I have looked all over and there is no mention anywhere of where this 4", hockey-stick shaped scar came from.
Maybe he had his conscience surgically removed?
He also has a crease in his earlobe.
The National Institute of Health conducted a study on the value of the ear-crease sign in predicting the presence of coronary artery disease. The study consisted of 340 consecutive patients who underwent coronary arteriography. In this selected population, 75.6% of whom had coronary artery disease, the sensitivity of the sign was 59.5%, the specificity 81.9% and the positive predictive value 91.1%.
The sign was associated with increasing age but was also independently associated with obstructive coronary artery disease. No significant correlation was found between the sign and the presence of risk factors or other signs of such disease, except for corneal arcus. In symptomatic patients the sign suggested the presence of more extensive coronary artery disease. In an asymptomatic population with a low prevalence of coronary artery disease it appears to be of limited value in predicting obstructive coronary artery disease. However, it may identify a subset of patients prone to early ageing and to the early development of coronary artery disease, whose prognosis might be improved by early preventive measures.
Coronary artery disease (CAD) is an important health concern for African Americans, who are diagnosed with CAD at high rates.
The scar itself is interesting.
In "Dreams..," there are two references to scars. One applies to Obama, and that resulted from a large cut on his arm he received as a child in Indonesia (page 49).
Obama also tells of a wound he received on his head (page 36), also in Indonesia, when struck by a rock, thrown by another child. In the book, Lolo Soetoro describes the wound -- "it's not bleeding."
There's no mention of Obama's wound in the single-page "doctor's note" he released during the campaign (by comparison, John McCain released 1,173 pages of his medical records to the press).
So where did that nasty "hockey stick-shaped" scar on Obama's head come from? -- an accident? -- a mugging? -- an operation? You know he remembers.