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A Reliable, Ultrasound-Based Method for the Diagnosis of Discoid Lateral Meniscus

Open AccessPublished:September 26, 2020DOI:https://doi.org/10.1016/j.arthro.2020.09.034

      Purpose

      To explore the feasibility and validity of ultrasound in the diagnosis of discoid lateral meniscus (DLM) by comparing quantitatively the morphologic difference between DLM and normal lateral meniscus.

      Methods

      This study was designed to develop and validate the ultrasound diagnostic criterion for DLM. In the development stage (July 2018 to June 2019), data from 180 subjects were used to derive the ultrasound diagnostic criterion, including 90 patients diagnosed as DLM by magnetic resonance imaging (DLM group) and 90 matched controls diagnosed as normal lateral meniscus diagnosed by magnetic resonance imaging (control group). Twelve distinct parameters of meniscus thickness, width, 0.5∗thickness/width, and angle were obtained through anterior, lateral, and posterior views with the probe oriented perpendicular to the lateral tibiofemoral joint line. In the validation stage (July 2019 to December 2019), data from 324 additional participants were used to validate the criterion derived from the development stage. Differences of the continuous variables and categorical variables between the 2 groups were analyzed by an independent t test and χ2 test, respectively. The diagnostic value of parameters was analyzed by the receiver operating characteristic curve.

      Results

      In the development stage, significant differences were found in the above 12 parameters between the 2 groups (P < .05 for all). Cut-off values of anterior meniscus angle, meniscus body angle, and posterior meniscus angle were 28.45°, 27.85°, and 29.15°, respectively. The area under the curve, sensitivity, and specificity of anterior meniscus angle (0.953, 95.6%, 91.1%), meniscus body angle (0.980, 95.6%, 95.6%), and posterior meniscus angle (0.942, 80.0%, 97.8%) were greater than other parameters. In the validation stage, the sensitivity, specificity, and accuracy of anterior meniscus angle, meniscus body angle, and posterior meniscus angle in diagnosing DLM were as high as 91.3%, 88.6%, and 89.2%; 94.2%, 93.3%, and 93.5%; and 76.8%, 95.7%, and 91.7%.

      Conclusions

      DLM can be reliably diagnosed by ultrasound measurements of anterior meniscus angle, meniscal body angle, and posterior meniscus angle.

