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1
Department of Diagnostic Radiology, Yale University School of Medicine, 333
Cedar St., Rm. MRC 147, New Haven, CT 06520.
2
Present address: Department of Radiology, Leids Universitair Medisch Centrum,
Albinusdreef 2, 2333 ZA Leiden, The Netherlands.
3
Present address: Department of Diagnostic Radiology, The Catholic University
of Korea, Kangnam St. Mary's Hospital, 505 Banpo-Dong, Seocho-Ku, Seoul
037-040, Korea.
Received June 7, 2000;
accepted after revision July 27, 2000.
Address correspondence to T.R. McCauley.
Abstract
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MATERIALS AND METHODS. Two hundred consecutive patients referred for MR imaging of the knee were evaluated for central osteophytes, articular cartilage defects, marginal osteophytes, meniscal tears, and anterior cruciate ligament tears. A 1.5-T scanner was used, and assessments were made by consensus of two experienced musculoskeletal radiologists. Seven patients were excluded, leaving 193 patients in the study population.
RESULTS. The prevalence of central osteophytes in the knee was 15% (35 central osteophytes in 29 patients). Patients with central osteophytes were older (mean age, 52 years versus 38 years), weighed more (mean weight, 204 lb [92 kg] versus 174 lb [78 kg]), had more articular cartilage defects (mean, 4.3 versus 1.3), and had more marginal osteophytes (mean, 3.9 versus 1.1) than patients without central osteophytes (p < 0.0001, Student's t test). Patients with central osteophytes were more likely to have a meniscal tear (p = 0.004, chi-square test), but they were not more likely to have an anterior cruciate ligament tear. All central osteophytes were associated with articular cartilage defects at the same location, which were full or near-full thickness on MR imaging for 32 of 35 central osteophytes.
CONCLUSION. Central osteophytes are common in patients referred for MR imaging of the knee. When central osteophytes are seen in the knee there is a high likelihood of an associated full thickness or near-full thickness articular cartilage defect.
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The purpose of our study was to determine the prevalence and location of central osteophytes in patients referred for MR imaging of the knee and to determine the relationship of central osteophytes to articular cartilage defects, marginal osteophytes, meniscal tears, and anterior cruciate ligament tears.
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All patients were imaged with a 1.5-T superconducting magnet (Signa; General Electric Medical Systems, Milwaukee, WI) using our routine clinical protocol that includes the following sequences: axial fat-suppressed fast spin-echo images (TR/effective TE, 4000/20; 4-mm thick; 0.5-mm intersection gap; 16-cm field of view; 256 x 256 matrix; echo train length, 10; 1 excitation); sagittal T1-weighted spin-echo images (TR range/TE, 450-600/14; 3.3-mm thick; no gap; 16-cm field of view; 256 x 192 matrix; 1 excitation); coronal fast spin-echo images (TR/effective TEs, 3000/20 and 80; 3.3-mm thick; 0.3-mm intersection gap; 14-cm field of view; 256 x 256 matrix; echo train length, 10; 1 excitation); sagittal fast spin-echo T2-weighted images through the anterior cruciate ligament (TR/effective TE, 2000/84; 3-mm thick; 0.3-mm intersection gap; 14-cm field of view; 256 x 160 matrix; echo train length, 10; 2 excitations); and fat-suppressed three-dimensional spoiled gradient-echo images in the sagittal plane that were also reformatted in the axial plane (TR/TE, 40/6; 1.5-mm thick; 14-cm field of view; 256 x 160 matrix; 0.75 excitations; flip angle, 40°; 60 images).
Two experienced musculoskeletal radiologists reviewed all images by consensus for the presence of focal articular cartilage defects, central osteophytes, marginal osteophytes, meniscal tears, and anterior cruciate ligament tears. Central osteophytes and articular cartilage defects were assigned to one of 10 locations; the medial or lateral femoral condyle; the medial or lateral tibial plateau; the medial facet, lateral facet, or median ridge of the patella; or the medial part, lateral part, or midline of the trochlear groove. A central osteophyte was recorded as associated with an articular cartilage defect when it was subjacent to the defect or at the margin of the defect.
