Rotator Cuff Relevant Anatomy and Mechanics.

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The acromion, the coracoid and the coracoacromial

Centers of ossification

These centers of ossification are usually united by age 22. When these centers fail to unite, the ununited portion is referred to as an os acromiale. This condition may have been first recognized by Schar and Zweifel (Schar and Zweifel, 1936) in 1936, as was mentioned by Pettersson. (Pettersson, 1942) Grant (Grant, 1972) found that 16 of 194 cadavers aged over 30 years demonstrated incomplete fusion of the acromion; the condition was bilateral in 5 subjects and unilateral in 11 subjects. In a large review of 1000 radiographs, Liberson (Liberson, 1937) found unfused acromia in 2.7 per cent; of these, 62 per cent were bilateral. Most commonly the lesion is a failure of fusion of the mesoacromion to the meta-acromion. He found the axillary view to be most helpful in revealing the condition. The size of the unfused fragment may be substantial, up to five by two centimeters. (Neer, 1972) Resection of a fragment this large creates a serious challenge for deltoid reattachment.

Norris and coworkers (Norris, Fischer, 1983) and Bigliani and associates (Bigliani, Norris, 1983) have pointed to an association of cuff degeneration and unfused acromial epiphysis. Mudge and coworkers (Mudge, Wood, 1984) found that 6 percent of 145 shoulders with cuff tears had an os acromiale; whereas Liberson found a 2.7 incidence of this finding in unselected scapulae. (Liberson, 1937) The statistical and clinical significance of this association remains unclear.

An additional anatomical feature of importance is the acromial branch of the thoracoacromial artery. This artery runs in close relation to the coracoacromial ligament and often is transected in the course of an acromioplasty and coracoacromial ligament section.

The coracoid arises from two or three ossification centers. (Samilson, 1980). It provides the medial attachment site for both the coracohumeral and coracoacromial ligament. In that their muscle bellies lie medial to it, the neighboring supraspinatus and subscapularis tendons must be able to glide by the coracoid with their full excursion during shoulder movement. Scarring of one or both these tendons to the coracoid can inhibit passive and active shoulder motion. While the coracoid does not normally contact the anterior subscapularis tendon, forced internal rotation, particularly in the presence of a tight posterior capsule, can produce such contact due to obligate translation. (Gerber, Terrier, 1985, Harryman, Sidles, 1990)

The coracoacromial ligament spans from the undersurface of the acromion to the lateral aspect of the coracoid and is continuous with the less dense clavipectoral fascia. It forms a substantial part of the superficial aspect of the humeroscapular motion interface (see figures 2 and 3). This ligament may be thought of as the spring ligament of the shoulder, maintaining the normal relationships between the coracoid and the acromion. Separation of these two scapular processes has been observed on sectioning this ligament in cadavers. (Flatow, Raimondo, 1996)

The coracoacromial arch is the inferiorly concave smooth surface consisting of the anterior undersurface of the acromion and coracoacromial ligament. It provides a strong ceiling for the shoulder joint along which the cuff tendons must glide during all shoulder movements (see figures 2 and 3). Passage of the cuff tendons and proximal humerus under this arch is facilitated by the subacromial-subdeltoid bursa, which normally is not a space, as is often shown on diagrams, but rather two serosal surfaces in contact with each other, one on the undersurface of the coracoacromial arch and deltoid and the other on the cuff. These sliding surfaces are lubricated by bursal surfaces and synovial fluid.

The recognition of the gliding articulation between the arch and the cuff is not new. Renoux et al credited Ludkewitch, who in 1900 recognized that the proper functioning of the "scapulohumeral articulation" requires the presence of a "secondary socket" which extends the glenoid fossa of the scapula above, in front and behind, and of which the coracoacromial arch forms the ceiling. (Renoux, Monet, 1986) In 1934 Codman stated that the coracoacromial arch was an auxiliary joint of the shoulder and that its roughly hemispheric shape was "almost a counterpart in the size and curvature of the articular surface of the true joint." He referred to the "gleno-coraco-acromial socket". (Codman, 1934b) His belief in the importance of the coracoacromial arch was great enough to state that "the coracoacromial ligament has an important duty and should not be thoughtlessly divided at any operation." Wiley has pointed to the severe superior instability when the ceiling of the shoulder is lost in association with cuff deficiency. (Wiley, 1991) Kernwein et al in 1961 stressed the importance of the "suprahumeral gliding mechanism" consisting of the coraco-acromial arch on one side and the rotator cuff and biceps tendon on the other separated by the subacromial bursa. ( Kernwein, Roseberg, 1961) They believed that these two opposing, gliding surfaces and interposed bursa constituted a fifth joint which contributed to shoulder motion. DePalma, in 1967, also recognized the intimate relationship between the arch and the structures below it. (DePalma, 1967) He referred to the arch, together with the head of the humerus, the rotator cuff and subacromial bursa, as the "superior humeral articulation."

