(Radiographics. 2002;22:765-774.)
© RSNA, 2002
MR Imaging of the Postoperative Knee: A Pictorial Essay1
Michael P. Recht, MD and
Josef Kramer, MD, PhD
1 From the Department of Radiology, Cleveland Clinic Foundation, 9500 Euclid Ave, Desk A-21, Cleveland, OH 44195 (M.P.R.); and Institut for CT and MRI, Linz, Austria (J.K.). Received March 23, 2001; revision requested June 4; final revision received March 18; accepted March 18. Address correspondence to M.P.R. (e-mail: rechtm@ccf.org).
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Abstract
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Magnetic resonance (MR) imaging of the postoperative knee has become more common because more arthroscopic repair procedures are being performed. The most common procedures include partial meniscectomy and meniscal repair, anterior cruciate ligament (ACL) reconstruction, and cartilage repair procedures. Specific findings of a retorn meniscus following meniscal repair or partial meniscectomy are increased signal intensity extending through the site of repair on T2-weighted images, displaced meniscal fragments, and abnormal signal intensity at a site distant from the repair. Findings of ACL graft disruption on T2-weighted MR images include absence of intact graft fibers and increased signal intensity similar to that of fluid within the expected region of the graft. Partial tears of the graft appear as areas of increased signal intensity affecting a portion of the graft with some intact fibers still present. An impinged ACL graft may appear to be draped over the anterior inferior edge of the intercondylar roof or be posteriorly bowed. Localized anterior arthrofibrosis appears on T1-weighted MR images as a focal nodular lesion of low signal intensity that is anterior to the ACL graft in the intercondylar notch and is indistinguishable from adjacent joint fluid. On T2-weighted images, the nodule is well differentiated from high-signal-intensity joint fluid. Finally, MR imaging has been shown to be accurate in the evaluation of cartilage repair tissue. Knowledge of the normal MR imaging appearance of the knee after the more common repair procedures will allow radiologists to recognize complications associated with such procedures.
© RSNA, 2002
Index Terms: Knee, injuries, 452.45, 452.485 • Knee, ligaments, menisci, and cartilage, 452.485 • Knee, MR, 452.12141
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LEARNING OBJECTIVES
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After reading this article and taking the test, the reader will be able to:
- Describe the normal appearance of postoperative menisci and the findings of retorn menisci on conventional MR images and MR arthrograms.
- Recognize common complications following ACL reconstructions.
- Identify the MR imaging appearances of osteochondral autograft transplantations and autologous chondrocyte implantations and their complications.
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Introduction
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Magnetic resonance (MR) imaging of the knee after surgical repair is becoming more common because of the increasing number of therapeutic knee arthroscopic procedures being performed. The most common arthroscopic repair procedures include partial meniscectomy and meniscal repair, anterior cruciate ligament reconstruction, and cartilage repair procedures. It is important to understand the surgical procedure performed as well as the normal MR appearance following such procedures. By understanding the normal MR appearance it is possible to diagnose complications following such procedures.
In this article, we discuss the MR imaging evaluation of the knee after meniscal surgery, review the indications for and the application of MR imaging to evaluating anterior cruciate ligament (ACL) reconstruction, and describe two cartilage repair procedures and the common findings at follow-up MR imaging.
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Meniscal Surgery
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Meniscus-sparing surgery has replaced total meniscectomy as the standard of care in meniscal surgery to preserve as much meniscal function as possible and to prevent the acceleration of degenerative changes of the knee. Following partial meniscectomy or meniscal repair, it can often be difficult clinically to distinguish knee pain caused by retearing of the meniscus from other causes of knee pain, and a reliable imaging method is needed.
