Perthes disease is a childhood hip condition where blood supply to the ball-shaped top of the thighbone (the femoral head) is temporarily disrupted, causing the bone to weaken, break down, and eventually regrow. It typically strikes children between ages 4 and 8, with an average age of diagnosis around 6.5 years. The full cycle of bone death and regrowth can take anywhere from two to five years, and the central goal of treatment is to keep the femoral head as round as possible while it heals.
Who Gets Perthes Disease
Perthes disease affects roughly 0.4 to 29 per 100,000 children under age 15, with rates varying significantly by geography and ethnicity. Boys are affected far more often than girls, making up about 81% of cases. Children from the Indian subcontinent tend to present later, with an average age closer to 9.5 years rather than the typical 6.5.
The exact cause remains unclear. Something interrupts blood flow to the femoral head, but researchers have not pinpointed a single trigger. It is not caused by an injury, though an injury may bring attention to a hip that was already affected. In about 10% to 15% of cases, both hips are involved.
How It Affects the Hip
The femoral head sits inside a deep socket in the pelvis, forming a ball-and-socket joint that allows the leg to move in many directions. When blood flow to the femoral head is cut off, bone cells begin to die. The bone softens and can fracture or collapse under normal body weight. Over time, the body absorbs the dead bone and replaces it with new bone, but during that vulnerable window the femoral head can flatten or become irregularly shaped. If the ball no longer fits smoothly into the socket, the hip loses range of motion and may develop arthritis later in life.
Symptoms to Recognize
The most noticeable early sign is a limp that gets worse with activity and improves with rest. Children often complain of pain in the hip, groin, or thigh, and sometimes the pain is felt only in the knee, which can delay diagnosis. The limp can look different from child to child. Some children walk with a pain-avoidance gait, shortening their stride on the affected side. Others develop a noticeable trunk lean toward the sore hip or a drop of the pelvis on the opposite side during each step.
Hip movement gradually becomes restricted, particularly when rotating the leg inward or spreading the legs apart. Tightness in the muscles along the inner thigh (the adductors) is common and contributes to the altered walking pattern. Some children lose muscle bulk in the thigh on the affected side because they instinctively use that leg less.
How the Disease Progresses
Perthes disease moves through four recognizable stages, each visible on X-rays. Understanding these stages helps explain why the condition takes so long to resolve and why treatment decisions change along the way.
In the initial (necrosis) stage, the bone becomes dense and hardened on imaging because dead bone stops being remodeled the way healthy bone is. This stage typically lasts around 4 to 6 months. Next comes the fragmentation stage, when the dead bone starts to break apart and be reabsorbed. Cracks appear in the femoral head, and this is actually when the hip is most vulnerable to deformation. Fragmentation can last anywhere from about 6 months in milder cases to nearly a year or more in severe ones.
During reossification, new bone begins forming at the edges of the femoral head and gradually fills in. This is the longest phase, averaging around 51 months in one large study, though the range is enormous (from just a couple of months to over 10 years in extreme cases). Finally, in the residual (remodeling) stage, the bone matures and takes on its final shape. Whatever shape the femoral head has at this point is largely what the child will carry into adulthood.
How Severity Is Classified
Doctors use the Herring lateral pillar classification to gauge how much of the femoral head is affected and predict the likely outcome. This system divides the femoral head into three columns and focuses on the outer (lateral) pillar during the fragmentation stage.
- Group A: The outer pillar keeps its full height. These children have uniformly good outcomes.
- Group B: The outer pillar loses some height but retains more than 50%. About two-thirds of these children end up with a good result.
- Group B/C border: The outer pillar is right around the 50% mark, or the remaining bone is thin and poorly formed. Outcomes are more unpredictable.
- Group C: The outer pillar collapses to less than 50% of its original height. These hips carry the highest risk of long-term problems, with only about 13% achieving a good outcome.
This classification can only be applied once fragmentation is underway, which means there is sometimes a waiting period before doctors can fully assess severity.
Non-Surgical Treatment
For many children, especially those diagnosed before age 8 with milder involvement, the hip heals well without surgery. The primary goal of non-surgical care is to keep the femoral head seated deeply in the hip socket (a principle called “containment”) and to preserve range of motion while the bone regenerates.
Treatment options range from simple observation to more active interventions. Activity modification is common: children may be told to avoid running, jumping, and high-impact sports during the fragmentation phase. Physical therapy focuses on stretching to maintain hip mobility and strengthening exercises for the muscles around the hip. Maintaining the ability to spread the leg outward (abduction) is especially important because it helps the socket act as a mold for the regrowing bone. Some children use crutches or a wheelchair during flare-ups to reduce weight on the hip.
In certain cases, doctors use braces, calipers, or Petrie casts (casts that hold both legs apart in an A-shape) to keep the femoral head contained within the socket. These are less commonly used today than in past decades, but they remain an option when other conservative approaches aren’t maintaining good hip position.
When Surgery Is Recommended
Surgery becomes a serious consideration for children over age 8 with Herring B or B/C hips, where studies show significantly better outcomes with surgical containment compared to observation alone. Younger children with the same classification tend to do well regardless of treatment, so surgery is often unnecessary for them. For the most severe cases (Herring C), outcomes are generally poor whether or not surgery is performed, though individual circumstances may still favor an operation.
The most widely used procedure is a femoral varus osteotomy, where the thighbone is cut and angled inward so the femoral head sits more deeply in the socket. This gives the bone a better environment in which to heal round. The bone is held in its new position with screws and plates, which are removed once healing is complete. A similar goal can be achieved from the pelvis side with a pelvic osteotomy, where the socket itself is repositioned to cover more of the femoral head. In children over 8, whose pelvic bones are less flexible, a triple pelvic osteotomy (cutting the pelvis in three places) may be needed to achieve adequate repositioning.
In the most severe cases, where the femoral head has become mushroom-shaped and the hip functions like a hinge rather than a ball-and-socket, a different type of osteotomy angles the thighbone outward to restore more normal joint mechanics. Surgery is most effective when performed early in the disease, during the initial or fragmentation stages, because the bone still has maximum potential to remodel into a good shape.
Long-Term Outlook
The younger the child at diagnosis, the better the prognosis. Children diagnosed before age 6 have the most remodeling time before their skeleton matures, and their femoral heads are more likely to round out even if significant collapse occurs. Children diagnosed after age 8 have less growth remaining, which limits the bone’s ability to reshape itself.
The total healing process typically spans two to five years. During that time, activity levels are adjusted based on symptoms and X-ray findings. Most children eventually return to full activity, including sports, though the timeline varies. High-impact activities are generally the last to be reintroduced, and the decision depends on how well the femoral head has healed and whether range of motion has been preserved.
Children who end up with a round or near-round femoral head can expect a hip that functions normally well into adulthood. Those whose femoral head heals in a flattened or irregular shape are at higher risk for hip stiffness and early-onset arthritis, sometimes requiring hip replacement in their 40s or 50s rather than the typical age range of 60s or 70s. Regular follow-up through adolescence helps catch any developing problems early.