Musculoskeletal Imaging
Review
MRI of Native Knee Cartilage Delamination Injuries
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OBJECTIVE. The purpose of this article is to describe the normal imaging appearance of cartilage and the pathophysiologic findings, imaging appearance, and surgical management of cartilage delamination.
CONCLUSION. Delamination injuries of knee cartilage signify surgical lesions that can lead to significant morbidity without treatment. These injuries may present with clinical symptoms identical to those associated with meniscal injury, and arthroscopic identification can be difficult, thereby creating a role for imaging diagnosis. A low sensitivity of imaging identification of delamination injury of the knee is reported in the available literature, although vast improvements in MRI of cartilage have since been introduced.
Keywords: cartilage, cartilage delamination, native knee
Cartilage delamination is the separation of the articular cartilage at the tidemark from the underlying subchondral bone and is considered a surgical lesion with the potential for significant morbidity if not appropriately recognized and treated. Preoperative identification of delaminated cartilage is important because arthroscopic identification may be challenging, and surgical planning is critical because cases require special instrumentation and implants. The literature suggests that delamination in native knee cartilage is underrecognized, although it is often described as a complication after surgical repair of cartilage. The purpose of this article is to describe the normal imaging appearance of cartilage and the pathophysiologic findings, imaging appearance, and surgical management of cartilage delamination.
| Normal Imaging Appearance of Articular Cartilage |   |
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Articular cartilage is formed from a matrix of hyaline cartilage and is supported by a base of collagen that is continuous with the collagen of the subchondral bone. This supporting base helps resist shear forces at the osteochondral junction. The basal layers of the articular cartilage calcify with advancing age after skeletal maturity. In children, the calcified cartilage is replaced by bone [1]. In skeletally mature individuals, the line between calcified and noncalcified cartilage is called the tidemark [1]. The tidemark forms a natural cleavage plane that separates the deepest calcified layer of articular cartilage from the subchondral bone.
Normal articular cartilage has a trilaminate appearance on MRI with a low-signal-intensity deep layer, a thicker intermediate- to high-signal-intensity middle layer, and a thin low-signal-intensity surface layer. High-resolution high-field-strength imaging best depicts the individual layers. Low-resolution or very-short-TE imaging will cause normal cartilage to appear to have homogeneously intermediate to high signal intensity. Collagen fibers are oriented horizontally at the cartilage surface (the superficial zone) and perpendicularly at the base (the radial zone), with a transitional zone of randomly oriented fibers located in between these areas. Normal cartilage shows regional variations in T2 signal intensity resulting from anisotropy, reflecting the changing orientation of the collagen relative to the magnetic field [2]. This normal signal gradient is smooth and continuous, in contrast to chondral lesions, which tend to be abrupt [3]. The central tibial plateau, central patella, and weight-bearing femoral condyles may have prominent low signal intensity in the deep articular cartilage as a normal variant [3]. Trochlear cartilage may show artifacts related to volume averaging secondary to its curved surface [3].
| Suggested MRI Sequences for Imaging Cartilage | |
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Cartilage-specific sequences are important for accurate assessment of cartilage morphologic findings. The most widely used and accurate cartilage-specific sequences include fluid-sensitive fat-suppressed sequences, such as intermediate-weighted, proton density (PD)–weighted, or T2-weighted fast spin-echo (FSE) sequences, and spoiled gradient-recalled echo (SPGR) imaging [4, 5]. SPGR imaging is considered the standard technique for evaluation of knee cartilage morphologic findings, but it may not be as accurate as fluid-sensitive FSE sequences for identification of small focal cartilage lesions [5–8]. Dual-echo steady state (DESS) sequences can be obtained faster than SPGR sequences, have a high signal-to-noise ratio, and, like SPGR sequences, are isotropic (i.e., 3D); however, the drawback of DESS sequences is that they may not reliably depict signal intensity changes in cartilage. High-resolution FSE imaging with a longer TE has been shown to correlate well with arthroscopy for depiction of chondral injury, and it has the advantage of being able to depict other knee structures exquisitely [9]. Fat-suppressed PD-weighted or intermediate-weighted sequences are used at many institutions [5]. The imaging committee of the International Cartilage Repair Society (ICRS) recommends the use of FSE sequences and fat-suppressed T1-weighted 3D gradient-echo sequences and suggests imaging parameters [10]. Three-dimensional spin-echo sequences are relatively newer and have theoretic advantages, but they are not yet in widespread clinical use. We prefer to use a combination of orthogonally acquired 2D FSE sequences, including fat-suppressed T2-weighted and PD-weighted sequences, and a sagittal fat-suppressed DESS sequence.