When it is performed on nonrepaired knees, MR imaging has been shown to be extremely valuable in the evaluation of primary meniscal injuries, with a sensitivity, specificity, and accuracy of approximately 85%–90% (1). Standard MR imaging criteria for the diagnosis of primary meniscal tears include regions of increased intrameniscal signal intensity (on short echo time images) that reach an articular surface and abnormal meniscal morphology (2). In the evaluation of postoperative menisci, however, these criteria have proved more problematic, particularly when more than 25% of the meniscus has been resected. If greater than 25% of the meniscus has previously been resected, the meniscus will appear truncated with a portion of the meniscus being absent (Fig 1). The margins of the meniscal remnant can be highly irregular, and abnormal signal intensity that extends to the new articular surface (Fig 2) can be seen in stable meniscal remnants (3,4). The MR imaging findings of contour abnormalities and signal-intensity abnormalities reaching an articular surface (as seen on short echo time images) are less accurate for the diagnosis of retorn menisci, with an accuracy of 66%–80%, which is substantially below that for primary meniscal tears (4,5). This decrease in accuracy is caused primarily by a decrease in specificity of these findings in postoperative menisci. The findings of high-signal-intensity joint fluid extending into a cleft within the meniscal fragment on T2-weighted images (Fig 3) or of a displaced meniscal fragment are specific but not sensitive signs of a retorn meniscus.
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Figure 1. Normal appearance of a partially resected meniscus in a 45-year-old man. Coronal fat-suppressed proton-density-weighted fast spin-echo image (repetition time msec/echo time msec = 4,000/14, echo train length of 5) demonstrates a small blunted body (arrowheads) of the medial meniscus following partial meniscectomy.
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Figure 2a. Signal-intensity changes in an intact meniscal remnant in a 35-year-old man following partial medial meniscectomy. (a) Preoperative sagittal proton-density-weighted spin-echo image (2,200/20) shows a complex tear of the posterior horn of the medial meniscus. This tear was treated with partial medial meniscectomy. (b) Sagittal proton-density-weighted spin-echo image obtained 6 months later shows intermediate signal intensity (arrows) within the meniscal remnant that abuts its inferior articular surface. The finding corresponds to the site of signal-intensity abnormality seen preoperatively in the meniscus. There is also blunting of the anterior horn secondary to the partial meniscectomy. The meniscus was intact at repeat arthroscopy.
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Figure 2b. Signal-intensity changes in an intact meniscal remnant in a 35-year-old man following partial medial meniscectomy. (a) Preoperative sagittal proton-density-weighted spin-echo image (2,200/20) shows a complex tear of the posterior horn of the medial meniscus. This tear was treated with partial medial meniscectomy. (b) Sagittal proton-density-weighted spin-echo image obtained 6 months later shows intermediate signal intensity (arrows) within the meniscal remnant that abuts its inferior articular surface. The finding corresponds to the site of signal-intensity abnormality seen preoperatively in the meniscus. There is also blunting of the anterior horn secondary to the partial meniscectomy. The meniscus was intact at repeat arthroscopy.
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Figure 3. Retorn meniscus in a 48-year-old man after partial medial meniscectomy. Sagittal T2-weighted spin-echo image (2,200/80) demonstrates increased fluidlike signal intensity entering a cleft (arrows) within the remnant of the posterior horn of the medial meniscus, a finding indicative of a retorn meniscus.
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In an attempt to improve the accuracy of MR imaging for diagnosing retorn menisci, the use of direct MR arthrography, which involves the injection of intraarticular contrast material, has been proposed. Advantages of MR arthrography over conventional MR imaging include the ability to obtain adequate joint distension; the greater penetration of contrast material–altered joint fluid into the meniscal tear because of its lower viscosity; and the ability to use T1-weighted images, which have higher signal-to-noise ratio and frequently greater spatial resolution than T2-weighted images. On MR arthrograms, meniscal tears are diagnosed as areas of increased signal intensity (equal to that of the intraarticular gadolinium contrast material) within the meniscal fragment (Fig 4) (4,5). Although use of arthrographic technique was previously believed to increase the accuracy of MR imaging in the evaluation of retorn menisci, a recent study comparing conventional MR imaging with MR arthrography found that this increase was not statistically significant (5).
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Figure 4. Retorn meniscus in a 53-year-old woman. Sagittal T1-weighted spin-echo MR arthrogram (800/12) obtained after an intraarticular injection of gadopentetate dimeglumine shows a linear region of increased signal intensity (arrows) that represents contrast material entering the retorn meniscal remnant.
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The diagnosis of retorn menisci following meniscal repair has also proved problematic because abnormal signal intensity reaching the articular surface at the site of meniscal repair can be seen in healed menisci on short echo time images (6). Specific signs of a retorn meniscus following meniscal repair are similar to those of a retorn meniscus after partial meniscectomy: increased signal intensity extending through the site of repair on T2-weighted images, displaced meniscal fragments, and abnormal signal intensity at a site distant from the site of repair (6).
