Perthes disease is a rare childhood hip condition that occurs when blood supply to the ball-shaped head of the thigh bone is temporarily interrupted, causing the bone to break down before eventually healing over several years.

This condition, also known as Legg-Calve-Perthes disease, affects children during a critical period of their growth and development. When the blood flow to the top of the femur bone stops or slows down significantly, the bone tissue begins to die because it no longer receives the oxygen and nutrients it needs to stay healthy. This process can be alarming for families, but understanding what happens and why can help everyone navigate the journey ahead with greater confidence.^[1]

The disease unfolds in stages over a period that can last anywhere from two to five years. During this time, the body works to remove the dead bone tissue and replace it with new, healthy bone. However, as the bone breaks down and rebuilds, the shape of the femoral head can become flattened or irregular instead of maintaining its normal round ball shape. This change in shape can affect how well the hip joint works, both during childhood and later in adult life.^[2]

How Common Is Perthes Disease?

Perthes disease is considered uncommon, affecting a relatively small number of children worldwide. The condition occurs in approximately one out of every 12,000 children, though some geographic areas report higher rates. The exact number of affected children can vary significantly depending on the population studied and the region of the world.^[3]

Boys are affected much more frequently than girls, with studies showing that boys are about five times more likely to develop this condition. When girls do develop Perthes disease, they often experience more severe cases with worse outcomes. The disease most commonly appears in children between the ages of four and eight years old, though it can occur in children as young as two or as old as fifteen. The peak age for diagnosis tends to be around five to seven years of age.^[2][4]

In most cases, only one hip is affected. However, in approximately 10 to 15 percent of children with Perthes disease, both hips become involved. When both hips are affected, they typically don’t develop symptoms at the same time, with one hip usually showing signs before the other. Children with bilateral involvement should always be carefully evaluated to rule out other conditions that can affect both hips.^[2][5]

The condition appears to be more common in white populations compared to other ethnic groups. Some studies have also found that children who are very physically active, smaller than average for their age, and those exposed to secondhand cigarette smoke may be at higher risk.^[3][4]

What Causes Blood Supply Problems in Perthes Disease?

Despite more than a century of medical study since Perthes disease was first described, the exact cause remains unknown. Doctors and researchers refer to it as an idiopathic condition, which simply means the underlying cause hasn’t been identified. What is clear is that something disrupts or completely blocks the blood vessels that supply the femoral head, but why this happens is still a mystery.^[1][3]

The blood supply to the femoral head in children comes through small arteries that can be vulnerable to disruption. When these blood vessels become blocked or compressed, the bone cells in the femoral head can no longer receive oxygen and nutrients. Without this vital blood supply, the cells begin to die in a process called avascular necrosis, which literally means bone death due to lack of blood flow.^[2]

Several theories have been proposed about what might trigger this blood supply interruption. Some researchers have suggested that blood clotting abnormalities might play a role, where small clots could block the tiny arteries feeding the femoral head. Others have pointed to possible trauma or repetitive stress injuries to the hip that could cause swelling, which in turn might compress blood vessels from the outside. Joint stress, inflammation, and metabolic factors have all been investigated as potential contributors.^[6][4]

Environmental and lifestyle factors may also contribute to the development of Perthes disease. Malnutrition, obesity, and exposure to cigarette smoke have all been studied as possible risk factors. Some research has explored whether genetic factors might make certain children more susceptible, though no single gene has been definitively linked to the condition in most cases.^[4]

Who Is Most at Risk?

While any child can potentially develop Perthes disease, certain characteristics appear more frequently in affected children. Understanding these risk factors doesn’t mean a child will definitely develop the condition, but it helps doctors maintain appropriate awareness when evaluating hip problems in young patients.^[7]

Age is one of the most significant factors. Children between four and ten years old are in the highest risk window, with the peak incidence occurring around ages five to seven. Very young children under age six generally have better outcomes when they do develop the disease, partly because they have more time remaining for their bones to grow and remodel. Children diagnosed when they are older than eight years tend to have more challenging cases with potentially poorer long-term results.^[6][4]

Gender plays a major role, as boys develop Perthes disease approximately four to five times more often than girls. However, when girls are diagnosed, they often experience more severe disease and worse outcomes. This gender difference remains one of the puzzling aspects of the condition that researchers continue to study.^[7][5]

Children who are small for their age or have delayed bone maturation appear to be at higher risk. Many children with Perthes disease are shorter than their peers and have what doctors call “bone age delay,” meaning their skeletal development lags behind their chronological age. These children are often described as being very physically active despite their smaller stature.^[3]

Exposure to secondhand tobacco smoke has been identified as a potential environmental risk factor. Children living in homes where adults smoke may have an increased chance of developing Perthes disease, though the exact mechanism for this association is not fully understood.^[3]

Some studies have suggested that low birth weight, lower socioeconomic status, and living in certain geographic regions might be associated with higher rates of Perthes disease, though these findings are not consistent across all populations studied.^[5]

Recognizing the Symptoms

The symptoms of Perthes disease often develop gradually, which can make the condition easy to overlook initially. Parents might first notice that their child has begun to walk with a slight limp, especially when tired or after physical activity. In many cases, this limping appears before the child complains of any pain, and it may come and go in the early stages.^[1][7]

When children do experience pain, it most commonly affects the hip area, but the location can be misleading. Pain may be felt in the hip itself, but it can also appear in the groin, thigh, or even the knee. This phenomenon, called referred pain, occurs because nerves in the body can transmit pain signals to areas away from the actual problem. Some children will complain only of knee pain, which is why doctors examining children with knee complaints always check the hips as well.^[1][6]

