Relationship between patellar tendon–lateral femoral condyle friction syndrome and patellofemoral instability Patellar tendon – lateral femoral kondil sürtünme sendromu ile patellofemoral instabilite arasındaki ilişki

Aim: The etiology of the patellar tendon–lateral femoral condyle friction syndrome (PTLFCFS) is not fully known and the number of studies on the diagnosis, treatment, and prognosis of PTLFCFS is limited. The aim of this study was to evaluate the relationship between PTLFCFS and patellofemoral instability in magnetic resonance imaging (MRI). Material and Methods: Six morphological parameters were measured in MRI to evaluate patellofemoral stability in patients with PTLFCFS (study group n = 82) and patients without detection PTLFCFS (control group n = 204) in the knee MRI examination. These parameters include the Insall–Salvati ratio, ventral trochlear prominence, sulcus angle, lateral trochlear inclination angle, patellar inclination angle, and patellar lateralization measurement. In addition, the chondromalacia patella relationship was also evaluated. Results: The Insall–Salvati ratio, patellar inclination angle, patellar lateralization, sulcus angle, and ventral trochlear prominence were significantly higher and lateral trochlear inclination angle was significantly lower in the study group than in the control group. The incidence of chondromalacia patella was significantly higher in the study group. In the study group, there was a positive correlation between the chondromalacia patella and the ventral trochlear prominence. Conclusion: Among the factors causing PTLFCFS, patellofemoral instability was found to be effective. The measured parameters having high sensitivity and specificity suggest that these parameters can be used as risk factors and for the diagnosis.


Introduction
The knee is a hinge type joint in which the patellar tendon, ligaments, and sections of fascia lata provide passive stabilization, whereas the quadriceps muscle and aponeurosis provide active stabilization [1]. Excessive movement of the joint is restricted externally by Hoffa (infrapatellar fat pad) and the femoral and tibial condyles [1,2]. Patellar tendon-lateral femoral condyle friction syndrome (PTLFCFS) is considered as one of the leading causes of anterior knee pain, especially seen in young people [3]. Although the etiology of PTLFCFS is not fully known, its pathology is considered to result from the deterioration of knee biomechanics due to previous trauma. Generally accepted view is that the development of inflammation in the Hoffa and the direct contact of the lateral femoral condyle and patellar tendon due to trauma lead to clinical symptoms [1,4].
Magnetic resonance imaging (MRI) is an excellent diagnostic tool for knee imaging. With MRI, all the structures of the knee can be evaluated simultaneously, and this method allows a comprehensive understanding of the causes of anterior knee pain.
In PTLFCFS, an MRI examination shows an increased focal signal to the superolateral of the infrapatellar fat pad due to edema [1,4,5].
The aim of this study was to evaluate the quantitative parameters demonstrating patellofemoral instability of the knee using MRI in patients with PTLFCFS

Materials and Methods
The ethical compliance of this retrospective study was approved in accordance with the Helsinki Declaration by the Hospital Local Ethics Committee, Ankara, Turkey. Between May 1, 2016, and February 1, 2019, 1542 patients who were admitted to the orthopedics and traumatology department were evaluated. The inclusion criteria were as follows: (1) patients between 18-40 years old; (2) patients who had knee MRI in our radiology database. The exclusion criteria were as follows: (1) a history of major trauma, surgery or arthroscopy, tumor, patellar dislocation, inflammatory knee disease; (2) patients with findings of internal derangement, meniscal pathology, joint effusion; (3) artifacts precluding a proper MRI examination.
Eighty-two patients (18-40 years old) who had for more than 3 weeks due to anterior knee pain and increased focal signal to the superolateral of the infrapatellar fat pad in fat-suppressed proton density-weighted images were evaluated as a study group, whereas 204 patients (23-40 years old) with absence of anterior knee pain and no pathological findings in the superolateral of the infrapatellar fat pad were evaluated as a control group. Quantitative parameters used to evaluate patellofemoral stability were performed in MRI and two groups were compared with each other.

MRI protocol
MRI examinations were performed using a 1.5 T scanner (Gy- Anahtar kelimeler: Patellar tendon-lateral femoral kondil sürtünme sendromu; patellar insitabilite; manyetik rezonans görüntüleme standard dedicated knee coil. During scanning, the patients were given a supine position with their knees at full extension.
The imaging protocol constituted the following five routine sequences: coronal fast spin-echo T1-weighted, sagittal fatsuppressed proton density-weighted, coronal fat-suppressed proton density-weighted, axial fat-suppressed proton densityweighted and sagittal fast spin-echo T2-weighted.    The relationship between patellofemoral instability and the chondromalacia patella was also evaluated using the same parameters. Chondromalacia patella type was evaluated using MRI-adapted Outerbridge classification for chondral defects. The patients with multiple cartilage injuries were classified according to the most serious injury.

Statistical analysis
Data were analyzed using Statistical Package for the Social Sciences (SPSS) software (IBM SPSS 25.0, IBM Corporation, Armonk, NY, USA). The data were analyzed using Kolmogorov-Smirnov for normal distribution. The Mann-Whitney U test (non-parametric) was used for analysis because the data did not conform to a normal distribution. Spearman (nonparametric) correlation test was used for correlation among the groups. Pearson's Chi-Square test was used for the analysis of categorical variables. Receiver operating characteristic (ROC) curve analysis was used to calculate the cut-off value (cut-off ), specificity and sensitivity of the data. Mean and standard deviation values were used for analysis. P < 0.05 was considered statistically significant.