It is important to remember that not all recurrent pain after partial meniscectomy is related to the meniscus. Chondral or osseous abnormalities can occur after meniscectomy and are not uncommon; thus, these regions should be evaluated carefully. Bone marrow changes similar to those seen in spontaneous osteonecrosis of the knee have been described following meniscectomy (7). These changes consist of a well-demarcated focal region of subchondral bone marrow abnormality with more diffuse changes in signal intensity in surrounding bone marrow (Fig 5). These changes most likely represent sequelae of mechanical changes related to the meniscectomy rather than a primary abnormality of bone marrow.
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Figure 5a. Bone marrow changes in a 36-year-old man after partial medial meniscectomy. (a) Preoperative coronal fat-suppressed proton-density-weighted fast spin-echo image (3,400/14, echo train length of 5) demonstrates a complex medial meniscal tear. Underlying bone marrow and articular cartilage are normal. Nine months after partial meniscectomy, the patient presented with recurrent medial knee pain. (b) Postoperative coronal fat-suppressed proton-density-weighted fast spin-echo image (3,400/14, echo train length of 5) shows a focal area of well-demarcated low signal intensity in the subchondral bone marrow (arrows) with a large surrounding area of increased signal intensity. There is also thinning of the overlying articular cartilage. The bone marrow changes are most likely secondary to a combination of microtrabecular fractures and vascular insufficiency of the subchondral bone.
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Figure 5b. Bone marrow changes in a 36-year-old man after partial medial meniscectomy. (a) Preoperative coronal fat-suppressed proton-density-weighted fast spin-echo image (3,400/14, echo train length of 5) demonstrates a complex medial meniscal tear. Underlying bone marrow and articular cartilage are normal. Nine months after partial meniscectomy, the patient presented with recurrent medial knee pain. (b) Postoperative coronal fat-suppressed proton-density-weighted fast spin-echo image (3,400/14, echo train length of 5) shows a focal area of well-demarcated low signal intensity in the subchondral bone marrow (arrows) with a large surrounding area of increased signal intensity. There is also thinning of the overlying articular cartilage. The bone marrow changes are most likely secondary to a combination of microtrabecular fractures and vascular insufficiency of the subchondral bone.
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ACL Reconstruction
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The most commonly reconstructed ligament in the knee is the ACL. Clinical evaluation of ACL reconstructions can be difficult, and MR imaging plays an important role in evaluating the integrity of the ACL graft, as well as in diagnosing complications associated with ACL reconstruction. The most common indications for evaluating ACL reconstructions with MR imaging include (a) failure of ACL reconstruction to stabilize the knee, with imaging used to evaluate for tunnel placement and integrity of the graft; (b) postoperative re-injury to the knee, with imaging performed to assess ACL graft integrity and meniscal and chondral status; (c) postoperative stiffness especially extension loss (flexion contracture), with imaging used to look for impingement and arthrofibrosis; and (d) preparation for revision of a failed ACL reconstruction, with imaging used to evaluate for meniscal and chondral integrity, tunnel placement, and tunnel size, all of which aid the surgeon in preoperative planning.
The most common methods of reconstructing the ACL are to use a bone–patellar tendon–bone autograft (ie, patellar tendon autograft) or hamstring autograft. The positions of the bone tunnels in the distal femur and proximal tibia are crucial for proper function of the ACL graft, and it is important to evaluate the bone tunnel position on MR images.
The position of the femoral tunnel is critical in obtaining isometry, which permits a constant length and tension of the graft through the range of flexion and extension of the knee. An anteriorly located femoral bone tunnel will cause elongation of the graft and result in instability of the knee. The position of the femoral tunnel should be at the intersection of the posterior femoral cortex and the posterior physeal scar corresponding to the posterior intercondylar roof on sagittal MR images (8). On coronal MR images, the femoral tunnel should be at the 11 o’clock and 1 o’clock positions in the right and left knees, respectively.
Proper position of the tibial tunnel is important to prevent graft impingement. Most cases of impingement develop because the graft contacts the intercondylar roof during knee extension. The position of the tibial tunnel should be parallel but posterior to the slope of the intercondylar roof (Blumensaat line) as seen on sagittal MR images (9). On coronal MR images, the tibial tunnel should open on the intercondylar eminence.
