DOI: 10.1148/rg.271065064
RadioGraphics 2007;27:207-220
© RSNA, 2007
Postoperative Evaluation of the Knee after Autologous Chondrocyte Implantation: What Radiologists Need to Know1
Yvonne Yiwan Ho, MB, BS, FRANZCR,
Anthony Joseph Stanley, MB, BCh, FRCR,
James Hoi-Po Hui, MB, BS, FRCS (Edin) and
Shih-Chang Wang, MB, BS, FRANZCR
1 From the Department of Diagnostic Imaging, National University Hospital of Singapore, 5 Lower Kent Ridge Rd, Singapore 119074. Presented as an education exhibit at the 2005 RSNA Annual Meeting. Received April 17, 2006; revision requested May 11 and received May 30; accepted May 31. All authors have no financial relationships to disclose.
Address correspondence to Y.Y.H. (e-mail: Yvonne_Ho{at}nuh.com.sg).
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Abstract
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Articular cartilage lesions occur commonly. Cartilage is relatively avascular and is unable to self-repair. A chondral lesion may become symptomatic. It may lead to osteoarthritis and increased morbidity. The aim of cartilage repair is to restore hyaline cartilage. There are many types of cartilage repair surgery, most of which result in fibrocartilage repair tissue that is suboptimal. Autologous chondrocyte implantation has been shown to produce hyaline-type repair tissue. Magnetic resonance (MR) imaging is performed preoperatively to define the ulcer and postoperatively to evaluate the technical success of implantation and the state of cartilage healing and to identify potential complications. Features of the autologous chondrocyte implantation graft that are assessed include the degree of filling by repair tissue, its integration with native cartilage and subchondral bone, the character of the graft substance and surface, and the underlying bone. MR arthrography is superior to unenhanced MR imaging because intraarticular contrast material allows the recipient site to be physically separated from adjacent structures so that it can be characterized more accurately. MR imaging and arthroscopy are complementary investigations in the follow-up of an autologous chondrocyte implantation in the knee. The appearance of the knee after autologous chondrocyte implantation varies among individuals and according to the time-course of healing. Familiarity with the surgical procedure and imaging appearance is essential for an accurate postsurgical assessment.
© RSNA, 2007
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Introduction
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Articular cartilage lesions occur commonly. They may be symptomatic or clinically silent. Many chondral lesions are first diagnosed at arthroscopy. In a study in which 993 knees were evaluated with arthroscopy on the basis of International Cartilage Repair Society (ICRS) criteria, articular cartilage lesions were diagnosed in 66% of the evaluations (1). Full-thickness lesions eligible for cartilage repair were found in 5%–11% of patients who underwent knee arthroscopy (1,2).
Since articular cartilage is relatively avascular, it has a limited ability to repair itself. Some initially asymptomatic chondral lesions degenerate and become symptomatic with time. The incidence of degenerative chondral disease is unknown; however, when a chondral defect with early joint space narrowing is found at radiography, progression toward osteoarthritis is highly likely (3). The degenerative process results in increased morbidity, particularly in young patients. The aim of cartilage repair is to restore the functional properties of the chondro-osseous unit. However, although a number of approaches have been investigated and described, there is still debate about which method of treatment is best for symptomatic cartilage defects.
The article reviews the structure and function of cartilage, the types of cartilage surgery, and the advantages and disadvantages of unenhanced magnetic resonance (MR) imaging versus MR arthrography in the postoperative evaluation, with emphasis on normal features and complications of autologous chondrocyte implantation that are commonly seen at imaging of the knee.
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Histologic and Physiologic Features of Cartilage
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Cartilage is a semirigid connective tissue that is characterized by the nature of its ground substance in extracellular matrix and by the absence of blood vessels, lymphatics, and nerves. Three subtypes of cartilage (hyaline, fibro-, and elastic) are distinguished by different proportions of collagen and elastic fibers in the ground substance.
Articular cartilage is composed of hyaline cartilage. Its role is to bear loads in different joint positions and activities and to minimize surface friction on articular surfaces. The extracellular matrix, which accounts for 95% of the total tissue volume of articular cartilage, consists mostly of type II collagen. Articular cartilage also contains sparsely interspersed chondrocytes. It is organized in four highly specialized layers or zones, each with different load-bearing properties (Fig 1) (4).
