As is the case with most subjects in orthopedics, the best way to start a discussion of the shoulder is to begin with the basic anatomy. This leads to some confusion right from the start because there really is no single structure that is accepted to be the “shoulder joint.” In the shoulder, three bones come together to make two joints, the glenohumeral joint and the acromioclavicular (AC) joint. Technically, there is also a third articulation between the scapula and the chest wall known as the scapulothoracic joint, but pathology in the scapulothoracic joint is rare, so we will limit our discussion to the glenohumeral and AC joints. The three bones that come together to form these two joints are the clavicle, the humerus, and the scapula (Figure 2-1). Of the three bones, the scapula has the most complicated shape, so we will study it in more detail. Figure 2-2 shows the scapula viewed from three different angles: a posterior view, a lateral view, and an anterior view. Each of these views highlights a different feature of the scapula. The posterior view shows the scapular spine, a long, thin bony prominence that terminates as the acromion process. The scapular spine is a subcutaneous bony prominence that is palpable on even the heaviest patients. The lateral view shows the acromion process, the coracoid process, and the glenoid fossa. Looking at the anterior view of the shoulder gives us the best view of the coracoid process and the anterior edge of the acromion. On all of the views, we can see the thin body (blade) of the scapula. The body of the scapula is designed to provide a large surface area for the origins and insertions of various muscles, and, as such, it is broad and flat. The coracoid process projects anteriorly and laterally, like a hooked finger. Several tendons, including the short head of the biceps and the coracobrachialis, attach to the coracoid process.
The two joints formed by the humerus, clavicle, and scapula are stabilized by a series of firm, but flexible, ligaments (Figure 2-3). The ligaments that stabilize the glenohumeral joint are bands of collagen-rich connective tissue that are embedded in the substance of the joint capsule, the thick membrane that originates from the rim of the glenoid socket and inserts around the neck of the humerus. At the point of its origin along the edge of the socket rim, the joint capsule becomes thick and dense. This tissue is known as the labrum. The AC joint has this same system of stabilizing ligaments in its capsule, but it also has an interesting, second set of ligaments that anchor the clavicle to the coracoid process. Since the coracoid process and the acromion are both part of the same bone, anchoring the clavicle to the coracoid process stabilizes the AC joint.
The next layer we encounter as we go from the deep to the superficial anatomy of the shoulder is the deepest set of shoulder muscles: the rotator cuff muscles (Figures 2-4, 2-5, 2-6, and 2-7). There are four rotator cuff muscles: the supraspinatus, the infraspinatus, the subscapularis, and the teres minor (see sidebar). Of the four muscles, the teres minor plays the least important role in shoulder function. If you were born without a teres minor, you may not even notice it. It is rarely involved in shoulder pathology, and I recommend that you forget about it entirely and concentrate on the other three, more important, muscles.
REMEMBERING THE NAMES IS EASY …
Remembering the names of the rotator cuff muscles is easy. Their names essentially tell us where they are located. The drawing on the left-hand side of Figure 2-2 shows the scapular spine on the posterior surface of the scapula. The muscle that originates inferior to the scapular spine is infraspinatus. The muscle that originates superior to the scapular spine is the supraspinatus (see Figure 2-6). The subscapularis muscle gets its name from the fact that it is on the deep surface of the scapula, underneath it and against the chest wall where it is hidden from view. You’d have to lift the scapula up off of the chest wall and look under it to see the subscapularis (Figure 2-A). The teres minor muscle plays a minor role in shoulder function or pathology. We don’t need to remember where it is located.
The next layer of shoulder anatomy contains the subacromial bursa (Figure 2-8). Bursae are interesting structures. They function the way bearings in machines function to lower the friction between articulating surfaces (for a more complete discussion of bursae, please refer to the section on the olecranon bursitis in Chapter 4). The subacromial bursa is interposed between the rotator cuff below it and the acromion directly above it. The final, most superficial, layer of shoulder anatomy that we will discuss is the deltoid muscle (Figure 2-9). This muscle is just below the skin. It originates from the clavicle and acromion and inserts a third of the way down the lateral side of the humerus. The deltoid muscle is big, strong, and important but rarely the cause of clinically relevant pathology.
