Blount Disease

Etiology

Blount disease has a multifactorial etiology involving both biological and mechanical factors. Although mechanical overloading of the proximal tibia is a significant contributor, especially in children who are overweight and start walking early, it is not the sole cause of the condition. The infantile form, which also affects normal-weight children, and the higher prevalence among African-American patients indicate a potential hereditary component. In addition to mechanical overload, genetic susceptibility is also implicated in the development of Blount disease.

Epidemiology

The prevalence of Blount disease in the United States is <1%. The infantile form predominates and is more prevalent in males than females. In 80% of cases, infantile Blount disease manifests bilaterally. Conversely, adolescent Blount disease, also recognized as adolescent genu varum, is generally less severe and more likely to be unilateral. This condition is most prevalent among children of African and Scandinavian ancestry.

Pathophysiology

Blount disease results from growth inhibition induced by excessive compressive forces, leading to cartilage damage and a subsequent delay in ossification. Mechanical overload leads to altered enchondral bone formation. As the compressive forces peak at the posteromedial aspect of the physis, patients undergo a heightened growth inhibition on the medial side of the knee, leading to the development of a continual varus deformity.[2]

History and Physical

Genu varum is typically considered a normal finding in children until age 2, after which alignment transitions to valgus, reaching its peak around age 3. In children who are young and overweight, persistent genu varum often serves as the initial indicator for diagnosis.[3] As the growth disorder progresses, knee deformities and associated abnormalities worsen gradually, resulting in a 3-dimensional deformity that combines varus, procurvatum, internal tibial rotation, and limb length discrepancy.

Infantile Blount Disease

Infantile Blount disease is typically diagnosed in children between the ages of 1 and 3. This condition manifests as a bilateral condition characterized by varus deformity of the tibia and internal tibial torsion. Pain is generally rare, and a palpable "beak" may be evident over the medial aspect of the proximal tibial condyle.

The identification of lateral thrust, indicating lateral translation of the knee joint during weight-bearing, is a crucial clinical observation. Irreversible asymmetric medial proximal tibial epiphysiodesis usually develops around the ages of 6 to 8, making conservative treatments ineffective. 

Adolescent Blount Disease

Adolescent Blount disease typically emerges in children in their late childhood or early adolescence period and is often accompanied by pain in the medial aspect of the knee. The condition is often associated with being overweight or obese. The presentation of the disease is often unilateral and may involve associated abnormalities of the distal femur.

Evaluation

History, physical examination, and plain radiography constitute sufficient means for diagnosing Blount disease. In the initial stages, clinicians use long-leg anteroposterior radiographs to screen for and measure varus. Accurate measurement requires bilateral projection of the radiograph from the hip to the ankle.

Indicators of Blount Disease

Findings suggestive of Blount disease include medial beaking of the epiphysis, widened and irregular medial physis, irregular ossification, and medial slope of the epiphysis and metaphysis in varus.

Angles For Blount Disease Detection

Healthcare professionals refer to specific angle measurements to detect Blount disease in children. Various angles, such as the Levine-Drennan angle, are used to assess the relationship between the tibia shaft and its upper growth plate. An angle measurement exceeding 11° typically indicates the presence of Blount disease. The angles necessary for Blount disease detection are mentioned below.

Metaphyseal-diaphyseal angle: The metaphyseal-diaphyseal angle (MDA) can predict the progression of Blount disease.

• The intersection occurs between a line drawn from the most distal point on the medial and lateral beaks of the tibial metaphysis to a line perpendicular to the long axis of the tibial diaphysis.[4][5]

• The disease progression can be predicted as follows:
• An angle >16° is associated with a 95% risk of deformity progression.[4][5]
• An angle <10° is likely physiological, with a 95% chance of spontaneous resolution.[4][5]
• An angle between 11° and 16° necessitates close observation for the potential tibia vara progression.[4][5]

• The angular abnormalities include intra- and extra-articular varus malalignment, internal tibial rotation, procurvatum, distal tibial valgus, lateral and medial laxity, and distal femoral deformities.[4][5]

Tibiofemoral angle: The tibiofemoral angle measures the severity of the varus deformity.

Medial metaphyseal beak angle: The medial metaphyseal beak angle (MMBA) is a potential diagnostic screening tool for individuals at risk of Blount disease. When combined with the MDA, MMBA can confirm the diagnosis, resulting in earlier diagnosis and improved patient outcomes.

Magnetic resonance imaging (MRI) effectively assesses cartilage, menisci, ligaments, and vascularity of the physis. In addition, MRI outperforms radiographs in detecting cartilaginous changes.[6] Thus, gadolinium-enhanced MRI proves beneficial to pediatric patients with neglected or delayed forms of Blount disease observed after age 4 but before the development of radiographic epiphysiodesis. 

