Blount Disease 

Blount disease is an uncommon growth disorder characterized by disordered ossification of the medial aspect of the proximal tibial physis, epiphysis, and metaphysis. This progressive deformity is manifested by varus angulation and internal rotation of the tibia in the proximal metaphyseal region immediately below the knee. The natural history of this disease leads to irreversible pathologic changes, especially at the medial portion of the proximal tibial epiphysis because of growth disturbances of the subjacent physis.^{[1, 2]}

Blount disease can occur in growing children of any age and is classified into 2 groups: early onset and late onset. Early onset (in children < 3 y) is termed the infantile type. The late-onset group includes the juvenile form (in children aged 4-10 y) and adolescent form (in those aged 11 years and older) of the disease. Juvenile tibia vara usually is discussed with the infantile type, and the remainder of this article addresses infantile and juvenile types as part of the broader grouping of the infantile type.^{[3, 4, 5]}

In 1922, Erlacher described the first case of tibia vara. However, Blount's article in 1937 prompted the recognition of this disorder. Blount presented a series of 13 new cases and reviewed the 15 cases in the literature.^[6] He delineated the similarities between infantile and adolescent tibia vara and emphasized the differences in their etiology. Because he was the first to identify the similar clinical, radiographic, and pathologic characteristics of the cases in the literature, the disease has become associated with Blount.^[7]

Tibia vara and osteochondrosis deformans tibiae are two other terms that have been used to describe the deformity of Blount disease. Blount suggested the anatomic term tibia vara, which is the generally accepted term.^[6] However, the term does not identify the specific location of the abnormality, nor does it indicate the etiology of the disease. The term osteochondrosis deformans tibiae is not accurate because it describes a disorder in which the primary or secondary centers of ossification undergo avascular necrosis.^[7] Avascular necrosis has never been found in either form of Blount disease.^[8] Hence, Blount disease and tibia vara continue to be the most commonly accepted terms for the disease.

Disordered growth of the proximal medial physis, epiphysis, and metaphysis of the tibia results in a progressive varus deformity below the knee.

Frequency

The estimated prevalence of infantile Blount disease in the population of young children with significant bowlegs in the United States is 0.007, or less than 1%; the prevalence of adolescent Blount disease may reach 2.5% in the population at greatest risk (see image below).^{[9, 10]} The exact frequency in persons of all ethnicities is unknown and most likely is less than 1%. In addition to race and body weight, the frequency is increased if other family members have been diagnosed as having Blount disease.^{[11, 12]}

The cause of Blount disease remains controversial, but it is most likely secondary to a combination of hereditary and developmental factors. Biomechanical overload of the proximal tibial physis due to static varus alignment and excessive body weight have been implicated in the etiology of infantile tibia vara. The compressive forces at the medial aspect of the knee appear to cause growth suppression. Although similar processes may be implicated in the development of adolescent tibia vara, static varus alignment is not a prerequisite.^[13] Dynamic gait variation secondary to increased thigh girth has been suggested to be implicated in the development of adolescent Blount disease.^[13]

A number of authors have noted a positive family history of Blount disease in some affected individuals. Data supporting inheritance are limited but worthy of mention. Sevastikoglou and Eriksson based this contention on the finding of 4 persons with tibia vara in the same family, of whom 2 were identical twins.^[12] Schoenecker et al also found a positive family history in 14 of 33 patients.^[11] However, no direct proof of a genetic relationship has been discovered.

Blount disease most likely is caused by a combination of excessive compressive forces on the proximal medial metaphysis of the tibia and altered endochondral bone formation.^{[6, 14, 15]} It is unclear whether the deformity is caused by an intrinsic alteration of bone formation that is exacerbated by compressive forces or by compressive forces that cause a disruption in normal endochondral bone formation.

Weight bearing must be necessary, since the disease does not occur in nonambulatory patients.^[16] Cook et al correlated epidemiologic and histologic findings in a model that provided evidence for the role of biomechanical overload in the pathogenesis of infantile tibia vara. They analyzed static single-limb stance in children and determined that 10° and 20° varus deformities, in children aged 2 years and 5 years, respectively, could generate compressive forces adequate to retard growth of the medial tibial physis.^[16]

The combination of mechanical and biologic factors in tibia vara most likely impacts the disease to varying extents. Furthermore, excessive physiologic bowing often is found in individuals with the infantile form of the disease. It is known that epiphyseal compression inhibits physeal growth (the Heuter-Volkmann law) and distraction stimulates growth.^[17] Delpech demonstrated this stimulation by showing that release of abnormal pressure from a physis causes increased vertical growth.^[17] Such compressive forces cause a relative inhibition of growth of the medial portion of the proximal tibial physis, as compared with the lateral portion.