A LITTLE BIOMECHANICS
In recent years, there has been a significant change in our understanding of the biomechanics of the shoulder. It wasn’t that long ago that we were all taught that the function of the supraspinatus was to accomplish the first 30 degrees of shoulder abduction. In other words, if you were standing, arms at your sides, and you wanted to abduct your shoulder joints and move your arms away from your sides (think of the motion your arms make when you make a snow angel), your supraspinatus muscles would be responsible for the first 30 degrees of that motion. The reason that we thought that is how things worked is that, when patients had large supraspinatus rotator cuff tears, they lost the ability to initiate abduction. If you brought their arm out away from their side for them (past 30 degrees), they could raise it the rest of the way on their own. If they went on to have the supraspinatus tear repaired, the operation restored their ability to initiate abduction. This was observed so consistently among so many patients that it was concluded that the supraspinatus must serve to raise the arm through the first 30 degrees of abduction. We understand now that this is not the case. To understand what’s really going on here, we need to review three important facts about the structure and function of the shoulder. The first fact is that the glenoid, the socket of the ball-and-socket joint of the shoulder, isn’t much of a socket at all. If you compare it to the acetabulum, the socket of the hip joint (Figure 2-B), you will see that it is much more shallow, almost like the surface of a golf tee, just a shallow dimple on which the ball rests. The second fact is that, when a patient’s arm is at their side, the deltoid force vector is essentially vertical (Figure 2-C). The last fact we have to review is that the deltoid attaches to the lateral humerus (Figure 2-D). Figure 2-E shows what our shoulder glenohumeral joints would look like if the glenoid were more of a cup-shaped socket, like the acetabulum of the hip. The center of rotation of the humeral head is marked with a +, and the deltoid force is labeled with the black arrow. In this configuration, the force that the deltoid exerts on the humerus is vertical, but the humerus cannot translate superiorly because it is captured by the roof of the socket. Unable to translate superiorly, the humerus rotates as the force is being applied lateral to the center of rotation (Figure 2-F). In reality, our shoulder glenohumeral joint is shaped more like the drawing in Figure 2-G. The socket is shallow and does not keep the humeral head from translating superiorly when the deltoid contracts with the arm at the side (Figure 2-H). If the arm is placed in greater than 30 degrees of abduction (Figure 2-I), the glenoid is in a position to prevent humeral head translation, and a deltoid contraction results in rotation. We have now come to understand that the rotator cuff muscles act as a unit to hold the ball against the socket of the shoulder, so that the shallow lip on the superior rim of the glenoid can capture the humeral head and prevent superior translation when the arm is at the side and the deltoid contracts (Figure 2-J). An intact rotator cuff enables the deltoid to abduct the shoulder by establishing a pivot point at the ball and socket joint. Without a functional rotator cuff, the patient cannot abduct through the first 30 degrees since the humeral head will just translate superiorly.
Now that we’ve completed a basic review of shoulder anatomy, we can begin our discussion of the shoulder problems you are likely to see in your office practice. It will help if you take the time to read the sidebar on biomechanics (Page 45) since these principles have guided the development of many of the treatment strategies we will discuss.
Of all the shoulder conditions you will encounter in your practice, subacromial space impingement is likely to be the condition you see most often. To understand subacromial space impingement, we first have to understand the subacromial space. Figure 2-10 shows the cross-sectional anatomy of the shoulder. The subacromial bursa lives in a tight space between the acromion (above it) and the rotator cuff (below it). Exactly which rotator cuff muscle is directly under the acromion depends on the position of the shoulder, but in most positions, it is the supraspinatus. Figure 2-11 shows the same set of structures from a lateral view, the way things look when the arm is at the side. The supraspinatus is beneath the acromion when the arm is in this position. Figure 2-12 shows how rotating the shoulder to put the arm out in front of us (forward flexion of the shoulder joint) places the subscapularis beneath the acromion (figure on the left) and how bringing the arm back behind us (shoulder extension) brings the infraspinatus underneath the acromion (figure on the right). These drawings help us appreciate how small the teres minor is, which explains its minor contribution to shoulder function. The drawings also illustrate how difficult it would be to rotate the shoulder into a position where the teres minor would come to reside beneath the acromion. That explains why it is rarely involved in subacromial impingement pathology (see discussion that follows).
The problem in impingement is that the dimensions of the subacromial space narrow as the arm is raised. Under normal conditions, the bursa and rotator cuff glide under the acromion as the arm is raised, but if the muscle or bursa is swollen and inflamed or if a layer of calcium accumulates on the undersurface of the acromion, the subacromial space can become so narrow that the acromion bone actually strikes the bursa or rotator cuff, causing it to swell and become inflamed (Figure 2-13). The more swollen and inflamed it becomes, the more likely it is to impinge against the acromion, and the more it impinges, the more swollen and inflamed it becomes. This cycle of swelling and impinging is what creates the condition of subacromial impingement, sort of like biting the inside of your cheek creates a tender, swollen bump that makes it easier to bite it a second time, which makes it more swollen, and so on. Subacromial space impingement is self-propagating and, as such, can become chronic. It is felt that chronic impingement of the rotator cuff against the acromion is what causes the rotator cuff to fray and eventually tear (see sidebar).