      Level of Evidence

      Level III, case-control study.
      Discoid lateral meniscus (DLM) of knee joint is fibrous cartilage with abnormal morphology and structure,
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      The postoperative shorter meniscal width was the risk factor of lateral meniscal extrusion in the middle portion for juvenile and adolescent knees with discoid lateral meniscus [published online July 30, 2020]. Knee Surg Sports Traumatol Arthrosc.
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      The fate of the contralateral knee in patients with a lateral discoid meniscus.
      and is symptomized by pain, swelling, snapping, buckling, and locking,
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      Widening of the popliteal hiatus on sagittal MRI view plays a critical role in the mechanical signs of discoid lateral meniscus [published online July 29, 2020].Knee Surg Sports Traumatol Arthrosc.
      seriously impacting the patient’s quality of life.
      Magnetic resonance imaging (MRI) is the “gold standard” in imaging diagnosis of DLM, with a clear diagnostic criterion and a high sensitivity (95%) and specificity (97%),
      • Samoto N.
      • Kozuma M.
      • Tokuhisa T.
      • Kobayashi K.
      Diagnosis of discoid lateral meniscus of the knee on MR imaging.
      but it has high costs, long examination times, and is contraindicated in patients with installed pacemakers, magnetic metal implanted, and claustrophobia.
      • Marino M.A.
      • Leithner D.
      • Sung J.
      • et al.
      Radiomics for tumor characterization in breast cancer patients: A feasibility study comparing contrast-enhanced mammography and magnetic resonance imaging.
      ,
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      "Sports Ultrasound", advantages, indications and limitations in upper and lower limbs musculoskeletal disorders. Review article.
      In addition, MRI is difficult to perform in young children, who may not remain motionless during the imaging study. Ultrasound has potential capacity for the diagnosis of DLM for its satisfactory soft-tissue resolution.
      • Blankstein A.
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      ,
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      • et al.
      Accuracy of high-resolution ultrasound in the detection of meniscal tears and determination of the visible area of menisci.
      Compared with MRI, ultrasound has advantages such as low cost (only 20%-30% that of MRI), short examination times, no contraindications, open examination environment, and an adjustable examination position that is especially suitable to young children and patients with compulsive position.
      • Baloch N.
      • Hasan O.H.
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      • Hattori S.
      • Yamada S.
      "Sports Ultrasound", advantages, indications and limitations in upper and lower limbs musculoskeletal disorders. Review article.
      ,
      • Blankstein A.
      Ultrasound in the diagnosis of clinical orthopedics: The orthopedic stethoscope.
      Moreover, ultrasound uses portable equipment, enables real-time and dynamical assessment, and early detects pathologic changes.
      • Baloch N.
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      • Jessar M.M.
      • Hattori S.
      • Yamada S.
      "Sports Ultrasound", advantages, indications and limitations in upper and lower limbs musculoskeletal disorders. Review article.
      ,
      • Blankstein A.
      Ultrasound in the diagnosis of clinical orthopedics: The orthopedic stethoscope.
      Ultrasound is widely used in the diagnosis of meniscus lesion, ligament, or tendon injury, etc.
      • Baloch N.
      • Hasan O.H.
      • Jessar M.M.
      • Hattori S.
      • Yamada S.
      "Sports Ultrasound", advantages, indications and limitations in upper and lower limbs musculoskeletal disorders. Review article.
      ,
      • Karpinski K.
      • Diermeier T.
      • Willinger L.
      • Imhoff A.B.
      • Achtnich A.
      • Petersen W.
      No dynamic extrusion of the medial meniscus in ultrasound examination in patients with confirmed root tear lesion.
      • Alvarez C.A.D.
      • Hattori S.
      • Kato Y.
      • et al.
      Dynamic high-resolution ultrasound in the diagnosis of calcaneofibular ligament injury in chronic lateral ankle injury: A comparison with three-dimensional magnetic resonance imaging.
      • Breukers M.
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      Diagnostic accuracy of dynamic ultrasound imaging in partial and complete anterior cruciate ligament tears: A retrospective study in 247 patients.
      • Lee S.H.
      • Yun S.J.
      Efficiency of knee ultrasound for diagnosing anterior cruciate ligament and posterior cruciate ligament injuries: A systematic review and meta-analysis.
      • Dickson D.M.
      • Fawole H.O.
      • Newcombe L.
      • Smith S.L.
      • Hendry G.J.
      Reliability of ultrasound strain elastography in the assessment of the quadriceps and patellar tendon in healthy adults.
      However, there are just a few, small-sample, uncontrolled case studies about diagnostic ultrasound for DLM, merely in children.
      • Arifa Achour N.
      • Tlili K.
      • Mhiri Souei M.
      • Gamaoun W.
      • Jemni H.
      • Mrad Dali K.
      • et al.
      Discoid menisci in children: ultrasonographic features.
      ,
      • Wang Y.
      • Geng H.
      • He M.
      • et al.
      Ultrasonic diagnosis of discoid meniscus of knee joint in children.
      These studies demonstrate that ultrasound can accurately discern DLM as well, compared with MRI or arthroscopy. In addition, the absence of a normal triangular shape, abnormally elongated and thick meniscus, and the presence of a heterogeneous central pattern on ultrasound images is the key to diagnose DLM in children.
      • Arifa Achour N.
      • Tlili K.
      • Mhiri Souei M.
      • Gamaoun W.
      • Jemni H.
      • Mrad Dali K.
      • et al.
      Discoid menisci in children: ultrasonographic features.
      ,
      • Wang Y.
      • Geng H.
      • He M.
      • et al.
      Ultrasonic diagnosis of discoid meniscus of knee joint in children.
      Nonetheless, the blocking effect of the femoral condyle on the ultrasound beam reduces the scanning range in the lateral knee joint space; thus, the true morphology of DLM cannot be displayed,
      • Baloch N.
      • Hasan O.H.
      • Jessar M.M.
      • Hattori S.
      • Yamada S.
      "Sports Ultrasound", advantages, indications and limitations in upper and lower limbs musculoskeletal disorders. Review article.
      ,
      • Lee M.
      • Chow K.
      Ultrasound of the knee.
      ,
      • Serafin-Krol M.
      • Maliborski A.
      Diagnostic errors in musculoskeletal ultrasound imaging and how to avoid them.
      which indicates that it is limited to diagnosing DLM relying solely on the descriptively morphologic difference between DLM and normal lateral meniscus (NLM) on the ultrasound image.
      The purpose of this study was to explore the feasibility and validity of ultrasound for diagnosing DLM by comparing quantitatively the morphologic difference between DLM and NLM. Our hypothesis was that measuring the thickness, width, 0.5∗thickness/width, and angle of the anterior, body and posterior of the lateral meniscus would reflect quantitatively the morphologic difference between DLM and NLM, thereby diagnosing DLM.

