Published online before print November 24, 2004, 10.1148/radiol.2341031302
(Radiology 2005;234:53-61.)
© RSNA, 2004
MR Imaging Evaluation of the Postoperative Knee1
Thomas R. McCauley, MD
1 From the Department of Diagnostic Imaging, Yale University School of Medicine, and Radiology Consultants, 40 Temple St, Suite 2B, New Haven, CT 06510. Received August 15, 2003; revision requested November 6; revision received December 22; accepted February 9, 2004. Address correspondence to the author (e-mail: troycemccauley@comcast.net).
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ABSTRACT
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The increased number of patients undergoing arthroscopy or surgery of the knee for sports medicine injuries is leading to increased numbers of patients who require imaging after surgery because of failure to improve, recurrent symptoms, or new injury. As in preoperative patients, magnetic resonance (MR) imaging is the most valuable imaging method for postoperative evaluation of the knee. Surgical changes increase the difficulty of diagnosis of abnormalities in the knee with MR imaging. MR arthrography with direct intraarticular injection of contrast material can help improve evaluation of the postoperative meniscus and possibly help improve evaluation of anterior cruciate ligament grafts in patients after surgery. Recognition of the normal postoperative MR imaging appearance of the structures in the knee and of abnormalities is essential to accurate MR imaging evaluation of these patients.
© RSNA, 2004
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INTRODUCTION
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There has been increasing demand for postoperative evaluation of the knee (1) because of the increased number of patients who undergo knee arthroscopy (2). Patients return for imaging because of either poor outcome after surgery or reinjury. MR imaging findings in these patients include tear or retear of the meniscus, failure of meniscal repair, tear of the native or reconstructed anterior cruciate ligament (ACL), localized anterior arthrofibrosis, articular cartilage damage, and osteonecrosis.
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IMAGING
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Postoperative MR imaging of the knee can be performed as conventional MR imaging without intraarticular contrast material or as MR arthrography. MR arthrography can be performed with direct injection of the joint with a dilute solution containing gadolinium or with indirect intravenous injection of contrast material. In my practice, direct MR arthrography is performed in almost all patients who have undergone prior meniscal surgery or ACL graft reconstruction, because the distention of the joint with intraarticular contrast material improves evaluation of the meniscus (3–6) and may improve evaluation of ACL graft integrity (7). In patients who have not undergone meniscal or ACL surgery or in whom the site of symptoms is separate from the site of prior surgery, we sometimes will perform conventional MR imaging without injection of contrast material.
Direct MR arthrography is performed with injection of a dilute mixture containing gadolinium. A dilution ranging from 1:250 to 1:100 has been shown to provide high-quality images (3–8). Precontrast images are generally not needed (3,4). Direct MR arthrography is preferred to indirect MR arthrography because of the distention of the joint achieved with direct injection, which is not achieved with indirect arthrography (3–5). MR imaging is performed in all three planes with a combination of short and long echo time conventional spin-echo (SE) and fast SE sequences. I currently use a phased-array extremity coil and a 1.5-T magnet and obtain all images with a 20.8-kHz bandwidth. Transverse fat-suppressed intermediate-weighted fast SE images are obtained with 3200/24 (repetition time msec/echo time msec), echo train length of 11, 5-mm section thickness with 1-mm intersection gap, 15-cm field of view, 256 x 256 matrix, and one signal acquired. Sagittal T1-weighted SE images are obtained with 800–950/14, 3-mm section thickness with 0.3-mm gap, 16-cm field of view, 384 x 256 matrix, and one signal acquired. Coronal fat-suppressed intermediate-weighted fast SE images are obtained with 2400/14, echo train length of five, 3.5-mm section thickness with 0.5-mm gap, 16-cm field of view, 256 x 224 matrix, and two signals acquired. Sagittal fat-suppressed T2-weighted fast SE images are obtained with 4000/70, echo train length of 10, 3.5-mm section thickness with 0.5-mm gap, 14-cm field of view, 320 x 256 matrix, and two signals acquired.
Metal rarely causes artifacts sufficient to prevent adequate imaging. Use of nonferromagnetic metals such as titanium has decreased the amount of artifact we see in the postoperative knee (9). Metal implants rarely affect interpretation because they are usually located far enough from the structures of interest. For the rare cases where metal artifact is severe, a number of techniques can be used to decrease artifact. The use of fast SE imaging, an increase in bandwidth (maximum available depends on the magnet), a decrease in the field of view, an increase the frequency-encoding dimension of the matrix, and a decrease in the echo time all reduce the size of the artifact (9,10). The artifact can be minimized by positioning the knee such that any screws are as close to parallel to the main magnetic field as possible. The artifact from metal is usually larger in the frequency-encoding direction; thus, phase encoding in the superior-to-inferior direction for coronal and sagittal sequences usually allows best demonstration of the tibiofemoral joint (9). If there is marked metal artifact, gradient-echo imaging and fat suppression are not used because of their sensitivity to magnetic field inhomogeneity (10). Pulse sequences that minimize metal artifact (11) have been developed but are not yet widely available.
