Last Updated: February 10, 2005
Strength is essential to carry out the functions of the shoulder.
About shoulder weakness
For normal function each muscle must be healthy conditioned securely attached and coordinated.
Weakness of the shoulder can come from deficits in coordination nerve muscle or tendon. Often weak shoulders will respond to a gradually progressive strengthening program. If shoulder weakness does not respond to these exercises it may be due to a rotator cuff problem or a nerve injury. Here we will focus our attention the evaluation and management of the most common mechanical cause of shoulder weakness rotator cuff failure.
There is a typical age distribution of patients presenting for evaluation of full thickness rotator cuff defects.
A typical history for degenerative cuff fiber failure in an older individual reveals an insidious onset of weakness of flexion and external rotation perhaps punctuated by episodes of "bursitis" or "tendonitis." Failure of weakened tendon tissue may not produce much in the way of pain bleeding or swelling. The shoulder may have been treated with steroid injections with some relief of discomfort but without improvement in strength. More acute incremental losses of strength from tear propagation may follow lifting or falls.
A greater injury is required to tear the cuff of individuals at the younger end of the age distribution. A history of sudden eccentric loading such as trying to support a falling load or trying to cushion a fall with the arm is commonly given by younger patients with cuff tears. Traumatic glenohumeral dislocations in individuals over the age of 40 have a strong association with rotator cuff tears. These traumatic cuff tears may also involve the subscapularis producing weakness in internal rotation.
Characteristic elements of the history of other common causes of shoulder weakness include:
- long thoracic nerve palsy: posterior protrusion of the scapula on attempts to elevate the arm;
- cervical radiculopathy: pain on top of the shoulder with radiation down the arm below the deltoid tubercle weakness of the biceps diminished biceps reflex and sensory changes on the lateral forearm;
- suprascapular neuropathy from brachial neuritis: acute onset of pain lasting several weeks followed by profound weakness of external rotation;
- suprascapular neuropathy from traction: external rotation weakness following an injury in which the shoulder was forced down and the neck forced to the opposite side (may be part of a full Erb's palsy); and
- suprascapular neuropathy from compression/entrapment: insidious onset of external rotator weakness. Facioscapulohumeral muscular dystrophy is suggested by the atraumatic onset of bilateral symmetrical weakness of the scapular musculature.
The Simple Shoulder Test provides a set of data for characterizing some functional impairment from rotator cuff tears. It is evident that sleeping on the affected side placing the hand behind the head lifting eight pounds and throwing overhand are particularly compromised by cuff tears.
Substantial information bearing on the reparability of a rotator cuff defect can also be determined from the history. Acute tears in younger healthy individuals without prior shoulder disease are likely to be repairable. Long-standing tears associated with major weakness in older patients carry a poor prognosis. The prognosis for a durable repair is even worse if the history reveals local or systemic steroids smoking or difficulties in healing previous injuries or surgeries. The surgeon can also determine preoperatively the patient's goals and functional expectations for surgical treatment to see whether these are reasonable in view of the likely prognosis.
Chronic rotator cuff tears are accompanied by atrophy of the spinatus muscles.
Signs and symptoms
Subtle atrophy can be seen most easily by casting a shadow from a light over the head of the patient. Rupture of the long head of the biceps is frequently evident on inspection of shoulders with rotator cuff tears. Defects in the cuff can often be palpated by rotating the proximal humerus under the examiner's finger placed at the anterior corner of the acromion. The defect is usually just posterior to the bicipital groove and medial to the greater tuberosity. Crepitance on rotation of the arm elevated to shoulder height may result from the abrasion of torn tendon margins against the coracoacromial arch a positive "abrasion sign". A boutonniere deformity is evident when no cuff can be palpated over the humeral head. Chronic massive cuff defects may present with anterosuperior instability of the humeral head on attempted elevation of the arm. This may be particularly severe after previous surgical compromise of the coracoacromial arch. Cuff tear arthropathy is indicated by bone on bone crepitance when the humeral head is rotated beneath the coracoacromial arch even in the unelevated position.
Three isometric tests are used to evaluate the strength of the different components of the cuff. Weakness or effort-limiting pain on isometric testing is considered a positive "tendon sign". The supraspinatus is challenged by isometric flexion of the internally rotated arm which is elevated 90 degrees in the plane of the scapula. The infraspinatus is challenged by isometric external rotation with the arm in neutral rotation at the side. The subscapularis is challenged by isometric internal rotation pushing the hand away from the waist in the posterior midline. The size of the cuff tear can be estimated by physical examination. Partial tears tend to demonstrate relatively more pain with minimal loss of strength. Small tears usually compromise only the function of the supraspinatus. Large tears involve the infraspinatus and compromise external rotation. Massive tears compromise the subscapularis and weaken internal rotation.
