Perthes disease is a rare childhood hip disorder that interrupts the blood flow to the ball of the hip joint, the femoral head. This temporary disruption causes the bone tissue to die, a process known as avascular necrosis, which weakens the bone structure. The condition involves a complex sequence of bone breakdown and regrowth that can take several years to complete. This article examines the statistical rarity of Perthes disease, its official designation, global incidence rates, and the demographic factors that influence its occurrence in different populations.
Understanding the Condition
Perthes disease, also known as Legg-Calvé-Perthes disease, typically affects the hip joint in children between the ages of four and ten years old. The temporary interruption of blood supply leads to avascular necrosis, causing the weakened bone of the femoral head to gradually flatten or collapse.
The disease runs a complex course generally divided into four stages: initial necrosis, fragmentation, reossification, and remodeling. During fragmentation, the body removes dead bone and replaces it with new, softer tissue. The reossification stage involves the new bone hardening and taking shape, which can take up to two years. The goal of treatment is to ensure the femoral head heals into a smooth, rounded shape that fits properly into the hip socket, preventing long-term joint damage.
Global and National Incidence Rates
Perthes disease is classified as a rare condition due to its low incidence rate among the general population of children. The annual incidence typically ranges from 0.4 to 29.0 cases per 100,000 children under the age of 15 per year. This translates to a general lifetime risk of approximately one in every 1,200 children developing the condition.
The low occurrence rate means Perthes disease meets the criteria for an “orphan disease” or “rare disease” under international definitions. In the United States, a rare disease affects fewer than 200,000 people at any given time, a threshold Perthes disease falls well below. This official designation helps direct research efforts toward finding better treatments.
The significant range in reported incidence rates highlights the geographical variability of the disease. Rates vary from a low of 0.4 per 100,000 in parts of India to a high of 29.0 per 100,000 in the Faroe Islands. Studies in the United States report incidences around 2.84 to 5.5 per 100,000 children, similar to the 2.8 per 100,000 reported in a British study. These variations suggest that environmental or genetic factors influence the disease’s distribution.
Demographic Factors Influencing Prevalence
The rarity of Perthes disease is not uniform across all groups, as specific demographic factors significantly alter the risk profile. The most pronounced factor is sex, with the condition being much more common in boys than in girls. The male-to-female ratio is generally cited as 4-to-1 or 5-to-1.
Age at diagnosis is another defining factor. The peak age of onset is often cited between five and seven years old. The highest incidence rates have been observed in children aged two to five years, with the rate declining significantly for children aged nine to twelve. Cases diagnosed outside this typical window are considerably less common, confirming the condition is age-specific.
Geographical and ethnic variations also create distinct high- and low-prevalence zones. Incidence is generally higher in Northern European populations and those living at higher latitudes. Children of African or East Asian descent have been found to have a very low rate of the disease. For example, the incidence in white patients (5.69 per 100,000) is significantly higher than the rate found in Asian patients (0.78 per 100,000).