Genu Valgum

► [Decreased distance between the knees with a wide gap separating the ankles when legs are fully extended]

In normal children, genu valgum - also termed knock-knee - represents the recovery phase from physiological bowleg and usually resolves by 6-7 years of age. Genu valgum persisting beyond that age is abnormal. There are several mechanisms involved in its development. Weakening of the muscles inserted at the knees and joint laxity certainly play a part. Articular joint diseases with preferential involvement of the lateral femorotibial articulation may be implicated. Congenital deficiency of the lateral portion of the distal femur or proximal tibia is another potential mechanism. Genu valgum occurring as a compensatory mechanism of ipsilateral progressive coxa vara has also been described (Shim et al. 1997).

Genu valgum is one symptom of a number of distinct disorders in which hypotonia and muscular weakness or ligamentous and capsular laxity are the cardinal manifestations (Swischuk and John 1995). For example, in homocystinuria (OMIM 236200),genu valgum and patella alta are manifestations of joint laxity. In this disorder, abnormal joint laxity at some joints typically occurs in conjunction with flexion contractures at other joints (Brenton et al. 1972). Multiple growth recovery lines, elongation of the limbs, and osteoporosis are additional features. Two distinct processes can lead to knee malalignment in osteogenesis imperfecta (OMIM 166200): fracture deformities, resulting in distortion of the articular surfaces and joint incongruity; and ligamentous and capsular laxity. Bowing of the long bones, especially of the tibias, can be associated with genu valgum in neurofibromatosis type 1 (OMIM 162220). Skeletal changes in this condition include pathologic fractures with defective healing and pseudarthrosis. In turn, failure of fracture healing may cause growth inhibition at

Fig. 5.30 a, b. Spondylometa-physeal dysplasia, Algerian type, in an 8-year-old boy. There is strikingly short stature (boy's height is 85 cm., the 3rd normal percentile being 115 cm), limb shortening (most prominent in the humeri), genu valgum, severe metaphyseal alterations (irregularities, fragmen-tation,sclerosis),grossly unremarkable epiphyses, and coxa vara. (From Kozlowski et al. 1988)

Fig. 5.30 a, b. Spondylometa-physeal dysplasia, Algerian type, in an 8-year-old boy. There is strikingly short stature (boy's height is 85 cm., the 3rd normal percentile being 115 cm), limb shortening (most prominent in the humeri), genu valgum, severe metaphyseal alterations (irregularities, fragmen-tation,sclerosis),grossly unremarkable epiphyses, and coxa vara. (From Kozlowski et al. 1988)

Genu Valgum Coxa Vara

the tibial physis, thereby resulting in genu valgum. Angular deformities of joints other than the knee are also possible (Gregg et al. 1982). A high incidence of genu valgum has been reported in patients with fluorosis, a condition caused by chronic intoxication with fluorine. As already mentioned, the main radiographic abnormalities in this disorder include diffuse osteosclerosis, periostitis, ligamentous calcifications, vertebral osteophytosis, and hypoplasia and irregularity of the teeth (Resnick 1995). Whether genu valgum is secondary to muscular weakening, a neurological complication of fluorosis, or to changes in the mechanical properties of the bones, is not known. Genu valgum is also described among the side-effects of deferoxamine, an iron-chelating agent used in patients with systemic hemochromatosis. The toxic effects of this drug to the skeleton are most prominent in the metaphyses of the tubular bones, particularly at the knee and wrist, and consist of metaphyseal widening, cupping, fraying, and cystic changes of the subchondral bone. In these cases genu valgum represents a mechanical adjustment to the primary metaphyseal lesions (de Virgiliis et al. 1988). Intrinsic disturbances of epiphyseal growth rate such as are encountered in multiple epiphyseal dysplasia (OMIM 132400) can result in either genu varum or genu valgum deformity, depending on whether the growth deficiency, and the resultant epiphyseal abnormalities, are predominant at the medial or at the lateral side of the knee, respectively. In addition to being flattened, the epiphyses are variably delayed in appearance, irregular, and fragmented. Short hands and feet, joint stiffness and pain, and limping are additional manifestations (Hunt et al. 1967). A distinct disorder of limb malalignment,termed St. Helena familial genu valgum (OMIM 137370) because of the geographical localization and anatomical distribution of the abnormalities, has been described by Beighton et al. (1986). In this disorder of autosomal dominant inheritance, malalignment is most prominent at the knees, but is also evident at the elbows and wrists. Severe genu valgum is due primarily to hypoplasia of the lateral femoral condyles and is further exacerbated by progressive degenerative osteo-arthropathy. Another condition in which genu val-gum deformity is a cardinal feature is spondylometa-physeal dysplasia, Algerian type (OMIM 184253). The clinical and radiographic manifestations of the disease include severe dwarfism, genu valgum, myopia, progressive kyphoscoliosis, wrist deformities, and severe metaphyseal dysplasia of the long tubular bones (Fig. 5.30a,b). The hands and feet are little affected. The metaphyseal changes in this disorder are intermediate in severity between the mild alterations of the common type of spondylometaphyseal dysplasia, Kozlowski type (OMIM 184252) and the severe changes seen in metaphyseal dysplasia, Jansen type (OMIM 156400). The delayed bone age in Kozlowski spondylometaphyseal dysplasia and the severe involvement of the hands in Jansen metaphyseal dys-

plasia are additional features distinguishing these from the Algerian type of spondylometaphyseal dysplasia (Kozlowski et al. 1988).

Genu valgum and degenerative osteoarthritis of the knee are closely intertwined. Osteoarthritic narrowing of the articular space on the lateral side of the knee leads to genu valgum deformity. On the other hand, in the presence of genu valgum, the weight-bearing forces pass through the lateral compartment of the knee, thus predisposing to osteoarthritis. In rheumatoid arthritis, osseous changes about the knee characteristically involve both the medial and the lateral femorotibial compartments. However, there are cases in which the lateral femorotibial compartment is affected more severely than the medial, thus resulting in valgus deformity of the knee.

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