Results
The mean age of the patients was 23.61 ± 2.94 years, and 51.2% of the patients were males. No difference was found between the study and control groups in terms of age and gender (p > 0.05) ( Table 1). In our study, the Insall-Salvati ratio, patellar inclination angle, patellar lateralization, sulcus angle, and ventral trochlear prominence were significantly higher and lateral trochlear inclination angle was significantly lower (p < 0.05) in the study group than in the control group (Table 1). The incidence of chondromalacia patella was also significantly higher in the study group (p < 0.05) ( Table 1).
In both study and control groups, chondromalacia patella was found to be positively correlated with Insall-Salvati ratio, patellar inclination angle, patellar lateralization, sulcus angle, and ventral trochlear prominence, whereas chondromalacia patella was found to be negatively correlated with lateral trochlear inclination angle (p < 0.05). In the study group, there was a positive correlation between the chondromalacia patella and the ventral trochlear prominence (p < 0.05), whereas no correlation was found between the chondromalacia patella and the Insall-Salvati ratio, patellar inclination angle, patellar lateralization, sulcus angle, and lateral trochlear inclination angle (p > 0.05) ( Table 2). The sensitivity, specificity, area under the curve and cut-off values for the Insall-Salvati ratio, patellar inclination angle, patellar lateralization, sulcus angle, lateral trochlear inclination angle, and ventral trochlear prominence are shown in Figure   4 and Table 3.

Discussion
PTLFCFS is a disease typically characterized by anterior knee pain that emerges with activity [1]. In the literature, the number of studies on the diagnosis, treatment, and prognosis of PTLFCFS is limited.
In the present study, the Insall-Salvati ratio, patellar inclination angle, patellar lateralization, sulcus angle, and ventral trochlear prominence increased significantly in the study group, whereas the lateral trochlear inclination angle was found to be significantly lower than in the control group. In addition to the quantitative parameters in the literature, to our knowledge, ventral trochlear prominence was measured for the first time in the present study for the diagnosis of PTLFCFS and we found a positive correlation between ventral trochlear promi-nence. And the incidence of chondromalacia patella was significantly higher in the study group and there was a positive correlation between the chondromalacia patella and the ventral trochlear prominence.
Studies in the literature have reported that biomechanical changes cause this syndrome in young patients under the age of 40 [4,6]. Subhawong et al. evaluated the quantitative parameters measured in patients with PTLFCFS with respect to gender and age and found no statistically significant difference [7]. Consistent with the literature, the patients in the present study were young; 51.2% of the patients were males, and both age and gender were similar to that of the control group [7].
We consider that the incidence of pathology is higher in young people than in elderly due to the fact that young people perform activities that constantly operate the knee joint [sports, training, etc.]; chondromalacia patella is common in young people, and the knee is more frequently exposed to trauma.
We consider that the pathology is similarly common in both genders due to the fact that both genders are at similar risk.   fat pad edema and the Insall-Salvati ratio [9]. Bonadio et al.
stated that a high patella leads to more tension of the patellar tendon during knee movements and that this can easily cause compression of the bursa between the lateral femoral condyle [10]. Consequently, inflammation can develop in the Hoffa and lead to edema and pain [10]. Li et al. found that the Insall-Salvati ratio rate was high in patients with PTLFCFS and that the sensitivity and specificity were 82% and 96.6%, respectively, at a cut-off value of 1.20 [1]. In the present study, the Insall-Salvati ratio was found to be significantly higher in the study group than in the control group. We believe that the patellar tendon is stretched and the structures underneath are compressed during knee flexion in patients with a high Insall-Salvati ratio. We also believe that inflammation and pain develop in the region in response to this.
It has been reported that the patellar inclination angle is the source of anterior knee pain in patients without clinical instability or clinical history. A patellar inclination angle of more than 13° has been reported to be a risk factor [4,11]. Matcuk et al. and Barbier-Brion et al. found that patellar inclination angle was significantly higher in patients with PTLFCFS [3,4]. In the present study, the patellar inclination angle was found to be significantly higher in the study group than in the control group. Patellar inclination angle exceeding the cut-off value may have led to the volume on the posterior side of the patella to be reduced and the posterior side of the patella to not sit properly on the joint. Insall-Salvati ratio was higher [12]. In the present study, it was found that the prevalence of PTLFCFS increased in patients with chondromalacia patella. In the entire sample, a positive correlation was found between the Insall-Salvati ratio, patellar inclination angle, patellar lateralization, sulcus angle, and ventral trochlear prominence, whereas a negative correlation was found among the lateral trochlear inclination angle.
Joint instability caused by chondromalacia patella, increased pressure on the bursas, especially the Hoffa, and increased inflammation may have led to an increase in the prevalence of PTLFCFS. In relation to the increase in the chondromalacia patella, we think that the instability of the knee is further deteriorated and that the inflammation is further exacerbated.
The major limitation of this retrospective study was the fact that radiologists who evaluated the parameters simultaneously determined the presence of the increased focal signal to the superolateral of the infrapatellar fat pad. Second, the diagnosis of articular cartilage damage in these patients could not be confirmed by any gold standard method, such as arthroscopy and surgery.

Conclusion
Patellofemoral instability was found to be effective among the factors causing PTLFCFS. The Insall-Salvati ratio, ventral trochlear prominence, sulcus angle, lateral trochlear inclination angle, patellar inclination angle, and patellar lateralization of the study group were found to be different than those of the control group. In addition, high sensitivity and specificity values identified for these parameters indicate that these parameters may be used both as a risk factor and for diagnostic purposes.