Total Knee Replacement: Nerves Affected, Risks, and Recovery

What Nerves Are Cut During Total Knee Replacement?

During a total knee replacement (TKR) surgery, the infrapatellar branch of the saphenous nerve is almost invariably cut or damaged due to its anatomical location within the surgical field, leading to sensory changes around the kneecap. While other major nerves are typically avoided, they can be at risk of injury from stretching, compression, or, rarely, direct trauma.

Introduction to Total Knee Replacement (TKR)

Total knee replacement, also known as total knee arthroplasty, is a highly effective surgical procedure designed to alleviate pain and restore function in individuals with severe knee arthritis or damage. During the procedure, the damaged bone and cartilage of the thigh bone (femur) and shin bone (tibia) are removed and replaced with prosthetic components made of metal alloys and high-grade plastics. The patella (kneecap) may also be resurfaced. While TKR is a common and generally safe operation, it is an invasive procedure that involves navigating complex anatomical structures, including nerves and blood vessels.

The Primary Nerve Affected: Infrapatellar Branch of the Saphenous Nerve

The most consistently affected nerve during a total knee replacement is the infrapatellar branch of the saphenous nerve.

• Anatomy and Function: The saphenous nerve is a sensory branch of the femoral nerve. It travels down the medial (inner) side of the thigh and leg. One of its terminal branches, the infrapatellar branch, typically crosses the anterior (front) aspect of the knee joint, often directly through the incision site used for TKR. Its primary function is to provide sensory innervation to the skin over the front of the knee, including the area around the patella.
• Why It's Affected: Due to its superficial course and proximity to the surgical incision, the infrapatellar branch of the saphenous nerve is almost always transected (cut) or significantly damaged during the surgical approach. This is often an unavoidable consequence of accessing the knee joint.
• Consequences: The cutting of this nerve results in a localized area of numbness or altered sensation (paresthesia) on the outer and lower aspect of the kneecap and sometimes extending down the shin. While this numbness is permanent, it is generally well-tolerated and rarely causes significant functional impairment, though some individuals may find it bothersome.

Other Nerves at Risk of Injury During TKR

While the infrapatellar branch is routinely affected, other major nerves, though not typically "cut," can be at risk of injury during a total knee replacement. These injuries are less common but can have more significant consequences.

• Common Peroneal Nerve:
  • Anatomy and Vulnerability: The common peroneal nerve is a branch of the sciatic nerve. It wraps superficially around the neck of the fibula (the smaller bone of the lower leg) on the lateral (outer) side of the knee. This superficial course makes it particularly vulnerable to injury.
  • Mechanism of Injury: It can be injured by direct trauma, prolonged compression (e.g., from a tourniquet used during surgery), or excessive stretching, especially in cases of severe pre-existing knee deformity (e.g., significant valgus or "knock-knee" deformity) that requires aggressive correction.
  • Consequences: Injury to the common peroneal nerve can lead to "foot drop," a condition where the individual has difficulty lifting the front part of the foot, causing the toes to drag during walking. This can also be accompanied by numbness on the top of the foot and outer lower leg.
• Tibial Nerve:
  • Anatomy and Vulnerability: The tibial nerve is the other major branch of the sciatic nerve, continuing down the back of the leg. It is generally more protected than the common peroneal nerve.
  • Mechanism of Injury: Injury is rare but can occur from direct trauma, excessive traction, or pressure.
  • Consequences: Damage can affect the muscles that point the foot down and curl the toes, as well as sensation in the sole of the foot.
• Femoral Nerve:
  • Anatomy and Vulnerability: The femoral nerve originates in the lumbar spine and travels down the front of the thigh, supplying the quadriceps muscles and providing sensation to the front of the thigh. It is generally well away from the primary surgical field of a TKR.
  • Mechanism of Injury: Injury is exceedingly rare but can occur due to direct trauma, hematoma (blood clot) formation, or pressure from retractors, particularly during complex revisions or in cases with unusual anatomy.
  • Consequences: Damage can lead to weakness in knee extension (difficulty straightening the leg) and numbness in the front of the thigh.

Understanding Post-Surgical Neurological Symptoms

It is crucial for patients and fitness professionals to understand the potential neurological changes following TKR:

• Expected Numbness: The numbness around the kneecap due to infrapatellar nerve damage is a common and expected outcome. Patients should be informed about this pre-operatively.
• Signs of More Serious Nerve Injury: Any new onset of significant motor weakness (e.g., inability to lift the foot, difficulty straightening the knee) or widespread, intense numbness/pain beyond the expected area, especially if it develops acutely after surgery, should be reported to the surgical team immediately. Early diagnosis and intervention are critical for optimizing outcomes in these rare cases.

Nerve Regeneration and Recovery

The ability of nerves to regenerate varies significantly based on the type of nerve, the extent of injury, and individual factors.

• Sensory Nerves (like the infrapatellar branch): Complete regeneration leading to full restoration of sensation is unlikely if the nerve has been transected. The body may form collateral branches or the surrounding nerves may take over some sensory function, but the area of numbness typically persists.
• Bruised or Stretched Nerves: If a nerve like the common peroneal or tibial nerve is merely stretched or compressed (neuropraxia) rather than severed, there is a good chance of recovery. This recovery can be slow, sometimes taking weeks to months as the nerve heals and remyelinates.
• Role of Physical Therapy: In cases of motor nerve injury, physical therapy is essential. It helps maintain joint range of motion, prevent muscle atrophy, and retrain affected muscles as nerve function returns.

Conclusion

While total knee replacement is an effective procedure for restoring mobility and alleviating pain, it inherently involves the risk of nerve involvement. The most common and expected neurological consequence is the transection of the infrapatellar branch of the saphenous nerve, leading to a localized area of numbness around the kneecap. Less commonly, other major nerves such as the common peroneal, tibial, or femoral nerves can be injured, potentially leading to more significant motor or sensory deficits. Understanding these potential neurological implications is vital for both patients undergoing TKR and the healthcare professionals guiding their recovery. Despite these potential risks, TKR remains a highly successful intervention that significantly improves the quality of life for countless individuals.