Rotator cuff tear patterns: MRI appearance and its surgical relevance

Advances in anatomy, imaging, and arthroscopy have provided opportunities for improved treatment strategies, based on a better understanding of the different patterns of RC tears.

Previous divergent perspectives on and complexity of RC disease have resulted in different types of classification systems [1], most of which are not standardized or universally accepted. The international Society of Arthroscopy, Knee Surgery and Orthopedic Sports Medicine (ISAKOS) group published an expert consensus manuscript trying to establish a guideline to predict patients’ outcome and to assist in therapeutic decision making [2] (Table 1). However, new tear patterns have recently been identified which do not fit in the ISAKOS or classic classifications [3, 4]. These new patterns can be explained by anatomical features that are visualized with MRI. MRI thus remains the gold standard imaging method for diagnosing cuff tears, for depicting features bearing prognostic implications, allowing therapeutic decision-making as well as planning surgery for those potential candidates for surgical reconstruction [2].

The goal of this paper is by providing a comprehensive anatomical and radiological description of the RC tear patterns in MRI. We will also briefly review the most common surgical techniques.

The anatomy of the RC is complex. It is well known that the insertions of the RC are not based on one single insertion, but they have main and secondary insertions where different tendons blend [6, 7]. RC tendons can be divided into an anterior tendon, the subscapularis, and posterior tendons, the supraspinatus, infraspinatus and teres minor.

The supraspinatus (SS) originates from the posterior aspect of the scapula in the SS fossa above the spine of the scapula and inserts in horizontal and anterior directions onto the greater tuberosity of the humeral head. The SS is divided into two components (anterior and posterior) and each of them is subdivided into three parts (superficial, middle, and deep) [3, 8]. The anterior belly of the SS is voluminous and tubular, has a bipennate configuration, and generates greater contractile forces than the posterior belly, partly because of its long intramuscular tendon [3, 9] (Fig. 1). In contrast, the posterior belly muscle is smaller, is unipennate, and its tendon is flatter and wider. The posterior belly generates less contractile forces. Despite the anterior muscle belly being almost double the size of the posterior belly, the ratio for the tendon cross-sectional area between anterior and posterior is 0,9:1. The difference in contractile load together with this discrepancy in tendon size, increases the stress in the anterior belly, making it especially susceptible to myotendinous injuries and tendon injuries [3].

The infraspinatus (IS) is the second RC muscle in size. It arises from the dorsal surface of the infraspinatus fossa of the scapula and inserts onto the greater tuberosity of the humerus, forming a functional unit with the SS tendon. Its fibers run parallel to the teres minor but are separated by a thick fascia.

The main osseous insertions of the SS and IS cuff tendons form a footprint that is a three-dimensional area on the humeral head. The SS insertion has a triangular shape extending from the bicipital groove anteromedially to the top of the greater tuberosity. The IS insertion has a trapezoid shape, is wider, covers the posterior border of the SS superiorly and inserts onto the anterolateral area and the middle facet of the greater tuberosity [10‐12] (Fig. 2). The insertion on this shared footprint suggests that the IS may be more frequently affected in RC tears than we usually thought and could be one of the reasons explaining the high incidence of fatty atrophy of the IS occurring in isolated tears of the SS. The footprint insertions are important in surgical planning to adequately restore shoulder function [12‐14].

Microscopically the fibers of the SS and IS are arranged in five layers, demonstrating the complex biomechanical role of the rotator cuff. Each layer has a different collagen fiber organization and orientation [7, 9]. Some of them can be identified with high-resolution MRI (Fig. 3). Layer 1 is the superficial layer of the coracohumeral ligament. Layer 2 is thicker (3–5 mm) with densely packed collagen fibers while layer 3 is less thick (3 mm) and formed by smaller bundles of collagen with a less uniform organization [8]. The fibers of layer 2 are more elastic, more resistant to rupture and are well vascularized. Conversely, the fibers of layer 3 are inelastic and more prone to tear and are poorly vascularized [8, 15]. The differences between layers 2 and 3 regarding the different movement between them, thickness, tensile properties, orientation of the collagen fibers, vascularization, and stress they support can explain the origin of partial and delaminating tears [4, 15, 16]. In the inferior part of the SS and IS, there is a bundle of connective tissue fibers perpendicular to the long axis of the SS and IS tendons, known as the rotator cable. The rotator cable is a capsular reinforcement formed by an extension from the deep layers of the coracohumeral ligament, this forms the 4th layer of the RC. Anteriorly the rotator cable has two insertions, on the superior facet of the lesser tuberosity and on the anterosuperior edge of the greater tuberosity. The posterior insertion is complex in the area between the middle and inferior facets of the greater tuberosity, between the insertion areas of the teres minor and the insertions of the SS and IS tendons [17]. The area lateral to the rotator cable is called the rotator crescent [17, 18]. The rotator cable acts as a suspension bridge, dissipating load from the rotator crescent which is particularly susceptible to tears, and inhibiting retraction of the SS or IS [18‐20]. The rotator cable can be observed in most high-resolution MRI, especially with Arthro MRI [20] (Fig. 4). Some studies suggest that it is better observed on the ABER views, thanks to the relaxation of the rotator cuff in abduction and external rotation [21]. The last layer, the 5th, is the articular capsule [7].