Articular cartilage defects were graded using a modification of the following method proposed by Shahriaree [5]: grade 0, absent (no abnormality in signal intensity or morphology); grade 1, signal abnormality (signal intensity was abnormal but the cartilaginous surface appeared intact); grade 2, less than 50% reduction of cartilage thickness; grade 3, 50% or greater reduction of cartilage thickness; grade 4, full thickness or near-full thickness cartilage defect (cartilage defects with either no high signal intensity over the cortex or a very thin rim of high signal intensity); and grade 5, same findings as grade 4 with associated underlying subchondral marrow signal abnormality. The depth of cartilage loss was measured by estimating the actual cartilage contour in relation to the expected normal cartilage contour.
Osteophytes were identified when focal excrescences extended from the cortical surface either with signal that was the same as the cortex or covered with signal that was the same as cortex and containing marrow signal. Osteophytes were identified as marginal when at the margin of the joint and as central when surrounded by articular cartilage on all sides. The size of each osteophyte was compared with that of adjacent articular cartilage with a normal MR imaging appearance. Central osteophytes were assessed using the following scale: grade 0, none; grade 1, small (<50% of cartilage thickness); grade 2, moderate (50-100% of cartilage thickness); and grade 3, large (>100% of cartilage thickness). The configuration of each central osteophyte was categorized as to whether it completely filled the base of the cartilage defect or incompletely filled the base of the cartilage defect. The locations of central osteophytes were categorized as weight-bearing versus non-weight-bearing surfaces. Weight-bearing surfaces were defined as the tibial plateau and the articular surfaces of the femoral condyles between the posterior margin of the posterior horn of the meniscus and the anterior margin of the anterior horn of the meniscus. The presence or absence of a rim of high signal covering the surface of the osteophyte on the fat-suppressed three-dimensional spoiled gradient-echo images was determined for each central osteophyte.
The presence or absence of marginal osteophytes was determined at each of seven sites in the knee: the medial femoral condyle, the lateral femoral condyle, the medial tibial plateau, the lateral tibial plateau, the patella, the medial margin of the trochlear groove, or the lateral margin of the trochlear groove. The maximum size of a marginal osteophyte at each of these sites was assessed using the following scale: grade 0, none; grade 1, minimal (<1 mm); grade 2, small (1-3 mm); grade 3, moderate (>3-5 mm); and grade 4, large (>5 mm). Size was measured from the base to the tip of the osteophyte.
For statistical analysis, the ages, weights, number of hyaline cartilage defects, and number of sites with marginal osteophytes for patients with and without central osteophytes were compared using the unpaired two-tailed Student's t test. The difference in the prevalence of central osteophytes in male versus female patients and the association of central osteophytes with meniscal tears and anterior cruciate ligament tears were compared using the chi-square test. Spearman's rank correlation coefficient was used to determine whether there was a correlation between the grade of central osteophytes and the maximum grade of marginal osteophytes. To quantitate the severity of changes of osteoarthritis, the mean number of articular cartilage defects, the number of sites with marginal osteophytes, and the highest grade of marginal osteophyte in each patient were determined. These values for patients with central osteophytes were compared with patients with marginal osteophytes alone (i.e., patients with marginal osteophytes without central osteophytes). The unpaired two-tailed Student's t test was used for number of articular cartilage defects and number of marginal osteophytes, and the Mann-Whitney test was used for grade of marginal osteophytes. For all comparisons, a p value of 0.5 or less was considered significant.
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Eighteen (51%) of the 35 central osteophytes were associated with grade 4 articular cartilage defects, and 14 (40%) of the 35 were associated with grade 5 defects (Fig. 4A,4B). Only two central osteophytes were associated with grade 3 articular cartilage defects, and one central osteophyte was associated with a grade 2 defect. Thus, 91% of the central osteophytes occurred in association with articular cartilage defects graded as full thickness or near-full thickness, and no central osteophyte was associated with a normal appearance of the adjacent or overlying articular cartilage. Patients with central osteophytes had more articular cartilage defects at other locations in the knee (mean, 4.3) than patients with marginal osteophytes alone (mean, 2.7) (p = 0.007, Student's t test) and those without central osteophytes (mean, 1.3) (p < 0.0001, Student's t test) (Fig. 2A,2B). High-signal-intesity tissue covered the surface of 91% (32/35) of the central osteophytes on fat-suppressed three-dimensional spoiled gradient-echo images (Figs. 1A,1B,1C,1D and 2A,2B); only three were not covered by a rim of high signal (Fig. 3).
