Background: The purpose was to analyze the influence of deltoid lengthening due to different implant designs and anatomic variations of the acromion and scapular spine (SS) in the parasagittal plane on strain patterns after reverse shoulder arthroplasty (RSA).
Methods: Ten cadaveric shoulders with strain rosettes placed on the surface of the acromial body (Levy II) and SS (Levy III) were tested using a shoulder simulator. RSA using humeral onlay (+3, +5, +8, +10, +13 mm) and glenosphere lateralization (0, +6 mm) was performed. Arm lengthening and magnitude of strain on acromion/SS were measured. The length of deltoid was assessed using validated computer modeling. Anatomic variance of the SS angle and position of acromion in relation to the scapular plane was examined. For comparison of strain as a function of deltoid lengthening, 25 mm was used as a threshold value for comparison based on previous literature demonstrating a decrease in Constant score and active anterior elevation in patients with arm lengthening >25 mm.
Results: At maximal deltoid lengthening (30.8 mm), average strains were 1112 με (acromion) and 1165 με (SS) (P < .01). There was an 82.6% increase in acromial strain at maximum lengthening compared with 25 mm (P = .02) and a strain increase of 79 με/mm deltoid lengthening above a threshold of 25 mm. The strain results delineated 2 anatomic groups: 5 of 10 specimens (group A) showed higher strain on SS (1445 με) vs. acromion (862 με, P = .02). Group A had a more posteriorly oriented acromion, whereas group B was anteriorly oriented (P < .001).
Conclusion: Deltoid lengthening above 25 mm produced large strains on the acromion/SS. Anatomic variation may indicate that as the acromion is more posteriorly oriented, the SS takes more strain from the deltoid vs. the acromion. Our study's data may help surgeons identify a high-risk population for increased strain patterns after RSA.
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