Diederichs, Issever, and Scheffler [7] reported that the AKP may affect up to more than 30% of adolescents at any time. The majority of their patients were female, and their symptoms most commonly occurred in their second and third decades of life; this was matching the present study where about 66% of the study sample was females with an average age of 27 years and 34% were males with an average age of 30 years.
In our study, we divided the pathological process causing the symptoms of anterior knee pain in a fashion similar to that proposed by McNally et al. [10] into five categories according to the location and the anatomical structure affected as follows: (a) patellar tendon disorders, (b) quadriceps tendon disorders, (c) patellar disorders, (d) Hoffa’s diseases, and (e) miscellaneous causes including anterior meniscal tear and cartilage injuries.
In the present study, each category represented the following percentages: (a) Patellar tendon disorders represent 9% of the sample size, and they include patellar tendinopathy at 4% and Osgood–Schlatter disease at 4%. (b) Quadriceps tendon disorders represent 7% of the sample size, and they include quadriceps tendinopathy at 3% and quadriceps tendon tear at 4%. (c) Patellar disorders represented 67% of the sample size, and they include chondromalacia patella at 43%, patellar instability at 19%, transient patellar dislocation at 7%, and the painful bipartite patella at 3%. (d) Hoffa’s diseases represented 14% of the sample size, and they include Hoffa impingement syndrome and Hoffa ganglion cyst. Finally, (e) the miscellaneous causes that represented 17% of the sample size, and they include torn anterior horn of the lateral meniscus at 13% and articular cartilage shear injury at 4% (Tables 5 and 8).
Patellar tendinitis (PT) or Jumper’s knee is considered as one of the most common tendon abnormalities in young active subjects. From the etiological point of view, it had been recently described as a degenerative tendinopathy rather than an inflammatory process [11]. It exhibits the characteristic imaging features in MRI including focal thickening of the proximal segment of the tendon (the tendon AP thickness is more than 7 mm) that may show an area of focal T2 high signal intensity, with a predilection to the medial portion of the affected tendon [12].
In our results, 4% of the patients presented with AKP showed MRI evidence of patellar tendinopathy (66% below the age of 30 years). In all cases, the hyperintense focal thickening was at the proximal third of the tendon, with the AP diameter of the patellar tendon being greater than 8 mm (Fig. 7), and this was concordant with the study of Mohammad Samim et al. [12], as regards the MRI findings of patellar tendinitis.
Osgood–Schlatter disease (OSD) was defined by Gottsegen et al. [13], as a traction apophysitis of the tibial tuberosity caused by repetitive microtrauma. It is more prevalent in adolescent male athletes.
Niitsu [14] considered the MRI as the modality of choice for the diagnosis of OSD as the tibial surface may not be yet ossified; thus, the plain radiography may play a limited role in the diagnosis of the early cases. In our study, about 4% had an MRI evidence of OSD (all of them were < 30 years) (Fig. 12), with a male predominance in the study sample (males represented 67% of OSD sample size).
Aparicio et al. [15] had related the presence of a patella alta (patellar height ratio of more than 1.3) (normal is 0.8 to 1.1) as one of the predisposing factors to OSD; in our results, 33% of the cases showed MRI evidence of patella alta (patellar height ratio equal to 1.7) as a predisposing factor for OSD (Fig. 15).
Quadriceps tendon rupture is one of the serious extensor tendon injuries, and its incidence had been reported as an uncommon problem; it is more seen in subjects older than 40 years. When it occurs, it usually takes place at the tendon–bone junction and may be a partial or a full-thickness tear; nevertheless, the partial thickness tear may be challenging for clinical diagnosis as there is some preservation of the extensor mechanism; hence, the MRI may play an important role in its early detection [16]. In our results, 4% (3 patients) had a quadriceps tendon rupture; 2 of them were above the age of 40 years; also, 2 of them showed MRI evidence of partial tear and one patient showed MRI evidence of complete tendon tear with all fibers were transected. All of these patients experienced a past history of trauma, and none of them had a predisposing factor for spontaneous tendon rupture.