Intact grafts appear with either low signal intensity (Fig 6) or intermediate signal intensity on short echo time images. On T2-weighted images, there may also be intermediate signal intensity within the graft, but there should be no area isointense relative to fluid that traverses the full thickness of an intact graft (10). Most studies have indicated that the increased signal intensity on the short echo time images decreases over time (10). The cause of the intermediate signal intensity is uncertain, but it may be caused by vascularized periligamentous tissue, graft revascularization, or graft impingement.
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Figure 6. Intact ACL graft in a 23-year-old man. Sagittal proton-density-weighted spin-echo image (2,200/20) demonstrates homogeneous low signal intensity throughout the ACL graft. The positions of the femoral and tibial bone tunnels are clearly seen. Note the relatively mild amount of metallic artifact related to the tibial interference screw (arrow).
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Graft disruption is relatively uncommon if there is no history of recurrent trauma, and patients with graft disruption will present with clinical findings of knee instability. Because both intact and torn ACL grafts can have intermediate signal intensity on short echo time images, T2-weighted imaging findings are crucial for detection of graft disruption. T2-weighted MR imaging findings of graft disruption include an absence of intact graft fibers and increased signal intensity similar to that of fluid within the expected region of the graft (Fig 7) (11). Partial tears of the ACL graft are demonstrated as areas of increased signal intensity affecting a portion of the graft with some intact fibers still present.
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Figure 7. Disrupted ACL graft in a 40-year-old man. Sagittal T2-weighted spin-echo image (2,320/80) demonstrates increased signal intensity along the expected course of the ACL graft (arrows). No intact graft fibers are visible.
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Extension loss or flexion contracture (sometimes known as "extension lag") is a disabling complication following ACL reconstruction that is most commonly caused by graft impingement or localized anterior arthrofibrosis. Clinically, it can be difficult to distinguish between these two conditions, but each has distinctive MR imaging findings. Graft impingement most commonly occurs when the tibial bone tunnel is anterior to the intersection of the slope of the intercondylar roof with the proximal tibia (12). As discussed previously, the tibial tunnel should be completely posterior to the intersection of the intercondylar roof and the tibia on a fully extended lateral view of the knee. With impinged grafts, the tibial tunnel is partially or completely anterior to the projected slope of the intercondylar roof in the extended knee. MR images allow direct visualization of the tibial tunnel and its relationship to the Blumensaat line and the relationship of the ACL graft to the intercondylar roof. On MR images, the impinged graft contacts the intercondylar roof and may appear to be draped over the anterior inferior edge of the intercondylar roof or be posteriorly bowed (Fig 8). Short echo time images demonstrate increased signal intensity within the distal two-thirds of the graft, the site of graft impingement (13). Notchplasty is performed to treat graft impingement, and the signal-intensity changes seen on MR images of impinged grafts usually resolve within 12 weeks (14).
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Figure 8. ACL graft impingement in a 36-year-old woman. On a sagittal proton-density-weighted spin-echo image (2,240/20), the tibial bone tunnel, although not well seen, is anterior to the intersection of the slope of the intercondylar roof and the proximal tibia. This causes the ACL graft to contact and be displaced posteriorly (arrows) by the roof of the intercondylar notch. Abnormal intermediate signal intensity is seen in the distal portion of the graft.
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Localized anterior arthrofibrosis is a second important cause of extension loss. It represents a focal fibrous lesion that sits anterior to and may be attached to the distal portion of the ACL graft (12). This nodule becomes trapped between the femur and tibia when the knee is extended and leads to a mechanical block of terminal extension. The pathogenesis of localized anterior arthrofibrosis is uncertain, although postulated causes include impingement and debris raised by drilling the tibial tunnel. At arthroscopy, the fibrous lesion has a headlike appearance with focal areas of discoloration that resemble an eye; thus, some refer to it as the "cyclops" lesion. On T1-weighted MR images, it appears as a focal nodular lesion of low signal intensity that is anterior to the graft in the intercondylar notch and can be indistinguishable from adjacent joint fluid. On T2-weighted images, the nodule is heterogeneous but predominantly of low signal intensity and is well differentiated from high-signal-intensity joint fluid (Fig 9) (15).
