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Cables, crescents and suspension bridges: the unique anatomy of the rotator cuff cable
Chris Mallac explores the anatomy of the ‘rotator cuff cable’ and the associated ‘crescent’. Why is this cable important when assessing and treating rotator cuff injuries, and what are the implications for clinicians?
The unique anatomy of the shoulder rotator cuff, is a genuine concern for the clinician dealing with large tears of the supraspinatus. However, not all large rotator cuff tears cause pain and dysfunction in the patient. This is due to the presence of the ‘suspension bridge’ ligament known as the rotator cable¹.
Anatomy and biomechanics
The analogy of the rotator cable acting having a suspension bridge-like function was first identified by Clark and colleagues in 1990, and expanded by Burkhart and colleagues in 1993¹,2. The rotator cable as it is colloquially known, is in fact officially called the ligamentum semicirculare humeri. It has also been called the ‘circular fibers system;’and the ‘transverse band’³.
The rotator cable
The rotator cable is a curved structure, which is an extension of the coroco-humeral ligament in an anterior direction (see figure 1). It also blends anteriorly with the subscapularis and/or anterior supraspinatus tendons. The cable runs from anterior to posterior under the supraspinatus tendons and the infraspinatus tendons to blend with the posterior infraspinatus and teres minor tendons. Importantly, it connects both the infraspinatus and supraspinatus to the head of the humerus, and its function is to transmit forces across the rotator cuff complex. This allows the rotator cuff to remain functional in the event of certain types of tears to the rotator cuff by acting as a bridge between the tendons¹’⁴(discussed below).
Figure 1: Structure of the rotator cable (anterolateral view)
The relatively recent ‘discovery’ of the shoulder cable is due to the fact that traditional shoulder imaging tests, such as magnetic resonance imaging (MRI) and ultrasound, lack the sensitivity to consistently detect it. Therefore, orthopedic specialists have historically relied on observational evidence in subjects undergoing shoulder surgery. More recently (in 2017), a group of researchers in Estonia carefully dissected the shoulders of 21 cadavers and found evidence of the rotator cable in all shoulders examined⁵. The structure was described as an
“Organized, parallel-running bundle of connective tissue fibers, forming a curved capsular–ligamentous structure in the superolateral part of the glenohumeral joint capsule”(Rahu 2017, pp. 2049)⁵.
They also determined that this ‘cable’ is tightly connected to the deep layer of the supraspinatus tendon and inserts firmly with the supraspinatus tendon. Specifically, it starts anteriorly at both the anterior part of the superior facet of the greater tubercle and the superior portion of the lesser tubercle of the humerus, creating a roof over the intertubercular groove anteriorly. It then courses posteriorly (in the superior part of the shoulder joint capsule), inserting posteriorly between the insertion areas of the infraspinatus, teres minor and supraspinatus on the humerus, in the area of the middle and inferior facets of the greater tubercle.
The rotator crescent
The rotator cable surrounds the more distal and lateral rotator crescent (see figure 1), a relatively avascular area which tears easily⁶. This crescent area represents the more distal attachments of the blended supraspinatus and infraspinatus tendons onto the head of the humerus. In a study by Burkhart et al (1993)¹, researchers dissected 20 cadavers aged 60-85, and found the following quantitative measurements of the crescent and cable:
- The greatest anteroposterior dimension of the crescent was 41.35mm.
- The greatest mediolateral dimension of the crescent was 14.08mm.
- The average thickness of the crescent was 1.82mm.
- The average width of the rotator cable was 12.05mm.
- The average thickness of the rotator cable was 4.72mm.
- The average ratio of cable to crescent was 2.59.
Stress shielding with the ‘suspension bridge’
The large ratio between the cable and the crescent (on average, the cable is 2.59 times thicker than the crescent) gives credence to the idea that the rotator cuff is able to transfer stress around the entirety of the cuff. This is because the relatively thick cable provides a ‘stress shielding’ effect to the thinner tissue within the ‘crescent’ ¹’⁴. The now famous suspension bridge analogy, first presented by Burkhart and colleagues in 1993, describes how the uppermost suspension cable transfers force to the towers of the suspension bridge¹(see figure 2).