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POSTOPERATIVE MENISCI
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As with preoperative MR imaging evaluation of the knee, the most common abnormalities in patients who have undergone prior knee surgery are in the menisci. The meniscus used to be completely resected; unfortunately, this was found to lead to early osteoarthritis, likely due to the loss of the function of the meniscus to distribute forces evenly between the femur and tibia at the tibiofemoral joint (12–14). Orthopedic surgeons now try to maintain as much meniscal tissue as possible by performing either meniscal repair or partial meniscectomy with resection only of the unstable portions of the meniscus at the tear (14,15). The outer meniscal rim is believed to be the most important portion that needs to be preserved (12,13). This is because the collagen fibers in the outer third of the meniscus are predominantly circumferential; these fibers maintain the hoop stresses in the meniscus and transmit most of the axial load from the femur to the tibia (12,13). Meniscal repair preserves the entire meniscus; however, it is associated with a higher short-term complication rate than is partial meniscectomy. Meniscal repair is associated with the best outcome for linear, vertical, or oblique tears in the periphery of the meniscus (likely due to the greater vascularity in the peripheral meniscus) and is generally reserved for younger patients (13,15,16). Meniscal repair is less commonly performed than is partial meniscectomy (performed in 22% of patients with meniscal tear at arthroscopy in one study [17]) because of the limited group of patients who are eligible, the higher short-term morbidity, and the longer postoperative recovery time (14–16).
When patients have poor outcome or reinjury, they are often referred for MR imaging. Patients can retear a previously repaired meniscus or partially resected meniscus or can have a new tear of a portion of the meniscus separate from the site of surgery. Unfortunately, the accuracy of MR imaging for evaluation of tears in repaired or partially resected menisci is lower than that in menisci prior to surgery (3,18–22), which has prompted some orthopedic surgeons to use diagnostic arthroscopy rather than MR imaging in patients who have undergone prior meniscal surgery (16). Diagnostic arthroscopy has a 1.4% (six of 433) major complication rate, with possible complications that include nerve damage, reflex sympathetic dystrophy, infection, hemarthrosis, adhesions, deep venous thrombosis, and instrument breakage (23). Thus, it would be desirable to use a less risky diagnostic technique.
The decreased accuracy of conventional MR imaging after meniscal surgery occurs because postoperative changes in the meniscus may mimic or obscure tears (3,18–22). Studies on meniscal repair (18–20) have shown that linear increased signal intensity extending to the surface can persist at the site of surgery for at least 1 year after repair (Fig 1). Thus, diagnosis of meniscal tear by using the usual criterion of linear increased signal intensity extending to the surface on conventional short echo time MR images may lead to a false-positive diagnosis in patients after meniscal repair. Use of the stricter criterion of fluid signal intensity within a linear defect in the meniscus on T2-weighted images (Fig 2) has been shown to provide high specificity (88%–92%) but low sensitivity (41%–69%) for tears (3,20,21).
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Figure 1. Sagittal T1-weighted (600/15) MR arthrogram shows meniscal scar (arrow) 2 years after repair of bucket handle tear in a 29-year-old woman. Meniscus was intact at surgery. Meniscal scars mimic meniscal tears on short echo time MR images obtained without intraarticular contrast material; with intraarticular contrast material, however, the scar can be differentiated from tear because signal intensity is lower than that of intraarticular gadolinium-based contrast material (arrowhead).
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Figure 2. Sagittal T2-weighted SE (2000/80) MR image obtained without intraarticular contrast material shows retear of repaired meniscus in a 48-year-old man 8 years after meniscal repair. Fluid can be seen in the tear (arrow). When fluid signal intensity extends into the meniscus on T2-weighted MR images, there is almost always a tear present; however, this sign for diagnosis of tear has low sensitivity.
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Thus, the use of this stricter criterion with conventional MR imaging will result in fewer false-positive diagnoses; however, many tears will be missed. Identification of displaced meniscal fragments allows detection of tears with high confidence; however, displaced fragments are only seen in the minority of tears. Postoperative changes may not only mimic meniscal tears but can also obscure meniscal tears at conventional MR imaging (3,20–22).
For patients who have undergone partial meniscectomy, the accuracy of conventional MR imaging for detection of a tear is 66%–82% (3–5,21). Studies have shown accuracy similar to that found for evaluation of preoperative menisci in cases when only a small resection is performed (3,6,22), with a small resection defined as removal of less than 25% of the meniscus (3,6) or removal of less than one-third with no contour irregularity (22). Unfortunately, most resections are not small (3,6,22). In a recent study by Magee et al (6), MR arthrography, rather than conventional MR imaging, was required for assessment of meniscal tear in just fewer than one-third (32 of 100) of patients. In that study, the surgical reports and preoperative MR images were available for all patients. In my practice, the surgical reports are usually not available, and the degree of resection is usually not evident on these reports. The preoperative MR images are often not available. Thus, I usually cannot determine if a given patient will require MR arthrography prior to imaging, and this is why we perform MR arthrography in almost all patients with a history of meniscal surgery.