THE GRAY HAIR AND WRINKLES OF THE SHOULDER
Rotator cuff muscle tears are interesting and unique. In most instances, when a muscle in our bodies tears, it is because normal, healthy tissue is exposed to a sudden abnormal force. A tennis player who lunges forward to get a ball and tears his or her calf muscle would be a good example of a typical mechanism for tearing a muscle. Rotator cuff tears are different. Though it is somewhat controversial, our best data indicate that the vast majority of rotator cuff tears are attritional tears, and that they result from chronic abrasive wear of the rotator cuff against the underside of the acromion. Often, there is some element of trauma that is the “straw that breaks the camel’s back. ” This can be something trivial, like lifting a sack of garbage to put it in a dumpster or throwing a ball in a recreational softball game. The trauma does cause the muscle to tear, but instead of a normal muscle tearing under an abnormal force, as was the case in the tennis player example, it is an abnormal muscle (frayed from chronic impingement) tearing when subjected to a normal force. Impingement is common, especially in older patients. As we age, it is natural for extra bone to grow and accumulate on the undersurface of the acromion. This narrows the already critically narrow subacromial space and starts the impingement process. Impingement is so common in people over 60 that it is hard to call it a pathologic condition. Like gray hair and wrinkles, it is a fact of life that most of us will have to face if we are lucky enough to live into our 60s, 70s, and beyond.
So far, we have discussed two of the occupants of the subacromial space, the subacromial bursa and the rotator cuff. The third occupant of the subacromial space is the tendon of the long head of the biceps muscle. This tendon enters the joint through a small gap between the supraspinatus and subscapularis muscles and is in a location that allows it, in certain positions of the shoulder, to impinge against the acromion (Figure 2-14). If the long-head biceps tendon becomes so damaged from impingement that it finally tears, the result is the “Popeye” deformity shown in Figure 2-15. Like most rotator cuff tears, tears of the long-head biceps tendon are typically attritional tears (see sidebar) that result from long-standing impingement against the underside of the acromion. The functional deficits that result from a long-head biceps tendon rupture are minimal (patients only lose about 5% of elbow flexion strength), so surgical repair is seldom recommended.
LONG HEAD BICEPS TENDON RUPTURES: NO BIG DEAL
The reason ruptures of the tendon of the long head of the biceps muscle don’t result in a significant strength defecit has to do with the shape of the muscle. The muscles illustrated below are both biceps (two headed) muscles. If one if the two superior heads of biceps mucle A were to rupture, all of the muscle below it would no longer be attached superiorly, and one would predict a profound (close to 50%) loss of strength. The human biceps looks more like biceps muscle B. If one of the two superior tendons of this mucle were to rupture, most of the muscle mass is still able to exert its force using the remaining tendon and loss of strength is minimal.
Subacromial space impingement is probably easiest to understand if we consider it a continuum of conditions ranging from bursitis to cuff tear arthropathy (Figure 2-16). In its early stages, it may only be the bursa that is inflamed and painful. This is the condition we refer to as subacromial bursitis or, sometimes, just “shoulder bursitis.” If the condition goes on for a longer period of time, the rotator cuff (usually the supraspinatus because it spends the most time under the acromion) or the biceps tendon may become swollen and inflamed. This we know as rotator cuff or biceps tendonitis. If impingement continues, the rotator cuff or biceps tendon may rupture, resulting in a rotator cuff or biceps tendon tear. The final point on the continuum of subacromial impingement is what has been termed “cuff tear arthropathy.” Figure 2-17 shows an anteroposterior (AP) x-ray of a shoulder. The occupants of the subacromial space are not visualized on a plain x-ray, so we cannot tell from this film if the patient has bursitis, a rotator cuff tear, or a completely normal shoulder. Compare the x-ray in Figure 2-17 to the x-ray in Figure 2-18, which shows an x-ray of a shoulder with cuff tear arthropathy. Two major differences between the x-rays are apparent. For one, on the x-ray of the patient with cuff tear arthropathy, the humeral head is superior to its normal position opposite the glenoid socket; second, there is no subacromial space. The humeral head is in direct contact with the underside of the acromion. As we learned in the sidebar on shoulder biomechanics, without the rotator cuff to hold the humeral head against the glenoid socket, the superiorly directed force of the deltoid, even its resting muscle tone, causes the humeral head to translate superiorly. In cases of cuff tear arthropathy, we can make the diagnosis of a rotator cuff tear based on plain x-ray alone. It is with certainty that we can say that the cuff is torn since there is no possible way that a structure as thick as the rotator cuff can exist in the paper-thin space between the humeral head and the acromion on this x-ray. The long-head biceps tendon is likely torn as well, and what’s left of the bursa, if anything, is probably severely inflamed. The appearance of the x-ray also tells us that the rotator cuff tear has been there a long time, and that it is massive. We know the process has been going on for a long time because the humeral head has ground a concavity into the underside of the acromion. Such bony changes take years to occur.
