Moby Parsons, MD
In this article, Somerson and colleagues queried the Manufacturer and User Facility Device Experience (MAUDE) database for failure modes of anatomic and reverse shoulder arthroplasty. The authors identified 2,390 reports on reverse total shoulder arthroplasty of which 32% were for dislocation/instability. This was more than twice the rate of the next most commonly reported complication of infection at 13.8%. This rate concurred with findings by Bohsali et al1 who reported 31.2% of reverse shoulder complications due to instability in an analysis of published reports.
However, given that most complications are not reported in the medical literature, the actual incidence of dislocation is not known and may be higher. Risk factors have included higher body mass index, fracture sequelae, axillary nerve injury, soft tissue and bone impingement, improper implant placement, and inadequate soft tissue tension. The role of the subscapularis in reverse arthroplasty stability is controversial with some reports of a higher risk of dislocation in the subscapularis deficient shoulder and other reports of no difference in instability rates regardless of integrity. In general, instability after reverse shoulder arthroplasty occurs relatively early in the postoperative setting and often requires revision surgery. It remains a substantial problem.
In general, instability after reverse shoulder arthroplasty occurs relatively early in the postoperative setting and often requires revision surgery. It remains a substantial problem.
Prosthesis design may play a role in instability although the effect of design parameters on stability biomechanics has not been fully defined. In this report, the two designs with the highest dislocation percentage are both humeral inlay designs. Considering that some cases of instability occur from bone or soft tissue impingement, particularly in adduction, it stands to reason that onlay designs which provide humeral lateralization may be less at risk for this problem. For example, the Equinoxe® shoulder system, which is an onlay system with a 145° neck/shaft angle, has shown low rates of scapular notching compared to traditional Grammont-style designs which are inlay systems with a 155-degree neck shaft angle.2 Glenosphere lateralization may also reduce this risk of instability.3 The Equinoxe shoulder system provides lateralized glenospheres, which when combined with an eccentric baseplate, may help protect against postoperative dislocation. The ExactechGPS® Shoulder Application’s preoperative planning tool and ExactechGPS® intraoperative navigation can also be used to improve implant position, and in cadaveric studies have shown to increase implant accuracy and precision when used together.4
When considering optimal soft tissue tension during surgery, there are no clear-cut guidelines on which to base implant configuration, and most surgeons rely on clinical experience to make this decision. It is critical, however, that surgeons take the arm through a range of abduction to see if tissue tension changes as the deltoid wrap reduces with abduction angle. Similarly, it is critical to check for rotational impingement in adduction to making sure there is no levering of the humerus due to implant/bone or implant/soft tissue contact. The latter may be particularly true in revision cases where dense scar tissue from revision surgery can impinge in adduction.
When optimizing stability during the trialing phase of the procedure, surgeons should consider lateralization using a +4 glenosphere prior to distalization with a thicker liner or baseplate. This has the potential effect of improving tension and reducing impingement without putting increased stretching on the brachial plexus through lengthening. This is supported by a report from Dr. Emilie Cheung of Stanford University who found that increasing liner thickness was often insufficient in correcting instability when reverse shoulders required revision due to recurrent dislocation. A larger diameter or lateral offset glenosphere was often required.5 This finding suggests that glenoid sided lateralization, in combination with an onlay humerus, is a stable biomechanical construct.
With 4 glenosphere diameters and lateralized options for the 38mm and 42mm designs, the Equinoxe shoulder system provides a wide range of implants which can optimize stability. While unrelated to the MAUDE database, Exactech’s large multicenter clinical database provides a substantial sample size to follow outcomes after reverse total shoulder arthroplasty for the Equinoxe implant. Looking only at dislocation as a complication, the dislocation rate for the Equinoxe shoulder system is less than 1% in primary cases. Despite this success, it is important to recognize that instability is the most frequent complication of reverse shoulder arthroplasty and careful assessment of on table stability is essential prior to final implant placement. Additional information on the Equinoxe Shoulder System can be found at www.exac.com or www.equinoxenumbers.com.
Moby Parsons, MD, is in private practice in New Hampshire. He completed his internship and residency at University of Pittsburgh and fellowships at the University of Washington, Southern California Orthopaedic Institute and the University of Sydney. His work has produced multiple published articles and international presentations.
1. Bohsali, et al.The Journal of Bone and Joint Surgery. 99(3):256–269, FEB 2017.
2. Mollon, B. et al. Journal of Shoulder and Elbow Surgery, Volume 26, Issue 7, 1253 – 1261.
3. Helmkamp, J. K. et al. Journal of Shoulder and Elbow Surgery, Volume 27, Issue 11, 2099 – 2107.
4. Greene, A. et al. Navigated vs. non-navigated results of a CT based computer assisted shoulder arthroplasty system in 30 cadavers. Presented at ISTA 2018.
5. Cheung, E. V. et al. Journal of Shoulder and Elbow Surgery, Volume 27, Issue 11, 1946 – 1952.