MR arthrography can improve the accuracy for detection of a meniscal tear in the postoperative knee, according to the results of three studies by Applegate et al (3), Sciulli et al (4), and Magee et al (6). The criteria used for diagnosis of a tear in these studies were (a) signal intensity similar to that of gadolinium on T1-weighted images (Fig 3) or to that of fluid on T2-weighted images that extends into the meniscus or (b) identification of displaced meniscal fragments.
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Figure 3a. Retear of previously repaired meniscus on sagittal fat-suppressed T1-weighted MR arthrograms in a 35-year-old man. (a) Image (800/14) obtained after meniscal repair shows gadolinium signal intensity extending into the meniscus, which is indicative of tear (arrow). (b) Image (600/14) obtained after subsequent partial meniscectomy for the retorn meniscus demonstrates mild deformity (arrow) of residual meniscal fragment, which is a normal postoperative appearance.
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Figure 3b. Retear of previously repaired meniscus on sagittal fat-suppressed T1-weighted MR arthrograms in a 35-year-old man. (a) Image (800/14) obtained after meniscal repair shows gadolinium signal intensity extending into the meniscus, which is indicative of tear (arrow). (b) Image (600/14) obtained after subsequent partial meniscectomy for the retorn meniscus demonstrates mild deformity (arrow) of residual meniscal fragment, which is a normal postoperative appearance.
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The study by Applegate et al (3) with both conventional MR imaging and MR arthrography performed in each patient showed that the accuracy for identification of postoperative meniscal tear was 88% (36 of 41) for MR arthrography as compared with 66% (27 of 41) for conventional MR imaging. Sciulli et al (4) compared four imaging techniques in each patient and showed that gadolinium-enhanced MR arthrography was the most accurate (92%, 12 of 13), followed by conventional MR imaging (77%, 10 of 13), MR arthrography with saline injection (75%, nine of 12), and conventional arthrography (58%, seven of 12). Magee et al (6) compared conventional MR imaging with MR arthrography in each patient and found that conventional MR imaging had 52% (13 of 25) sensitivity for detection of tears in patients with resection of greater than 25% of the meniscus, while MR arthrography had 100% (25 of 25) sensitivity. Magee et al also concluded that all patients with meniscal repair require MR arthrography because all 16 of their patients with meniscal repair showed no difference in the conventional MR appearance between their postoperative MR images and their preoperative MR images.
White et al (5) compared groups of patients who were imaged with either conventional MR arthrography, indirect MR arthrography, or direct MR arthrography. The sensitivity and specificity, respectively, for direct MR arthrography were 89% (17 of 19) and 78% (seven of nine), which were higher than the 86% (25 of 29) and 67% (10 of 15) found for conventional MR imaging; however, this difference was not statistically significant. Indirect arthrography in this study had a sensitivity of 83% (19 of 23) and a specificity of 78% (seven of nine), which also was not significantly different from the other two groups. White et al (5) compared randomized patient groups rather than perform conventional MR imaging and MR arthrography in each patient, as was done in the other cited studies (3,4,6). Such a design would have required larger percentage differences or larger patient groups to reach statistical significance than would a design where each patient could serve as his or her own control. Unfortunately, statistical tests were not performed in the studies by Applegate et al (3), Sciulli et al (4), and Magee et al (6) to prove that the higher accuracy of MR arthrography compared with that of conventional MR imaging was statistically significant. Despite these limitations, the results of these studies considered together indicate that MR arthrography likely is more accurate than conventional MR imaging for evaluation of menisci after partial resections of greater than 25% and after meniscal repair.
When examining the postoperative meniscus at MR arthrography, one should attempt to identify the site of prior surgery because standard criteria can be used for accurate assessment of the unrepaired portions of the meniscus. At the sites of prior partial meniscectomy or repair, a tear should be diagnosed only if gadolinium-based contrast material enters the meniscus or if there is a displaced fragment (3–6). If there is a small apparent tear (less than one-third the height and width of the meniscus [24]) at the site of prior surgery, the size of the tear should be described because small portions of tears are sometimes left in place at partial meniscectomy, and small stable tears do not necessarily require reoperation (13,15,24). In these patients, the decision to reoperate is made by the surgeon on the basis of clinical symptoms.
Meniscal transplantation is occasionally performed, most commonly in patients with symptomatic degenerative changes in a meniscus-deficient compartment of the knee (25–27). MR imaging well demonstrates the appearance and location of the allograft (25–27); unfortunately, the MR imaging appearance has not been shown to be predictive of clinical outcome (25,28). Thus, the role of MR imaging in the clinical evaluation of these patients is currently uncertain.
