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

Abstract

Objective: 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.

Materials and methods: 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.

Results: 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.

Conclusion: 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.