Shoulder Dislocations: Glenohumeral Joint, Acromioclavicular Joint
Normal Anatomy
The shoulder joint is the second largest and most mobile joint in the human body and can be easily understood if divided into three layers. The first layer is the deepest layer made up of the bones that form the foundation. There are three bones consisting of the arm bone (humerus), the shoulder blade (scapula), and the collar bone (clavicle). There is a rounded head on top of the arm bone called the humeral head and a saucer shaped surface of the shoulder blade called the glenoid which complements the humeral head and together they form the shoulder joint. A layer of cartilage (blue area) covers the articular surfaces of the humeral head and glenoid to provide a cushioned surface which allows for smooth pain free shoulder motion. These three bones unite to form three joints, which are held together by the second layer which keeps the bones from separating during shoulder movement, allowing the bones to function as a unit.
The second layer is comprised of collagen tissue fibers that attach to the bones on both sides of the joint and form the joint capsule. Within the joint capsule, are dense cordlike collagen bundles that unite to form ligaments that in strategic locations further enhance the stability of the joint by resisting the forces created when we perform our routine daily activities.
The rim of the glenoid is encircled by a rubbery fibro-cartilage tissue called the labrum. The labrum is an anchor point for the ligaments of the shoulder capsule and the long head of the biceps tendon which attach to it creating a suction seal mechanism when the humeral head is in contact with the glenoid. This sealing mechanism adds another joint stabilizing effect. The three shoulder bones form there joints called the Glenohumeral (GH) joint, Acromioclavicular (AC) joint, and the sternoclavicular (SC) joint.
The third layer in the shoulder is the muscle layer. Within the muscle layer, there are several potential spaces that carry the arteries which supply the bodies’ nutrition and nerves that deliver the commands sent by the brain and spinal cord for muscle movement. The most important muscle group in the shoulder is called the rotator cuff muscles. They are comprised of four muscles that begin (originate) in the shoulder blades’ flat surfaces and attach to the arm bone in an area called the greater tuberosity footprint as a common tendon unit. The four tendons formed by the four rotator cuff muscles interdigitate with one another forming a protective mechanism that prevents rotator cuff tendon tears from quickly spreading once they occur. The interdigitation also allows the four muscles to work together as a unit, carefully balancing the shoulder joint against the forces of the deltoid muscle to create precise shoulder movement. The deltoid muscle, biceps muscle and other muscles about the shoulder that balance the shoulder blade, called the scapular stabilizers complete the shoulder muscles that make up the third layer and together create a dynamic balanced force which allows for normal function. The major function of the rotator cuff muscles is to provide shoulder joint dynamic stability, shoulder joint motion and strength to the arm.
Pathology
A shoulder dislocation is classified according to the direction of the dislocation (Anterior, posterior or multi-directional), the amount of force it took to dislocate the shoulder (Traumatic or Atraumatic), and whether it is accompanied by a fracture (fracture dislocation). When a shoulder dislocation presents, the mechanism of injury including the position of the arm when the dislocation occurred and the amount of force involved in the dislocation at impact are important parameters to understand which treatment category is selected. The age of the patient and the patient activity level are also important details in selecting an operative vs. non-operative approach.
When a major force causes an anterior shoulder dislocation, the shoulder ligaments and the labrum most commonly are torn in the front of the joint between the 3 to 6 O’clock position on the right shoulder and between the 6 to 9 O’Clock position when looking at the left shoulder. When the labrum and capsule rip as a unit, the injury is called a Bankart lesion when the dislocation is in the front and a reverse Bankart lesion when it dislocates in the back. The dislocation also typically produces a small compression fracture in the humeral head called a Hillsachs lesion with an anterior dislocation and a reverse Hill sacks lesion with a posterior dislocation. The dislocation can at times also create a fracture in the anterior glenoid rim.
