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Original Research |
1 Department of Orthopaedic Surgery, Osaka University Medical School, 2-2
Yamadaoka, Suita, Osaka 565-0871, Japan.
2 Department of Radiology, Osaka University Medical School, Osaka 565-0871,
Japan.
3 Department of Radiology, Osaka Seamen's Insurance Hospital, Osaka 552-0021,
Japan.
Received August 26, 2004; accepted after revision October 8, 2004.
Address correspondence to T. Nishii.
OBJECTIVE. Our objective was to assess the diagnostic ability of MDCT arthrography for acetabular and femoral cartilage lesions in patients with hip dysplasia.
MATERIALS AND METHODS. A disorder of the articular cartilage was evaluated in 20 hips of 18 patients with acetabular dysplasia who did not have osteoarthritis or who had early stage osteoarthritis before undergoing pelvic osteotomy surgery. The findings on fat-suppressed 3D fast spoiled gradient-echo MRI and MDCT arthrography of the hip were evaluated by two independent observers, and sensitivity, specificity, and accuracy were determined using arthroscopic findings as the standard of reference. Kappa values were calculated to quantify the level of interobserver agreement.
RESULTS. The sensitivity and specificity for the detection of any
cartilage disorder (grade 1 or higher) were (observer 1/observer 2) 49%/67%
and 89%/76%, respectively, on MRI, and 67%/67% and 89%/82%, respectively, on
CT arthrography. The sensitivity and specificity for the detection of
cartilage lesions with substance loss (grade 2 or higher) were (observer
1/observer 2) 47%/53% and 92%/87%, respectively, on MRI, and 70%/79% and
93%/94%, respectively, on CT arthrography. CT arthrography provided
significantly higher sensitivity in the detection of grade 2 or higher lesions
than MRI for both observers. Interobserver agreement in the detection of grade
2 or higher cartilage lesions was moderate (
= 0.53) on MRI and
substantial ( = 0.78) on CT.
CONCLUSION. MDCT arthrography is a sensitive and reproducible method for assessing articular cartilage lesions with substance loss in patients with hip dysplasia.
Osteoarthritis is the most common joint disorder in the hip joint. In particular, acetabular dysplasia is one of the major causes of hip osteoarthritis [1], and accurate and sensitive detection of osteoarthritis progression is important for determining effective conservative or surgical treatment in patients with hip dysplasia. Conventional radiography has long been the major imaging technique for patients with osteoarthritis; however, inaccurate relationships between radiographic findings and the status of the articular cartilage have been identified [2, 3]. Other imaging techniques available for the evaluation of articular cartilage disorder and osteoarthritic involvement are arthrography, bone scintigraphy [4, 5], CT arthrography [6, 7], and MRI [8–11]. Although CT arthrography has excellent diagnostic ability for cartilage disorder with satisfactory in-plane imaging resolution and tissue contrast in the knee joint [6, 7], its diagnostic ability in the hip joint has been shown to be inferior because of susceptibility to partial volume averaging of the joint surface by limited longitudinal imaging resolution.
Recent advances in helical CT technology with multidetector arrays allow submillimeter spatial imaging resolution in the longitudinal direction [12–14]. This new method has great potential to minimize partial-volume-averaging problems and provide superior diagnostic ability for disorders of the articular cartilage in the hip joint, especially around the weight-bearing area. The purpose of the our study was to evaluate the diagnostic ability of MDCT arthrography compared with that of MRI in the hip joint.
Materials and Methods
Twenty hips in a consecutive series of 18 patients (age range, 12–49 years; one man, 17 women) who underwent pelvic osteotomy surgery because of symptomatic hip dysplasia were included in this study. According to the conventional radiologic classification by Lane et al. [15], 10 hips were grade 0 (no joint space narrowing and no osteophytes) and 10 hips were grade 1 (mild joint space narrowing, mild osteophytes, or both). The mean center edge angle of Wiberg [16] of those hips was –3° (range, –21° to 10°).
