Total Knee Arthroplasty (TKA), commonly known as total knee replacement, is a highly successful orthopedic procedure performed to relieve pain and restore mobility in a severely damaged knee joint. This surgery involves resurfacing the ends of the femur and tibia with metal and plastic components. While generally safe, the procedure carries a risk of complications, including damage to the surrounding nervous tissue, known as peripheral neuropathy. Peripheral neuropathy is damage to the nerves outside of the brain and spinal cord, leading to symptoms like weakness, numbness, or pain. While TKA can cause these nerve issues, they are a recognized complication that is often temporary.
Prevalence of Post-Surgical Neuropathy
The incidence of a new, clinically apparent nerve injury following Total Knee Arthroplasty is relatively low, typically reported to be in the range of 0.12% to 0.4% of cases. Most cases involve a mononeuropathy, meaning only a single nerve is affected. Clinically significant, permanent nerve damage is rarer still, and most reported cases of nerve palsy are transient.
The common peroneal nerve is the most frequently affected nerve, accounting for the majority of major post-TKA neuropathies. Persistent neuropathic pain, which is distinct from a nerve palsy, can be more common, affecting a percentage of patients months or years after the operation. The low rate of actual nerve injury confirms that major motor or sensory deficits are uncommon following knee replacement surgery.
Mechanisms of Nerve Injury During Knee Replacement
Nerve injury during TKA can occur through three main mechanisms: direct trauma, excessive stretching or traction, and compression or ischemia.
Direct Trauma
Direct trauma involves accidental cutting or burning of the nerve tissue during the surgical procedure itself. This can happen with a surgical saw blade, a drill, or from the heat generated by electrocautery instruments used to control bleeding.
Traction or Stretching
Traction or stretching of the nerve is a frequent cause of injury, particularly in patients undergoing correction of a significant knee deformity. When the surgeon manipulates the limb to realign the joint, the nerves are stretched. This is especially true for the common peroneal nerve in cases of severe valgus (knock-kneed) deformity, as it is anatomically vulnerable where it wraps around the head of the fibula near the knee joint.
Compression or Ischemia
Compression can occur from prolonged use of a tourniquet during the operation, leading to temporary ischemia or lack of blood flow. Post-operatively, swelling, fluid accumulation, or a hematoma can also compress a nerve as it travels through a tight space. While the common peroneal nerve is the most susceptible to major injury, the infrapatellar branch of the saphenous nerve is almost always affected because it must be cut during the standard surgical incision, resulting in a localized patch of numbness near the scar.
Identifying Symptoms and Diagnosis
The symptoms of post-surgical neuropathy depend on which nerve is injured and whether the damage affects motor function, sensory function, or both. Injury to the common peroneal nerve typically results in “foot drop,” which is the inability to lift the front part of the foot, leading to a dragging gait. This motor weakness is often accompanied by sensory symptoms like numbness or tingling (paresthesia) along the outer calf and the top of the foot.
If the infrapatellar branch of the saphenous nerve is affected, the primary symptom is a patch of numbness on the medial (inner) side of the knee below the kneecap. Other general symptoms of nerve irritation can include sharp, shooting, or burning pain. These symptoms may appear immediately after the anesthesia wears off or develop hours to days later, with the mean onset for new neuropathy being around two days following the operation.
Diagnosis begins with a physical examination, assessing muscle strength, reflexes, and the extent of sensory changes. To confirm the diagnosis and determine the severity of the damage, specialized electrodiagnostic tests are often performed. These tests include Nerve Conduction Studies (NCS), which measure how quickly electrical signals travel through the nerve, and electromyography (EMG), which assesses the electrical activity of muscles to see if they are receiving proper nerve signals.
Treatment and Prognosis for Nerve Damage
The management of nerve injury following TKA is determined by the severity of the damage, but initial treatment is typically conservative. For mild cases of nerve irritation, known as neurapraxia, the nerve sheath remains intact, and the injury is expected to resolve spontaneously. Conservative measures include observation, physical therapy to maintain muscle function, and the use of an ankle-foot orthosis (brace) to manage foot drop and prevent tripping.
Medications are often prescribed to manage the discomfort associated with nerve pain, such as specific nerve pain medications or anti-inflammatory drugs. The prognosis for nerve recovery is generally favorable, as most nerve injuries related to TKA are temporary and resolve over time. Recovery can be a slow process, with most patients seeing improvement within weeks to a year, though some may require up to 18 months for full recovery.
The degree of recovery correlates with the initial severity of the nerve damage; patients with incomplete palsies have a higher rate of full recovery compared to those with complete loss of function. In rare instances where a nerve is severely damaged or fails to show signs of recovery after several months, surgical intervention, such as nerve decompression or a nerve repair procedure, may be considered to relieve pressure or reconnect the damaged nerve.