Mechanical leg pain – Pathophysiology

Most episodes of leg pain related to mechanical causes can be divided into chronic overuse injuries and acute traumatic or contact injuries. Overuse injuries usually occur insidiously over time as a result of a repetitive number of relatively small loads. They are characterized by pain, tenderness to palpation, and inflammation. Overuse injuries may produce variable discomfort with daily activities and more severe pain when the activity that produced them, such as running, is attempted. Examples of mechanical pain related to overuse are stress fractures, iliotibial band syndrome, and osteoarthritis.

Traumatic injuries occur as a result of acute massive mechanical overload. Examples include fracture, joint dislocation, and ligament rupture. These injuries are characterized by pain, swelling, ecchymosis, and significant dis-ability. In the majority of cases a thorough history and physical examination will provide the correct diagnosis. Adjuvant testing such as plain radiographs, magnetic resonance imaging (MRI), and computed axial tomography (CT) scanning may be required to verify a diagnosis.

Pathophysiology

Soft tissue injury

Muscle strains represent a tear in the muscle–tendon complex and usually occur at the muscle–tendon junction or occasionally at the muscle–tendon ori-gin from bone. The exact mechanism of injury is still not known but one theory is that the antagonist muscle or muscle group (usually the stronger muscle) undergoes a very forceful contraction while the affected muscle is still in the contraction phase, stretching the contracted muscle to the point of injury. This causes a sudden forced change in the length of the musculotendinous unit, with rupture of the muscle fibers and subsequent bleeding into the tissues. For example, this sequence commonly occurs during sprinting, with flexion of the hip and extension of the knee, which places the muscle under maximal stretch.

Ligaments and other soft tissue components of the joints of the lower extremity are not directly connected to muscle groups and their tendons. They are subject to injury when there are abnormal mechanical stresses placed upon the joint. The stress may be extremely strong, with joint dislocation and severe ligamentous and capsular damage. On the other hand, even minimal stress may produce injury to ligaments and other joint components such as the meniscus of the knee, particularly after previous injury or when the joint is loose.

Fracture

Fractures of the lower extremity are generally associated with severe trauma which may also result in significant soft tissue injury. In patients who are sus-ceptible, such as those with osteoporosis or bone metastases, even minimal trauma may result in fracture. Stress fractures of the pelvis and lower extrem-ity represent fatigue fracture of bone caused by repetitive submaximal stresses over time that exceed the bone’s capacity to remodel in response to stress. The most common cause of stress fracture is an increased level of intensity and duration of impact activities such as running.

Degenerative osteoarthritis

Chronic wear and tear with repeated traumatic insult to the articular surfaces of joints may result in hypertrophy of bone underlying the weight-bearing points in the joint, articular spur formation, loss of joint surface cartilage, and a reactive inflammatory process in the joint space. Chronic pain and loss of joint mobility may lead to secondary muscular atrophy and musculotendinous or arthritic conditions related to antalgic gait and skeletal imbalance.

History and physical examination

As emphasized earlier, the evaluation of pain begins with a detailed history and physical examination. It is imperative to be aware of any history of trauma and the mechanism of injury. In this way any potentially severe injuries that might otherwise be dismissed as trivial at first glance can be identified. Current symptoms including the severity of pain, location, char-acter of the pain (sharp, dull, ache), amount of disability caused by the pain and any prior history of leg pain should be noted. The ability to get to sleep at night and whether the pain awakens the patient are valuable indicators of severity.

Differential diagnostic points must be kept in mind. A history of lower back pain may be associated with complaints of paresthesias and weakness in the leg. In these cases the complaints of sensory loss and pain will usually be in the dermatome corresponding to the level of back pathology. Any history of sys-temic vascular disease or diabetes should lead to inquiry regarding possible vascular claudication. When pain is related to increased activity, the course of resolution after cessation of activity may provide important clues to etiology. For example, vascular claudication resolves after a few minutes of rest, whereas neurogenic claudication may take a minimum of 30 min to dissipate.

Physical examination should always be carried out systematically, compar-ing the normal and symptomatic legs with one another. Visual examination is performed comparing symmetry, areas of swelling, discoloration, previous scars, and alignment. The patient is observed standing and walking to detect any evidence of alignment or gait abnormalities. Range of motion, ligament stability, skin integrity, localized maximal point of tenderness and a thorough neurovascular exam including strength and sensory testing are carried out. In cases where it is difficult to isolate tenderness the patient is asked to point one finger to the most painful area of the leg. A thorough examination of the back should be carried out whenever there are complaints of dysesthesias or weakness.

 

Jean-Paul Marat

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