Knee discomfort after total hip replacement is a common, though often surprising, occurrence for many patients. Since the surgery’s primary focus is alleviating chronic hip pain, new pain in the lower leg can be concerning. This knee discomfort is frequently temporary, representing the body’s necessary adjustments following a major orthopedic procedure. Understanding the various reasons behind this phenomenon, from mechanical changes to nerve pathways, can help manage expectations and guide recovery.
Biomechanical Shifts and Alignment
Replacing a damaged hip joint fundamentally changes the mechanics of the entire leg. A frequent cause of post-operative knee pain is the alteration in limb length. Severe hip arthritis often shortens the leg, and the replacement procedure restores it to a more natural position. This restoration places new stress on the knee joint, requiring the knee, ankle, and foot to adapt to a new alignment and loading pattern.
The body must also establish a new gait pattern after the hip has been replaced. For years, the patient likely compensated for the painful hip by walking differently, and these old habits do not disappear immediately. This temporary, altered walking style can force the knee to absorb shock and bear weight in an unusual way, leading to strain and inflammation. Furthermore, the muscles surrounding the hip and thigh, which may have become weak or imbalanced due to prolonged disuse, now have to work harder to stabilize the new joint. This muscle compensation can inadvertently transmit tension down the leg, leading to discomfort felt specifically around the knee.
Referred Pain and Nerve Irritation
Pain felt in the knee may not actually originate from the knee joint itself, but rather from the hip or thigh through a neurological phenomenon known as referred pain. The hip and knee joints share an interconnected network of sensory nerves, originating from the lumbar region of the spine, including the femoral, obturator, and sciatic nerves. This shared pathway means that irritation or inflammation occurring near the hip can send pain signals that the brain interprets as coming from the knee.
During the hip replacement procedure, the nerves adjacent to the hip joint can become temporarily irritated, stretched, or compressed due to surgical manipulation and swelling. Irritation to the femoral nerve or the obturator nerve, both of which supply sensation to the thigh and knee, can manifest as discomfort in the knee area. This type of nerve-related pain, which can present as a deep ache or tingling, is often a temporary side effect that subsides as the post-surgical inflammation decreases and the nerve tissue heals.
Exacerbation of Pre-existing Knee Conditions
In some cases, the knee pain is not a new issue, but rather the surfacing of a previously silent or minor condition within the knee joint itself. Many individuals who develop hip arthritis also have some degree of knee arthritis or meniscus wear. Once the hip pain is relieved by the replacement, the pain from the pre-existing knee condition is no longer “masked” and becomes noticeable.
The physical stress of the surgery and the subsequent rehabilitation program can also trigger symptoms in a vulnerable knee. The temporary increase in activity, combined with swelling and the initial awkwardness of movement, places acute demands on the knee structures. This acute demand can aggravate a chronic, underlying issue, causing a flare-up of pain in an area that had been asymptomatic before the operation. Addressing these conditions often involves working with a physical therapist to strengthen the surrounding muscles and reduce the stress on the knee joint.
Knowing When to Contact Your Surgeon
While a certain amount of temporary knee discomfort is expected, certain symptoms indicate a potentially serious complication that requires immediate medical attention. Any sudden, severe, or unrelenting knee pain that is not relieved by rest or prescribed medication should be reported to your surgical team immediately. This type of pain, particularly if it worsens over time, may suggest an issue beyond the normal recovery process.
Deep Vein Thrombosis (DVT)
Monitoring for signs of a deep vein thrombosis (DVT), a blood clot in the leg, is also very important. Symptoms of DVT can include sudden or significant swelling in the calf or thigh, warmth in the leg, or a change in skin color to red or purple.
Infection
The presence of systemic infection should be treated as an urgent matter. Warning signs include a fever of 100.5 degrees Fahrenheit or greater, chills, or excessive redness and drainage from the hip incision site. Finally, any sudden inability to bear weight on the operated leg, or a major change in stability, warrants an urgent call to your surgeon for guidance.