Attenuation In Meniscus Body Acl Graft Thickness To 13 Mm

MRI is currently the most widely used noninvasive, cross-sectional imaging technique to assess the knee joint. MRI is accepted widely as a reliable means

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Fig. 1. (A) Scout view from CTarthrogram of left knee shows long lateral sideplate and screws used to treat tibial fracture. Extensive hardware in close proximity to joint would limit visualization on MR, but does not preclude diagnostic CT examination. (B) Coronal reformation from CT arthrogram shows normal medial (arrow) and lateral (open arroW) menisci. Normal-thickness articular cartilage is outlined by contrast media in joint space. (C) Sagittal reformation from CT arthrogram, anterior to image left. Anterior and posterior horns of normal medial meniscus (arrowheads) are sharply defined. Artifact from metal sideplate (asterisk) does not interfere with joint visualization.

to assess knee meniscal tears, cruciate ligament disruption, bone marrow pathology, collateral ligament sprains, and cartilage integrity. CT arthrography has a similar diagnostic accuracy for a subset of these conditions and, for this reason, it is important to recognize that CTA is an excellent substitute for MRI in selected patients.

CTA of the knee usually is performed as a single-contrast examination using iodinated contrast media, or as a double-contrast examination using a large amount of air and a smaller quantity of iodinated contrast media to coat the joint surfaces. At the author's institution, the single-contrast technique is preferred, because it is simpler to perform and because some of our examinations are done following a failed MRA; the MRA always is performed as a single-contrast examination. There is some evidence to suggest that visualization of intra-articular osseous loose bodies is facilitated by the double-contrast technique. Because the osseous fragment rarely is exactly the same density as iodin-ated contrast media, osseous loose bodies can be detected within the contrast when viewed with appropriate window and level settings.

In 1985 one of the earliest reports was published on the use of CTA for the detection of meniscal tears as an adjunct to conventional double-contrast knee arthrography using a single slice CT scanner [5]. Today, detection of meniscal tears using single-contrast CTA and modern multichannel CT scanners has a reported sensitivity and specificity of 98% and 94% to 96% respectively, which are comparable to MRI [6]. A meniscal tear is diagnosed on CTA using criteria similar to those of MRI: deformity of meniscal contour or size or contrast tracking into the meniscus (Fig. 2).

More challenging is the detection of a meniscal tear in the postoperative patient. Conventional MRI has a reported accuracy of 66% to 80% or more [7-9]; MRA increases the accuracy to 88% to 92% [7,8]. A limited study using CTA showed a sensitivity of 79% to 93%, and specificity of 89% [10], which compares favorably with MRI. The diagnosis of a meniscal retear at CTA is

Mri Meniscus Retear

Fig. 2. (A) Sagittal reformation from single-contrast CT arthrogram, anterior to image left. Contrast outlines horizontal tear (arrow) of posterior horn of medial meniscus. (B) Coronal reformation from same arthrogram. Tear of medial meniscus extends into body region (arrow). Partial thickness cartilage defect of medial femoral articular surface (arrowhead) is visualized.

Fig. 2. (A) Sagittal reformation from single-contrast CT arthrogram, anterior to image left. Contrast outlines horizontal tear (arrow) of posterior horn of medial meniscus. (B) Coronal reformation from same arthrogram. Tear of medial meniscus extends into body region (arrow). Partial thickness cartilage defect of medial femoral articular surface (arrowhead) is visualized.

established when there is abnormal meniscal shape (small or deformed) and in-trameniscal contrast is seen in more than one third of the meniscus length and height, or if peripheral meniscal capsular separation is detected [10]. Minor irregularities of the meniscal surface are considered to be within the range of normal for the postoperative meniscus.

Although CT cannot visualize the cruciate ligaments directly, CTA can be used to assess for ACL tears. The reported sensitivity and specificity for the detection of complete ACL tears at CTA are 90% to 95%, and 96% to 99% respectively, similar to the accuracy of MRI [11]. The normal ACL is outlined by contrast during arthrography, and is seen on sagittal and coronal reformations as an uninterrupted line parallel or nearly parallel with the roof of the intercon-dylar notch (Blumensaat's line) (Fig. 3). Tears of the ACL are diagnosed if there is discontinuity of the ligament, abnormal course (sagging), and abnormal contour. These findings are similar to some of the criteria used in MRI; secondary signs such as anterior tibial translation, depression of the lateral femoral condylar notch, and fracture of the posterior lateral tibial plateau can be used to confirm the diagnosis. Although it is normal to encounter a small linear contrast collection parallel with the course of the ACL, intra-ligamentous contrast appearing in a cleavage plane is abnormal. Findings and accuracy of partial ACL tears at CTA are unknown.

Imaging of the postoperative ACL graft is possible using CTA [12]. This is particularly important because evaluation of the graft may be difficult with MRI if ferromagnetic hardware has been used to secure the graft, or if extensive ligament reconstruction elsewhere in the knee obscures the menisci. The interference screws, staples, and other surgical hardware usually placed do not result in significant artifacts on CT. The normal ACL graft appears as a band of tissue outlined by contrast. Small linear regions of contrast may outline fascicles of a graft, and should not be considered to be abnormal. Interruption of the graft is an indication of graft failure. The graft tunnel, graft

Normal Acl Mri Condylar Roof
Fig. 3. Sagittal reformation from CTarthrogram illustrates normal ACL (arrows).

alignment, and presence or absence of femoral notch osteophytes all can be assessed using CTA.

Cartilage defects can be graded reliably and detected using CTA. Most grading systems follow an adaptation of Shahriaree's system [13] used to assess chondromalacia: grade 1 is surface blistering or fibrillation, grade 2 is a focal defect involving less than 50% thickness, grade 3 is a focal defect involving more than 50% thickness, and grade 4 represents bare bone. The superior soft-tissue contrast of MRI translates to a superior ability to visualize grade 1 defects. For other grades, CTA is comparable to MRI [14,15], with sensitivity and specificity of 80% and 88% respectively for detecting grade 2 lesions, and 85% and 94% for detecting grade 3 lesions [14]. According to some authors evaluating the detection of loose bodies in the knee [16], MRA is the best technique, with a statistically significant advantage over conventional MRI. In this same study, 3-mm-thick axial CT performed with air contrast only performed better than conventional MRI [16]. The take-home message is that the reliable detection of surface cartilage defects and intra-articular loose bodies in the knee is best with CT or MR arthrography (Fig. 4).

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