Matsen and Romeo described the humeroscapular motion interface as an articulation (see figures 2 and 3) between the cuff, humeral head and biceps on the inside and the coracoacromial arch, deltoid and coracoid muscles on the outside (Matsen, Lippitt, 1994) and measured up to 4 cm of gliding at this articulation in normal shoulders in vivo.

Recent investigations (Burns and Whipple, 1993, Flatow, Raimondo, 1996, Flatow, Soslowsky, 1994, Regan and Richards, 1989, Wuelker, Plitz, 1994, Ziegler, Matsen III, 1996) have pointed to the importance of contact and load transfer between the rotator cuff and the coracoacromial arch in the function of normal shoulders, including the provision of superior stability. Because there is normally no gap between the superior cuff and the coracoacromial arch, the slightest amount of superior translation compresses the cuff tendon between the humeral head and the arch. Superior displacement is opposed by a downward force exerted by the coracoacromial arch through the cuff tendon to the humeral head. Ziegler et al (Ziegler, Matsen III, 1996) demonstrated this "passive resistance" effect in cadavers by showing that the acromion bent upwards when a superiorly directed force was applied to the humerus in the neutral position. The amount of acromial deformation was directly related to the amount of superior force applied to the humerus; the load being transmitted through the intact superior cuff tendon. Furthermore these authors found that the amount of superior humeral displacement resulting from a superiorly directed humeral load of 80 N was increased from 1.7 mm to 5.4 mm when the cuff tendon was excised (p < .0001) (see figures 4 and 5). These results indicate that

1. the intact superior cuff tendon is subject to compressive loading between the humeral head and the coracoacromial arch and
2. that the presence of this tendon provides passive resistance against superior displacement of the humeral head when superiorly directed loads are applied.

Flatow et al (AAOS 1996) also noted that, in a dynamic cadaver model, the presence of the supraspinatus tendon limited superior translation of the humeral head, even when there was no tension in the tendon from simulated muscle action.

The spacer effect of the superior cuff tendon is evident in comparing shoulders with intact cuffs (see figure 6) with those in which the superior tendon is deficient (see figure 7).

Both Ziegler et al and Flatow et al cautioned that the superior stability of the shoulder is dependent on an intact coracoacromial arch. Surgical sacrifice of the arch can lead to severe superior stability.

Changes in the coracoacromial arch have been described in association with cuff disease (Soslowsky, An, 1994 July) along with variations of acromial shape.(Bigliani, Norris, 1983, Neer, 1972) Bigliani and colleagues (Bigliani, Morrison, 1986) studied 140 shoulders in 71 cadavers. The average age was 74.4 years. They identified three acromial shapes: Type I (flat) in 17 per cent, Type II (curved) in 43 per cent, and Type III (hooked) in 40 per cent. Fifty-eight per cent of the cadavers had the same type of acromion on each side. Thirty-three per cent of the shoulders had full-thickness tears, of which 73 per cent were seen in the presence of Type III acromia, 24 per cent in Type II, and 3 per cent in Type I. The anterior slope of the acromion in shoulders with cuff tears averaged 29 degrees, slightly more than the slope of those without cuff tears which averaged 23 degrees. The clinical significance of this relatively small difference is not known. A number of other authors have reported that patients with cuff defects are more likely to have hooked or angled acromia. (Morrison and Bigliani, 1987, Toivonen, Tuite, 1995, Tuite, Toivonen, 1995) Nicholson et al (Nicholson, Goodman, 1996) demonstrated on a review of 420 scapulas that spur formation of the anterior acromion was an age-related process such that individuals under age of 50 had less 1/4 the prevalence of those over 50 years of age. The status of the cuffs of these shoulders is unknown.