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Figure 1. Histologic zones of cartilage. A = superficial zone (consists of collagen fibers parallel to the articular surface, with flattened chondrocytes; provides resistance to shearing forces), B = intermediate zone (consists of collagen fibers oriented obliquely to the articular surface, with spherical chondrocytes; provides resistance to shearing and compressive forces), C = deep zone (consists of collagen fibers and chondrocytes in perpendicular orientation to the articular surface; resists compressive forces), D = calcified layer (consists of degenerated chondrocytes and osteoblasts, is penetrated by blood vessels from the diaphysis; joins cartilage to underlying bone). Sub. = subchondral.
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The survival of articular cartilage depends on the ability of healthy chondrocytes to synthesize the extracellular matrix and contribute to various zones of hyaline cartilage. 
Restoration of hyaline cartilage is the goal of autologous chondrocyte implantation. Most surgical techniques produce predominantly fibrocartilage repair tissue (5,6). Unfortunately, this tissue is less durable and does not resist compression and shear loads as well as does hyaline cartilage.
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Types of Surgery
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An abridged review of common surgical techniques is presented here. The best method of treatment for cartilage defects is not yet known; none of the existing methods enables the consistent reproduction of normal hyaline cartilage.
Mechanical Repair
Unstable cartilage at the chondral defect may be removed with arthroscopic débridement and lavage (7). The calcified layer of cartilage is abraded to allow new tissue to form at the base of the ulcer.
Marrow Stimulation Techniques
Abrasion Arthroplasty.—
Chondral ulcers are diagnosed and treated with arthroscopic techniques. In abrasion arthroplasty, 1–3 mm of superficial subchondral bone is removed with a motorized instrument to expose the interosseous vessels. As a result, a fibrin clot forms. This eventually develops into fibrocartilage repair tissue (8).
Puncture of Subchondral Bone.—
With this method, a pathway is created to allow immature chondrocytes and blood cells to migrate from bone marrow to the chondral defect, where the cells proliferate and gradually form fibrous or fibrocartilage mosaic repair tissue (9). The pathway between the marrow and the chondral lesion may be created with a drill (by using the Pridie technique [10]), with a pick by using the microfracture technique (a modification of the Pridie procedure, introduced by Steadman et al [11]), or with a chisel.
Resurfacing Techniques
Osteochondral Autograft or Mosaicplasty.—
In this one-step open surgical procedure, a plug of intact hyaline cartilage and subchondral bone obtained from a non-weight-bearing region of the knee is transferred to a chondral defect in a weight-bearing region. Ideally, such a defect should have an area of approximately 1–4 cm2 (12).
Autologous Chondrocyte Implantation.—
This biologic technique was the first application of cell engineering in orthopedic surgery. The procedure was introduced by Peterson et al (13) in 1987 and made popular by Brittberg et al (14). Two different author groups reported moderately good histologic findings at the recipient sites after treatment, and the repairs to osteochondral lesions lasted up to 11 years after treatment (14, 15). Promising results also were reported by Micheli et al (16) and Minas (17). At the time when the present article was written, it was estimated that autologous chondrocyte implantation had been performed in more than 10,000 patients worldwide (18).
Traditionally, autologous chondrocyte implantation is indicated for large chondral lesions in which there is minimal involvement of the subchondral bone. A marrow-stimulating procedure or an osteochondral autograft transplantation is performed for treatment of smaller lesions (19).
Autologous chondrocyte implantation is traditionally a two-stage procedure. The first stage comprises an arthroscopic assessment with harvesting of healthy cartilage from the edges of the patient’s ulcer and in vitro culture of healthy hyaline cartilage. In the second stage, approximately 1 month later, an open surgical procedure is performed in which cultured chondrocytes are reimplanted beneath an autologous periosteal patch that has been sutured and sealed to the edges of the chondral lesion (Figs 2, 3).
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Figure 2a. Intraoperative photographs of an autologous chondrocyte implant in a patellar ulcer. (a) Photograph obtained before implantation shows a large central patellar ulcer (arrow) highlighted with gentian violet. (b) Photograph obtained after implantation shows a chondrocyte-filled periosteal pouch that has been attached with microsutures to the patella (arrow).
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Figure 2b. Intraoperative photographs of an autologous chondrocyte implant in a patellar ulcer. (a) Photograph obtained before implantation shows a large central patellar ulcer (arrow) highlighted with gentian violet. (b) Photograph obtained after implantation shows a chondrocyte-filled periosteal pouch that has been attached with microsutures to the patella (arrow).
