Stable or unstable tear of the anterior cruciate ligament of the knee: an MR diagnosis?

Transcription

1 Skeletal Radiol (2012) 41: DOI /s SCIENTIFIC ARTICLE Stable or unstable tear of the anterior cruciate ligament of the knee: an MR diagnosis? Pieter Van Dyck & Jan L. Gielen & Filip M. Vanhoenacker & Kristien Wouters & Lieven Dossche & Paul M. Parizel Received: 21 January 2011 /Revised: 15 March 2011 /Accepted: 31 March 2011 /Published online: 19 April 2011 # ISS 2011 Abstract Purpose To determine the usefulness of magnetic resonance (MR) imaging to distinguish stable from unstable tears of the anterior cruciate ligament (ACL) of the knee. Materials and methods MR images of 97 patients with surgically confirmed ACL tear were retrospectively reviewed. According to arthroscopic and clinical examination, these patients had 36 stable and 61 unstable (9 partial and 52 complete) ACL tears. MR images were interpreted by two blinded reviewers and scored with respect to previously reported primary and secondary MR signs of ACL injury. Based on a comprehensive assessment of all the MR findings, ACLs were categorized as being stable or unstable. MR accuracy was calculated considering only primary MR signs and considering both primary and secondary MR signs of ACL injury, separately. Accuracy of each individual primary and secondary MR sign was calculated. Results Considering only primary MR signs, sensitivity, specificity, and accuracy of MR were 77, 92, and 82%, P. Van Dyck (*) : J. L. Gielen : F. M. Vanhoenacker : P. M. Parizel Department of Radiology, University Hospital Antwerp and University of Antwerp, Wilrijkstraat 10, 2650 Edegem, Belgium pieter.van.dyck@uza.be J. L. Gielen jan.gielen@uza.be F. M. Vanhoenacker filip.vanhoenacker@uza.be F. M. Vanhoenacker filip.vanhoenacker@telenet.be P. M. Parizel paul.parizel@uza.be respectively. Considering both primary and secondary MR signs, sensitivity, specificity, and accuracy of MR were 59, 81, and 67%, respectively. Of all MR signs, discontinuity and abnormal orientation had highest test accuracy (79 and 87%, respectively). Anterior tibial translation, uncovering of the posterior horn of the lateral meniscus, and hyperbuckled PCL were only seen in unstable ACLs (specificity 100%), but these secondary findings had low sensitivity (23%). Bone contusion around the lateral knee compartment was seen in both unstable and stable ACLs (accuracy 64%). Conclusion Previously reported MR imaging signs do not allow accurate distinction between clinically stable and unstable ACL injuries. Anterior tibial translation, uncovering of the posterior horn of the lateral meniscus, and hyperbuckled PCL, if present, are helpful signs in the diagnosis of an unstable tear. The presence of bone marrow edema around the lateral knee compartment is not predictive of ACL insufficiency. F. M. Vanhoenacker Department of Radiology, AZ St-Maarten Duffel/Mechelen, Rooienberg 25, 2570 Duffel, Belgium K. Wouters Department of Scientific Coordination and Biostatistics, University Hospital Antwerp and University of Antwerp, Wilrijkstraat 10, 2650 Edegem, Belgium kristien.wouters@uza.be L. Dossche Department of Orthopedics, University Hospital Antwerp and University of Antwerp, Wilrijkstraat 10, 2650 Edegem, Belgium lieven.dossche@uza.be

2 274 Skeletal Radiol (2012) 41: Keywords Knee. Ligaments. Articular. Anterior cruciate ligament. Knee injury. Magnetic resonance imaging Introduction The reported accuracy of magnetic resonance (MR) imaging in the diagnosis of anterior cruciate ligament (ACL) ruptures is as high as 95% [1, 2]. Most MR studies have considered complete and partial tears of this ligament [3 5], but only a few investigators have been concerned with the ability of MR imaging to allow a distinction between stable and unstable ACL tears [6]. This distinction may influence patient management and prognosis [4, 7 9]. Axial MR imaging of the ACL may provide important diagnostic information for patients who have ACL injury. According to Roychowdhury et al. [6], axial findings that were predictive of ACL insufficiency included the cloudlike mass sign, the isolated ACL bundle sign, and nonvisualization of the ACL, whereas stable ACLs were elliptical, attenuated, or showed areas of increased intrasubstance signal intensity. However, these results have not been confirmed by other authors. Furthermore, secondary signs of ACL injury, when present, have been reported to be indicators of an unstable ACL tear [10, 11]. The purpose of this study was to determine the usefulness of MR imaging to distinguish clinically stable from unstable ACL tears using previously described primary and secondary MR signs of ACL injury. Materials and methods Patients This study correlating MR imaging with arthroscopy findings was approved by the hospital ethics committee, and patient informed consent was granted. Medical records of 690 patients who