Mri Posterior Cruciate Ligament Demonstrates Increased Signal

Bl V'VJ

Fig. 6. Sagittal T2-weighted fat-suppressed (TR/TE 3500/70) direct MR arthrographic image demonstrating a recurrent tear of the anterior horn of the medial meniscus following a prior partial meniscectomy. There is imbibition of the diluted gadolinium mixture into the recurrent tear cleft (arrow).

MRI has been extensively used in the assessment of patients following ACL reconstruction presenting with recurrent symptoms of instability or complications including impingement, arthrofibrosis, cystic degeneration, tunnel position, and widening, as well as donor site complications. Susceptibility artifacts from metallic fixation devices such as interference screws are generally not problematic enough to preclude adequate MRI and the use of techniques to reduce metallic hardware-related artifacts such as increased bandwidth and matrix, reduction of slice thickness, and interecho spacing and use of short tau inversion recovery sequence (STIR) rather than spectral fat suppression [32] is not generally required. Bioabsorbable fixation devices have also gained popularity with even less artifact and distortion generated at MRI [17,33].

Following a BPTB reconstruction, during the first 3 postoperative months, the reconstruction graft tendon typically demonstrates uniformly low signal intensity on T1- and T2-weighted images. Thereafter, there is progressive vascularization of the periligamentous soft tissues with subsequent synovialization and modeling resulting in graft ''ligamentization'' [34]. During this phase, which may last 12 to 18 months, the graft may display enhancement post IV contrast administration as well as increased signal intensity on T1- and T2-weighted imaging because of the synovial proliferation [35]. The graft construct may also appear inhomogeneous and be difficult to visualize adequately on MRI particularly at 1 year postoperatively [36,37]. Variable degrees of marrow edema may be seen at MRI around the reconstruction graft tunnels, particularly the femoral tunnel, for up to 15 months after surgery [33]. The donor site initially demonstrates thickening and increased T1 and T2 signal as well as a gap within the central third of the tendon [38] (Fig. 7). With time, the central defect gradually diminishes in size and may disappear [39]. By 2 years following surgery the tendon demonstrates normal low signal intensity but may display residual thickening [16,27]. Development of donor site morbidity such as patellar tendon rupture, patellar fractures, and anterior knee pain has been cited by investigators as one rationale for more widespread use of hamstring graft reconstruction techniques.

The normal hamstring tendon ACL graft displays similar MRI signal changes to the BPTB graft over the postoperative period (Fig. 8) but in addition may demonstrate areas of linear increased signal intensity and fluid in-between the four bundles of the graft construct in the absence of a graft tear [40], a finding that would be abnormal in BPTB graft [41]. The hamstring tendon donor sites commonly demonstrate fluid within the harvest tracks during the first month postoperatively, before progressive regeneration of the tendons. Apart from possible residual thickening at their insertion sites, the regenerated hamstring tendons typically resume a normal MRI appearance by 1 year and can be difficult to differentiate from normal tendons [42].

Crucial factors in accomplishing a successful surgical outcome following ACL reconstruction relate to avoidance of graft impingement and maintenance of isometry. These factors are partially governed by tunnel positioning with the femoral tunnel being of importance in maintaining isometry, while the tibial

Mri Hamstring

Fig. 7. Axial T2-weighted fat suppressed image (TR/TE, 3800/75) (A) illustrating a small residual defect (white arroW) within the central third of the patellar tendon following harvest of a BPTB graft. The sagittal proton density MR image (TR/TE, 2200/15) (B) demonstrates a bony defect along the anterior surface of the patella (white arrow) as well as residual postoperative thickening of the proximal patellar tendon (black arrow).

Fig. 7. Axial T2-weighted fat suppressed image (TR/TE, 3800/75) (A) illustrating a small residual defect (white arroW) within the central third of the patellar tendon following harvest of a BPTB graft. The sagittal proton density MR image (TR/TE, 2200/15) (B) demonstrates a bony defect along the anterior surface of the patella (white arrow) as well as residual postoperative thickening of the proximal patellar tendon (black arrow).

Imaging Matrigel

Fig. 8. Sagittal proton density-weighted image (TR/TE, 2000/15) (A) following a hamstring ACL reconstruction demonstrates a normal low signal intensity graft (white arroW) with intermediate signal intensity tissue surrounding the graft (black arrow). Tl-weighted image with fat saturation (TR/TE, 660/10) following intravenous gadolinium (B) shows enhancement of the periligamentous tissues consistent with vascularization and synovialization (white arrows).

