As the use of magnetic resonance imaging (MRI) is increasingly diverse in diagnosing a variety of injuries, the value of studies that analyze the clinical efficacy and further define the role of MRl increases. The purpose of this research was to measure the sensitivity, specificity, and accuracy of mid-field MRI evaluations of the knee, compared to actual pathology observed by arthroscopy. Seventy-four knees were evaluated, and arthroscopy was taken to be the true diagnosis of the menisci and the anterior cruciate ligament (ACL). Meniscal topography also was included secondarily in the analysis of MRI diagnostic validity. This study showed good overall validity with the use of mid-field MRI to diagnose knee structures. Accuracy was highest in the detection of ACL tears (96%) and lowest in finding medial meniscal tears (89%). Sensitivity was lower in the lateral meniscus compared to the medial meniscus (69% vs 94%, respectively), and lower specificity occurred with the medial meniscus compared to the lateral meniscus (88% vs 94%, respectively). MRI can be a valuable adjunct in medical diagnoses, and the use of MRI will continue to evolve as an important part of surgical planning.

Since the discovery of the use of nuclear magnetic resonance in imaging by Lauterbur in 1973,1 magnetic resonance imaging (MRI) has come to be accepted as a noninvasive modality to examine the anatomy and pathology of various organs2,3 and soft tissues as well as the musculoskeletal system.4,5 Other authors have reported on the specific usefulness of MRI.6-10

The use of MRI offers many advantages, which include excellent inherent soft-tissue contrast, multiplanar imaging ability, lack of exposure to ionizing radiation, no need for intraarticular contrast material, and the absence of artifact caused by the superimposition of osseous structures. MRI also is painless, noninvasive, and easily and rapidly performed.

Studies have been published that involve comparisons of the findings of MRI with those of arthroscopy. In series ranging in size from 25 to 1,014 patients,11-22 the accuracy of meniscal imaging ranged from 45% to 98% and the accuracy of imaging the anterior cruciate ligament (ACL) ranged from 92% to 100%.

The purpose of this study was to evaluate the sensitivity, specificity, and accuracy of our local experience using mid-field MRI evaluations of the knee, compared to actual pathology as observed by arthroscopy.

Methods and materials

Seventy-four knees in 72 patients were evaluated with MRI followed by arthroscopy. Data collected from a period of February 1991 to July 1994 were evaluated by an independent researcher. The patient series represented a random sampling of the population, who would be evaluated clinically for suspected acute or chronic lesions of the meniscus and/or ACL. There were 40 male and 32 female patients involved in the study, and the average age was 34.7 years (standard deviation = 11.2 years), with a range of 13 to 65 years. The series included 41 right knees and 33 left knees.

Arthroscopy was performed from 1 day to 13 weeks after MRI evaluation and was carried out under general anesthesia. Distention of the joint cavity was achieved with an electrolyte-free solution (saline). Thirty-degree rod-lens arthroscope and a probe were introduced using anterolateral and anteromedial portals. All arthroscopic findings and remedial procedures were recorded on videotape. Only one arthroscopist performed all surgeries (ALO).

MRI images were obtained from a 0.6T (Tesla) Health Images (HI) standard magnet. A dedicated full-volume (360°) knee surface coil was utilized. The patient's knee within the surface coil usually was in approximately 10° of flexion and 10° to 15° of external rotation, based primarily on the patient's comfort level.

The routine knee protocol included a T1-weighted transaxial and coronal spin-echo sequence (TR 600-650; TE 30) with a field of view (FOV) of 16,21 cm or 17,21 cm; YRES 160; 2 NSQ; 5-mm-thick slices with a 20% gap. Spin-echo proton-density and T2-weighted sagittal images (TR 2000; TE 30/90); FOV 16,21 cm or 17,21 cm; YRES 184; 2 NSQ; with 4-mm-thick slices and a 25% gap, also were performed. At the technologist's discretion, if a normal ACL was not seen on the routine sagittal images, an additional gradient-echo sagittal sequence angled through the region of the ACL was performed with a TR 440, TE 24 and a flip angle of 13°. The latter sequence utilized an FOV of 16,21 cm or 17,21 cm; YRES 160; and a 3-mm slice thickness with a 20% gap.

Arthroscopy was taken to be the true diagnosis to evaluate the sensitivity, specificity, and accuracy of MRI findings. Sensitivity was calculated using the number of true-positive results divided by the sum of the true-positive and false-negative results. Specificity was calculated using the number of true-negative results divided by the sum of the true-negative results and false-positive results. Accuracy was determined using the total number of patients minus the sum of false-positive and false-negative MRI results divided by the total number of patients.

Predictive values also were calculated. The negative predictive values were calculated by dividing the number of true-negative results by the sum of true-negative results plus false-negative results. Positive predictive values were determined by the number of true-positive results divided by the sum of true-positive results plus false-positive results.

MR images were interpreted primarily by one radiologist (SMS) experienced in MR radiology. The criteria used to determine meniscal tears were as follows.

* Criteria involved abnormal increase of intrameniscal signal (best demonstrated on the proton-density sagittal sequences and the next T1-weighted coronal sequence), which extended confidently into an articular surface (again designated as a grade III tear).

* Abnormal increase of intrameniscal signal presenting as a global (grade I) or linear (grade II) configuration without definite extension into an articular surface was considered to represent internal degeneration rather than a frank tear (figure 2).

• Morphologic distortion, blunting, or truncation of the meniscus also was used as a criterion in determining the presence of a tear, often associated with the abnormal intrameniscal signal described previously.