underwent both MR examination and arthroscopy of the knee in our institution over a 3-year period (November 2007 to March 2010) were retrospectively reviewed. Patients included in our study met the following criteria: first, MR examination on a 1.5 or 3.0 T MR magnet; second, available medical record with the patient s relevant history (prior trauma or not) and clinical assessment of the ACL including the anterior drawer, Lachman, and/or pivot shift test; third, no history of prior knee surgery; fourth, surgically confirmed complete or partial ACL tear. After elimination of patients based on these criteria, we identified a group of 97 patients (62 male, 35 female; ages years, mean age 49 years) as the patient population of this study. The mean time interval between MR and arthroscopy was approximately 43 days (range 1 80 days). Forty-one patients were referred after sustained acute trauma, 56 patients presented with chronic knee pain and disability. MR imaging protocol In 74 patients, MR imaging was performed with a 1.5 T superconducting magnet (Magnetom Symphony TIM; Siemens Medical Systems, Erlangen, Germany) using a flexible, circular, dedicated knee coil. Imaging parameters included the following: axial and coronal frequencyselective fat-suppressed (FS) turbo spin-echo (TSE) intermediate-weighted images (WI) [TR/TE=3,470 3,590/ 28 ms, 4 mm slice thickness (ST), and turbofactor (TF) 7]; coronal SE T1-WI (TR/TE=555/15 ms, 4 mm ST); sagittal TSE proton density (PD) and T2-WI (TR/TE=3,340/21 85 ms, 3 mm ST, and TF 5). All images were acquired with a base resolution of 384 and phase resolution of 80%, fieldof-view (FOV) of 160 mm and distance factor (DF) 10%. In 23 patients, MR imaging was performed with a 3.0 T superconducting magnet (Trio TIM Magnetom; Siemens Medical Systems, Erlangen, Germany) using a dedicated send/receive eight-channel phased-array knee coil. Imaging parameters included the following: axial and coronal FS TSE intermediate WI (TR/TE=3,560 3,670/21 ms, 3 mm ST and TF 7); coronal SE T1-WI (TR/TE=620/12 ms, 3 mm ST); sagittal TSE PD and T2-WI (TR/TE=3,000/17 86 ms, 3 mm ST, and TF 5). All images were acquired with a base resolution of 384 and phase resolution of 75 80%, FOV of mm, and DF 10%. Image review and surgical correlation MR images were interpreted in consensus by two musculoskeletal radiologists in a retrospective fashion. Reviewers were informed of the age of the patient and the presence of recent knee trauma prior to the MR evaluation but were blinded to clinical ACL assessment and arthroscopic results. The ACL was assessed on the sagittal, coronal, and axial images. The following previously described primary MR signs of ACL tear were evaluated [1, 4, 12 14]: (1) abnormal ACL signal (isointensity on T1-WI and increased intrasubstance signal intensity, either focal or diffuse, on T2-WI); (2) discontinuity of the ACL, either focal or diffuse, complete or partial; (3) abnormal orientation of the ACL with horizontalization or bowing of the ACL with respect to the Blumensaat s line on the sagittal images. Furthermore, the reviewers assessed the configuration of the origin and midportion of the ACL on axial MR images using criteria described by Roychowdhury et al. [6] without reliance on sagittal or coronal images. The ACL was classified according to its primary axial imaging characteristics as follows: (4) elliptical (normal oval

3 Skeletal Radiol (2012) 41: appearance); (5) attenuated (ACL having a subjectively narrower mediolateral waist as compared with the ovoid shape); (6) hyperintense intrasubstance signal (within an otherwise continuous ligament); (7) cloudlike hyperintense mass replacing the ACL; (8) isolated ACL bundle (continuous fiber bundle in the expected course of the ACL being much smaller); (9) nonvisualized ACL (any sequential discontinuity along the course of the ACL). The following secondary MR signs of ACL injury were assessed, as described in literature [4, 12 14]: (10) bone bruises or osteochondral fractures in the lateral knee compartment, located at the mid-weight-bearing surface of the lateral femoral condyle (with or without deepened lateral femoral notch) and posterolateral tibial plateau; (11) anterior tibial translation assessed at the mid-sagittal portion of the lateral knee compartment (anterior tibial translation relative to the femur of more than 5 mm was considered positive); (12) uncovering of the undersurface of the posterior horn of the lateral meniscus measured relative to the back of the mid lateral tibial plateau; (13) hyperbuckled PCL with vertical straightening of the mid- and distal PCL fibers. Each of the primary and secondary signs was individually assessed and recorded. The reviewers were then asked to categorize ACL lesions as stable or unstable based on a comprehensive assessment of all of the primary MR findings as follows: ACLs appearing elliptical or