It is also known that damaged cartilage ossifies more slowly.^[18] Histologic sections of cartilage in the infantile form show damaged cartilage. If the cartilage on the medial aspect of the plateau is damaged, ossification is delayed on the medial side of the tibia compared with the lateral side. The result is a progressive varus angulation below the knee and an increase in the compressive forces on the physis, which changes the direction of the weightbearing forces on the upper tibial epiphysis from perpendicular to oblique. The obliquity of this force tends to displace the tibial epiphysis laterally. The trabecular pattern of the metaphyseal region in the tibia curves medially to align itself to the deviation of the stress.^[19]

Many authors believe that disease progression is the result of this cycle of growth disturbance, varus deformity, and further growth disturbance.^{[8, 14]} Distal femoral valgus or varus deformity and/or distal tibial varus or valgus deformities also can occur in conjunction with tibia vara.^{[20, 21]} Whether these occur as compensatory mechanisms or are due to intrinsic factors of Blount disease is unknown. These deformities should be corrected at the same time the tibial vara deformity is corrected.

Histologic specimens from the medial tibial condyle in the infantile form of the disease show changes principally in the zone of resting cartilage in the proximal tibial physis. These changes consist of (1) islands of densely packed cells that exhibit a greater degree of hypertrophy than would be expected from their topographical location, (2) islands of almost acellular fibrous cartilage, and (3) abnormal groups of capillary vessels.^[14]

The pathogenesis of the adolescent form of the disease remains less clear than that of the infantile form. Some authors consider the 2 forms to have similar pathophysiology, while other authors consider them to be separate entities. Adolescent Blount disease does not appear to be as progressive or as common as the infantile form. Factors such as injury or infection of the physis have been suspected to play an etiologic role; however, most patients have no history of trauma or infection, leading many authors to discount them as the only possible causes.^{[6, 8, 22, 23]}

The clinical presentation of the different types of tibia vara varies according to the age of onset. In infantile tibia vara, children generally start to walk early, usually when aged 9-10 months.^[14] At the onset of the disease, differentiating between early infantile Blount disease and marked physiologic bowlegs is difficult.

Physiologic genu varum is a common torsional deformity that occurs secondary to normal in utero positioning. The tight posterior hip capsule causes an external rotation of the thigh at the hip. When combined with internal tibial torsion, the resulting appearance is a varus deformity. This physiologic deformity usually resolves spontaneously by the time the child is aged 2 years. In contrast to physiologic genu varum, infantile Blount disease can progress to severe deformity.

The infantile form is generally more prevalent in females, blacks, and those with marked obesity. It is associated with a prominent metaphyseal beak, internal tibial torsion, and leg-length discrepancy; involvement is bilateral in approximately 80% of cases.^[19] The metaphyseal prominence, or beak, may be palpable over the medial aspect of the proximal tibial condyle. Patients usually do not complain of pain. However, the deformity of the lower extremity can be quite pronounced.^{[6, 7, 11]}

In contrast, patients with adolescent tibia vara usually complain of pain at the medial aspect of the knee. These patients are typically overweight or obese. In contrast to infantile tibia vara, involvement is unilateral in 80% of cases; the involved leg sometimes is shorter than the opposite leg by as much as 2-3 cm. The degree of varus deformity usually is not as severe as in individuals with the infantile form and usually does not exceed 20°.^[19]

Indications for operative treatment include increasing severity of symptoms or progression of deformity.

For direct exposure for osteotomies on the medial aspect of the knee or pin fixations, surgeons must be aware of the location of the infrapatellar branch of the saphenous nerve. On the lateral side, it is the course of the peroneal nerve around the fibula that deserves attention. One must be aware that lengthening or shortening procedures can cause injury to the anterior tibial artery. Avoidance of injury to the neurovascular structures is paramount in obtaining a good result.

Osteotomies in the epiphyseal or metaphyseal region of the proximal knee must necessarily avoid the epiphyseal plate in the growing child to prevent premature closure. It is important to remember that the epiphyseal plate is often "V" shaped, with the apex pointing inferiorly in the proximal tibia and superiorly in the tibia.

Surgical intervention is contraindicated in children who are younger than 2 years because it is difficult at this age to differentiate between Blount disease and excessive physiologic bowing that may resolve spontaneously. In patients with adolescent Blount disease, surgical intervention is recommended only when the patient complains of pain associated with the deformity.