Figure 2: Suspension bridge and stress shielding (superior view)
The suspension bridge analogy: the relatively thick rotator cable provides a ‘stress shielding’ effect to the thinner tissue within the crescent by distributing forces across the span of the humeral head.
This shielding analogy holds true in all but younger and more athletic shoulders. In young athletes, the crescent often appears thickened and therefore, not ‘stress shielded’ in the same way as a thinner crescent. This younger type of rotator cuff structure is known as a crescent dominant rotator cuff¹. In shoulders of older athletes, the crescent is much thinner and therefore the cable is better able to shield it from loads. This latter type is called a cable dominant rotator cuff⁴.
Tears of the rotator cuff
Injuries to the rotator cuff that do not involve the rotator cable may present as completely functional due to the ‘suspension bridge’ mechanism described above⁵. This suggests that the cable-crescent anatomy and the analogy of the load-bearing suspension bridge makes the location of a rotator cuff tear more important than the size of the tear in terms of its effect on shoulder function. Indeed, rotator cable tears may be biomechanically much more debilitating than a tear that involves ‘only’ the rotator crescent.
A recent study in Poland looked at the morphology of the rotator cable in cadavers that had both normal rotator cuff anatomy, and those that had suffered tears to the rotator cuff – either partial or full thickness³. The researchers concluded certain types of capsular-sided cuff tears may split the tendon away from the rotator cable and thus make the ‘suspension bridge’ ineffective. However, partial tears, and even full-thickness tears that do not involve the rotator cable do not seem to affect the function of the ‘suspension bridge’ (confirming the original thoughts of Burkhart and colleagues in 1993¹). Force can still be transferred from the damaged rotator cuff muscle to the humeral head; therefore, this type of patient would not completely lose rotator cuff function.
This explains how some patients who have full thickness tears of the rotator cuff can still have essentially normal and pain-free function¹’³’⁴. Furthermore, Namdari et al (2014)⁷propose that the anterior part of the supraspinatus tendon and its cable attachment has more potential to cause functional disability of the shoulder if damaged. Although not well described, it appears as if the anterior part of the cable has a more important role to play in the function of the ‘suspension bridge’.
As stabilizers, the supraspinatus, infraspinatus and subscapularis provide a force couple to depress the humeral head and prevent it from migrating superiorly during abduction. With large rotator cuff tears, the humeral head easily displaces superiorly towards the acromion process. However, if the rotator cable remains intact then the humeral head remains centered, since the force couple between subscapularis and infraspinatus remains balanced⁸.
Diagnosis of cable vs. crescent tears
It is not possible for the clinician to distinguish between cable tears and crescent tears. The nature of the injury can only be inferred based on how the patient presents functionally. Also, the age of the patient may be an indicator. That is, young athletic patients with radiological evidence of a rotator cuff tear who cannot abduct their shoulder will most likely have a ‘crescent dominant’ rotator cuff tear, which has made the ‘suspension bridge’ useless. Therefore, if a young athlete with even a less than 50% tear is no better after four to six weeks of conservative rehabilitation, consider that they may be structurally unable to regain function without surgery.
Conversely, if the patient is older and has evidence of a full thickness tear but the cable is intact, then they can present with almost normal function and respond successfully to conservative treatment. Numerous studies show variability in the sensitivity of both MRI radiography and diagnostic ultrasound in identifying cable or crescent dominant tears⁹⁻¹³. Ultimately, time and response to rehabilitation provide insight into the location of the tear.
The rotator cuff incorporates a unique anatomical feature known as the rotator cable. Integrated into the tendons of the rotator cuff muscles, the cable functions as a stress shield across the humerus in the same way a suspension bridge transmits loads across to its supporting towers. Large tears of the rotator cuff tendons may therefore not debilitate the function of the shoulder and the presentation of the patient may be optimistic. Tears that involve the function of the rotator cable will most likely require surgical management. Therefore, the prognosis of rotator cuff tears may rely more on the location of the tear in relation to the cable and crescent and not so much on the size of the tear.
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