Grand Rapids Shoulder

Tim Lenters, MD

Shoulder Surgeon in Grand Rapids, MI

Let’s clarify something – it isn’t the “rotary cup”, the “rotator”, or the “rotor”. It doesn’t need replacing. It can’t be changed. Shoulder specialists hear these words so often that we just chuckle a little inside when we hear them. The “rotator cuff” is a collection of muscles and tendons that wrap around the shoulder joint, much like a cuff of a shirt. Although four different muscles contribute to the cuff, their tendons blend together to wrap around the joint in one continuous layer that creates a near complete wrap around the shoulder. As the muscles contract, the humeral head is compressed onto the concavity of the shoulder socket and secured in position (see concavity compression).

Functional Motor Unit

Understanding what creates movement and strength requires one to understand the structural components needed to move one bone in relation to another. Nerves must be intact and function to send an electrical signal to the junction of nerve and muscle. This signal stimulates a muscle to contract. The contraction of the muscle then pulls on the tendon that comes out of the muscle belly. The tendon then pulls on its insertion to bone, which moves the bone.

Weakness can result from dysfunction anywhere along the chain, including the nerve, the neuromuscular junction, the muscle, the tendon, and the insertion onto bone. The predominant cause of shoulder weakness results from an avulsion of a portion of the cuff from bone (a “rotator cuff tear”), usually involving the area of the cuff supplied by the supraspinatus muscle. When the tear is large, it can involve cuff tissue supplied by more than one muscle. But this doesn’t mean that multiple tears are present – it just means that more of the cuff is detached. One can now see that rotator cuff surgery does not involve replacing anything to do with rotors or changing out a rotary. It involves repair of tendon to bone.

However, shoulder surgeons are aware of other issues including nerve injuries/compression, muscle injuries, and overall deconditioning. Often, when patients present with profound weakness of the shoulder, the source is neurological: the nerve supply has been disrupted. Most cuff tears produce more subtle levels of weakness.

Major vs Minor muscles

The major movers of the shoulder joint include the deltoid, pectoralis, latissimus, trapezius, and teres major muscles. The cuff muscles assist with movement, but their major role relates to securing the humeral head to the glenoid via the mechanism of concavity compression. They provide power for rotational movements, and massive tears can lead to significant weakness of internal and external rotation of the shoulder. A strong deltoid can often compensate for even large rotator cuff tears. When profound levels of weakness are present, it can be related to dysfunction of the major movers, especially the deltoid muscle. It’s nerve, the axillary nerve can be injured in trauma. It has been said: “the loss of the deltoid is the loss of the shoulder”. Higher up the chain, the brachial plexus or cervical nerve roots can all lead to significant shoulder weakness.

Don’t be surprised if your shoulder surgeon requests an evaluation by a spine specialist. Neck problems can sometimes lead to painful, weak shoulders.

Summary

Normal strength, one of the essential “S”‘s of normal shoulder function, requires intact nerve, muscle, and tendon. The predominant cause of shoulder weakness is avulsion of the rotator cuff tissue from bone, although nerve dysfunction can lead to more profound loss of strength. The challenge with treating weakness lies in determining the cause of the weakness. The challenge of rotator cuff tear treatment lies in determining which ones should be repaired, and which should be treated without surgery. Future posts will discuss further.