Neurogenic and Muscular Causes
The most devastating injury to a muscle is the loss of its nerve supply. Losses of the innervation of the deltoid, supraspinatus, and infraspinatus can be congenital (Erb’s Palsy), inflammatory (brachial neuritis), degenerative (cervical radiculopathy), traumatic (penetrating or stretch), compressive (a ganglion in the spinoglenoid notch), or iatrogenic (dissection or screw placement). These conditions along with myopathies, such as facioscapular muscular dystrophy, must be considered when the shoulder is weak.
The day this paragraph was written, we saw a patient sent in for a cuff repair. He had been in a water-skiing accident and landed on the water in a way that pressed his head to the left and his right shoulder down. At the moment of impact he experienced a shock down his right arm and noted severe weakness of his right shoulder. His physician sent him for an MRI that showed a partial tear of his supraspinatus. On exam in our office he had, six weeks after injury, atrophy and weakness of his spinati, deltoid, and biceps – a classic Erb’s type injury.
Electromyography, nerve conduction velocities, and muscle biopsies may be needed to sort out these diagnoses.
Loss of the Humeral Fulcrum
If the humeral head does not remain centered during muscle contraction, the forces intended to cause rotation will cause translation. As an example, the shoulder with posterior instability: when the arm is elevated forward, the humeral head slips out the back, losing its fulcrum. In this situation, valuable muscle excursion is consumed by displacement of the head and the strength of elevation is lost. The effects of loss of centering are also seen with the cuff and superior labral deficient shoulder in which contraction of the deltoid produces ascension of the humeral head rather than abduction
However, if the glenoid concavity is intact, the humeral head may remain stabilized in the glenoid in spite of superior cuff deficiency. This is why many shoulders with cuff tears remain functional.
Alternatively, if the coracoacromial arch remains intact, the humeral head of the cuff deficient shoulder may be secondarily stabilized in a new position, superiorly displaced against the arch.
Thus in the presence of cuff deficiency, it is vitally important to protect and maintain the coracoacromial arch. Without it, the humeral head will escape anterosuperiorly when the deltoid contracts – a complication that can result from acromioplasty and resection of the coracoacromial ligament.
This phenomenon is nicely demonstrated in this short video clip of a patient before surgery.
The treatment of anterosuperior instability may require a reverse total shoulder.
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Here are some videos that are of shoulder interest
Shoulder arthritis - what you need to know (see this link).
How to x-ray the shoulder (see this link).
The ream and run procedure (see this link).
The total shoulder arthroplasty (see this link).
The cuff tear arthropathy arthroplasty (see this link).
The reverse total shoulder arthroplasty (see this link).
The smooth and move procedure for irreparable rotator cuff tears (see this link).
Shoulder rehabilitation exercises (see this link).