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MRI was performed in the coronal and sagittal sections using a 1.5-T imager (Signa Horizon, GE Healthcare) equipped with a unilateral surface coil (TORSO, GE Healthcare). A fat-suppressed 3D fast spoiled gradient-echo pulse sequence was used with a TR of 24.4 msec, a TEeff of 5.7 msec, and a flip angle of 20°. The section thickness was 1.5 mm, and in-plane resolution was 0.63 mm (field of view, 160 mm; matrix, 256 x 256 pixels). The two signals were averaged. The acquisition time was 10 min 43 sec for each direction. Frequency encoding was from head to foot across the hip joint. During MRI, a leg traction system was used, as described in a previous report [8], to show the acetabular and femoral cartilages distinctly by interposition of a layer of joint fluid between the two cartilages (Fig. 1A, 1B, 1C, 1D). Briefly, this device comprises a leg apparatus to pull the leg caudally by approximately 15 kg of force and a pelvic apparatus to pull the pelvis cranially by approximately 10 kg of force, which exerts a traction force to the hip.
The midsagittal MR images were used for subsequent analysis because cartilage disorder was often observed at the anterosuperior area of the acetabulum in arthroscopic studies of dysplastic hips [18, 19]. On the three consecutive midsagittal images, abnormalities of the acetabular and femoral cartilages were evaluated in three zones (superoanterior zone, superior zone, and superoposterior zones) that were defined by dividing the weight-bearing area of the acetabulum into three 30° ranges [8] (Fig. 1A, 1B, 1C, 1D). The degree of abnormalities was classified according to a modification of the classification of Vande Berg et al. [13] (Table 1): Grade 0 was assigned to normal cartilage; grade 1, to a focal superficial signal intensity change without surface irregularity; grade 2, to a contour defect less than the full thickness of the cartilage in depth; and grade 3, to a full-thickness contour defect. The most severe grade observed on the three consecutive images for each zone was recorded. Cartilage abnormalities were recorded for a total of six zones in each patient, including the three defined zones in the acetabular and femoral cartilages, respectively. The MR images were evaluated independently by two musculoskeletal radiologists, each with more than 10 years of experience, without knowledge of the clinical or surgical data.
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For CT arthrography, a 20-gauge needle was placed into the affected hip joint, and under fluoroscopic observation, 10 mL of iotrolan (Isovist 300, Nihon-Schering) was injected. Active exercise of the hip joint was encouraged for approximately 20 min to facilitate extensive coating of the articular surface by the contrast material. CT was performed using a 4-MDCT system (Aquilion, Toshiba). Helical scanning was conducted at 120 kVp and 150 mAs, and the collimation beam was 2.0 mm. The field of view was 256 mm, and the section thickness was 0.5 mm. The table speed was 3 mm/sec, and the acquisition time was 36 sec. During CT, the leg traction apparatus was used to visualize acetabular cartilage and femoral cartilage separately, as used during MRI.
The imaging data were transferred digitally to a workstation, and coronal and sagittal reformations of the hip joint were obtained with a 2-mm interval and an in-plane resolution of 0.5 x 0.5 mm. Using the two consecutive midsagittal reformatted CT images, the same two musculoskeletal radiologists evaluated abnormalities in the acetabular cartilage and femoral cartilage in the superoanterior zone, superior zone, and superoposterior zones (Fig. 1A, 1B, 1C, 1D) using the same grading system as that for MRI except grade 1 was assigned to loss of smooth surface without penetration of contrast material within the cartilage [13] (Table 1). The most severe grade observed on the two consecutive images at each zone was recorded. The CT images were evaluated independently by the two reviewers 3 weeks after the MRI evaluation.
