Evaluation of ACL Graft by MRI: A Pictorial Review The Journal of Global Radiology

The reconstruction of a ruptured anterior cruciate ligament (ACL) is a well established procedure for repair of ACL injury. Knowledge of normal appearances, the expected postoperative changes over time, and potential acute and chronic complications of this reconstruction procedure are essential. This study illustrates the role of MRI in evaluation of ACL reconstructions and their complications. MR imaging is the modality of choice for evaluation of ACL graft reconstruction. ACL graft complications such as abnormal tunnel positioning, partial and complete graft tears, arthrofibrosis, and tunnel cysts can be reliably assessed using MRI.


Introduction
The anterior cruciate ligament (ACL) is one of the important stabilizers of the knee that is commonly torn in sports injuries. ACL reconstruction is a common and often successful surgical intervention. The increased number of ACL reconstruction surgeries being performed has led to an increased demand for the postoperative knee evaluation when symptoms persist or recur after these procedures. In order to guide treatment management, it is essential to have knowledge of the knee's normal imaging appearances, expected postoperative changes over time, and potential complications from this reconstruction procedure.
The two primary ACL reconstruction procedures are: • The autologous bone-patella tendon-bone graft • The autologous four-strand hamstring graft, which is also known as the doubled semitendinosus and gracilis tendon graft [ Figure 1] [1].
The ACL graft fixation is done using a wide range of fixation devices. These differ in function, shape, size, material, and biomechanical properties [ Figure 16]. The outcome of this procedure is generally good, though graft rupture or clinical failure is known to occur.
Patients with postoperative complications present with symptoms of persistent pain, instability, joint swelling, infection, and stiffness. The possible causes of reconstruction failure and patient complications are graft discontinuity, inappropriate position of the femoral and/or tibial tunnel, hardware failure, infection, and arthrofibrosis [5]. The goals of revision ACL surgery are to achieve stabilization of the knee and to prevent further injury to the articular cartilage and menisci [10].
The indications for evaluating ACL reconstructions with magnetic resonance imaging (MRI) include [5]: • Instability of the knee joint • Postoperative re-injury to the knee • Postoperative stiffness, especially extension loss due to flexion contracture • Preparation for the revision of a failed ACL reconstruction, which can aid the surgeon in preoperative planning.

Imaging Protocol
MRI examination of the knee should be done in three planes: the axial, coronal, and sagittal planes. Patient should be positioned in a supine position with the knee placed at 10°-15° of external rotation. A knee joint-specific extremity coil should be used. An axial acquisition is taken through the patellofemoral joint. It is used as an initial localizer for the subsequent sagittal and coronal plane images. Axial images should be obtained from the distal portion of the quadriceps tendon to the insertion of the patellar tendon on the tibial tuberosity. Sagittal images should be obtained from the medial to lateral femoral condyles. Coronal images should be obtained with a line parallel to the femoral condyles. The images should be analyzed for the location of the femoral and tibial tunnels, graft characteristics, and graft-related complications.

Graft Signal Intensity
ACL grafts appear in MR images at a uniformly low signal intensity, similar to the signal characteristics of the native graft harvest tissue (patellar tendon or hamstring tendon) [ Figure 2]. Increased intrasubstance graft signal changes develop within the first year after surgery and are thought to represent changes related to synovial proliferation, vascularisation, and "neoligamentization" of graft constructs.
Complete resolution of such graft signal changes are often described 18-24 months after surgery [3]. Due to ongoing ligamentization of the graft for up to one and a half years, postoperative ligament evaluation MRI may be warranted in patients experiencing severe discomfort or reinjury [11].

Femoral Tunnel
The position of the femoral tunnel is the primary factor in maintaining the isometry of a graft [4]. It should be evaluated in both the sagittal and coronal planes. In the sagittal plane the tunnel is assessed by drawing a line along the posterior cortex of the femur and another line along the roof of the intercondylar notch. The inferior portion of the tunnel should be located at the intersection of these two lines.
On the coronal MR image, the intra-articular portion of the femoral tunnel should open at the superolateral posterior margin of the intercondylar notch [ Figure 5]. If a clock face is superimposed on a coronal MR image with the center at the intercondylar notch, the tunnel should be oriented between ten and eleven o'clock on the right knee or between one and two o'clock on the left knee [ Figure 3]. An anteriorly placed femoral tunnel will cause elongation of graft and instability of knee [14].

Tibial Tunnel
The tibial tunnel sagittal should be oriented parallel to the Blumensaat line, which is a line drawn along the intercondylar roof. The distal portion of the tunnel should start near the tibial tuberosity and the intra-articular opening of the tunnel should be completely posterior to the Blumensaat line [1].

ACL Reconstruction Complications
Based on clinical symptoms, the main complications of the reconstruction are divided into two groups: decreased range of motion and laxity [6]. Complications leading to decreased range of motion are an anteriorly placed tibial tunnel, arthrofibrosis, cystic degeneration of the ACL graft, and intraarticular loose bodies [ Figure 4].

Complications Leading To Laxity and Graft Tear
Grafts are most susceptible to injury during the process of remodelling, which occurs approximately four to eight months after surgery. The primary signs include graft signal abnormalities including increased signal on T2-weighted sequences, increased graft thickness, and fiber discontinuity [13] [ Figures 8,9].
Hardware failure is uncommon but is important to recognize because it can cause instability of the graft which can be confused with graft disruption. Hardware failure includes screw displacement and dislodgment of the bone plug [2].
Arthroscopic reconstruction of the ACL is a procedure performed frequently and generally with good results [ Figures 2,3]. Complications of the surgical procedure, however, are not infrequent. Technical causes include nonanatomic tunnel placement, hardware failure, improper graft fixation, and insufficient graft material [12]. Biologic causes include failed ligamentization, infection, and arthrofibrosis [ Figure 15]. External causes for failure include traumatic rupture, secondary instability of the knee, and improper rehabilitation.
MRI is the most valuable imaging method for postoperative evaluation of the knee [ Figures 17-21]. It is non-invasive and has multiplanar imaging capabilities that are useful for assessing the tunnel positioning and other structures of the knee. It offers the added benefit of direct visualization of the graft with excellent soft tissue contrast [9].
Arthrofibrosis is the abnormal proliferation of fibrous tissue in a joint that leads to the loss of motion, pain, muscle weakness, swelling, and functional limitation. It is most commonly associated with joint trauma or surgery. Arthrofibrosis following ACL reconstruction can present as a focal or diffuse process that limits the mobility of the knee.
Other factors that can lead to knee stiffness and restriction in motion after ACL reconstruction may also play a role in the development of arthrofibrotic lesions. These factors might include suboptimal femoral or tibial tunnel placement or a stretched and overtensioned ACL graft.

Conclusion
Given the increasing number of patients to undergo ACL reconstruction, it is critical to be familiar with both normal postoperative imaging appearance and the appearance of complications in the graft reconstruction. MRI is the modality of choice for evaluation of failed ACL graft reconstruction surgery, normal appearance, and all the intermediate and long term complications of the surgery.