| Delamination Injury | |
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Cartilage delamination is the separation of the articular cartilage at the tidemark from the underlying subchondral bone [11] (Fig. 1). The tidemark separates more readily than the junction between cartilage and subchondral bone [1, 11]. Because skeletally immature individuals have little calcified cartilage and thus do not have a well-formed tidemark, osteochondral fractures predominate over chondral injuries such as delamination [12, 13]. Delaminated cartilage may completely separate from the parent bone and become displaced.
 View larger version (124K) | Fig. 1 —Illustration of cartilage delamination injury (dashed arrow) extending along tidemark (solid arrow) and through deep, transitional, and superficial cartilage layers to articular surface. (Drawing by Rizer M, used with permission) |
Delamination is thought to result from traumatic shearing stress to the cartilage. This stress occurs parallel to the joint surface along the surface of the tidemark. The overlying cartilage may not be initially violated, and the tidemark separation often occurs beyond the margin of the overlying cartilage defect [14]. Thus, delamination injuries may appear intact at arthroscopy and may be seen only as a bulge of the cartilage surface [10, 14]. This is in contradistinction to cartilage flaps, the cartilaginous surface of which is always violated. Consequently, delamination may not always easily fit into a chondral lesion grading system. If there is superficial cartilage injury, the modified Outerbridge classification would usually be grade 3 (i.e., deep ulceration or a chondral flap involving 50% or more of the depth of the articular cartilage without exposure of subchondral bone) or grade 4 (i.e., exposed bone) [15–17]. According to the arthroscopic system of classification of chondral injury proposed by Bauer and Jackson [12], delamination injuries are either flap or crater types. The modified ICRS classification categorizes delamination injuries as ICRS grade 3b or 3d lesions, depending on the status of the superficial cartilage [10].
Although cartilage delamination is commonly regarded as a shearing injury, other mechanism theories have been suggested. Rubin et al. [18] proposed that subchondral bone injury may precede cartilage injury, citing observations in animal models. This suggests that the delamination mechanism could result from a primary shallow osteochondral defect with osseous disruption or edema acting as an ischemic insult secondarily affecting the cartilage. Mankin [19] provides a histologic discussion of cartilage response to mechanical injury, describing characteristics of healing and progressive degeneration at different cartilage zones [20]. Of note, deep chondral extension (as in delamination) is expected to elicit multiple healing phases, including necrosis, inflammation, and repair [19]. Conversely, the hematoma and osseous edema expected in deep injury are not noted on MRI of delamination injury, whereas progressive degenerative changes of deep injury are. Although this could be a factor of time, with hemorrhage or edema resolving before MRI is performed, neither the expected artifact from hemosiderin deposition nor subchondral bone depression is reported in conjunction with delamination injury. Imaging follow-up of nontreated delamination injury to evaluate the presence of scar formation (hyaline versus fibrous repair) could provide beneficial information regarding the healing process but was not identified in a literature review [19, 20].
Patients with articular cartilage injuries typically present with knee pain and effusion, probably as a result of associated soft-tissue or bone injury because cartilage is aneural. A small case series of delamination reported that 70% of patients reported recurrent episodes of swelling at the knee [11]. Less commonly, patients report symptoms of pain and locking that may be confused with meniscal abnormalities. Patients with chondral injury may not recall the inciting event [12, 21]. Those patients indicating a suspected cause of injury commonly report a sports-related injury [22]. Physical examination may reveal joint-line tenderness, joint effusion, or crepitus [11]. Palpation of the condylar defect may reveal focal tenderness; however, this is not always the case. Intraarticular cartilage fragments may cause clicking, catching, or locking as symptoms, and because free intraarticular fragments can migrate around the knee, it is possible to have symptoms in varying locations. Patients can also present with meniscal abnormalities (Figs. 2A and 2B) or with signs of instability resulting from concomitant anterior cruciate ligament injuries leading to positive results of the drawer test, Lachman test, and meniscus-specific tests.