      Methods

      This study was designed the study to derive and validate the ultrasound diagnostic criterion for DLM. In the development stage, ultrasound data of the lateral meniscus from 180 subjects was used to derive the ultrasound diagnostic criterion for DLM. Ninety consecutive patients (90 knees) diagnosed with incomplete or complete DLM by MRI (MRI diagnostic criterion: the ration of the meniscus to the tibia ≥20% or the percent coverage of the meniscus ≥75%
      • Samoto N.
      • Kozuma M.
      • Tokuhisa T.
      • Kobayashi K.
      Diagnosis of discoid lateral meniscus of the knee on MR imaging.
      ) were enrolled in the DLM group from July 2018 to June 2019. Simultaneously, a control group including 90 consecutive patients (90 knees) with NLM demonstrated on MRI was created by a 1:1 matching method based on sex, age ± 2 years, and body mass index (BMI) ± 2. Participants of both DLM group and control group were examined by 2 experienced musculoskeletal ultrasonologists, and an ultrasound diagnostic criterion of DLM was derived by comparing ultrasonic findings of DLM group and control group in a quantitative method. In the validation stage, 324 additional participants with knee symptoms during July 2019 to December 2019 were included to make a qualitative judgment on the lateral meniscus by another experienced musculoskeletal ultrasonologist according to the ultrasound diagnostic criteria derived from the development stage, and the lateral meniscus of these participants were diagnosed by MRI simultaneously. Data from MRI and ultrasound were used to validate the specificity and sensitivity of the criterion. The flowchart of included participants is shown detail in Figure 1. This study was approved by the institutional ethics board, and written informed consent was obtained from all participants.
      Figure thumbnail gr1
      Fig 1The detail flowchart of the participants’ inclusion in the study. (DLM, discoid lateral meniscus; MRI, magnetic resonance imaging; NLM, normal lateral meniscus.)
      Ultrasonologists were blinded to the purpose of the study, clinical history, other imaging findings, and clinical diagnosis. During ultrasonography, participants were in a lateral decubitus position, keeping the knee straight. A Philips high-frequency (3.0-12.0 MHz) linear array probe perpendicular to the lateral tibiofemoral joint line was used to detect the anterior, body, and posterior of the lateral meniscus (Fig 2).
      Figure thumbnail gr2
      Fig 2The illustration of ultrasound measurement of the lateral meniscus. The examinee takes the right decubitus position, with the lateral surface of the left knee joint facing upward. The body surface locations of the anterior, body, and posterior of lateral meniscus are shown in (A). 1: lateral tibiofemoral joint line; 2: patella; 3: fibular head; 4: extension line of the lateral edge of the patella; 5: extension line of the front edge of the fibular head; 6: extension line of the posterior edge of the fibular head; 7: meniscus anterior (intersection area between 4 and 1); 8: meniscus body (intersection area between 5 and 1); 9: meniscus posterior (intersection area between 6 and 1). During ultrasonic inspection, participants are in a lateral decubitus position, with the knee straight. The meniscus anterior (B), meniscus body (C), and meniscus posterior (D) are detected by the ultrasonic probe perpendicular to the lateral tibiofemoral joint line and the body surface.
      According to the characteristics of ultrasound image in the lateral space of the knee joint, the lateral meniscus in the ultrasound scan section perpendicular to the joint line is approximately an inverted isosceles triangle
      • Baloch N.
      • Hasan O.H.
      • Jessar M.M.
      • Hattori S.
      • Yamada S.
      "Sports Ultrasound", advantages, indications and limitations in upper and lower limbs musculoskeletal disorders. Review article.
      (Fig 3A). As Figure 3B shows, the bottom of the triangle is the meniscus thickness at the synovial edge; the 2 waists of the triangle are chords of the largest upper and lower arc-shaped surface of meniscus that can be displayed by ultrasound; the height of the triangle is the maximum width of the meniscus that can be displayed by ultrasound; the apex angle of the triangle is the angle formed by the upper and lower arc-shaped surface of meniscus at the free edge (referred to as α). Measuring the thickness, width, and angle of the anterior, body, and posterior of the lateral meniscus, we obtained the following parameters: anterior meniscus thickness (AMT), anterior meniscus width (AMW), 0.5∗thickness/width of anterior meniscus (TW-AM), anterior meniscus angle (AMA), meniscus body thickness (MBT), meniscus body width (MBW), 0.5∗thickness/width of meniscus body (TW-MB), meniscus body angle (MBA), posterior meniscus thickness (PMT), posterior meniscus width (PMW), 0.5∗thickness/width of posterior meniscus (TW-PM), and posterior meniscus angle (PMA). The measurements of distance and angle were performed on an HD11-XE Color Doppler System (Philips, Tokyo, Japan) by a mouse-point cursor and an automated computer calculation. The accuracy of distance and angle were 0.01 mm and 0.01°, respectively, which were rounded to 1 decimal. The 2 ultrasonologists independently measured the aforementioned parameters of the subjects. No statistical difference was found as to the data measured by 2 ultrasonologists, so the data were averaged and recorded.
      Figure thumbnail gr3
      Fig 3Schematic diagram measuring the thickness, width, and angle of the lateral meniscus. Ultrasound image of the DLM posterior of a left knee is shown in (A), the lateral meniscus (2) is located between the lateral femoral condyle (1) and the lateral tibial plateau (3), which is approximately an inverted isosceles triangle. The green arrow shows the synovial edge of the meniscus, the white arrow shows the free edge of the meniscus, the red arrow shows the upper arc-shaped surface of the meniscus, and the yellow arrow shows the lower arc-shaped surface of the meniscus. As shown in (B), in inverted isosceles triangle, the bottom of the triangle is the meniscus thickness at the synovial edge (green line); the 2 waists of the triangle are chords of the largest upper and lower arc-shaped surface of meniscus that can be displayed by ultrasound (white line); the height of the triangle is the maximum width of the meniscus that can be displayed by ultrasound (yellow line); the apex angle of the triangle is the angle formed by the upper and lower arc-shaped surface of meniscus at the free edge (α), tan(α/2) = 0.5∗thickness/width. (DLM, discoid lateral meniscus.)

