Here we showed a comprehensive investigation of the insertion widths and capsular thickness of the whole glenohumeral articular capsule. From the inferior edge of the subscapularis insertion, via the humeral anatomical neck, to the inferior edge of the teres minor insertion, the inferior part of the articular capsule had a wider attachment on the humerus than other areas. In addition, the midsubstance of the articular capsule corresponding to the above-metioned area was thicker than the superior part of the capsule.
Previously, the superior capsule of the shoulder joint had been assumed to be a thin continuous sheet which was the deepest layer of rotator cuff tendons.(Clark and Harryman 2nd, 1992) Therefore, the functional significance of the superior capsule of shoulder joint were mostly overlooked before the following papers. Recently, Mihata et al.(Mihata et al. 2013; Mihata et al. 2012) reported the biomechanical significance and the good clinical results of the superior capsule reconstruction for irreparable rotator cuff tears. In addition, Nimura et al.(Nimura et al. 2012) recently reported that the articular capsule has a wide attachment on the humerus, at the border from the infraspinatus to the teres minor, and that it compensates for the lack of tendinous insertion. In addition, they described that it was consistent with the posterior pole that holds up the bridge cable(Burkhart 1993) or the posterosuperior glenohumeral ligament.(Pouliart et al. 2007) Additionally, the wide attachments of the joint capsule have been reported to be related with the stabilizing structures of the elbow and knee joints.(Nasu et al. 2017; Shimura et al. 2016) In the current study, we further demonstrated that the articular capsule, compared to other areas, had a wider attachment on the humerus from the inferior edge of the subscapularis insertion to the inferior edge of the teres minor insertion, where no rotator cuff tendons insert. The glenohumeral joint has been well known as the inherently unstable joint, and thought to be stabilized by the interaction between the static, including the labrum and capsule, and dynamic, including musculotendinous structures.(Johnson and Ellis 2005) Therefore, the attachment of the inferior capsule could be speculated to make up for the lack of rotator cuff insertion.
The inferior glenohumeral ligament complex (IGHL), which is a hammock-like structure of the articular capsule extending from anteroinferior to posteroinferior areas of the shoulder joint,(O'Brien et al. 1990) corresponds to the inferior part of the capsule as described in the current study. In previous reports, the dimensions of the IGHL attachment on the humerus have remained controversial. O’Brien et al.(O'Brien et al. 1990) and Ticker et al.(Ticker et al. 1996) reported that IGHL has 2 variable attachments on the humerus: a collar-like attachment close to the articular cartilage, and a V-shaped attachment pointing inferiorly. In addition, Sugalski et al.(Sugalski et al. 2005) also described 2 attachment variations: split-type and broad-type attachment on the humerus. To the contrary, Pouliart and Gagey(Pouliart and Gagey 2005) observed the IGHL attachment on the humerus by dissection and arthroscopic viewing, and reported that the IGHL consistently attaches to the humerus with a V-shape as viewed from outside, and it attaches close to the cartilage rim, when viewed from the intra-articular side. They explained that the discrepancy of the IGHL attachment findings compared to previous studies originates from these biases according to dissection method. The results of the present study were compatible with the description of Pouliart and Gagey. The current study showed that widths of the capsular attachment at the inferior edge of the subscapularis and teres minor insertion were relatively narrow, and they widely expanded toward the metaphysis of the humeral neck. Through the perspective of the whole capsular membrane, we could interpret that area of humerus as a wide attachment of the inferior part of the articular capsule in the continuous capsular membrane and its attachment, which corresponds to the capsuloligamentous complex such as IGHL.
The posterior band of the inferior glenohumeral ligament (PIGHL) has been described as the most significant stabilizer in the posterior loading position, however, the posterior capsule itself seemed relatively thin and the biomechanical performance is not robust unlike the thick anteroinferior capsule.(Bey et al. 2005; Ticker et al. 1996) In the current study, the red dotted line in Fig. 7 seems to correspond to the superior margin of PIGHL. Based on the current study, the humeral and glenoid side edges, which superiorly continued from the thick inferior part of the articular capsule could be interpreted as the PIGHL. Morphologically, the PIGHL could be speculated to act as the hammock suspension to superiorly pull in the thick inferior part of the articular capsule.
As for clinical implications, the detailed anatomic findings of the current study of the whole glenohumeral capsule could provide some clues to understand the significance of stabilizing techniques for shoulder instability. Some surgeons have proposed the importance of AIGHL tensioning for stabilization with lifting the AIGHL superiorward in the procedure of Bankart repair.(Blasier et al. 1992; Post 1996) In addition, more recent articles have indicated that the Hill-Sachs remplissage procedure together with the Bankart procedure provide satisfactory biomechanical stabilization and clinical outcomes.(Boileau et al. 2012; Cho et al. 2016; Merolla et al. 2015; Omi et al. 2014) The Bankart repair and remplissage have been thought to have no anatomic relationships, because both techniques affected to different parts of the glenohumeral capsule. In the current study, we revealed that the inferior part of the articular capsule is thicker than the superior part in the whole capsule, and the humeral and glenoid side edges continue superiorly. Bankart repair involves the anterior advance of the glenoid side in the thick inferior capsule. On the other hand, remplissage involves the superior advance of the humeral side of the thick inferior capsule as a continuous membrane (dagger in Fig. 6a). Based on the current study, these two techniques apply tension to the same structure but at different areas diagonally continued. Thus, these anatomic findings might support the efficacy of the Bankart repair combined with remplissage. However, the question whether the appropriate tension can be applied to the diagonally continuing capsule still remains. Next, based on the “circle stability concept”, the shoulder capsule has been thought to be damaged on both the posterior and anterior side to allow the complete dislocation. (Bowen and Warren 1991) Taking into consideration of this concept, the anatomical knowledge about the superior thinner part of the capsule could be some clue for understanding the pathomechanism of the anterior instability, differently from the IGHL. Based on the above, the anatomical findings in the current study could then serve as a take off point for further research regarding the pathology of the shoulder instability using the biomechanical and clinical studies.
The current study has several limitations. First, we used cadavers which were embalmed with formalin for measurements of the capsular attachment on bones. The position of the glenohumeral joint at the time of fixation may affect the local thickness of the whole capsule. However, the width of the capsular attachment should not be affected by the fixation methods or by the arm position. Additionally, Thiel’s embalming method was used for the analysis of capsular thickness to reduce the positional bias. The advantage of formalin-fixed specimens is thought to be that membranous structures can be clearly separated between the rotator cuff and capsule, as previously reported.(Nimura et al. 2012) Second, the current study was a pure anatomic study. Therefore, to show the significance of the thick inferior part of the articular capsule, biomechanical analyses and clinical findings are also recommended. Third, the results of the current study may be affected with the bias of age and female predominance. Fourth, regarding the thickness measurement of the whole capsule, we have not statistically analyzed the differences, and the method using micro-CT for the thickness measurement could not be validated by other methods, because we had no choice for non-contact measurement of the thickness. Fifth, the long head of the triceps muscle had the superficial fibers originating from the outer surface of the shoulder joint capsule. In the current study, the superficial fibers of the long head of the triceps muscle were cut at the base of the glenoidal origin. Therefore, the inferior part of the capsule could include the superficial fibers originating from the outer surface of the shoulder joint capsule.