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Young boy with pseudoachondroplasia

Pseudoachondroplasia is a severe type of dwarfism characterized by joint laxity and severe deformities affecting the hips, knees, and ankles. Pseudoachondroplasia has many of the phenotypical features of Achondroplasia. If differs greatly from achondroplasia because it also affects the joint cartilage and not just the growth of the cartilage. Pseudoachondroplasia patients have more growth stenting than achondroplasia patients. Furthermore, they have significant malalignment deformities of the lower limbs. Because the joint cartilage is affected, there is deformation of the hips, knees, and ankles. They also have spinal involvement. In contrast, achondroplasia, although it produces varus and valgus deformities of the knees, only has mild deformities affecting the hips and ankles.

The typical deformities of the knee seen with pseudoachondroplasia are flexion deformity of the distal femur and recurvatum deformity of the proximal tibia. In addition, there is often varus or valgus deformities of the femur and tibia. Another common facet is a windswept deformity, which is characterized by valgus knee deformity on one side and varus knee deformity on the other side.

At the hip joints, there is often a flexion deformity of both hips with overgrowth of the greater trochanter and the femoral heads are misshapen into a more elliptical or mushroom shape. There is also an association with dysplastic acetabulum, usually bilateral.

At the ankle joints, there is often a valgus procurvatum deformity of the joints. The fibula is often short and the ankle subluxes (partially dislocates) laterally.

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