The biomechanical effect of specific labral and capsular lesions on posterior shoulder instability

In contrast to anterior shoulder instability there seems to be no single key lesion in posterior shoulder instability. Therefore, the purpose of this study was to determine the biomechanical effect of specific posterior capsulolabral lesions. Our hypothesis was that a posterior capsule lesion will have a predominant effect compared to a labrum detachment (Bankart lesion).

Stability testing of 16 cadaveric human shoulders was performed. The specimens were distributed to two groups: the labrum lesion group and the capsular lesion group. In the labrum lesion group three different conditions were tested consecutively: posteroinferior Bankart lesion, additive pHAGL lesion, additive posterosuperior Bankart lesion. In the capsular lesion group two conditions were tested: posteroinferior capsule cut including a glenoidal transection of the pIGHL, additive rotator interval and superior capsule lesion (SGHL and CHL cut). All lesions were set arthroscopically. Biomechanical testing was performed in two positions: the sulcus-test position and the jerk-test position each with a passive humerus load of 50 N in the posterior, posteroinferior and inferior direction.

A posteroinferior Bankart lesion resulted in a percentage increase of 86% posterior translation in the jerk position and an increase of 31% inferior translation in the sulcus position. An additional pHAGL lesion resulted in a significant increase of posterior and inferior translation given by 31 and 41% in the jerk position. Regarding the capsular lesions, a cut of the posteroinferior capsule and the pIGHL resulted in a significant increased inferior translation of 53% in the sulcus position but did not cause a significant increase of posterior translation in the jerk position. If an additional rotator interval lesion is set the inferior translation is again significantly increased.

On the basis of our results traumatic posterior shoulder instability must be suspected to be bidirectional posteroinferior independently if a posterior capsule lesion or a posterior Bankart lesion is evident.

Capsular and labral lesions both have a significant biomechanical effect but differ in the predominant direction of instability, which is posterior for the Bankart lesion and inferior for the capsular lesion. An additional pHAGL or rotator interval lesion aggravates the posteroinferior instability and must be respected in the surgical treatment strategy.