Quantitative analysis of dynamic patellar tracking in patients with lateral patellar instability using a simple video system

What is the J-sign and why is it important?

PURPOSE OF REVIEW

Recurrent lateral patellofemoral instability is a complex condition that requires a thorough evaluation to optimize treatment. The J-sign test is classically part of the physical examination, but its significance and importance remain unclear. This review aims to describe how to perform the test and classify the observation as well as to analyze the most recent literature on its clinical applications.

RECENT FINDINGS

The J-sign test has been described as positive (present) or negative (absent), and classified using the quadrant method and the Donnell classification. Suboptimal inter-rater reliability has been shown for both classifications, making comparison between clinicians and studies challenging. The J-sign is most predominantly associated with patella alta, trochlear dysplasia, lateral force vector, and rotational abnormalities. A growing number of studies have shown a correlation between a positive J-sign and lower clinical outcome scores and higher rate of surgical failure.

SUMMARY

The J-sign is an important aspect of the physical examination in patients with recurrent lateral patellofemoral instability. Although there is no consensus on how to perform or classify the test, it can be used as a marker of severity of patellofemoral instability and is one of the tools available to guide the treatment plan.

Inter-rater Reliability of the Classification of the J-Sign Is Inadequate Among Experts

OBJECTIVE

The purpose of this study was to determine the inter-rater and intra-rater reliability of the symmetry, classification, and underlying pathoanatomy associated with the J-sign in patients with recurrent lateral patellofemoral instability.

STUDY DESIGN

Blinded, inter-rater reliability study.

SETTING

N/A.

PARTICIPANTS

Thirty patellofemoral joint experts.

INTERVENTIONS

Thirty clinicians independently assessed 30 video recordings of patients with recurrent lateral patellofemoral instability performing the J-sign test.

MAIN OUTCOME MEASURES

Raters documented J-sign symmetry and graded it according to the quadrant and Donell classifications. Raters indicated the most significant underlying pathoanatomy and presence of sagittal plane maltracking. Intra-rater reliability was assessed by 4 raters repeating the assessments. Mean pairwise simple and/or weighted Cohen's kappa were performed to measure inter-rater and intra-rater reliability, as well as calculation of percent agreement.

RESULTS

J-sign symmetry demonstrated fair inter-rater reliability (k = 0.26), whereas intra-rater reliability was moderate (k = 0.48). Inter-rater reliability for the quadrant and Donell classifications indicated moderate agreement, k = 0.51 and k = 0.49, respectively, whereas intra-rater reliability was k = 0.79 and k = 0.72, indicating substantial agreement. Inter-rater reliability of the foremost underlying pathoanatomy produced only slight agreement (k = 0.20); however, intra-rater reliability was substantial (k = 0.68). Sagittal plane maltracking demonstrated slight inter-rater agreement (k = 0.23) but substantial intra-rater agreement (k = 0.64).

CONCLUSIONS

The symmetry, classification, and underlying pathoanatomy of the J-sign demonstrated fair to moderate inter-rater reliability and moderate to substantial intra-rater reliability among expert reviewers using video recordings of patients with recurrent lateral patellofemoral instability. These findings suggest individual raters have a consistent standard for assessing the J-sign, but that these standards are not reliable between assessors.

LEVEL OF EVIDENCE

III.

Accuracy and Reliability of the Visual Assessment of Patellar Tracking

BACKGROUND

Abnormal patellar tracking is described as a "J-sign" when the patella exhibits excessive lateral displacement during active knee extension.

PURPOSE

To determine (1) the accuracy and reliability of the visual assessment of patellar tracking when viewed by surgeons with patellofemoral expertise and (2) whether surgeon experience (in years) correlates with the ability to accurately identify the presence and severity of patellar maltracking.

STUDY DESIGN

Cohort study (diagnosis); Level of evidence, 2.

METHODS

Using a web-based assessment, 32 orthopaedic surgeon members of the International Patellofemoral Study Group determined the presence or absence of maltracking (≥2 quadrants of lateral translation) in 10 single-knee videos of patients with patellar instability during active knee extension (qualitative analysis). Surgeons then graded patellar tracking in 20 single-knee videos as follows: 0 (<1 patellar quadrant of lateral translation), 1 (1 to <2 quadrants), 2 (2 to <3 quadrants), or 3 (≥3 quadrants) (quantitative analysis). Responses were compared with a previously described grading system using patellar bisect offset measured with 4-dimensional computed tomography. We evaluated the association between number of years of surgeon experience and the ability to correctly identify and grade patellar tracking. A total of 22 surgeons repeated the survey 3 months later, and their answers were matched to the first survey, allowing for assessment of intraobserver reliability.