The ACL was considered present when the entire ligament could be identified. Contiguous sagittal images were reviewed to verify the intact length of the ACL. The ligament was considered torn when the entire length could not be identified (figure 3).

Results

Comparisons of the MR diagnoses and arthroscopy diagnoses are summarized in tables 1 and 2. For the medial meniscus, 31 true-positive and 43 true-negative tears were revealed by arthroscopy. Of the 74 patients, MR diagnoses resulted in 6 false-positive and 2 false-negative images (table 1). Accuracy was 89%; sensitivity was 94%; specificity was 88%; negative predictive value (NPV) was 96%; and positive predictive value (PPV) was 84% (table 2).

With regard to the lateral meniscus, arthroscopy identified 11 true-positive and 63 true-negative tears (table 1). Comparisons of the findings of MRI with arthroscopy revealed one false-positive and five false-negative images. Accuracy was 92%; sensitivity was 69%; specificity was 94%; NPV was 93%; and PPV was 92%, as shown in table 2.

For the ACL in table 1, there were 15 true-positive, 59 true-negative, 1 false-positive, and 2 false-negative results. Accuracy, sensitivity, and specificity were 96%, 88%, and 98%, respectively. NPV was 97%, and PPV was 94% (table 2).

Meniscal topography was included in the analysis of the MRI diagnostic validity (figure 1). When each of the three segments of the menisci were evaluated individually, the anterior aspect of the medial and lateral meniscus had the lowest sensitivity values (67% for both), compared to other segments. Also, somewhat low sensitivity resulted in the middle and posterior aspects of the lateral meniscus (78% and 71%, respectively). Overall, MRI results were generally accurate in all meniscal components with very good specificity (figure 1).

Diagnoses other than meniscal and ACL included Baker's and popliteal cysts, articular cartilage and osteochondral defects, bone tumors, meniscal cysts, extensor and patellar tracking problems, subchondral sclerosis, tibial plateau fractures, patellar tendinitis, and collateral ligament sprains and tears.

Discussion

This study resulted in generally good validity in the use of MRI to diagnose deranged knee structures. Accuracy was highest in detecting ACL tears (96%) and lowest in detecting medial meniscus (89%), as shown in table 2. Sensitivity was lower in the lateral meniscus compared to the medial meniscus (69% vs 94%, respectively), and lower specificity occurred with the medial meniscus compared to the lateral meniscus (88% vs 94%, respectively).

Reduced specificity in MRI evaluations of the medial meniscus appears to be a common tendency and has been reported in other studies.12,18-21,23 It has been suggested that because medial meniscal tears occur more often than lateral meniscal tears, radiologists tend to expect medial meniscus tears and consequently "over-read" images of the medial menisci. Conversely, lateral meniscus images are "under-read" as having no tears.12 In this series, table 1 shows that arthroscopy revealed almost three times as many tears of the medial meniscus compared to the lateral meniscus, with nearly orthogonal results in false-positive and false-negative diagnoses of the menisci with MRI.

We considered the possibility of other factors that could have acted as potential confounders in over- or under-reading meniscal MR images. Factors such as the amount of effusion, grade or location of chondromalacia, chronicity, accompanying bone injury, or previous injury were reviewed and compared in patients with false-positive and negative results versus patients with true-positive and negative results. No specific or apparent trends in the above factors were present.

Our algorithm for the use of MRI in the management of traumatic knee injuries is similar to others that have been described.13 In our clinic, a history and an objective and subjective clinical evaluation are used to determine the functional status of the ACL. MRI is an essential tool in a difficult-to-examine knee due to pain, swelling, or muscle spasms; these complications occur in about 20% of our patients. If the ACL is deficient with significant laxity and the patient is a candidate for surgery based on age and activity level, MRI can be used to determine the structural integrity of the ACL and, consequently, augment a surgical decision. In patients who present with painful joint range of motion but minor joint instability, MRI can be useful in determining the condition of the menisci. If the patient is not a candidate for ACL surgical intervention and conservative, nonoperative treatment is elected, MRI can define meniscal pathology and possibly reduce the need for diagnostic arthroscopy.

MRI is especially indicated when the clinician is doubtful regarding the diagnosis and treatment or when an expeditious diagnosis is required. Athletes and select self-employed individuals particularly desire an immediate diagnosis and typically are anxious to begin treatment and rehabilitation after an injury. Those patients' situations are such that prolonged time off could hinder their level of performance and/or place them in financial jeopardy.

Excellent results using MRI evaluations to help guide surgical intervention decisions have been reported. In a series of 45 athletic patients, surgical treatment of all MRI grade II lesions of the menisci resulted in the elimination of symptoms in almost all patients in the series who previously had severe complaints and disability.24 Although signal grade was not part of this investigation, it appears to be another potentially useful component in successful clinical outcomes.

The economic efficacy of diagnostic tests continues to emerge as an issue in health care and medical management. We are challenged to provide the highest level of care yet keep costs to a minimum. The results of our local series reveal very good validity and reliability using MRI to diagnose knee injuries. Total overall combined accuracy, PPV, and NPV (calculated for the medial and lateral meniscus and ACL) were 92%, 90%, and 95%, respectively; these are very high validity rates compared to similar studies.

MRI has been instrumental in preventing unnecessary surgery when there has been a reasonable clinical doubt and, in some cases, the use of MRI has been important in surgical planning. MRI can be a useful tool in the decision-making process to treat knee injuries successfully and can help direct the efficient use of diagnostic dollars. AR

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