Introduction

Genu valgum or "knocked knees" are part of the coronal plane deformities of the lower extremity. The majority of patients are asymptomatic and have no functional limitations. This condition can be preceded by flat feet and occasional medial foot and knee pain. Children start developing physiologic genu valgum starting by age 2, and it becomes most prominent between ages 3 to 4. After that, it typically decreases to a stable, slightly valgus position by age 7 years. In the adolescent age group, minimal, if any, change in this alignment is expected. Intermalleolar distance has been used to assess the degree of genu valgum. It is the distance between the medial malleoli in a standing patient with touching medial femoral condyles. Intermalleolar distances greater than 8 cm is considered pathologic.[1] Rarely, in cases where valgus alignment continues to increase, it can be associated with an out-toed gait, lateral subluxation of the patella, and rubbing of the knees together as the child ambulates.[2][3]

Etiology

Bilateral genu valgum

• Physiologic genu valgum
• Skeletal dysplasias
• Metabolic bone diseases
• Lysosomal storage diseases

Unilateral genu valgum

• Post-traumatic
• Tumors
• Infection

Epidemiology

Most patients present to the clinic between ages 3 to 5 for the evaluation of genu valgum. The most common site of pathologic deformity is the distal femur, however, it can arise from the tibia as well.

History and Physical

Most patients present to the clinic between ages 3 to 5 when parents generally become concerned about knocked kneed appearance. Bilateral genu valgum in this age group is typically physiologic but can also be secondary to skeletal dysplasia such as spondyloepiphyseal dysplasia and chondroectodermal dysplasia (Ellis van Creveld syndrome), metabolic bone diseases such as rickets (renal osteodystrophy and hypophosphatemic rickets) and lysosomal storage disease such as Morquio syndrome. Unilateral genu valgum is most often secondary to physeal or metaphyseal trauma. Radiographs should be assessed for physeal narrowing, premature closing, and the presence of growth recovery lines (Park-Harris lines), giving attention to their morphology.

Cozen phenomenon is a post-traumatic valgus deformity seen after proximal tibial fractures.[4] Of note, this can be seen even in the presence of non-displaced fractures.[5] The most accepted theory of this phenomenon is the increased vascularity that occurs during fracture healing resulting in medial metaphyseal overgrowth. Other causes of genu valgum include radiation, infection, and tumors (osteochondromas, multiple hereditary exostoses, fibrous dysplasia).

Evaluation

Gait and rotational profile analysis are important aspects in the workup of angular deformities and help providers to identify the etiology of angular deformities, especially in the pediatric population. Primary or true valgus deviations about the knee can present as a stance-phase valgus thrust as seen in metabolic bone disease like renal osteodystrophy and longitudinal deficiency of the fibula that is associated with lateral femoral condyle hypoplasia. Secondary or apparent valgus gait deviations are associated with both axial and sagittal plane deviations. For example, increased femoral anteversion has an apparent valgus angulation attributed to internal rotation of the distal femur. 

Radiographs are not indicated in children in the physiologic valgus phase. However, they are indicated in the setting of asymmetrical findings, excessive genu valgum clinically, age group beyond which is expected of physiologic changes, patients whose height falls below the tenth percentile for their age, and a history of trauma or infection. Radiographic assessment begins with obtaining weight-bearing long leg alignment images in which both patellae are facing forward.  Coronal plane angulation of the lower extremities can be analyzed based on the deviation of the center of the knee from the mechanical axis and the tibiofemoral angle. The mechanical axis is a line connecting the center of the femoral head to the center of the ankle. In normal coronal alignment, the mechanical axis passes through the center of the knee. There is lateral and medial deviation of the center of the knee with respect to the mechanical axis of the lower extremity in genu varus (bowed legs) and genu valgum (knocked knee), respectively.