The subscapularis (SSC) is the largest and most powerful of the rotator cuff muscles [22]. It arises from the anterior scapula and has multiple tendinous bands that merge laterally to insert onto the lesser tuberosity of the humeral head in a broad comma-shaped footprint (Fig. 5). The two upper thirds of the insertion are tendinous and wider with variable attachment onto the lesser tuberosity, bicipital groove and greater tuberosity. The lower third has a muscular insertion on the inferior margin of the lesser tuberosity and in the anterior aspect of the humeral metaphysis [22, 23].

The two upper thirds are stronger, stiffer and subjected to greater tension while the inferior third is mechanically weaker [24, 25]. These factors may explain why some tears may be confined to either the superior or inferior portions of the SSC [22].

The upper fibers of SSC tendon interdigitate with the anterior fibers of SS tendon and the structures of the rotator interval. This anatomical disposition may explain why the SSC tendon tears are frequently associated with massive RC tears or rotator interval injuries [26].

The location of RC tears can be divided according to ISAKOS [2] into posterosuperior, when they affect the supraspinatus (SS), the infraspinatus (IS) or unfrequently the teres minor (TM), and into anterior when they affect the subscapularis (SSC).

SSC tears are different from posterosuperior cuff tears, in their mechanism of injury and management. They are more difficult to recognize at both MRI and arthroscopy and therefore remain underestimated. Although the prevalence of SSC injuries is therefore not well known, it has been reported to be between 20 and 37% [65, 66].

Degenerative tears are the most frequent and begin in the upper third of the SSC and progress caudally and are associated in up to 25% of cases with posterosuperior RC tears [67].

Despite there not being universally accepted classification system for degenerative SSC tears, the one proposed by Lafosse is often used and was endorsed by ISAKOS because it allows differentiation between reparable and non-repairable tears [68]. Type 1 is a partial thickness tear of the upper third at the articular surface only. Garavaglia subclassified this type in 1A when the tear is superficial or 1B when they involve the deeper fibers [66]. In type 2, there is full thickness involvement with complete detachment of the upper third. In type 3, the upper and middle third is involved. Type 4 is a full thickness tear of the tendon of the SSC while the humeral head remains centered in the glenoid fossa, and fatty atrophy is less or equal than Goutallier type 3. Type 5 is like a type 4 lesion, but with anterior subluxation of the humeral head, coracoid impingement and fatty infiltration is equal or greater than Goutallier type 3 (Figs. 19 and 20). The close relationship of the SSC muscle with the anterior fibers of the SS means that type 1 and 2 injuries are frequently associated with biceps injuries and tears of the anterior margin of the SS [66, 67], whereas types 3 and 4 are associated with extensive tears of the RC [68]. Isolated SSC tears are uncommon and are mainly secondary to a traumatic event with hyperextension or external rotation of the abducted arm and occur more frequently near the insertion site on lesser tuberosity [69, 70] (Fig. 21).

The diagnostic yield of MRI for the diagnosis of SSC tears seems to be less compared to the posterosuperior rotator cuff tears, especially for type 1 tears [66]. A combination of axial and sagittal planes together with higher resolution imaging have improved our accuracy. For subtle tears, a small amount of fluid between the lesser tuberosity and the SSC tendon is a useful radiological sign. MR arthrography has a higher sensitivity and specificity for the diagnosis of these types of injuries [71]. In our experience, adding a sequence in the axial plane with external rotation of the shoulder during MR arthrography improves the visualization of the SSC attachment and outlines better the extent of subtle tears.

Large tears of the SSC might involve the anterior fibers of the SS and part of the rotator interval.

In patients with RC tears, a band-like structure can be observed connecting the cranial portion of SSC and the anterior margin of SS through the subcoracoid and subacromial space. This attachment of both tendons creates the appearance of an anterior bridge (“bridging sign”) or comma shape corresponding to a combined tear of the SSC tendon, anterior portion of SS, coracohumeral ligament, and superior glenohumeral ligament adhered to the anterior margin of SS tendon on arthroscopy [26, 72].

Also, this sign is associated with more chronic shoulder pain and more severe rotator cuff tears [26] (Fig. 22).

Chronic tears of the rotator cuff with involvement of the myotendinous unit lead to retraction with subsequent muscle atrophy and fibrosis of the fibers and fatty infiltration in the interfascicular and intrafascicular structure of the muscle. This phenomenon is progressive and irreversible. Deposit of lipids in muscle causes loss of elasticity and diminishes its compliance, making it more difficult to mobilize and repair RC components and increasing the chances of surgical failure [75]. The role of the suprascapular nerve has also been proposed as a factor that increases fatty infiltration of rotator cuff muscles. Retraction of the tendon and muscle induces excessive tension to the nerve, decreasing the propagation of neuromuscular junction potentials and reducing muscle contraction causing nerve stretching and, if chronic, denervation. This suprascapular nerve factor might explain the frequently seen fatty atrophy of the infraspinatus in supraspinatus tears [76].