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POSTOPERATIVE ACL
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Tears of the ACL cause instability, which can lead to meniscal tear and articular cartilage damage (29–31). This has led surgeons to recommend reconstruction of ACL tears in most patients, especially in young patients who wish to return to their preinjury level of activity (32). ACL tears are reconstructed rather than repaired because of the poor outcomes associated with repair (33,34). Synthetic reconstructions have not performed as well as tendon reconstructions; thus, most surgeons reconstruct by using portions of other tendons from the patient, including patellar, quadriceps, iliotibial band, semitendinosus, or gracilis tendons. Most grafts are now placed intraarticularly with tibial and femoral tunnels.
MR imaging can be used to demonstrate graft failure, graft placement, impingement, and arthrofibrosis, as well as other causes of poor outcome. The appearance of the graft varies with the type of graft used and with the time after graft placement (32,35–37). With patellar grafts, increased signal intensity in the graft may be seen for 1–2 years after graft placement, likely owing to vascular ingrowth (35–37). After 2 years, the graft should be of uniformly low signal intensity on images obtained with all routinely used sequences (Fig 4).
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Figure 4. Sagittal T1-weighted SE (600/14) MR arthrogram in a 43-year-old man shows normal placement and signal intensity of ACL reconstruction performed 10 years earlier. Graft has low signal intensity (arrow), and its insertion in the tibia is located posterior to a line drawn along the roof of the intercondylar notch (oblique line). Graft origin from the femoral tunnel should be posterior to a line drawn along the posterior cortex of the femoral shaft (vertical line).
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When examining MR images of patients with an ACL graft, note should be made of tunnel placement and graft position (38,39). The anterior margin of the graft tunnel in the tibia should be behind a line drawn along the roof of the femoral notch, and the center of the graft tunnel should be one-quarter to one-half the distance from the anterior to the posterior tibial cortex (38,39). If it is too far forward, roof impingement can occur, which limits extension. If it is too far back, instability can occur (38). The tunnel in the femur should be at or behind a point formed by a line drawn along the posterior femoral cortex and a line drawn along the roof of the intercondylar notch. If this is too anteriorly placed, the knee can be unstable or have decreased range of motion (38).
Studies on conventional MR imaging for detection of tears of an ACL graft reconstruction have been limited to small groups of patients with arthroscopic correlation (35,40). One study (35) showed 100% accuracy for a group of 12 patients with arthroscopic correlation, with two cases of graft tear and 10 intact grafts. A study with 16 patients (40), four of whom had a full-thickness graft tear, showed 50% sensitivity and 100% specificity for detection of tear. There has been only one study (7) of which I am aware on MR arthrography for graft tear, and that study included 27 patients with arthroscopic correlation, nine of whom had graft tear. In that study, the sensitivity for graft tear was 100% for all readers, with specificity of 89%–100%. ACL graft tear was diagnosed in that study when the graft fibers could not be identified as extending from the femoral tunnel to the tibial tunnel, especially when gadolinium-based contrast material extended through a discontinuity in the graft fibers.
Detection of graft failure on MR images can also be assisted by looking for signs of dysfunction of the ACL graft, which manifest as anterior displacement of the tibia with respect to the femur (Fig 5). The anterior displacement can cause buckling of the posterior cruciate ligament and relative posterior displacement of the posterior horn of the lateral meniscus relative to the tibial plateau (41,42). Anterior displacement of the tibia can also be measured by drawing a vertical line that is tangential to the posterior cortex of the lateral femoral condyle and by measuring the distance from this line to the posterior cortex of the lateral tibial plateau. This measurement is normally less than 5 mm, with greater than 7 mm indicating abnormality and 5–7 mm as an equivocal finding (42). This measurement was abnormal in eight of nine patients with graft tear in the study on MR arthrography (7); thus, this measurement can be a helpful secondary sign for detection of graft tear.
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Figure 5a. Sagittal fat-suppressed T1-weighted SE (500/14) MR arthrograms show torn ACL graft in a 19-year-old male football player with pain. (a) The proximal and distal portions (arrowheads) of the graft are seen, with discontinuity in the graft traversed by intraarticular contrast material (arrow). (b) Image through the lateral compartment shows that posterior cortex of lateral tibial plateau (arrow) is more than 7 mm anterior to vertical line drawn along the posterior cortex of the femur (arrowhead). This finding is consistent with insufficient ACL graft due to the tear.
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Figure 5b. Sagittal fat-suppressed T1-weighted SE (500/14) MR arthrograms show torn ACL graft in a 19-year-old male football player with pain. (a) The proximal and distal portions (arrowheads) of the graft are seen, with discontinuity in the graft traversed by intraarticular contrast material (arrow). (b) Image through the lateral compartment shows that posterior cortex of lateral tibial plateau (arrow) is more than 7 mm anterior to vertical line drawn along the posterior cortex of the femur (arrowhead). This finding is consistent with insufficient ACL graft due to the tear.
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Laxity can be seen even in an intact graft. Thus, when there are signs of anterior displacement of the tibia with respect to the femur on MR images, the referring orthopedic surgeon should be alerted to the possibility of graft insufficiency. In these cases, the degree of instability at physical examination determines the need for graft revision (Fig 6).
