomechanical studies have been performed comparing the effects of humeral lateralization, glenosphere lateralization, and humeral distalization on deltoid force.  These data demonstrate that glenosphere lateralization and humeral distalization increase deltoid force and joint loads, while humeral lateralization paradoxically decreases the deltoid force required to raise the arm.  Similarly, acromial stress is increased with more glenosphere lateralization and decreased with humeral lateralization.3,4

Scapular spine fractures are multifactorial complications that need to be minimized as we continue to utilize and develop modern implant designs. Surgeons must understand that different prostheses vary in many respects, even when a similar concept (such as the “onlay” humerus) is used. In my practice, I attempt to mitigate risk factors for scapular spine fractures by avoiding the combination of glenosphere and humeral lateralization except in cases of extreme glenoid bone loss.  Reverse shoulder arthroplasty is not a “one size fits all” procedure, and we must be cognizant of the myriad factors that can lead to various complications.

References:

  1. Cheung, et al. Complications in reverse total shoulder arthroplasty. JAAOS. 2011 Jul;19(7):439-49
  2. Routman HD, et al. Reverse Shoulder Arthroplasty Prosthesis Design Classification System.  Bulletin of the Hospital for Joint Diseases 2015; 73 (Suppl 1):S5-14
  3. Giles JW, et al. Implant Design Variations in Reverse Total Shoulder Arthroplasty Influence the Required Deltoid Force and Resultant Joint Load. Clin Orthop Relat Res. 2015 Nov;473(11):3615-26
  4. Kerrigan A. Acromial Insufficiency Fractures in Reverse Total Shoulder Arthroplasty: A Biomechanical Study (2018). Electronic Thesis and Dissertation Repository. 5977. https://ir.lib.uwo.ca/etd/5977

Ian Byram, MD, is a shoulder and elbow specialist at the Bone and Joint Institute of Tennessee. Dr. Byram received his medical degree from the University of North Carolina School of Medicine and completed his residency at Vanderbilt University School of Medicine. He went on to complete a shoulder, elbow and sports medicine fellowship at Columbia University Medical Center. Dr. Byram is an accomplished author of more than 20 peer-reviewed articles and book chapters, serves on the AAOS shoulder and elbow program committee, and is a member of both ASES and ACESS.

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