Radiographs acquired before total hip replacement in dogs underestimate femoral canal flare and misjudge trochanteric overhang

Abstract Objective To compare measurements of canal flare index (CFI) and greater trochanter overhang (TrO) from ventrodorsal (VD) and craniocaudal horizontal beam (CCHB) radiographic views to measurements from contemporaneously acquired CT scans and to evaluate the impact of size, age, radiographic view, severity of osteoarthritis, hip subluxation, and femoral rotational malposition on CFI and TrO measurement accuracy. Methods This was a retrospective study of femurs imaged from June 28, 2018, through March 27, 2023. The CFI and linear TrO index measured from VD and CCHB radiographs and from CT-derived surface renderings of the femur prepared with −10°, −5°, 0°, +5°, and +10° of rotation using computer-aided design software were compared. Results 80 femora from 43 dogs were included. Radiographs measured CFI with errors > 0.2 in 81% of VD views and 77% of CCHB views and yielded linear TrO measurements with errors > 20% of canal radius in 75% of VD and 74% of CCHB views. The TrO grade was incorrect for 44% of femurs on VD views and 30% of femurs on CCHB views. Internal femoral rotation of 10° significantly influenced CT measurements of CFI and TrO. Severity of osteoarthritis and hip subluxation did not influence measurements. Conclusions Measurements of CFI and TrO from VD and CCHB views are inaccurate relative to CT measurements. Clinical Relevance Radiographic measurements underestimate CFI and poorly predict TrO. A CT of the femur should be considered when accurate measurements of CFI and TrO are sought, particularly for femurs with abnormal geometry.