Fatty infiltration of the rotator cuff is a major prognostic factor for the structural and functional outcome of rotator cuff surgery and is associated with a higher rate of retear [75, 76]. After repair, atrophy of the muscle belly may improve but the degree of fatty infiltration does not change [77].

It is important to assess the degree of fatty infiltration of the different RC muscles. Different systems have been described. Goutallier created a classification system with five stages [78] for assessing fatty infiltration of supraspinatus muscle based on CT (Grade 0 = normal muscle, Grade 1 = some fatty streaks; Grade 2 = less than 50% fatty muscle atrophy, i.e., more muscle than fat; Grade 3 = 50% fatty muscle atrophy, i.e., equal muscle and fat; Grade 4 = more than 50% muscle atrophy, i.e., more fat than muscle) (Fig. 23). Later, Fusch et al. validated this system to be used in MRI. Fatty atrophy is evaluated on the sagittal plane at the level of the coracoid process on T1-weighted images [79]. Other classification systems based on MRI have been used, such as the tangent sign described by Thomazeau. This is an easy method that categorizes volume loss of the SS on the sagittal plane as minimal, moderate, or severe with the disadvantage that it only considers the upper middle of the SS [80].

It is important to asses terer minor integrity and its fatty atrophy. The functional importance of teres minor is critical, especially in IS full thickness tear, since it becomes the only external rotator of the shoulder. A tear or severe fatty atrophy of the teres minor can affect postoperative clinical outcomes in patients with rotator cuff pathology and with latissimus dorsi tendons transfers [44, 81].

The definition of massive tears is still under controversy. Initially, De Orio and Coefield defined massive RC tears as those tears being equal or greater than 5 cm in the coronal oblique plane [82]. Later, Gerber et al. considered massive rotator cuff tears when there were full thickness tears of two or more tendons [83]. Burkhart defined them based on shape and mobility of the tear and considered anatomical and functional factors that cannot be assessed by imaging [84]. Gerber’s definition is more consistent and has implications on functional and surgical outcome and is preferred by the surgeons [2].

Massive RC tears, which are more frequent in posterosuperior rotator cuff tendons [85] result in structural changes in the myotendinous unit with loss of elasticity, scarring, adhesion, retraction, fatty infiltration, and hypovascularity [86, 87]. Significant loss of mechanical and cohesive stability of the glenohumeral joint ensues. The force imbalance produced by massive tears generates superior migration of the humeral head. This migration is enhanced by the superior force exerted by the deltoid muscle as the posterosuperior RC do not longer act as a hinge and inferior force vectors, causing a clinical condition known as pseudoparalysis [88]. This severe impact of massive RC tears on shoulder biomechanics leads to a complex pattern of progressive secondary osteoarthritis [88, 89]. Therefore, early treatment is indicated.

The “term RC tear arthropathy” is also variable but the three major features are as follows: RC massive tear, glenohumeral degenerative changes, and superior migration of humeral head [85]. It is still unknown why some patients with RC tears progress to RC arthropathy and others do not. Several theories from crystal deposition induced arthropathy to mechanical factors have been proposed [87].

Hamada et al. described in 1990 the radiological findings of rotator cuff arthropathty and classified it into five consecutive stages [90]. Although they were described on plain films, MRI is reliable for diagnosing and staging rotator cuff arthropathy [89] (Fig. 24).

They suggested that massive cuff tears will progress to RC arthropathy through a set of pathomechanics. Initially, the deltoid contraction during elevation causes superior migration of humeral head decreasing the acromiohumeral interval. The mechanical friction of the humeral head and the acromion and the increasing stress into the long head of the biceps tendon leads to a tear of the tendon and this subsequently contribute to the narrowing of acromiohumeral interval. If the superior migration continues, the humeral heads abuts the acromion leading to acromial acetabularization, femoralization of the humeral head, chondral damage, bone loss, and glenoid remodelation and retroversion.

In the last few years, several salvage surgeries, including augmentation techniques, have been advocated and used for the treatment of irreparable massive tears, especially in patients with high functional demand.

Rotator cuff tears are a common cause of pain and disability, being variable in location, tear pattern, functional impairment, and reparability.

Radiological findings of the RC tears have a decision-making impact for patients, depending on tear pattern, extension, retraction and fatty atrophy.

We recommend following ISAKOS consensus guidelines as they use radiological features in the decision-making process for partial, complete, and massive rotator tears. However, some specific patterns of tears, such as delaminated or myotendinous, do not fit in this classification and should be reported in addition.

MRI can depict almost all of the various features of rotator cuff tears.

Radiologists should be familiar with the different surgical techniques and their decision-making factors to be able to deliver a useful radiological report and to help manage the patient.