Blount’s disease is a growth disorder that causes the shinbone (tibia) to bow outward just below the knee. It happens when excessive pressure on the inner part of the knee’s growth plate slows or stops normal bone development on that side, creating a progressive bowing that worsens over time if left untreated. Unlike the mild bow-leggedness that most toddlers naturally outgrow by age three, Blount’s disease gets worse rather than better.
What Happens Inside the Bone
The top of the tibia has a growth plate, a strip of cartilage where new bone forms as a child grows. In Blount’s disease, the inner (medial) side of this growth plate gets compressed beyond what it can handle. That compression damages the cartilage and delays the process of turning cartilage into solid bone. Meanwhile, the outer side of the growth plate keeps growing normally. The result is an increasing tilt: the inner side falls behind, and the leg angles outward into a bowleg shape that deepens over time.
Two Types Based on Age
Blount’s disease comes in two distinct forms. The infantile type appears before age four, often in children who started walking early. It frequently affects both legs and tends to progress through recognizable stages on X-ray if not caught early. The adolescent (or late-onset) type develops after age ten, usually affects just one leg, and is strongly linked to excess body weight during the rapid growth of puberty.
The distinction matters because the two types behave differently and respond to different treatments. Infantile Blount’s disease caught early can sometimes be corrected with bracing alone. Adolescent Blount’s disease almost always requires surgery.
Risk Factors
The strongest risk factor is excess weight. Heavier children put more compressive force through their knee growth plates with every step. A study of children with infantile Blount’s disease in Ghana found that 23% of affected children were overweight or obese, compared to just 5% in a control group of children without the condition. Their average weight-for-age was at the 57th percentile versus the 37th percentile for unaffected children.
Early walking age also plays a role. Children who begin walking before 12 months load their growth plates before the bone is fully prepared for that stress, particularly if they already have some degree of natural bowing. African and African-descended populations have higher rates of Blount’s disease. Research on 125 patients with late-onset disease found that Black children had significantly higher average BMI (37 vs. 32) and were three times more likely to be on Medicaid than non-Black patients, pointing to the intertwined roles of biology, body weight, and socioeconomic access to early care. Notably, Blount’s disease has also been reported at considerable rates in Scandinavian countries despite lower obesity prevalence, suggesting genetics contribute independently of weight.
How It’s Diagnosed
Distinguishing Blount’s disease from normal toddler bowing is one of the trickiest parts of diagnosis. Most children under two have some degree of bow-leggedness that resolves on its own. Doctors use X-rays and a specific measurement called the metaphyseal-diaphyseal angle to help tell the difference. This angle measures how sharply the top of the tibia is tilted.
In children with normal physiologic bowing, the angle averages about 9 degrees. In children with Blount’s disease, it averages about 19 degrees. An angle of 16 degrees or higher is a strong indicator of Blount’s disease, while anything between 9 and 16 degrees falls into a gray zone where the diagnosis isn’t certain from that measurement alone. Doctors in that range typically monitor the child with repeat X-rays every few months to see whether the bowing is improving (normal) or worsening (Blount’s).
Radiologists also use a six-stage classification system to grade how advanced the disease is based on the appearance of the growth plate and surrounding bone on X-ray. The earliest stages show subtle irregularities, while later stages show a characteristic beak-shaped deformity and, eventually, a bony bridge forming across the growth plate that permanently halts growth on the inner side of the knee.
Bracing for Early Cases
For infantile Blount’s disease caught early, a knee-ankle-foot orthosis (a long leg brace) can work by taking pressure off the compressed inner growth plate and allowing it to resume more normal development. Bracing works best when started before age three and when X-rays show only the earliest stages of disease (stages I or II). Beyond that window, the growth plate changes are typically too advanced for a brace to reverse.
The brace is worn for an extended period, often 12 or more hours a day. Compliance can be challenging, especially with toddlers. If the bowing hasn’t improved after a full course of bracing, surgery becomes the next step.
Surgical Options
Several surgical approaches exist, chosen based on the child’s age, how much growth remains, and how severe the deformity has become.
- Guided growth (tension band plating): A small plate with screws is placed on the outer side of the growth plate. This slows growth on that side while the inner side catches up, gradually straightening the leg over months. Because the plate sits outside the growth plate itself, it’s reversible: once the leg is straight, the hardware is removed and growth resumes normally. One study reported an 89% success rate with this technique in children with early-stage disease. It’s the least invasive surgical option and works well when significant growth remains.
- Corrective osteotomy: The surgeon cuts the tibia below the knee and realigns it into a straighter position. This is used when guided growth isn’t an option, such as in older children with little growth remaining, or when a rapid, complete correction is needed. Several techniques exist, including opening wedge (spreading the cut bone apart), closing wedge (removing a wedge of bone), and dome-shaped cuts. Some specialized cuts can correct rotation and angulation simultaneously, addressing up to 25 degrees of bowing in a single procedure.
- External fixation: A frame attached to the bone through pins allows gradual correction over weeks. This approach is reserved for severe or complex deformities involving multiple planes of malalignment, or cases where the leg also needs to be lengthened. It carries lower risks of nerve and blood vessel injury than a large one-time correction because the adjustment happens slowly. Patients can bear weight during the correction period.
In advanced cases where a bony bridge has already formed across the inner growth plate, surgery becomes more complex. The bridge itself may need to be addressed, and additional procedures to equalize leg length are sometimes required.
What Happens Without Treatment
Untreated Blount’s disease, particularly the infantile form, can lead to severe and permanent deformity. By about age six to eight, a bony bridge typically forms across the inner growth plate, permanently stopping growth on that side. At that point, simple corrective surgery is no longer enough. Patients need more involved reconstruction including correction of the angular deformity, rotational abnormalities, and leg length differences that have accumulated over years.
Even with treatment, some patients reach skeletal maturity with residual alignment issues. The abnormal loading pattern through the inner part of the knee over years of childhood can set the stage for premature osteoarthritis of the knee in adulthood. In those cases, a corrective osteotomy later in life may help delay the eventual need for knee replacement.
Outlook With Early Treatment
The earlier Blount’s disease is identified, the simpler and more effective treatment tends to be. Children diagnosed before age three with mild disease have the best outcomes, sometimes correcting fully with bracing alone. Those diagnosed later or with more advanced disease typically do well with surgery but may need more than one procedure as they grow. Adolescent patients generally need surgical correction but recover well, with most returning to full activity after healing. The key variable across all age groups is timing: catching the disease before irreversible growth plate damage occurs makes every treatment option more likely to succeed.