Patellofemoral Instability in Children: Correlation Between Risk Factors, Injury Patterns, and Severity of Cartilage Damage

Pediatric Imaging Original Research Kim et al. Patellofemoral Instability in Children Pediatric Imaging Original Research Hee Kyung Kim 1 Sahar Shiraj 1 Chang Ho Kang 2 Christopher Anton 1 Dong Hoon Kim 3 Paul S. Horn 4 Kim HK, Shiraj S, Kang CH, Anton C, Kim DH, Horn PS Keywords: children, cartilage damage, patellofemoral instability, T2 relaxation time mapping DOI:10.2214/AJR.15.15217 Received June 25, 2015; accepted after revision October 19, 2015. 1 Department of Radiology, Cincinnati Children s Hospital Medical Center, 3333 Burnet Ave, Cincinnati, OH 45229. Address correspondence to H. K. Kim (Hee.Kim@cchmc.org). 2 Departments of Radiology, Korea University Anam Hospital and Korea University College of Medicine, Seoul, Korea. 3 Korea University College of Medicine, Seoul, Korea. 4 Divisions of Neurology and Biostatistics and Epidemiology, Cincinnati Children s Hospital Medical Center, Cincinnati, OH. AJR 2016; 206:1321 1328 0361 803X/16/2066 1321 American Roentgen Ray Society Patellofemoral Instability in Children: Correlation Between Risk Factors, Injury Patterns, and Severity of Cartilage Damage OBJECTIVE. The purpose of this study was to compare MRI findings between groups with and without patellofemoral instability and to correlate the MRI findings with the severity of patellar cartilage damage. MATERIALS AND METHODS. Fifty-three children with patellofemoral instability and 53 age- and sex-matched children without patellofemoral instability (15.9 ± 2.4 years) were included. Knee MRI with T2-weighted mapping was performed. On MR images, femoral trochlear dysplasia, patellofemoral malalignment, medial retinaculum injury, and bone marrow edema were documented. The degree of patellar cartilage damage was evaluated on MR images by use of a morphologic grading scale (0 4) and on T2 maps with mean T2 values at the medial, central, and lateral facets. MRI findings were compared between the two groups. In cases of patellofemoral instability, MRI findings were correlated with the severity of cartilage damage at each region. RESULTS. Trochlear structure and alignment were significantly different between the two groups (Wilcoxon p < 0.0001). In patellofemoral instability, a high-riding patella was associated with central patellar cartilage damage with a higher morphologic grade and T2 value (Spearman p < 0.05). The severity of medial retinacular injury and presence of bone marrow edema at either the medial patella or the lateral femoral condyle were associated with a higher grade of medial patellar cartilage damage (Wilcoxon p < 0.05). None of the other findings correlated with the severity of patellar cartilage damage. CONCLUSION. Patients with patellofemoral instability have significantly different trochlear structure and alignment than those who do not, and these differences are known risk factors for patellofemoral instability. However, the only risk factors or injury patterns that directly correlated with the severity of patellar cartilage damage were patella alta, medial stabilizer injury, and bone marrow edema. P atellofemoral instability (PFI) is fairly common, particularly in female adolescents who are physically active. The overall incidence of PFI is 6 77 cases per 100,000 persons [1, 2], and PFI is the most common cause of clinical visits for anterior knee pain in children and adolescents [3, 4]. PFI is a consequence of incongruence between the patella and the femoral trochlear groove. Altered mechanics from patellar malalignment and abnormal tracking cause excessive contact stress and considerable shearing forces [5] that result in patellar cartilage degeneration and damage [6]. MRI has been primarily used to evaluate PFI, and imaging findings can be analyzed for prospective risk factors and to evaluate injury patterns. Risk factors for PFI seen with MRI include femoral trochlear dysplasia and abnormal patellofemoral alignment, such as patella alta and increased lateral distance between the tibial tubercle and the trochlear groove (TT-TG distance). Injury patterns include injuries to the medial patellar stabilizers, osteochondral injury, and bone marrow edema of the medial aspect of the patella and the lateral femoral condyle [7]. In contrast to the adult population, who have completed skeletal maturation and have stationary anatomic structures in the patellofemoral joint, children and young adolescents undergo developmental changes both in patellofemoral joint congruency [8] and in the integrity of the patellar cartilage [9, 10]. Therefore, to determine the presence of PFI in the pediatric population, it is mandatory to develop reference values standardized for the pediatric population. AJR:206, June 2016 1321

