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1
Sports Orthopedics and Rehabilitation, 2884 Sand Hill Rd., Ste. 110, Menlo
Park, CA 94025.
2
Present address: Vail Orthopaedics, 181 W. Meadow Dr., Ste. 800, Vail, CO
81657.
3
Department of Radiology, S-062A, Stanford University Medical Center, 300
Pasteur Dr., Stanford, CA 94305.
Received April 30, 1999;
accepted after revision August 9, 1999.
Address correspondence to D. M. Bradley.
Abstract
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SUBJECTS AND METHODS. Thirty-three MR studies of 31 patients with residual persistent extension loss after ACL reconstruction using patellar tendon autograft were reviewed and compared with results of second arthroscopy. We used MR imaging to describe the ACL graft signal intensity and course, tibial and femoral tunnel placement, quantitative measurements of notch size and shape, and the presence or absence of cyclops lesions. When a cyclops lesion was revealed on MR imaging, the signal-intensity characteristics, location, and size were documented. Preoperative MR imaging findings were then correlated with findings at arthroscopy.
RESULTS. The sensitivity, specificity, and accuracy of revealing a cyclops lesion on MR imaging were 85.0%, 84.6%, and 84.8%, respectively. We found no statistically significant differences in the size of intercondylar notches for patients with and patients without cyclops lesions.
CONCLUSION. MR imaging was sensitive, specific, and accurate in revealing cyclops lesions in a subgroup of patients with extension loss after ACL reconstruction.
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Because of its ability to define bony and softtissue detail, MR imaging can be used to examine patients with extension loss after ACL reconstruction. Studies indicate a high accuracy for MR imaging of ACL graft integrity, tunnel placement, and the size of intercondylar notches [15,16,17,18,19,20,21,22,23]. Additionally, reports describe the MR appearance of cyclops lesions [9, 19, 24, 25]. However, because of the small number of patients in these studies, it is impossible to determine the sensitivity and specificity of MR imaging for cyclops lesions. We asked a musculoskeletal radiologist and an orthopedist to interpret MR imaging of patients with residual symptoms of extension loss after ACL reconstruction. The observers were unaware of the patients' clinical and arthroscopic findings. We correlated the radiologists' interpretations with MR results at second arthroscopy, and determined the sensitivity, specificity, and accuracy of MR imaging for the diagnosis of cyclops lesions.
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A musculoskeletal radiologist, unaware of the patients' clinical and arthroscopic findings, retrospectively reviewed 33 MR examinations. MR images were obtained using a 1.5-T scanner (Signa; General Electric Medical Systems, Milwaukee, WI) (n = 11), a 0.5-T scanner (Gyroscan; Philips, Best, the Netherlands) (n = 12), and a 0.38-T scanner (RX5000; Resonex, Redwood City, CA) (n = 10). An extremity surface coil was used in all examinations. Images were obtained in the sagittal, coronal, and axial planes, with 3- to 4-mm slice thickness and 0.5- to 1-mm spacing. MR sequences included proton density- and T2-weighted images on the 0.5- and 0.38-T scanners, and spin-echo T1-weighted, fast spin-echo proton density—weighted, and fast spin-echo fat-saturated T2-weighted images on the 1.5-T scanner.
When present, cyclops lesions were examined for size in three dimensions: axial (width), sagittal (depth), and coronal (height) (Figs. 2,3,4). We noted the locations of lesions relative to the graft (anterior, medial, lateral) and the graft's signal-intensity characteristics on proton density- and T2-weighted images.
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ACL grafts were examined for MR signal-intensity characteristics, continuity, and course. For the purpose of signal-intensity examination, the intraarticular portion of the ACL graft was divided into thirds (as seen on the sagittal images): the proximal third (near the femoral tunnel), the middle third, and the distal third (at the tibial tunnel) [17, 20, 21].
The tibial tunnel location was examined in the sagittal plane. The anterior extent of the tibial tunnel was examined relative to a line extending from the roof of the femoral notch (Fig. 3). The tibial tunnel location was measured relative to the anteroposterior diameter of the tibia measured on an axial image (Fig. 5). We calculated the percentage of the distance from the anterior margin of the tibial plateau to the tibial tunnel center relative to the distance from the anterior margin to the posterior margin of the tibial plateau [15, 20, 21].
