A novel ultrasound scanning approach for evaluating femoral cartilage defects of the knee: comparison with routine magnetic resonance imaging

In this prospective study, 64 consecutive patients (65 knees) scheduled for knee arthroscopy between April 2010 and July 2011 were prospectively evaluated by US and MRI at our hospital. The study was approved by the research ethics committee of the Third Affiliated Hospital of Sun Yat-sen University. All patients signed an informed consent prior to participation in the study.

Patients with a chief complaint of knee pain or disability and scheduled for arthroscopy were enrolled in this study. The patients underwent arthroscopy of the knee because of suspicion of internal derangement. Patients with prior knee surgery (e.g., total knee arthroplasty) and contraindications to MRI were excluded.

US examination of the knee joint was conducted a week before arthroscopy using a LOGIQ 700 (GE Medical Systems, Milwaukee, WI, USA) with a 7–9-MHz high-resolution linear transducer. A novel US scanning approach based on the functional anatomy of the knee (Fig. 1) [15] was proposed, considering articular motions and both transverse and longitudinal scanning (Fig. 2). (1) In the supine position with a fully extended knee (0°), transverse and longitudinal scanning of bilateral sides of the patella for the anterior portions of both condyles was carried out (Fig. 2a). (2) In the prone position, longitudinal and transverse scanning of the popliteal fossa for the posterior portions of both condyles was performed (Fig. 2b). (3) In the supine position with maximum knee flexion, transverse and longitudinal scanning of the suprapatellar recess for the trochlear surface was undertaken (Fig. 2c). (4) Finally, in the same position, transverse and longitudinal scanning of bilateral sides of the patella was performed again, for the weight-bearing portion of the condyles (Fig. 2c). The US beam was always kept perpendicular to the femur surface. Maximum flexion angle of each patient was recorded, and an angle > 135° was considered good flexion. This angle was confirmed using a lateral photography and the Angler app (an iOS app for angle measurement which was available only on Apple Store China but is unfortunately no longer available). The patient was in the supine position with maximum knee flexion, then a lateral photography was taken and the flexion angle was measured by this app.

A US grading system for knee femoral cartilage defects, based on the International Cartilage Repair Society (ICRS) classification [5, 7, 19, 20], was used: grade 0, normal cartilage; grade 1, nearly normal cartilage with blurred margin or partial lack of clarity without thickness change; grade 2, abnormal cartilage with blurring or obliteration of the margin, lack of clarity, and overt local thinning of the cartilage (< 50% of cartilage depth); grade 3, severely abnormal cartilage with blurring and obliteration of the margin and obvious focal thinning for > 50% of the cartilage depth but intact cartilage-bone interface; and grade 4, severely abnormal cartilage with complete loss of cartilage and coarse or irregular cartilage-bone interface.

Two musculoskeletal radiologists (JYC and JR, with 5 and 15 years of clinical experience, respectively) were trained for 2 months to perform the novel examination method before the present study. This training involved ten volunteers. The radiologists repeatedly scanned those volunteers, alone and together, and compared their results. In this study, the radiologists scanned each patient immediately one after another (according to their availability, without a predefined order) and made assessments independently during real-time scanning, based on individual findings. A form with a schematic drawing of the articular surfaces of the femoral cartilage [19] was used. The trochlear surface and lateral and medial femoral condyles were evaluated separately. In the presence of multiple cartilaginous defects on the same articular surface, the worst score was attributed.

MRI examinations were performed within 1 week before arthroscopy on a Signa Excite 1.5 Tesla MR System (GE Medical Systems, Milwaukee, WI, USA), using a phased array knee coil. Patients were placed in the supine position with the knee fully extended. All MRI examinations consisted of sagittal T1- and T2-weighted, sagittal fat-saturated (fs) proton density (PD)-weighted, coronal fs PD-weighted, and axial fs PD-weighted fast spin echo (FSE) sequences. The imaging parameters are presented in Additional file 1: Table S1.

All MRI images were interpreted by one radiologist (XCM) with over 15 years of experience in musculoskeletal imaging. The radiologist was blinded to the clinical history and US findings. The cartilage was morphologically graded according to a modified ICRS classification system [19]: grade 0, normal cartilage; grade 1, cartilage with intact surface and no tissue loss, but fibrillation and superficial fissures; grade 2, deep defects, but < 50%; grade 3, lesions representing > 50% of the cartilage thickness; and grade 4, defects extending into the subchondral bone. The same schematic drawing of articular surfaces of the femoral cartilage was used to mark defect locations and degrees. The trochlear surface and lateral and medial femoral condyles were evaluated separately. In the presence of multiple cartilaginous defects on the same articular surface, the worst score was attributed.

Two orthopedic surgeons (HDL and KW, with 15 and 25 years of clinical experience, respectively) were involved in this study. They were blinded to the US and MRI findings. Knee arthroscopy with fluid irrigation was performed using the standard anterolateral and anteromedial portals to assess the articular cartilage defects and potential comorbidities. The surgeons were free to flex and extend the patients’ knees. For cartilage evaluation by arthroscopy, the same schematic drawing of the femoral cartilage surfaces of the knee was used. The surgeons performed their assessment in consensus. The trochlear surface and lateral and medial femoral condyles were evaluated separately. The cartilage lesions were graded analogously to the US and MRI using the ICRS classification [19]. In the presence of multiple cartilaginous defects on the same articular surface, the worst score was attributed. Arthroscopic grading served as the gold standard.