The alteration of the biomechanics of the tendon can lead to tendon degeneration resulting in chronic tendinosis that may present by AKP [17]; in MRI, it may appear as a chronic enthesopathy; in our results, the quadriceps tendinopathy was detected in about 3% of the patients who presented by AKP.
Chondromalacia patella could be described as a denudation of the patellar cartilage with surface irregularity on arthroscopic probing; this condition may present patellofemoral pain (PFP) and is more likely to be seen in adolescents especially the females [18]; this was matched with our study where (70% female, 30% male, and 25 years age average). Considering the 0.05 level of significance, the P value of chi-square test (0.03) showed that there was a significant difference between the prevalence of chondromalacia patella between males and females.
The degree of cartilage loss can be graded into four degrees by arthroscopy according to the Outerbridge grading system; however, the International Cartilage Repair Society (ICRS) found a good MRI correlation to the Outerbridge grading system and classified the cartilage loss into four degrees as well (Table 3) [19, 20].
Mattila et al. [20] reported that the MRI diagnostic accuracy is higher for high-grade lesions (grades III and IV), and this was greatly matching our study, where 43% of the cases of chondromalacia patella were grade IV, 27% were grade III, 23% were grade II, and 7% were grade I (Table 9).
MRI plays an important role in the diagnosis of the patellar instability; moreover, it can provide a clue for the diagnosis of the anatomical variants that may predispose to the condition [21].
The study done by Kirsch et al. had found that the prevalence of patellar dislocation among 1450 patient was 2%. The most constant finding detected in all of the patients with transient patellar subluxation was the presence of knee effusion (100%). Abnormalities in the medial patellar retinaculum were found in about 96%, and abnormal lateral patellar position in about 92%, whereas the contusions of the outer femoral condyle and the inner patellar facet were found in about 81% [22].
In our study, the patellofemoral instability (i.e., transient patellar dislocation) was detected in 7% of the patients who were presented by AKP, with female predominance (60% female and 40% male); almost all patients had MRI evidence of joint effusion, and also almost all of them showed abnormalities in the medial patellar retinaculum. Eighty percent of our patients showed abnormal lateral patellar position and 60% of them showed contusions of the outer femoral condyle and the inner patellar facet (kissing contusions).
Trochlear dysplasia had been described as a predisposing factor for patellar instability and was classified into four main types by Dejour and Coultre [23] (types A, B, C, and D) (Table 4).
In our study, we founded that 19% of the patients had PFP with malalignment (i.e., chronic patellar instability) and showing MRI evidence of trochlear dysplasia. Thirty-eight percent were categorized as type A, 38% as type B, and 24% as type C according to Dejour et al.’s classification (Figs. 13, 14, and 15) (Table 10).
For the assessment of the trochlear dysplasia, the lateral trochlear angle inclination was instrumented using the MRI by Carrillon et al. [8]; they found the lateral trochlear inclination of 11° to be a threshold value to distinguish between patients with patellofemoral instability (trochlear dysplasia) and those with a nonspecific knee pain.
Another method was by measuring the depth of the trochlear groove and the trochlear facet asymmetry being measured 3 cm proximal to the tibiofemoral cleft; this was described by Pfirrmann et al. [9] who found that trochlear groove depth less than 3 mm and facet asymmetry of less than 40% were of diagnostic value for the trochlear dysplasia.
MRI had been proved to be highly accurate with reproducible measurement of the femoral sulcus from the subchondral bone as well as from the articular hyaline cartilage as the measurements from the articular cartilage was considered a true measurement rather than the plain radiography measurements as the former is more representative of the actual joint space [7].