 View larger version (177K) | Fig. 2A —39-year-old man with full-thickness tibial articular cartilage delamination injury. A, Coronal proton density–weighted fat-saturated MR image (TR/TE, 2050/30) of knee shows full-thickness tibial articular cartilage delamination injury involving superficial and deep cartilage (arrowhead) and extending along tidemark (arrows). Medial meniscus tear and mild femoral subchondral osseous edema (asterisk) are also visible. |
 View larger version (231K) | Fig. 2B —39-year-old man with full-thickness tibial articular cartilage delamination injury. B, Sagittal T2-weighted fat-saturated MR image (TR/TE, 3377/72) of same knee as in A shows full-thickness tibial articular cartilage delamination injury involving superficial and deep cartilage (arrowhead) and extending along tidemark (arrows). Medial meniscus tear, mild subchondral femoral osseous edema (asterisk), and tibial impaction injury are also present. |
Most reported cases of cartilage delamination are located over the femoral side of the knee joint, especially the medial femoral condyle [11, 14, 18, 21]. Although not specifically evaluated, most single Outerbridge grade 4 lesions were located at the femoral or patellar sides of the knee joint, with medial and lateral tibial plateau lesions accounting for approximately 5% of lesions [15].
The true frequency of cartilage delamination of the knee is unclear. A review of focal full-thickness cartilage defects of the knees of athletes reported a prevalence of 36% [23]. Two large series reported that isolated full-thickness cartilage injury of the knee—including but not limited to delamination—occurred in 11% of 993 arthroscopies and accounted for 5.2% of 2931 cartilage injuries identified by arthroscopy [24, 25]. Similarly, a 4% incidence of chondral fracture, as delamination was sometimes known historically, was reported in a series of 312 consecutive knee arthroscopies [26]. Additional smaller case series have noted a cartilage fracture incidence ranging from less than 1% to 10% [27, 28].
| Imaging of Chondral Delamination | |
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At MRI, delamination typically appears as a thin line of near–fluid intensity interposed between the deep layer of articular cartilage and the underlying bone (Fig. 3). Edemalike marrow signal may be present in the subchondral bone and can help to identify the site of injury (Figs. 3 and 4). The presence of a joint effusion may increase the conspicuity of a delamination injury by interposing additional fluid beneath the injured cartilage [11, 29] (Figs. 2B and 3). With MR arthrography, a line of contrast material may be present at this interface.
 View larger version (299K) | Fig. 3 —13-year-old boy with extensive delamination injury. Sagittal proton density–weighted fat-saturated MR image (TR/TE, 4000/37) shows extensive delamination injury (arrow) at lateral femoral condylar articular cartilage extending to articular surface. There is mild subchondral bone edema (asterisk) near inferior extent. Joint effusion (E) is also present. |
 View larger version (214K) | Fig. 4 —39-year-old man with delamination injury. Coronal proton density–weighted fat-saturated MR image (TR/TE, 3770/16) shows delamination injury (arrow) at lateral tibial plateau with subtle adjacent osseous edema (asterisk). |
Although highly specific, the use of MRI for the detection of cartilage injury is reported to have wide variance in sensitivity when compared with arthroscopy, even when cartilage-specific sequences are incorporated [30, 31]. Superficial cartilage lesions, including delamination injuries, generally are one of the more difficult types of lesions to visualize and assess accurately with MRI [32]. Imaging pitfalls include poor spatial resolution secondary to a small matrix, chemical shift and truncation artifacts, and the use of conventional spin-echo or gradient-recalled sequences instead of an FSE technique [9]. An early small case series of knee delamination injury in high-level athletes reported that preoperative MRI had a diagnostic sensitivity of 21% [11]. However, extensive progress in MRI technique has been made since most of these reports were published.
