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DOI:10.2214/AJR.04.1809
AJR 2006; 186:449-453
© American Roentgen Ray Society

Original Research

Sensitivity of MR Arthrography in the Evaluation of Acetabular Labral Tears

Glen A. Toomayan1, W. Russell Holman1, Nancy M. Major1, Shannon M. Kozlowicz1 and T. Parker Vail2

1 Department of Radiology, Duke University Medical Center, Durham, NC 27710.
2 Division of Orthopaedic Surgery, Department of Surgery, Duke University Medical Center, Durham, NC 27710.

Received November 22, 2004; accepted after revision January 27, 2005.

 
Presented at the 2003 annual meeting of the American Roentgen Ray Society, San Diego, CA.

Address correspondence to G. A. Toomayan, Department of Radiology, Duke University Medical Center, Box 3808, Durham, NC 27710 (glen.toomayan{at}duke.edu).


Abstract
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 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
OBJECTIVE. MRI has historically provided suboptimal visualization of tears of the acetabular labrum. Degenerative fraying and underlying cartilage abnormalities can often mimic tears of the labrum on conventional MRI. Administration of intraarticular gadolinium enhances the MRI appearance of the labrum to improve detection of labral abnormalities. This study examined the improved diagnostic sensitivity of MR arthrography compared with conventional MRI and the importance of confining the study to a small field of view.

MATERIALS AND METHODS. Fifty-one hips were imaged in 48 patients. Fourteen hips underwent conventional MRI with a large field of view (30-38 cm). Seven hips underwent conventional MRI with a small field of view (14-20 cm). Thirty hips underwent MR arthrography with a small field of view (14-20 cm). Labral tears were diagnosed when contrast material was identified within the labrum or between the labrum and the acetabulum, when a displaced fragment was noted, or when a paralabral cyst was identified. All study results were compared with findings at the time of hip arthroscopy.

RESULTS. Conventional MRI with a large field of view was 8% sensitive in detecting labral tears compared with findings at the time of arthroscopy. Diagnostic sensitivity was improved to 25% with a small field of view. MR arthrography with a small field of view was 92% sensitive in detecting labral tears.

CONCLUSION. A combination of MR arthrography and a small field of view is more sensitive in detecting labral abnormalities than is conventional MRI with either a large or a small field of view.

Keywords: hip • MRI • MRI contrast agents • musculoskeletal imaging


Introduction
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 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Acetabular labral tears are a recently recognized cause of hip pain in the absence of plain radiographic abnormalities [1-3]. Typical presenting signs and symptoms in patients with labral tears include persistent pain, clicking, locking, and decreased range of motion. As orthopedic surgeons become more comfortable performing arthroscopic surgery on the hip joint, labral abnormalities are more frequently being addressed by means of minimally invasive surgery. Accurate preoperative identification of labral tears by the radiologist has therefore become an area of increasing interest.

MRI has historically provided suboptimal visualization of the acetabular labrum, with large variability in appearance of the labrum in asymptomatic patients [4-6]. Distention of the hip capsule with intraarticular gadolinium enhances the MRI appearance of the labrum and allows improved detection of labral abnormalities [7-9]. The present study showed the superior sensitivity of MRI for detecting labral tears when both administering intraarticular gadolinium and confining the study to a small field of view.


Materials and Methods
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 Materials and Methods
Results
 Discussion
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This study was a retrospective analysis that included 48 patients (51 hips), with three patients imaged twice because of recurring symptoms. The patients included in this series were referred for MRI of the hip during a 12-month interval to rule out acetabular labral abnormalities. Twenty-one male patients and 27 female patients were included (age range, 14-63 years; mean, 35 years). Each patient underwent MRI using one of three techniques followed by hip arthroscopy.

Thirty hips underwent MR arthrography, and 21 underwent conventional MRI. Of the 21 hips undergoing conventional MRI, 14 were imaged with a large field of view (30-38 cm) and seven with a small field of view (14-20 cm). In the group undergoing conventional MRI with a large field of view, patient age ranged from 16 to 63 years (mean, 32 years). Two of these patients had Legg-Calvé-Perthes disease, one had developmental dysplasia of the hip, and one had evidence of osteoarthritis on hip radiography. In the group undergoing conventional MRI with a small field of view, patient age ranged from 20 to 44 years (mean, 33 years). None of the patients in this group had radiographic findings of hip disease. All hips undergoing MR arthrography were imaged with a small field of view (14-20 cm). In this group, patient age ranged from 14 to 49 years (mean, 36 years). One patient in this group had findings consistent with osteoarthritis on hip radiography. All hip arthroscopy was performed by the same orthopedic surgeon. All imaging was done with a 1.5-T Signa MRI scanner (GE Healthcare). All MRI studies were retrospectively interpreted by one musculoskeletal radiologist who was unaware of the abnormalities.

