Radiologic Measurements in the Assessment of Patellar Instability: A Systematic Review and Meta-analysis

Abstract

Background: Numerous diagnostic imaging measurements related to patellar instability have been evaluated in the literature; however, little has been done to compare these findings across multiple studies. Purpose: To review the different imaging measurements used to evaluate patellar instability and to assess the prevalence of each measure and its utility in predicting instability. We focused on reliability across imaging modalities and between patients with and without patellar instability. Study Design: Systematic review; Level of evidence, 4. Methods: We performed a systematic review of the literature using the PubMed, SCOPUS, and Cochrane databases. Each database was searched for variations of the terms “patellar instability,” “patellar dislocation,” “trochlear dysplasia,” “radiographic measures,” “computed tomography,” and “magnetic resonance imaging.” Studies were included if they were published after May 1, 2009, and before May 1, 2019. A meta-analysis using a random effects model was performed on several measurements, comparing instability and control groups to generate pooled values. Results: A total of 813 articles were identified, and 96 articles comprising 7912 patients and 106 unique metrics were included in the analysis. The mean patient age was 23.1 years (95% CI, 21.1-24.5), and 41% were male. The tibial tubercle–trochlear groove (TT-TG) distance was the most frequently included metric (59 studies), followed by the Insall-Salvati ratio and Caton-Deschamps index (both 26 studies). The interobserver intraclass correlation coefficients were excellent or good for the TT-TG distance and Insall-Salvati ratio in 100% of studies reporting them; however, for the Caton-Deschamps index and Blackburne-Peel ratio, they were excellent or good in only 43% and 40% of studies. Pooled magnetic resonance imaging values for TT-TG distance ( P < .01), Insall-Salvati ratio ( P = .01), and femoral sulcus angle ( P = .02) were significantly different between the instability and control groups. Values for tibial tubercle–posterior cruciate ligament distance ( P = .36) and Caton-Deschamps index ( P = .09) were not significantly different between groups. Conclusion: The most commonly reported measurements for evaluating patellar instability assessed patellar tracking and trochlear morphology. The TT-TG distance was the most common measurement and was greater in the patellar instability group as compared with the control group. In addition, the TT-TG, tibial tubercle–posterior cruciate ligament, and patellar tendon–trochlear groove distances were highly reproducible measurements for patellar tracking, and the Insall-Salvati ratio had superior reproducibility for assessing patellar height.

The Tibial Tuberosity–Rotational Angle as a Novel Predisposing Parameter for Patellar Dislocation

“…This study may partially explain why there is no significant difference in TT-PCL distance between patients with patellar instability and healthy individuals, as a recent systematic review showed. 29 Individualized measurements of the TT-TG distance were also proposed for diagnosing a lateralized TT in patients with recurrent patellar dislocation. 15 , 17 , 21 , 22 TT-RA is not affected by individual tibial size; therefore, the individual variability of pathologic values may be minimized.…”

“…The tibial tuberosity (TT)–trochlear groove (TT-TG) distance is commonly used for the assessment of TT lateralization (TTL) in patients with patellar dislocation. 9 , 12 , 18 , 29 However, recent studies have suggested that the TT-TG distance is affected by several factors, such as knee rotation, femoral anteversion, and trochlear groove medialization (TGM), rather than TTL alone. 1 , 3 , 7 , 16 , 23 , 30 In addition, it is difficult to measure the TT-TG distance in patients with trochlear dysplasia because the deepest point of the trochlea is unclear in these patients.…”

Redefining Trochlear Dysplasia: Normal Thresholds Vary by Measurement Technique, Landmarks, and Sex

“…31 The ideal imaging modality for each measurement is unknown, and the utility of originally described cutoff values across imaging modalities has not been validated. 13,36 On MRI scans, measurements utilizing bony versus cartilaginous landmarks are often performed interchangeably, despite evidence that they may differ in the evaluation of trochlear dysplasia. 33 Trochlear dysplasia has been more commonly reported in female patients, suggesting that increased anatomic risk factors may account for sex-specific differences in patellar instability.…”

Radiographic measurement of the congruence angle according to Merchant: validity, reproducibility, and limits

“…Assessment of patellofemoral congruence is performed in routine practice in cases of patellofemoral instability [ 1 , 2 ], painful patellar syndrome [ 3 ], or patellofemoral osteoarthritis [ 4 ]. In these cases, the analysis of patellofemoral congruence makes it possible to justify a surgical treatment choice in case of chronic instability or chronic patellofemoral pain (tibial tuberosity osteotomy, external patellar release, axial correction and/or derotation osteotomy, medial patellofemoral ligament reconstruction) [ 5 – 8 ].…”

“…However in 2013, a meta-analysis demonstrated that there was insufficient evidence to determine the reliability, validity, sensitivity, and specificity of this congruence angle [ 12 ]. In fact, the various studies found were conducted either on a population with healthy knees, representative of only 20% of the general population, or on series of cases of patellofemoral instability, with great variability in the radiographic acquisition protocols [ 2 ]. Moreover, no intraobserver or intraobserver analysis was performed of this radiographic measurement according to the level of trochlear or patellar dysplasia.…”