Kim et al. A B Fig. 1 16-year-old girl with patellofemoral instability. Example of T2 relaxation time mapping of patellar cartilage. A and B, Gray-scale (A) and color-coded (B) maps show patellar cartilage evenly segmented into six locations: medial 1 (M1), medial 2 (M2), central 1 (C1), central 2 (C2), lateral 2 (L2), and lateral 1 (L1). With technical advances, cartilage imaging can be performed with both morphologic and biochemical imaging. T2 relaxation time mapping (T2 mapping) is highly sensitive to changes in collagen content and the anisotropic orientation of collagen fibers within cartilage [11]. T2 map biochemical imaging is used to detect early changes in the cartilage microstructure [12] that can precede gross morphologic damage [11, 13]. The purposes of this study were to determine the MRI findings of PFI in a pediatric and young adolescent population and to correlate risk factors and injury patterns with the severity of patellar cartilage damage in PFI. To determine the pediatric reference values, age- and sex-matched groups with and without PFI were included. Patellar cartilage damage was evaluated with morphologic imaging that included semiquantitative MRI and with biochemical imaging that included T2 mapping. Understanding the relation between MRI findings (risk factors and injury pattern) and the severity of cartilage damage should be helpful in determining the best therapeutic option for minimizing further cartilage damage in patients with PFI. Materials and Methods This retrospective study was HIPAA compliant and approved by the institutional review board. The requirement for informed consent was waived. A Fig. 2 10-year-old girl without patellofemoral instability. A, MR image shows lateral trochlear inclination angle (red lines) as angle between lateral trochlear facet and posterior aspect of femoral condyle. Trochlear facet asymmetry (yellow lines) is ratio of medial (M) to lateral (L) facet length. Trochlear depth (green lines) is distance between lateral facet (A), sulcus (B), and medial facet (C) and posterior aspect of femoral condyle. Trochlear depth is calculated from formula (A + C) / 2 B. Blue lines indicate tibial tubercle trochlear groove distance. B, MR image shows tibial tubercle trochlear groove distance (blue line) measured as mediolateral distance between tibial tuberosity and deep sulcus (green line, A). C, MR image shows lateral patellofemoral angle (white lines) measured between posterior aspect of patella lateral facet and anterior aspect of condyles. separate departmental review board approved the additional MRI sequence, which was performed for research purposes and added less than 10 minutes to the imaging time. The requirement for informed consent for this additional sequence was waived. All consecutive knee MRI examinations performed during a 14-month period were reviewed. During this time, axial T2 maps were generated as part of a routine knee MRI protocol across all MRI examinations. A total of 1224 examinations were performed for a clinical indication and were stored in the institutional PACS (Merge PACS, Merge Healthcare). The MRI reports and images were reviewed by a single radiologist, who had both pediatric radiology and musculoskeletal radiology fellowship training and 12 years experience in MRI. Inclusion criteria were age 5 22 years, MRI performed with a 1.5-T system (HDxt, GE Healthcare), and MRI examination performed with axial T2 mapping that included at least the patellar cartilage. Subjects The study subjects were sorted to fulfill two groups. One group included patients with PFI, and the other included age- and sex-matched subjects without PFI. The age and sex-matched design was used because