All patients underwent arthroscopic examination during pelvic osteotomy surgery to evaluate the acetabular and femoral articular cartilages under an in-house-developed navigation system using an optical sensor (OPTOTRAK3020, Northern Digital) [20]. Although accurately defining the location where the cartilage is inspected by the arthroscope in the hip joint is difficult, this system guides the surgical instruments and allows surgeons to recognize the arthroscopic position with respect to the acetabular and femoral articular cartilages correctly. Abnormalities were recorded at the six locations for each patient (superoanterior, superior, and superoposterior zones in the acetabular cartilage and femoral cartilage) using a modification of the Outerbridge classification system [21] (Table 1): Grade 0 was assigned to intact cartilage surface without softening; grade 1, to softening or fibrillation without substance loss; grade 2, to fragmentation, fissuring, or cartilage defect without bone exposure; and grade 3, to full-thickness cartilage erosion with bone exposure. Two experienced orthopedic surgeons who were not involved in the image evaluations participated in each examination without prior knowledge of the imaging results and arrived at a consensus interpretation.
One of the authors who was not involved in the image interpretation sessions performed statistical analysis. The arthroscopic findings were compared with evaluations by MRI and CT arthrography on a region-by-region basis (120 regions from three acetabular and three femoral regions in 20 hips). Sensitivity, specificity, and accuracy for detecting any cartilage abnormality (grade 1 or higher) and for detecting a cartilage abnormality with substance loss (grade 2 or higher) were calculated (Table 2) by considering an area in which at least a grade 1 lesion or at least a grade 2 lesion, respectively, was present in the image evaluations as a test-positive area. Kappa values were calculated to quantify the level of agreement at interobserver comparison. Kappa values were considered to indicate slight (0–0.20), fair (0.21–0.40), moderate (0.41–0.60), substantial (0.61–0.80), and almost perfect (0.81–1.00) agreement, according to Landis and Koch [22]. Differences in detecting cartilage abnormality between the two imaging techniques were tested for statistical significance using the two-tailed McNemar test, and statistical significance was set at a p value of less than 0.05.
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Results
On arthroscopy, a high prevalence of disorder in the acetabular or femoral articular cartilage was noted, although most hips showed no or low-grade osteoarthritis on conventional radiography. All hips had grade 1 or higher lesions on the acetabular cartilage in at least one zone, and 12 of 20 hips had grade 1 or higher lesions on the femoral cartilage in at least one zone. On a region-by-region basis, of 120 cartilage regions from the 20 hips, 66 regions showed a normal appearance, 21 showed grade 1 lesions, 24 showed grade 2 lesions, and nine showed grade 3 lesions. The cartilage lesions at grade 1 or higher were located most often in the superoanterior zone of the acetabulum (n = 20), followed by the superior zone of the acetabulum (n = 15), and superior zone of the femoral head (n = 8). In one hip, MR images were subject to metal artifact, which was thought to be caused by fine metal particles disseminated during open-reduction surgery performed when the patient was 2 years old; these images were not evaluated.
On MRI, interobserver agreement was moderate both for the detection of any
cartilage lesions of grade 1 or higher ( = 0.52) and for the detection
of cartilage substance loss of grade 2 or higher ( = 0.53). On CT
arthrography, interobserver agreement was moderate for the detection of any
cartilage lesions of grade 1 or higher ( = 0.52) and was substantial
for the detection of cartilage substance loss of grade 2 or higher ( =
0.78).
The diagnostic ability of MRI and that of CT arthrography are summarized in Table 2. For both observers, all the sensitivity, specificity, and accuracy values for the detection of cartilage lesions with substance loss (grade 2 or higher) were higher than those for the detection of any cartilage abnormality (grade 1 or higher) on both imaging methods except sensitivity on MRI. The accuracy for the detection of grade 2 or higher lesions was (observer 1/observer 2) 80%/78% on MRI and 87%/90% on CT arthrography (Fig. 2A, 2B, 2C). In 21 cartilage lesions without substance loss (grade 1) detected on arthroscopy, observers 1 and 2 found that 16 and eight lesions, respectively, showed normal appearance on MRI and 11 and 13 lesions, respectively, showed normal appearance on CT arthrography. Conversely, in those 21 lesions at grade 1, observers 1 and 2 detected cartilage lesions with substance loss (grade 2 or higher) in four and five lesions, respectively, on MRI and in five and three lesions, respectively, on CT arthrography (Fig. 3A, 3B, 3C).
