The portion of the cross-sectional area of the tibia-fibula cortical bone that corresponds to the area circumscribed by the endosteum, the tissue lining the medullary or marrow cavity in the shaft of a long bone, i.e. the cross-sectional area of the medullary cavity of the tibia-fibula cortical bone. The cross-sectional area is the extent of a plane through, that is, perpendicular to the longest axis of the tibia-fibula, the portion of the two bones (tibia, the medial and larger bone, and fibula, the lateral and smaller bone) of the lower leg of bipeds or hindlimb of quadrupeds which are fused to form a single bone, e.g. the distal third of the rodent tibia and fibula. Cortical or compact bone is the relatively solid type of bone tissue found in the middle of long bones which is a hierarchical composite material consisting of a collagen-hydroxyapatite composite at the lowest level forming a complex network of cylindrical units of laminated bone at the highest level.