Study Guide

Genu Valgum

Key Points:

Description:

Pathologic genu valgum refers to the pathologic condition of persistent or worsening genu valgum in a patient older than 7 years of age. This needs to be differentiated from physiologic valgus, which is normal during early childhood growth and generally resolves by age 7 (Hensinger, 1986; Klin, 1983; Salenius, 1975; Heath, 1993; Green, 1994). Knock knee is the layman’s term for genu valgum. Primary care physicians and orthopedic surgeons should encourage evaluation for genu valgum, especially in the growing child or adolescent.  Treatment options become more limited after skeletal maturity.

Epidemiology:

Clinical Findings:

Physiologic femoral-tibial valgus first becomes apparent at 2 years of age and increases to a maximum of generally 8-10 degrees by 3-4 years of age (Klin, 1983; Heath, 1993; Green, 1994). This valgus then gradually decreases over the next few years reaching a stable adult knee valgus of 5-7 degrees by 7 years of age (Salenius, 1975; Heath, 1993; Green, 1994). However, a broad range of physiologic knee valgus has been reported (Salenius, 1975; Heath, 1993; Green, 1994). At 3 to 4 years of age, a femoral-tibial angle from 2 degrees varus to 20 degrees valgus is considered normal while neutral to 12 degrees valgus is normal over the age of 7 (Hensinger, 1986; Klin, 1983; Salenius, 1975).
 
While physiologic knock-knee normally occurs during early childhood and spontaneously resolves by age 7, genu valgum refers to the pathologic condition that tends to develop in early adolescence and does not spontaneously resolve after age 7. Asymmetric genu valgum is more likely to be pathologic than physiologic. Most children younger than 6 years of age who are being evaluated for genu valgum actually have physiologic knock-knee and are considered normal. While valgus deformity can arise from the distal femur or the proximal tibia, it is most often primarily from the femur.
 
Clinical examination should include torsional profile.  In the prone position, the amount of femoral anteversion and tibial torsion should be assessed and recorded.  Often the appearance of knock-knee alignment in adolescents and younger children is due to increased femoral anteversion and external tibial torsion.  However, if the patellae are positioned pointing directly forward, genu valgum may not be present. 
 
Genu valgum may appear worse clinically than radiographically. Besides deformity, knee pain is the most common patient complaint. Other associated features may include an out-toed gait and lateral patellar subluxation. The presence or absence of knee pain and/or functional limits with activities should be determined.

Imaging Studies:

An AP standing long-length film (bilateral hips to ankles) is recommended when clinical examination is consistent with pathologic genu valgum. It allows for assessment of the mechanical axis and joint deviation. The mechanical axis of the lower extremity is defined as the line from the center of the femoral head through the center of the ankle mortise, and generally passes through the medial tibial spine of the knee.
 
The proximal tibia can be divided into 4 equal-width partitions based upon a center-line between the tibial spines. The lateral 2 partitions are assigned positive values whereas the medial 2 partitions are assigned negative values. A normal mechanical axis passes through the middle half of the knee (Zone +1 or -1). Genu valgum that causes a mechanical axis to pass lateral to the tibial plateau (Zone +3) is considered pathologic. Correction of pathologic genu valgum will not only improve appearance and mechanical axis, but also will generally improve pain in the symptomatic patient (Herring, 1985; Stevens, 2004; Bartel, 1992).
 
When evaluating any knee deformity it is important to determine whether the deformity originates from the femur or tibia. The mLDFA (mechanical lateral distal femoral angle) is the lateral angle between a line from the center of the femoral head to the center of the tibia spines versus a line along the distal femoral condyles. The MPTA (medial proximal tibial angle) is the medial angle between a line from the center of the tibia spines to the center of the ankle mortise versus a line along the tibial plateau. Both angles are considered normal at 87°, and are definitively abnormal if they fall outside of the normative ranges of 85 to 90 degrees (Paley, 1992).

Treatment:

Physiologic knock-knee is expected to spontaneously resolve and therefore managed conservatively with observation. Since pathologic genu valgum does not self-correct and may cause significant morbidity, surgical treatment is indicated.
 
Surgical correction of a pathologic genu valgum with a mechanical axis lateral to the tibial plateau (Zone +3) or through the lateral fourth of the tibial plateau (Zone +2) with associated knee pain will not only improve appearance and mechanical axis, but may also delay the potential development of early-onset arthritis (Paley, 2002; Stevens, 1999). The threshold severity of genu valgum sufficient to cause degenerative changes is unknown.
 
Hemiepiphysiodesis and osteotomy are the operative treatments of choice (Paley, 2002; Stevens, 1999; Mielke, 1996). Hemiepiphysiodesis is used in skeletally immature children and is most often achieved by placement of medial physeal-spanning plates. Fixation can be placed on the distal femur, proximal tibia, or both depending on the location of deformity and expected growth time remaining. Fixation position is very important, with the plates placed extraperiosteally. Fixative devices should be placed centrally on the lateral view to prevent sagittal plane deformity, unless additional sagittal deformity is present and correction is desirable.  Other techniques such as transphyseal screw and tension band screw placement have been reported (Park, 2016).
 
Close follow-up every 3-4 months after hemiepiphysiodesis fixation placement is important to monitor for abnormal angular or rotational correction as well as overcorrection. Fixation removal within 24 months is sometimes recommended to prevent permanent physeal growth arrest, although this time interval has not be rigorously studied. Slight overcorrection before removal of fixation may be considered in children who begin hemiepiphysiodesis treatment before the age of 10 due to the risk of rebound medial overgrowth and loss of correction. Radiolographic rebound of genu valgum has been reported following implant removal (Farr, 2016).  However, factors leading to rebound still need to be better defined, and treatment of children under 10 years of age is reasonable as long as the patients and families are aware that repeat treatment is a possibility.
 
Osteotomy can be performed when immediate correction of the valgus deformity is desired or if the patient is skeletally mature (Davis, 1998; Rajacich, 1992). While a variety of osteotomy techniques are available, it is advisable to correct the bone(s) with deformity based on analysis of mLDFA and MPTA. While the distal femur is most often the site of correction; when correction of valgus is performed in the tibia, the peroneal nerve is at risk, and decompression of the peroneal nerve with a larger acute correction or gradual correction with external fixation should be considered.

Complications:

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References:

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  3. Farr, Sebastian, et. al.  Rebound of frontal plane malalignment after tension band plating. J Pediatr Orthop 2016 (In review)
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