Magnetic Resonance Imaging of Post Operative Ligament

Transcription

1 Magnetic Resonance Imaging of Post Operative Ligament Richard Kijowski, M.D. Associate Professor of Radiology University of Wisconsin School of Medicine and Public Health Madison, Wisconsin Injury to the ligaments of the knee joint is common especially in the younger athletic population. Many patients experience persistent pain, instability, and restricted motion following ligament reconstruction surgery, and these complications are commonly evaluated using imaging studies. Magnetic resonance imaging (MRI) is the non invasive modality of choice for evaluating post operative ligaments due to its ability to directly visualize the reconstructed ligament and to evaluate other potential sources of knee pain including the menisci, tendons, and articular cartilage. This lecture will review the normal MRI appearance and common complications of reconstruction surgery of the anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL), which are the most commonly injured and subsequently reconstructed ligaments of the knee joint. MRI of ACL Reconstruction Surgery ACL tear is one of the most common sports related injuries (1). Partial thickness tears and full thickness tears in sedentary patients may be treated conservatively (2, 3). However, most surgeons recommend surgical reconstruction for young active patients with full thickness tears to restore knee function and allow return to sports activity (4 6). ACL reconstruction is typically performed using a bone patellar tendon bone or hamstring tendon autograft. Both types of grafts have been successfully used to reconstruct the torn ACL and have certain advantages and disadvantages (7). Cadaver allografts have also been used for ACL reconstruction but carry a potential risk for disease transmission (8, 9). The goal of ACL reconstruction surgery is to create an isometric ACL graft which maintains constant length and tension throughout knee range of motion and to avoid graft impingement. The location of the femoral tunnel is the most important factor for creating an isometric graft, while the location of the tibial tunnel is the most important factor for preventing graft impingement. The femoral tunnel should lie at the point

2 where Blumenstaat s line intersects the posterior femoral cortex. A femoral tunnel located too far anteriorly will cause stretching of the graft with resultant graft laxity and knee instability. The anterior margin of the tibial tunnel should lie posterior to the point where Blumenstaat s line intersects the tibia but not posterior to the midpoint of the tibia. A tibial tunnel located too far anteriorly will cause graft impingement with resultant loss of knee extension, while a tunnel located too far posteriorly will cause laxity of the graft with resultant knee instability (10 13). The appearance of the normal ACL graft on MRI depends upon the type of graft used and the time following reconstruction surgery. The normal bone patellar tendon bone graft exhibits uniform low T1 and T2 signal similar to the native ACL immediately following surgery which is due to the avascular nature of the graft. However, between 1 to 3 months following surgery, the graft undergoes revascularization and synovial proliferation during a process referred to as ligamentization. During this time, the graft exhibits increased T2 signal which should never be as bright as the signal intensity of fluid. Ligamentization is completed 12 to 18 months following surgery at which time the graft exhibits uniform low T1 and T2 signal. The appearance of the normal hamstring tendon graft on MRI is similar to that of the normal bone patellar tendonbone graft. However, the hamstring tendon graft consists of 4 separate bundles doubled back on one another and then sutured together. For this reason, increased T1 signal can be seen between the separate bundles during the immediate post operative period and more than 18 months following surgery. Furthermore, fluid like signal on T2 weighted images may be seen between the separate bundles during the ligamentization phase (14 17). The most common complication following ACL reconstruction surgery is traumatic reinjury (18). Re injury may occur at any time following surgery, but is most common during the ligamentization phase at which time the graft is the weakest (16). The integrity of the ACL graft on MRI is best assessed on T2 weighted images. Full thickness tears are characterized by the absence of intact fibers coursing between the femur and tibia and fluid like signal in the expected location of the graft. Partial thickness tears are characterized by areas of fluid like signal within an intact graft. However, linear fluidlike signal between the separate bundles of a hamstring tendon graft should not be confused with a partial thickness tear. The presence of an intact ACL graft in a patient with knee instability raises the possibility of graft stretching rather than disruption. In this situation, MRI may show a more anteriorly located femoral tunnel and posterior bowing of the lax ACL graft (19 22). Graft impingement is another common complication following ACL reconstruction surgery. Impingement of the graft against the intercondylar notch results in loss of full knee extension and a high likelihood of progressive graft injury and eventual rupture. On MRI, graft impingement is characterized by a more anteriorly located tibial tunnel and increased T2 signal within the distal third of the ACL graft. The graft may also appear to drape over the anterior inferior edge of the intercondylar roof. Increased T2