Fig. 8. Sagittal proton density-weighted image (TR/TE, 2000/15) (A) following a hamstring ACL reconstruction demonstrates a normal low signal intensity graft (white arroW) with intermediate signal intensity tissue surrounding the graft (black arrow). Tl-weighted image with fat saturation (TR/TE, 660/10) following intravenous gadolinium (B) shows enhancement of the periligamentous tissues consistent with vascularization and synovialization (white arrows).

tunnel position is critical in preventing impingement. Tunnel positioning can be directly visualized at MRI. At MRI evaluation, optimal position of the femoral tunnel is at the intersection of the posterior femoral cortex and the posterior edge of the intercondylar roof on sagittal MR images [43]. The anterior margin of the tibial tunnel opening should lie completely posterior to a line tangention-al to the intercondylar roof (Blumensaat's line) as imaged with the knee in a fully extended position at MRI (Fig. 9). The center of the tibial tunnel opening ideally needs to be located 42% of the entire sagittal distance of the tibial plateau from the anterior edge of the tibia [44]. Reconstructions in which the tibial tunnel is located anterior to Blumensaat's line are prone to mechanical impingement of the graft by the distal intercondylar aspect of the femur upon knee extension. Graft impingement may lead to symptoms of limited terminal extension of the joint and may result in graft fibrosis, partial tearing, or eventual complete graft tearing.

Impingement may be depicted at MRI by increased signal intensity within the distal two thirds of the graft [45]. However, as similar increased signal intensity may be visualized as a result of ''ligamentization'' of the graft, other findings such as anterior positioning of the tibial tunnel and contact between the distal graft and the anteroinferior edge of the intercondylar roof need to be demonstrated to ascribe signal changes directly to graft impingement [46] (Fig. 10). It has been suggested, however, that MRI findings of increased signal within the distal graft on its own may be the result of unrecognized impingement [44]. In either case, demonstration of increased T2-weighted signal similar

Tibial Plateau Abnormal

Fig. 9. Sagittal proton density MR images (TR/TE, 2200/15) demonstrating the optimal positions for the femoral (A) and tibial (B) tunnels. The femoral tunnel is located at the intersection of the posterior femoral cortex and the roof of the intercondylar notch (A). The tibial tunnel lies completely posterior to Blumensaat's line (B).

Fig. 9. Sagittal proton density MR images (TR/TE, 2200/15) demonstrating the optimal positions for the femoral (A) and tibial (B) tunnels. The femoral tunnel is located at the intersection of the posterior femoral cortex and the roof of the intercondylar notch (A). The tibial tunnel lies completely posterior to Blumensaat's line (B).

to fluid signal intensity, particularly within or through a portion of the substance of a BPTB graft, is suggestive of a graft tear, whether the result of ongoing impingement or other causative etiology. In an attempt to prevent impingement, notchplasty is commonly performed at the time of ACL

Fig. 10. Sagittal fat-suppressed T2-weighted MR image (TR/TE, 3500/70) in a patient with graft impingement. The tibial tunnel is located anterior to Blumensaat's line; there is contact between the graft and the roof of the intercondylar notch causing focal posterior bowing and signal change of the graft (arrow).

reconstruction by resection of part of the lateral wall and roof of the notch [47]. This results in concave or flat contour to the notch on coronal and axial imaging. Indistinct margins are initially seen after notchplasty on MRI, although cortical bone and fibrocartilage may form over the operative site and in some cases may become hypertrophic causing delayed graft impingement

Graft failure may occur insidiously as a result of impingement or as a result of reinjury. The graft is most vulnerable during the period of ''ligamentiza-tion.'' Useful signs for demonstrating an intact graft include continuity of graft fibers on coronal images and uniform graft thickness on coronal and sagittal images. Complete graft discontinuity particularly on coronal images is indicative of graft tears (Fig. 12). Of the secondary signs of an ACL graft tear, anterior tibial translation by more than 5 mm is most useful, but this has a low sensitivity as a diagnostic criteria of graft tearing on it own [49]. Direct MR arthrography has also been evaluated for assessment of graft failure. Using discontinuity of the graft with extension of gadolinium into the defect as a sign of a tear, sensitivity of 100% and specificity of 89% to 100% has been achieved [50]. In the presence of recurrent instability but demonstration of intact graft fibers, graft stretching is the likely cause of the symptoms. In such a situation, posterior bowing of the graft may be illustrated on the sagittal images [41].

Cyclops lesions or localized arthrofibrosis may be seen in 1% to 10% of ACL reconstructions resulting in an inability to gain full extension [46]. This localized accumulation of fibrous, synovial, and osseous tissue is typically seen anterior to the distal ACL graft characterized by low to intermediate signal on T1- and proton-density-weighted pulse sequences and heterogeneous but

Mri Posterior Cruciate Ligament
Fig. 11. Coronal intermediate-weighted MRI (TR/TE, 3700/35) illustrates the characteristic scalloped margin (arrow) to the lateral aspect of the intercondylar notch consistent with prior notchplasty.
Illustrated GraftingCyclops Anterior Cruciate Ligament

Fig. 12. Sagittal T2-weighted fat-suppressed image (TR/TE, 3500/70) (A) demonstrating graft failure as illustrated by a transverse fluid signal intensity defect and discontinuity of the graft fibers (arrow). Coronal intermediate-weighted image (TR/TE, 3700/35) (B) in another patient with graft rupture fails to demonstrate any intact graft fibers (arrow).

predominantly low T2 signal [51] (Fig. 13). MRI has a sensitivity and specificity of 85% for demonstration of localized arthrofibrosis, and if only lesions larger than 1 cm are considered the specificity of MRI in the diagnosis of a cyclops lesions climbs to 100% [52].