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Figure 9a. Localized anterior arthrofibrosis (cyclops lesion) in a 33-year-old woman who presented with extension loss 3 months after ACL reconstruction. (a) Sagittal T2-weighted spin-echo image (2,200/80) shows a mild amount of intermediate signal intensity (arrowheads) anterior to the distal attachment site of the ACL graft. (b) Sagittal T2-weighted spin-echo image (2,350/80) obtained 9 months after reconstruction demonstrates enlargement of the mass of heterogeneous signal intensity (arrows), a finding indicative of progression of the localized anterior arthrofibrosis.
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Figure 9b. Localized anterior arthrofibrosis (cyclops lesion) in a 33-year-old woman who presented with extension loss 3 months after ACL reconstruction. (a) Sagittal T2-weighted spin-echo image (2,200/80) shows a mild amount of intermediate signal intensity (arrowheads) anterior to the distal attachment site of the ACL graft. (b) Sagittal T2-weighted spin-echo image (2,350/80) obtained 9 months after reconstruction demonstrates enlargement of the mass of heterogeneous signal intensity (arrows), a finding indicative of progression of the localized anterior arthrofibrosis.
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Hardware failure is uncommon but is important to recognize because it may cause graft instability and thus be confused with graft disruption. Hardware failure includes screw displacement and bone plug dislodgment.
Patellar fracture is a complication unique to the use of the patellar tendon autograft. Both signal-intensity changes and altered morphology of the patellar tendon are usually seen following the use of a patellar tendon autograft. In the early postoperative period, the tendon shows loss of definition and diffuse thickening with increased signal intensity within and around the surgical defect (Fig 10). These signal-intensity and morphologic abnormalities often revert to normal within 12–18 months, although thickening and a defect within the tendon can persist (10).
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Figure 10. Patellar tendon changes in a 26-year-old man after ACL reconstruction. Sagittal proton-density-weighted spin-echo image (2,220/20) obtained 8 months after ACL reconstruction with a bone-patellar tendon-bone autograft shows diffuse intermediate signal intensity and thickening of the patellar tendon (arrows). These are normal findings following surgery and the patient was not symptomatic.
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Cartilage Repair Procedures
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Articular cartilage injury is very common, being present in up to 63% of arthroscopic procedures (16). It is even more frequent in association with ACL deficiency, with up to 79% of patients with an ACL deficient knee having some form of chondral damage (17). With the advent of new cartilage repair procedures that have the potential to either form hyaline-like repair tissue or transplant hyaline cartilage to the damaged area, there is an increased need for an accurate noninvasive method to evaluate the results of such repair procedures. MR imaging is currently the best method for such evaluation.
Two of the more common cartilage repair procedures being performed are osteochondral autograft transplantation (also known as mosaicplasty) and autologous chondrocyte implantation. Osteochondral autograft transplantation is performed by harvesting osteochondral plugs from a relatively nonweightbearing area of the joint andimplanting them into the chondral defect (Fig 11). The goal of the procedure is to fill the defect as completely as possible while maintaining a congruent bone-bone and cartilage-cartilage interface. Osteochondral autograft transplantation is primarily recommended for relatively small lesions up to 2 cm2 in size, although lesions as large as 8 cm2 have been treated (18). The osteochondral plugs range in size from 2.7 to 15 mm in diameter and 10 to 15 mm in depth. Follow-up arthroscopic procedures with histologic biopsies have demonstrated hyaline-like cartilage surfaces with fibrocartilage-like tissue filling the gaps between plugs (19,20).
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Figure 11. Chondral defect treated with osteochondral autograft transplantation in a 25-year-old man. Intraoperative photograph shows 10 osteochondral plugs used to treat a full-thickness defect of the medial femoral condyle. (Resemblance of the plugs to a mosaic has led to the term mosaicplasty.)
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Initial studies have demonstrated that MR imaging can allow an accurate evaluation of (a) the relationship of the graft with the surrounding cartilage and bone (Fig 12), (b) the subchondral bone and incorporation of the graft, (c) the vascularity of the plugs, and (d) the donor site (21,22). Potential complications of osteochondral autograft transplantation are donor site pain, condylar fracture, avascular necrosis of the donor site, loose bodies, and incongruence of the grafts (Fig 13). Graft incongruence can develop secondary to harvesting and implanting the grafts at the wrong angle or from subsidence or degeneration of the grafts.
