Reliability of the modified lateral pillar classification for Legg Calvé Perthes disease performed by a large group of international paediatric orthopaedic surgeons

Abstract

Purpose: The modified lateral pillar classification (mLPC) is used for prognostication in the fragmentation stage of Legg Calvé Perthes disease. Previous reliability assessments of mLPC range from fair to good agreement when evaluated by a small number of observers with pre-selected radiographs. The purpose of this study was to determine the inter-observer and intra-observer reliability of mLPC performed by a group of international paediatric orthopaedic surgeons. Surgeons self-selected the radiograph for mLPC assessment, as would be done clinically.

Methods: In total, 40 Perthes cases with serial radiographs were selected. For each case, 26 surgeons independently selected a radiograph and assigned mLPC and 21 raters re-evaluated the same 40 cases to establish intra-observer reliability. Rater performance was determined through surgeon consensus using the mode mLPC as 'gold standard'. Inter-observer and intra-observer reliability data were analysed using weighted kappa statistics.

Results: The weighted kappa for inter-observer correlation for mLPC was 0.64 (95% confidence interval: 0.55 to 0.74) and was 0.82 (range: 0.35 to 0.99) for intra-observer correlation. Individual surgeon's overall performance varied from 48% to 88% agreement. Surgeon mLPC performance was not influenced by years of experience (p = 0.51). Radiograph selection did not influence gold standard assignment of mLPC. There was greater agreement on cases of mild B hips and severe C hips.

Conclusions: mLPC has low good inter-observer agreement when performed by a large number of surgeons with varied experience. Surgeons frequently chose different radiographs, with no impact on mLPC agreement. Further refinement is needed to help differentiate hips on the border of group B and C.

Level of evidence: III.

Keywords: Legg Calvé Perthes disease; inter-observer reliability; intra-observer reliability; lateral pillar classification; paediatric hip.

Fig. 1

Percent agreement of each individual rater’s mLPC relative to the group mode (i.e. gold standard) mLPC. The top five performer raters are raters 45, 61, 57, 50, 39.

Fig. 2

Percent rater agreement in mLPC on a case-by-case basis. The bars represent the percent of raters who were in agreement for the case. The mode mLPC for each case is denoted in brackets next to the case number along the y-axis.

Fig. 3

Radiograph of Case 14, with 100% rater agreement of a Group C hip.

Fig. 4

Radiograph of Case 25, with 90% rater agreement of a mild Group B hip.

Fig. 5

Radiographs of two cases with the lowest rater agreement (38%) for mLPC. (a) For this case (21), the mode mLPC was Group B, and the top five highest-performing raters assigned three different classifications (group B, B/C border, and C) and selected four different radiographs. (b) For this case (7), the mode mLPC was Group C, and the top-five highest raters reported three different classifications (B, B/C border, C) and three different radiographs were selected.

Fig. 6

Percent of raters that selected the gold standard mLPC and/or radiograph. The black bars represent the raters who assigned the same mLPC as the mode of the group (i.e. the gold standard mLPC) and, used the most commonly selected radiograph (gold standard radiograph). The dark-grey bars represent raters who selected the correct mLPC as the gold standard, but seleted a different radiograph to reach this conclusion. The medium-grey bars represent the group of raters who selected the gold standard radiograph, but did not assign the gold standard mLPC. The light-grey bars represent the group of raters who did not slect the gold standard mLPC nor the gold standard radiograph.