Ganglion cyst formation may be encountered following ACL reconstruction, causing pain and limited motion when large. Ganglion cysts may develop within the tibial tunnel leading to tunnel enlargement, and may extend proxi-mally into the articulation or distally through the tibial tunnel to a subcutaneous location [53] (Fig. 14). Such cysts may arise secondary to cystic degeneration or partial tear of the graft and are more common with hamstring grafts. However it has been demonstrated that small amounts of fluid are frequently visualized on MRI within the tibial tunnel following hamstring grafts during the first postoperative year without progression to ganglion cyst formation or symptomatic complication [54].

Hardware complications relating to displaced hardware or malpositioned fixation devices can also be demonstrated at MRI (Fig. 15). Such hardware problems may lead to restricted motion or locking.

Other Ligamentous Reconstructions

Posterior cruciate ligament (PCL) reconstructions are far less frequently performed in comparison with ACL reconstructions even though PCL injuries comprise 3% to 20% of all ligamentous injuries in the knee. However, the majority of these are partial injuries that may heal with conservative treatment [55]. Because of the demonstration of development of osteoarthritis resulting from chronic instability, a more surgically oriented approach has been

Proton Density Weighted Image
Fig. 13. Sagittal proton density- (TR/TE, 2000/15) (A) and T2-weighted fat-suppressed (TR/ TE, 3500/70) (B) MR images display a localized area of low to intermediate signal intensity extending anterior to the distal ACL graft (arrows) consistent with localized arthrofibrosis.

advocated in patients with chronic symptomatic PCL laxity, patients with multiple ligament injuries, and isolated PCL injuries with significant laxity [56,57]. Because of the limited number of PCL ligament reconstructions, however, the MRI appearances of PCL reconstructions have not been as widely evaluated as the postoperative ACL. The BPTB graft is the most common choice of graft material with femoral fixation achieved through a femoral tunnel and an

Fig. 14. Sagittal T2-weighted fat-suppressed image (TR/TE, 3500/70) illustrating a tibial tunnel ganglion cyst causing tunnel enlargement (arrow).

Imaging Cyclops Lesions
Fig. 15. Sagittal proton density MR image (TR/TE, 2200/15) demonstrating an interference screw protruding through the articular surface of the proximal tibia (arrow).

interference screw. A tibial tunnel or a tibial inlay technique using a unicortical window and fixation with a bicortical screw and washer may accomplish tibial fixation. The latter technique results in a greater degree of metal-related artifact at MRI, which may partly obscure the distal PCL graft [58]. Malpositioning of the femoral tunnel of a PCL reconstruction has been correlated with recurrent instability with the optimal tunnel position located within the anterior 25% of Blumensaat's line [59]. Studies have suggested that the normal PCL graft demonstrates uniform low signal intensity on T1- and T2-weighted images on long-term follow-up with a curved or straight appearance through the intercondylar notch [59]. A more recent study has demonstrated frequent mild to moderate increased signal intensity within the graft during the first year suggesting a syn-ovialization process similar to ACL grafts [58] (Fig. 16). No significant change in signal intensity was observed with time, although a reduction in signal intensity was demonstrated in some patients. Fluid signal intensity traversing a graft with discontinuity of graft tendon fibers on MRI is indicative of graft disruption, although longitudinal fluid signal intensity may be seen within hamstring graft constructs without signs or symptoms of graft dysfunction, instability, or disruption.

Collateral ligament repairs have not been extensively evaluated by MRI as the vast majority of injuries are partial and treated conservatively. In surgically treated cases a primary repair is often used. With posterolateral corner injuries graft material may also be used [30]. There is often metallic artifact related to the use of surgical staples but the ligaments may still be evaluated. Postopera-tively increased signal intensity and thickening is demonstrated and although the signal may diminish, a degree of thickening is invariable at follow-up MRI assessment [41] (Fig. 17).

Arthrofibrosis Mri

Fig. 16. Sagittal proton density- (TR/TE, 2200/15) (A) and fat-suppressed T2-weighted (TR/ TE, 3500/70) (B) images following PCL reconstruction. Despite susceptibility artifact and poor fat suppression the distal aspect of the intact PCL graft can be clearly visualized. The graft appears thickened and heterogeneous (arrows), which can be a normal postoperative finding.

Fig. 16. Sagittal proton density- (TR/TE, 2200/15) (A) and fat-suppressed T2-weighted (TR/ TE, 3500/70) (B) images following PCL reconstruction. Despite susceptibility artifact and poor fat suppression the distal aspect of the intact PCL graft can be clearly visualized. The graft appears thickened and heterogeneous (arrows), which can be a normal postoperative finding.

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Responses

  • virginio
    Does ACL cyst cause instability?
    7 years ago
  • juhani
    Why do acl grafts have moderate signal intensity on mri?
    7 years ago
  • tewolde
    What causes thickening and increased signal on MRI of knee?
    6 years ago

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