Shoulders with incomplete thickness cuff lesions often manifest limitation of motion particularly in flexion internal rotation and cross-body movement due to selective tightness of the posterior capsule.
The examination of a patient with a weak shoulder must include the neck and brachial plexus. Placing the head in extension and rotating the chin to the affected side usually exacerbates symptoms of cervical radiculopathy. The neurologic examination tests the cutaneous distribution of the nerve roots from C5 to T1. The biceps reflex and the triceps reflex help to screen C5/6 and C7/8 respectively. The next component of the neurologic examination requires recognition of the segmental innervation of joint motion:
- Abduction C5 adduction C6 7 and 8.
- External rotation C5 internal rotation C6 7 and 8.
- Elbow flexion C5 and 6 elbow extension C7 and 8.
- Wrist extension and flexion C6 and 7.
- Finger flexion and extension C7 and 8.
- Finger adduction/abduction T1.
A set of screening tests checks the motor and sensory components of the major peripheral nerves:
- the axillary nerve (the anterior middle and posterior parts of the deltoid and the skin just above the deltoid insertion);
- the radial nerve (the extensor pollicis longus and the skin over the first dorsal web space);
- the median nerve (the opponens pollicis and the skin over the pulp of the index finger);
- the ulnar nerve (the first dorsal interosseous and the skin over the pulp of the little finger); and
- the musculocutaneous nerve (the biceps muscle and the skin over the lateral forearm).
The long thoracic nerve is checked by having the patient elevate the arm 60 degrees in the anterior sagittal plane while the examiner pushes down on the arm seeking winging of the scapula posteriorly. The nerve of the trapezius is checked by observing the strength of the shoulder shrug. Lesions of the suprascapular nerve produce weakness of elevation and external rotation without sensory loss.
Other diagnostic techniques
Standard radiographs are of limited assistance in evaluating shoulder weakness. Small avulsed fragments of the tuberosity may be seen in younger patients with cuff lesions. Chronic cuff disease may be accompanied by sclerosis of the undersurface of the acromion or traction spurs in the coracoacromial ligament from forced contact with the cuff and the humeral head. In large cuff tears the head of the humerus may be subluxated upwards toward or against the undersurface of the acromion. In cuff tear arthropathy the humeral head may have lost the prominence of the tuberosities (become "femoralized") and the coracoid acromion and glenoid may have formed a deep socket (become "acetabularized").
A number of different studies are available for imaging the rotator cuff. The single contrast arthrogram can reveal full thickness cuff defects by revealing leakage of injected contrast material from the joint into the subacromial subdeltoid bursa. Magnetic resonance imaging can reveal some information about the tendon and muscle. Ultrasonography can reveal the thickness of the various components of the cuff and the extent of cuff defects. Each of these tests adds expense to the evaluation of the patient.
Resources can be conserved by not ordering imaging tests unless it will change the management of the patient. Patients under the age of 40 without a major injury are unlikely to have significant cuff defects; thus cuff imaging will not be helpful in their evaluation. At the other extreme patients with weak external rotation and atrophy of the spinatus muscles whose plain radiographs show the head of the humerus in contact with the acromion do not need cuff imaging to establish the obvious diagnosis of a rotator cuff defect. Finally the management of patients with nonspecific shoulder symptoms and an unremarkable physical examination is unlikely to be changed by the results of a cuff imaging test. In summary cuff imaging is usually not needed where a cuff tear is very unlikely (a 35-year-old with the minimally traumatic onset of shoulder pain) or where it is very likely (a 70-year-old with gradual onset of shoulder weakness spinatus atrophy and radiographic evidence of contact between the head of the humerus and the acromion). The primary indication for cuff imaging is to establish the diagnosis in situations where it would affect treatment such as a 47-year-old with weakness of flexion and external rotation after a major fall on the outstretched arm.
Electromyography can be an important diagnostic test for the patient with shoulder weakness in the absence of cuff lesions. It is particularly helpful in younger patients with a history suggestive of cervical radiculopathy or suprascapular nerve lesions and a physical examination showing neurological signs.