Knee Dislocation: Understanding Common Peroneal Nerve Injury, Symptoms, and Management

What is the most common nerve injured in knee dislocation?

The most common nerve injured in a knee dislocation is the common peroneal nerve, primarily due to its anatomical course and superficial position as it wraps around the fibular head, making it highly susceptible to stretch or compression injuries during the traumatic event.

Understanding Knee Dislocation

A knee dislocation is a severe and often limb-threatening injury characterized by the complete displacement of the tibia relative to the femur at the knee joint. Unlike a patellar (kneecap) dislocation, a true knee dislocation involves the main joint surfaces. These injuries typically result from high-energy trauma, such as motor vehicle accidents or falls from significant heights. Given the immense forces involved, knee dislocations are frequently associated with extensive damage to multiple structures, including ligaments (often multiple), menisci, articular cartilage, and crucially, neurovascular structures.

The Most Commonly Injured Nerve: The Common Peroneal Nerve

Among the various nerves that traverse the lower limb, the common peroneal nerve (also known as the common fibular nerve) is overwhelmingly the most frequently injured nerve in the context of a knee dislocation. Its susceptibility is a critical concern, as damage to this nerve can lead to significant functional impairment and long-term disability if not promptly recognized and managed.

Anatomy and Vulnerability of the Peroneal Nerve

To understand why the common peroneal nerve is so vulnerable, it's essential to trace its anatomical path:

• The common peroneal nerve originates as one of the two main terminal branches of the sciatic nerve (the other being the tibial nerve) in the posterior thigh.
• It descends along the lateral aspect of the popliteal fossa (the hollow behind the knee).
• Crucially, it then courses superficially around the neck of the fibula, just below the knee joint, before dividing into its two main branches: the superficial peroneal nerve and the deep peroneal nerve.

This superficial trajectory around the rigid fibular head makes it particularly exposed and susceptible to injury during the violent forces of a knee dislocation. Mechanisms of injury include:

• Stretch Injury: The most common mechanism, occurring when the knee joint is hyperextended or severely dislocated, stretching the nerve beyond its physiological limits.
• Compression: Direct compression against the fibular head by displaced bones or surrounding soft tissues.
• Direct Trauma: Less common, but can occur from a direct blow or laceration.

Signs and Symptoms of Peroneal Nerve Injury

Recognizing common peroneal nerve injury is crucial for timely intervention. The symptoms primarily affect the muscles responsible for foot dorsiflexion (lifting the foot) and eversion (turning the sole outwards), as well as sensation to the top of the foot and lateral shin. Key signs include:

• Foot Drop: This is the hallmark sign, characterized by an inability to actively lift the front part of the foot (dorsiflexion). When walking, the foot may drag, leading to a characteristic "steppage gait" where the individual lifts their knee high to clear the foot off the ground.
• Weakness in Foot Eversion: Difficulty or inability to turn the sole of the foot outwards.
• Sensory Deficits: Numbness, tingling (paresthesia), or burning sensations on the top (dorsum) of the foot, the first web space between the big toe and second toe, and/or the lateral aspect of the lower leg.
• Difficulty with Toe Extension: Weakness in extending the toes.

Diagnosis of Peroneal Nerve Injury

Diagnosis of peroneal nerve injury typically involves:

• Clinical Examination: A thorough neurological assessment, including testing muscle strength (manual muscle testing for dorsiflexion and eversion), evaluating sensation in the affected dermatomes, and checking reflexes.
• Imaging Studies: While X-rays and MRI are essential for assessing the extent of skeletal and soft tissue damage from the knee dislocation, they do not directly visualize nerve function. However, they can identify structures that may be compressing the nerve.
• Electrophysiological Studies: Nerve conduction studies (NCS) and electromyography (EMG) are often performed once the acute phase has passed (typically 3-4 weeks post-injury). These tests can confirm the presence and severity of nerve damage, help differentiate between different types of nerve injury (e.g., neurapraxia, axonotmesis, neurotmesis), and provide prognostic information.

Management and Prognosis

Immediate management of a knee dislocation prioritizes rapid reduction of the joint and a comprehensive assessment of neurovascular status, as vascular injury (e.g., popliteal artery damage) is a limb-threatening emergency. For peroneal nerve injury:

• Conservative Management: For less severe injuries (e.g., neurapraxia, a temporary conduction block), conservative management is often initiated. This includes splinting (e.g., an ankle-foot orthosis or AFO) to prevent foot drop and protect the foot, along with physical therapy to maintain range of motion, prevent muscle atrophy, and facilitate nerve recovery.
• Surgical Intervention: If there is evidence of complete nerve transection (neurotmesis) or if there is no sign of recovery after a reasonable period (typically 3-6 months), surgical exploration, nerve repair (suturing the severed ends), or nerve grafting (using a segment of another nerve to bridge a gap) may be considered.
• Prognosis: The prognosis for peroneal nerve recovery after knee dislocation varies widely depending on the severity and type of nerve injury. Mild stretch injuries may fully recover over weeks to months. More severe injuries, involving axonal damage, have a poorer prognosis, and full recovery is not always achieved, potentially leading to persistent foot drop and sensory deficits.

The Critical Importance of Prompt Medical Evaluation

Given the high incidence of neurovascular complications, any knee dislocation is considered a medical emergency. Immediate assessment by an orthopedic surgeon is paramount to:

• Reduce the dislocated joint to restore alignment and reduce tension on neurovascular structures.
• Thoroughly assess vascular integrity (e.g., checking pulses, ankle-brachial index) to rule out popliteal artery injury, which requires urgent surgical repair.
• Assess neurological function, specifically looking for signs of common peroneal nerve damage.

Failure to promptly diagnose and manage these associated injuries can lead to devastating long-term consequences, including chronic pain, permanent disability, or even limb loss.

Conclusion

The common peroneal nerve stands out as the most vulnerable and frequently injured nerve in the context of a knee dislocation. Its superficial anatomical course around the fibular head makes it highly susceptible to stretch and compression during these high-energy traumatic events. Awareness of the characteristic signs and symptoms—especially foot drop and sensory changes—is critical for both healthcare professionals and individuals recovering from such an injury. Prompt medical evaluation, accurate diagnosis, and appropriate management are essential to optimize recovery and mitigate the potential for long-term functional impairment.