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Femoral tunnel location was examined relative to several landmarks: in the sagittal plane, the junction of the intercondylar roof and the posterior femoral cortex, in the coronal plane, and the 11-o'clock (right knee) or 1-o'clock (left knee) positions [15, 17] (Fig. 6).
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Notch dimensions were measured according to a method described by Herzog et al. [22]. We measured the width of the anterior outlet at the articular margin, at the popliteal recess, and at the superior two-thirds level of the notch in the axial (Fig. 7A,7B) and coronal (Fig. 8A,8B) planes. The width of the anterior outlet was measured at the level of the popliteal recess. If necessary, we determined the level of the popliteal recess by transferring the level from subsequent images onto the anterior outlet on MR images. Notch ratio (notch width at the popliteal recess divided by the maximum femoral condylar transverse width) was calculated. Notch height, lateral wall angle, and notch angle at the anterior outlet were measured in the axial (Fig. 7A,7B) and coronal (Fig. 8A,8B) planes [22]. If fat pad fibrosis was present, its location was described.
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During the second examination, each patient underwent a complete diagnostic arthroscopy. We paid particular attention to the course and integrity of ACL grafts. If cyclops lesions were present, their location and size were noted. We examined the location of the tibial tunnel relative to the roof and the lateral wall of the intercondylar notch. If we noted graft impingement, we tried to determine whether it was caused by an anterior placement of the tibial tunnel, insufficient notchplasty, bone formation at the notch margins after notchplasty, or the presence of a cyclops lesion. In nine patients with cyclops lesions, biopsy specimens were taken during second arthroscopy.
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Cyclops Lesion Examination
In group 1, arthroscopic findings included a well-defined nodule or mass of
fibrosis anterior to the ACL graft (Fig.
1A). When the knee was fully extended, the cyclops lesion was
adjacent to the anterior margin of the roof of the intercondylar notch.
In group 1, MR images revealed cyclops lesions in 17 of 20 examinations, yielding a sensitivity of 85.0%. The mean axial dimension of cyclops lesions was 12.7 ± 5.5 mm, the mean sagittal dimension was 11.7 ± 6.1 mm, and the mean coronal dimension was 11.6 ± 5.2 mm. The location of the cyclops lesion was anterior to the graft in 15 patients, medial to the graft in one, and lateral to the graft in one. Proton density—weighted MR images revealed intermediate signal intensity in 15 lesions and high signal intensity in two. T2-weighted images revealed intermediate signal intensity in 14 lesions, high signal intensity in one, and low signal intensity in two.
Group 2 included 13 patients without cyclops lesions. The absence of lesions was shown on MR images for 11 of 13 patients. This result yielded a specificity of 84.6% and an accuracy of 84.8%.
ACL Graft Examination
Twenty MR images for 19 patients with cyclops lesions revealed 10 grafts
with low signal intensity throughout (Fig.
3), and 10 grafts with areas of intermediate signal intensity on
proton density— or T2-weighted images. No lesions showed areas of high
signal intensity or evidence of graft tear. In all 10 patients, intermediate
signal intensity was present in the distal third of the intraarticular portion
of the ACL graft. Of these, four patients also had signal-intensity changes in
the middle third of the ACL graft and four had also signal-intensity changes
distributed over the entire graft. Five grafts had mild bowing with posterior
convexity (Fig. 9A).
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In 13 patients without cyclops lesions, MR imaging revealed nine grafts with low signal intensity throughout, and four grafts with areas of intermediate signal intensity. For patients with intermediate signal intensity, intensity was distributed over the entire graft for three patients and in the middle third of the graft for one patient. One graft appeared posteriorly bowed.
Femoral and Tibial Tunnel Locations
For the patients with cyclops lesions, the femoral tunnel (as examined on
MR imaging in the sagittal plane) entered the notch within 2 mm of the
intersection of the intercondylar roof and the posterior femoral cortex. In
the coronal plane, femoral tunnels were located between the 10:30- and
11:30-o'clock positions for right knees and between the 12:30- and
1:30-o'clock positions for left knees (Fig.
