A hip dislocation occurs when the head of the thigh bone, known as the femur, is forced out of its natural resting place in the hip socket, or acetabulum, located in the pelvis. This is a serious injury that typically requires prompt medical attention to prevent further complications. When the hip dislocates, it can cause severe pain and significantly limit movement.
Types of Hip Dislocation
Hip dislocations can arise from various circumstances, leading to different classifications. Traumatic hip dislocation results from significant external force. Developmental dysplasia of the hip (DDH) is a condition present from birth where the hip joint does not form correctly. This can cause the hip to be unstable or even dislocated from an early age. Prosthetic hip dislocation refers to instances where an artificial hip joint, typically following replacement surgery, comes out of its socket.
Traumatic Causes
Traumatic hip dislocations result from substantial forces impacting the hip joint. These injuries often occur in high-impact accidents, such as motor vehicle collisions, where a knee striking the dashboard can force the femoral head backward. This commonly leads to a posterior dislocation, accounting for approximately 85% to 90% of all traumatic hip dislocations. Falls from significant heights or industrial accidents can also cause dislocation. Hip dislocations can also occur during sports activities involving high speed, collisions, or falls, such as football or skiing, which may involve direct impacts or extreme hip movements leading to an anterior dislocation.
Developmental and Predisposing Factors
Hip dislocations can also stem from underlying conditions.
Developmental Dysplasia of the Hip (DDH)
Developmental dysplasia of the hip (DDH) is a condition where the hip joint does not develop properly, often meaning the hip socket is too shallow to fully contain the femoral head. This shallow socket can lead to instability or dislocation, sometimes apparent at birth or developing during infancy. Genetic factors and the baby’s position in the womb, especially breech presentation, can increase the likelihood of DDH.
Neuromuscular Conditions
Neuromuscular conditions can predispose individuals to hip dislocation. Disorders affecting muscle control and tone, such as cerebral palsy or spina bifida, can create muscle imbalances around the hip joint. Stronger hip flexor and adductor muscles may overpower weaker hip extensors and abductors, progressively pulling the hip out of alignment over time. This imbalance can lead to subluxation or dislocation, particularly in growing children.
Connective Tissue Disorders
Connective tissue disorders, including Ehlers-Danlos syndrome and Marfan syndrome, affect the strength and elasticity of ligaments and other tissues throughout the body. Individuals with these conditions often have hypermobile joints, meaning their joints have an unusually wide range of motion. This increased laxity in the ligaments and joint capsules makes the hip inherently less stable, increasing the likelihood of dislocation even with minimal force.
Septic Arthritis
A severe infection in the hip joint during childhood, known as septic arthritis, can damage the joint surfaces and surrounding structures. If not treated promptly, this damage can lead to lasting instability or even dislocation of the hip.
Causes Related to Hip Replacement
Dislocation is a known complication following total hip replacement surgery.
Patient and Muscle Factors
Certain patient movements, particularly extreme hip flexion, adduction, or internal rotation, can cause the prosthetic femoral head to detach from its socket. This is especially true in the early period after surgery when the surrounding tissues are still healing. Weakened or damaged muscles and soft tissues around the hip, such as the gluteal muscles, can also compromise the new joint’s stability. Patient-specific factors, including cognitive impairment, a history of previous dislocations, or failure to follow post-operative precautions, also contribute to dislocation risk.
Surgical Factors
Surgical factors play a role in the stability of a prosthetic hip. Improper positioning of the artificial components, such as the acetabular cup or femoral component, can affect the joint’s range of motion and increase the risk of dislocation. The type and size of the prosthetic components can also influence stability, with larger femoral heads generally providing more stability.