attenuated, or showing hyperintense signal on axial images without obvious discontinuity in any anatomic plane were considered stable; obvious discontinuity in any anatomic plane, the cloudlike mass and isolated bundle sign, or nonvisualization of the ACL were all considered primary MR signs of unstable ligaments. The presence or absence of secondary findings was noted. Secondary findings were considered present if the presence of at least one secondary sign was noted. We did not make an attempt to distinguish complete from partial ACL tears, as this is frequently not possible [4]. ACL lesions were categorized as acute (MRI within 6 weeks of injury) or chronic (MRI more than 6 weeks after injury). At arthroscopy, the ACL was diagnosed as partially or completely torn. Partial tears were diagnosed according to the standard nomenclature of the American Medical Association Injury Classification System (grade 2 or incomplete ligament injury with moderate loss of function or increase in laxity) [15]. At arthroscopy, partial ACL tear was diagnosed when residual ACL fibers remained in continuity and exhibited resistance to deformation upon physical probing [4]. On the basis of the clinical examination and arthroscopic evaluation of the ACL, patients were categorized as having stable or unstable partial ACL tear: mild elongation of ACL fibers and/or laxity of the ACL during probing of the ligament with clinically stable ACL was considered a stable partial tear, frank tearing of ligament fibers seen at arthroscopy with clinically unstable ACL requiring an ACL reconstruction was considered an unstable partial tear. Our orthopedic surgeons did not specifically designate tears in the anteromedial or posterolateral bundle of the ACL as this is frequently difficult during arthroscopy [7 9, 16]. Surgically confirmed complete ACL tears were considered unstable. With arthroscopy and clinical examination as the standard of reference, sensitivity, specificity, and accuracy of MRI for the diagnosis of stable and unstable ACL tears were then calculated as follows: first, considering only primary MR signs, and second, considering both primary and secondary MR signs of ACL injury (primary stable ACL with secondary signs of ACL injury and primary unstable ACL without secondary signs of ACL injury regarded as unstable and stable ACLs, respectively). Validity parameters of each individual primary and secondary MR sign were calculated. The incidence of positive primary and secondary MR signs in stable and unstable ACL injuries was compared using Fisher s exact test Fig. 1 Axial FS intermediate-weighted MR images showing the ACL configurations of unstable ligaments (arrow), as described by Roychowdhury et al. [6]: isolated ACL bundle (a), nonvisualized ACL (b), and cloudlike mass (c)

4 276 Skeletal Radiol (2012) 41: Table 1 MR diagnoses in 97 patients with arthroscopic and clinical correlation Clinical and arthoscopic findings Stable Unstable Total Partial Partial Complete SF Secondary findings, + present, absent MRI Primary stable SF SF Primary unstable SF SF Total (statistical significance P<0.05). All analyses were performed using the Statistical Package for Social Sciences for Windows (version 17.0; SPSS, Chicago, IL, USA). Results On MR images, we diagnosed 47 ACLs as being stable (attenuated n=21; increased intrasubstance signal intensity n=15; elliptical n=11) and 50 as being unstable (cloudlike mass n=24; isolated bundle n=4; nonvisualization n=22) (Fig. 1). Thirty-six stable and 61 unstable (9 partial and 52 complete) ACL tears were diagnosed at arthroscopy. Fortyone were acute and 56 were chronic ACL tears. Secondary MR signs of ACL injury were seen in 19% (7/36) of stable and 59% (36/61) of unstable ACL lesions. Anterior tibial translation (14/36), uncovering of the posterior horn of the lateral meniscus (14/36), and hyperbuckled PCL (4/36) were only seen in unstable ACLs. Bone contusion around the lateral knee compartment was seen in both unstable (33/ 36) and stable (7/7) ACLs. MR diagnoses in 97 patients with arthroscopic and clinical correlation are summarized in Table 1. Considering only primary MR signs, sensitivity, specificity, and accuracy of MR were 77, 92, and 82%, respectively. Three acute ACL tears with cloudlike mass sign and bone marrow edema (without fractures or deepened lateral femoral notch) around the lateral knee compartment were categorized as unstable on MR, but stable partial tears were found at arthroscopy (Fig. 2). Fourteen unstable ACL lesions were missed on MR (attenuated n=11; hyperintense signal n=1; elliptical n= 2). These were chronic ACL tears with secondary findings of ACL injury (anterior tibial translation and uncovering of the posterior horn of the lateral meniscus) only present in two patients. Considering both primary and secondary MR signs, sensitivity, specificity, and accuracy of MR were 59, 81, and 