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Figure 3. Schematics of autologous chondrocyte implantation in a flexed knee viewed from a cranial perspective. A shows the articular aspect of the femoral condyle, a common site of cartilage ulceration. In the first stage of the procedure, the ulcer (red oval) is debrided arthroscopically. B provides a close-up of the ulcerated femoral condyle (FC) and the site of harvesting of healthy chondrocytes (lavender oval) in the non-weight-bearing surface of the joint (eg, intertrochanteric notch of the femur [F]). C and D represent the in vitro culture of harvested chondrocytes. E shows the second stage of the procedure, in which autologous periosteum (white oval) is harvested from the proximal tibia (T), a non-weight-bearing location that is commonly used as a harvest site. In F, the harvested periosteum is fashioned into a patch and sutured to the edges of the debrided ulcer. In G and H, cultured chondrocytes are injected beneath the periosteal patch. In I, the periosteal patch is securely fastened and sealed with fibrin glue.
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There are many possible variations of this general implantation procedure. In second-generation autologous chondrocyte implantation, a bi-layered collagen membrane (eg, Chondro-Gide; Geistlich Biomaterials, Wolhusen, Switzerland) is used instead of a periosteal flap to simplify the second stage of implantation. In third-generation autologous chondrocyte implantation performed with a one-step mini-arthrotomy, a three-dimensional (3D) biologic scaffolding that has been optimized for the culture of seeded chondrocytes (eg, matrix-induced autologous chondrocyte implantation, MACI; Verigen, Leverkusen, Germany, and Genzyme, Boston, Mass) is implanted (20). This biologic material can be trimmed and glued to the debrided cartilage ulcer.
The ideal repair tissue is restored hyaline cartilage. However, the best repair tissue obtained in classic autologous chondrocyte implantation studies with histologic analysis was hyaline-type tissue that was not morphologically or histochemically identical to normal hyaline cartilage, and it was found only in some specimens. Histologic analysis of other specimens revealed fibrocartilage (5,6). Studies of second- and third-generation autologous chondrocyte implants show promising outcomes (20).
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MR Assessment before and after Implantation
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Preoperatively, diagnosis and surgical planning can be undertaken by identifying, measuring, and characterizing a cartilage defect. A thorough assessment of the knee joint should be performed to exclude other abnormalities. Arthroscopic assessment of articular cartilage may be performed by using the Outerbridge classification system. In this system, a defect is categorized according to (a) its diameter and (b) its relationship to subchondral bone or whether the lesion exposes bone. This classification system has proved moderately accurate when used to grade chondral lesions of the knee (21). However, it does not include a description of lesion depth. More recently, the ICRS has developed a comprehensive system for assessing outcomes of cartilage resurfacing or reconstructive surgery (Table). The assessment focuses on the depth of the chondral lesion and the area of damage (22). Correlation between MR findings and the ICRS hyaline cartilage lesion classification is currently imperfect but allows preliminary characterization.
Postoperative visualization of the repair tissue also is recommended for the purpose of assessing the technical success of the procedure and the state of cartilage healing, as well as for identifying possible complications. The tissue may be evaluated with arthroscopy; however, a less invasive method, such as MR imaging, is ideal.
The ability of MR imaging to directly depict subchondral bone and bone marrow represents an advantage over arthroscopy (Fig 4). However, the true dimensions of a cartilage defect may be underestimated at MR imaging if the defect is not imaged in all three planes or if a detached fragment conceals its margin.
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Figure 4a. (a) Coronal short inversion time inversion recovery (repetition time msec/echo time msec/inversion time msec, 5300/25/170) image obtained after autologous chondrocyte implantation in a 39-year-old man shows signal intensity characteristic of edema in the marrow adjacent to the site of cartilage repair at the right femoral condyle (arrow). F = femur, T = tibia. (b) Arthroscopic image obtained after implantation shows a healed ulcer with fibrillation at the graft site ( ) but does not depict the change in marrow signal intensity found at MR imaging.
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Figure 4b. (a) Coronal short inversion time inversion recovery (repetition time msec/echo time msec/inversion time msec, 5300/25/170) image obtained after autologous chondrocyte implantation in a 39-year-old man shows signal intensity characteristic of edema in the marrow adjacent to the site of cartilage repair at the right femoral condyle (arrow). F = femur, T = tibia. (b) Arthroscopic image obtained after implantation shows a healed ulcer with fibrillation at the graft site () but does not depict the change in marrow signal intensity found at MR imaging.
