Total hip replacement, or total hip arthroplasty, is a common procedure used to treat severe hip arthritis and restore mobility. While generally highly successful, a known, though uncommon, complication is the development of foot drop. Foot drop describes difficulty lifting the front part of the foot, which significantly affects a patient’s ability to walk. Understanding the mechanisms behind this complication is important for both patients and healthcare providers.
What Foot Drop Is and Its Symptoms
Foot drop is primarily a functional impairment defined by the inability to actively dorsiflex the foot. This weakness results from a disruption in the nerve signals that control the muscles in the front of the lower leg. Since the foot cannot be lifted, the toes may drag along the floor during the swing phase of walking.
To compensate for this dragging, a person with foot drop often develops a characteristic “steppage gait.” They must raise their thigh higher than usual, resembling climbing stairs, so the foot can clear the ground. The foot then tends to slap down onto the floor with each step, which is known as a “slapping gait.” The condition may also be accompanied by sensory changes, such as numbness, tingling, or a pins-and-needles sensation in the top of the foot and toes.
The Specific Nerve Affected by Hip Surgery
The development of foot drop after hip surgery almost always points to an injury of the sciatic nerve or one of its major branches. The sciatic nerve runs from the lower back through the hip and down the back of the leg. Because it passes near the hip joint, it is vulnerable during replacement surgery.
Specifically, the motor function loss that causes foot drop is linked to the common peroneal division of the sciatic nerve, also called the common fibular nerve. This division controls the muscles that perform dorsiflexion (pulling the foot upward). The peroneal portion of the sciatic nerve is particularly susceptible to injury because it contains fewer protective nerve fibers compared to the tibial division.
How Surgical Procedures Cause Nerve Damage
The nerve damage leading to foot drop is a consequence of several mechanical injuries that can occur during total hip arthroplasty. One common cause is excessive stretching, known as traction injury, often resulting from limb lengthening. If the surgeon restores a significant difference in leg length, the sudden tension on the sciatic nerve can cause injury. Excessive limb lengthening, particularly over 4 centimeters, is a known risk factor.
Direct trauma from surgical instruments or excessive pressure from retractors, which are tools used to hold tissue aside, can also damage the nerve. The nerve can be compressed or bruised by these tools, especially during procedures that use a posterior or posterolateral approach, which places the sciatic nerve at higher risk. While rare, direct laceration or transection of the nerve by a surgical instrument is possible.
Another mechanism involves post-operative complications, such as the formation of a large hematoma, or blood clot, near the surgical site. This mass can expand and compress the sciatic nerve, causing injury due to external pressure and restricted blood flow. Additionally, components used in the procedure, like bone cement or migrated hardware, can cause impingement by pressing directly on the nerve. In some cases, the cause of the nerve palsy remains undetermined.
Recovery Timelines and Treatment Options
The prognosis for foot drop after hip surgery depends heavily on the severity and type of nerve injury. If the injury was primarily a temporary stretch or compression, often referred to as a neurapraxia, recovery can be expected. Improvement often occurs gradually, taking several weeks to months, with maximum recovery sometimes requiring 6 to 18 months.
Immediate management focuses on preventing further complications and supporting function. An Ankle-Foot Orthosis (AFO), a brace, is commonly used to hold the foot in a neutral position. This assists with foot clearance during walking and prevents the development of contractures. Physical therapy is started early to strengthen the available muscles and maintain joint mobility.
Diagnostic tools like electromyography (EMG) and nerve conduction studies (NCS) are used to assess the extent of nerve damage and monitor for regeneration. If a compressive cause, such as a large hematoma or misplaced hardware, is identified, surgical intervention to relieve the pressure on the nerve is often pursued promptly. If no recovery of motor function is observed after a prolonged period, typically 3 to 7 months, a specialist may consider late surgical exploration, nerve repair, or tendon transfer procedures to restore some function.