Keywords: Shoulder joint, Rotator Cuff, MR Arthrography, Glenoid labrum
|How to cite this article:|
Doshi R, Maheshwari S, Singh J. Review article : MR anatomy of normal shoulder. Indian J Radiol Imaging 2002;12:261-6
| Introduction|| |
Normal shoulder function is essential for day-to-day life and many popular sports. Imaging of the shoulder and its dysfunctions is one of the most challenging regions for all sports medicine practitioners. The aim of this article is to provide a background in the functional and radiological anatomy of the shoulder joint. Magnetic Resonance Imaging is a very useful modality for evaluation of shoulder, because of its multiplanar capability and excellent soft tissue resolution.
| Functional Anatomy|| |
The glenohumeral joint is a ball and socket joint. The glenoid cavity is a shallow socket and is inherently unstable. The additional stability to the joint is provided by static constraints - the glenohumeral ligaments, glenoid labrum and capsule - and dynamic constraints, predominantly the rotator cuff muscles.
The main static stabilizer of the shoulder in the abducted or functional position is the inferior glenohumeral ligament (IGHL). IGHL is attached to the labrum, which in turn, attaches directly to the margin of the glenoid fossa. The dynamic stabilizers of the glenohumeral joint are the rotator cuff muscles, which serve to control the position of the humeral head in the glenoid fossa. The rotator cuff muscles, principally the supraspinatus and, to a lesser extent, infraspinatus, teres minor and subscapularis, counteract the action of the deltoid by preventing the head of the humerus from moving superiorly when the arm is raised. An imbalance between the deltoid and the rotator cuff muscle strength may result in excessive superior movement of the humeral head, causing impingement of subacromial structures.
| MRI Technique|| |
For a higher resolution study, it is preferable to use phased array flex coils. The shoulder and arm are positioned in mild external rotation with the patient in the supine position . External rotation causes the anterior capsular structures to appear more taut and sharply defined. Routine shoulder evaluations start with a T2-weighted axial localizer. The oblique coronal and sagittal 4mm slices with a maximum FOV of 14 cm are planned over the axial scout and include T1-weighted and T2-weighted fat suppressed images . The coronals scans are planned parallel to the supraspinatus tendon, while the sagittal scans are perpendicular to this plane [Figure - 1] and [Figure - 2] . Fat-suppressed T2-weighted fast spin-echo axial images, obtained with a 12-14 cm FOV, are also included ,,,,,. Optional sequences include 3D-Gradient echo images for evaluation of cartilage and picking up calcification and loose bodies.
MR arthrography with intra-articular contrast is performed with gadolinium-based contrast material . The advantages of capsular distention are twofold. Firstly, it allows distinction of individual structures by improved soft-tissue contrast and physical separation by the intra-articular contrast material. Secondly, it allows analysis of the distribution of contrast material in and around the joint. Magnetic resonance (MR) arthrography is more accurate than standard MR imaging for assessing the capsulolabral complex, undersurface of the rotator cuff, glenohumeral ligaments and rotator cuff interval. A routine MR arthrography protocol uses fat-suppressed T1-weighted images at a 3 mm slice thickness in all three planes (coronal oblique, axial and sagittal oblique) post injection . Fat-suppression helps to avoid mistaking areas of normal fat from high signal intensity contrast, increasing the conspicuity of the paramagnetic contrast agent . The gadolinium is diluted (0.1 cc into 20 cc of saline) and approximately 12 to 16 cc is then injected into the joint .
| Basic Anatomy|| |
The shoulder joint is composed of three bones and five articulations. This includes the glenohumeral joint, the Acromio- Clavicular joint, the scapulothoracic joint, the sternoclavicular joint and the Coracoclavicular joint .
Clavicle is an "S" shape structure, which connects axial to the appendicular skeleton. The clavicle articulates with the sternoclavicular joint medially and acromioclavicular joint laterally [Figure - 3].
The scapula consists of the scapular body, the scapular spine, the scapular neck, the acromion, the glenoid fossa and the coracoid process.
The acromion is classified into three types according to its morphology ,,.
Type 1 is a flat undersurface with a high angle of inclination.
Type 2 is a curved arc and decreased angle of inclination.
Type 3 is hooked anteriorly with a decreased angle of inclination.
The proximal humerus consists of the head, anatomical neck and the greater and lesser tuberosities. The intertubercular or bicipital groove is located between the greater and lesser tuberosities along the anterior surface of the humerus.
The articular capsule completely encircles the joint; it is attached to the circumference of the glenoid cavity beyond the glenoidal labrum. Inferiorly it is attached to the anatomical neck of the humerus [Figure - 4] a and b. Usually there is an opening inferior to the coracoid process in the capsule, through which there is a communication between the joint and a bursa beneath the tendon of the Subscapularis ,.