CUFF TEAR ARTHROPATHY, MY DAUGHTER’S JEANS, AND THE REVERSE TOTAL SHOULDER REPLACEMENT
After reading the sidebar on the biomechanics of the shoulder, you understand how important a normal rotator cuff can be to proper shoulder function. In patients with cuff tear arthropathy, there is not enough intact rotator cuff muscle to keep the humeral head pressed tightly against the glenoid socket, and the pivot point for deltoid-mediated shoulder rotation is lost. Attempts to raise the arm result in superior translation of the humeral head instead of shoulder joint rotation. Even at rest, the deltoid muscle tone translates the head of the humerus superiorly into a subluxed position above its normal location opposite the glenoid socket. Over time, the superior translation of the humeral head up against the underside of the acromion wears the underside of the acromion, causing it to have a concave geometry matching the shape of the humeral head. Figure 2-18 shows both the superior position of the humeral head and the abnormally shaped acromion typical in cases of cuff tear arthropathy.
Figure 2-L shows a typical rotator cuff tear on the top and one of these massive, chronic, cuff tear arthropathy tears on the bottom. Rotator cuff tears as big and as chronic as these tears are not surgically repairable. To fix these tears, we would have to be granted three miracles. Miracle number one: We would have to be able to take the leading edge of the torn rotator cuff and somehow stretch it all the way over to the lateral side of the humeral head, where it normally inserts. This would truly be a miracle. The reason it is so difficult is that this isn’t just a tear; there is actually a loss of material. Furthermore, the edges of the tear are frayed and ragged and won’t hold sutures well. I tell patients that this particular type of rotator cuff tear is like the holes on the knees of my teenaged daughter’s favorite pair of jeans (Figure 2-K). There is a large area of missing material, and the edges of the defect are of poor quality. To accomplish a repair, we would first have to trim the edges back to healthy tissue, which would only make the already-too-big tear even bigger. We would then have to bring the edges together to meet, so that we could sew them together. This would be impossible. There just isn’t enough material. Miracle number two: The repaired rotator cuff tissue would have to heal. The sutures used to accomplish the repair will only last so long. Eventually, the only way the repair will work is if the tissue heals. Unfortunately, tissue doesn’t heal well under tension, and this repair would be under massive tension. Miracle number three: If we are granted the first two miracles and we are able to bring the cuff over to the humerus and repair it and it heals, the result may still be poor. A proper, healthy set of rotator cuff muscles is one of the most elegant symphonies of coordinated motion found anywhere in the human body. Like firemen around a fireman’s net, each member of the rotator cuff muscle group adjusts its contractile force so that the tension on the shoulder joint is balanced, and the ball stays centered in the socket regardless of the position of the shoulder and what task it is performing. It is unrealistic to expect a rotator cuff muscle that has been detached and unused for years or decades to perform in such a complicated way, and any patch-type material we use to cover the defect will not have the contractile properties of muscle, so it won’t work to actively center the humeral head. So, for most of the history of the field of orthopedics, we had nothing to offer our patients with cuff tear arthropathy. Some orthopedists attempted to solve the problem by installing a conventional shoulder replacement. Unfortunately, the unopposed, superiorly directed force of the deltoid in a rotator cuff–deficient artificial shoulder is no different from that in a cuff-deficient native shoulder, and the artificial ball also subluxes superiorly out of the artificial socket to rest against the underside of the acromion (Figure 2-M). In an attempt to prevent superior humeral head subluxation, designs of an artificial glenoid with a larger superior lip were considered (Figure 2-N), but when the humeral head would ride up against the superior lip of the device, a tilting force developed that tended to loosen the bond that held the artificial glenoid to the scapula (think of pressing down on the rim of your dinner plate. The plate flips up as a result of the tilting force your fingers apply to the plate). For decades, we struggled with addressing the needs of patients with cuff tear arthropathy and were not able to offer any meaningful solutions until the recent introduction of a new and novel type of shoulder joint replacement called the reverse total shoulder. The operation gets its name from the fact that the ball is placed on the scapula and the socket is placed on the humerus (Figures 2-O and 2-P). As the deltoid contracts, the humerus cannot translate superiorly because the ball bonded to the scapula is there to stop it. The socket-shaped humeral component glides along the surface of the ball, and the shoulder rotates. The procedure is relatively new, and there are concerns about forces at the ball/scapula interface causing premature loosening, but so far the device appears to be performing well without the problems that the lipped glenoid and other designs had experienced.
Figure 2-K.
The author’s daughter’s favorite pair of jeans. The loss of material and frayed edges around the holes in the knees mimic the appearance of the chronic, massive rotator cuff tears seen in patients with cuff tear arthropathy.
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