CHAPTER 115

Angular Deformities of the Lower Extremity: Bowlegs and Knock-Knees

Kier Maddox Blevins, MD; Andrew K. Battenberg, MD; and Carol D. Berkowitz, MD, FAAP

With normal growth and development, the angular alignment of children’s legs progresses through a series of developmental stages, from relative bowlegs to knock-knees and eventually straight legs. Rotational problems, such as in-toeing and out-toeing, are deformities in the transverse plane that occur when a bone rotates internally or externally, respectively, along its long axis (Figure 115.1). Angular deformities, such as bowlegs and knock-knees, are deformities in the frontal plane. In bowleg deformity (ie, genu varum), the lower extremity distal to the knee joint is angled or tilted toward the midline of the body (tibial varus). In contrast, in knock-knee deformity (ie, genu valgum), the lower extremity distal to the knee is tilted away from the midline of the body (Figure 115.2). Variations in the knee angle that fall outside the normal range (eg, more than ±2 standard deviations of the mean) are referred to as genu varum for bowlegs and genu valgum for knock-knees (Box 115.1). Appreciation of the normal developmental sequence in conjunction with a careful history and physical examination can help pediatricians identify pathologic cases of bowlegs and knock-knees and initiate prompt management.

Epidemiology

Bowlegs and knock-knees are common in infants and children. Although all babies are born bowlegged, parents and guardians usually do not appreciate this finding until infants begin to walk. Knock-knees occur less frequently than bowlegs, occur more often in females than males, and are commonly associated with generalized ligamentous laxity. Although the frequency of these conditions is generally unknown, a study of more than 3,000 children aged 7 through 11 years found a prevalence of 7.9% for bowlegs and 2% for knock-knees. Other literature has shown that males are up to 4 times more likely than females to have bowlegs, whereas females were 3 times as likely to have knock-knees. Weight has also been shown to be an important associated factor in prevalence of these conditions. Bowlegs are more prevalent among underweight populations than in overweight individuals. In contrast, knock-knees are more common in overweight individuals compared with individuals with normal body weight. Pathologic cases of bowlegs and knock-knees are uncommon and are defined by the degree of angulation.

Clinical Presentation

Children with bowlegs have a characteristic wide-based stance with increased distance between the knees. They may walk with a waddling gait. In-toeing may be noted as a result of associated internal tibial torsion. Evaluation for pathologic bowlegs is required in cases in which the intercondylar distance (ie, distance between the knees) is more than 10 cm with the child lying supine with the medial malleoli touching.

Severe knock-knees may produce an awkward gait with the knees rubbing. Children may walk with the feet apart in an effort to avoid knee-to-knee contact. They may need to place 1 knee behind the other to stand with both feet together. Evaluation for pathologic knock-knees is warranted in cases in which the intermalleolar distance (ie, distance between the ankles) is more than 10 cm with the child lying supine with the knees touching.

Figure 115.1. Select anatomic reference and rotational planes. Rotational problems (double-sided arrow) are deformities in the transverse plane. Angular deformities are assessed in the frontal plane.

Figure 115.2. Illustration of varus (A) and valgus (B) deformities of the lower extremities. A, Bowleg deformity. B, Knock-knee deformity.

Pathophysiology

The normal variation in the angular alignment of the lower extremities changes with age. During the first 2 years of age, relative bowing of the legs is common. Although physiologic bowing of the lower leg may be appreciated at birth, it is most prominent during the second year after birth, when it most commonly involves both the tibia and the femur. In patients in whom the deformity is associated with internal tibial torsion, it may appear more striking. Physiologic knock-knees manifest between 3 and 4 years of age. The knock-knee stage resolves between 5 and 7 years of age, when normal adult alignment develops. A slight knock-knee appearance remains in normal adults. Bowlegs or knock-knees that do not follow the normal variation in angular alignment of the lower extremities require further evaluation for pathologic causes.

The tibiofemoral angle, that is, the angle between the long axis of the femur and the long axis of the tibia, is used to assess the angular alignment of the leg. At birth, the tibiofemoral angle is approximately 15° varus, and it decreases to 0° between ages 18 and 24 months. By age 3 to 4 years, the angle peaks at approximately 10° degrees of valgus angulation. Between 5 and 7 years of age it decreases to the normal range of approximately 7° to 9° in girls and 4° to 6° in boys (Figure 115.3).

Recent evidence has shown that weight-bearing activity may affect the alignment of lower extremity alignment at the knee in adolescent children. An association exists between normal, healthy, active adolescents who participate in weight-bearing field sports and some degree of genu varum. This small degree of bowlegs is not pathologic, although it is noted in older adolescent children who have displayed increased weekly activity levels and more years of participation in weight-bearing sports than their counterparts of the same age.

Differential Diagnosis

The most common causes of genu varum and genu valgum are presented in Box 115.2. Typically, physiologic bowlegs and knock-knees are usually bilateral, and they occur in a sequence that follows the normal developmental pattern. Lateral bowing of the tibia is commonly noted during the first year after birth. Bowleg, involving both the femur and the tibia, are pronounced during the second year after birth, and knock-knees become prominent between 3 and 4 years of age. This is the normal sequence of angular development. Typically, bowlegs is the result of normal physiologic processes, Blount disease (ie, tibia vara), or rickets. Common causes of pathologic knock-knees are severe renal rickets and a history of proximal tibia fracture.

Blount disease, a growth disturbance involving the posteromedial aspect of the proximal tibia (ie, physis, epiphysis, and metaphysis), is the most common cause of pathologic bowlegs and is the result of idiopathic undergrowth of the medial side of the tibia. The 2 forms of the disease are early-onset (ie, infantile) and late-onset and are defined by disease development before or after 4 years of age. The late-onset form is further subdivided into juvenile (4–10 years of age) and adolescent (>10 years of age). At all ages, Blount disease is more common in black individuals than those of other ethnicities.

Figure 115.3. Graph showing the development of the tibiofemoral angle.
Reprinted with with permission from Salenius P, Vankka E. The development of the tibiofemoral angle in children. J Bone Joint Surg Am. 1975; 57(2):259–261.

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