Figure 1. Anterior-posterior skull radiograph.
Figure 2. Lateral skull radiograph.
Inca bones are a result of faulty ossification in the interparietal area of the occipital squama.1 The occipital squama consists of a supraoccipital area that develops in cartilaginous bone and an interparietal area that is ossified in membranous bone. Inca bones form when an additional pair of ossification centers develop in the interparietal area and do not fuse with the remainder of the occipital bone. They are located at the lower border of the interparietal occipital suture and slightly above the external occipital protuberance. The Inca bones fuse completely by age 40 years.1
The distinction between the Inca bone and a skull fracture is accomplished by noticing the irregular contour of the edges of the Inca bone compared with the smoother, linear course of a skull fracture.2 The distinction is important in avoiding the patient and the family experiencing an evaluation for nonaccidental trauma.
Accepted for publication September 22, 1999.
Reprints: Kimberly Parente, MD, Medical College of Georgia, 1120 15th St, Augusta, GA, 30912-3770.
Radiological Case of the Month. Arch Pediatr Adolesc Med. 2001;155(6):732. doi:10.1001/archpedi.155.6.731