For patients who underwent MR arthrography, the scanning parameters included spin-echo T1-weighted images with fat suppression (TR/TE range, 500-800/15-20) and fast spin-echo T2-weighted images with fat suppression (4,000-6,000/50-80). All these patients were imaged with a field of view of 14-20 cm in both the axial and the coronal planes and a matrix of 256 x 192 with 1 or 2 excitations. Before imaging, the hip joints of these patients were injected anteriorly at the level of the femoral neck with 10-15 mL of a 1:200 dilution of gadolinium in sterile saline without epinephrine under fluoroscopic guidance. A torso phased-array or flexible coil was used for patients imaged with a small field of view.

For patients who underwent conventional MRI, the scanning parameters included spin-echo T1-weighted images (500-800/15-20) and fast spin-echo T2-weighted images (4,000-6,000/50-80). Seven hips were imaged with a field of view of 14-20 cm, and 14 were imaged with a larger field of view, 30-38 cm. For all hips, a matrix of 256 x 192 with 1 or 2 excitations was used. Both groups were imaged in the axial and coronal planes. A torso phased-array or body coil was used for patients imaged with either a large or a small field of view.

Labral tears were diagnosed by one of six musculoskeletal radiologists when contrast was identified within the labrum or between the labrum and acetabulum, when a displaced fragment was noted, or when a paralabral cyst was identified. Abnormal morphology alone was not diagnostic of a tear by MRI. At arthroscopy, our orthopedic surgeon does not consider findings of "degenerative fraying" to be a labral tear.

Sensitivities and specificities were calculated for each group. Chi-square analysis was used to compare the three diagnostic techniques and to compare the gold standard, arthroscopy, with the MRI technique that had the highest sensitivity. This analysis was performed on data corresponding to all hips with evidence of labral tears based on arthroscopy. A significance level of 0.05 was used.

The procedures stipulated by the institutional review board were followed in performing this study.


Results
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 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Tears of the acetabular labrum were diagnosed at arthroscopy in 40 of the 51 hips in this study. Of the 14 hips undergoing conventional MRI with a large field of view, 12 were found to have acetabular labral tears at arthroscopy. Conventional MRI revealed a tear in one of those 12 hips, representing an 8% sensitivity for conventional MRI with a large field of view, compared with arthroscopy, in detecting acetabular labral tears (Table 1). Of the seven hips undergoing conventional MRI with a small field of view, four were found to have acetabular labral tears at surgery. One of those tears was diagnosed on conventional MRI, for a sensitivity of 25%.


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  		TABLE 1: Sensitivity of Detecting Labral Tears by MRIs

 

Of the 30 hips that were imaged with MR arthrography and a small field of view, 24 were found to have acetabular labral tears at arthroscopy. Of those 24 hips, 22 were diagnosed with labral tears on MR arthrography, representing a 92% sensitivity for MR arthrography with a small field of view compared with arthroscopy in detecting acetabular labral tears. In this group, 19 tears were anterosuperior and three were posterior.

None of the three groups had false-positive results. Specificity was therefore 100% for all groups (Table 2).


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  		TABLE 2: Specificity of Detecting Labral Tears by MRI

 

Based on chi-square analysis, no significant difference existed between unenhanced MRI with a small field of view and unenhanced MRI with a large field of view in detecting acetabular labral abnormalities in patients with arthroscopy-proven tears (Table 3). Significant differences were found between MR arthrography and unenhanced MRI with a large field of view (p < 0.001) and unenhanced MRI with a small field of view (p = 0.01). This finding was further substantiated by sensitivities of 92%, 25%, and 8%, respectively. There was no significant difference between arthroscopy and MR arthrography (p = 0.20).


