A shoulder dislocation occurs when the head of the humerus is forcibly displaced from the glenoid fossa, the shallow socket of the shoulder blade. This injury, involving the glenohumeral joint, is a common orthopedic trauma often resulting from high-impact events or falls. The forceful displacement of the bone puts surrounding soft tissues under extreme stress, and it can cause damage to nearby nerves. This neurological complication is a serious and frequent consequence of shoulder dislocation that requires immediate medical evaluation.
How a Dislocated Shoulder Harms Nerves
The mechanism of nerve injury during a shoulder dislocation is primarily biomechanical, involving two main types of forces. The initial displacement of the humeral head violently pulls on surrounding structures, causing a stretching or traction injury to the nerves. This sudden, excessive elongation can disrupt the nerve fibers and their protective myelin sheaths.
The second mechanism involves direct compression from the humeral head itself. As the bone is driven out of the socket, it can press directly against nearby nerve tissue, crushing it against the bone or surrounding ligaments. Nerve injury can also occur during the reduction process if the maneuver used to relocate the shoulder is overly aggressive. The overall severity of the dislocation, including the degree of displacement and any associated fractures, often correlates with the extent of the resulting nerve damage.
Identifying the Affected Nerves
The nerves most susceptible to damage are those located closest to the joint capsule, with the Axillary Nerve being the most commonly compromised structure. This nerve courses just beneath the shoulder joint and is responsible for both sensation and movement in the shoulder area. Studies indicate that some form of nerve damage may be present in up to 65% of shoulder dislocations, although many cases are mild and temporary.
More severe dislocations can affect the larger network of nerves known as the Brachial Plexus. This complex bundle originates in the neck and supplies the entire arm. Injury to the Brachial Plexus can involve multiple nerves, such as the musculocutaneous or radial nerves, leading to a wider range of sensory and motor deficits extending down the arm and forearm.
Recognizing Symptoms of Nerve Injury
Damage to the Axillary Nerve typically presents as a combination of sensory and motor impairments in the affected arm. Sensory loss is noted as numbness, tingling, or a pins-and-needles sensation on the lateral, or outer, aspect of the shoulder. This area is often described by clinicians as the “regimental badge area” due to the distinct patch of skin supplied by the Axillary Nerve.
Motor deficits manifest as weakness or an inability to contract the muscles the nerve controls, particularly the deltoid muscle. Patients will have difficulty initiating abduction, which is the action of lifting the arm away from the body. Because the deltoid is the primary muscle for this movement, its paralysis results in a functional limitation. Reporting these specific symptoms to medical staff helps differentiate nerve injury from other complications like a rotator cuff tear.
Diagnosis and Prognosis
The initial diagnosis of nerve injury is made through a physical examination where a clinician tests the patient’s muscle strength and sensation. If neurological symptoms are present, specialized electrodiagnostic tests are ordered to assess the nerve’s function and the extent of the damage. Electromyography (EMG) and Nerve Conduction Studies (NCS) measure the electrical activity in the muscles and the speed at which signals travel along the nerves. These tests pinpoint the specific nerve involved and determine the severity of the injury.
Most nerve injuries resulting from a shoulder dislocation are classified as neurapraxia, a temporary conduction block where the nerve structure remains intact. These milder injuries generally have a favorable prognosis and recover spontaneously, showing improvement within a few weeks to months. Recovery typically spans from three to six months, though it can take longer for full strength to return. More severe injuries, known as axonotmesis, involve damage to the nerve fibers themselves. These may require a longer recovery period or surgical exploration and repair if no improvement is seen after four to six months. Prompt reduction of the dislocated shoulder minimizes long-term nerve injury.