Hip subluxation is a partial or incomplete dislocation of the hip joint. The femoral head (the ball of the thigh bone) temporarily slips partway out of the acetabulum (the socket in the pelvis). Unlike a full dislocation, the joint surfaces remain in partial contact, maintaining some alignment. This displacement creates instability, ranging from mild and intermittent to severe and painful.
Understanding the Partial Displacement
The hip joint is a highly stable ball-and-socket joint, where the round femoral head fits snugly into the cup-shaped acetabulum. This inherent stability comes from the joint’s bony architecture, strong surrounding ligaments, and a ring of cartilage called the labrum. Subluxation occurs when the femoral head shifts laterally or superiorly, moving slightly out of its normal resting position within the socket.
A helpful way to visualize this is imagining a golf ball sitting in a golf tee. In a subluxation, the ball has rolled to the edge but has not completely fallen off, which is the key difference between subluxation and a full dislocation (luxation). Because the displacement is partial, the hip often self-reduces, or slips back into place, but the underlying instability persists. The ligaments and joint capsule are stretched and stressed during this event, compromising the soft tissues that normally hold the ball securely. This trauma can lead to a chronically loose joint, increasing the risk of future subluxations and long-term wear on the joint cartilage.
Factors Contributing to Instability
The reasons a hip joint may become unstable enough to sublux are broadly divided into developmental issues and acquired factors. Developmental Dysplasia of the Hip (DDH) is a primary cause, particularly in infants and children. In DDH, the acetabulum is abnormally shallow or the femoral head is incorrectly shaped, preventing a secure fit in the socket. This structural inadequacy allows the femoral head to slide out easily under normal pressure. DDH is the most common context for subluxation in newborns, often requiring early intervention to promote proper joint development.
Acquired factors usually involve trauma that disrupts a previously normal joint. High-impact trauma, such as a motor vehicle collision or a severe fall, can generate forces strong enough to momentarily push the femoral head partially out of the socket. Subluxation can also complicate a total hip replacement, as the prosthetic ball may slip out of the artificial socket due to specific movements or insufficient soft tissue tension. Furthermore, conditions causing generalized ligamentous laxity, such as Ehlers-Danlos Syndrome, predispose individuals to chronic hip subluxation without major trauma.
Recognizing the Physical Signs
The physical signs of hip subluxation vary considerably between infants and adults, depending on the cause and severity. Adults and older children often experience deep pain, usually localized in the groin area, which worsens with weight-bearing activities. They may report a distinct feeling of the hip “giving way” or a momentary “pop” or “click” as the joint slips and relocates. Changes in walking gait are common, often leading to a limp or difficulty standing for extended periods as the body shifts weight to avoid stressing the unstable joint. Symptoms may progress to significant functional limitation and muscle spasms around the hip and buttock.
In infants, developmental subluxation signs are often less obvious and are typically detected during routine physical examinations. A pediatrician may notice an asymmetry in the skin folds of the thigh or buttocks. Other signs include a noticeable difference in the range of motion, especially when moving the leg outward (abduction), or a leg length discrepancy where one leg appears shorter than the other.
Diagnosis and Management
Diagnosis begins with a detailed physical examination assessing the range of motion, stability, and pain response. For infants, specific maneuvers like the Ortolani and Barlow tests check for joint instability. Physical findings are typically confirmed through medical imaging.
In infants, ultrasound is the preferred imaging modality because the hip bones are still primarily cartilage. For older children and adults, X-rays are routinely used to visualize bony alignment and identify structural abnormalities. Advanced imaging, such as Magnetic Resonance Imaging (MRI) or Computed Tomography (CT) scans, may be ordered to assess the condition of the labrum, ligaments, and cartilage.
Management aims to stabilize the joint, relieve pain, and prevent long-term damage like early-onset arthritis. Non-surgical approaches are the first line of treatment, especially for developmental causes in infants. This frequently involves using a soft brace or harness, such as a Pavlik harness, to hold the femoral head securely and promote normal joint formation.
For adults and older children, non-surgical care includes physical therapy to strengthen surrounding muscles and improve function. Activity modification, including avoiding movements that trigger subluxation, is also necessary. If the joint remains persistently unstable, or if the underlying cause is a severe anatomical defect or significant ligament damage, surgical intervention may be required to restore normal hip mechanics.