      Statistical Analysis

      All the analyses were performed using the statistical software package for SPSS 23.0 (IBM Corp., Armonk, NY). Continuous variables were described as mean ± standard deviation. Differences of the continuous variables and categorical variables between the 2 groups were analyzed by an independent t test and χ2 test, respectively. The diagnostic value of parameters was analyzed by the receiver operating characteristic (ROC) curve, through which the area under the curve (AUC), cut-off value, sensitivity, specificity, Youden’s index, and accuracy were calculated. AUC was interpreted as 4 grades: poor (<0.5), marginal (0.5-0.7), good (0.7-0.9), and better (>0.9). P < .05 was considered significant.

      Results

      Baseline Characteristics

      From July 2018 to June 2019 and from July 2019 to December 2019, a total of 1939 outpatients or inpatients with knee symptoms were enrolled. According to included and excluded criterion, 180 participants and 324 participants were included in the development and validation stage of the study, respectively (Fig 1). As shown in Table 1, in the development stage, the DLM group consisted of 21 men and 69 women, with a mean age of 38.5 years (3-60 years) and a mean BMI of 22.3 (16.1-31.2). The control group comprised of 21 men and 69 women, with a mean age of 38.6 years (3-60 years) and a mean BMI of 21.7 (16.3-32.2). Sex, age and BMI between the 2 groups did not differ significantly (P > .05). In the validation stage, participants consisted of 155 men and 169 women, with a mean age of 32.0 years (3-76 years) and a mean BMI of 22.2 (13.8-35.1).
      Table 1Baseline Characteristics of the Participants
      CharacteristicsDevelopment StageValidation Stage (n = 324)
      DLM Group (n = 90)Control Group (n = 90)χ2/tP
      Sex, n, male/female21/6921/690.0001.000155/169
      Age, y (range)38.5 ± 12.9 (3-60)38.6 ± 13.3 (3-60)–0.051.95932.0 ± 15.9 (3-76)
      BMI (range)22.3 ± 2.5 (16.1-31.2)21.7 ± 2.9 (16.3-32.2)1.559.12122.2 ± 3.6 (13.8-35.1)
      NOTE. Age and BMI are presented as mean ± SD. P <.05 is considered significant.
      BMI, body mass index; DLM, discoid lateral meniscus; SD, standard deviation.

