A total hip replacement (total hip arthroplasty) alleviates severe pain and restores mobility in a damaged hip joint. While focus is often on hip recovery, discomfort in the knee on the same side is a common concern. This occurs because the body’s major joints function as a single, connected system. Understanding the mechanics of the leg, both before and after surgery, reveals why the knee is often affected by changes happening at the hip.
The Hip-Knee Biomechanical Connection
The lower extremity operates as a kinetic chain, meaning movement or misalignment in one joint—the hip, knee, or ankle—directly influences the mechanics of the others. Before surgery, chronic hip pain, often from osteoarthritis, causes the body to adopt compensatory movement patterns to avoid discomfort. This adjustment attempts to redistribute weight and reduce the load on the painful hip.
This compensation typically involves an altered gait, where the person shifts their weight and changes the rotation of the leg. Such chronic misalignment places abnormal, twisting forces on the knee joint, which is designed primarily for hinge-like motion. Over time, this unnatural stress can strain the knee’s internal structures, including the cartilage, menisci, and supporting ligaments.
When the hip joint degenerates, the resulting stiffness and restricted motion can force the knee to absorb excessive rotational and shearing forces during everyday activities like walking and standing. This prolonged biomechanical stress can lead to the development of secondary knee issues or exacerbate pre-existing, unnoticed knee arthritis.
New Stresses Caused by Surgical Correction
The knee pain experienced after total hip arthroplasty often stems from the sudden change in mechanics introduced by the new joint. The body must adapt to a surgically corrected hip that now moves differently and without pain. The most common mechanical change is a slight leg length discrepancy, which is difficult to avoid entirely, with minor differences of less than one centimeter being common.
Even a small change in limb length can significantly alter the biomechanics of the entire leg, forcing the knee to adjust its position and rotation. This imbalance can lead to an uneven distribution of weight and altered tension in the long muscle groups that cross both the hip and the knee. Muscles like the quadriceps and the iliotibial (IT) band, which stabilize the knee, may experience new strain due to the hip’s restored alignment and altered muscle length.
Furthermore, the surgical approach itself can sometimes irritate or inflame tissues that influence the knee. For example, procedures that require significant maneuvering around the hip can affect nerves or tendons that run down the leg, leading to referred pain that is felt near the knee. In many cases, the knee pain is a temporary adaptation as the body re-learns a normal, pain-free walking pattern, requiring the knee to quickly shed its long-standing compensatory habits.
Addressing Post-Operative Knee Discomfort
Managing post-operative knee pain focuses on encouraging the body to fully adapt to its newly aligned hip and lower limb mechanics. Physical therapy is fundamental, focusing on re-training the patient’s gait for symmetrical and balanced walking. Therapists guide patients through exercises designed to strengthen muscles surrounding both the hip and the knee, stabilizing the joint and normalizing force distribution.
Non-surgical interventions are often employed to manage localized discomfort while the knee adjusts. Applying ice to the knee for sessions of 20 to 30 minutes can help reduce swelling and inflammation. Clinicians may also recommend the RICE protocol—Rest, Ice, Compression, and Elevation—along with non-opioid medications such as acetaminophen or non-steroidal anti-inflammatory drugs (NSAIDs) to control pain.
Patients should maintain consistent communication with their orthopedic surgeon, particularly if the knee pain is severe, affects daily activities, or does not improve over time. Worsening pain, especially when accompanied by signs of infection like warmth, redness, or excessive swelling, warrants immediate medical attention. For persistent symptoms, a careful evaluation is necessary to determine if the discomfort is purely mechanical adaptation or if a pre-existing knee condition requires separate treatment.