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  		TABLE 3: Chi-Square Analysis and p Values

 


Discussion
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References
 
Injury to the acetabular labrum can result from trauma, degenerative changes, or dysplasia [1]. Mechanisms of traumatic injury include motion involving pivoting or twisting of the hip, as occurs in tennis, golf, football, soccer, hockey, and other sports [1]. Degenerative changes of the labrum may be seen in patients with inflammatory arthritis and acetabular dysplasia [10]. Patients may present with sudden or insidious pain or mechanical symptoms in the affected hip. Pain is often located in the anterior groin or thigh and is worse with combined flexion and rotation of the hip [2]. Mechanical symptoms commonly include clicking, catching, locking, giving way, or a restricted range of motion.

The acetabular labrum can be difficult to image given its small size, variable morphologic appearance, and redundancy of the joint capsule when not distended. It measures 3-11 mm in width and 2-5 mm in height [4] and most frequently appears triangular on conventional coronal or axial MR images [5]. A good signal-to-noise ratio and excellent spatial resolution are needed for adequate visualization of this small structure. Because of this small size and variable morphology, degenerative changes, seen as irregular labral margins, can be difficult to distinguish from small tears. Similarly, the iliopsoas tendon can mimic a labral abnormality as it crosses over the anterior labrum (Fig. 1), and a posteroinferior sublabral groove may be mistaken for a labral injury [11]. These pitfalls should be known to radiologists interpreting labral images.


Figure 1
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  		Fig. 1 —36-year-old man with normal hip MRI. Axial spin-echo T1-weighted image without intraarticular contrast material (TR/TE, 650/15; field of view, 19 cm) shows normal anterior labrum having typical low-signal-intensity triangular appearance (arrowhead). Low-signal-intensity structure just anterior to anterior labrum represents normal iliopsoas tendon (arrow) and should not be misinterpreted as tear of labrum.

 
Analogous to evaluation of the labrum in the shoulder, evaluation of tears and detachment is better when contrast material [7, 8] or an effusion is present. MR arthrography allows excellent assessment of the acetabular capsular-labral complex [12]. The increased signal intensity of joint fluid or intraarticular contrast material can be more readily identified against the low signal intensity of the normal labrum. Cartilage undercutting the labrum can mimic high-intensity signal from a labral tear (Fig. 2). Contrast material in the joint can make this distinction more apparent because the contrast material and cartilage are of different signal intensities.


Figure 2
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  		Fig. 2 —32-year-old woman with normal hip MRI. Coronal spin-echo T1-weighted image with fat suppression after administration of intraarticular gadolinium (TR/TE, 500/15; field of view, 14 cm) shows normal, triangular low-signal-intensity superior labrum (arrow) with area of intermediate signal intensity undercutting labral attachment to acetabulum (arrowhead). This finding represents normal cartilage and should not be mistaken for labral abnormality.

 
Use of a large field of view is a good screening tool for hip and pelvis abnormalities but is not useful for diagnosing labral tears given the small size of the labrum (Fig. 3). In this study, use of conventional MRI with a large field of view (30-38 cm) was only 8% sensitive in detecting the labral tears of patients who subsequently underwent arthroscopy. Decreasing the field of view to 14-20 cm moderately improved sensitivity with conventional MRI to 25%. However, labral tears are frequently missed with unenhanced MRI with a small field of view because of a normal, diffusely low-signal-intensity appearance of the labrum without definite evidence of a tear (Fig. 4).


Figure 3
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  		Fig. 3 —Axial fat-suppressed fast spin-echo T2-weighted image without intraarticular contrast material (TR/effective TE, 4,000/71; field of view, 32 cm) in 35-year-old female long-distance runner with right hip pain and "clicking." Anterior labrum (arrow) appears diffusely low in signal intensity with no evidence of tear. Anterior labrum is poorly distinguished from adjacent iliopsoas tendon because of large field of view. At surgery, this patient had anterior labral tear.

 

Figure 4
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  		Fig. 4 —Axial fat-suppressed fast spin-echo T2-weighted image without intraarticular contrast material (TR/effective TE, 4,000/75; field of view, 20 cm) in 38-year-old woman with hip pain and "locking." Anterior labrum appears diffusely low in signal intensity (arrow) and was therefore interpreted as normal. However, at arthroscopy, patient was found to have anterior labral tear.

 


Figure 5
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  		Fig. 5 —Coronal spin-echo T1-weighted image after intraarticular gadolinium administration with fat suppression (TR/TE, 750/5; field of view, 20 cm) in 28-year-old woman with right hip pain and "locking." Contrast material is undercutting superior labrum (arrow), consistent with tear confirmed at arthroscopy.