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Only three patients (10%) with central osteophytes did not have marginal osteophytes. More marginal osteophytes were present in patients with central osteophytes (mean, 3.9 sites) than in those with marginal osteophytes alone (mean, 2.6 sites) (p = 0.011, Student's t test) or in those without central osteophytes (mean, 1.1 sites) (p < 0.0001, Student's t test; Fig. 2A,2B). Patients with central osteophytes had larger marginal osteophytes (mean grade, 2.3) than patients with marginal osteophytes alone (mean grade, 1.7) (p = 0.001, Mann-Whitney test). Increased size of the central osteophytes was associated with increased size of the largest marginal osteophyte in the knee (Spearman's rank correlation coefficient = 0.46 with p = 0.022). Patients with central osteophytes were more likely to have a meniscal tear than were patients without central osteophytes (p = 0.004, chi-square test), but they were not more likely to have a meniscal tear than patients with marginal osteophytes alone (p = 0.15, chi-square test). A meniscal tear occurred in 69% (20/29) of the patients with central osteophytes, 40% (66/164) of the patients without central osteophytes, and 53% (36/68) of the patients with marginal osteophytes alone. There was no significant relationship between central osteophytes and anterior cruciate ligament tears; anterior cruciate ligament tears were present in 14% (4/29) of the patients with central osteophytes and in 15% (24/164) of the patients without central osteophytes (p > 0.05).
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All the central osteophytes occurred in association with a cartilage defect, most (91%) of which were graded as full or near-full thickness (grade 4 or 5). A prior study found 73% of central osteophytes were accompanied by an overlying articular cartilage abnormality on MR imaging [2]. The prevalence of articular cartilage abnormality in our study is higher most likely because of the use of an MR imaging sequence designed specifically for articular cartilage evaluation. Our results indicate detection of central osteophytes with any imaging modality makes it probable that an adjacent articular cartilage defect, which usually is full thickness or near-full thickness, is present. When central osteophytes are seen in the knee on radiographs, CT scans, or MR images obtained without cartilage-specific sequences, an adjacent articular cartilage defect should be assumed to be present. It has been suggested that MR imaging is more sensitive for detection of central osteophytes than radiographs because the curved articular surfaces often lead to obscuration of central osteophytes on radiographs [2, 3].
We found a rim of high-signal-intensity tissue covers the surface of almost all central osteophytes on fat-suppressed three-dimensional spoiled gradient-echo images. We do not have histologic proof of the nature of this tissue. We believe it most likely is a thin rim of cartilage on the basis of previous histologic descriptions that indicate overlying cartilage is required for development of central osteophytes and is seen overlying most central osteophytes [1, 2, 6]. The covering tissue along with osteophytes either being present at the margin of cartilage defects or filling the base of cartilage defects would explain the fact that our referring orthopedic surgeons rarely report seeing central osteophytes at arthroscopy. The inability to see central osteophytes at arthroscopy is consistent with the pathologic description that central osteophytes are usually not visible on inspection of the articular surface but are visible with longitudinal sectioning of the articular surface [6].
Of the 35 central osteophytes, 13 (37%) diffusely filled the base of the cartilage defect. Central osteophytes with a diffuse configuration can lead to an underestimation of the depth of cartilage loss at arthroscopy. Identification of these osteophytes is important for cartilage therapies, such as chondrocyte transplantations, because the central osteophyte decreases the volume available for placement of cells and the volume available for regrowth of cartilage.
