With regard to the fixation of joints of the lower limbs often seen in individuals with SA/CRS, there is a reason proffered for why the fixation occurs. Arthrogryposis (arth-ro-grip-OH-sis) is the medical term to describe a child born with joint contractures. This means some of their joints don’t move as much as normal and may even be stuck in one position. Often the muscles around these joints are thin, weak, stiff or missing. Extra tissue may have formed around the joints, holding them in place. Read more information about fetal akinesia here.

The main cause of arthrogryposis is fetal akinesia. This means the baby does not move around inside the womb as much as normal. Starting in early pregnancy, moving helps a baby’s joints, muscles, and tendons develop. If a baby does’t move much, these parts may not develop well, and extra tissue may form in the joints, making movement harder. For children with SA/CRS, because part of the spinal cord is absent, nerve signals don’t reach the baby’s muscles, which is why the baby’s lower limbs do not move as much. The baby’s muscles, tendons, bones, or joints do not form normally. Therefore, the lack of development of the complete spine and spinal cord affects the movement of the baby’s legs during pregnancy, and reduced movement (akinesia) then impacts the way that the baby’s legs grow.

Although the suggestion that fetal akinesia is responsible for the characteristic of fused or fixed lower limb joints among children with SA/CRS, it does not completely explain hallmark the webbing that may be present only for children with SA/CRS. Nor does it explain why other conditions that impact movement of the limbs in the womb, such as spina bifida, do not produce the extremely small size of pelvis and legs and fixed joints in the same way as what is seen in SA/CRS. It also does not explain why many children diagnosed with arthrogryposis do not have any spinal conditions at all.