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Few reports have assessed the articular cartilages in the hip compared with ongoing active investigations of these cartilages in the knee joint. This lack of investigation is primarily because of the difficulty in obtaining satisfactory images of the relatively thin acetabular and femoral cartilages that are in close proximity to each other in the tight ball-and-socket configuration of the hip [23]. As shown in this study and previous arthroscopic studies [18, 24], cartilage disorder occurs frequently even in the preosteoarthritic hip (grade 0 according to the Lane classification [15]) and in early stage osteoarthritis (grade 1 [Lane classification]) of the hip on conventional radiographs. In addition to the use of conventional radiography, the development of other imaging techniques that are sensitive and accurate enough to detect early osteoarthritic change is necessary. This article documents an initial attempt to assess the hip articular cartilages using high-spatial-resolution CT arthrography under a continuous traction system [8] and compare its diagnostic ability with MRI, which is widely recognized as the major imaging technique for visualizing the articular cartilage in the knee joint.
A large number of investigations during the past 10 years have evaluated osteoarthritic change using MRI to directly image the articular cartilage. Its superior soft-tissue contrast, noninvasiveness, and feasibility of 3D evaluation have encouraged physicians to develop an MRI technique for clinical use, especially in knee patella cartilage, which is the thickest cartilage of the body. Through comparative studies of various imaging sequences, a fat-suppressed 3D gradient-echo pulse sequence with thin slices has been established as the most adequate pulse sequence for clinical imaging of cartilage [10, 11, 25]. Excellent results for the detection of knee cartilage lesions have been reported, with sensitivities of 75–93% and specificities of 94–97% using arthroscopic findings as the standard of reference [9–11].
In the current study, the diagnostic ability of MRI regarding specificity for cartilage lesions with substance loss ranged from 87% to 92% and was comparable with the results of previous studies in the knee joint. However, the results for sensitivity on MRI, ranging from 47% to 53%, are inferior to those in previous reports. Possible explanations are that the signal-to-noise ratio was lower because of the location of the hip joint deep inside the body and that using dedicated small surface coils was difficult. Furthermore, the sensitivity for the detection of grade 2 or higher lesions on MRI was not improved compared with that for the detection of grade 1 or higher lesions. This may be caused partly by insufficient traction effect of the hip joint because of the elevated resistant force by surrounding thickened or adhesive capsules or ligaments in the early stages of osteoarthritis with grade 2 or higher cartilage lesions.
Conventional CT arthrography with superior in-plane resolution allows accurate evaluation of patella cartilage, including small-cartilage disorders such as fissures and fibrillation [6]. This imaging technique has previously provided a sensitivity of 65% and a specificity of 100% in detecting chondromalacia of the patella using arthroscopic findings as the standard of reference [7]. However, evaluation of femoral and tibial cartilages in the knee joint or acetabular and femoral cartilages in the hip joint was unsuccessful using conventional CT arthrography. Because longitudinal resolution, defined by CT slice thickness, was largely inferior to in-plane resolution, evaluation of the articular cartilage parallel to the imaging plane was difficult even if coronal or sagittal images were reconstructed. The new CT technology with multidetector arrays offers superior spatial resolution, including in the longitudinal axis, as thin as 0.5 mm and allows successful assessment of the femoral and tibial cartilages and the anterior cruciate ligament in the knee joint [13, 14].
The results of our present study also revealed successful delineation of cartilage lesions with substance loss in the hip joint, with sensitivities of 70–79%, and specificities of 93–94%. In the detection of cartilage lesions with substance loss, the sensitivity of CT arthrography was significantly superior to that of MRI. Most likely, a shorter imaging time, the higher spatial resolution of images, and arthrographic effect for distinct separation of the acetabular and femoral cartilages account for the superior results of CT arthrography. These advantages also might lead to higher interobserver agreement for CT arthrography compared with MRI. However, CT arthrography, like MRI, remained relatively insensitive to cartilage lesions without substance loss. Higher-spatial-resolution techniques may be required to detect subtle morphologic changes of the cartilage using an MR scanner with a magnetic field strength higher than 1.5 T that can acquire images at increased resolution with sufficient signal-to-noise ratio [26] or a CT scanner beyond the capabilities currently available.