the patellar cartilage undergoes age- and sex-dependent changes with skeletal maturation [9, 10]. Subjects were included in the PFI group if they had a history of a single or multiple episodes of lateral patellar dislocation, subluxation, or general patellar instability in one or both knee joints as documented in MRI reports or clinical notes. Subjects with PFI associated with a developmental condition or syndrome, such as Down syndrome, cerebral palsy, or Ehlers-Danlos syndrome, were excluded. A total of 53 subjects were included (38 girls, 15 boys; mean age, 15.9 ± 2.4 [SD] years; range, 8 21 years). Images from a single MRI examination were analyzed for each subject. The group without PFI was formed with the same inclusion criteria as for the group with PFI. 1322 AJR:206, June 2016

Patellofemoral Instability in Children Fig. 3 17-year-old girl with patellofemoral instability. Sagittal proton density weighted MR image shows prominent bony protrusion of femoral condyle (arrow), indicating grade 4 femoral trochlear dysplasia. The exclusion criteria were as follows: MRI reports with clinical history of PFI or anterior knee pain or MRI findings of patellar dislocation, subluxation, or malalignment; MRI findings of patellar cartilage abnormalities with either gross morphologic abnormalities or changes in signal intensity; any condition affecting the congruency of the patellofemoral joint, such as congenital structural anomalies, developmental delay, non weight-bearing condition, or underlying diagnosis of any kind of syndrome; any pathologic condition affecting the patella, including neoplasm, inflammation, infection, fracture, or bone marrow edema of the patella; and internal derangement possibly affecting the patellar cartilage, such as anterior cruciate ligament tear or posterolateral corner injury. A total of 268 subjects met these criteria. From these, age- and sexmatched subjects were randomly selected to comprise 53 subjects in the group without PFI. MRI Knee MRI was performed. A quadrature transmit-receive knee coil was used (Signa Excite 1.5HD, eight-channel array, GE Healthcare). All imaging studies included conventional MRI and axial T2 maps and were performed with the same protocol and consistent MRI parameters across examinations for both groups. Total acquisition time was less than 60 minutes. All MRI examinations were monitored by the radiologist to achieve the optimal level of imaging quality. Conventional MRI was performed with the following pulse sequences: axial fast spin-echo (FSE) intermediate-weighted imaging (TR/TE, 3000/40; echo-train length [ETL], 6; bandwidth, 15.6 khz; FOV, 14 cm 2 ; slice thickness, 3 4 mm; slice gap, 1 mm; matrix, 256 192; number of signals averaged, 2); axial, sagittal, and coronal FSE T2-weighted imaging with fat suppression (TR/TE, 3000/60; ETL, 8; bandwidth, 15.6 khz; FOV, 14 cm 2 ; slice thickness, 4 mm; slice gap, 1 mm; matrix, 256 192; number of signals averaged, 2); sagittal FSE proton density weighted imaging (TR/TE, 1700 2000/12; ETL, 8; bandwidth, 15.6 khz; FOV, 14 cm 2 ; slice thickness, 3 mm; slice gap, 1 mm; matrix, 320 224; number of signals averaged, 2); coronal T1-weighted imaging (TR/TE, 350/10 15; ETL, 6; bandwidth, 15.6 khz; FOV, 16 cm 2 ; slice thickness, 3 mm; slice gap, 1 mm; matrix, 256 192; number of signals averaged, 1); 3D gradient sequences on request with a fat-suppressed spoiled gradient-recalled echo sequence (TR/TE, 50/6 7; flip angle, 60 ; FOV, 13 16 cm 2 ; matrix, 256 128; slice thickness, 2 mm; 45 slices; number of signals averaged, 1). Axial T2 relaxation time mapping was performed to include the entire patellar cartilage according to the following MRI parameters: TR/TE, 1500/9, 18, 27, 36, 45, 54, 63, 72, 81, 90, and 99; number of echoes, 11; FOV, 14 cm 2 ; slice thickness, 3 4 mm; slice gap, 1 mm; matrix, 256 160. Each T2 map included at least 8 26 slices. T2 maps were generated with the linear least squares curve-fitting algorithm for pixel-by-pixel analysis. A monoexponential model was used for the signal intensity of each pixel to be fitted as a function of different TEs. The gray-scale T2 map was gener- B A Fig. 4 17-year-old girl with patellofemoral instability and patella alta. A, MR image shows patellar height ratio as ratio of patella (P) to patellar tendon (T) length. Patellar height ratio in this case is 1.79. B, Axial fat-suppressed T2-weighted MR image shows deep fissuring of patellar cartilage (morphologic grade 3) at C1 (arrow). C, Axial T2-weighted color-coded map shows elongation of mean T2 relaxation time at central portion of patellar cartilage (arrowheads). C AJR:206, June 2016 1323