Langenskiöld Classification System

Blount disease has been classified into 6 stages by Langenskiöld. The stages indicate a rise in severity and medial physeal collapse, and the staging is specifically applied to the infantile form. A physeal bar becomes evident from stage V onward due to an injury or infection due to the disruption of normal cartilage in a growing physis. The healing process involves bone instead of cartilage. Physeal bars cause angular deformities and limb length discrepancies in skeletally immature children. Although MRI-based classifications, such as Fort-de-France (FDF), have become more prevalent recently, x-ray-based classification remains the most widely used.[7]

Stages of ligament laxity: The 4 recognized stages of ligament laxity are as follows:

• Stage 0: Normal laxity
• Stage +: Medial laxity 
• Stage ++: Lateral laxity 
• Stage +++: Multidirectional laxity 

Langenskiöld classification system: The 6 stages of the Langenskiöld classification system are as follows:[7][8]

• Stage I: Irregularity of metaphyseal zone 
• Stage II: Medial metaphyseal beaking 
• Stage III: Development of "step" in the metaphyseal beak
• Stage IV: Epiphysis beaking and occupies a pit in the medial metaphysis
• Stage V: Double epiphyseal plate
• Stage VI: Bony bar formation

Treatment / Management

The treatment of Blount's disease depends on the child's age and the severity of the deformity. The discrepancy between chronological and skeletal age diminishes as the chronological age increases. Due to the possibility of limited remaining growth in some patients, a preoperative assessment of skeletal age is crucial as it can affect the timing and magnitude of the correction. When indicated, surgical intervention aims to restore normal joint and limb alignment, achieve equal limb lengths at skeletal maturity, and prevent recurrence.

Brace

KAFOs are a potential treatment option for children diagnosed before age 4 and with Langenskiöld stage I or II disease. Bracing extends from the upper thigh to the foot and applies valgus force to the knee. Orthotic treatment is most likely to be successful when initiated before age 3 in non-obese children who primarily wear the brace at night. The treatment duration continues for 1 year. In cases where orthoses prove ineffective, osteotomy should be considered before age 4 when indicated. However, fitting a brace on a boisterous child aged 3 or younger poses a significant challenge and limitation to this treatment.[9] Notably, 80% of pediatric patients with progressive early-onset Blount disease undergoing surgical axis realignment before age 4 achieve a full recovery. The emergence of lateral thrust during weight-bearing is a characteristic sign indicating the onset of mechanical knee failure.

Guided Growth

Hemiepiphysiodesis, or guided growth, is a surgical technique designed to correct angular limb deformities in skeletally immature patients progressively. As an alternative to corrective osteotomies, hemiepiphysiodesis offers cost-effectiveness, reduced pain for patients, shorter immobilization times, and diminished surgical risks. 

The most common procedure is hemiepiphysiodesis of the lateral epiphysis with extraperiosteal implants such as staples, pins, or tension band plating. Bone growth on the side of the growth plate where pinning or plating occurs stops or slows down, allowing the other side of the growth plate to grow normally and gradually straighten the bone over time. An advantage of this approach is that the entire physis can resume growth after hardware removal. 

The child should have at least 4 years of growth for this procedure to be successful. The Hueter-Volkmann principle, which posits that compression inhibits longitudinal growth, forms the basis of this treatment modality. 

In contrast to genu varum associated with other disorders, the results of guided growth in Blount disease are less predictable, potentially due to the involvement of the diseased proximal medial physis. As per Schroerlucke, patients with Blount disease experience a 44% risk of tibial metaphyseal screw breakage after hemiepiphysiodesis.[10] For patients with moderate-to-severe Blount disease, it is recommended to use two parallel plates or non-cannulated solid stainless steel screws. [11] Hemiepiphysiodesis and guided-growth systems are now viable options for individuals with late-onset Blount disease, presenting a varus deformity of <15°, limb shortening not exceeding 1 cm, and having at least 2 years of skeletal growth. Surgical intervention is not deemed appropriate for children aged 2 and younger.[12](B2)

Osteotomy

Realignment osteotomy is typically performed before age 4 in pediatric patients with documented and progressive Blount disease or FDF stage I who demonstrate risk factors.[13] Due to the high recurrence rate in infantile Blount disease, it is expected to overcorrect osteotomies to achieve between 5º and 15° of valgus. The objectives of the osteotomy include lateral translation, 10° to 15° of lateral derotation, and 5° to 10° of valgus.