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The advantage of arthroscopy is that it allows immediate tissue biopsy for histologic assessment of the implant. MR imaging findings are not useful for predicting the histologic structure of an autologous chondrocyte graft (23).
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MR Imaging Techniques
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MR imaging and MR arthrography are very useful for evaluating the morphologic status of cartilage defects and repair tissue throughout the postoperative period (24–31). In the remainder of this article, MR imaging refers to an MR examination performed without the use of an intraarticular contrast medium, and MR arthrography refers to a study for which an intraarticular contrast medium is administered. The term MR encompasses both MR imaging and MR arthrography.
As with all imaging, with MR techniques the accurate assessment of a lesion depends on the signal-to-noise ratio of the system, the size of the lesion, and the contrast resolution between the lesion and adjacent tissues.
The greatest challenge in the imaging of cartilage is partial volume artifacts, which appear on MR images because the cartilage is thin (<4 mm in thickness) and articular surfaces are generally curved. Spatial and contrast resolution of cartilage imaging can be improved with volumetric 3D acquisition and MR arthrography. Volumetric 3D acquisition allows imaging of very thin contiguous sections. In MR arthrography, an intraarticular injection of a solution containing gadolinium is used to outline cartilage defects and to improve the conspicuity of lesions.
At our institution, MR imaging of the knee is performed by using a 1.5-T unit (LX EchoSpeed Plus; GE Healthcare, Milwaukee, Wis) with a quadrature extremity transceiver coil. Image acquisition includes the application of a coronal short inversion time inversion recovery sequence, sagittal intermediate-weighted and T2-weighted fast spin-echo sequences, and 3D gradient-recalled echo (GRE) sequences in two planes tangential to the site of the graft. MR imaging is followed by direct MR arthrography, which is performed after the intraarticular injection of contrast material by using aseptic technique and a medial or lateral approach to the knee. A 1:200 solution that contains 0.5 mmol/mL gadopentetate dimeglumine (Magnevist; Schering, Berlin, Germany) is prepared, and 40 mL of this solution is introduced into the knee joint by using a 23-gauge (3.8-cm-long) needle. After the contrast material is administered, a 3D spoiled GRE sequence with spectral fat suppression is applied in axial, coronal, and sagittal planes.
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Features of the Postoperative Knee
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The postoperative knee is evaluated with MR imaging and MR arthrography to determine whether implantation was successful. Features of the autologous chondrocyte implantation graft that are assessed with MR include the extent of filling of the defect by repair tissue, integration of the graft with native cartilage and subchondral bone, the appearance of the graft surface and underlying tissue, and the underlying bone (32). Abnormal findings are considered to indicate complications that may require treatment.
Filling of the Defect by Repair Tissue
The volume of repair tissue generally decreases slightly after the immediate postoperative period. Stabilization of the repair tissue volume has been shown to occur approximately 3 months after autologous chondrocyte implantation (6).
MR allows depiction of the depth and volume (percentage) of filling of a cartilage defect. Underfilling (Fig 5) is subdivided into filling greater than 50% or less than 50% of the depth of adjacent native cartilage. In approximately 2% of cases, underfilling is severe enough to require further surgical intervention (32).
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Figure 5. Sagittal MR arthrogram obtained with a spoiled GRE sequence (repetition time msec/echo time msec, 6.1/1.6; flip angle, 15°) in a 36-year-old man 11 months after autologous chondrocyte implantation shows focal underfilling of a cartilage defect (arrow) at the central aspect of the right patella. F = femur, T = tibia.
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Repair tissue with a thickness greater than that the native cartilage is considered hypertrophy (Fig 6). Hypertrophy, which is commonly seen between the 3rd and 7th postoperative months, may cause the knee joint to catch in an estimated 25% of patients. Rarely, persistent symptoms of necessitate the arthroscopic removal of hypertrophic tissue to the level of native cartilage (32).
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Figure 6a. Coronal (a) and sagittal (b) spoiled GRE (6.1/1.6; flip angle, 15°) MR arthrograms obtained in an 18-year-old man 44 months after autologous chondrocyte implantation show heterogeneous hypertrophic repair tissue at the articular portion of the right medial femoral condyle (arrow). F = femur, T = tibia.