MRI has since been well established as highly sensitive, specific, and accurate in the diagnosis of chondral lesions, specifically with the use FSE and 3D DESS sequences [9, 31, 33]. Potter et al. [9] noted that MRI had a sensitivity of 87% in the diagnosis of both low- and high-grade cartilage defects. Kohl et al. [31] reported that MRI had a high sensitivity for diagnosis of Outerbridge grade 3 (74%) and grade 4 (83%) lesions when compared with arthroscopy. Brown et al. [34] noted high positive and negative predictive values for the detection of traumatic chondral delamination when comparing MRI to arthroscopy. The sensitivity and specificity of MRI in the diagnosis of delamination injury specifically, with the use of modern techniques, is unclear.
Although cartilage is radiolucent and thus is not directly discernable by either radiography or CT, CT arthrography can depict chondral lesions in patients who cannot undergo MRI (Fig. 5). Spiral CT arthrography has been shown to evaluate knee articular cartilage lesions accurately and can depict full-thickness cartilage delamination [35–37]. Findings from CT arthrography are similar to those from MR arthrography, with a thin line of contrast material (and sometimes air) outlining the cartilage at the interface between deep articular cartilage and subchondral bone. Radiographs may be obtained at the time of initial evaluation to evaluate joint alignment. These may include long-leg radiographs obtained with the patient standing, because malalignment may need to be addressed with an osteotomy to help offload the area of defective cartilage.
 View larger version (381K) | Fig. 5 —41-year-old woman with full-thickness chondral defect. Coronal reconstruction of CT arthrogram shows arthrographic contrast material insinuating through full-thickness chondral defect (short arrow) caused by lateral tibial osteochondral fracture (long arrow). Focal cartilage thinning from osteoarthritis is present at margin of lateral femoral condyle (arrowhead). |
| Significance and Treatment | |
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Although it is a relatively uncommon injury, delamination can lead to early-onset degenerative changes with resultant pain and loss of function, and because it is amenable to surgical treatment, it is therefore considered a surgical lesion [38]. Accurate diagnosis and treatment are especially important because delamination injury most commonly occurs in the active young adult population [25]. Delamination is also recognized as one of the most common complications after surgical repair of cartilage, with separation of repaired cartilage from subchondral bone occurring, and it leads to unplanned surgical revision [10, 39]. Chondral injuries have limited healing potential, especially when compared with osteochondral fractures [19].
After the lesion is identified on MRI, diagnostic arthroscopy is typically performed with or without treatment of the lesion at initial surgery. Current treatment options include microfracture or bone marrow stimulation, osteochondral autograft transplantation, osteochondral allograft transplantation, autologous chondrocyte implantation, particulated juvenile cartilage allograft transplantation, and cell-based and scaffold treatments [40, 41]. Surgeons typically débride the loose cartilage to a stable base and then measure the lesion. Articular cartilage treatment is based on patient age, the size of the lesion, and patient preference regarding allograft versus autograft. The goal of treatment is to either regenerate hyaline cartilage or replace the area with a fibrocartilage scar because not all treatments create hyaline cartilage [40]. Depending on the surgery, the procedure is performed arthroscopically, with arthroscopic assistance, or as an open procedure. No clear guidelines currently exist regarding the best treatment modality. Therefore, surgeon preference is generally considered the most important decision in terms of treatment.
| Conclusion | |
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Delamination injuries of knee cartilage are surgical lesions that can lead to significant morbidity if they are not treated. These injuries may present with clinical symptoms identical to those of meniscal injury, and arthroscopic identification can be difficult without a superficial defect, thereby creating a role for imaging in diagnosis. A low sensitivity of imaging identification of delamination injury of the knee is reported in the available literature, although vast improvements in MRI of cartilage have since been introduced. A discussion of delamination injury, its mechanisms, and optimal imaging parameters is presented to improve the accuracy of diagnosis of this lesion.
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