Coraco-humeral ligament: It is a broad band, which strengthens the superior part of the capsule. The ligament arises from the coracoid process, passes obliquely inferolaterally towards the greater tubercle of the humerus and blends with supraspinatus [Figure - 5].
Gleno-humeral ligaments: The glenohumeral ligaments are band like thickenings of anterior capsule, which strengthen the capsule anteriorly .
Superior glenohumeral ligament (SGHL) has two sites of origin, supraglenoid tubercle just anterior to long head of biceps origin on superior labrum and the base of coracoid [Figure - 6], while inserts superior to lesser tuberosity. MRI plays an important role as arthroscopically SGHL may be hidden by the biceps tendon.
Middle Glenohumeral Ligaments (MGHL): MGHL arises from the anatomic neck and inserts into the mid-anterior labrum. Middle part of the ligament is seen just posterior to subscapularis [Figure - 7]a andb. It shows maximum variation in size and shape among all three gleno-humeral ligaments. MGHL may appear cord like and frayed with absent anterior-superior labrum. It is then called as Buford complex or physiologic antero-superior sublabral hole [Figure - 7]c, ,. MGHL limits external rotation at 45 deg of abduction.
Inferior glenohumeral ligament (IGHL):
IGHL is the most important glenohumeral ligament
IGHL attaches to inferior two thirds of glenoid via glenoid labrum [Figure - 8]a. It has two bands like thickening, anteriorly (anterior band, AB) and posteriorly (posterior band, PB) [Figure - 8]b and a lax part, axillary pouch (AP), in between. IGHL is lax in adducted position. IGHL is the main static stabilizer of the shoulder in the abducted or functional position.
| The Glenoid Labrum|| |
It is a fibrocartilaginous rim attached around the margin of the glenoid cavity [Figure - 9]a. It increases the superior-inferior diameter of the glenoid by 75% and the anterior-posterior diameter by 50%. The base of the glenoid labrum is fixed to the circumference of the cavity, while the free edge is thin and sharp [Figure - 9]b. It is continuous above with the tendon of the long head of the Biceps brachii, which blends with the fibrous tissue of the labrum. It deepens the articular cavity and protects the edges of the bone. ,,
| Biceps long head|| |
The tendon of long head of biceps forms "biceps-labral complex with superior glenohumeral ligament and inserts on the supraglenoid tubercle. The tendon traverses laterally in the rotator cuff interval [Figure - 10]a, b and c to lie in the bicipital groove. Glenohumeral joint is in connection with the sheath of biceps tendon sheath. ,
| Rotator Interval Capsule|| |
The rotator interval is the portion of the shoulder joint capsule which lies between the supraspinatus and subscapularis tendons. This lies between the superior and middle glenohumeral ligaments [Figure - 11]. The interval is reinforced by the coracohumeral ligament and underlying joint capsule. It acts to limit flexion and external rotation. Tears of the rotator interval capsule may mimic rotator cuff tears. Impingement signs may be present along with biceps tendon tenderness.
| Rotator cuff|| |
Four muscles and their tendon attachments form the rotator cuff. The muscles in the group are Supraspinatus, Infraspinatus, Teres minor and Subscapularis [Figure - 12] a, b and c. The group of flat tendons fuses together and surrounds the front, back and top of the shoulder joint. They connect to muscles originating from the scapula. When the muscles contract, they cause the rotator cuff tendon to rotate either inward, outward or upward. Supraspinatus lies directly over top of humeral head and is an abductor. It is predisposed to degenerative changes because of its location between humeral head and acromion, which compress the tendon during shoulder movement. Infraspinatus and teres minor cover the back of humeral head and are external rotators. Subscapularis crosses the front of the shoulder joint and is an internal rotator and reinforces anterior capsule. ,
| Conclusion|| |
Thorough understanding of the normal MR anatomy of the shoulder joint is an essential prerequisite to precise diagnosis of pathological conditions of the joint. MRI imaging due to its excellent soft tissue resolution and multiplanar capability is very useful in understanding complex anatomy of shoulder joint. MR Arthrogram helps in accurate identification and demonstration of gleno-humeral ligaments, labrum and capsule.
| Acknowledgement|| |
Authors extend their sincere thanks to Prof.Dr.Curtis Hayes, Division Head, Department of Musculoskeletal Radiology, University of Michigan, Ann Arbor, MI, USA for allowing them to use MR Arthrogram images performed in the department.
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Department of MRI, Jaslok Hospital & Research Centre, 15 Dr. G. Deshmukh Marg, Mumbai-400026
Source of Support: None, Conflict of Interest: None
[Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5], [Figure - 6], [Figure - 7], [Figure - 8], [Figure - 9], [Figure - 10], [Figure - 11], [Figure - 12]