On the basis of the results in this initial study, the following specific roles of each imaging technique may be proposed: MRI is suitable for determining 3D locations of cartilage lesions having morphologic change without the need for an invasive procedure or radiation exposure; however, sensitivity in detecting cartilage disorders is limited. CT arthrography allows accurate diagnosis of cartilage disorders with morphologic change and is probably useful for assessing disease progression or the effects of pharmacologic or surgical therapy on the articular cartilage during follow-up examinations. It also can be used to plan femoral or acetabular osteotomy surgery and evaluate the congruency of the acetabular and femoral articular cartilages and the congruency of the bones in the hip joint [27]. Further studies are important to determine whether these advantages justify CT arthrography in clinical use, despite the associated invasiveness and radiation.
Our study had several limitations. First, we have concerns about the small sample size and the selection criteria. Although arthroscopy is commonly used in knee surgery, an indication of hip arthroscopic surgery in pathologic diagnosis and treatment has not been determined yet. Therefore, we selected patients believed to require osteotomy surgery to prevent osteoarthritic progression together with arthroscopic investigations, and this may have resulted in a small number of patients and a high likelihood of involvement of cartilage disorder. The advent of hip arthroscopy equipment and techniques [19] will allow further investigations of a large number of unselected patients for a definitive comparison of the diagnostic ability of MRI and that of CT arthrography.
Second, the inferior results of MRI, as compared with those of CT arthrography, may be caused partly by its failure to show a separation between the acetabular and femoral cartilages; thus, MRI with intraarticular injection of contrast material or saline solution may provide superior visualization of cartilage. Schmid et al. [28] studied the diagnostic ability of MR arthrography for hip articular cartilage lesions using direct inspection at surgery as the standard of reference and found sensitivities of 50–79% and specificities of 77–84%. However, using other pulse sequences, some investigators reported the depiction of articular cartilage on MRI after intraarticular injection of contrast material was inferior to that on MRI without injection [29], and the influence of contrast material on the evaluation of cartilage morphology remains unknown. Furthermore, noninvasiveness of MR examinations cannot be preserved by the addition of intraarticular injection to the procedure.
Finally, although arthroscopy is used as our best available standard of reference, its evaluation of surface characteristics or softening of manual probing is limited, and degenerative change within the cartilage or subchondral bone hypertrophy (Fig. 3A, 3B, 3C) may be undetected. This limitation may have led to underestimated specificity and accuracy for the detection of cartilage lesions on MRI or CT arthrography.
The objective of our study was to evaluate the diagnostic ability of MDCT arthrography in comparison with that of MRI using the pulse sequence that is currently established as the most adequate for showing cartilage disorders clinically. Many other MRI techniques that may provide superior diagnostic ability for the cartilage lesion have been explored, including magnetization transfer contrast [30], driven equilibrium Fourier transform [31], T2 relaxation time in association with distribution of water content or collagen organization [32], and assessment of filtration of ionic contrast agents inside the cartilage as related to a decrease in proteoglycans in the cartilage [33].
Another approach to sensitively detect cartilage disorders is the evaluation of subchondral bone marrow edema. High involvement of subchondral bone abnormalities was shown in patients with symptomatic knee osteoarthritis [34], and the finding of subchondral bone abnormalities was more predictive for the progression of osteoarthritis on MRI than conventional radiography and arthroscopy [35]. Additional research comparing MDCT arthrography with MRI using other pulse sequences or concomitant evaluation of subchondral bone abnormality may result in different relative assessments of diagnostic ability for the cartilage lesion.
Nonetheless, we have shown that the evaluation of articular cartilage lesions with substance loss using MDCT arthrography is satisfactory in patients with hip osteoarthritis. From the results of higher interobserver agreement and sensitivity for the detection of cartilage lesions, CT arthrography may be more reliable than 3D fast spoiled gradient-echo MRI in the detection of cartilage lesions with substance loss in patients with hip dysplasia.
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