Kim et al. ated from the slope of best fit model and converted to a color-coded map. The color in the color scale represented the range of T2 relaxation times in milliseconds (Fig. 1). Generation of T2 maps was performed with institutional postprocessing software written in Interactive Data Language (IDL, ISS). Interpretation of conventional MR images was performed to evaluate for femoral trochlear dysplasia, patellofemoral alignment, injury patterns, and morphologic grading of patellar cartilage damage. Femoral trochlear dysplasia Femoral trochlear dysplasia was evaluated with measurements of the osseous morphologic features in a pediatric population as described and modified by Kim et al. [14] and was divided into four morphologic subtypes according to the classification of Dejour et al. [7, 15]. The measurements of the osseous morphologic features included lateral trochlear inclination angle, trochlear facet asymmetry, and trochlear depth. All measurements were primarily performed by a single reader (a medical graduate student with 1 year of experience in MRI and research) under supervision of the senior investigator after training in MRI measurement. The key images used in measurements with scales were saved and reviewed by the senior investigator and were consistent and optimal in both groups. The lateral trochlear inclination angle was the angle between the lateral trochlear facet and the posterior aspect of the femoral condyle on axial intermediate-weighted images (red lines, Fig. 2A). Trochlear facet asymmetry was the length of the medial and lateral facets of the femoral condyle on axial intermediate-weighted images at the level with the largest area of the femoral condyles. The ratio of medial to lateral facet length was calculated as a percentile (yellow lines, Fig. 2A). Trochlear depth was measured on the same image and at the level used for trochlear facet asymmetry. It was the distance between the medial and lateral facets and sulcus and the posterior aspect of the femoral condyle. The average of the medial and lateral facet distances was measured and the sulcus distance subtracted (green lines, Fig. 2A). The morphologic subtypes of femoral trochlear dysplasia were graded from mild to severe (1 4) by two radiologists in consensus (senior investigator and radiologist with pediatric radiology fellowship training and 15 years experience in MRI). Grades 1, 2, and 3 were evaluated on the axial T2-weighted fatsuppressed image. Grade 4 was determined on the sagittal FSE proton density weighted image. The morphologic grading of femoral trochlear dysplasia was performed only in the group with PFI. The grades were as follows: 1, preserved trochlear structure but shallow sulcus; 2, flat and horizontal orientation of the trochlea; 3, flat and oblique orientation of the trochlea; 4, same as grade 3 on axial image with presence of a prominent bony protrusion (Fig. 3). Patellofemoral alignment Patellofemoral alignment was evaluated by measuring the patellar height ratio, TT-TG distance, and the lateral patellofemoral angle by use of the method described and modified by Kim et al. [14]. Measurement of patellofemoral alignment was performed in both groups. Patellar height ratio was the ratio of the patellar tendon length to the greatest length of the patella on sagittal images (Fig. 4A). The TT-TG distance was the mediolateral distance between the tibial tuberosity and the deep sulcus. TT-TG distance was measured in millimeters on consecutive axial images (blue lines, Figs. 2A and 2B). The lateral patellofemoral angle was the angle between the anterior aspect of the condyles and the posterior aspect of the patellar lateral facet. The lateral opening of the angle was defined as a positive angle, and medial opening was defined as a negative angle and patella tilting (Fig. 2C). Injury patterns Injury patterns were