Hip arthritis frequently causes pain felt in the knee, a phenomenon that can confuse both patients and clinicians. This disconnect between the disease location and the pain perception arises from two distinct mechanisms: the physical alteration of leg movement and the shared neurological wiring of the joints. Understanding these connections is important for correctly diagnosing the true source of chronic knee discomfort.
The Biomechanical Link Between the Hip and Knee
The hip and knee joints are fundamental components of the lower extremity’s kinetic chain, meaning a problem in one area can mechanically influence the other. Hip arthritis leads to cartilage degradation, causing stiffness, pain, and limited range of motion. This often results in a restricted or altered gait, commonly known as antalgic gait, as the body attempts to protect the painful hip.
This change in walking mechanics forces the knee to absorb improper or excessive load and rotation during everyday movements. Reduced hip extension or internal rotation due to arthritis leads to compensatory movements at the knee, placing abnormal stress on its cartilage and ligaments. This strain results in inflammation and pain, even in the absence of primary knee disease. While this mechanical compensation can eventually lead to secondary knee problems, the initial source of the pain remains the arthritic hip joint.
Understanding Referred Pain Pathways
Beyond the mechanical relationship, hip arthritis can cause knee pain through referred pain, a neurological mechanism where the brain misinterprets the origin of a pain signal. The hip joint capsule, which becomes inflamed in arthritis, and the knee joint share common nerve pathways originating from the lumbar spine.
Branches of the femoral nerve and the obturator nerve supply sensation to both the hip joint and the structures around the knee, particularly the anterior and medial aspects. When the inflamed hip joint sends a pain signal along these shared nerve routes, the brain sometimes incorrectly localizes the discomfort to the more distal knee area. Because of this shared innervation, the knee pain can be the only symptom a patient reports, even when the hip is the true source of the problem.
Differential Diagnosis: Locating the True Source of Pain
Accurately distinguishing between primary knee pain and pain referred from the hip is a necessary step in treatment. The initial physical examination often provides the strongest clues. Clinicians frequently test the hip’s passive range of motion, as severely limited internal rotation is a highly indicative sign of hip arthritis, even when the patient’s only complaint is knee pain.
Imaging studies are employed to confirm the diagnosis. While knee X-rays may appear normal or show only minor changes, hip X-rays often reveal signs of advanced osteoarthritis, such as joint space narrowing or the presence of osteophytes. If initial knee X-rays are unremarkable, the physician must consider imaging the hip and even the lumbar spine to rule out other sources of referred pain.
The most definitive diagnostic tool is a targeted local anesthetic injection. An image-guided injection of a numbing agent, such as lidocaine, is performed directly into the hip joint. If the patient’s knee pain immediately and significantly resolves following this procedure, the hip is confirmed as the pain generator. This diagnostic injection provides a temporary but conclusive answer to the source of the chronic knee discomfort.
Targeted Treatment Approaches
Since knee pain is a symptom of underlying hip pathology, effective treatment must focus on addressing the hip arthritis itself. Non-surgical management begins with physical therapy aimed at restoring proper hip function and correcting biomechanical faults. This includes strengthening the muscles surrounding the hip, such as the gluteals, to improve stability and normalize gait mechanics, thereby reducing strain on the knee.
Anti-inflammatory medications, such as non-steroidal anti-inflammatory drugs (NSAIDs), reduce inflammation within the arthritic hip joint. Injections of corticosteroids directly into the hip joint may provide temporary pain relief and reduce inflammation, often alleviating secondary knee symptoms. For advanced hip arthritis that does not respond to conservative measures, total hip replacement surgery serves as the definitive treatment. This procedure removes the damaged joint surfaces and replaces them with artificial components, resulting in the resolution of the referred knee pain by eliminating its source.