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Figure 12. Follow-up evaluation in a 34-year-old woman after osteochondral autograft transplantation. Coronal proton-density-weighted fast spin-echo image (3,400/43, echo train length of 7) demonstrates mild irregularity (arrowhead) of the surface of the repair tissue at the transplantation site on the medial femoral condyle.
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Figure 13a. Graft incongruence in a 27-year-old man after osteochondral autograft transplantation. (a) Sagittal fat-suppressed T1-weighted three-dimensional spoiled gradient-echo image (50/10, 45° flip angle), obtained 3 months after transplantation to treat an osteochondral defect of the medial femoral condyle, reveals incongruity between the native cartilage-plug interface (arrows) because of subsidence of the osteochondral plugs. (b) Sagittal fat-suppressed T1-weighted three-dimensional spoiled gradient-echo image (50/10, 45° flip angle) obtained 39 months after transplantation demonstrates hypertrophic repair tissue (arrows) that has formed over the osteochondral plugs. The repair tissue has lower signal intensity than that of the native articular cartilage, and there is a clear interface (arrowheads) between the repair tissue and the native articular cartilage.
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Figure 13b. Graft incongruence in a 27-year-old man after osteochondral autograft transplantation. (a) Sagittal fat-suppressed T1-weighted three-dimensional spoiled gradient-echo image (50/10, 45° flip angle), obtained 3 months after transplantation to treat an osteochondral defect of the medial femoral condyle, reveals incongruity between the native cartilage-plug interface (arrows) because of subsidence of the osteochondral plugs. (b) Sagittal fat-suppressed T1-weighted three-dimensional spoiled gradient-echo image (50/10, 45° flip angle) obtained 39 months after transplantation demonstrates hypertrophic repair tissue (arrows) that has formed over the osteochondral plugs. The repair tissue has lower signal intensity than that of the native articular cartilage, and there is a clear interface (arrowheads) between the repair tissue and the native articular cartilage.
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Autologous chondrocyte implantation was introduced in Sweden in 1994 (23) and involves harvesting chondrocytes from the femoral trochlea or intercondylar notch, growing them in culture, and reimplanting them into the chondral defect covered by a periosteal flap. After implantation, there is a three-stage healing process of the transplanted cells (24). In the first stage, the proliferative stage, a soft jellylike tissue is formed. From 7 to 12 weeks, the transition stage occurs, which involves the production of type II collagen framework and of proteoglycans. In the final stage, the remodeling and maturation stage, which takes from 13 weeks to 3 years after implantation, the collagen framework reorganizes, integrates into the subchondral bone, and forms a tissue as firm as native articular cartilage. Hyaline-like repair tissue has been demonstrated in up to 81% of follow-up biopsies (25).
MR imaging has been shown to be accurate in the evaluation of the repair tissue, the status of the underlying subchondral bone, the interface of the repair tissue and native articular cartilage, and complications associated with the repair tissue such as delamination (Fig 14) and repair tissue hypertrophy (18,26).
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Figure 14a. Follow-up evaluation in a 37-year-old man 14 months after autologous chondrocyte transplantation on the medial femoral condyle. (a) Coronal fat-suppressed proton-density-weighted fast spin-echo image (3,000/25, echo train length of 8) demonstrates high-signal-intensity joint fluid (arrows) undercutting the transplant, a finding consistent with delamination in situ. (b) Permission to reprint this figure electronically was denied by the publisher. Please see print version.
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Figure 14b. Follow-up evaluation in a 37-year-old man 14 months after autologous chondrocyte transplantation on the medial femoral condyle. (a) Coronal fat-suppressed proton-density-weighted fast spin-echo image (3,000/25, echo train length of 8) demonstrates high-signal-intensity joint fluid (arrows) undercutting the transplant, a finding consistent with delamination in situ. (b) Permission to reprint this figure electronically was denied by the publisher. Please see print version.
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Conclusion
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Meniscal, ligamentous, and cartilage repair procedures of the knee are increasing in prevalence as are MR imaging examinations of patients after such procedures. Therefore, it is important to be able to recognize the normal MR imaging appearance of the knee after the more common procedures, as well as complications associated with such procedures.
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Footnotes
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Abbreviation: ACL = anterior cruciate ligament
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References
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Abstract
LEARNING OBJECTIVES