evaluated by two radiologists (the senior investigator and radiologist with pediatric radiology fellowship training and 15 years experience in MRI) in consensus on the basis of the following findings: medial stabilizer injury, in which the medial retinaculum and medial patellofemoral ligament injury were graded from 0 to 3 on axial fat-suppressed T2-weighted images (0, normal; 1, periligamentous edema; 2, partial tear; 3, complete tear; R, repaired); presence of bone marrow edema of the medial aspect of the patella on axial fat-suppressed T2-weighted images; and presence of osteochondral injury or bone marrow edema of the A C Fig. 5 12-year-old girl with patellofemoral instability and bone marrow edema in lateral femoral condyle and medial patellar bone. A, Coronal fat-suppressed T2-weighted MR image shows bone marrow edema in lateral femoral condyle (arrow). B, Axial T2-weighted fat-suppressed MR image shows superficial fissuring involving less than 50% of depth (grade 2 cartilage damage) at medial location 1 (white arrow) and grade 3 cartilage damage at medial location 2 (black arrow). Dotted line indicates cartilage defect. C, Axial T2-weighted color-coded map shows elongation of mean T2 relaxation time at medial locations 1 (solid arrowheads) and 2 (open arrowheads). B 1324 AJR:206, June 2016

Patellofemoral Instability in Children lateral femoral condyle on axial and coronal fatsuppressed T2-weighted images. Patellar cartilage damage Patellar cartilage damage was evaluated with conventional MR images and T2 maps. The morphologic grading (0 4) of patellar cartilage damage was performed on fat-suppressed T2-weighted images or 3D gradient-echo images according to the system initially described as the Outerbridge classification [16] and modified by the International Cartilage Repair Society [17]. Morphologic grading was performed in the PFI group by two radiologists in consensus (radiologist with musculoskeletal radiology fellowship training and 15 years of experience in MRI, radiologist with pediatric radiology fellowship training and 15 years experience in MRI). The grading scale was as follows: 0, normal; 1, intrasubstance high T2 signal intensity without defect; 2, superficial fissuring involving less than 50% of the entire cartilage thickness; 3, deep fissuring involving more than 50% of the cartilage thickness; 4, full-thickness defect with subchondral bone involvement. A single reviewer under supervision of the senior investigator manually drew the ROI on T2 maps to extract the patellar cartilage. All images with the ROI were saved and later reviewed by the senior investigator. The mean T2 relaxation time was automatically calculated from the ROI by use of our institutional postprocessing software (IDL, ISS). Mean T2 relaxation times were obtained in both groups. The patellar cartilage was divided into three regions (medial, central, and lateral facets). Each region was equally divided into two areas, and both morphologic grade and mean T2 relaxation times were obtained at all six locations (M1, M2, C1, C2, L2, L1) of the patellar cartilage for each subject (Fig. 1A). Therefore, morphologic grades should have been obtained from 318 locations in the 53 patients with PFI. However, the T2 relaxation time was not available for two subjects who had grade 4 injury at M1 (medialmost aspect of the medial facet) with no remaining cartilage to be measured, so 316 mean T2 relaxation times were obtained in the PFI group. Statistical Analysis The Wilcoxon rank sum test was used to compare the measures of femoral trochlear dysplasia and patellofemoral alignment between the two groups. Lower and upper 95% confidence limits (t test) for each group were obtained. In the group with PFI, correlation coefficients were derived between MRI findings and the severity of the patellar cartilage damage. Spearman correlations were used for continuous numeric values (lateral trochlear inclination angle, trochlear facet asymmetry, trochlear depth, patellar height ratio, TT-TG distance, lateral patellofemoral angle, mean T2 relaxation times) and chromatic values (morphologic subtypes of the femoral trochlear dysplasia, medial stabilizer injuries, and morphologic grade of patellar cartilage damage). For the relation