RESULTS

In the qualitative analysis, surgeons correctly identified videos as showing patellar maltracking 68% of the time (κ = 0.45). In the quantitative analysis, 53%, 51%, 48%, and 68% of surgeons correctly identified maltracking of grades 3, 2, 1, and 0, respectively (κ = 0.42). Surgeon experience did not correlate with ability to identify the presence (P = .59) or grade (P = .35) of patellar maltracking. Respondent answers from the second survey demonstrated inadequate intraobserver reliability (κ = 0.48).

CONCLUSION

Using visual assessment alone, surgeons correctly identified patellar maltracking in approximately two-thirds of videos and correctly graded patellar maltracking in half. Inter- and intraobserver reliability were inadequate to support the use of visual assessment alone for detecting the presence or grade of patellar maltracking. Surgeon experience did not correlate with ability to identify the presence or grade of patellar maltracking.

Dynamic Evaluation of Patellofemoral Instability: A Clinical Reality or Just a Research Field? A Literature review

Patellofemoral instability (PFI) is one of the most disabling conditions in the knee, often affecting young individuals. Despite its not uncommon presentation, the underlying biomechanical features leading to this entity are not entirely understood. The suitability of classic physical examination manoeuvres and imaging tests is a matter of discussion among treating surgeons, and so are the findings provided by these means. A potential cause for this lack of consensus is the fact that, classically, the diagnostic approach for PFI has relied on statically obtained data. Many authors advocate for the study of this entity in a dynamic scenario, closer to the actual situation in which the instability episodes occur. In this literature review, we have compiled the available data from the last decades regarding dynamic evaluation methods for PFI and related conditions. Several categories are presented, grouping the related techniques and devices: physical examination, imaging modalities (ultrasound (US), magnetic resonance imaging (MRI), computed tomography (CT) and combined methods), arthroscopic evaluation, and others. In conclusion, although a vast number of quality studies are presented, in which comprehensive data about the biomechanics of the patellofemoral joint (PFJ) are described, this evidence has not yet reached clinical practice universally. Most of the data still stays in the research field and is seldom employed to assist a better understanding of the PFI cases and their ideal treatment targets.

Research Methods and Progress of Patellofemoral Joint Kinematics: A Review

Patellofemoral pain syndrome has a high morbidity, and its pathology is closely associated with patellofemoral joint kinematics. A series of in vivo and in vitro studies have been conducted to explore patellofemoral kinematics, and the findings are relevant to the diagnosis, classification, and management of patellofemoral diseases and even the whole knee joint. However, no definite conclusion on normal patellofemoral kinematics has been established. In this study, the measurement methodologies of patellofemoral kinematics (including data collection methods, loading conditions, and coordinate system) as well as their advantages and limitations were reviewed. Motion characteristics of the patella were analyzed. During knee flexion, the patellar flexion angle lagged by 30–40% compared to the tibiofemoral joint flexion. The patella tilts, rotates, and shifts medially in the initial stage of knee flexion and subsequently tilts, rotates, and shifts laterally. The finite patellar helical axis fluctuates near the femoral transepicondylar axis or posterior condylar axis. Moreover, factors affecting kinematics, such as morphology of the trochlear groove, soft tissue balance, and tibiofemoral motion, were analyzed. At the initial period of flexion, soft tissues play a vital role in adjusting patellar tracking, and during further flexion, the status of the patella is determined by the morphology of the trochlear groove and patellar facet. Our findings could increase our understanding of patellofemoral kinematics and can help to guide the operation plan for patients with patellofemoral pain syndrome.

Patella tracking calculation from patellofemoral positions at finite angles of knee flexion

Patellofemoral (PF) pain is a common knee disease. Patella tracking has a significant correlation with PF pain, therefore it could be used as an index for diagnosis and treatment evaluation. Previous research has proposed a method for measuring in vivo patella tracking by means of an interpolation algorithm. The present study aimed to quantify the effect of the interpolation parameters on the accuracy of the patella tracking with a motion capture experiment. The precise patella tracking of 5 knee specimens was collected and compared with the interpolated tracking. The results showed that the total interpolation error decreased to 2 mm with the number of interpolation angles increasing to 6. The number of interpolation reference points had a slight influence on the accuracy. The findings consolidated the feasibility of using interpolation to measure the in vivo patella tracking, and can help to optimize the accuracy and efficiency of the methodology.

Editorial Commentary: Is Magnetic Resonance Imaging Imaged Knee Patellar Tracking Relevant in Assessing the Patient With Patellar Instability?

Patellar pain and instability are common presentations to surgeons, yet assessment is more a static art than a dynamic science. In addition to resource-intensive gait laboratory, computed tomography (CT) and magnetic resonance imaging (MRI) have been used to measure patellar tracking. CT has the limitation of radiation and MRI has the limitation of software processing times. With an updated MRI protocol and software, it is now possible to dynamically view patellar tracking. Determining how this will be used to help in the diagnosis and treatment of patients will be the next goal.