Although these data indicate a strong association between aging, the presence of cuff tears, and alterations of acromial contour, it has been unclear whether the change in acromial shape was caused by or resulted from the cuff defect, or whether both were consequences of aging. As pointed out by Neer, (Neer, 1972) disease of the rotator cuff causes characteristic changes on the undersurface of the coracoacromial arch. In a remarkable study, Ozaki et al (Ozaki, Fujimoto, 1988) correlated the histology of the acromial undersurface with the status of the rotator cuff in 200 cadaver shoulders. Cuff tears which did not extend to the bursal surface were associated with normal acromial histology, whereas those which extended to the bursal surface were associated with changes in pathological changes in the acromial undersurface. They concluded that most cuff tears are related to tendon degeneration and that acromial changes are secondary to pathology of the bursal side of the cuff. These results are similar to those reported Fukuda et al. (Fukuda, Hamada, 1990)

Recent studies suggest that type II and III acromia are acquired, rather than being developmental. (Yazici, Kapuz, 1995) In that most acromial "hooks" lie within the coracoacromial ligament (see figure 8), it seems likely that they are actually traction spurs in this ligament (analogous to the traction spur seen in the plantar ligament at its attachment to the calcaneus) (see figure 9). The traction loads producing this hook may result from loading of the arch by the cuff and may be increased with increasing dependency on the coracoacromial arch for superior stability in the presence of cuff degeneration. (Flatow, Raimondo, 1996, Flatow, Soslowsky, 1994, Ziegler, Matsen III, 1996) The concept of the "hook" as a traction phenomenon was first forwarded by Neer over 25 years ago. (Neer, 1972) More recently, Putz and Reichelt (Putz and Reichelt, 1990) reported that three quarters of 133 operative specimens of the coracoacromial ligament showed chondroid metaplasia near the acromial insertion, suggesting that this metaplastic area becomes the acromial "hook" by enchondral bone formation. (Ogata and Uhthoff, 1990) Because this "hook" lies within the ligament and points toward the coracoid (see figure 8), it seems unlikely that it would jeopardize the passage of the cuff beneath the coracoacromial arch (see figure 9). Even in the severest cases of cuff tear arthropathy, the undersurface of the coracoacromial arch commonly presents a smooth articulating concavity (see figures 9 and 10).

In view of the forgoing, it is instructive to consider the humeroscapular articulation as consisting of two concentric spheres, the humeral head sphere and the sphere represented by the inferior surface of the coracoacromial arch. Together these two spheres enhance both shoulder stability and the surface available for scapulohumeral load transfer (see figure 11). (Codman, 1934b) Normally, the spheres of the humeral head and coracoacromial arch share the same center. The difference in radius of the two spheres (R and r in figure 11) is provided by the thickness of the rotator cuff which serves as a spacer. In the presence of posterior capsular tightness, shoulder flexion and/or internal rotation cause obligate anterosuperior translation of the humeral head and loss of the concentricity of the two spheres (see figure 12). (Cofield and Simonet, 1984, Harryman, Sidles, 1990) As a result, the convex cuff-covered head is forced against the anterior undersurface of the concave coracoacromial arch, rather than rotating concentrically beneath it (see figure 11). In the presence of degeneration of the cuff tendon, the shoulder may lose the concentricity of the humeral head and coracoacromial arch spheres (see figure 13). Taken together, these observations reinforce the shoulder's need for

1. normal posterior capsular laxity,
2. a smooth, concentric and congruent coracoacromial undersurface, and
3. a normally thick and uniform cuff interposed between the humeral head and coracoacromial arch.

The acromioclavicular joint. Osteophytes from the acromioclavicular joint may encroach on the space normally occupied by the cuff tendons (see figure 14). In a series of 47 patients with arthrographically confirmed supraspinatus tendon ruptures, Peterson and Gentz (Peterson and Gentz, 1983) found that 51 per cent had distally pointing acromioclavicular joint osteophytes. A similar incidence was found in their series of 170 autopsy specimens with cuff defects. The incidence of distally pointing acromioclavicular osteophytes in normal shoulders was 14 per cent and 10 per cent in the clinical and cadaver studies, respectively. It is recognized, however, that degenerative changes in the cuff and degenerative changes in the acromioclavicular joint may coexist in the aging population without the former being causally related to the latter. It is furthermore recognized that acromioclavicular osteophytes may be sufficiently medial that they not jeopardize the cuff.

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