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Figure 6a. Intact ACL graft that was insufficient in a 44-year-old woman 1 year after surgery. (a) Sagittal fat-suppressed T1-weighted SE (600/14) MR arthrogram shows normal appearance and position of graft (arrow). (b) Sagittal T1-weighted SE (600/14) MR image shows anterior translation of tibia with respect to femur, with posterior cortex of lateral tibial plateau (arrowhead) more than 7 mm anterior to a line drawn vertically tangent to posterior cortex of the lateral femoral condyle. Fibrosis in infrapatellar fat pad is from arthroscopic portal (arrow). The patient had instability but a firm end point at examination. This graft required revision.
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Figure 6b. Intact ACL graft that was insufficient in a 44-year-old woman 1 year after surgery. (a) Sagittal fat-suppressed T1-weighted SE (600/14) MR arthrogram shows normal appearance and position of graft (arrow). (b) Sagittal T1-weighted SE (600/14) MR image shows anterior translation of tibia with respect to femur, with posterior cortex of lateral tibial plateau (arrowhead) more than 7 mm anterior to a line drawn vertically tangent to posterior cortex of the lateral femoral condyle. Fibrosis in infrapatellar fat pad is from arthroscopic portal (arrow). The patient had instability but a firm end point at examination. This graft required revision.
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A postoperative complication can occur when nodular fibrous tissue forms anterior to the ACL graft above the tibial plateau (43,44). This fibrosis is called localized anterior arthrofibrosis, or cyclops lesion (so named because of the lesions resemblance to an eyeball at arthroscopy). The cause of this abnormality is uncertain. Possible theories include formation from residual native ACL, formation from tissues lifted during drilling of the tibial tunnel, or formation from the ACL graft, possibly as a result of graft impingement. The cyclops lesion can cause inability to fully extend the knee or pain at extension. Symptoms are relieved with surgical resection. This lesion usually appears on MR images as a low- to intermediate-signal-intensity structure anterior to the ACL graft on images obtained with all sequences (Fig 7) (43,44). Fibrous lesions usually have low signal intensity on MR images, but cyclops lesions have higher signal intensity than is typically seen in fibrous lesions, likely due to irritation of the lesion because it is pinched between the two bones. Conventional MR imaging was shown to be 85% accurate for detection of localized anterior arthrofibrosis in one study (44). That study showed higher accuracy for detection of lesions that are greater that 1 cm in at least one dimension.
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Figure 7a. Cyclops lesion in a 26-year-old man with decreased range of motion after ACL reconstruction. Sagittal (a) intermediate-weighted SE (1800/14) and (b) T2-weighted SE (1800/80) MR arthrograms shows cyclops lesion (arrow) anterior to ACL graft (arrowhead). Note that cyclops lesion has intermediate signal intensity even though it is a fibrous lesion.
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Figure 7b. Cyclops lesion in a 26-year-old man with decreased range of motion after ACL reconstruction. Sagittal (a) intermediate-weighted SE (1800/14) and (b) T2-weighted SE (1800/80) MR arthrograms shows cyclops lesion (arrow) anterior to ACL graft (arrowhead). Note that cyclops lesion has intermediate signal intensity even though it is a fibrous lesion.
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Graft impingement is a complication that can occur with an ACL graft when the graft abuts the roof or wall of the intercondylar notch. This complication is associated with anterior placement of the tibial tunnel, osteophytes at the margins of the intercondylar notch, and small intercondylar notch. Patients may present with pain and/or inability to fully extend the knee. Increased signal intensity may be seen in the graft owing to impingement; however, increased signal intensity, attributed to vascular ingrowth (35–37), can be seen in normal grafts from 3 months to 2 years after graft placement.
No large studies of MR imaging for detection of impingement have been performed; thus, there are no well-defined criteria for diagnosis. One of the difficulties in the diagnosis of impingement by using MR imaging is that there are no uniform criteria at arthroscopy for diagnosis to define a reproducible standard of reference. In one study on MR arthrography (7), impingement was diagnosed when the graft contained increased signal intensity or was enlarged and was associated with either placement of the tibial tunnel anterior to a line drawn along the roof of the femoral notch or deformation of the superior surface of the graft by the roof of the femoral notch (Fig 8). Unfortunately, the use of these criteria resulted in high interobserver variability, with sensitivity of 33%–89%, specificity of 52%–100%, and accuracy of 59%–96% for diagnosis among three reviewers (7). When findings of impingement are seen at MR imaging the diagnosis can be suggested, but the limitations of MR imaging for facilitating this diagnosis should be kept in mind.
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Figure 8. ACL graft impingement in a 26-year-old man 2 years after ACL reconstruction. Sagittal T1-weighted SE (600/14) MR arthrogram shows increased signal intensity in graft (long arrow). Spur (arrowhead) at anterior margin of intercondylar notch deforms the superior surface of the graft, which bulges (short arrow) anterior to the spur.