6). On MR images in the axial plane, tibial tunnels were located
20.9 ± 3.3 mm from the anterior tibial margin. The ratio of this
distance to the maximum anteroposterior diameter of the tibial plateau was
39.3% ± 5.1%. In all patients, the front of the tibial tunnel appeared
even with or posterior to a line extended along the slope of the intercondylar
roof.
In 13 patients without cyclops lesions, the femoral tunnel entered the notch within 2 mm of the intersection of the intercondylar roof and the posterior femoral cortex. In the coronal plane, femoral tunnels were located between the 10:30- and 11:30-o'clock positions for right knees and between the 12:30- and 1:30-o'clock positions for left knees. In the axial plane, tibial tunnels were centered 21.9 ± 5.7 mm from the front of the tibia. The ratio of this distance to the maximum plateau depth was 37.4% ± 7.6%. For all patients, the front of the tibial tunnel was even with or posterior to the slope of the intercondylar roof.
Other Findings
Table 1 summarizes the
measurements of the anterior outlet of the notch for patients with and without
cyclops lesions. Fat pad fibrosis was present in 17 of 20 cases with cyclops
lesions and in all cases without cyclops lesions. Its location was central in
all patients except for one patient who had fibrosis noted at the apex of the
fat pad. Additional involvement of the synovial margin was present in seven
patients, and additional involvement of the apex in six patients.
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All cyclops lesions in group 1 patients were resected. Four patients required revision notchplasty because of bony overgrowth of the notch margins. In group 2, seven patients were treated with joint manipulations, 10 with arthroscopic débridement, four with partial meniscectomy, and two with notchplasty. No graft ruptures were noted. More than one surgical procedure was performed in five patients: one patient had a recurrent cyclops lesion, and one patient had a cyclops lesion with subsequent generalized arthrofibrosis and a meniscal tear (the patient who was included in both groups). One patient had a cyclops lesion that was resected with subsequent development of arthrofibrosis, and two patients had recurrent arthrofibrosis. These three patients did not undergo repeat MR imaging, so they were not included in our study for a second time.
The specimens of cyclops lesions from nine knees were sent for histopathologic examination. Six had dense fibrosis, two had bony fragments, two had healthy synovium, and one had chronic synovitis. The chronicity of cyclops lesions did not appear to affect histopathologic diagnoses.
Four patients underwent MR imaging after cyclops lesion resection. Three of the four patients had findings conclusive for complete resection, but one revealed residual or reformed cyclops lesion at 4 months. The lesion measured 10 x 10 x 6 mm, and a second resection was performed. Three knees without lesions had resolution of intermediate signal intensity previously present in the distal intraarticular portion of the ACL graft (Fig. 9B).
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The interpretation of MR images for patients who undergo ACL reconstruction can be difficult because of the morphology and signal-intensity characteristics of ACL grafts, the presence of joint effusion and scar formation, and metal artifacts related to hardware. However, autograft ACL reconstructions have been examined for graft integrity with more than 90% sensitivity and specificity [15,16,17,18,19]. Approximately 90% of intact ACL grafts appear as continuous low-signal-intensity bands on MR imaging, and the remaining 10% show some degree of intermediate signal in the graft [15,16,17,18,19]. The early presence of high signal intensity in the graft, probably resulting from revascularization, normally resolves in 1 year [15, 17,18,19,20]. Several articles have reviewed MR imaging of graft impingement as a complication of ACL reconstruction [9, 20, 21]. Most suggest that a persistent increase in signal intensity in the distal two thirds of an ACL graft indicates the presence of impingement [20, 21]. Additionally, two small series describe the MR imaging features of cyclops lesions [9, 24].
With more patients than in previous studies, we were able to determine the sensitivity (85.0%), specificity (84.6%), and accuracy (84.8%) of MR imaging of cyclops lesions in patients with persistent symptoms after ACL reconstruction. When cyclops lesions measured more than 10 mm in at least one dimension, the sensitivity, specificity, and accuracy of MR imaging improved to 85.0%, 100.0%, and 91.0%, respectively. Additionally, our larger series allowed us to better describe the diagnostic criteria for cyclops lesions. The lesions had predominantly intermediate signal intensity on proton density— and T2-weighted MR images, were located anterior to the ACL graft, and extended anteriorly to a line along the intercondylar roof (Fig. 3).
