Background
Anterior knee pain (AKP) is a problematic complaint, considered to be the most frequent cause of orthopedic consultancy for knee problems [
1]. AKP’s differential diagnosis falls within a wide range, which is still overlapping and lacking clear understanding [
2]. It is more frequent in young people between 15 and 30 years and is more frequent in women than in men. Despite its prevalence, AKP nature and causes remain inadequately understood and can be bothersome for patients and clinicians as it causes chronic disability, limited sports and activity, and a negative impact on the quality of life [
3,
4]. Many authors linked AKP to the patellofemoral pathology, especially the patellofemoral instability, while others clearly reported that structural anomalies did not provide a complete explanation of the pain [
5]. Moreover, structural anomalies were found to be minor among AKP patients, and there was no obvious correlation between patellofemoral malalignment and long-term results of AKP treatment [
6].
The patient’s clinical history and physical examination are of paramount importance in diagnosing the cause of AKP. The physical examination is complemented by imaging examinations, and the combination should yield a precise diagnosis that will be the cornerstone in developing an appropriate therapeutic program [
7]. Imaging workup is important to evaluate the extent of the bone and soft tissue abnormality and guide therapeutic intervention if needed [
8,
9]. The most helpful diagnostic techniques for evaluating soft tissue changes are ultrasonography and magnetic resonance imaging (MRI). Plain radiography is of limited value, and computed tomography is not recommended [
10,
11]. Most orthopedics rely on MRI as the method of choice in knee imaging as it provides high contrast resolution images not only for the soft tissue but also for the underlying bone and allowing a precise assessment of the underlying cause [
3,
12]. Moreover, MRI has replaced diagnostic arthroscopy as the primary diagnostic modality for many knee pathologies [
13]. Ultrasonography has become more popular because it is safe, quick, inexpensive, and reliable. It has the ability to assess soft tissues in the anterior aspect of the knee, which could be the main source of pain [
6].
Although the literature is full of several researches about the value of ultrasonography examination of the knee, the AKP dilemma makes the need for evidence-based value, regarding various diagnoses, useful for guiding choices in value-based health care in imaging, a point that should be stressed. Therefore, we conducted our prospective study to clarify the diagnostic accuracy of ultrasonography as a fast imaging technique in the assessment of patients with AKP and comparing the results with MRI.
Discussion
The current study highlighted the role of ultrasonography in the diagnosis of AKP. The overall results are encouraging and demonstrated the high diagnostic accuracy of ultrasonography (85.3% sensitivity, 100% specificity, 100% PPV, and 57.4% NPV) compared to MRI. Ultrasonography detected 195 out of 259 findings detected by MRI. Some of these findings could be the cause of the AKP like tendinopathy, fat impingement, and chondropathy, whereas others were just a sign of associated pathology like joint effusion and anterior subcutaneous edema, which may strengthen an equivocal or suspected clinical diagnosis. The ultrasonography detected most of the pathological findings in this study. The early and fast diagnosis by ultrasonography could allow the orthopedic to start the patient management plane without the usual delay for requesting and scheduling MRI examinations that would have a good impact on patient recovery.
According to the study done by Artul et al. [
19], 34% of ultrasonography reports were negative, and 66% were positive. In the present study, 29% (45 knees) of ultrasonography reports and 16.8% (26 knees) of MRI reports were normal.
Lee and Chow [
20] reported that ultrasonography is a sensitive tool to assess knee joint effusion, and a minimal amount of joint effusion as low as 7 to 10 ml could be optimally detected. According to the Draghi et al. [
21] study, ultrasonography had 81.3% sensitivity and 100% specificity in detecting knee effusion. In the present study, ultrasonography showed a sensitivity of 91.5% and specificity of 95.8% in detecting knee effusion. Ultrasonography failed to detect five knees as the effusions were minimal and presented in front of the anterior cruciate ligament. On the other hand, possible improvement in the gap period between ultrasonography and MRI may be responsible for the false-positive results.
In the relevant literature, the detection of cartilage defects commonly refers to trochlear cartilage. The patellar cartilage is not routinely assessed by ultrasonography, as it is usually shaded by the patella. Cao et al. [
22] stated that ultrasonography is considered a promising screening tool for assessment of trochlear cartilage defects as it showed a sensitivity range of 62.2 to 69.4% and specificity range of 90.5 to 92.9%. In the present study, ultrasonography showed a sensitivity of 71.9% and a specificity of 98.4% in detecting trochlear cartilage defects. In the 9 false-negative knees, the trochlear defects were overlooked by ultrasonography as they were deeply located in the intercondylar fossa. All patellar cartilage defects were overlooked by ultrasonography because they were obscured by the patellar shadow during the examination.
Unlu et al. [
23] reported that anterior subcutaneous edema is a common finding (82.7%) on routine knee MRI and is significantly associated with old age, overweight, and patellofemoral chondral changes. In the present study, ultrasonography showed a sensitivity of 77.4% in detecting subcutaneous edema.