 


Figure 6
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  		Fig. 6 —Coronal spin-echo T1-weighted fat-suppressed image after intraarticular gadolinium administration (TR/TE, 616/15; field of view, 20 cm) in 44-year-old male tennis professional with left hip pain aggravated by rotation. Contrast material is undercutting anterosuperior labrum, consistent with tear (arrow) that was proven at surgery. Incidentally noted is herniation pit (arrowhead).

 
Combining use of a small field of view with administration of intraarticular contrast material dramatically increased the sensitivity of detecting labral tears to 92%. This study showed that the ability of MR arthrography to allow correct identification of acetabular labral tears is significantly better than that of unenhanced MRI with either a large or a small field of view. In addition, although the number of hips with a normal labrum in this study was small (six), no false-positive findings were encountered, suggesting 100% specificity of MRI in detecting labral tears. MR arthrography is therefore the most sensitive imaging technique for identifying labral tears. This finding of 92% sensitivity for MR arthrography in detecting labral tears is similar to the 90% sensitivity of MR arthrography in a study of 20 surgically proven cases [9] and better than the 63% sensitivity found in another study of 16 surgically proven cases of labral tear [13]. Unlike previous studies, the current study showed that in addition to the use of intraarticular contrast material, the choice of a small field of view is an important consideration in improving sensitivity for detecting labral tears.

These results highlight the importance of both a small field of view and intraarticular contrast material in the accurate diagnosis of labral abnormalities. The presence of gadolinium in the hip joint readily allows the detection of contrast material undercutting the labrum in patients with labral tears (Figs. 5 and 6). Likewise, linear high signal intensity in the labrum may indicate a labral tear. Limiting the images to a small field of view allows small structures about the hip, such as the acetabular labrum, to be identified with improved clarity.

Treatment techniques for labral tears have historically been conservative [3]. Rest and antiinflammatory medications have been advocated, followed by a period of physical therapy with protected weight bearing. Open surgery of the hip is avoided when possible because of its many complications, including osteonecrosis of the femoral head, heterotopic bone formation, infection, neurovascular injury, thromboembolic disease and resultant muscle weakness [14], and the need for inpatient hospitalization and lengthy rehabilitation [15]. Although arthroscopic surgery of the knee, shoulder, and wrist is common, only recently has arthroscopic surgery of the hip been performed by some surgeons. Orthopedic surgeons with experience in hip arthroscopy often perform this procedure for intractable hip pain and mechanical symptoms in the absence of a demonstrable radiographic abnormality [15]. This procedure is done when undetected internal derangement of the hip joint is suspected. Labral abnormalities can be one such cause of radiographically undetectable hip pain, particularly when conventional MRI is performed with a large field of view.

At our institution, a labral tear is diagnosed preoperatively if a patient complains of consistent mechanical symptoms but radiography findings are negative, if a labral abnormality is evident on MRI, or if the pattern of pain suggests labral abnormality [15]. Visualization of a labral tear allows for appropriate surgical treatment, which involves débridement of the torn labrum back to a stable base of healthy-appearing tissue [14] or occasional partial resection [16]. Débridement of a labral tear resolves mechanical symptoms and significantly decreases pain in as many as 91% of patients undergoing arthroscopy for labral tears at our institution [15]. If the sensitivity and specificity of MRI for detecting internal derangements of the hip are sufficiently high, an orthopedic surgeon can confidently rely on using MRI to determine which patients will benefit from arthroscopic intervention. Therefore, it is important to refine protocols to improve diagnostic accuracy in the detection of labral tears.

It is likely that more studies for the evaluation of labral abnormalities will be requested by referring clinicians as orthopedic surgeons become increasingly comfortable performing hip arthroscopy. Administration of intraarticular gadolinium increases the sensitivity of MRI in detecting tears of the acetabular labrum. Furthermore, use of a small field of view is critical in assessing this small structure. Use of a small field of view is therefore recommended for patients in whom an acetabular labral abnormality is suspected.

A limitation of this study was the small number of patients. Future efforts will focus on confirming these results in a larger series of patients. Ongoing evaluation of our results and a greater number of surgeons performing hip arthroscopy at our institution will increase our institutional experience with labral tears and help refine our protocols to improve the diagnostic accuracy of MR arthrography for acetabular labral tears.


References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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