Introduction

Lateral patellar dislocation (LPD) is a disabling condition associated with the risk of further dislocating events, patellofemoral pain, reduced quality of life (QOL), and concern for patellofemoral joint osteoarthritis in the long term [1,2,3,4]. While the treatment concept of first-time patellar dislocation remains controversial [5], current treatment algorithms favor surgical stabilization of the patella in patients with recurrent LPD [3, 6,7,8]. Due to the complexity and the broad variety of factors associated with patellar instability, surgical treatment strategies remain challenging [9].

Multiple studies have demonstrated that pathoanatomic risk factors represent a major cause of patellar instability, that their individual and combined prevalence influence the risk of recurrent dislocation and that they impact the clinical results after reconstruction of the medial patellofemoral ligament (MPFL-R) [1, 10,11,12,13,14]. However, Hiemstra et al. [1] found no correlation between postoperative outcome scores and pathoanatomic factors of LPD when focusing on disease-specific QOL parameters. Thus, basing surgery (solely) on given thresholds for anatomic risk factors, which are influenced by nuances of imaging modalities and by observer-dependent reliability (e.g., grading of trochlear dysplasia), appears critical when deciding for or against a bony procedure for patellar stabilization.

Recent studies have tried to bridge this gap between biomechanics and the clinic and have emphasized the importance of a more clinically derived patellar instability assessment protocol [15,16,17,18]. Therefore, two clinical parameters have gained increased significance in recent years. First, the J-sign evaluates patellar maltracking in patients with LPD, and several studies demonstrated that a preoperative high-grade J-sign was associated with inferior clinical outcome scores and increased residual MPFL-graft laxity [16, 17, 19]. Second, the dynamic evaluation of patellar instability (reversed dynamic patellar apprehension test, ReDPAT) indicates the patient-specific end of stable patellar tracking and the beginning of patellar stabilizer insufficiency at certain degrees of knee joint flexion [18]. In the majority of patients with recurrent LPD, test results and the flexion angle at which the provocative sense of apprehension became positive correlated with the severity of pathoanatomic risk factors [15, 18].

Patellar instability typically presents a myriad of clinical symptoms, influenced by various individual parameters, which makes it difficult to weight all of them according to their impact on instability and, in particular, on patients’ QOL. As the aim of surgical patellar stabilization is to achieve the best possible result with the least possible invasiveness, the goal of this study was to determine which demographic, clinical and pathoanatomic risk factors for LPD contribute most relevantly to subjective disease-specific QOL in patients with recurrent LPD, aiming to provide implications on the overall treatment decision-making process. The hypothesis was that patients’ QOL is determined by the severity of patellar instability expressed clinically by the J-sign and by the ReDPAT.

Materials and methods

This study received approval from the local ethics committee (Medical Council Baden-Württemberg F-2019-070). Between September 2019 and October 2020, the senior author of this study treated a total of 233 consecutive patients (male/female 98/135, mean age 24.2 ± 6.6 years) for LPD. To be included in this study, patients had to (1) have ≥ 2 objective lateral patellar dislocations without previous surgery, (2) a first-time patellar dislocation requiring surgery because of failed conservative treatment over a minimum of 6 months. The exclusion criteria were as follows: (1) first-time patellar dislocation undergoing primary surgery due to an osteochondral flake fracture or due to primary acute patellar stabilization procedure because of massive disruption of medial ligamentous stabilizers, (2) previous MPFL-R or previous bony procedures at the distal femur or proximal tibia including tibial tubercle osteotomy, trochleoplasty, varization or torsional osteotomy, (3) patellofemoral pain without objective findings of LPD, and (4) other previous knee ligament surgery (e.g., ACL reconstruction). Finally, a total of 182 patients met the inclusion criteria (male/female 70/112; mean age 23.6 ± 7.3 years) and comprised the study group for this investigation.

