Blount's Disease
Blount's Disease Definition
Blount's disease is a growth disorder involving the tibia (shin bone) that causes the lower leg to angle inward, resembling a bowleg.
Blount's Disease Pathophysiology
The common factor in tibia vara cases is abnormal stress placed on the posteromedial proximal tibial epiphysis that leads to growth suppression. Predisposing factors for the development of the condition have included obesity, early walking, and black ancestry. Black children have been noted to have an excessive amount of ligamentous laxity, and they begin to walk at an earlier age. Both of these factors expose them to a risk of Blount disease. Obesity and early walking exaggerate the impact of physiologic bowing and heighten the stress placed on the physis of the proximal tibia. Altered mechanical forces in the proximal tibia lead to abnormal axial loading, which results in a change in the direction of the weight-bearing forces from the perpendicular to the oblique. The oblique angle tends to displace the tibial epiphysis in a lateral direction, overloading the posteromedial segment and stopping its growth. A cycle of further longitudinal growth is established, and this leads in progressive varus deformity. Unless the disease is diagnosed and treated early, the condition can worsen. Histologic evaluation generally confirms the physeal changes. A disordered columnar arrangement of the cartilage cells and suppression of normal endochondral growth are taken note of, especially on the medial side of the proximal growth plate. The three major types of tibia vara are the following: infantile, late-onset juvenile, and late-onset adolescent. Infantile tibia vara is the most common type of this disease. The late-onset types may represent untreated or unrecognized forms of the infantile type or may occur after a neutral mechanical axis has been established.
Blount's Disease Prevalence
The incidence of Blount disease in the United States is not known.
Blount's Disease Symptoms and Signs
Clinically, children with Blount's disease present with bowing and length discrepancy in their lower limbs. A nontender bony protuberance can be palpated along the medial aspect of the proximal tibia, this representing the deformed medial tibial metaphysis. In late-onset tibia vara, shortening of the leg may be associated with pain and tenderness over the medial prominence of the proximal tibia. Obesity and bowing are usually evident on physical examination. Bowing can be unilateral or bilateral. An estimated 80% of infantile cases and 50% of late-onset cases are bilateral. On observing the standing child from behind, the bowing is centered just below the knee without involvement of the femur. Uncorrected internal tibial torsion is commonly found. On walking, a lateral thrust of the knee or sudden lateral knee movement with weight bearing may be seen as evidence of progressive tibia vara.