A: Malleolar fractures may be coded on either the tibia or fibula. AIS 2005 Update 2008 provides codes for isolated medial malleolar fractures under tibia and codes for bimalleolar and trimalleolar fractures under the fibula. Be sure to read the boxed bold directive under the fibula which reminds the coder to use Fibula NFS if the specific site of the bimalleolar fracture is unknown.