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- MATERIALS AND METHODS
This pilot study presents a technique for three-dimensional and quantitative analysis of meniscus shape, position, and signal intensity and compares results in knees with (n = 20) and without (n = 11) radiographic osteoarthritis. 3-T MR images with 2mm section thickness were acquired using a proton density–weighted, fat-suppressed, coronal, fast spin-echo sequence. Segmentation of the tibial, femoral, and external surface of the medial meniscus and the tibial joint surface was performed. Three-dimensional parameters were computed to describe the shape, position, and signal intensity of the entire meniscus and three subregions (body, anterior, and posterior horn). Key results included a greater size (i.e., volume, surface areas, and thickness), increased medial extrusion (i.e., greater extrusion distance, greater meniscal area uncovered by tibial surface), and elevated signal intensity of the medial meniscus in osteoarthritis than in nonosteoarthritis knees, particularly in the meniscus body. These results need to be confirmed in larger cohorts, preferably under weight-bearing conditions. Magn Reson Med 63:1162–1171, 2010. © 2010 Wiley-Liss, Inc.
The meniscus plays an important role in normal knee function by providing a higher degree of joint conformity (congruity) and by distributing loads over a wider area (1). Meniscal pathology (i.e., extrusion and tears) is frequent in the general population, even among asymptomatic individuals, and becomes more common with increasing age (2). Meniscal pathology has also been associated with knee pain (3–6) and osteoarthritis (OA) progression, specifically with increased rates of cartilage loss (7–12). These findings have generally relied on semiquantitative scoring of the meniscus (4, 13), and only few studies have utilized quantitative measures of meniscus position (subluxation) or shape (14–16).
Gale et al. (14) used coronal fat-saturated proton density and T2-weighted images to investigate the relationship between meniscal extrusion (subluxation) and joint space narrowing in 233 participants with symptomatic knee OA and in 58 asymptomatic controls. They determined the degree of extrusion to the nearest millimeter in the image where the greatest distance between the most peripheral aspect of the meniscus and the border of the tibia (excluding osteophytes) was observed. The OA participants displayed more extrusion of the medial meniscus (MM) than controls (5.1 versus 2.8mm; P = 0.001). Modest degrees of meniscal extrusion were common in controls, but severe degrees (>7mm) were unique to OA cases. Hunter et al. (15) explored the role of meniscal tears (semiquantitative scoring (13)), extrusion, and height as risk factors for cartilage loss in 257 subjects. Extrusion and height were measured quantitatively in the coronal MR image showing the maximal medial tibial spine volume and in two sagittal images, one through the medial and one through the lateral tibia. Meniscal coverage and height were smaller in knees with meniscal extrusion, and coverage, height, and extrusion were all associated with a higher risk of cartilage loss (13).
However, the above two-dimensional studies (14, 15) did not perform a full three-dimensional (3D) analysis of the meniscus. Two-dimensional measurements rely on the particular slice selected and its orientation, which may be difficult to reproduce in longitudinal trials. Also, two-dimensional measurements do not permit full analysis of defined meniscal subregions, such as the anterior and posterior horn and the body. With regard to articular cartilage, recent research reported 3D, quantitative outcomes of cartilage morphology to be more effective in describing longitudinal progression and the relationship with other OA risk factors than semiquantitative cartilage readings (12). Given the important role of the meniscus for symptoms and structural progression in OA, the objective was to develop a technique for fully quantitative 3D analysis of meniscus shape, position, and signal intensity (SI), including the body, the anterior horn, and the posterior horn. In this pilot analysis, we also explored whether such quantitative measures differ significantly between knees with radiographic OA and healthy knees.
- Top of page
- MATERIALS AND METHODS
Here we propose a technique for fully quantitative 3D analysis of meniscus shape, position, and SI, including the body, as well as the anterior and posterior horns. Few studies dealt quantitatively with the meniscus. In a cadaver study, Bowers et al. (26) reported MRI-based meniscus segmentation to provide volume estimates within 5% of the true values (determined by water displacement), with a test-retest error of 4% (26). Our work extends previous quantitative investigations on meniscus shape and position in OA (14, 15) and is the first to apply quantitative measurement technology to full 3D analysis. A high spatial resolution was required and contiguous 2mm slices were used, whereas previous work in OA was based on 3mm slices with 1mm gaps (14, 15). To compensate for the lower signal- and contrast-to-noise ratios in the higher-resolution images, two excitations were averaged, and the acquisition time of 4:44 min was well tolerated by all participants. The test-retest (resegmentation) precision was satisfactory for most parameters: for the tibial surface and the meniscus thickness, it was similar to that for resegmentation of the joint surface and cartilage thickness using cartilage-specific high-resolution sequences reported previously (27). In contrast, the meniscal width and the minimal and maximal medial extrusion distance displayed errors larger than the intersubject variability. This is plausible as these latter measures were based on few measurement points only, which are sensitive to small inconsistencies in segmentation. A limitation of the study is that test-retest precision was not determined for repositioning of the knee or between different observers.