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ACL graft ganglia are a rare complication that can cause pain. Graft ganglia appear as cyst formation in or extending from the graft tunnel (Fig 9) (32).
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Figure 9a. Presumed ACL graft ganglion in tibial tunnel in a 26-year-old woman with pain after multiple ACL graft revisions, with the last revision performed with a portion of the Achilles tendon. (a) Sagittal T1-weighted SE (500/14) MR image obtained without intraarticular contrast material shows expansion of ACL graft to fill an expanded tibial tunnel (arrowheads). Graft in intercondylar notch shows abnormal signal intensity that is higher than normal (arrow). (b) Sagittal T2-weighted fast SE (2000/84) MR image obtained without intraarticular contrast material shows cysts (arrowheads) as high-signal-intensity structures in expanded graft within tunnel, with noncystic portion of graft in the intercondylar notch having low signal intensity (arrow). Graft tunnel and cysts had expanded when compared with earlier MR findings (not shown). Patient was not treated surgically after this MR examination because of a history of multiple surgeries.
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Figure 9b. Presumed ACL graft ganglion in tibial tunnel in a 26-year-old woman with pain after multiple ACL graft revisions, with the last revision performed with a portion of the Achilles tendon. (a) Sagittal T1-weighted SE (500/14) MR image obtained without intraarticular contrast material shows expansion of ACL graft to fill an expanded tibial tunnel (arrowheads). Graft in intercondylar notch shows abnormal signal intensity that is higher than normal (arrow). (b) Sagittal T2-weighted fast SE (2000/84) MR image obtained without intraarticular contrast material shows cysts (arrowheads) as high-signal-intensity structures in expanded graft within tunnel, with noncystic portion of graft in the intercondylar notch having low signal intensity (arrow). Graft tunnel and cysts had expanded when compared with earlier MR findings (not shown). Patient was not treated surgically after this MR examination because of a history of multiple surgeries.
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ARTICULAR CARTILAGE DAMAGE AND OSTEOARTHRITIS
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Articular cartilage damage and osteoarthritis are the major long-term concerns after arthroscopic treatment for meniscal injury or ACL injury (14,31). Both ACL tears and meniscal tears, with or without treatment, can result in articular cartilage damage (29,30,45–48) (Fig 10). Partial meniscectomy and meniscal repair have replaced complete meniscectomy to decrease the rate of development of osteoarthritis after meniscal surgery (12,14). Articular cartilage thus should be assessed when imaging the knee in patients who have undergone prior surgery.
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Figure 10a. Articular cartilage defect and loose body in a 54-year-old man with prior medial meniscal repair. (a) Coronal intermediate-weighted SE (2000/20) MR arthrogram shows defect (arrow) in articular cartilage of the medial femoral condyle. (b) Coronal T2-weighted SE (2000/80) MR arthrogram shows defect (arrow), which is more clearly seen than on a because of increased contrast between high-signal-intensity fluid and low-signal-intensity articular cartilage. (c) Coronal T2-weighted SE (2000/80) MR arthrogram at posterior joint shows loose body (arrow), which required arthroscopic removal. Loose bodies often grow after detachment, which likely explains the lobular contours and the fact that the loose body is much larger than the defect in articular cartilage. Note that increased signal intensity in a (arrowhead) in the body of the meniscus extends to the articular surface but is not as high as gadolinium signal intensity and is not seen on b. Meniscus was not torn at surgery.
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Figure 10b. Articular cartilage defect and loose body in a 54-year-old man with prior medial meniscal repair. (a) Coronal intermediate-weighted SE (2000/20) MR arthrogram shows defect (arrow) in articular cartilage of the medial femoral condyle. (b) Coronal T2-weighted SE (2000/80) MR arthrogram shows defect (arrow), which is more clearly seen than on a because of increased contrast between high-signal-intensity fluid and low-signal-intensity articular cartilage. (c) Coronal T2-weighted SE (2000/80) MR arthrogram at posterior joint shows loose body (arrow), which required arthroscopic removal. Loose bodies often grow after detachment, which likely explains the lobular contours and the fact that the loose body is much larger than the defect in articular cartilage. Note that increased signal intensity in a (arrowhead) in the body of the meniscus extends to the articular surface but is not as high as gadolinium signal intensity and is not seen on b. Meniscus was not torn at surgery.
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Figure 10c. Articular cartilage defect and loose body in a 54-year-old man with prior medial meniscal repair. (a) Coronal intermediate-weighted SE (2000/20) MR arthrogram shows defect (arrow) in articular cartilage of the medial femoral condyle. (b) Coronal T2-weighted SE (2000/80) MR arthrogram shows defect (arrow), which is more clearly seen than on a because of increased contrast between high-signal-intensity fluid and low-signal-intensity articular cartilage. (c) Coronal T2-weighted SE (2000/80) MR arthrogram at posterior joint shows loose body (arrow), which required arthroscopic removal. Loose bodies often grow after detachment, which likely explains the lobular contours and the fact that the loose body is much larger than the defect in articular cartilage. Note that increased signal intensity in a (arrowhead) in the body of the meniscus extends to the articular surface but is not as high as gadolinium signal intensity and is not seen on b. Meniscus was not torn at surgery.