      Development of Ultrasound Diagnostic Criterion

      As shown in Figure 4, ultrasonic images of DLM and NLM were different in morphology, and the former is slender, but the latter is dumpy. As displayed in Table 2, compared with control group, AMT, TW-AM, AMA, MBT, TW-MB, MBA, PMT, TW-PM, and PMA in DLM group were significantly decreased (P < .05 for all). However, AMW, MBW, and PMW of DLM group were significantly greater than those of control group (P < .001 for all).
      Figure thumbnail gr4
      Fig 4The ultrasound measuring images of the anterior, body, and posterior of the DLM and NLM. The ultrasound measuring images of the anterior, body and posterior of the DLM are shown in (A), (B), and (C), and the corresponding images of NLM were displayed in (D), (E), and (F). The ultrasound images of the lateral meniscus in the DLM and NLM were all approximately inverted triangles, and the former is slender, but the latter is dumpy. The yellow arrow shows the measurement of thickness, width and angle of corresponding parts. (DLM, discoid lateral meniscus; NLM, normal lateral meniscus.)
      Table 2Differences of Each Parameter Between the 2 Groups in the Development Stage
      ParametersDLM Group (n = 90)Control Group (n = 90)P
      AMT, mm4.36 ± 0.814.93 ± 0.79.000
      AMW, mm10.11 ± 2.248.29 ± 1.46.000
      TW-AM0.22 ± 0.040.30 ± 0.04.000
      AMA, °22.93 ± 4.3733.24 ± 4.43.000
      MBT, mm4.77 ± 0.985.29 ± 0.90.000
      MBW, mm11.44 ± 2.859.00 ± 1.69.000
      TW-MB0.21 ± 0.030.30 ± 0.04.000
      MBA, °22.18 ± 3.5033.31 ± 4.18.000
      MBA, mm5.58 ± 1.065.91 ± 1.18.049
      PMW, mm11.46 ± 2.379.39 ± 1.79.000
      TW-PM0.25 ± 0.040.32 ± 0.04.000
      PMA, °26.31 ± 4.1436.19 ± 4.45.000
      NOTE. Data are presented as mean ± SD. P < .05 is considered significant.
      AMA, anterior meniscus angle; AMT, anterior meniscus thickness; AMW, anterior meniscus width; DLM, discoid lateral meniscus; MBA, anterior meniscus angle; MBT, meniscus body thickness; MBW, meniscus body width; MBA, meniscus body angle; PMA, posterior meniscus angle; PMW, posterior meniscus width; SD, standard deviation; TW-AM, 0.5∗thickness/width of anterior meniscus; TW-MB, 0.5∗thickness/width of meniscus body; TW-PM, 0.5∗thickness/width of posterior meniscus.
      The results of ROC curve analysis were shown in Table 3 and Figure 5. Cut-off values of AMT, AMW, TW-AM, AMA, MBT, MBW, TW-MB, MBA, PMT, PMW, TW-PM, and PMA in the diagnosis of DLM were 4.55 mm, 9.85 mm, 0.2473, 28.45°, 4.95 mm, 9.75 mm, 0.2536, 27.85°, 5.15 mm, 10.45 mm, 0.2544, and 29.15°, respectively. Among the aforementioned cut-off values, the diagnostic performance (AUC, sensitivity, specificity, Youden’s index, accuracy) of AMA (0.953, 95.6%, 91.1%, 0.867, 93.4%), MBA (0.980, 95.6%, 95.6%, 0.912, 95.6%), and PMA (0.942, 80.0%, 97.8%, 0.778, 88.9%) were greater than other parameters. Furthermore, combined diagnostic test demonstrated that sensitivity, specificity, Youden’s index, and accuracy of series test (AMA, MBA, and PMA) were 0.878, 76.6%, 98.9%, 0.756, 87.8%, and that the corresponding values of parallel diagnostic test (AMA or MBA or PMA) were 0.993, 97.8%, 88.9%, 0.867, and 93.3% (Table 4).
      Table 3The Diagnostic Value of Each Parameter for DLM in the Development Stage
      ParametersAUCCut-off Value, mm/°Sensitivity, %Specificity, %Youden’s IndexAccuracy, %
      AMT0.6874.5560.071.10.31165.6
      AMW0.7459.8551.191.10.42271.1
      TW-AM0.9250.247386.794.40.81190.6
      AMA0.95328.4595.691.10.86793.4
      MBT0.6664.9563.371.10.34467.2
      MBW0.7679.7567.873.30.41170.6
      TW-MB0.9570.253691.191.10.82291.1
      MBA0.98027.8595.695.60.91295.6
      PMT0.5915.1541.177.80.18959.5
      PMW0.76410.4568.975.60.44572.3
      TW-PM0.8960.254471.196.70.67883.9
      PMA0.94229.1580.097.80.77888.9
      AMA, anterior meniscus angle; AMT, anterior meniscus thickness; AMW, anterior meniscus width; AUC, area under the curve; DLM, discoid lateral meniscus; MBA, anterior meniscus angle; MBT, meniscus body thickness; MBW, meniscus body width; MBA, meniscus body angle; PMA, posterior meniscus angle; PMW, posterior meniscus width; SD, standard deviation; TW-AM, 0.5∗thickness/width of anterior meniscus; TW-MB, 0.5∗thickness/width of meniscus body; TW-PM, 0.5∗thickness/width of posterior meniscus.
      Figure thumbnail gr5
      Fig 5ROC curve area of each parameter. AMT, AMW, TW-AM and AMA were shown in (A). MBT, MBW, TW-MB and MBA were shown in (B). PMT, PMW, TW-PM, and PMA were shown in (C). (AMA, anterior meniscus angle; AMT, anterior meniscus thickness; AMW, anterior meniscus width; MBA, anterior meniscus angle; MBT, meniscus body thickness; MBW, meniscus body width; MBA, meniscus body angle; PMA, posterior meniscus angle; PMW, posterior meniscus width; ROC, receiver operating characteristic; TW-AM, 0.5∗thickness/width of anterior meniscus; TW-MB, 0.5∗thickness/width of meniscus body; TW-PM, 0.5∗thickness/width of posterior meniscus.)
      Table 4The Diagnostic Value of the Combination Tests on the Basis of AMA, MBA, and PMA for DLM in the Development Stage
      Combination TestsSensitivity (%)Specificity (%)Youden’s indexAccuracy (%)
      Series test76.798.90.75687.8
      Parallel test97.888.90.86793.3
      AMA, anterior meniscus angle; DLM, discoid lateral meniscus; MBA, meniscus body angle; PMA, posterior meniscus angle.