The presence of central osteophytes is associated with more severe changes of osteoarthritis than marginal osteophytes alone, as indicated by more numerous articular cartilage defects and larger and more numerous marginal osteophytes. Marginal osteophytes have previously been shown to be associated with knee pain [7]. Future studies are needed to determine whether central osteophytes are associated with more severe symptoms or a worse prognosis than are marginal osteophytes alone.
Increased weight and age are known predisposing factors for osteoarthritis [8] and likely account for the association seen between central osteophytes and increased weight and age. Despite the association of central osteophyte formation with increased age, the youngest patient with a central osteophyte was only 19 years old, which indicates that central osteophytes can occur in young patients. Meniscal tears have been shown to predispose patients to develop osteoarthritis, likely accounting for the association between central osteophytes and meniscal tears [9]. The lack of association of central osteophytes with anterior cruciate ligament tears is surprising given the known association between anterior cruciate ligament tears and osteoarthritis [9]. It is possible that the mechanism of development of osteoarthritis caused by an anterior cruciate ligament tear differs from that caused by a meniscal tear, thus resulting in the lack of association with central osteophyte formation. Alternatively, the lack of association possibly occurred because of factors not controlled in this study. We did not determine the time interval between anterior cruciate ligament injury and MR examination, and we did not assess the degree of instability, both of which could influence the association between the presence of central osteophytes and anterior cruciate ligament tears.
Proposed explanations for the development of osteophytes include aging, mechanical instability of the joint, proliferative response caused by adjacent synovial membrane inflammation, and tissue response to stretching at the insertion site of the joint capsule [2, 4, 10,11,12]. These factors are believed to induce production of chemical or hormonal transducers including insulin, insulinlike growth factor-1, and transforming growth factor-b, which have been found to stimulate osteophyte growth [10,11,12,13,14,15,16]. Unlike marginal osteophytes, central osteophytes do not have an adjacent joint capsule or synovium, and, thus, stretching of the joint capsule and the adjacent synovial inflammation are not responsible for their formation. The exact mechanism or mechanisms resulting in formation of central osteophytes and marginal osteophytes have not yet been determined, and future studies are needed to determine whether similar mechanisms are responsible for formation of both types of osteophytes.
One limitation of this study is that we did not have arthroscopic or pathologic confirmation of our findings. Arthroscopic confirmation of central osteophytes is not possible because central osteophytes are generally not visible at arthroscopy, which is most likely because of the thin layer of covering cartilage and because of their morphology. Our referring orthopedic surgeons report seeing central osteophytes only when they débrided the overlying tissue. Our referring orthopedic surgeons rarely use débridement in their treatment of articular cartilage defects and, thus, rarely visualize subarticular osteophytes. The diffuse form of central osteophytes would not be expected to be visible even with débridement because they fill the base of the cartilage defect (Fig. 3). Despite the inability to obtain arthroscopic correlation, the MR imaging findings are typical of osteophytes seen at the margins of the knee, and the configuration and appearance of the osteophytes agree with prior studies [2, 3]. Although cartilage defects were not arthroscopically confirmed, the fat-suppressed three-dimensional spoiled gradient-echo imaging sequence used in this study has been shown to be accurate for detection of cartilage defects [17, 18]. An MR imaging grading system similar to that used in this study showed good concordance with arthroscopic grade in a prior study [18]. Consensus interpretation was used in this study because the study was designed to determine the prevalence of central osteophytes and their association with cartilage defects and other MR imaging findings rather than to determine the accuracy or interobserver reliability for MR imaging. We also did not design this study to compare different MR imaging sequences because this was not the goal of this investigation.
In conclusion, central osteophytes are commonly found in patients referred for MR imaging of the knee and are associated with cartilage defects at the same location that are usually full thickness or near-full thickness. Thus, when central osteophytes are identified in the knee on radiographs, CT scans, or MR images obtained without articular cartilage-specific sequences, it is likely that an associated articular cartilage defect is present. Central osteophytes are associated with more severe changes of osteoarthritis than are marginal osteophytes alone, including a greater number of articular cartilage defects and a greater number and larger size of marginal osteophytes. Future studies are needed to determine whether central osteophytes are associated with more severe symptoms of osteoarthritis or a worse prognosis.
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