Demographics (sex, age, body mass index (BMI), number of dislocations), clinical evaluation, assessment of pathoanatomic risk factors, and patient disease-specific QOL were assessed in all patients prior surgery. The clinical evaluation comprised the ReDPAT and evaluation of the J-sign [18, 20]. The ReDPAT extends the knee joint from a deep flexion angle with lateral force applied to the medial side of the patella and checks for the first onset of a subjective apprehensive reaction. This flexion angle is measured with a goniometer and determines the patient-specific end of stable patellar tracking and the beginning of patellar stabilizer insufficiency [18]. For evaluation of the J-sign, the patient was asked to actively extend the knee from a seated position with knees flexed to 90°. The J-sign was defined as positive if there was a lateral patellar shift during active terminal extension. Grading of the J-sign was performed according to a modified clinical grading system published by Zhang et al. [16, 17]: grade 0: normal = absence of J-sign; grade 1: mild = 1–2 quadrants of motion; grade 2: apparent = 2–3 quadrants of motion; grade 3: severe =  > 3 quadrants of motion or complete patellar dislocation at extension. The evaluation was conducted at the time of initial outpatient presentation. Patients were examined by one orthopedic resident with 3 years of clinical experience and by the senior orthopedic surgeon with more than 20 years of clinical experience, with agreement reached by consensus. In case of a disagreement, the senior author made the final decision.

Preoperative MRI data and routine radiographs (standing long leg axis radiograph and lateral view radiograph of the knee joint) were obtained for each patient and evaluated for the following: severity of trochlear dysplasia (absent, low grade (Dejour type A), or high grade (Dejour type B–D) [21]; patellar height (Caton-Deschamps index), with ≥ 1.3 recorded as elevated [22]; tibial tuberosity–trochlear groove (TT-TG) distance, with ≥ 16 mm recorded as elevated [12]; tibial tuberosity–posterior cruciate ligament (TT-PCL) distance, with ≥ 25 mm recorded as elevated [23]; and varus–valgus alignment [24]. Values were summarized to a cumulative anatomic risk factor score as previously published [18]. The validated Banff Patellofemoral Instability Instrument 2.0 (BPII 2.0) was used to assess patient-reported disease-specific QOL [25].

Statistical analysis

Data are presented as the means and standard deviation (SD). Categorical and dichotomous data are presented as frequency tabulations. The preliminary data analysis consisted of Spearman rank correlation and one-way ANOVA for determining the ability of the individual variables to predict the BPII 2.0 and exploring the possible correlation among the variables themselves. Following the preliminary analysis, all variables that fulfilled at least one of the following two criteria, ANOVA p ≤ 0.1 or Spearman’s abs (rho) > 0.1, were entered into a multivariate linear regression model. Backward-stepwise regression was then used to reach the final prediction model with all of the variables at p ≤ 0.05 significance. The recommended “rule of thumb” for a regression analysis is ten patients per each independent variable being assessed. [1, 26]. Having 12 predictor variables, the study population of n = 182 exceeded the recommended number of 10 × 12 = 120 patients, thus the sample size was sufficient to perform the analyses [26]. All analyses were performed using R (R Core Team 2020; www.R-project.org/; please view Supplement File for additional information).

Results

The demographic results, clinical evaluation results and anatomic values are summarized and presented in Table 1. Within the study group, 14% (n = 25) of the participants had failed conservative treatment after primary patellar dislocation, 18% (n = 33) reported a second episode of LPD, and 68% (n = 124) had multiple (> 3) patellar dislocations. High-grade trochlear dysplasia (Dejours’ types B-D) was present in 79% (n = 143) of the study population. The TT-TG distance was increased in 40% (n = 72), while the TT-PCL distance exceeded 25 mm in 32% (n = 59) of the patient population. An increase in patellar height was found in 41% (n = 75) of the patients, and 17% (n = 30) of the patients had ≥ 3° valgus malalignment. The ReDPAT became on average positive at 46 ± 22° of knee joint flexion, whereas a high-grade J-sign (Grade II and III) was present in 51% (n = 93). The mean BPII 2.0 score value was 37.8 ± 15.9 points.