3 signal in the setting of impingement may be difficult to distinguish from the normal appearance of an ACL graft during the ligamentization phase. However, graft impingement should be suspected if the increased signal within the graft worsens over time or persists 12 to 18 months following surgery (11, 12, 23, 24). Focal arthrofibrosis is another complication following ACL reconstruction surgery. The process is of unknown etiology and is characterized by synovial hyperplasia, excessive fibrous tissue production, and inflammatory cell infiltration which forms a nodule anterior to the distal portion of the ACL graft. The nodule may become entrapped between the femur and tibia and create a mechanical block to full knee extension. Focal arthrofibrosis can be readily detected using MRI and is best visualized on sagittal images. The nodule has low signal intensity on T1 weighted images and may be difficult to distinguish from adjacent joint fluid. However, the nodule has heterogeneous but primarily low signal intensity on T2 weighted images and is thus readily distinguishable from the high signal intensity fluid within the knee joint (25 28). Additional complication following ACL reconstruction surgery includes hardware failure and ganglion cyst formation. Hardware failure is rare and typically consists of bone plug dislodgement due to fracture or displacement of the fixation hardware. Displacement of the bone plug results in knee instability, and MRI is commonly performed to exclude the possible of graft rupture. The displaced bone plug and associated graft laxity is best visualized on T1 weighted images. Cystic degeneration or ganglion formation is a late complication following ACL reconstruction surgery. Ganglion cysts typically arise from the ACL graft within the tibial tunnel and are more common in ACL reconstructions using hamstring tendon grafts and endobutton fixation hardware. The cysts expand the bone tunnel and may extend out into the intercondylar notch and the adjacent soft tissue of the knee. Ganglion cyst formation is not associated with graft laxity or knee instability. (20, 29 31). Donor site complications are not uncommon following ACL reconstruction surgery with bone patellar tendon bone grafts which has led to the more frequent use of the hamstring tendon as alternative autograft material. On MRI, the normal patellar tendon in the immediate post operative period shows diffuse thickening, irregularity, and increased T1 and T2 signal. The signal intensity changes resolve within 12 to 18 months at which time the tendon has uniform low T1 and T2 signal although it may remain mildly thickened with a central defect. However, significant thickening and persistence of increased T1 and T2 signal with the patellar tendon more than 18 months following surgery indicates an inflammatory response which may weaken the tendon and lead to anterior knee pain. Other donor site complications associated with the use of bonepatellar tendon bone grafts include patellar fracture, patellar tendon rupture, and Hoffa s fat pad disease (29, 32, 33).

4 MRI of PCL Reconstruction Surgery PCL tears are less common that ACL tears but still comprise 3% to 20% of all ligamentous injuries of the knee joint. Most PCL injuries are partial thickness tears which are treated conservatively. For this reason, PCL reconstruction surgery is much less common that ACL reconstruction surgery. Reconstruction of the torn PCL is reserved for full thickness tears with symptomatic knee instability and PCL tears combined with other ligamentous injuries. Surgery is performed using bone patellar tendon bone and hamstring tendon autographs similar to ACL reconstruction surgery. On MRI, the normal graft in the immediate post operative period shows diffuse thickening and increased T1 and T2 signal. The signal intensity changes resolve within 12 at which time the tendon has uniform low T1 and T2 signal similar to the native PCL. Graft tears are characterized by the absence of intact fibers coursing between the femur and tibia and fluid like signal in the expected location of the graft. Focal arthrofibrosis may also occur anterior to the PCL graft and may be symptomatic and require surgical removal (34 37). References 1. Miyasaka KC, Daniel DM, Stone ML, Hirshman P. The incidence of knee ligament injuries in the general population. American Journal of Knee Surgery. 1991;4: Messner K, Maletius W. Eighteen to twenty five year follow up after acute partial anterior cruciate ligament rupture. Am J Sports Med. 1999;27(4): Epub 1999/07/29. PubMed PMID: Delince P, Ghafil D. Anterior cruciate ligament tears: conservative or surgical treatment? A critical review of the literature. Knee Surg Sports Traumatol Arthrosc Epub 2011/07/21. doi: /s x. PubMed PMID: Otto D, Pinczewski LA, Clingeleffer A, Odell R. Five year results of single incision arthroscopic anterior cruciate ligament reconstruction with patellar tendon autograft. Am J Sports Med. 1998;26(2): Epub 1998/04/21. PubMed PMID: Spindler KP, Warren TA, Callison JC, Jr., Secic M, Fleisch SB, Wright RW. Clinical outcome at a minimum of five years after reconstruction of the anterior cruciate ligament. J Bone Joint Surg Am. 2005;87(8): Epub 2005/08/09. doi: 87/8/1673 [pii] /JBJS.D PubMed PMID: Bach BR, Jr., Tradonsky S, Bojchuk J, Levy ME, Bush Joseph CA, Khan NH. Arthroscopically assisted anterior cruciate ligament reconstruction using patellar tendon autograft. Five to nine year follow up evaluation. Am J Sports Med. 1998;26(1):20 9. Epub 1998/02/25. PubMed PMID: Goldblatt JP, Fitzsimmons SE, Balk E, Richmond JC. Reconstruction of the anterior cruciate ligament: meta analysis of patellar tendon versus hamstring tendon autograft. Arthroscopy. 2005;21(7): Epub 2005/07/14. doi: S [pii] /j.arthro PubMed PMID:

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