Nelitz et al. did a study on 80 knees of 78 patients, aiming to evaluate whether specific measurements of the femoral trochlea can be assigned to the qualitative classification system of Dejour using the above-mentioned measurements. They concluded that, by using a descriptive statistics using boxplot diagrams, none of the objective measurements of the femoral trochlea described in the literature could be assigned to the four-grade descriptive classification of trochlear dysplasia of Dejour as the median and average range of these measurements allowed no discrimination between trochlear dysplasia types B, C, and D. However, the threshold values used to discriminate between low-grade (Dejour type A) and high-grade dysplasia (Dejour types B–D) could be identified [24].
In our results, types B and C trochlear dysplasia showed nearly similar median and average values of both the trochlear groove depth and lateral inclination angle (median value of trochlear groove depth for both B and C was 2 mm and for A was 5 mm; the median values of the lateral inclination angles for both B and C were 9° and 9.4° respectively and for type A was 20°). The median values of the trochlear facet asymmetry were different between different types of trochlear dysplasia (the median value for type A was 39%, for type B was 55%, and for type C was 33%) (Table 11).
Fithian et al. [25] also reported that most patients with patellar dislocation were young and active subjects, with the females in their 2nd decade of life being considered as a more vulnerable group. In our study, we found that 66% of the patients were female and 34% of them were male.
An association between the patellar subluxation with both the chondromalacia patella and the patellar tendinopathy had been found by McNally et al. [26]; in our results, only 7% of the patients with patellar subluxation had MRI evidence of a chondromalacia patella and 7% had MRI evidence of patella Alta (Fig. 15).
Bipartite patella occurs when the secondary ossification centers of the patella did not fuse together; Mohammad Samim et al. [12] described three types of bipartite patella according to Saupe et al.’s classification. The first one involves the lower pole of the patella; the second one involves the lateral margin of the patella, and the third one—which is considered as the most common type—involves the upper-lateral pole.
In a study done by Kavanagh et al. [27], the bone marrow edema that was detected within the bipartite fragment was considered as the sole finding in 49% of the patients who had a symptomatic bipartite patella. In our study, 3% of the patients (2/70) with AKP had a symptomatic bipartite patella. Both of them were of the third type (at the superolateral patellar pole) and had MRI evidence of bone marrow edema within their bipartite fragments (Fig. 6).
Chung et al.’s [28] study enrolled 50 patients with AKP and reported that 50% of the sample had Hoffa impingement syndrome with a female predominance. In our study, only 14% of patients showed edema within the Hoffa pad of fat with respect to female predominance to account for about 70% of the sample size, and male 30% (Tables 5, 6, and 8). So, our study is concordant with Chung et al.’s study as regards the gender but differs in the prevalence and this might be explained by the difference in the criteria of the selected sample between the two studies, as Chung et al. excluded patients outside the ages of 14–50 years or those with a history of direct trauma to the knee, while in our study both were included.
Anterior meniscal tear and cartilage injuries were considered by McNally et al. [26], among the main causes of AKP. MRI is considered as the study of choice in cases of the meniscal tear; moreover, it may replace the unnecessary arthroscopy for such entity of disease [10]. In our results, about 13% of the patients in the study sample had MRI evidence of a tear in the anterior horn of lateral knee meniscus with a female predominance (67% females and 33% males). Their age average was 27 years, 78% of them had a history of trauma. Only 22% were above the age of 45 and had no past history of knee trauma (Tables 6 and 8).
The articular cartilage injury may mimic the meniscal tears and might be associated with less satisfactory arthroscopic results; thus, it would be beneficial to have an MRI examination prior to intervention to sort out the patients who might get benefit from cartilage replacement therapies [29].
A variety of articular cartilage injuries had been described as post-traumatic sequelae including fissures, chondral flaps or tears, and loss of a segment of articular cartilage; however, these lesions may be detected as a sole finding or might be associated with other abnormalities.
In our results, cartilage injuries were detected in 6% of the study sample, showing a male predominance (75% were male and 25% were female) with average age 32 years. All had a past history of trauma. It is worth mentioning that 25% of cartilage injury cases were associated with the anterior meniscal tear (Tables 6 and 8).