The role of bone in glenohumeral stability

Abstract

Shoulder stability depends on several factors, either anatomical or functional. Anatomical factors can be further subclassified under soft tissue (shoulder capsule, glenoid rim, glenohumeral ligaments etc) and bony structures (glenoid cavity and humeral head).Normal glenohumeral stability is maintained through factors mostly pertaining to the scapular side: glenoid version, depth and inclination, along with scapular dynamic positioning, can potentially cause decreased stability depending on the direction of said variables in the different planes. No significant factors in normal humeral anatomy seem to play a tangible role in affecting glenohumeral stability.When the glenohumeral joint suffers an episode of acute dislocation, either anterior (more frequent) or posterior, bony lesions often develop on both sides: a compression fracture of the humeral head (or Hill-Sachs lesion) and a bone loss of the glenoid rim. Interaction of such lesions can determine 're-engagement' and recurrence.The concept of 'glenoid track' can help quantify an increased risk of recurrence: when the Hill-Sachs lesion engages the anterior glenoid rim, it is defined as 'off-track'; if it does not, it is an 'on-track' lesion. The position of the Hill-Sachs lesion and the percentage of glenoid bone loss are critical factors in determining the likelihood of recurrent instability and in managing treatment.In terms of posterior glenohumeral instability, the 'gamma angle concept' can help ascertain which lesions are prone to recurrence based on the sum of specific angles and millimetres of posterior glenoid bone loss, in a similar fashion to what happens in anterior shoulder instability. Cite this article: EFORT Open Rev 2018;3:632-640. DOI: 10.1302/2058-5241.3.180028.

Keywords: glenoid track; instability; shoulder.

Fig. 1

During arm motion towards the end-range, the glenoid moves along the contact zone on the posterior margin of the humeral articular surface, shifting from inferomedial to posterolateral. Reproduced with permission from Itoi E. ‘On-track’ and ‘off-track’ shoulder lesions. EFORT Open Rev 2017;2:343-351.

Fig. 2

The “on-track”/”off-track” concept in anterior shoulder instability from Di Giacomo et al. If HSL falls within the medial margin of the GT, there is still glenoid track support for bone stability (“on-track” HSL) and the HSL will not engage (above). If the HSL extends medial to the medial margin of the GT and there is concomitant loss of bone support at the anterior glenoid rim, the HSL will engage (“off-track HSL”)(below). Reproduced with permission from Elsevier.

Fig. 3

Illustrations of the measurements performed to determine the defect size and localization in posterior shoulder instability as proposed by Moroder et al. (A) Best-fit circle placed on the remainder of the humeral articular surface to create a reference centre for the measured angles; (B) alpha, defined as the angle between the anterior and posterior defect margin; (C) beta, defined as the angle between the anterior defect margin and the bicipital groove; (D) gamma, defined as the angle between the bicipital groove and the posterior defect margin; (E) delta, defined as the angle between the posterior defect margin and the posterior glenoid rim; and (F) epsilon, defined as the angle between the posterior defect margin and the anterior glenoid rim. Reproduced with permission from SAGE Publications.

Fig. 4

The gamma angle concept, as proposed by Moroder et al. Reproduced with permission from SAGE Publications.

Fig. 5

Schematic representation of the gamma angle concept applied to posterior instability. (A) Gamma angle <90°, (B) internal rotation does not engage the posterior glenoid. (C) Gamma angle >90°, (D) internal rotation engages the posterior glenoid. (E) When posterior GBL is present, about 2.3 degrees per mm of bone loss are lost on the delta angle. (F) In this case, concomitant posterior glenoid defects might lead to the engagement of noncritical RHSLs. Note. BG: Bicipital Groove.