Straight-line measurement of posterior wall as described by Calkins et al. for calculation of “approximate acetabular fracture index” (ApAFI). A, Straight line medial-lateral measurement is made of remaining intact articular posterior wall acetabular segment at level of greatest amount of fracture involvement (X). B, Length of posterior acetabular arc is determined from uninjured, contralateral hip at same level (Y). X divided by Y multiplied provides the index percentage.