Original Research

Assessment of Scapular Morphology and Surgical Technique as Predictors of Notching in Reverse Shoulder Arthroplasty

Am J Orthop. 2015 May;44(5):E148-E152

References

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Regarding the effects of notching severity on outcomes in our study cohort, there were no significant differences between groups 1 and 2 in postoperative function, including forward flexion (123° vs 112.4°; P = .11), external rotation (18.8° vs 16.7°; P = .76), positive lag sign (P = .2), and VAS pain scores (1.2 vs 2.1; P = .15). There were also no significant differences between groups in the rate of complications (P = .92). Regression analysis determined that PSNA, PGRD, glenosphere inclination, glenosphere overhang, and implant manufacturer were not significant predictors of complications.

Discussion

RSA has provided good pain relief and restored function in patients with irreparable rotator cuff disease associated with arthritis.^5,12,17,18 Scapular notching is a complex, multifactorial process. Nevertheless, surgeons remain cautious about the implications of inferior scapular notching, which is being reported by a significant number of patients. Our cohort’s high incidence of scapular notching (82%) in the early postoperative period clearly highlights the importance of predictive models, such as the notching index.⁸ Although concerns about consequences of notching have been expressed, notching severity did not affect outcomes or increase complications in this cohort.^{5,8,11,12,17-19}

We conducted this study to examine use of a predictive tool for scapular notching, the notching index, in a large cohort of patients who underwent primary RSA. This index combines 2 well-established factors that contribute to notching—craniocaudal position and PSNA—into a predictive formula based on statistical analyses performed in a prospective cohort study.^4,5,8,12,18 In their clinical study, Simovitch and colleagues⁸ found that both craniocaudal position and PSNA were tightly coupled with inferior scapular notching, and they developed a notching index that accounts for this relationship. We hypothesized that patients with a notching index of less than the recommended 35 would not develop notching and that patients with a notching index of more than 35 would have increased incidence and severity of notching. With our cohort, the recommended index of 35 was not an appropriate threshold predictive of notching. Furthermore, the 35 threshold applied to our cohort had 89% sensitivity and 21% specificity in predicting notching. Although the sensitivity is high, and correctly predicted true instances of notching, the low specificity compromises the precision of the notching formula ([PSNA × 0.13] + PGRD).

From the formula, it can be inferred that higher PSNA values can be compensated for by decreasing PGRD and inferiorizing the glenosphere. However, this recommendation appears limited based on increasing PSNA values, as in our cohort. The previously described notching formula cannot be universally applied to all patients treated with RSA because of the complexity of this relationship and patient-specific anatomy.

We assessed other possible anatomical and surgical factors, specific to scapular morphology, that could contribute to scapular notching. In other studies, reaming that produced an inferior tilt of the glenoid increased the likelihood of inferior notching.^8,20,21 Furthermore, we expected less inferior glenoid overhang and smaller glenosphere would predispose patients to more notching.^8,12,19 In our cohort, notching grade was not correlated with inferior tilt, glenoid overhang, or glenosphere size, which may be attributed to minimal variability in glenosphere size and a small range of glenosphere overhang.

There were limitations to this study. We examined only 2 types of RSA systems, and they had very similar Grammont designs. Other RSA designs might not have similar shortcomings with respect to inferior notching. In addition, we examined patient cases at a single time point and did not evaluate the effect of notching over time.

Overall, our results suggest that PGRD and PSNA have little effect on development of higher grade notching, particularly with use of Grammont prostheses. With newer surgical techniques, the recommendation is for inferior craniocaudal placement of the glenosphere, but this may not prevent notching with some types of patient-specific scapular morphology. Clearer surgical guidelines and techniques may help delineate the contribution of each parameter causing inferior scapular notching. Surgeons must weigh the evidence to determine how to correct patient-specific glenoid pathology and orient the glenosphere. Recent studies on bony increased-offset reverse shoulder arthroplasty (bio-RSA) techniques or newer prosthetic designs that considerably alter PSNA and the center of rotation may prevent inferior notching and provide a promising alternative to Grammont designs. Ultimately, longer follow-up is also needed to understand the clinical relevance of increased scapular notching.