Developmental Dysplasia of the Hip (DDH) is a condition where the hip joint does not form correctly. The answer to whether it is genetic is complex: yes, genetics play a significant role, but it is not the sole cause. This common disorder involves a mix of inherited predisposition and environmental influences, making it a multifactorial condition. DDH can range from a mild looseness of the joint to a complete dislocation of the ball from the socket.
Understanding Developmental Dysplasia of the Hip
The hip is a classic ball-and-socket joint, where the head of the thigh bone (femur) acts as the ball, fitting securely into a cup-shaped hollow in the pelvis called the acetabulum, or socket. Tough connective tissues and ligaments surround and reinforce this joint structure.
DDH occurs when there is an abnormal formation or alignment of these components, with the socket often being too shallow or the ligaments surrounding the joint being too lax. This prevents the femoral head from being held securely within the acetabulum, leading to instability. The condition can manifest as a slight instability (subluxation) or a complete displacement (dislocation) of the femoral head from the socket. The left hip is affected more often than the right, and the condition can sometimes occur in both hips.
The Role of Inherited Factors
DDH has a strong tendency to run in families, which is the clearest evidence of its genetic component. Studies have shown high heritability estimates, suggesting that a significant portion of the risk is passed down through generations. The risk of developing DDH is substantially higher for first-degree relatives of an affected individual, sometimes reported to be as high as 12 times the risk of the general population.
The inheritance pattern is generally considered polygenic, meaning that multiple genes, rather than a single gene, contribute to the overall susceptibility. Researchers have identified several genes involved in forming connective tissue, bone, and cartilage that are associated with DDH risk. These genes affect the development and structure of the hip joint components, such as the depth of the acetabulum and the elasticity of the ligaments. This genetic predisposition creates a susceptible joint structure that is more vulnerable to physical or hormonal influences.
Non-Genetic and Environmental Risk Factors
While genetics create the susceptibility, several non-inherited factors interact with this predisposition to influence whether the condition develops. Female sex is a major factor, with girls being affected far more often than boys, often at a ratio of six to one or higher. This disparity is likely due to the effects of maternal hormones, such as relaxin, which can circulate in the fetus and cause temporary ligamentous laxity, making the female hip joint more unstable.
Mechanical factors related to the baby’s position in the womb also contribute significantly to risk. The most recognized of these is a breech presentation, where the baby’s feet or buttocks are positioned to be delivered first. Frank or single breech presentation is especially associated with a higher risk because the position forces the hips into an unfavorable, extended posture.
Other prenatal factors include firstborn status and conditions like oligohydramnios, which is a low volume of amniotic fluid that restricts fetal movement and increases mechanical pressure on the hip joint.
Even after birth, certain practices can influence the risk, especially in an infant who already has a genetic or mechanical predisposition. Improper swaddling, which tightly wraps the baby’s legs together in an extended and adducted position, is a postnatal risk factor. This positioning restricts the natural outward and upward movement of the hips, potentially hindering the development of a stable hip socket. Safe swaddling techniques allow the baby’s hips and knees to bend and move freely.
Translating Risk into Early Detection
The combined understanding of genetic and non-genetic factors forms the basis of early screening programs. Because DDH is most successfully treated when detected early, identifying infants with known risk factors is a priority. Infants with a family history, those born via breech presentation, and female infants are routinely monitored more closely.
Screening begins with a physical examination performed by a pediatrician, which includes specific maneuvers like the Ortolani and Barlow tests to check for hip instability. These tests attempt to gently dislocate or relocate the femoral head, and a palpable “clunk” can indicate a problem. However, these physical signs can sometimes be subtle or resolve spontaneously in the first few weeks of life.
For infants with risk factors, even with a normal physical exam, selective ultrasound screening is often recommended between four and six weeks of age. Ultrasound is highly effective in this age range because the femoral head is primarily cartilage and is not visible on X-ray, allowing for a detailed visualization of the socket’s development and joint stability. Early detection allows for non-surgical treatment, which significantly improves the long-term prognosis.