Source: See citation below

Pearl:  When applying the Ottawa Ankle Rule, we specifically palpate the posterior maleoli.

Explanation: When reviewing the Ottawa Ankle Rule today during morning report the question came up: why do we specifically palpate the posterior lateral and medial maleoli to assess for bony tenderness? The image above, from the initial derivation study and also reproduced on MDCalc, describes the intended areas of palpation. Looking into the original paper, the authors explain that they chose the posterior aspect of the lateral maleolus in order to avoid confusing ATFL (most common ankle sprain tear) tenderness with bony pain. The posterior lateral maleolus keeps you clear of the ATFL. The posterior medial maleolus appears to have been chosen because it had the best inter-rater reliability on that side. So there you have it.

Source:  Stiell IG, Greenberg GH, McKnight RD, Nair RC, McDowell I, Worthington JR. A study to develop clinical decision rules for the use of radiography in acute ankle injuries. Ann Emerg Med. 1992; 21:384–90.