between dichromatic values (presence of osteochondral injury, bone marrow edema of the patella and femoral condyle, and patellar cartilage damage), a t test was used. Values of p < 0.05 were considered statistically significant. All hypotheses and tests were conducted at the 0.05 level of statistical significance. All statistical analyses were performed with a SAS statistical software package (version 9.3, SAS Institute). Results A statistically significant difference was found between the two groups in terms of femoral trochlear dysplasia and patellofemoral alignment (Table 1). The lateral trochlear inclination angle, trochlear facet asymmetry, trochlear depth, and lateral patellofemoral angle were significantly smaller in the group with PFI (p < 0.0001 for both). The patellar height ratio and TT-TG distance were significantly greater in the group with PFI (p < 0.0001 for both) (Table 1). For the group with PFI, categorized grading data, including femoral trochlear dysplasia and medial stabilizer injuries, are summarized in Table 2. A statistically significant positive correlation was seen between patellar height ratio and degree of patellar cartilage damage at the central area (C1) measured with both morphologic grading (Fig. 4B) and mean T2 relaxation times (Fig. 4C) (p < 0.05). Statistically significant correlation was found between severity of medial stabilizer inju- TABLE 1: Comparison of Femoral Trochlear Dysplasia and Patellofemoral Alignment Between Groups With and Without Patellofemoral Instability (PFI) Measurement Group With PFI (n = 53) Group Without PFI (n = 53) p a MR measurements of trochlear dysplasia Lateral trochlear inclination ( ) < 0.0001 Mean ± SD 10.2 ± 5.1 19.1 ± 4.4 Lower 95% confidence limit 8.8 17.9 Upper 95% confidence limit 11.6 20.3 Trochlear facet asymmetry (%) < 0.0001 Mean ± SD 53.8 ± 18.7 69.1 ± 11.5 Lower 95% confidence limit 48.6 65.9 Upper 95% confidence limit 58.9 72.2 Trochlear depth (mm) < 0.0001 Mean ± SD 3.1 ± 1.4 5.2 ± 1.3 Lower 95% confidence limit 2.7 4.8 Upper 95% confidence limit 3.4 5.5 MR measurements of patellofemoral alignment Patellar height ratio < 0.0001 Mean ± SD 1.3 ± 0.2 1.2 ± 0.2 Lower 95% confidence limit 1.3 1.1 Upper 95% confidence limit 1.4 1.2 Tibial tubercle-trochlear groove distance (mm) < 0.0001 Mean ± SD 10.4 ± 4.8 6.2 ± 3.7 Lower 95% confidence limit 9.1 5.2 Upper 95% confidence limit 11.8 7.2 Lateral patellofemoral angle ( ) < 0.0001 Mean ± SD 0.6 ± 7.0 11.1 ± 4.2 Lower 95% confidence limit 1.3 9.9 Upper 95% confidence limit 2.6 12.3 a Wilcoxon rank sum test. 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Kim et al. TABLE 2: Distribution of Grades of Femoral Trochlear Dysplasia and Medial Stabilizer Injury in the Group of Patients With Patellofemoral Instability Grade Femoral Trochlear Dysplasia ry and degree of patellar cartilage damage at the medial areas (M1 and M2) with both morphologic grading (p < 0.05) and mean T2 relaxation times (p < 0.05). The central and lateral areas did not exhibit any significant correlation. In the group with PFI, 27 subjects had bone marrow edema in the medial aspect of the patella, and 30 subjects had bone marrow edema or osteochondral injury of the lateral femoral condyle (Fig. 5A). The subgroup with medial patellar bone marrow edema (n = 27) had a higher grade of cartilage damage (Fig. 5B) (p < 0.05) and longer T2 relaxation times (Fig. 5C) (p < 0.05) in the medial facets of the patellar cartilage (M1 and M2) than did the subgroup without marrow edema. The mean T2 relaxation times at M1 were 40.2 ms (with marrow edema) versus 34.3 ms (without marrow edema) and at M2 were 40.1 ms (with marrow edema) versus 34.3 ms (without marrow edema) (Figs. 6A and B). The subgroup with lateral femoral condyle bone marrow edema or osteochondral injuries (n = 30) had a higher grade of cartilage damage (p < 0.05) and longer T2 relaxation times (Figs. 6C and 6D) (p < 0.05) in the medial facet (M1 and M2) of the patellar cartilage than did the subgroup without such lesions: 40.4 versus 34.1 ms at M1 and 39.2 versus 34.3 ms at M2. Regarding femoral trochlear dysplasia, none of the MRI measurements, including lateral trochlear inclination angle, trochlear facet asymmetry, and trochlear depth, and none of the