Capsule function following anterior dislocation: Implications for diagnosis of shoulder instability

Abstract

During shoulder dislocation, the glenohumeral capsule undergoes non-recoverable strain, leading to joint instability. Clinicians use physical exams to diagnose injury and direct repair procedures; however, they are subjective and do not provide quantitative information. Our objectives were to: (1) determine the relationship between capsule function following anterior dislocation and non-recoverable strain; and (2) identify joint positions at which physical exams can be used to detect non-recoverable strain in specific capsule regions. Physical exams were simulated at three joint positions including external rotation (ER) using robotic technology before and after anterior dislocation. The resulting joint kinematics, strain distribution in the capsule, and non-recoverable strain were determined. Following dislocation, anterior translation increased by as much as 48% (08 ER: p ¼ 0.03; 308 ER: p ¼ 0.03; 608 ER: p < 0.01). Capsule sub-regions with less non-recoverable strain required more ER to detect differences in the strain ratios between the intact and injured joint. Strain ratio changes on the humeral side of the posterior axillary pouch (0.31 AE 0.32) were significant at all joint positions (08 ER: p ¼ 0.03; 308 ER: p ¼ 0.048; 608 ER: p ¼ 0.04), whereas strain ratio differences on the humeral side of the anterior axillary pouch (0.18 AE 0.21) were significant only at 608 of ER (p ¼ 0.03). Therefore, standardizing physical exams for joint position could help surgeons identify specific locations of non-recoverable strain that may have been ignored. ß

Location and magnitude of capsular injuries varies following multiple anterior dislocations of the shoulder: Implications for surgical repair

“…To minimize the effects of venting, the location of venting on the capsule was within the rotator interval and the size was minimized. Dessication of the capsule was prevented by spraying saline continuously during the experimental protocol and inflation of air was used as it has been validated in prior studies 12,17,33 …”

“…The maximum principal strain was computed at the centroid of each element for each specimen. The repeatability of the maximum principal strains based on the entire testing procedure is 3.5% 12,14,17 . Therefore, any maximum principal strains less than 3.5% can be considered negligible (∼0%).…”

“…The capsular injury that occurs during an anterior dislocation can be quantified as the permanent deformation or nonrecoverable strain of the capsule 8–11 . Nonrecoverable strain occurs when the capsule is loaded beyond its elastic limit and is a contributor to to recurrent anterior instability 12 . Prior studies have evaluated the nonrecoverable strain of the glenohumeral capsule during a simulated clinical exam and anterior subluxation of the shoulder, 13–16 but only following a single dislocation of the glenohumeral joint in the anterior direction 12,17 …”

“…[8][9][10][11] Nonrecoverable strain occurs when the capsule is loaded beyond its elastic limit and is a contributor to to recurrent anterior instability. 12 Prior studies have evaluated the nonrecoverable strain of the glenohumeral capsule during a simulated clinical exam and anterior subluxation of the shoulder, [13][14][15][16] but only following a single dislocation of the glenohumeral joint in the anterior direction. 12,17 During surgical repair, the glenohumeral capsule is often plicated and shifted to reduce joint laxity, especially following multiple dislocations.…”

“…12 Prior studies have evaluated the nonrecoverable strain of the glenohumeral capsule during a simulated clinical exam and anterior subluxation of the shoulder, [13][14][15][16] but only following a single dislocation of the glenohumeral joint in the anterior direction. 12,17 During surgical repair, the glenohumeral capsule is often plicated and shifted to reduce joint laxity, especially following multiple dislocations. [18][19][20][21][22][23] However, the location and amount of capsulorraphy is subject to surgeon preference and bias since quantification Despite current techniques, 6%-20% of shoulders sustain recurrent dislocations following surgical stabilization 19,21,24,25 and 19%-25% cannot return to sports at the same level.…”