With arthroscopic findings as a reference, sensitivity, specificity, and accuracy of US and MRI for femoral cartilage defects were assessed. In addition, positive predictive value (PPV) and NPV were calculated and adjusted for the prevalence of femoral cartilage lesions documented in arthroscopy at our institution (51.9%), based on previous recommendations [21]. The threshold of femoral cartilage abnormality was set between grade 0 as negative and grades 1–4 as positive. Furthermore, detection rates of each grade of cartilage defects and proportion of cartilage lesions graded identically and within one grade in arthroscopy and imaging modalities were calculated. The McNemar chi-square test was used to compare the groups.

Weighted κ statistics for the level of agreement among different methods and between radiologists (US) were used [22]. The Student t test was used to compare the κ values between grades assigned by arthroscopy and US or MRI. Error analysis was performed for the first blinded US evaluation by the same radiologists after the revelation of arthroscopic findings. All false-positive or false-negative results were analyzed, and the most likely reason for the diagnostic error was recorded.

Sixty-four consecutive patients (65 knees) with various abnormalities of the knee were assessed, including 21 men and 43 women, with a mean age of 42 years (range, 18–75). Final diagnoses included OA (25 knees), meniscus injury (11 knees), traumatic arthritis (11 knees), chondromalacia patella (7 knees), anterior cruciate ligament tears (6 knees), dislocation of the patella (3 knees), and meniscus cyst (2 knees). Among the 65 knees, 90.8% (59/65) had a good flexion. Altogether, 195 knee femoral cartilage surfaces, including the trochlear surface (TS), lateral femoral condyles (LC), and medial femoral condyles (MC), were evaluated by US, MRI, and arthroscopy. Arthroscopy revealed that 84, 20, 30, 38, and 23 surfaces had grades 0, 1, 2, 3, and 4 defects, respectively (Table 1). The distribution of cartilage defects identified by arthroscopy is shown in Table 1.

Table 2 shows the overall sensitivity, specificity, accuracy, and crude and adjusted PPV and NPV for US and MRI in detecting knee femoral cartilage defects, using arthroscopy as the gold standard. Figures 3 presents typical cases of each grade of cartilage defects. The sensitivity (62.2 and 69.4%), specificity (92.9 and 90.5%), and accuracy (75.4 and 78.5%) were similar between two independent radiologists performing the US. Interobserver agreement was obtained between the two radiologists, as indicated by a weighted κ value of 0.76. Considering the prevalence of cartilage lesions of 51.9% at our institution, obtained based on the analysis of 832 knee arthroscopies from January 2002 to December 2011, adjusted PPV was 88.7–90.4% (false PPV = 9.6–11.3%), and adjusted NPV was 69.5–73.3% (false NPV = 26.7–30.5%). Only a lower sensitivity of US was observed for radiologist 1 (JYC, 5 years of clinical experience) compared with MRI (P = 0.042). Other parameters showed non-significant differences between US and MRI (P = 0.060 to 1.000), as shown in Table 2.

For all cartilage defect grades on each articular surface, some differences were seen between US and MRI for specific surfaces (Table 3), but there were differences between the two radiologists.

The respective detection rates and comparisons between US and MRI for cartilage defects of each grade are shown in Tables 4 and 5. Generally, compared with MRI, no significant differences were obtained in detecting grades 0, 2, 3, and 4 defects (P = 0.083 to 0.317), but a lower detection rate with US for grade 1 defects was obtained for both radiologists (P = 0.025). Indeed, for radiologist 1, ten lesions were diagnosed as grade 0, eight as grade 2, and two as grade 3. For radiologist 2, eight lesions were diagnosed as grade 0, six as grade 2, and six as grade 3. The proportions of cartilage defects graded identically and matched within one grade of arthroscopic values using US by the two radiologists were 80.0 and 80.5%, respectively, with no significant difference between both radiologists (P = 0.179 and P = 0.224, respectively). A moderate agreement between arthroscopy and US for grade assignment (weighted κ = 0.60, 95% CI 0.50–0.71, and 0.63, 95% CI 0.52–0.74, for the two radiologists) was obtained, with no significant difference (P = 0.57 and P = 0.11, respectively) in comparison with MRI data (weighted κ = 0.73, 95% CI 0.62–0.84). In addition, substantial agreement was found between grades assigned using US and MRI (weighted κ = 0.61, 95% CI 0.50–0.72).

Error analysis was carried out to determine the reasons for false-positive and false-negative diagnoses by US, and the results are shown in Table 6. The majority of false negatives were due to the lesions being located at certain sites, including the femoral condyles near the intercondyloid fossa, where no defects were detected by US (Fig. 4). The majority of false positives were attributed to the partial volume effect (Fig. 5).