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Figure 6b. Coronal (a) and sagittal (b) spoiled GRE (6.1/1.6; flip angle, 15°) MR arthrograms obtained in an 18-year-old man 44 months after autologous chondrocyte implantation show heterogeneous hypertrophic repair tissue at the articular portion of the right medial femoral condyle (arrow). F = femur, T = tibia.
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Filling usually is considered complete when the original articular contour is restored and the repair tissue is of the same thickness as the adjacent native cartilage. An exception is made if an osseous defect was part of the original lesion. In this case, repair tissue that is thicker than usual is necessary to fill the original bone defect (33).
Integration Zone between Repair Tissue and Native Cartilage
Integration between the repair tissue and the native cartilage is considered complete if the margin between the two is continuous, without a discernible interface. A defect between the native cartilage and the graft tissue is considered to indicate marginal delamination, whether it consists of a thin linear fissure or a large irregular gap.
It is difficult at arthroscopy to distinguish between marginal delamination and normal maturation tissue at the margin between the graft and native cartilage(19,32). However, with MR arthrography, it is easier to differentiate between the two, because contrast material seeps into a delamination defect, whereas normal maturation tissue appears as a high-signal-intensity interface without contour deformity on T2-weighted fast spin-echo images.
Integration between Repair Tissue and Subchondral Bone
Incomplete integration between repair tissue and subchondral bone may lead to partial or complete delamination of a graft. Delamination from the underlying subchondral bone occurs in 5% of all autologous cartilage implants. It is most common during the first 6 months after surgery. The predominant symptom is painful catching or locking of the knee.
Partial delamination appears as a fissure between the repair tissue and underlying subchondral bone. Delamination may occur with or without displacement. If displacement is present, the repair tissue appears as a cartilage flap (Fig 7), which may require arthroscopic excision. If delamination is complete, the entire graft may separate from the defect site and appear as a loose body elsewhere in the joint. If delamination is debilitating, arthroscopic removal of the detached graft is indicated (33). MR is highly sensitive and specific for the detection of delamination. Fluid or contrast material can be seen beneath the delaminated portion of the repair tissue.
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Figure 7. Sagittal spoiled GRE (6.1/1.6; flip angle, 15°) MR arthrogram, obtained in a 30-year-old man 10 months after a right patellar autologous chondrocyte implantation, shows displaced delamination with folding of a retropatellar cartilage flap (arrow) in the superior direction and resultant denudation of the inferior articular surface. F = femur, P = patella, T = tibia.
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Signal Intensity of Repair Tissue
The signal intensity of repair tissue is judged on the basis of GRE images. The signal in repair tissue may be isointense (Fig 8), hypointense (Fig 9), or hyperintense (Fig 10) when compared with that in the native cartilage. In a study of 3D spoiled GRE imaging, Wada et al found low signal intensity of healthy repair tissue immediately after autologous chondrocyte implantation. The initial signal intensity of repair tissue increased with time and, 6–9 months later, resembled that of native cartilage. At approximately 9–12 months after autologous chondrocyte implantation, the signal intensity of normal repair tissue reached a plateau (6).
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Figure 8. Variations in signal intensity of repair tissue on GRE (8.8/4.2; flip angle, 60°) images obtained in three patients after autologous chondrocyte implantation at the right femoral trochlea. Axial image obtained in a 21-year-old man 17 months after implantation shows isointense signal with minimal irregularity at the cartilage reconstruction site (arrow).
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Figure 9. Variations in signal intensity of repair tissue on GRE (8.8/4.2; flip angle, 60°) images obtained in three patients after autologous chondrocyte implantation at the right femoral trochlea. Sagittal image obtained in a 39-year-old man 12 months after implantation shows hypointense signal with perfect depth of cartilage filling at the repair site (arrow).
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Figure 10. Variations in signal intensity of repair tissue on GRE (8.8/4.2; flip angle, 60°) images obtained in three patients after autologous chondrocyte implantation at the right femoral trochlea. Sagittal image obtained in a 40-year-old man 11 months after implantation shows hyperintense signal with perfect depth of cartilage filling at the repair site (arrow). F = femur, P = patella, T = tibia.
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Hyperintense signal in a cartilage graft may be attributed to softening, a common finding at arthroscopy. However, studies have shown that cartilage softening cannot be reliably detected with MR (27,34,35).
Surface Changes
A graft surface is considered intact when it is congruent with the native cartilage surface. Incongruence may be secondary to surface changes such as fissures (Fig 11) and ulceration (Fig 12), which are detectable with MR (33).