We analyzed the arthroscopic and clinical findings of three false-negative findings on MR imaging in group 1 patients. One patient had a large diffuse area of fibrosis along the anterior aspect of the ACL graft but did not have a discrete cyclops nodular component on MR images. The second patient had a nodule originating from the tibial plateau anterior to the tibial tunnel, but the nodule was not seen along the ACL graft on MR images. The third patient had a small recurrent lesion in the same position as one that had been previously resected.
During the analysis of two false-positive findings of cyclops lesions in group 2, both lesions were found to be small, measuring 5 x 7 x 8 mm and 7 x 5 x 8 mm. Arthroscopically, neither knee had ACL graft impingement, and no discrete cyclops nodule was found. Both of the false-positive findings were in patients who had undergone previous resection of cyclops lesions. Therefore, three of the five false-positive results were in knees that had undergone previous cyclops resection.
The frequency of intermediate signal-intensity changes in the ACL grafts increased for patients with cyclops lesions. In group 1, there was an increased incidence of intermediate signal intensity located in the distal portion of the graft, and some grafts had a bowed appearance. Ten (50%) of 20 grafts with cyclops lesions had some intermediate signal intensity on proton density—weighted images, whereas only four (31%) of 13 grafts without cyclops lesions had intermediate signal intensity. However, this difference was statistically insignificant. Cyclops lesions caused impingementlike changes such as posteriorly convex bowing in some ACL grafts (Fig. 9A). These changes resolved in three of four patients whose MR images were available after cyclops resection.
The presence of primary impingement leading to graft injury and secondary formation of cyclops lesions may be the cause of increased signal intensity in ACL grafts. Therefore, it is important to examine the location of the tibial tunnel in patients with cyclops lesions. To avoid impingement, the anterior edge of the tibial tunnel must lie behind the projection of a line through the intercondylar roof in the sagittal plane with the knee fully extended [17, 20, 21]. In our patients, none of the tibial tunnels extended anterior to this projection of the intercondylar roof. Howell et al. [21] suggest the tibial tunnel should be positioned at 42% ± 2% of the total sagittal tibial plateau depth, closer to the front than the back rim. Our two groups showed no significant difference in tibial tunnel placement: 39.3% ± 5.1% (group 1) and 37.4% ± 7.6% (group 2) of the sagittal distance from the anterior tibia.
Notch size or adequacy relative to the ACL graft is important to address when examining graft impingement. Herzog et al. [22] describe the accuracy of notch dimension measurements using MR imaging in cadaver knees. Although the accuracy of this technique is not established in postoperative knees, it is useful to extrapolate for comparison purposes. Notch dimensions measured on MR images were equal in groups 1 and 2. The notch measurements of the control population in the study by Herzog et al. were the same as or smaller than those in both our groups. The larger dimensions in our groups are consistent with the fact that knees in our study underwent notchplasty at the time of ACL reconstruction. These findings indicate that the notchplasties were equally adequate in groups 1 and 2.
What is the origin of cyclops lesions? They may originate from a residual tibial ACL stump, a tibial tunnel trapdoor, infrapatellar fat pad metaplasia, intercondylar fibrosis, or the graft itself [6, 9, 10]. Research shows evolution of the cyclops lesion progressing from early fibrosis to fibrocartilage [6, 9, 10]. Fibrocartilage is consistent with the kind of regenerative tissue that forms intraarticularly because of compression or impingement. In our study, microscopic examination of biopsy specimens showed the presence of fibrosis but also of bone and synovium. Tissues that may contribute to the formation of cyclops lesions include fibrous tissue, fibrocartilage, bone, synovium, and fat from the infrapatellar fat pad. The term "anterior metaplasia" best describes the general tissue source of cyclops lesions.
Despite the presence of many postoperative changes, MR imaging was useful for examining patients with extension loss after ACL reconstruction. Cyclops lesions were detected with excellent sensitivity, specificity, and accuracy. Results were best for nodules measuring more than 10 mm in any dimension and for knees without previous cyclops resection. Because of its ability to define bony and soft-tissue detail, MR imaging provides a noninvasive method of detecting cyclops lesions and helps to identify patients who need a second arthroscopy.
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