ICD-9-CM
Chapter 13. Diseases of the musculoskeletal system and connective tissue (710-739) gives a broad definition of what's included in each 5th digit.
i.e.
4 Hand
carpus, metacarpus, phalanges(fingers)
5 Ankle and Foot
Ankle joint, Digits (toes), metatarsus, Phalanges-foot, Tarsus, Other joints in foot.
Hope this helps, Happy Holidays