67%, respectively. Of seven (acute) ACL tears that were categorized as unstable tears on MR, but proved stable (partial) tears at arthroscopy, four had bone contusion around the lateral knee compartment, but primary stable Fig. 2 A 43-year-old female patient with knee pain and disability after valgus trauma. a Axial FS intermediate-weighted image reveals cloudlike mass (large arrow) at the ACL origin. Also note sprain of the medial collateral ligament (small arrow). b Coronal FS intermediate-weighted image showing disruption of ACL fibers at its origin (large arrow). Also, extensive bone marrow edema at the lateral knee compartment (small arrow) can be seen. MR findings were interpreted as an unstable ACL tear, but stable (partial) tear was found at arthroscopy. Patient was treated conservatively and had no clinical signs of ACL insufficiency at 1 year follow up

5 Skeletal Radiol (2012) 41: Fig. 3 A 45-year-old male patient with knee pain and disability after ski trauma. a Axial FS intermediate-weighted image showing intraand periligamentous hyperintense signal without obvious discontinuity of the ACL origin (large arrow). Also note medial capsular injury (small arrow). b Coronal FS intermediate-weighted image showing bone marrow edema at the lateral knee compartment (large arrow) and medial capsular injury (small arrow). c Sagittal PD-weighted image demonstrates hyperintense ACL that runs normally along the Blumensaat s line. Large joint effusion is seen. Despite the presence of secondary MR findings suggestive of a high grade or unstable ACL tear, stable partial tear was found clinically and arthroscopically ACL configuration (Fig. 3). Two of these had associated osteochondral fracture at the posterior aspect of the lateral tibial plateau. Of 25 missed unstable ACL lesions on MR, 13 were categorized erroneously due to the absence of secondary findings despite having primary unstable ACL configurations. Parameters for validity (sensitivity, specificity, positive predictive value, negative predictive value, and accuracy) of each individual primary and secondary MR sign to diagnose unstable ACL injury are summarized in Table 2. The primary MR signs discontinuity and abnormal orientation had the highest test accuracy (79 and 87%, respectively). Other individual primary and secondary findings were less reliable. The incidence (%) of primary and secondary MR signs in stable and unstable ACL lesions is summarized in Table 3. The following MR signs were significantly more prevalent in unstable ACL tears (P<0.05): discontinuity, abnormal orientation, cloudlike mass sign, nonvisualized ACL, bone bruise, anterior tibial displacement, and uncovering of the posterior horn of the lateral meniscus (Fig. 4). Discussion The role of MR imaging in the diagnosis of partial and complete ACL tears is well established [1 5, 17 20]. However, relative few data are available in the literature Table 2 Validity data of primary and secondary MR signs for diagnosis of unstable tear of the anterior cruciate ligament PPV Positive predictive value, NPV negative predictive value, Ant. tib. translation anterior tibial translation, UPHLM uncovering posterior horn lateral meniscus, PCL posterior cruciate ligament Sensitivity Specificity PPV NPV Accuracy 1. Signal intensity Discontinuity Orientation Axial configuration 4. Ellipse Attenuated Hyperintensity Cloudlike mass Isolated bundle Nonvisualization Bone contusion Ant. tib. translation UPHLM Hyperbuckled PCL

6 278 Skeletal Radiol (2012) 41: Table 3 Incidence (%) of primary and secondary MR signs in stable and unstable tear of the anterior cruciate ligament Ant. tib. trans Anterior tibial translation, UPHLM uncovering posterior horn lateral meniscus, PCL posterior cruciate ligament, CI confidence interval Stable (%) Unstable (%) P value CI lower CI upper Odds ratio 1. Signal intensity Discontinuity < Orientation < Axial configuration 4. Ellipse Attenuated Hyperintensity < Cloudlike mass Isolated bundle Nonvisualization < Bone contusion Ant. tib. trans UPHLM Hyperbuckled PCL on the ability of MR to allow distinction between clinically stable ligaments (that are either normal or partially torn) and unstable ligaments (that are either partially or completely torn). This distinction is important because it may influence patient management and prognosis [4, 7 9]. To our knowledge, the combined value of primary and secondary MR signs of ACL injury to predict which ACLs are stable and which are unstable has not been specifically assessed in previous studies. Utilizing a comprehensive assessment of previously described primary MR signs of ACL injury, we could not correctly distinguish between stable and unstable ligaments in 17 of 97 patients (accuracy 82%). The primary MR signs discontinuity (identified in any anatomic plane) and abnormal orientation (identified in the sagittal plane) had the highest test accuracy (79 and 