When a shoulder dislocation is the result of a minor injury in a young patient between the ages of 20-25 years of age, a multidirectional instability (MDI) is suspected and occurs in patients whose joint anatomy predisposes the joint to dislocation due to congenital deficiencies in the ligaments and labrum. These deficiencies include ligaments that are more elastic and a labrum that is very thin in the front that fails to act as a bumper and fails to seal to the humerus with the suction cup mechanism that is normally the case. It is common for both of these anatomic differences to occur together and once a dislocation occurs, recurrent dislocations often result. The patients with multidirectional loose shoulders, on close examination are also found to have loose joints or hyperlaxity in other areas of the body including the elbow and knee (hyper extend), and the ability for the thumb to bend towards the forearm without difficulty.
A related condition causing an unstable feeling in the shoulder is called a SLAP lesion. The acronym stands for Superior Labrum Anterior Posterior Lesion which describes a labral injury occurring in front and behind the long head of the biceps tendon origin. This injury can occur from either a pulling arm injury or following a compression-sliding injury.
A Shoulder ACJ Dislocation occurs when the ligaments of the Acromioclavicluar joint are disrupted. This is most commonly the result of a direct blow to the shoulder. There are two sets of ligaments that hold the ACJ together. They are called the ACJ ligaments and the coracoclavicular ligaments. These ligaments allow the collar bone to have stability and limited motion when the shoulder blade moves as if follows the arm bone. These ligaments restrict superior, inferior, translational and rotatory forces. When both sets of ligaments are torn, the collar bone displaces from the shoulder blade’s anterior extension called the acromion. The normal and torn ligaments of the ACJ are depicted above. There are six patterns of ACJ tears and type III, IV, V and VI involve the complete tear of both sets of ligaments and are distinguished in the direction that the collar bone displaces. These are the types that require reconstruction if they develop chronic pain.
Treatment
The Treatment of a shoulder dislocation is the repair of the second layer restraints, including the Bankart lesion or tear of the labrum and capsule that can tear individually. This is done either through an open approach or through an arthroscopic procedure. The arthroscopic approach uses tiny incisions, a camera system inserted in the shoulder joint and instruments inserted through the various small incisions to fix the tear. The use of small suture anchors which are tiny screws with special strong thread attached to the anchor are placed in the bone and the suture is tied over the torn tissue in exactly the position it belongs. Initial healing of the repair takes 6-8 weeks and we typically use a sling during this period with a passive motion home exercise program. During the beginning of the third month, an active exercise regimen is begun and in the beginning of the fourth month, strengthening is begun.
When a multidirectional unstable shoulder is encountered, rehabilitation and changes in normal activities to reduce the incidence of recurrent dislocation is initiated. The rehabilitation program focuses on building up the dynamic stabilizers of the shoulder which are the rotator cuff muscles. If a six month trial of therapy is unsuccessful, then surgery is considered. Surgery for recurrent dislocations has several reconstructive surgical options that should be considered depending on what prior treatment that has been done and what is the anatomic problem. The goal is to treat the recurrent dislocations allowing normal function but also to prevent humeral head arthritis from developing secondary to humeral head cartilage injury from the recurrent dislocations. Above is a picture of a loose joint capsule and ligaments which are arthroscopically tightened (Capsulorrhaphy). The repair is allowed to heal with limited motion for the first 8 weeks coupled with an isometric strengthening program followed by progressive strengthening in all directions after eight weeks.
When a recurrent dislocations have worn the anteroinferior Glenoid rim or when the capsule that was either repaired, or shifted and imbricated and has failed, then widening the surface of the anteroinferior glenoid rim, called a Laterjet/Bristow procedure shown above, is an option that is very successful. It is not our first surgical choice since it is a reconstructive procedure that does not restore normal anatomy but in Europe, surgeons often use this option as a first choice because of its high success rate. This procedure is done through a cosmetic anterior shoulder incision on an outpatient basis.
The treatment of an ACJ chronic dislocation (Grade III-IV) is called a Weaver Dunn Procedure. This involves using the Coracoacromial ligament that is close by and transferring it into the lateral end of the collar bone to replace the ligaments of the coracoclavicular ligaments which prevent the collar bone from migrating superiorly. This procedure is done in conjunction with the resection of a small part of the lateral end of the collar bone.