Tibial hemimelia (TH) manifests as a shortened leg with knee and ankle deformities. Most often those deformities are varus in the ankle and a knee flexion contracture in the knee. There is also typically instability of these joints due to a lack of collateral ligaments. The patella and quadriceps muscles may be present or absent, but will have limited function if present. The tibia may be shortened, dysplastic, or absent. The tibia may also present as an anlage—a fibrous remnant that does not appear on x-rays. The fibula may be normal or dysplastic and is often dislocated from the knee. There are usually associated deformities of the foot as well.
To learn more about Tibial Hemimelia, download Dr. Paley's recent book chapter on Tibial Hemimelia in Pediatric Lower Limb Deformities, published by Springer: Chapter 24: Tibial Hemimelia
Tibial hemimelia is extremely rare, occurring in only one out of one million live births. It can be diagnosed by ultrasound as early as sixteen weeks gestation. There have been multiple studies on the genetic mechanisms of tibial hemimelia with reports suggesting parent to child transmission in families with multiple siblings affected. Tibial hemimelia is associated with several syndromes: Werner’s syndrome, tibial hemimelia-polysyndactyly-triphalangeal thumb syndrome (THPTTS), and CHARGE syndrome, to name a few. Tibial hemimelia can be unilateral or bilateral, with 30% of cases reported bilateral. For some unknown reason, TH seems to favor the right side (72% of unilateral cases). Tibial hemimelia may also be present alongside additional congenital deformities, such as congenital femoral deficiency.
The deficiency in tibial hemimelia is a spectrum of pathology, ranging from a congenitally short tibia with relative fibular overgrowth to complete absence of the tibia. Tibial hemimelia is to be distinguished from fibular hemimelia (progressive deficiency of the fibula). An important distinction between the two conditions is that the foot and ankle are always in varus (pointing inward) with tibial hemimelia and almost always in valgus (pointing outward) in fibular hemimelia. Furthermore, the progression of deficiency of the tibia in TH is from distal to proximal (ankle to knee), contrary to FH where the progression is proximal to distal (knee to ankle). Tibial hemimelia should also be distinguished from congenital deficiency of the knee. Both of these conditions demonstrate congenital deficiency of the tibia and relative fibular overgrowth. In order to prevent confusion, TH should be classified as an incremental deficiency from distal to proximal (ankle to knee).
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