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In my experience, fat-suppressed fast SE T2-weighted images are the most helpful for detection of cartilage defects when performing MR arthrography with gadolinium enhancement because the low-signal-intensity articular cartilage has markedly lower signal intensity than does high-signal-intensity fluid. T1-weighted images show lower contrast between fluid and cartilage because of the relatively higher signal intensity of articular cartilage on T1-weighted images compared with that on T2-weighted images. Articular cartilage damage is recognized at MR arthrography when gadolinium-based contrast material enters a defect. Although articular cartilage defects and osteoarthritis in the postoperative knee are rarely treated, identification of articular cartilage damage can explain symptoms and, thus, may obviate diagnostic arthroscopy.
Loose bodies are occasionally seen after surgery, and gadolinium-enhanced MR arthrography is the most sensitive imaging technique for detection of loose bodies in the knee (49). Most loose bodies are removed arthroscopically to eliminate symptoms of pain and locking and to prevent further articular cartilage damage due to the presence of the loose body (Fig 10).
Treatment of articular cartilage defects is becoming more common with increased interest in autologous articular cartilage transplants, which allow repair of defects with hyaline or hyaline-like cartilage (50,51). Two increasingly often used techniques are autologous osteochondral transplantation and autologous chondrocyte transplantation (50,51).
Autologous osteochondral transplantation is performed by removing osteochondral plugs from non–weight-bearing margins of the joint (eg, the margins of the femoral articular surface at the inferior portion of the patellofemoral joint) and placing these plugs in holes drilled in the cartilage defects at the weight-bearing joint surfaces. Autologous chondrocyte transplantation is performed by harvesting chondrocytes from non–weight-bearing portions of the joint, growing them in culture for 4–6 weeks, and then placing them into the cartilage defect with a covering made from periosteum.
MR imaging can be used to image these cartilage transplants with or without intraarticular or intravenous contrast material (50,51). I most commonly use a fast SE or a fat-suppressed three-dimensional spoiled gradient-echo sequence without intraarticular contrast material (51). MR imaging can be used to assess the appearance of the grafts, including bone healing in osteochondral grafts, congruence of the articular surface, graft incorporation, graft hypertrophy, graft collapse, and graft separation (51) (Fig 11).
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Figure 11a. Osteochondral transplant 1 year after surgery for defect in lateral femoral condyle in a 16-year-old girl. (a) Sagittal fat-suppressed three-dimensional spoiled gradient-echo (40/5, 40° flip angle) MR image obtained without intraarticular contrast material shows slightly depressed articular surface with slight step-off (arrow); however, the plug appears solidly incorporated, with margins not evident on this image. (b) Sagittal intermediate-weighted fast SE (3833/30) MR image obtained at same location as a does not as clearly show step-off (large arrow) at the articular surface but more clearly shows incorporation of bone plug (arrowheads) and marrow fat that now fills distal end of the marrow defect at the donor site (small arrow).
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Figure 11b. Osteochondral transplant 1 year after surgery for defect in lateral femoral condyle in a 16-year-old girl. (a) Sagittal fat-suppressed three-dimensional spoiled gradient-echo (40/5, 40° flip angle) MR image obtained without intraarticular contrast material shows slightly depressed articular surface with slight step-off (arrow); however, the plug appears solidly incorporated, with margins not evident on this image. (b) Sagittal intermediate-weighted fast SE (3833/30) MR image obtained at same location as a does not as clearly show step-off (large arrow) at the articular surface but more clearly shows incorporation of bone plug (arrowheads) and marrow fat that now fills distal end of the marrow defect at the donor site (small arrow).
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OSTEONECROSIS
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A potentially debilitating complication of knee arthroscopy is osteonecrosis (52). In a recent study (6), osteonecrosis was seen in nine of 100 patients who required reimaging because of new or persistent pain after meniscal surgery. Usually no evidence of osteonecrosis is seen on preoperative MR images. Osteonecrosis is believed to occur because of abnormal chondral and subchondral stresses due to the changes in biomechanics after meniscal surgery (52). Patients with preexisting articular cartilage abnormalities adjacent to the site of meniscal surgery appear to be at greatest risk. In severe cases, osteonecrosis can result in collapse of the articular surface (Fig 12). At MR imaging, the typical appearance of osteonecrosis is subcortical low signal intensity on T1-weighted images with or without central high signal intensity on T2-weighted images (52).
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Figure 12a. Osteonecrosis 6 months after meniscal repair in a 56-year-old woman. Moderate degeneration of articular cartilage adjacent to the tear was found at surgery. (a) Sagittal T1-weighted SE (600/14) MR image obtained without intraarticular contrast material shows meniscal tear (arrow) with normal appearance of medial condyle. (b) Sagittal T1-weighted SE (600/14) MR image obtained without intraarticular contrast material 6 months after surgery shows low-signal-intensity band (arrow) in subcortical marrow, with mild deformity of cortex due to partial collapse. (c) Sagittal fat-suppressed T2-weighted fast SE (3000/65) MR image at same location as b shows high signal intensity (arrow) in subcortical marrow with mild cortical collapse, which was better seen on coronal images (not shown).