Table 1 Demographics, clinical evaluation results, and anatomic values of the study group
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Analysis of the individual variables’ ability to predict BPII 2.0 score values revealed ‘age’, ‘BMI’, ‘ReDPAT’, ‘high grade of trochlear dysplasia’ (Dejour B-D), and ‘high-grade J-sign’ (Grade II and III) as factors of potential relevance (one-way ANOVA (p ≤ 0.1); Spearman’s correlation [abs(rho) > 0.1)] (Table 2). In addition, one-way ANOVA and Spearman’s rank correlation suggest that ‘BMI’ correlated with ‘Age’ and that the parameters ‘J-sign (Grade II and III)’, ‘ReDPAT’, and ‘Trochlear dysplasia (Dejour B-D)’ depend on each other (Table 3).

Table 2 One-way ANOVA with BPII 2.0 score values for all variables
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Table 3 One-way ANOVA for pairs of possible BPII 2.0 prediction variables
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Considering ‘Age’, ‘BMI’, ‘ReDPAT’, ‘J-sign (Grade II and III)’ and ‘trochlear dysplasia (Dejour B–D)’, backwards-stepwise multivariate regression analysis yielded a final parsimonious model that included ‘BMI’ and ‘J-sign (Grade II and III)’ as the most relevant parameters influencing BPII 2.0 score values (adjusted R2 = 0.418; p < 0.001; Table 4), with a cutoff value for the BMI found at 28 kg/m2 (Tukey test; p = 0.01).

Table 4 Final multivariate linear regression model for BPII 2.0 score values
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Standard assumptions of linear models were checked for the final model using the Shapiro–Wilk normality test (n.s.) and visual inspection of the QQ plot to confirm the normal distribution of residuals, the Durbin-Watson test (n.s.) to confirm their independence, and the Breusch-Pagan test to confirm homoscedasticity (n.s.).

Discussion

The main results of this study indicate that the presence of a high-grade J-sign and an increased BMI contribute significantly to inferior QOL score values in patients with recurrent LPD when evaluated with the BPII 2.0. Because BMI is a modifiable factor, it should be considered in the clinical decision-making process.

The J-sign has been considered a useful examination tool to clinically assess patellar maltracking during active knee joint motion [20]. Using a dynamic kinematic computed tomography study, Tanaka et al. [27] reported that the severity of the J-sign maltracking pattern correlated with the presence of symptomatic patellar instability. In their study, 79% of those participants with a grade-2 J-sign and 93% of those with a grade-3 J-sign had symptoms of recurrent LPD. Based on computed tomography-assessed patellar quadrants of motion, they proposed that the clinical J-sign should be quantified in a similar manner, with grade 1 showing greater than one quadrant of motion, grade 2 showing greater than two quadrants of motion, and grade 3 showing greater than three quadrants of patellar motion. This grading system has been used in recent clinical studies with good intra- and interobserver reproducibility and was adopted for this investigation [16, 17].

Multiple reasons for positive J-sign findings have been defined in the literature. Contributing factors include trochlear dysplasia [19, 28], patella alta [29], increased TT-TG distance [30], increased femur and tibia torsion [31], and disbalance between the vastus medialis and lateralis muscle [32]. All factors are well-known parameters of patellar instability and, in addition, potentially affect clinical outcome scores after MPFL-R, when certain anatomical thresholds and parameter characteristics of patellofemoral malalignment (i.e., trochlear dysplasia, patella alta, femur antetorsion, etc.) are exceeded [12, 13, 16, 17, 19]. Therefore, it is reasonable that patients with a preoperative high-grade J-sign achieved inferior clinical results and yielded higher postoperative MPFL-graft laxity after isolated MPFL-R, as MPFL-R alone fails to substantially restore patellar kinematics and patellar tracking when anatomical patellofemoral malalignment is present [33, 34].

The results of this study indicate that the severity of patellar maltracking (expressed as the grade of the J-sign) correlated negatively with BPII 2.0 score values, i.e., the more severe the J-sign is, the lower the disease-specific QOL. This correlation was not found for any single anatomical risk factor or for their cumulative incidence (risk factor score), a finding that initially appears counterintuitive, as patellofemoral (patho)anatomy impacts patellar tracking and, therefore, determines the presence and severity of the J-sign. However, this is in line with previous results, where no anatomic risk factor was found to be predictive of QOL outcome scores 2 years after MPFL-R surgery [1]. One possible explanation might be the fact that the clinical presence and grade of the J-sign better reflect the overall picture of an individual’s patellar (mal)tracking than can be expressed by the simple addition of individual anatomic risk factors. In particular, if a risk factor value is on the upper limit but does not exceed a given threshold, it is formally not included in the overall individual risk factor assessment, although it is likely that this parameter influences overall patellar tracking in the orchestra of patellofemoral alignment parameters.

