Scans were obtained at multiple sites on 1-T (50 patients) and 1.5-T (42 patients) systems. The standard imaging protocol (73 patients) consisted of double-echo sagittal non???fat-saturated spin-echo (SE) and fast spin-echo (FSE) proton density???weighted and T2-weighted sequences (TR range, 2100???4960 ms; TE range for echo 1, 14???20 ms; TE range for echo 2, 73???93 ms; slice thickness, 3???5 mm; echo-train length, 1???5; bandwidth, 100???150 Hz/pixel; matrix, 256 ?? 208???512 ?? 512; FOV, 12???17 cm) and a double-echo coronal fat-saturated FSE proton density???weighted sequence (TR range, 2000???5960 ms; TE range for echo 1, 13???16 ms; TE range for echo 2, 84???98 ms; slice thickness, 3???5 mm; echo-train length, 3???7; bandwidth, 115???150 Hz/pixel; matrix, 256 ?? 256???512 ?? 512; FOV, 13???17 cm); these scans were reviewed for this study. Additional axial fat-saturated FSE T2-weighted, coronal non???fat-saturated SE T1-weighted, and 3D gradient-echo sequences were also performed as part of the routine protocol, but the images were not reviewed for this study.

Two fellowship-trained musculoskeletal radiologists with 4 and 6 years of experience reviewed the MRI examinations in consensus. Both radiologists were blinded to the operative results. The medial and lateral menisci were individually assessed for the presence of a tear and were graded on a scale ranging from 1 to 5 (1 = definitely not a tear, 2 = probably not a tear, 3 = indeterminate or equivocal, 4 = probably a tear, 5 = definitely a tear). Grade 5 was defined as a clear alteration of the morphology of the meniscus, increased proton density???weighted signal clearly touching a meniscal surface on at least two slices, and no artifacts limiting evaluation. Grade 4 was defined as increased signal on proton density???weighted images clearly extending to an articular surface on only one image, minimal alteration of the meniscal morphology, and grade 5 lesions with artifacts limiting evaluation. Grade 3 was defined as increased signal approaching the articular surface but not clearly extending to the surface on proton density???weighted images; questionable alteration of the meniscal morphology; or any lesion not classifiable as grade 1, 2, 4, or 5. Grade 2 was defined as increased signal not extending to the articular surface on proton density???weighted images, normal morphology, or grade 1 lesions with artifacts limiting evaluation. Grade 1 was defined as no increased signal in the meniscus, normal morphology, and no artifacts limiting evaluation.

For menisci that were identified as grades 2, 3, or 4, specific MRI findings were also noted, including information about intrameniscal signal, surface morphology, and extrameniscal abnormalities. The intrameniscal signal was classified as amorphous or linear, and the presence or absence of increased signal that was less than fluid brightness on T2-weighted images, fluid-bright signal on T2-weighted images, signal approaching the surface but not definitely touching the surface, signal touching the surface on one slice, signal touching the surface on more than one slice, signal discretely touching the surface, and signal broadly touching the surface were noted. The surface morphology was assessed for the presence or absence of surface and free-edge irregularities. Assessment of extrameniscal findings included the presence or absence of a parameniscal cyst and marrow edema in the ipsilateral compartment. Additionally, the cartilage in the ipsilateral compartment was graded on a scale ranging from 1 to 4 (1 = normal, 2 = surface irregularity, 3 = partial-thickness loss, 4 = full-thickness loss).

Surgery was performed by one of 13 orthopedic surgeons. Each patient???s operative report and arthroscopic images were reviewed by a staff orthopedic surgeon with 5 years of experience and a postgraduate year 4 orthopedic surgery resident; both were blinded to the preoperative MR images and report. The presence or absence of a tear, fraying, and degeneration was noted for both menisci. Degeneration was defined as an irregular appearance of the meniscus such as a roughened surface or yellowish color without the presence of a tear. Fraying was limited to free-edge or inner margin fraying of the meniscus without the presence of a tear. For the purposes of data analysis, each meniscus was classified by the most severe abnormality that was present as follows in decreasing order of severity: tear, fraying, degeneration, or normal.

After this prospective review of MRI scans and operative findings, we performed a retrospective review, in consensus, of all the cases with discordant results between MRI and operative findings and of all cases with indeterminate or equivocal results on MRI.

The sensitivity and specificity of MRI were calculated as the proportion of patients with a positive test result among patients with a tear and the proportion of patients with a negative test result among patients without a tear, respectively. A positive test result was defined in three ways: First, only MRI grade 5 was considered positive; second, MRI grades 4 and 5 were considered positive; and, third, MRI grades 3, 4, and 5 were considered positive. The operative findings were considered the gold standard, with a tear considered a positive result and fraying, degeneration, and normal meniscus considered negative results.

Asymptotic methods were used to construct 95% CIs for sensitivity and specificity. For estimates of 1, a 95% CI was constructed using the rule of threes [12]. Predictive values were calculated for each of the five possible grades. The Youden index was computed for the sensitivity and specificity of each MRI finding [13] and findings with the highest Youden index were included in a multiple-variable logistic regression model.