Posterior plagiocephaly involves a fusion of the lambdoid suture. This suture is located on the back of the skull. It has a right and left side and is shaped like an upside down "V." Usually, only one side fuses shut, but rarely both sides will be closed. When viewed from above, the affected side of the back of the head is flatter than the opposite side. One key marker for fusion of the lambdoid suture is a low bump behind the ear on the same side as the fused suture. Another good way to determine if the lambdoid suture is closed is to look and see if the ear on the same side as the posterior skull flatness is pulled backwards and sticks out more. If the ear is forward on the flat side, with respect to the opposite ear, then a skull deformation should be suspected instead of a fused suture. It is critically important to determine whether or not the child truly has a fused suture because skull deformations almost never need to be surgically treated. There have been a number of articles in the news media about children who were treated surgically for flatness on the back of the head for presumed lambdoid synostosis when, in fact, the child had a deformation and could easily have been treated without surgery, doing nothing or by using a headband. Making the diagnosis even more difficult is the fact that on plain skull x-rays the lambdoid suture is frequently misdiagnosed as being fused shut. Ideally, the diagnosis of lambdoid synostosis is made by CT scans, read at an experienced center. Children who have lambdoid synostosis and significant flattening of the skull do require surgery to fix this condition.

The incidence of posterior plagiocephaly is currently unknown and is probably even more rare than one in 100,000 births. The reason that we cannot give an accurate estimate of the incidence of this condition is that it has been over diagnosed in the past. It is probably one of the most rare forms of craniosynostosis treated at busy craniofacial centers. The treatment for this condition is discussed in the treatment section.