Literature review March 2019 - Cables and Crescents!

Intro 

The rotator cable was first described in detail by Burkhart et al 1993 in his landmark paper titled “The Rotator Crescent and Cable: An anatomic description of the Shoulder’s Suspension Bridge” (https://www.ncbi.nlm.nih.gov/pubmed/8305096).

This piece of anatomic investigation has become a classic in shoulder circles for orthopaedic surgeons and physical therapists alike. It succinctly describes the anatomic configuration of the cable and crescent and also its relevance in symptomatic rotator cuff tears.

Let’s break it down…

The above picture illustrates the rotator cable and crescent. The cable is a thick projection of the supraspinatus and infraspinatus (C). The crescent is a thinner structure that attaches to the humerus (B). Together these two structures are vital in the functionality of the shoulder complex.

Quick snapshot

20 cadavers were dissected and their cuff configuration measured and recorded.

The rotator cable and crescent were present in 100% of specimens.

The rotator cable was shown to be significantly thicker than the crescent (2.6 times thicker on average).

The rotator crescent was thinner, weaker and had less vascular supply.

Let's discuss

The study confirmed a stress shielding relationship between the cable and crescent. The cable was shown to function as a suspension bridge and was able to distribute force to the humerus bone in an asymmetrically larger fashion than the crescent. Meaning, the cable could compensate for a pathological crescent and maintain a functional shoulder joint.

An interesting outcome of this paper was the relative difference in cable-crescent relationship between younger and older shoulders. Younger shoulders tend to be crescent dominant and older shoulders tend to be cable dominant. Therefore, as we age we become more reliant on the rotator cable to disperse force to the humerus to trigger movement of the shoulder. This is likely due to the increase in rotator cuff pathology that occurs with age. Ageing is the biggest risk factor in the development of a rotator cuff tear BUT they are more often asymptomatic than symptomatic (https://www.ncbi.nlm.nih.gov/pubmed/24403741).

This raises the intriguing question, does location matter more than size when it comes to a rotator cuff tear? Possibly, yes.

Does a disruption of the cable system of the rotator cuff have greater consequences than disruption to the crescent? Yes.

This becomes relevant more so, when we think of why some people with seemingly large and comprehensive pathology of the rotator cuff can still easily raise their arm into the air without pain and also exhibit adequate strength on testing? It is possible that many of these patients exhibit an intact cable system with the tear location being located in the thin crescent tissue.

Hsu and Keener 2015 reported that most rotator cuff tears initiate in the rotator crescent, due to the relative reduced vascularity and weakness of the crescent tissue compared to the cable (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4695395/).

Imagine the rotator cable as a suspension bridge. In between the 2 pillars that run vertically to the head of the humerus is where the crescent is located. The dark shaded horizontal line is the cable. Now let's perform a thought experiment; if there was a tear in the rotator crescent, would there still be fibres of the rotator cuff that projected to the humerus via the cable? The answer is yes. As you can see from the picture, the cable provides fibrous connections to the humerus that circles around the crescent. This allows for continuity of force transfer from the cable to the humerus DESPITE potential deterioration of the crescent.

Clinical implication

It appears the location of the rotator cuff tear is paramount in how we choose to manage it. Whilst size does matter, it ain't everything! (some of us will be relieved to hear).

A rotator cuff tear that disrupts the cable will likely significantly alter the biomechanics of the shoulder joint, producing weakness, reduced ROM and possibly pain. It may also result in fatty infiltration to the rotator cuff muscles, which is associated with poor outcomes. In this instance, I would consider a timely referral to an orthopaedic surgeon for review.

If you are dubious about the location of the tear, please get on the phone and make a call to the radiologist who interpreted the scan, they are often receptive and willing to chat specifically about tear location.

As alluded to earlier, location is important, but it's not the only thing that matters. You also need to consider; the size of the tear, mechanism of injury (traumatic Vs non-traumatic), age of the patient, psychosocial factors (kinesiophobia, expectations, beliefs etc.), and surrounding rotator cuff health (any other tears, fatty infiltration, atrophy).

Below is a table that neatly demonstrates some PHYSICAL factors that determine how we manage a cuff tear (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4827371/).

In closing, the rotator cable functions as a suspension bridge that can adequately disperse force around the crescent to enable a functioning shoulder EVEN in the presence of a full thickness tear of the rotator crescent. Therefore, location of a tear is vital when determining how to manage a person with a symptomatic rotator cuff. If in doubt, please engage and discuss with radiologist. If a disruption to the rotator cable is identified, consider a timely referral to our surgical colleagues. If cable is in tact, I would urge a concerted period of conservative management, consisting of progressive resistance training and education.

Next review, I will focus primarily on psychosocial factors associated with outcomes of rotator cuff pathology.