Scanogram leg length measurement after total hip arthroplasty: not all landmarks are created equal

To compare the magnitude of post-arthroplasty leg length discrepancy (LLD) and incidence of clinically significant LLD measured on CT scanogram using a commonly used measurement method (from the acetabular apex to tibial plafond) to an alternative technique avoiding the use of the acetabular prosthesis as a landmark and to assess inter-observer and intra-rater reliability of the new technique.

In this retrospective study, post-arthroplasty LLD measurements were conducted in 100 hips by two interpreters on CT scanogram scout views from the acetabular apex to the tibial plafond (AA-TP) and the inter-teardrop line to the tibial plafond (IT-TP). Aggregate means and proportions of clinically relevant LLD (≥ 10 mm) were compared between methods. Inter-rater reliability was calculated, and both interpreters repeated measurements on ten randomly selected patients to calculate intra-rater reliability.

The commonly used AA-TP technique overestimated LLD by 3.7 mm compared to the IT-TP technique. The odds of LLD measurement exceeding the clinically significant threshold of 10 mm were 3.8 times higher when using the AA-TP technique. Excellent inter-rater (ICC 0.984, 0.958) and intra-rater reliability (ICC > 0.9) were found for both techniques.

CT scanogram measurements from the acetabular apex to the tibial plafond often overestimate operative limb length due to reference landmarks in different axial planes. Measurements from the inter-teardrop line to the tibial plafond yield significantly lower LLD values, possibly reflecting actual limb length better. The authors recommend using the inter-teardrop line and tibial plafond as reference landmarks to improve LLD assessment accuracy post-arthroplasty.