      Validation of Ultrasound Diagnostic Criterion

      The results of the development stage indicated that AMA, MBA, and PMA have a superior diagnostic performance in the diagnosis of DLM. Thus, we further validated the diagnostic value of these 3 parameters to DLM on an independent patient population, the result of which is shown in Table 5 and Table 6. Table 5 presents that the exact number of subjects with DLM or NLM detected respectively by ultrasonography and MRI. Table 6 displays that the sensitivity, specificity, and accuracy of AMA, MBA, and PMA in diagnosing DLM were as high as 91.3%, 88.6%, and 89.2%; 94.2%, 93.3%, and 93.5%; and 76.8%, 95.7%, and 91.7%.
      Table 5Details of the Ultrasound and MRI in Discriminating DLM and NLM in the Validation Stage
      UltrasoundMRI
      DLMNLM
      AMA
       DLM6329
       NLM6226
      MBA
       DLM6517
       NLM4238
      PMA
       DLM5311
       NLM16244
      AMA, anterior meniscus angle; DLM, discoid lateral meniscus; MBA, meniscus body angle; MRI, magnetic resonance imaging; NLM, normal lateral meniscus; PMA, posterior meniscus angle.
      Table 6The Diagnostic Value of AMA, MBA, and PMA for DLM in the Validation Stage
      ParametersSensitivitySpecificityAccuracy
      AMA91.388.689.2
      MBA94.293.393.5
      PMA76.895.791.7
      AMA, anterior meniscus angle; DLM, discoid lateral meniscus; MBA, meniscus body angle; PMA, posterior meniscus angle.