In a recent study, Zhang et al. [16] demonstrated that patients with a preoperative high-grade J-sign showed inferior clinical outcomes, more MPFL residual graft laxity, and greater residual patellar maltracking, even after complex patellar stabilizing surgery that included femoral derotational osteotomy, tibial tubercle osteotomy and MPFL-R. The residual J-sign rate was 38%, indicating that there have to be more factors contributing to the formation of the J-sign than the corrected bony abnormalities. Trochlear dysplasia was the only untreated deformity and, as such, might explain the inferior clinical outcome scores and the persistent J-sign in the high-grade group. This is consistent with previous findings, where severe trochlear dysplasia but not increased torsion was identified as a significant predictor of patellar instability [35, 36], although degrees of femur antetorsion typically present with some correlation of trochlear dysplasia severity [37].

An increase in BMI has been shown to impact clinical outcome scores, revision rates, and patient satisfaction after various types of knee surgery (e.g., ACL-reconstructive surgery, total knee arthroplasty, lower limb osteotomy) [38,39,40]. In addition, BMI has been linked to the development and severity of chronic knee pain and cartilage degeneration and may increase the risk for early onset of patellofemoral osteoarthritis [41]. Because patellar instability often leads to considerable patellofemoral pain, immobility and fear of reinjury (redislocation), it is reasonable to assume that a higher BMI impacts the overall QOL in those particular patients by further limiting their ability to engage in everyday and athletic activities, consequently influencing their overall sense of well-being [42]. However, an increased BMI and obesity in general are known to play a significant role in various aspects of patients’ QOL, influencing their daily routines, overall motivation, self-body image, and social integration [43, 44]. In addition, obesity is considered to have a strong impact on mental and psychological well-being, and an association with mental disorders has been described [44]. Thus, it appears likely that BPII 2.0 score values were also influenced by the overall mental health status of the participants, regardless of their body weight, a possible correlation that deserves further investigation.

There are several limitations that deserve mention. (1) The J-sign assessment is a qualitative, observer-dependent clinical investigation tool. In this context, concern has been raised regarding the low intra- and inter-examiner reliability of this clinical test [45, 46]. Most recently, Hiemstra et al. reported that the J-sign demonstrated only fair to moderate inter-rater reliability, but concluded on the overall low reliability of the most common clinical assessment tools for patellar instability [45]. To minimize potential bias the assessment of the J-sign was performed with agreement reached by consensus, strictly following the previously determined parameters of J-sign grading [16, 17, 27]. However, a further objective quantification of the J-sign appears mandatory and is a desired goal for future studies. (2) This study provides some important information about QOL determinants in patients with LPD that can potentially help in the overall treatment decision-making process. However, it remains to be investigated whether and, if so, to what extent these presented parameters will impact postoperative outcome scores. Even if there is increasing evidence that a high-grade J-sign influences both the preoperative QOL and the postoperative patient-reported outcomes, it remains to be investigated whether a more aggressive surgical correction of the J-sign will be required for the sake of better outcome scores. However, Zimmermann et al. [47] showed most recently that a consequent correction of bony risk factors led to good QOL results in patients who underwent revision surgery for failed MPFL-R. (3) Regarding the statistical analysis, previous studies have determined that ten patients per variable provide acceptable power for regression analysis [26]. Accordingly, this study included a sufficiently large cohort of patients for the regression analysis. However, considering the multifactorial nature and high variability of risk factors for patellofemoral instability, a larger study population might yield a stronger correlation of the presented factors or potentially determine other variables as relevant. (4) Finally, patient-reported outcome scores are known to be strongly affected by individual mindsets, everyday stress levels, work-related demands, athletic motivation and various psychological conditions [35].

Conclusion

The results of this study indicate that in patients with lateral patellar instability, a high-grade J-sign and an increased BMI significantly impact subjective disease-specific QOL. Thus, information on both factors can help physicians in the overall treatment decision-making process.