Various osteotomy techniques have been described for Blount disease, including opening and closing wedges, opening wedges, serrations, domes, and inclined osteotomies. The correction can be acute or gradual with external fixation. Gradual correction leads to more precise mechanical axis and leg length discrepancy corrections. A systematic review comparing acute versus gradual correction for Blount disease reveals weak evidence favoring gradual correction, whereas an acute correction results in a higher prevalence of transient peroneal nerve palsy. No difference in the reoperation rate exists between the 2 procedures.[14] With acute correction, there is a risk of peroneal nerve injury and compartment syndrome regardless of the kind of osteotomy and fixation method.[15](B2)

Acute Correction

During acute correction, the varus distal fragment is fixed in translation and external rotation to correct the internal rotation deformity. Other necessary surgical procedures, such as physeal bar resection and medial plateau elevation, are carried out concurrently with the osteotomy. If the bar is more than 50% of the size of the physis, hemiepiphysiodesis is appropriate. The osteotomy level should be positioned below the tibial tuberosity to prevent patella baja, which can lead to extensor insufficiency and knee pain.

Children aged 3 or older, regardless of the stage, or patients with stage III Blount disease, irrespective of age, are considered suitable for an osteotomy. Accurate measurement of limb alignment after acute or gradual correction can be challenging. To visualize mechanical axis alignment, some researchers utilize intraoperative fluoroscopy with the electrocautery cord placed across the skin, overlaying the center of the hip and ankle.[16]

The advantage of acute correction is its ability to correct the deformity immediately. However, this approach increases the risk of compartment syndrome and peroneal nerve injury due to acute lengthening. 

Gradual Correction

An osteotomy is performed during gradual correction, and a frame is connected to enable progressive correction. Commonly used devices include the Taylor Spatial Frame or Ilizarov Ring External Fixator. Postoperatively, a treatment duration of 12 to 18 weeks is typically required. Gradual correction minimizes the risk of neurovascular compromise and compartment syndrome while also allowing for correction of deformity in all planes. However, a potential drawback is pin site infection, considering the required length of treatment.

Asymmetrical Physeal Distraction

Asymmetrical physeal distraction is a procedure that is not used frequently. This procedure involves inserting 2 half-pins of 6 mm into the proximal tibial epiphysis and 2 pins into the diaphysis. After the pins are inserted, progressive distraction is done at a rate of 1.5 mm/d. A monolateral fixator typically achieves an average of 13° of angular correction. The limited popularity of this procedure may be attributed to the risks of septic arthritis, discomfort during distractions, and the potential for premature closure of the growth plate.[17]

Physeal Bar Resection

Unlike patients who develop physeal bars after trauma, patients with Blount disease patients typically do not have a distinct region of osseous tether that lends itself to surgical excision. Physeal bar resection aims to restore normal growth and prevent further deformity. Children who receive physeal bar resection or epiphysiolysis before age 7, combined with a valgus osteotomy at that time, demonstrate improved outcomes. However, children aged 7 and older are not considered suitable candidates for this surgery, and epiphysiolysis as a standalone operation has minimal use in these patients. [18][19](B2)

Medial Tibial Plateau Elevation

Blount disease may progress to the extent that the tibia can laterally translate, and the medial femoral condyle enters the posteromedial depression, leading to a varus thrust gait. Children aged 6 and older with severe Blount disease, at Langenskiöld stage V or VI, and exhibiting a substantial posterior depression of the medial tibial plateau are considered suitable candidates for posteromedial tibial plateau elevation. Surgeons recommend conducting an intraepiphyseal or transepiphyseal osteotomy that hinges on the articular cartilage of the intercondylar notch to preserve the medial tibial plateau. The emphasis should be on simultaneously correcting the posterior depression of the medial plateau by incorporating a larger portion of the graft. To prevent the recurrence of a varus thrust gait, healthcare professionals must perform lateral proximal tibial and fibular epiphyseodesis concurrently.[20][21](B3)

Differential Diagnosis

Distinguishing infantile tibia vara from physiological bowing of the legs can be challenging. Physiological bowing is characterized by the gradual curve development of both the tibia and femur, whereas proximal tibial bowing in Blount disease is acute. An MDA >11° is indicative of Blount disease in affected patients.

Other potential diagnoses on the differential include rickets, Ollier disease, proximal tibial physeal injury resulting from trauma, radiation, or infection, osteomyelitis, metaphyseal chondrodysplasia, and thrombocytopenia absent radius syndrome. Common findings in Blount disease include asymmetrical beaking and sharp angular deformities, which are typically absent in rickets. The presence of multiple enchondromas distinguishes Ollier disease.[22][23]

Prognosis

The prognosis for Blount disease depends on the age and severity at the time of presentation. Infantile Blount disease generally has a favorable prognosis, with low deformity recurrence rates when treated early. Partial or complete regression is achievable in patients in the early stages. However, those in the later stages will likely progress without early intervention. Patients with the late-onset form may experience progression and significant joint deformity if left untreated.[24]

Complications

Disease Complications

Disease complications can lead to the recurrence of deformity and joint degeneration.

Operative Complications

Operative complications may include deep venous thrombosis, vascular impairment, pathologic fractures, wound infection, malalignment, compartment syndrome, premature physeal closure, rebound accelerated growth, and hardware migration.