Hospital for Special Surgery, New York, NY.
New England Baptist Hospital, Boston, Massachusetts.
J Bone Joint Surg Am. 2020 Aug 5;102(15):1358-1364. doi: 10.2106/JBJS.19.01118.
Scapular fractures following reverse total shoulder arthroplasty (RSA) are devastating complications with substantial functional implications. The role of the coracoacromial ligament (CAL), which is often transected during surgical exposure for RSA, is not fully known. We hypothesized that the CAL contributes to the structural integrity of the "scapular ring" and that the transection of this ligament during RSA alters the scapular strain patterns.
RSA was performed on 8 cadaveric specimens without evidence of a prior surgical procedure in the shoulder. Strain rosettes were fixed onto the acromial body (at the location of Levy type-II fractures) and the scapular spine (Levy type III). With use of a shoulder simulator, strains were recorded at 0°, 30°, and 60° glenohumeral abductions before and after CAL transection. The deltoid and glenohumeral joints were functionally loaded (middle deltoid = 150 N, posterior deltoid = 75 N, and joint compression = 300 N). Maximum principal strains were calculated from each rosette at each abduction angle. A repeated-measures analysis of variance with post hoc analysis was performed to compare the maximum principal strain at each abduction angle.
With the CAL intact, there was no significant difference between strain experienced by the acromion and scapular spine at 0°, 30°, and 60° of glenohumeral abduction. CAL transection generated significantly increased strain in the scapular spine at all abduction angles compared with an intact CAL. The maximum scapular spine strain observed was increased 19.7% at 0° of abduction following CAL transection (1,216 ± 300.0 microstrain; p = 0.011). Following CAL transection, acromial strains paradoxically decreased at all abduction angles (p < 0.05 for all). The smallest strains were observed at 60° of glenohumeral abduction at the acromion following CAL transection (296 ± 121.3 microstrain; p = 0.048).
The CAL is an important structure that completes the "scapular ring" and therefore serves to help distribute strain in a more normalized fashion. Transection of the CAL substantially alters strain patterns, resulting in increased strain at the scapular spine following RSA.
CAL preservation is a modifiable risk factor that may reduce the risk of bone microdamage and thus the occurrence of fatigue/stress fractures in the scapular spine following RSA.
反向全肩关节置换术（RSA）后肩胛骨骨折是一种破坏性的并发症，对功能有重大影响。喙肩弓韧带（CAL）的作用在手术暴露 RSA 时通常会被切断，但这一韧带的作用尚未完全明确。我们假设 CAL 有助于“肩胛骨环”的结构完整性，并且在 RSA 中切断该韧带会改变肩胛骨的应变模式。
对 8 具无肩部既往手术史的尸体标本进行 RSA。应变花固定在肩峰体（在 Levy Ⅱ型骨折的位置）和肩胛骨脊柱（Levy Ⅲ型）上。使用肩部模拟器，在 CAL 切断前后记录 0°、30°和 60°肩肱外展时的应变。三角肌和肩肱关节被功能性加载（中间三角肌=150 N，后三角肌=75 N，关节压缩=300 N）。从每个花盘在每个外展角度计算最大主应变。采用重复测量方差分析和事后分析比较各外展角度的最大主应变。
CAL 完整时，肩峰和肩胛骨脊柱在 0°、30°和 60°肩肱外展时的应变无显著差异。与完整的 CAL 相比，CAL 切断后，所有外展角度的肩胛骨脊柱应变明显增加。CAL 切断后，肩胛骨脊柱的最大应变增加了 19.7%，外展 0°（1216±300.0 微应变；p=0.011）。CAL 切断后，所有外展角度的肩峰应变都反常下降（所有 p<0.05）。CAL 切断后，肩峰在 60°肩肱外展时的应变最小（296±121.3 微应变；p=0.048）。
CAL 是一个重要的结构，它完成了“肩胛骨环”，因此有助于以更正常的方式分布应变。CAL 切断后，肩胛骨的应变模式发生了显著改变，导致 RSA 后肩胛骨脊柱的应变增加。
CAL 保留是一个可改变的危险因素，它可能降低肩胛骨脊柱骨微损伤的风险，从而降低 RSA 后疲劳/应力性骨折的发生风险。
