Chinese character 腂

腂 has several dictionary meanings, not one fixed sense.

Chinese references record meanings involving redness or red swelling, raised skin, and a medicinal herb. In the Eastern Han, the Chinese word-origin dictionary 《释名》 used 腂 in a gloss for 踝, the usual character for “ankle.” Rhyme dictionaries also preserve its readings in separate entries.

Is 腂 another form of 踝?

Only in its historical ankle sense.

《释名》 uses 腂 as an alternative form in an ankle gloss; 踝 is the ordinary modern character for that body part. The other dictionary senses of 腂 are separate, so don’t substitute it for 踝 in ordinary writing.