Total Knee Arthroplasty (TKA) is a highly successful orthopedic procedure designed to relieve pain and restore function in a severely damaged knee joint by replacing the worn-out surfaces with artificial components. Peripheral neuropathy, which is damage to nerves outside the brain and spinal cord, is a known, though uncommon, complication following TKA. Nerve injury is a possibility because of the close proximity of major nerves to the surgical site. The reported incidence of a significant nerve injury, such as a major motor deficit, ranges from approximately 0.12% to 0.53% of all primary TKA procedures.
Understanding Post-Operative Neuropathy
Patients experiencing post-operative neuropathy typically notice symptoms that go beyond the standard pain and swelling expected after major surgery. These symptoms often include sensations like persistent numbness, tingling, or a burning feeling. In more severe cases, the neuropathy can manifest as muscle weakness or even paralysis in the lower leg and foot.
One of the most concerning motor deficits is “foot drop,” the inability to lift the front part of the foot, which requires the patient to lift their knee higher than normal when walking to prevent dragging the toes. While some mild and temporary numbness around the surgical incision is normal due to the cutting of small sensory nerve branches, persistent or widespread symptoms suggest a more substantial nerve issue that requires medical attention.
The pattern of symptoms helps clinicians determine which nerve has been affected. For example, sensory loss along the outer side of the leg and the top of the foot, often accompanied by foot drop, points toward damage to the common peroneal nerve. Sensory issues isolated to the medial side of the knee and lower leg are more often related to the saphenous nerve. The timing and severity of these post-surgical symptoms are key indicators distinguishing expected recovery from a potential neuropathic complication.
Mechanisms of Nerve Injury During Knee Replacement
Nerve injury during TKA can occur through several distinct mechanisms related to the surgical process itself. One common cause is direct trauma from surgical instruments. Although surgeons take great care, a nerve can be inadvertently stretched, crushed by retractors used to hold tissue aside, or nicked by a saw blade or other instruments during bone preparation or implant placement.
A second mechanism involves the use of a surgical tourniquet, often inflated around the thigh to minimize blood loss. The pressure and duration of this tourniquet can lead to temporary or permanent nerve damage by restricting blood flow, a condition known as ischemia. This lack of oxygen can cause a temporary conduction block in the nerve, known as neuropraxia, which is the most common and mildest form of nerve injury.
The most common major nerve affected is the common peroneal nerve, which wraps around the fibular neck near the knee joint, making it susceptible to mechanical stress. This nerve is particularly vulnerable to stretching when a surgeon corrects a significant pre-existing joint deformity. For instance, in patients with a valgus deformity (knock-kneed), the correction to a straight alignment can severely stretch the common peroneal nerve beyond its physiological limit.
The intentional injury to a nerve is another mechanism, most often involving the infrapatellar branch of the saphenous nerve. This small sensory nerve runs close to the standard surgical incision and is often divided as part of the necessary surgical exposure. While this typically results only in a patch of numbness on the inner side of the knee, in some cases, the severed nerve end can form a painful growth called a neuroma, leading to chronic pain.
Diagnosis and Management of Nerve Damage
The diagnostic process begins with a detailed clinical assessment, including a thorough physical examination to test the patient’s reflexes, muscle strength, and sensation in the affected limb. This examination helps pinpoint the specific nerve or nerves involved and the severity of the motor or sensory loss. If a nerve injury is suspected, further testing is typically ordered to confirm the diagnosis and determine the extent of the damage.
Specialized diagnostic tools include Nerve Conduction Studies (NCS) and Electromyography (EMG). The NCS measures the speed and strength of electrical signals traveling through the nerve, revealing if the signal is slowed or blocked. EMG assesses the electrical activity within the muscles, indicating whether the muscle is receiving proper signals from the nerve or has begun to atrophy due to denervation.
Management of post-TKA neuropathy is initially conservative, as most nerve injuries are mild and resolve spontaneously. For the common form of injury, neuropraxia, recovery can take several months to a year as the myelin sheath regenerates. Immediate care often includes removing external compression, such as tight dressings, and positioning the knee in a slightly flexed position to minimize nerve stretch.
Physical therapy plays an important role in conservative management, helping to maintain muscle function and joint range of motion while awaiting nerve recovery. For patients with foot drop, a brace known as an Ankle-Foot Orthosis (AFO) is often prescribed to support the foot and prevent falls. Medications, such as anti-inflammatory drugs or specific nerve pain medications like gabapentin, may be used to manage discomfort. Surgical intervention is reserved for rare cases where severe symptoms persist without recovery after three to six months, involving nerve exploration, decompression, or surgical repair for transection. The prognosis for post-TKA neuropathy is generally positive, with most patients achieving a complete or near-complete motor recovery within one year.