The tibiofemoral angle is the acute angle formed between the longitudinal axes of the tibial and femoral shafts.  At birth, there is between 15 to 20 degrees of varus tibiofemoral angulation. As the child grows, this corrects to neutral by about age 2 and between 10 to 15 degrees of valgus tibiofemoral angulation between ages 3 and 4. At this point, the limb’s valgus angulation then starts to gradually decrease to approximately 3-5 degrees of valgus by age 7. This is the residual normal coronal plane angulation of the lower extremity that will be carried to adulthood and should not increase.[6]

It is important to determine whether the deformity is primarily originating from the femur or tibia. This is done by measuring the mechanical lateral distal femoral angle (angle between the femoral shaft and the mechanical axis of the femur) and medial proximal tibial angle (angle between the tibial plateau and mechanical axis of the tibia). The normal range of these angles is between 85 and 90 degrees.[7]

Treatment / Management

Observation is indicated for physiologic genu valgum or if the tibiofemoral angle is <15 degrees in a child <6 years of age. The use of bracing is typically not indicated in treating genu valgum.[8] In cases of post-traumatic valgus deformity of the tibia, the maximum deformity is often seen at 1 year following injury. Observation for 1 to 2 years is also indicated in this setting because most will resolve spontaneously with no functional deficit.[9][10][11] Medical management is the first-line treatment of angular deformities attributed to metabolic disorders. Some of these may resolve as the underlying disorder is treated.[12]

Guided growth with hemiepiphysiodesis is typically used to correct angular deformities. Implant choices include screws, plates, or staples that are placed extraperiosteally. Indications of medial sided hemiepiphysiodesis about the distal femur and/or proximal tibia include >15 to 20 degrees in a child <10 years of age and if the mechanical axis falls on the lateral quadrant of the proximal tibia in a patient >10 years of age. Once skeletal age and physeal width have been established, the Green-Anderson growth charts can help predict the timing of epiphysiodesis for angular correction.[13] In general, the younger the patient, the more rapid a correction is obtained. In these cases, removal of hardware may need to be performed to avoid over-correction to varus. Since it is a tendency for “rebound” growth, it is recommended to allow over-correction prior to hardware removal.[14][15] Patients are closely monitored between 4 to 6-month intervals.

Osteotomies about the distal femur are also used in the treatment of genu valgum. A medial closing wedge osteotomy or a lateral opening wedge osteotomy can be performed when the deformity is primarily about the distal femur (most common). Indications are patients who are skeletally mature or near skeletal maturity, where there is insufficient growth remaining to benefit from guided growth.[16][17] An external fixator can also be used, especially when lengthening is required as well.[16][18]

Differential Diagnosis

Physiologic genu valgum must be differentiated from its pathologic counterpart. Further evaluation is indicated when patients present with asymmetrical findings, excessive genu valgum clinically, in an age group beyond which is expected of physiologic changes, patients whose height falls below the tenth percentile for their age, and a history of trauma or infection.

Prognosis

Physiologic genu valgum and Cozen's phenomenon almost always correct spontaneously. Pathologic genu valgum associated with metabolic bone diseases may improve when the underlying disorder is treated. The rate and threshold of pediatric genu valgum that leads to adult degenerative joint disease is unknown.

Complications

Complications can arise when there is a failure to recognize pathologic genu valgum. These are most often related to the underlying etiology. 

Some of the complications arising from surgical intervention include:

• Physeal injuries when implants are placed subperiosteal
• Over or under correction of the deformity
• Neurovascular insult
• Infection

Deterrence and Patient Education

Most patients present to the clinic between ages 3 to 5 when parents generally become concerned about the knocked kneed appearance. Parents should be educated that genu valgum in this age group is often physiologic that improves spontaneously with age.

Enhancing Healthcare Team Outcomes

Genu valgum is often a clinical diagnosis. Patients with this condition are typically asymptomatic but may present with medial sided knee and/or ankle pain. While a history and physical exam are sufficient for diagnosis, there are certain indications that warrant further evaluation for pathologic conditions. While the primary care provider is usually the first clinician involved in the care of these patients, it is important to consult with an interprofessional team of specialists that include an orthopedist, geneticist, and a pediatric endocrinologist depending on the underlying etiology. The radiologist also plays a vital role in determining the cause. Without providing a proper history, the radiologist may not be sure what to look for or what additional radiologic exams may be needed. The Pediatric Orthopaedic Society of North America recommends long-leg alignment imaging for evaluating pathologic genu valgum. Further, hemiepiphysiodesis is the treatment of choice in patients who are skeletally immature and osteotomies in patients who are skeletally mature. Outcomes are improved with prompt consultations and good communication with subspecialty groups.