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Figure 12b. Osteonecrosis 6 months after meniscal repair in a 56-year-old woman. Moderate degeneration of articular cartilage adjacent to the tear was found at surgery. (a) Sagittal T1-weighted SE (600/14) MR image obtained without intraarticular contrast material shows meniscal tear (arrow) with normal appearance of medial condyle. (b) Sagittal T1-weighted SE (600/14) MR image obtained without intraarticular contrast material 6 months after surgery shows low-signal-intensity band (arrow) in subcortical marrow, with mild deformity of cortex due to partial collapse. (c) Sagittal fat-suppressed T2-weighted fast SE (3000/65) MR image at same location as b shows high signal intensity (arrow) in subcortical marrow with mild cortical collapse, which was better seen on coronal images (not shown).
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Figure 12c. Osteonecrosis 6 months after meniscal repair in a 56-year-old woman. Moderate degeneration of articular cartilage adjacent to the tear was found at surgery. (a) Sagittal T1-weighted SE (600/14) MR image obtained without intraarticular contrast material shows meniscal tear (arrow) with normal appearance of medial condyle. (b) Sagittal T1-weighted SE (600/14) MR image obtained without intraarticular contrast material 6 months after surgery shows low-signal-intensity band (arrow) in subcortical marrow, with mild deformity of cortex due to partial collapse. (c) Sagittal fat-suppressed T2-weighted fast SE (3000/65) MR image at same location as b shows high signal intensity (arrow) in subcortical marrow with mild cortical collapse, which was better seen on coronal images (not shown).
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HARDWARE MALPOSITION
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Occasionally, the hardware placed at surgery will fail or become dislodged or displaced. Attention should be directed to the interference screws, pins, bone tunnels, and bone plugs to detect these rare complications (Fig 13).
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Figure 13a. Bone plug dislodgement causing graft failure in a 34-year-old woman. (a) Sagittal fat-suppressed T1-weighed (733/14) MR arthrogram shows that bone plug (arrow) at distal end of graft is in the interchondylar notch, displaced from its normal position (arrowhead) in the tibial tunnel. (b) Lateral radiograph obtained 1 year prior to a shows bone plug (arrow) in the tibial tunnel adjacent to interference screw, which was placed to wedge the plug in the tunnel. (c) Lateral radiograph obtained after a confirms that bone plug (arrow) has become dislodged from the tunnel.
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Figure 13b. Bone plug dislodgement causing graft failure in a 34-year-old woman. (a) Sagittal fat-suppressed T1-weighed (733/14) MR arthrogram shows that bone plug (arrow) at distal end of graft is in the interchondylar notch, displaced from its normal position (arrowhead) in the tibial tunnel. (b) Lateral radiograph obtained 1 year prior to a shows bone plug (arrow) in the tibial tunnel adjacent to interference screw, which was placed to wedge the plug in the tunnel. (c) Lateral radiograph obtained after a confirms that bone plug (arrow) has become dislodged from the tunnel.
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Figure 13c. Bone plug dislodgement causing graft failure in a 34-year-old woman. (a) Sagittal fat-suppressed T1-weighed (733/14) MR arthrogram shows that bone plug (arrow) at distal end of graft is in the interchondylar notch, displaced from its normal position (arrowhead) in the tibial tunnel. (b) Lateral radiograph obtained 1 year prior to a shows bone plug (arrow) in the tibial tunnel adjacent to interference screw, which was placed to wedge the plug in the tunnel. (c) Lateral radiograph obtained after a confirms that bone plug (arrow) has become dislodged from the tunnel.
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In conclusion, patients who have persistent or recurrent knee symptoms after knee surgery can be accurately assessed with MR imaging. MR arthrography is preferable to conventional MR imaging in many patients because of increased accuracy for meniscal evaluation and possible benefit for ACL graft evaluation.
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ESSENTIALS
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Evaluation of the postoperative knee with MR imaging is less accurate than evaluation of the preoperative knee.
Compared with conventional MR imaging, MR arthrography improves accuracy for detection of meniscal tear after meniscal surgery, with tears diagnosed when gadolinium signal intensity is seen to extend into the meniscus.
Anterior cruciate ligament graft integrity can be assessed with MR arthrography.
Articular cartilage damage, which often contributes to symptoms in the postoperative knee, can be detected with MR imaging and MR arthrography.
Patients with persistent or recurrent knee symptoms after knee surgery can be accurately assessed with MR imaging and MR arthrography.
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FOOTNOTES
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Abbreviations: ACL = anterior cruciate ligament,
SE = spin echo
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TOP
ABSTRACT
INTRODUCTION