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Figure 11a. Coronal (a) and sagittal (b) spoiled GRE (6.1/1.6; flip angle, 15°) MR arthrograms obtained in a 51-year-old man 8 months after autologous chondrocyte implantation show near-perfect incorporation of cartilage at the right medial femoral condyle, with only a tiny cartilage fissure (arrow). F = femur, T = tibia.
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Figure 11b. Coronal (a) and sagittal (b) spoiled GRE (6.1/1.6; flip angle, 15°) MR arthrograms obtained in a 51-year-old man 8 months after autologous chondrocyte implantation show near-perfect incorporation of cartilage at the right medial femoral condyle, with only a tiny cartilage fissure (arrow). F = femur, T = tibia.
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Figure 12a. (a) Arthroscopic image obtained before autologous chondrocyte implantation in a 40-year-old man shows an ulcer of the right femoral trochlea. (b) Axial spoiled GRE (6.1/1.6; flip angle, 15°) image obtained 11 months after implantation shows near-perfect incorporation of cartilage at the recipient site, with a minor irregularity of the cartilage at the central part of the trochlea (arrow). F = femur, P = patella.
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Figure 12b. (a) Arthroscopic image obtained before autologous chondrocyte implantation in a 40-year-old man shows an ulcer of the right femoral trochlea. (b) Axial spoiled GRE (6.1/1.6; flip angle, 15°) image obtained 11 months after implantation shows near-perfect incorporation of cartilage at the recipient site, with a minor irregularity of the cartilage at the central part of the trochlea (arrow). F = femur, P = patella.
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Changes in Substance
The substance of the graft also is assessed for homogeneity. If there is a cleft or cyst (Fig 13) in the repair tissue, the substance is considered inhomogeneous (24).
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Figure 13a. Coronal (a) and sagittal (b) spoiled GRE (6.1/1.6; flip angle, 15°) MR arthrograms obtained in a 59-year-old woman 1 month after autologous chondrocyte implantation show a small cyst (arrow) and adjacent partial delamination (arrowhead) in the interface between native and transplanted cartilage at the left lateral femoral condyle. F = femur, T = tibia.
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Figure 13b. Coronal (a) and sagittal (b) spoiled GRE (6.1/1.6; flip angle, 15°) MR arthrograms obtained in a 59-year-old woman 1 month after autologous chondrocyte implantation show a small cyst (arrow) and adjacent partial delamination (arrowhead) in the interface between native and transplanted cartilage at the left lateral femoral condyle. F = femur, T = tibia.
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Change in Subchondral Lamina and Subchondral Bone
If the subchondral black line that separates the autologous implant from bone marrow on MR images is normal in shape and signal intensity, then the subchondral lamina is intact. If the line is disrupted or irregular, then there is a break in the subchondral lamina.
The subchondral bone marrow may be intact or may demonstrate alterations such as cyst formation or signal intensity characteristic of edema (Fig 4a). Edema-like signal intensity is common in the early postoperative period, but the persistence or progression of a signal intensity abnormality in marrow may indicate a failure of graft incorporation (33). Some investigators have suggested that the persistence of signal intensity characteristic of edema beyond 1 year heralds a poor outcome (29,32). Others have noted that abnormal marrow signal intensity may be seen for this duration without failure of the graft (23).
Other Complications
Less common complications specific to autologous chondrocyte implantation are intraarticular adhesions and hypertrophic synovitis. General complications of surgery, such as superficial and deep wound infection, hemarthrosis and hematoma, and deep venous thrombosis and pulmonary embolism, also may occur (24,36).
Intraarticular adhesions (Fig 14) occur in 5%–10% of implants. They appear as areas of fibrosis with low to intermediate signal intensity on the cartilage surface. Adhesions most commonly are located in the infrapatellar fat pad and the suprapatellar pouch. Adhesions occur most frequently in patients who have had extensive surgery (32). Their appearance may mimic that of a displaced delaminated graft (if they are completely surrounded by fluid or intraarticular contrast material) or a hypertrophic graft (if they are attached to cartilage repair tissue).
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Figure 14. Sagittal GRE (8.8/4.2; flip angle, 60°) image, obtained in a 58-year-old man 18 months after autologous chondrocyte implantation and surgical elevation of the tibial tubercle, shows low-signal-intensity material (arrow) suggestive of intraarticular adhesions in the infrapatellar fat pad. F = femur, P = patella, T = tibia.
