Knee pain following total hip replacement is a common experience, even though the surgery was performed on a different joint. This post-operative discomfort can be confusing for patients who expected full relief after their hip joint was successfully replaced. The pain is often manageable and typically points to an identifiable underlying cause that requires investigation. Understanding the connection between the hip and the knee is the first step in addressing this discomfort and ensuring a smooth recovery.
Changes in Leg Biomechanics
Total hip arthroplasty restores the hip joint’s alignment and function, which inevitably shifts the load distribution across the entire leg. Before surgery, patients often developed an abnormal gait to compensate for the damaged hip, causing the knee to absorb stress differently. When the hip is corrected, the entire mechanical chain of the leg must adjust to the new, healthier alignment, often placing temporary strain on the knee.
One significant change is the correction of a pre-operative leg length discrepancy, which occurs when the new hip component lengthens the limb slightly for stability. Even a small change in leg length, often a few millimeters, can alter the angle at which the knee joint bears weight during walking. This change in mechanics can temporarily irritate the soft tissues, such as tendons and ligaments, surrounding the knee, leading to inflammation and pain.
Muscle weakness and imbalance around the hip and thigh are common after surgery and rehabilitation. Specific muscles, like the hip abductors, may be temporarily weak, forcing the quadriceps and hamstring muscles around the knee to overcompensate. This excessive exertion places abnormal rotational or shearing forces on the knee joint. As the patient progresses through physical therapy, these muscle imbalances usually resolve, and the knee pain often subsides as the body adapts to its corrected biomechanics.
Understanding Referred Pain
Pain felt in the knee may be a form of referred pain originating from the hip region. This occurs because the hip and knee joints share nerve pathways that transmit sensory signals to the brain. Specifically, the obturator nerve and the femoral nerve provide sensation to both the hip capsule and the surrounding tissues, as well as parts of the inner knee.
During the hip replacement, these nerves can become stretched, irritated, or compressed by surgical retractors or post-operative swelling. When the irritated nerve sends a signal to the central nervous system, the brain sometimes misinterprets the source, localizing the pain to the knee. This misinterpretation means the knee is simply the messenger for irritation actually located higher up in the hip or thigh.
This neurogenic pain typically presents as a deep, sometimes burning, ache that does not worsen with direct manipulation of the knee joint. As the surgical site heals and swelling subsides, pressure on the shared nerve pathways often decreases, leading to a gradual resolution of the referred knee pain. Persistent nerve-related pain may require specific medications or targeted physical therapy interventions.
Pre-existing or Unrelated Knee Issues
The knee pain may be entirely separate from the hip replacement, representing a condition present before the surgery. Patients with severe hip osteoarthritis often experience a “masking effect,” where intense hip pain overshadows mild knee discomfort. Once the primary source of hip pain is removed, the previously unnoticed knee issue becomes apparent.
Mild knee osteoarthritis or an old ligament injury may have been present but was not a major complaint until the hip pain was relieved. Increased post-operative mobility allows the patient to engage in greater activity, which can stress a pre-existing knee condition. This increased stress reveals the underlying knee problem.
The knee now requires its own evaluation and management plan. Diagnostic imaging, such as an X-ray, can confirm conditions like knee osteoarthritis. Effective treatment may involve physical therapy focused on strengthening or targeted injections to manage the inflammatory process.
Signs Requiring Immediate Medical Review
While many causes of post-operative knee pain are temporary, certain symptoms warrant an immediate call to the surgical team. These “red flag” symptoms can indicate a serious complication, such as infection, blood clots, or component instability, requiring immediate assessment and intervention.
Urgent concerns include:
- Signs of infection, such as a sudden increase in pain accompanied by fever, chills, or excessive redness and warmth around the knee.
- Any unusual drainage from the hip incision site.
- Symptoms of a deep vein thrombosis (DVT), which may manifest as sudden, severe swelling in the calf or thigh along with pain and tenderness.
- Knee pain so severe that it prevents bearing weight on the leg, or new, loud clicking or grinding sounds from the hip.