Knee’s synovial plica syndrome is a commonly overlooked cause of AKP [
24]. Anatomically, the infrapatellar plica is the most common plica, followed by suprapatellar plica, and lastly, the medial patellar plica, which is considered the most symptomatic [
25]. In this study, ultrasonography showed a sensitivity of 78.5% and specificity of 100% in detecting synovial plicae. The infrapatellar plica was the least common type of plica in our study, contrary to what was reported by Kheiralla [
25], probably due to the type of our group, as they were symptomatic for AKP.
Patellar tendinopathy is a common cause of AKP, especially in athletes. It mainly affects the proximal aspect of the tendon. Ultrasonography had 87% sensitivity in detecting patellar tendinopathy [
26,
27]. In this study, the sensitivity of ultrasonography was 84%.
To the best of our knowledge, detection of quadriceps tendon pathology has been reported while commonly referring to the quadriceps tendon tear in athletes, and most of the related literature focused only on MRI [
25,
28]. In this study, ultrasonography showed 87.5% sensitivity, compared to 72.5% reported by King et al. [
29].
Fat pad impingement syndromes most often affect the suprapatellar and superolateral Hoffa’s fat pads [
30,
31]. These fat pads normally act to promote both joint lubrication and joint stability [
32]. When impinged due to patellar maltracking, these fat pads display obscuration of normal fat and increased vascularity in these regions [
33]. Gutierrez et al. [
34] ensured the ability of ultrasonography in the detection of suprapatellar impingement. Tsavalas and Karantanas [
35] reported a prevalence rate of 12% among patients with AKP. In our results, the prevalence rate of the suprapatellar fat impingement was 12.3%. Ultrasonography had 84.2% sensitivity and 100% specificity in detecting suprapatellar fat impingement. Color Doppler ultrasound showed increased vascularity in 3 knees due to concomitant inflammation.
Draghi et al. [
21] reported that MRI is the modality of choice in the assessment of pathological changes of the Hoffa’s fat pad. Mikkilineni et al. [
36] prospectively emphasized that ultrasonography may be valuable for the diagnosis of impingement of the Hoffa’s fat pad, which needs more research. In this study, the sensitivity and specificity of ultrasonography were 66.7% and 100%, respectively for diagnosing impingement of the Hoffa’s fat pad. To our knowledge, no results were published in the literature to compare.
According to Draghi et al. [
37], ultrasonography had 100% sensitivity and specificity in detecting deep infrapatellar bursa. In the present study, the sensitivity and specificity of ultrasonography were 66.7% and 100%, respectively.
Our results matched Blankstein et al. [
38] in the assessment of the bipartite patella and reported 100% sensitivity and specificity of ultrasonography.
Gel stand-off pad is an aqueous, flexible, easily available, disposable spacer, widely used in B-mode ultrasonography approach of superficial lesions or difficult-to-visualize areas. Moreover, it allows the detection of otherwise-missed peri- or intra-lesional flow signals on Doppler imaging [
39]. In our study, we did not use the gel pad and used plenty of thick gel to avoid loss of contact. Therefore, further studies discussing the value of using gel stand-off pad in musculoskeletal ultrasonography are recommended.
Although ultrasonography had high diagnostic accuracy, a substantial number of lesions were missed. The main lesions missed by ultrasonography and detected by MRI were the patellar cartilage defects (ultrasonography missed all lesions) and the trochlear cartilage defects (ultrasonography missed 9 lesions). Accordingly, ultrasonography can be used in the diagnosis and screening of patients with AKP and can be used as an alternative to MRI when MRI was unavailable or contraindicated. MRI is indicated if a patellar cartilage defect is clinically suspected or the ultrasonography yielded negative results.
Our study has some limitations: First, there was a wide variety of findings; some of these findings may not be the actual cause of the AKP, and the others had no previously published results in the literature to compare. This generalizability may result in substantial compromises on the quality of findings. However, this study was an attempt to publish a comprehensive review about the diagnostic accuracy of ultrasonography in patients with AKP. Second, there was higher sensitivity of ultrasonography in detecting joint effusion, which was at the same time the most common finding in the current study. The perfection of ultrasonography in detecting joint effusion increased the overall sensitivity of ultrasonography in the detection of pathological findings in the AKP patients and masked its weakness in detecting other entities like patellar cartilage defect and infrapatellar plica. Third, unfortunately, all the ultrasonography examinations were performed by one radiologist. Hence, there was no chance to make the intra- and inter-observer agreement. Further studies discussing the possible variability of ultrasonographic signs among radiologists and providing data for operator intra- and inter-observer agreement are recommended. Fourth, the patients included in this study were already scheduled for an MRI examination, which could produce a selection bias. Nevertheless, not all patients scheduled for an MRI examination in this study were severed or complicated. Finally, lack of surgical data and clinical follow-up.
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