The pain associated with Perthes disease typically worsens with activity and improves with rest. Children may be more uncomfortable after running, jumping, or playing sports. The pain is usually mild to moderate rather than severe, though during periods of increased inflammation, it can become more intense. Some children experience very little pain despite significant changes visible on imaging studies.^[1][5]

As the disease progresses, children often develop stiffness and reduced range of motion in the affected hip. They may have difficulty spreading their legs apart or rotating the hip inward. This stiffness occurs because the hip joint becomes inflamed and irritated as the bone breaks down. Activities that require hip flexibility, such as putting on shoes or getting in and out of a car, may become more challenging.^[7]

Over time, muscle changes can become noticeable. The thigh muscles on the affected side may appear thinner or smaller than those on the other leg, a condition called muscle atrophy. This happens because the child unconsciously uses that leg less to avoid discomfort, causing the muscles to weaken and shrink from lack of use. Similarly, the affected leg may appear shorter than the other leg, leading to an uneven gait.^[6]

In later stages, children may develop an unusual walking pattern called Trendelenburg gait, where the pelvis tilts down on the affected side with each step. This occurs because the hip muscles have weakened and can no longer properly support the pelvis during walking. Parents might notice that their child’s hips appear uneven when they walk or stand.^[6]

Can Perthes Disease Be Prevented?

Unfortunately, with current medical knowledge, there is no way to prevent Perthes disease from developing. Because the exact cause remains unknown, there are no specific interventions that can stop the blood supply interruption from occurring in the first place. This can be frustrating for families who naturally want to protect their children from health problems.^[8]

However, while the initial development of Perthes disease cannot be prevented, its progression and long-term complications can potentially be minimized with early detection and appropriate management. This makes awareness of the symptoms important, especially for parents of children in the at-risk age group. Seeking medical evaluation promptly when a child develops a persistent limp or hip pain can lead to earlier diagnosis and treatment.^[7]

For children already diagnosed with Perthes disease, preventing further damage to the femoral head becomes the primary goal. Doctors typically recommend avoiding high-impact activities such as running and jumping during the active phases of the disease. These activities can put extra stress on the weakened bone and potentially cause more deformation. Instead, children are encouraged to participate in low-impact activities like swimming and biking, which allow them to stay active while protecting the healing hip.^[7][9]

Maintaining a healthy lifestyle may support better outcomes. Ensuring children have good nutrition to support bone health, maintaining a healthy weight to reduce stress on the hip joint, and avoiding exposure to secondhand smoke are all sensible measures, even though their direct impact on Perthes disease outcomes hasn’t been definitively proven.^[4]

Regular follow-up with healthcare providers is essential for children with Perthes disease. Close monitoring through physical examinations and imaging studies allows doctors to track how the disease is progressing and adjust treatment plans as needed. This vigilant approach helps ensure that any concerning changes are caught early and addressed before they lead to more serious complications.^[7]

How the Disease Changes the Hip

Understanding what happens inside the hip during Perthes disease can help families appreciate why certain treatments are recommended and what the healing process involves. The disease follows a predictable sequence of stages, each involving specific changes to the bone structure of the femoral head.^[2]

The hip joint is normally a ball-and-socket joint, where the rounded ball-shaped head of the femur fits snugly into a cup-shaped socket in the pelvis called the acetabulum. This design allows for a wide range of motion while providing stability. The smooth, round shape of the femoral head is critical for proper joint function.^[2]

In the initial stage of Perthes disease, called the necrosis or initial stage, blood supply to the femoral head becomes disrupted. Without oxygen and nutrients from blood, the bone cells begin to die. The area becomes intensely inflamed and irritated, triggering the body’s immune response. During this phase, the child may begin showing symptoms like limping or hip discomfort. On x-rays, the femoral head may appear smaller or denser than normal. This initial stage typically lasts several months to about one year.^[2][3]

The second stage, known as fragmentation, occurs when the body begins removing the dead bone. Special cells called osteoclasts break down and absorb the necrotic bone tissue. At the same time, the body starts forming new, softer bone to replace what was lost. On imaging studies, the femoral head looks like it’s breaking into pieces, which is how this stage got its name. This is the most critical phase because the bone is in its weakest state and most vulnerable to becoming flattened or misshapen from normal weight-bearing forces. The fragmentation stage can last from six months to two years.^[3][10]

During fragmentation, soft tissue changes around the hip also occur. The joint develops synovitis, which is inflammation of the tissue lining the joint. The cartilage covering the femoral head may become thickened, and the ligament inside the hip joint can become enlarged. These changes, combined with muscle spasm, can cause the femoral head to shift outward from its normal position in the socket, a condition called extrusion. When the femoral head is extruded, it’s no longer properly contained within the socket, which increases the risk of deformation.^[11]

The third stage, reossification or healing, is when new, stronger bone continues to develop and the femoral head begins to solidify again. Blood flow has been restored, allowing the bone to receive nutrients for growth and remodeling. The shape that the femoral head takes during this stage will largely determine the long-term outcome. This is often the longest stage of the disease, potentially lasting one to three years.^[3][10]

The final stage, called the healed or remodeling stage, occurs when bone regrowth is complete. Normal bone cells have replaced the newer, weaker bone formed during reossification. However, instead of maintaining its original round ball shape, the femoral head may have taken on a flatter, wider, or more mushroom-like appearance. The degree of deformation varies greatly among children and depends on multiple factors including age at onset, severity of involvement, and effectiveness of treatment during earlier stages.^[10]

The socket side of the joint can also undergo changes during Perthes disease. In some cases, the acetabulum may become shallower or more vertically oriented as it responds to the changing shape of the femoral head. These socket changes can affect how well the joint functions even after the femoral head has finished healing.^[11]