There is currently no known cause of Pectus Excavatum (PE). Several ideas have been hypothesized, but none are fully supported by data:
1. Intrauterine compression, rickets, pulmonary restriction, diaphragm alterations resulting in posterior traction on the sternum, and failure of osteogenesis or chondrogenesis.
2. Overgrowth of costal cartilage (this is the current prevailing theory).
Even though this is still a widely accepted view, this theory has been proven false in several research studies. The latest can be found here.
3. Abnormal metabolism of Collagen Type II, a major component of rib cartilage
For example, abnormalities such as (1) altered distribution and arrangement of Type II collagen in costal cartilage, (2) abnormal high levels of zinc (citation), and (3) low levels of magnesium and calcium have been demonstrated in the past. Electron microscopy has shown (4) degenerated chondrocytes with fat droplets and other osmiophilic inclusions as well as (5) atypical fibrils with long-spacing collagen.
4. Alteration in endochondral ossification and metabolism of acid mucopolysaccharides.
5. Premature cartilage aging.
6. Retraction of the weak costal cartilage during fetal development causing retraction at birth.
7. Inherited disorder
There have been several cases described in the literature of family members across generations having PE. However, there are also many instances when those with PE have no affected family members. Therefore, it seems that in certain cases, PE may be inherited, and in others, it may not be. This is currently an active area of research with no clear answers yet.