87%, respectively) for unstable ACL lesions. Although the cloudlike mass and nonvisualized ACL sign were significantly more prevalent in unstable ACL injury in our study, accuracy rates of the primary axial configurations to differentiate between stable and unstable ACLs were rather low. Our results are not in concordance with the findings of Roychowdhury et al. [6]. These authors successfully distinguished stable ACLs (having elliptical or attenuated appearance or showing hyperintense intrasubstance signal) from unstable ACLs (showing a cloudlike mass in place of the ACL, an isolated ACL bundle, or nonvisualized ACL) relying solely on axial MR images. In our study, most discrepancies between MR and surgery findings were seen in chronic ACL tears (14/ 17). The low accuracy of MR in the detection of chronic ACL tears was already described by other authors and is related to the presence of scar tissue complicating the MR Fig. 4 A 37-year-old male patient with chronic derangement of the knee. a Axial FS intermediate-weighted image reveals an attenuated ACL origin (arrow). Sagittal PD-weighted images showing laxity of the ACL and hyperbuckling of the PCL (b) and anterior tibial displacement greater than 5 mm (c). At arthroscopy, a chronic completely torn ACL that was scarred to the posterior cruciate ligament was found, and surgical ACL reconstruction was performed

7 Skeletal Radiol (2012) 41: imaging analysis: with scarring to the PCL or the roof of the intercondylar notch, which is most typical, the ACL may appear to have a normal course and continuity when evaluated on MR images [17, 21]. This issue was not specifically addressed in the study by Roychowdhury et al. [6] because these authors did not make an attempt to classify ACL injuries as acute or chronic. On the basis of both primary and secondary MR findings, we had 7 false positive and 25 false negative diagnoses (32/97, accuracy 67%). There were seven stable ACL tears showing bone contusion around the lateral knee compartment on MR images, four of them had primary stable ACL configuration. These lateral compartment bone contusions with intact or stable ACLs have been reported to occur typically in adolescents (<20 years), probably due to increased ligamentous laxity at that age [22]. In our study, however, the mean age of patients with lateral bone bruises and stable ACL was 36 years with only two patients being younger than 20 years. Whereas bone contusion around the lateral knee compartment was seen in both unstable and stable ACLs, the presence of anterior tibial translation, uncovering of the posterior horn of the lateral meniscus, and hyperbuckled PCL was 100% specific for an unstable ACL tear, even with primary MR signs of a stable ligament. Sensitivity and accuracy of these secondary MR signs was, however, low. In summary, our results are in concordance with the findings of Brandser et al. [14], who stated that it is the primary signs that form the basis for diagnosing the status of the ACL. Several limitations of this retrospective study have to be considered. First, MR findings were compared to arthroscopy, which should be viewed as an imperfect gold standard because direct visual inspection of ligament integrity is an inadequate indicator of the extent of failure, intrasubstance injury, and future functional capabilities of that ligament [7, 8, 14]. Furthermore, the physical examination may be inadequate in certain situations, and difficulty may arise as a result of pain, muscle spasm, and guarding [1, 8, 23]. However, in many cases, clinical assessment of the ACL was performed at the time of knee arthroscopy with the patient under anesthesia. Furthermore, of seven patients with unstable ACL tear diagnosed on MR but stable partial tear at arthroscopy, six had a clinically stable knee at 1 year follow up without requiring surgical ACL reconstruction. One patient was lost at clinical follow up. Second, findings at arthroscopy could have been biased by the availability of the MR reports introducing surgical bias and limiting the reference standard. Third, selection bias was introduced in our study because we included only those patients who later underwent surgery. Finally, consensus review by two radiologists may not reflect true clinical practice. However, it does give an idea of the best possible result attainable. In summary, the MR imaging signs tested in this study are not sufficiently accurate to determine which patients will require ACL reconstruction and which patients will not. Whereas bone bruises around the lateral knee compartment seen on MR may occur in clinically stable ACL tears, anterior tibial translation greater than 5 mm and uncovering of the posterior horn of the lateral meniscus, if present, are helpful signs in the diagnosis of an unstable tear, even if a primary stable ligament is presumed on MR. 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