      Discussion

      In this study, we found that ultrasound-based measurements of AMA, MBA, and PMA are reliable in the diagnosis of DLM. Reportedly, the ability of ultrasound to assess internal derangement of the knee is similar to MRI,
      • Khan Z.
      • Faruqui Z.
      • Ogyunbiyi O.
      • Rosset G.
      • Iqbal J.
      Ultrasound assessment of internal derangement of the knee.
      but there are few studies regarding the diagnosis of DLM by ultrasound. Arifa Achour et al.
      • Arifa Achour N.
      • Tlili K.
      • Mhiri Souei M.
      • Gamaoun W.
      • Jemni H.
      • Mrad Dali K.
      • et al.
      Discoid menisci in children: ultrasonographic features.
      performed ultrasound on 8 children with suspected DLM (6-11 years), 3 of whom underwent MRI. All children were confirmed by arthroscopy, which found that the qualitative judgment of ultrasound is consistent with MRI or arthroscopy. Wang et al.
      • Wang Y.
      • Geng H.
      • He M.
      • et al.
      Ultrasonic diagnosis of discoid meniscus of knee joint in children.
      conducted ultrasound on the knee joints of 28 children diagnosed as having DLM by MRI and found that ultrasound diagnosis is consistent with MRI. In these studies, the characteristics of ultrasound image—absence of a normal triangular shape, presence of abnormally elongated and thick meniscus, and a heterogeneous central pattern—are the ultrasound diagnostic evidence of DLM.
      • Arifa Achour N.
      • Tlili K.
      • Mhiri Souei M.
      • Gamaoun W.
      • Jemni H.
      • Mrad Dali K.
      • et al.
      Discoid menisci in children: ultrasonographic features.
      ,
      • Wang Y.
      • Geng H.
      • He M.
      • et al.
      Ultrasonic diagnosis of discoid meniscus of knee joint in children.
      However, the present study found that the ultrasound images of the lateral meniscus in the DLM group and the control group were all approximately inverted triangles, which maybe arise from the limitedly ultrasonic scanning on the depth of the knee joint space and incompletely display of DLM.
      • Baloch N.
      • Hasan O.H.
      • Jessar M.M.
      • Hattori S.
      • Yamada S.
      "Sports Ultrasound", advantages, indications and limitations in upper and lower limbs musculoskeletal disorders. Review article.
      ,
      • Serafin-Krol M.
      • Maliborski A.
      Diagnostic errors in musculoskeletal ultrasound imaging and how to avoid them.
      Owing to the resistance of the lateral femoral condyle on the ultrasound beam, the ultrasonic scanning range in the lateral knee joint space is gradually reduced to a funnel shape. Furthermore, due to the abnormal broad in the shape of DLM, the free edge of DLM observed by ultrasound is not its real condition but the largest edge of DLM that can be displayed by ultrasound, which is mostly wedge-shaped, and rarely abnormally thicken or plate-like. Therefore, ultrasonic images of both DLM and NLM are approximately inverted triangles, and the former is slender, whereas the latter is dumpy (Fig 4). In addition, the current study found that heterogeneous central pattern existed in the ultrasound images of some patients of both the DLM group and the control group on the ultrasound images and that MRI of these patients showed tear and degeneration of the lateral meniscus, which is reasonable to speculate that heterogeneous center patterns may be correlated with the combined meniscus tear or degeneration,
      • Lee M.
      • Chow K.
      Ultrasound of the knee.
      not with the ultrasound morphologic characteristics of DLM. Therefore, just depending on the descriptively morphologic features by ultrasound to diagnose DLM is unreliable. It is essential to further explore a new ultrasound diagnosis method of DLM based on quantitative parameters.
      Thus far, few studies on quantitatively ultrasonic diagnosis of DLM have been reported. Our study found that AMT, TW-AM, AMA, MBT, TW-MB, MBA, PMT, TW-PM, and PMA in DLM group were significantly lower than those in control group (P < .05 for all), and that AMW, MBW, and PMW of DLM group were significantly greater than those of control group (P < .001 for all), indicating that it is feasible to distinguish between DLM and NLM by measuring the aforementioned ultrasound quantitative parameters. Discordantly, the MBW measured in Wang et al.
      • Wang Y.
      • Geng H.
      • He M.
      • et al.
      Ultrasonic diagnosis of discoid meniscus of knee joint in children.
      is significantly larger than that in this study (18.58 ± 4.44 vs 11.44 ± 2.85), which may because that they used the waist of the inverted triangle as the MBW, but our study acted the height of the inverted triangle as the MBW (Fig 3). The MBT in Wang et al.
      • Wang Y.
      • Geng H.
      • He M.
      • et al.
      Ultrasonic diagnosis of discoid meniscus of knee joint in children.
      study is slightly lower than that in our study (4.53 ± 1.41 vs 4.77 ± 0.98), which may be correlated with data in Wang et al. that are all from children with DLM. In addition, Wang et al.
      • Wang Y.
      • Geng H.
      • He M.
      • et al.
      Ultrasonic diagnosis of discoid meniscus of knee joint in children.
      found that the DLM thickness measured by ultrasound was greater than that by MRI (4.53 ± 1.41 vs 4.00 ± 1.44, P < .05), and the DLM width measured by ultrasound is smaller than that by MRI (18.58 ± 4.44 vs 25.82 ± 4.16, P < .05), and the consistency of the thickness (kappa = 0.158) or width (kappa = 0.111) between MRI and ultrasound is poor. Due to the different imaging principles of ultrasound and MRI, as well as the abnormal width and the ataxinomic thickened free edge of DLM, the morphology of DLM shown by the 2 methods is quite different. As shown in Figure 4, A-C and Figure 6, MRI can measure AMT, MBT, PMT, and MBW but cannot measure AMA, MBA, PMA, AMW, and PMW. As described previously, the width and thickness of DLM are of low diagnostic value for DLM, and it is unnecessary to compare the aforementioned ultrasound parameters between MRI and ultrasound. Therefore, in this study, we didn’t use MRI to measure these parameters; it just served as the “gold standard” for identifying DLM.
      Figure thumbnail gr6
      Fig 6Representative MRI of 1 patient with DLM. (A) MRI coronal scan image of the DLM body. The free edge of DLM (white arrow) is thickened and plate-like, the thickness of which is approximate to that of DLM synovial edge. Therefore, unlike ultrasound, MRI isn’t able to measure the meniscus body angle. (B-D) Three consecutive MRI sagittal scan images near the center of the knee joint. The anterior and posterior of DLM are connected (white arrow), and the thickness in connected position is close to that in DLM synovial edge. Thus, unlike ultrasound, MRI isn’t able to measure the width and angle of anterior meniscus and posterior meniscus. (DLM, discoid lateral meniscus; MRI, magnetic resonance imaging.)
      Through ROC curve analysis, the current study found that diagnostic performance of AMA, MBA, and PMA was significantly greater than other parameters. Given that the difference between DLM and NLM in thickness is small and in width is large, the meniscus width is more effective to discern DLM and NLM. However, the measurement of meniscus width is affected by the obstruction of femoral condyle on the ultrasound beam, thus the DLM width measured by ultrasound is significantly smaller than its true width and is closer to the NLM width, thereby reducing the diagnostic efficiency of the DLM width. The ratio parameter not only can reduce the standard deviation of the direct measurement parameter but can also decline the effect of individual differences such as sex, age, height, and weight on the result,
      • Jiang W.
      • Li X.
      • Su H.
      • Yang C.
      A new method to diagnose discoid lateral menisci on radiographs.
      thus the diagnostic efficiency of 0.5∗TW is greater than thickness and width. The measurement of meniscus angle is less impacted by the femoral condyle, and it also can cut down the effect of individual differences on the results. Therefore, the diagnostic efficiency of meniscus angle is greater than thickness and width. There is a calculable relation between meniscus angle and 0.5∗TW (tan [α/2] = 0.5∗TW) (Fig 3B). We found that the effectiveness of meniscus angle is slightly greater than 0.5∗TW in diagnosing DLM, which may be because that the measurement error of 0.5∗TW is relatively large compared with meniscus angle, as 0.5∗TW is calculated by 2 direct measurement parameters. Consequently, meniscus angle is the best parameter to distinguish between DLM and NLM. Furthermore, we performed the combined diagnostic test on DLM on the basis of AMA, MBA, and PMA. Compared with the independent diagnostic test of these 3 criteria, the parallel diagnostic test’s sensitivity is increased to 97.8% and specificity is reduced to 88.9%, which is suitable for screening of DLM; the series diagnostic test’s specificity is increased to 98.9% and sensitivity is reduced to 76.7%, which can be used to confirm the diagnosis of DLM. Therefore, the combined diagnostic test is a double-edged sword, which should be determined properly based on the specific needs in clinical application.
      In the validation stage, we further verified the aforementioned ultrasound diagnostic criterion by increasing the sample size, expanding the scope of the tested population, and changing ultrasonologist. Satisfactorily, the diagnostic value of AMA, MBA, and PMA for DLM were very close to the result in the development stage. Hence, the diagnostic effectiveness of ultrasound for DLM is stable and reliable, with clinical value. On the one hand, it provides an ideal and rapid diagnostic method for a population in whom is difficult to obtain an MRI, such as children and patients with MRI contraindications. On the other hand, it provides reliable and convenient examination methods for the screening, etiological research, and epidemiologic study of DLM. In addition, it provides a basis for further research on ultrasound in the diagnosis of DLM classification and DLM tear.

