The mendosal suture (also known as the accessory occipital suture) is a normal calvarial suture.
The suture extends through the occipital bone, lying superior to the occipitomastoid suture and inferomedial to the lambdoid suture. It closes in utero or in the first few days of life. It is usually seen bilaterally but can be unilateral.
The occipital bone has complex development, ossifying from six centers. The foramen magnum is surrounded by four ossification centers. On each side are the exoccipitals, ventrally located is the basioccipital and dorsally, the supraoccipital center contains the midline occipital fissure which can sometimes persist antenatally. This pattern of development can therefore give rise to numerous accessory sutures that could be mistaken for fractures especially with plain film evaluation alone.
An accessory occipital suture can occasionally persist into childhood, however the vast majority close before 6 years of age. One study of 500 dried skulls from adults and children in an Indian subcontinent population demonstrated a mendosal suture in 3% of skulls1.