Traumatic Dislocations

Etiology and mechanism of development

All dislocations are fundamentally divided into two major groups: congenital and acquired. Congenital forms develop during the prenatal period and are associated with impaired development of articular ends (the most striking example being congenital hip dislocation). Acquired forms can be pathological (when the joint is destroyed due to a tumor, tuberculosis, or osteomyelitis) or traumatic.

The primary cause of traumatic displacements is indirect trauma. Its mechanism of action lies in the fact that the traumatic force is applied at a distance from the joint. This creates a specific bone lever, the fulcrum of which is located directly in the area of the articular ends. The classic scenario for this mechanism is a fall on an outstretched hand or sharp hyperextension of the limb. Direct trauma (a blow directly to the joint area) leads to dislocation much less frequently.

Biomechanics of typical displacements

The anatomical structure of each joint strictly dictates the type and direction of displacement during trauma. Knowledge of these patterns is critically important for diagnosis.

• Shoulder joint: The humeral head most commonly shifts anteriorly and inferiorly. This is because the joint capsule is weakest in this zone, and muscular reinforcement is insufficient. Upward displacement of the head is physically impossible, as it is securely blocked by the bony roof formed by the acromial end of the scapula.
• Hip joint: Due to similar biomechanical reasons (specific features of the ligamentous apparatus and capsule), the femoral head most commonly shifts posteriorly relative to the acetabulum.

Clinical presentation and specific symptoms

Clinical diagnosis begins with palpation. The patient experiences acute pain in the affected area, and the physician identifies the articular end in a completely unusual location. A characteristic sign is the «empty joint» sign — a palpable depression of tissues in the zone where the bone should normally be located.

Special attention is paid to examining movements. Active movements are impossible, while passive movements are sharply limited and painful. Two specific symptoms are distinguished:

1. Springy fixation sign: When the physician attempts to move the limb out of its forced pathological position, a distinct springy resistance is felt. As soon as the limb is released, it immediately returns to its initial state.
2. Key sign (piano key sign): This phenomenon is characteristic of a dislocation of the acromial end of the clavicle. Pressing on the protruding bone end returns it to its normal anatomical position. However, when the pressure is removed, it springs right back up.

Classification by duration and pathogenesis of changes

The age of the injury is of absolute critical importance for choosing treatment tactics and the overall prognosis of reduction. Dislocations are classified into three groups:

Why is timing so important? The pathogenesis of local changes over time explains the difficulties of late treatment. First, edema rapidly escalates in the tissues. Shortly after, muscle retraction develops — their persistent contraction, which creates a powerful physical obstacle to reduction. Concurrently, a hematoma forms, which eventually organizes and scars. The conclusion is unequivocal: the more time that has passed since the injury, the more pronounced the degenerative changes in the joint become, and the harder it is to return the bone to its rightful place.

Instrumental and differential diagnosis

The gold standard for confirming the diagnosis remains radiography. Its tasks are not limited to simply stating the fact of displacement. X-rays are necessary to accurately determine the position of articular surfaces and identify concomitant fractures (the presence of bone fragments).

As part of differential diagnosis, it is important for the traumatologist to distinguish other bone tissue pathologies, such as delayed union of a fracture and a false joint (pseudarthrosis). With delayed union, pathological mobility, hyperemia, swelling, muscle atrophy, and sharp pain upon axial loading (e.g., tapping on the heel) are observed. In this case, the radiograph shows sclerosis of the bone fragment ends and a persisting gap between them. If the pathology progresses to the pseudarthrosis stage, the radiological picture changes: complete overgrowth (obliteration) of the medullary canal occurs at the ends of the fragments.

Class and exam questions

What else to study on this topic

• Congenital hip dislocation: pathogenesis and prenatal development
• Pathological dislocations secondary to tuberculosis and osteomyelitis
• Delayed fracture union and pseudarthrosis formation
• Techniques for closed reduction of recent dislocations

Other topics in "CHAPTER 10. GENERAL ISSUES IN TRAUMATOLOGY"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.