Other limitations of the study include the small sample size and the fact that only women were studied, the use of coronal slices only, and that measurements of meniscal shape and position were made without joint loading, while the meniscus performs its mechanical function primarily under load bearing. Nevertheless, meniscal surfaces, volume, and thickness were significantly greater in OA than in non-OA knees, although body height and joint surface areas of MT were similar between the groups and although participants with meniscus surgery were excluded from the study. A potential explanation is that the MM may undergo swelling with degeneration and tears, which are known to be more frequent in OA than in healthy knees. Interestingly, meniscal hypertrophy was recently also reported by Jung et al. (16) in late-stage OA, with two-dimensional measurements of meniscal height being similar to our 3D measurements of maximal meniscus thickness.
The medial extrusion (MM.mEx.Me and MM.mEx.Max) was elevated in OA versus non-OA knees, our 3D results being quantitatively similar to previous two-dimensional measurements (14). The fact that the distance of the internal meniscus margin to the external border of MT.ACdAB (MM.OvD), the area of MT.ACdAB covered by the MM, and the area of MM covered by MT.ACdAB did not differ between OA and non-OA knees indicates that the relative position of the internal margin of MM to the tibial surface was similar in OA and non-OA knees. These observations are explained by the larger size of the tibial surface (TA), which results in the EA being farther away from the external border of ACdAB. Hence, the medial extrusion (MM.mEx.Me) and uncovered areas of the tibial meniscal surface (MM.TA.Uncov) were significantly greater in OA than in non-OA knees, whereas MM.TA.Cov (and MT.ACdAB.Cov and MT.ACdAB.Uncov) were unchanged. Differences in the medial extrusion (MM.mEx.Me) between OA and non-OA knees were most obvious in the meniscus body.
The higher signal of MM in OA knees was expected since meniscus damage is characterized by increased signal, mostly in the posterior horn (13). A limitation of our approach is that the signal was not normalized to an external calibration standard or to another tissue in the MR images. Further work is necessary to test whether quantitative analysis of SI change is potentially superior to semiquantitative scoring (13, 28).
The observations from this study need to be confirmed in larger cohorts, including men, since the current pilot study only involved a small number of female participants. Larger cohorts will also be required to establish normal values and the normal variation in young healthy participants. Semiquantitative readings (T.M.L.) of the MR images of the KLG0 participants revealed that none of the KLG0 knees had cartilage changes in the medial femorotibial compartment (where the meniscus was measured), but one participant had a cartilage defect of <50% (thickness) in the lateral femoral condyle, one had a >50% defect in the lateral tibia, and seven had cartilage defects in the femoropatellar joint (four full thickness, three <50%). It is unclear how these defects (in other compartments) affect the status of the MM. Another limitation is that the imaging protocol used (coronal images) did not allow us to measure anterior extrusion of the meniscus, as described previously (15) in sagittal slices. As the slice thickness of the spin-echo sequence used here was 2mm, the spatial resolution for providing measurements of anterior subluxation was insufficient to provide this measurement. A viable solution is to analyze radially orientated images rotated around the center of the tibial plateau, to use isotropic imaging, or to perform additional analyses on sagittal images. It has been shown that semiquantitative MR readings of meniscal pathology are associated with knee pain (3–6) and OA progression (i.e., cartilage loss) (7–12). Future applications of the technique presented here will have to test whether the quantitative endpoints proposed display stronger relationships with clinical (pain and function) and structural outcomes (cartilage loss) than previous semiquantitative MRI readings, and these analyses may also include the lateral meniscus. In this context, longitudinal studies are needed as the current cross-sectional analysis cannot reveal whether quantitative differences of MM are causes or consequences of OA.
The measurement technology proposed is not confined to OA. For example, Kessler et al. (29) reported a 10% decrease in meniscus volume after a 20-km distance running, and recovery within 1 h (30), and our 3D technology can be used to measure the impact of load bearing and various physical activities on 3D parameters of meniscus shape and position, including subregions. Similarly, shape and position of the meniscus could be determined in variable functional load-bearing positions of the knee (24), particularly when using open magnets (23).
In conclusion, this study presents a technique for fully quantitative 3D analysis of meniscus shape, position, and SI, including the body segment, as well as the anterior and posterior horns. The presented pilot data indicated a greater size, increased medial extrusion, and elevated SI of the MM in OA compared with non-OA knees, particularly in the body. The quantitative measurement technology proposed may be used to further explore the relationship of meniscus shape, position, and signal with symptoms and progression of OA and to elucidate the function of the meniscus during load bearing and motion.