morphologic subtypes correlated with severity of patellar cartilage damage. The only measure of patellofemoral alignment that correlated with severity of cartilage damage was the patellar height ratio (patella alta). Neither TT-TG distance nor patellofemoral angle correlated with severity of cartilage damage. Medial Stabilizer Injury Description No. of Subjects Description No. of Subjects 0 Normal 0 Normal 21 1 Shallow 21 Surrounding edema 7 2 Flat and horizontal 6 Partial tear 12 3 Flat and oblique 12 Complete tear 9 4 Bony protrusion 14 Reconstruction 4 T2 Relaxation Time (ms) T2 Relaxation Time (ms) 100 80 60 40 20 0 100 80 60 40 20 0 No Edema No Edema M1 M1 Edema Edema A Discussion Unlike adults, in whom developmental changes cease and all osseous structures are stationary, growing children continue to undergo developmental changes with skeletal maturation in their knee joints [8, 14, 18]. In particular, the TT-TG distance increases with chronologic age during skeletal maturation [8]. Furthermore, cartilage microstructures change substantially with age; T2 relaxation times of the patellar cartilage decrease with chronologic age [9, 10]. In our study, we included groups with and without PFI. To eliminate age- and sex-related changes affecting both patellofemoral joint congruity and cartilage integrity, we included age- and sex-matched subjects as a control group. In this matched study, we found significant differences in measurements of femoral trochlear dysplasia and patellofemoral alignment between the two groups. However, none of the measurements reflecting the severity of femoral trochlear dysplasia in the PFI group correlated with the severity of patellar cartilage damage in morphologic grading or on T2 relaxation time maps. Our results are similar to those of a study conducted in the adult population in which cartilage damage was evaluated with morphologic imaging only [19]. Although morphologic imaging was found to be limited in the evaluation of early chondromalacia, the study did show that femoral trochlear dysplasia was related to the presence of patellar cartilage damage but not to the degree of damage. Cartilage damage in the knee joint occurs at variable locations, and different injury mechanisms account for the different locations of cartilage damage [20, 21]. More detailed and localized evaluations of the cartilage can improve the precision of as- T2 Relaxation Time (ms) T2 Relaxation Time (ms) 80 60 40 20 0 80 60 40 20 0 No Edema No Edema M2 Edema Edema C D Fig. 6 Relation between T2 relaxation time and presence of edema. A D, Box plots show comparison of subgroups with and without bone marrow edema of medial aspect of patella (A and B) and lateral femoral condyle (C and D). Subgroup with bone marrow edema had significantly longer T2 relaxation times at medial aspect of patella (M1 and M2). Boxes represent area between first and third quartiles of response; line, median response; whiskers, extreme low and high ends of response. M2 B 1326 AJR:206, June 2016

Patellofemoral Instability in Children sessments of cartilage damage location and severity [22]. In our study, subjects with a higher-riding patella had more severe cartilage damage at the central portion of the patella. High-riding patella (patella alta) is known to be a main contributor to patellofemoral malalignment [7]. Patella alta results in reduced contact area between the patella and femoral trochlea, and instability occurs in shallow degrees of flexion [7]. Therefore, patella alta is considered an independent predisposing factor for patellar cartilage damage [23]. In patients with PFI, the femoral condyle is shallow or flat. Unlike in normal knees, in which the distance between the patellar cartilage and the femoral condyle is even and the patella is stably lodged, in knees with PFI the two structures are closest at the patellar apex, and the patella does not stay in the femoral condyle. Therefore, the central patellar cartilage is more vulnerable to friction and shearing forces. This may be seen more in patients with patellar maltracking than in those with acute dislocation, as evidenced in a study that showed relatively lower-grade injury in the central patellar cartilage than in the medial area in patients with PFI [24]. Bone marrow edema of the patellar cartilage and bone marrow edema or osteochondral injury to the lateral femoral condyle are characteristic MRI findings reflecting acute patellar dislocation and relocation injuries. During this event, the medial aspect of the patella smashes the lateral femoral condyle with concurrent injury to the medial stabilizers. In our study, the patients with PFI with acute injuries had a higher grade of cartilage damage at the medial aspect of the patella. This is not unexpected and was previously observed during morphologic imaging [7] and was confirmed in our study with both morphologic imaging and quantitative cartilage imaging. Our study showed two different areas of patellar cartilage damage in patients with PFI. Although the injury mechanisms are different in patella maltracking and acute patellar dislocation and therefore account for different patterns of patellar cartilage damage, we did not subclassify our patient population. The extent and severity of cartilage damage directly correlate with recurrence and disease duration of PFI [20, 24]. However, we did not include recurrence rate or duration of disease because most of our patient population were children and young adults who presented with acute symptoms. A future study with a larger sample size in each subgroup (maltracking vs acute dislocation, acute vs recurrent dislocation) of PFI would be helpful for identifying the different mechanisms of patellar cartilage damage. Another limitation of our study was that the subjects in the group without PFI were selected on the basis of inclusion and exclusion criteria to achieve normal data on the patellar cartilage and patellofemoral alignment. However, most of the subjects in this group presented with knee pain, and our data from the control group may not represent absolutely normal cartilage. We did not use the new method for measuring patellar height ratio suggested by Ali et al. [23], which is recognized to more accurately reflect patellar-trochlear cartilage congruence than the commonly used method. However, we provided several different measurements reflecting the patellofemoral trochlear congruency and alignment. In our study, most of the patients with PFI had low-grade cartilage injury. A small number of subjects with PFI had grade 4 injury, which was predominantly seen at the far medial aspect of the patellar cartilage. This is not unexpected, because although patellar cartilage damage accounts for most cartilage damage in the knee joint, high-grade injury is before the age of 40 years [21], and the size of the lesion positively correlates with age in the adult population [20]. Therapeutic options for PFI include nonsurgical and surgical approaches that are optimized to prevent patellar cartilage damage from resulting in recurrent patellar dislocations. The strategy behind currently used surgical techniques is to restore altered mechanics with medial stabilizer reinforcement [25, 26] or lateral release [27] or to eliminate predisposing risk factors through reconstruction of the dysplastic femoral trochlea [28] or tibial tuberosity transfer [29]. Conclusion Children and young adolescents with PFI have significantly more femoral trochlear dysplasia and patellofemoral malalignment than age- and sex-matched control subjects. However, the degree of trochlear dysplasia is not related to the severity of patellar cartilage damage. Among the subjects with PFI, the central patellar cartilage was more severely damaged in patients with patella alta, and the medial patellar cartilage was more damaged in patients with acute patellar dislocation. To prevent progressive cartilage damage in children and young adolescents, different mechanisms of injury should be considered in patients with PFI to help determine the best therapeutic options. References 1. Fithian DC, Paxton EW, Stone ML, et al. Epidemiology and natural history of acute patellar dislocation. Am J Sports Med 2004; 32:1114 1121 2. Sillanpää P, Mattila VM, Iivonen T, Visuri T, Pihlajamäki H. Incidence and risk factors of acute traumatic primary patellar dislocation. Med Sci Sports Exerc 2008; 40:606 611 3. Cook C, Hegedus E, Hawkins R, Scovell F, Wyland D. 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