| Bone: Glenoid cavity | |
|---|---|
| Glenoid fossa of right side. | |
| Left scapula. Lateral view. | |
| Latin | cavitas glenoidalis, fossa glenoidalis |
| Gray's | subject #50 207 |
The glenoid cavity (or glenoid fossa of scapula from Greek: gléne, "socket") is a shallow pyriform, articular surface, which is located on the lateral angle of the scapula. It is directed laterally and forward and articulates with the head of the humerus; it is broader below than above and its vertical diameter is the longest.
This cavity forms the glenohumeral joint along with the humerus. This type of joint is classified as a synovial, ball and socket joint.
The cavity surface is covered with cartilage in the fresh state; and its margins, slightly raised, give attachment to a fibrocartilaginous structure, the glenoid labrum, which deepens the cavity.
Compared to the acetabulum (hip-joint) the glenoid cavity is relatively shallow. This makes the shoulder joint prone to luxation. Strong ligaments and muscles prevents luxation in most cases.
By being so shallow the glenoid cavity allows the glenohumeral joint to have the greatest mobility of all joints in the body, allowing 120 degrees of unassisted flexion. This is also accomplished by the great mobility of the scapula (shoulder blade).
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Interpretations of the fossil remains of Australopithecus africanus (STS 7) and A. afarensis (AL 288-1; a.k.a. Lucy) suggest that the glenoid fossa was oriented more cranially in these species than in modern humans. This reflects the importance of overhead limb postures and suggests a retention of arboreal adaptations in these hominoid primates, where as the lateral orientation of the glenoid in modern humans reflects the typical lowered position of the arm. [1]
The glenoid has interindividually different degrees of retroversion or anteversion. Precise measurement of the glenoid properties, including the glenoid version, are important to analyse instability or loading of the gleno-humeral joint in order to assess preoperatively any reconstructive and replacement surgery. The measurement should be standardized and reproducible as exact as possible. In living individuals the version cannot be measured directly, and therefore precise imaging is required. Conventional X-rays on an axillary view are insufficient. They showed a clinically unusable wide range of data.
CT seems to be the more accurate choice. Computed tomography is significantly less sensitive to variations of the scapular orientation, which has been detected as a main reason for non-reproducible examinations. The orientation in the scout view of the patient is determining : The glenoid articular surface has to be perpendicular to the plane of the CT cut.[2][3]
Checking of the function of each active (scapula stabilizing muscles and the rotator cuff) and passive stabilizer and knowledge concerning the interaction of these factors are essential in the treatment of shoulder instability. While in traumatic instability the isolated failure of a passive stabilizer is paramount (eg, Bankart lesion), are in the non-traumatic instability both multi-etiological insufficiencies of active stabilizers (rotator cuff muscles, scapula stabilizing muscles) and changes in the passive factors as glenoid version, gleno-humeral index or glenoid cavity to be considered.
The role of the orientation of the glenoid cavity is seen contradictory, especially as a passive bony stabilizing factor in the atraumatic shoulder instability. Per definition indicates the glenoid version the angle between the transverse diameter of the cup perpendicular to the scapular plane. An average retroversion of 2-7 degrees is physiological. Increased glenoid retroversion is thereby seen by some authors as a predisposition to a posterior instability, while decreased retroversion or even an anteversion are considered to be a disposition for an anterior shoulder instability. However, other authors found no significant differences compared to healthy control groups, neither in recurrent anterior nor in posterior shoulder instability.
