Injuries of the extensor apparatus of the hand

Anatomical and biomechanical features of the joint

The acromioclavicular joint is formed by the articulation of the distal clavicle and the acromion process of the scapula. Its biomechanical role is to ensure the mobility of the shoulder girdle: the main movements here occur when the patient abducts the elevated arm.

The key factor determining the stability of this joint is the state of its ligamentous apparatus. It is what prevents the pathological displacement of bone structures. The ligamentous complex includes two main structures:

• Acromioclavicular ligament. Located intra-articularly (inside the joint), it closely wraps around the articulation and provides basic fixation.
• Coracoclavicular ligament. Located extra-articularly (outside the joint). It connects the coracoid process of the scapula to the tubercle region on the distal end of the clavicle. This powerful ligament is structurally divided into two portions: conoid and trapezoid.

Any pathological displacement in this joint is a direct consequence of the rupture of one or both of the described ligaments.

Classification of injuries

In traumatology, injuries to this articulation are classified according to two main criteria: the direction of clavicle displacement and the degree of destruction of the ligamentous structures.

By the vector of displacement of the distal end of the clavicle:

1. Supra-acromial dislocations. The most common clinical variant. The distal clavicular fragment is displaced cranially (upward) relative to the acromion.
2. Sub-acromial dislocations. A casuistically rare type of injury where the clavicle goes under the acromion process.

By the degree of destruction of the ligamentous apparatus (for typical supra-acromial forms):

• Subluxation (incomplete dislocation). Occurs as a result of isolated damage to only the acromioclavicular ligament. The coracoclavicular ligament remains intact, which prevents complete displacement.
• Complete dislocation. Is the result of a combined, total rupture of both the acromioclavicular and coracoclavicular ligaments. In this case, the articular surfaces completely lose contact.

Mechanisms of injury

Traumatization of the acromioclavicular complex can occur under the influence of various force vectors. Two main mechanisms are distinguished:

• Direct mechanism. The damaging force (load vector) is directed strictly from top to bottom, with the impact directly on the region of the acromion process of the scapula.
• Indirect mechanism. Occurs through indirect action. It includes axial loading on the abducted shoulder, or lateral impact on the shoulder joint in a position where the arm is adducted to the trunk and internally rotated. In the classical literature of the early 20th century, this specific indirect mechanism was vividly described as "falling on the side while trying to reach into a tailcoat pocket".

Clinical presentation and diagnostics

Physical examination of a patient with suspected injury to this articulation reveals a number of characteristic signs. Patients complain of local pain in the joint area (which, as a rule, is not excessively intense), local edema, and marked tenderness upon palpation. Visual inspection reveals shoulder girdle deformity: the distal end of the clavicle is asymmetrically prominent upward compared to the healthy (contralateral) side.

Specific clinical tests are used to confirm the diagnosis:

• Acromioclavicular test. The physician abducts the patient's shoulder to 90 degrees, then shifts the elbow joint anteriorly, moving it past the midline (sagittal plane) of the trunk. This maneuver creates an isolated load on the affected articulation. In the presence of a ligament tear, the test provokes a sharp, jerky increase in pain.
• "Piano key" sign. An absolutely pathognomonic clinical sign of a complete rupture of the acromioclavicular articulation. The testing technique begins with rigid fixation of the adducted shoulder of the patient, after which the physician applies pressure to the prominent end of the clavicle (it springs back like a piano key).

Class and exam questions

What else to study on this topic

• Detailed anatomy and functions of the conoid and trapezoid portions of the coracoclavicular ligament
• Radiological criteria for assessing the degree of displacement of the distal end of the clavicle
• Technique of execution and interpretation of the "piano key" sign over time
• Conservative and surgical methods for treating supra-acromial dislocations
• Differential diagnosis with fractures of the distal third of the clavicle

Other topics in the "Specialized Adult Traumatology" section

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