The aim of this retrospective cohort study was
to identify any difference in femoral offset as measured on pre-operative
anteroposterior (AP) radiographs of the pelvis, AP radiographs of
the hip and corresponding CT scans in a consecutive series of 100
patients with primary end-stage osteoarthritis of the hip (43 men
and 57 women with a mean age of 61 years (45 to 74) and a mean body
mass index of 28 kg/m2 (20 to 45)). Patients were positioned according to a standardised protocol
to achieve reproducible projection and all images were calibrated.
Inter- and intra-observer reliability was evaluated and agreement
between methods was assessed using Bland-Altman plots. In the entire cohort, the mean femoral offset was 39.0 mm (95%
confidence interval (CI) 37.4 to 40.6) on radiographs of the pelvis,
44.0 mm (95% CI 42.4 to 45.6) on radiographs of the hip and 44.7
mm (95% CI 43.5 to 45.9) on CT scans. AP radiographs of the pelvis
underestimated femoral offset by 13% when compared with CT (p <
0.001).
No difference in mean femoral offset was seen between AP radiographs
of the hip and CT (p = 0.191). Our results suggest that femoral offset is significantly underestimated
on AP radiographs of the pelvis but can be reliably and accurately
assessed on AP radiographs of the hip in patients with primary end-stage
hip osteoarthritis. We, therefore, recommend that additional AP radiographs of the
hip are obtained routinely for the pre-operative assessment of femoral
offset when templating before total hip replacement.