Hip replacement surgery, or total hip arthroplasty, is a common and successful procedure used to relieve pain and improve mobility. While generally safe, a potential complication is foot drop, characterized by the inability to lift the front part of the foot. This weakness causes the toes to drag during walking, often requiring the patient to lift the leg higher to compensate. The incidence of this nerve injury is low, typically less than 4% in primary surgeries. The condition arises from trauma to major nerves near the hip, categorized into factors related to anatomy, direct mechanical trauma during the operation, and indirect pressures from the surgical aftermath.
Understanding the Nerve Pathway
The ability to lift the foot is controlled by the common peroneal nerve, which is a division of the larger sciatic nerve that runs down the back of the leg. The sciatic nerve originates from nerve roots in the lower spine (L4 to S3) and exits the pelvis, traveling close to the hip joint capsule and the back of the thigh. Because of its location, the sciatic nerve is the most frequently injured nerve during total hip replacement surgery. The peroneal division is structurally more susceptible to damage than the tibial division, making it the primary cause of foot drop following the procedure.
The peroneal nerve is more superficial and contains fewer protective connective tissue fibers, making it less tolerant of stretching or compression. Injury to this specific nerve pathway results in the loss of function in the muscles responsible for dorsiflexion (the upward movement of the foot and toes). This anatomical vulnerability links hip surgery to this distinct lower leg symptom.
Direct Mechanical Injury During Surgery
A primary cause of nerve dysfunction is direct trauma that occurs during the surgical manipulation of the hip joint and surrounding tissues. The use of surgical instruments, such as retractors, to move muscles and expose the joint can inadvertently compress or stretch the sciatic nerve. Retractor placement, especially in the posterior and inferior area of the hip socket, is considered a high-risk zone for nerve contact. Excessive traction is often necessary to dislocate the hip joint and subsequently place the new implant, which can place significant strain on the nerve.
The placement of the hip replacement components themselves can also cause direct injury to the nerve. If the surgeon uses bone cement, the heat generated during its hardening process can cause thermal damage to the adjacent nerve tissue. Similarly, screws or other fixation hardware used to secure the socket component might be placed too deeply or impinge upon the nerve pathway. These mechanical injuries are known as etiologies and can sometimes lead to a delayed onset of foot drop.
Indirect Factors Leading to Compression
Beyond direct contact, several indirect factors related to the overall surgical process and recovery can lead to nerve compression and foot drop.
Limb Lengthening
One of the most common indirect causes is excessive lengthening of the limb, which is sometimes required to achieve stability in the new hip joint. When the leg is lengthened beyond its original measure, the sciatic nerve is stretched, and this tension can impair its function. Studies suggest that lengthening the limb by more than 2.7 centimeters significantly increases the risk of injury to the peroneal division of the nerve.
Hematoma Formation and Positioning
Another element is the formation of a hematoma, a collection of blood, in the surgical area following the operation. This internal bleeding creates a mass effect that exerts pressure on the nearby sciatic nerve, causing compression and subsequent nerve palsy. Patient positioning on the operating table, particularly the duration and degree of hip flexion or hyperextension, can also contribute to nerve strain.
Pre-existing Conditions
Individuals with pre-existing conditions, such as diabetes or peripheral neuropathy, have nerves that are already metabolically compromised. They are more susceptible to injury from these indirect pressures, a concept known as a “double crush” phenomenon.