      Limitations

      We acknowledge that there are some limitations to our study. First, MRI was acted as the “gold standard” for DLM,
      • Sohn D.-W.
      • Bin S.-I.
      • Kim J.-M.
      • Lee B.-S.
      • Kim S.-J.
      Discoid lateral meniscus can be overlooked by magnetic resonance imaging in patients with meniscal tears.
      without further confirmation through arthroscopy. Second, we did not differentiate between complete and incomplete types of DLM. Third, tear or degeneration of the lateral meniscus existed in some subjects, which may be a confounding factor to our study. Fourth, this study was conducted in only one medical institution, and multicenter research is needed to verify. Finally, the accuracy of ultrasound diagnosis of DLM is greatly dependent on the experience of the ultrasonologist, and a long learning curve is required to achieve satisfactory diagnostic accuracy.
      • Baloch N.
      • Hasan O.H.
      • Jessar M.M.
      • Hattori S.
      • Yamada S.
      "Sports Ultrasound", advantages, indications and limitations in upper and lower limbs musculoskeletal disorders. Review article.

      Conclusions

      DLM can be reliably diagnosed by ultrasound measurements of AMA, BMA, and PMA.

      Acknowledgments

      The authors gratefully thank all patients who participate in the study and ultrasound physicians (Yan Luo and Li Qiu). This project would not have been successful without their support and assistance.

      Supplementary Data

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