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Hypertrophic synovitis, a rare complication, may result from a chemical reaction induced by graft delamination. Hypertrophic synovitis is associated with a joint effusion and is often an indirect sign of failure of cartilage repair.
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MR Arthrography versus MR Imaging
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Unenhanced MR imaging is noninvasive. Although some might argue that the insertion of a needle into the knee joint and the instillation of intraarticular contrast material convert noninvasive MR imaging to invasive MR arthrography, MR arthrography is much less invasive than arthroscopy.
In studies in which conventional MR imaging was compared with MR arthrography for the detection and staging of chondral defects, MR arthrography was shown to be superior because the instillation of intraarticular contrast material enables the physical separation of the recipient site from adjacent structures so that the recipient site can be characterized more accurately (Fig 15) (28,30,31). MR arthrography proved markedly superior in the assessment of intermediate-stage lesions and small lesions in cartilage (30). Our experience is in agreement with these study results.
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Figure 15a. (a) Axial spoiled GRE (6.1/1.6; flip angle, 15°) MR arthrogram obtained in a 40-year-old man 8 months after autologous chondrocyte implantation shows a focal cartilage defect (arrow) at the medial patellar facet of the left knee. (b) Axial GRE (8.8/4.2; flip angle, 60°) MR image obtained without the use of intraarticular contrast material does not show the defect (arrow). F = femur, P = patella.
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Figure 15b. (a) Axial spoiled GRE (6.1/1.6; flip angle, 15°) MR arthrogram obtained in a 40-year-old man 8 months after autologous chondrocyte implantation shows a focal cartilage defect (arrow) at the medial patellar facet of the left knee. (b) Axial GRE (8.8/4.2; flip angle, 60°) MR image obtained without the use of intraarticular contrast material does not show the defect (arrow). F = femur, P = patella.
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MR imaging and MR arthrography are equally useful for assessing high-grade cartilage lesions and equally poor for assessing low-grade chondral defects because the latter show only signal intensity change and minimal or no contour deformity. Similarly, near-perfect incorporation of cartilage (Fig 16) makes localization of the recipient site difficult with both techniques.
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Figure 16a. (a) Axial GRE (8.8/4.2; flip angle, 60°) MR image obtained in a 21-year-old man 17 months after autologous chondrocyte implantation shows near-perfect graft incorporation at the right patella (arrow). (b) Postoperative arthroscopic image obtained 1 month later shows minimal fibrillation of cartilage at the patella (arrows). Because minor fibrillation is not always visible on MR images, the findings at arthroscopy generally were considered to correlate well with those at MR imaging. F = femur, P = patella.
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Figure 16b. (a) Axial GRE (8.8/4.2; flip angle, 60°) MR image obtained in a 21-year-old man 17 months after autologous chondrocyte implantation shows near-perfect graft incorporation at the right patella (arrow). (b) Postoperative arthroscopic image obtained 1 month later shows minimal fibrillation of cartilage at the patella (arrows). Because minor fibrillation is not always visible on MR images, the findings at arthroscopy generally were considered to correlate well with those at MR imaging. F = femur, P = patella.
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The Future
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It has been suggested that the intraarticular contrast agent gadopentetate dimeglumine has the potential to demonstrate structural changes of hyaline cartilage at T1-weighted imaging (37). Delayed gadolinium-enhanced MR imaging of cartilage also has shown potential for the measurement of cartilage glycosaminoglycans and the assessment of graft maturity after autologous chondrocyte implantation (38). If these techniques are introduced as standards of care after future successful preclinical and clinical research studies, their use could lead to earlier detection of cartilage degeneration. They also could be used to monitor disease progression and therapeutic effectiveness.
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Conclusions
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MR and arthroscopy are complementary examinations for follow-up of autologous chondrocyte implants in the knee. Arthroscopy is better used for assessing the cartilage surface, and MR is better used for assessing lesions deep within the cartilage and the underlying bone. MR arthrography is superior to MR imaging because it allows more accurate characterization of the recipient site.
The appearance of the knee after an autologous chondrocyte implantation varies among individuals and in accordance with the time-course of healing. Familiarity with the surgical procedure and imaging appearance is essential for accurate postsurgical assessment.
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Footnotes
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Abbreviations: GRE = gradient-recalled echo, ICRS = International Cartilage Repair Society, 3D = three-dimensional
See the commentary by Disler following this article.
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References
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Abstract
Introduction