In the literature the specifications on the glenoid vary highly. This is partly due to the fact that it is not clear which reference points for defining the levels should be used. Additionally have conventional x-rays in vivo a low reproducibility and validity due to their lack of hardly reproducible exact same adjustments and their projection artifacts. More recent studies using 2-D CT showed the possibility of a precise description of the morphology in one plane. However, there could again be shown that the retroversion angle is highly conditioned both by the layer height relative to the glenoid and the adjustment of the patient at the CT.[4]
Anterior and posterior osseous glenoid rim define the glenoid plane. The scapular plane is defined through the center of the glenoid (50% of the distance between the anterior and posterior osseous glenoid rim) and margo. Conclusively is to say that the 3D-CT technology permits a reproducible determination of the retroversion, independent of the layer height and positioning of the patient. Between the 2D- and 3D-CT technology the glenoid version determination had an average deviation of ± 3 degrees. Subsequently except in extreme cases the 2D-CT technique under standardized conditions is not sufficient and a 3D-CT is required. Atraumatic unstable shoulders had on average on the affected side a significantly higher glenoid retroversion compared to the healthy control group, which showed a high variability of values. The extent of the changes varied between individuals and should be identified in order to initiate an efficient causal therapy. In patients with traumatic instability, however, no significant difference was observed compared to the healthy shoulders of the control group.[5]
Glenoid curvature analysis reflects mainly the osseous anatomy. This allows a comparison with the values described in the literature, which were determined by x-ray or CT techniques. Both due to the inhomogeneous distribution of the articular glenoid cartilage, which increases towards the periphery, and to the glenoid labrum the concavity of the glenoid is clearly increased. It cannot be excluded that the at non-traumatic instability observed lower concavity of the bony glenoid is balanced to a certain extent by these structures mentioned above. Both the cartilage thickness and the shape and position of the labrum are to be determined, even though they are more variable and harder to determine than osseous structures. Therefore, based on these parameters the implementation of valid studies is difficult.
Von Eisenhart-Roth et al.‘s results regarding the glenoid size and humerus head radius in healthy shoulders are in good agreement with previous studies based on measurements of anatomical specimens. There are also significantly higher values in male subjects. An additional gender difference is the relative ratio between glenoid size and humerus head radius at the 3D gleno-humeral index. Men showed a bigger glenoid related to the head.
In numerous previous cadaver and x-ray studies the glenoid cavity radius in healthy individuals has been determined at in between 32 and 37 mm. Only a few data exist about the exact glenoid cavity measurements in patients with shoulder instability. Inui et al. observed in their semi-quantitative analysis of the glenoid shape that 78% of healthy individuals have a concave osseous shape in the lower portion of the glenoid. But on the contrary this was not found in any of the shoulders with a posterior non-traumatic instability.
Patients with non-traumatic unstable shoulders showed significant changes in the passive stabilizers, like a reduced gleno-humeral index, a significantly decreased glenoid cavity and an increased glenoid retroversion. Comparison of these results with the gleno-humeral centering and position of the scapula shows that isolated changes exist exclusively in the active stabilizers. Von Eisenhart-Roth et al. classified 4 different groups with typical changes at the stabilizers, although the extent of individual changes varied greatly between the individuals and the group size was small.[6]
(3 patients):
Antero-inferior instability with a gleno-humeral decentration and a changed position of the scapula in both planes during an isometric muscle activity. Which suggests an isolated insufficiency of the active stabilizers. Speculative therapy recommendation: Conservative treatment with strengthening of the scapula stabilizing muscles and the rotator cuff.
(4 patients):
Combination with a decreased gleno-humeral index, decreased glenoid cavity and gleno-humeral decentration mainly in the horizontal plane. Speculative therapy recommendation: Given that it is not easy to correct the passive stabilizers, strengthening of the rotator cuff, especially of the Mm. subscapularis, teres minor and infraspinatus seems reasonable.
(3 patients):
Postero-inferior instability with an increased glenoid retroversion in combination with an isolated increased internal rotation of the scapula. Cave: It is not apparent whether the scapula position is physiologically conditioned in order to attain a normal glenoid adjustment or if those changes are currently causative for the instability.
(4 patients):
Combination of increased glenoid retroversion and decentration of the humerus head during isometric muscle activity. Speculative therapy recommendation: In the first instance conservative treatment of the rotator cuff. If not satisfying, correction of the glenoid.
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This article was originally based on an entry from a public domain edition of Gray's Anatomy. As such, some of the information contained within it may be outdated.
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