How Reliable is a J-sign Severity Scale When Assessing Lateral Patellar Instability?

Abstract

Background: Patellar instability is a common cause of anterior knee pain and can limit function and sports participation. To help assess patellar instability, the clinical J-sign test consists of observing the patella translate laterally in the shape of an inverted J over the anterolateral femur proximal to the trochlear groove during active knee extension. Only positive or negative categorization of the J-sign test has typically been used without rating the severity. The purpose of this study was to assess the inter- and intra-observer reliability of a grading/severity scale of the J-sign test.Methods: A scale for J-sign severity was utilized as follows: grade 0: unable to complete J-sign due to pain or apprehension; grade 1: ≤1 quadrant of translation; grade 2: >1quadrant of translation; grade 3: >2 quadrants of translation; grade 4: complete patellofemoral dislocation. This retrospective cross-sectional study assessed J-sign ratings (0 to 4) from videos of patients undergoing evaluation for patellar instability. Six healthcare professionals rated the severity of the J-sign using the proposed scale, two different times, for all knees presented in random order. Inter- and intra-observer reliability were calculated using a Fleiss Kappa, κ.Results: Forty-four patients (87 knees) ages 10-18 were included in this study. Both knees were rated, including unaffected knees to serve as a control. The proposed standardized grading scale for the J-sign had fair agreement for inter-observer reliability, κ = 0.31, and moderate agreement for intra-observer reliability, κ = 0.58.Conclusion: The proposed scale for determining J-sign severity yielded fair inter-observer reliability and moderate intra-observer reliability, similar to the Kappa scores evaluating only the presence or absence of the J-sign. Further study into developing a standardized scale for J-sign severity grading might improve clinical descriptors of the test and expand on other factors, including clarity of knee extension ability, video standardization, and training materials.Level of Evidence: Level III, retrospective cross-sectional study.

Key concepts: •Inter-observer agreement on a 0 to 4 rating scale of J-sign for lateral patellofemoral maltracking was fair while intra-observer agreement was moderate.•The four-quadrant scale of patellofemoral instability may not reproducibly assess lateral patellar maltracking.•Further development of a revised grading scale for lateral patellar maltracking is warranted.

Figure 1

Visual depiction of J-sign severity scale.

Figure 2

Example still pictures of participant with knee flexed (A) and knee extended (B). Pictures are intended to show how the test was completed. Videos were used for rating scale.