Anterior cruciate ligament (ACL) tears are the most common cause of internal derangement of the knee. ACL deficiency leads to early osteoarthritis, and in high-performance athletes it is essential to replace the torn ACL to prevent loss of function of the knee. The ACL originates from the lateral femoral condyle in the intercondylar notch and inserts onto the lateral aspect of the anterior tibial spine.1 When viewed using magnetic resonance imaging (MRI), a normal ACL appears as a linear hypointense structure on all sequences in its anatomical location.1 Focal hyperintensity with loss of continuity signifies an ACL tear.2
An ACL graft can be harvested from two sites: the patellar tendon and the hamstring muscle. In the patellar tendon bone graft, the mid-segment of the patellar tendon, with a small bony fragment from both ends, is extracted and used for the reconstruction.3 This is the most common site for ACL graft extraction as the extraction is relatively easy and efficient. However, this site is also prone to more complications, such as patellar tendon tenderness and anterior knee pain, than by using the hamstring muscle.4 The semitendinosus is the most common hamstring muscle used for an ACL graft as the muscle is thin, sleek and can easily removed from its point of attachment. However, the extraction and subsequent folding and braiding of the graft into a quadruple-thickness strand requires skill and experience (Figure 1). This graft is also known to slip as there is no bone attachment at either end.5
The next important step for ACL grafting is the positioning of the tibial and femoral bony tunnels. The aim of this part of the procedure is to maintain adequate tension and length of the ACL graft on all ranges of knee motions in order to mimic a native ACL.6 Such a graft with adequate tension and length is referred to as an ‘isometric graft’. In an isometric graft, the tibial tunnel should be positioned parallel to the slope of the intercondylar roof, as can be seen on a sagittal image.7
The radiological evaluation of a patient with an ACL graft begins with a radiograph of knee (Figure 2). This radiograph serves as a baseline for further evaluation of the patient and displays the type of orthopaedic hardware used in the reconstruction.8 An isometric tibial tunnel should be placed in the medial tibial condyle and is laterally inclined with an exit just anterior to the anterior tibial spine. The isometric femoral tunnel begins in the superolateral intercondylar notch and continues laterally along the lateral femoral condyle. In an anteroposterior view, both the tibial and femoral tunnels should be in alignment. In the lateral view, the isometric point in the femoral condyle is seen at the centre of the intercondylar shelf. If the femoral tunnel is found outside this point, it signifies non-alignment. A plain radiograph resolves all these issues and can also assess any hardware failure in the form of fractures or displacement of the screws.8
Failures of ACL graft reconstruction can be classified into early and late failures.7 Early failures are occur within 6 months of ACL reconstruction and late failures occur after 6 months. Causes of early failures include poor surgical technique, failure of graft incorporation or errors in rehabilitation.9 Other complications of ACL graft include arthrofibrosis, tunnel cysts, iliotibial band friction syndrome and hardware failures.
Poor surgical technique, which leads to graft failure, includes the abnormal placement of the tibial tunnel. This leads to excessive tension on the ACL graft, subsequently leading to graft impingement.9 Graft impingement is the presence of a focal indentation in the femoral surface of the graft ACL at the femoral tunnel coupled with abnormal signal and posterior bowing (Figure 3). Using MRI, abnormal tunnel placement can be interpreted by sagittal images. A tangential line is drawn along the femoral intercondylar roof up to the anterior aspect of the tibial surface. If the ACL graft is located anterior to the line, this represents an abnormal tunnel placement (Figure 4).10
Ligamentization of the anterior cruciate ligament graft
Anterior cruciate ligament grafts are known to have abnormal areas of hyperintensity within the fibres. This hyperintensity is a normal process of ligamentization of the graft, which is extracted from the muscle and should not be interpreted as a tear.11 On a magnetic resonance (MR) image, this appears as focal T2 hyperintensity within the graft fibres (Figure 5) without any thinning or loss of continuity of any of these fibres.
Partial anterior cruciate ligament graft tear
Partial graft tears are secondary to many conditions. In the first 6 months post surgery, a tear could be secondary to abnormal tunnel placement or trauma if the patient continues to engage in active sports in the early days without proper rehabilitation.12 On an MR image, partial graft tears appear thinned out with a loss of continuity of some or most of the fibres (Figure 6).
Complete anterior cruciate ligament graft tear
Complete tears are secondary to recurrent stress on the abnormally positioned graft or due to trauma.7 On an MR image, complete tears on the ACL graft appear the same as those seen on the native ACL. There is diffuse hyperintensity with complete loss of continuity of the graft (Figure 7).2 Repeat arthroscopy is essential for complete tears and the graft needs to be freshly harvested from another site and reconstruction repeated after excising the previous, torn graft.13
Localized anterior arthrofibrosis
Localized anterior arthrofibrosis is also known as a ‘cyclops lesion’. This is a condition in which the knee joint motion is restricted by scar tissue, which may be a result of trauma or surgery.14 Localized anterior arthrofibrosis is predominantly a clinical diagnosis made when a patient is unable to extend his or her knee completely.15 In ACL reconstruction, localized anterior arthrofibrosis is usually seen in the anterior intercondylar region and within 6 months of ACL reconstruction.16 MRI is undertaken to look for the cause of limited flexion. On an MR image, the cyclops lesion appears as focal area of T1 and T2 hypointensity along the anterior intercondylar region (Figure 8).17 In severe cases this may encase the entire joint including the supra- and infrapatellar bursae. If the MRI reveals a cyclops lesion, then this needs to be excised by arthroscopy in order to relieve the limited flexion.18
Tunnel cysts are produced by the extension of the synovial fluid into the bony tunnel. This may be due to the incomplete incorporation of the ACL graft into the bony tunnel.19 Tibial tunnel cysts are more common than femoral tunnel cysts7 and appear on an MR image as oblong fluid collection along the tibial tunnel, with a small swelling formed at the tibial site of the graft due to subcutaneous extension of the fluid (Figure 9). Tunnel cysts found on an MR image need to be explored surgically. Arthroscopic treatment of the tunnel cyst includes excision of the cyst followed by packing the channel with cancellous bone in order to seal off the water channel.20
Iliotibial band friction syndrome
Iliotibial band friction syndrome is a mechanical complication caused by transfixed devices. This complication is seen when using a hamstring graft.21 The femoral fixation devices cause thickening of the iliotibial band as a result of constant friction. This presents as a dull, aching pain along the lateral aspect of the knee.22 On an MR image, a focal area of thickening can be seen along the site of friction in the iliotibial band (Figure 10). Treatment of iliotibial band friction syndrome includes the removal of this implant.23
The breakage of screws and the prosthesis used in reconstruction can result in graft failure. Hardware failures are best viewed on a plain radiograph (Figure 11). MRI is undertaken to look at the condition of the ACL graft. Fractured and displaced prostheses need to be removed.24
Anterior cruciate ligament grafting is a commonly performed surgery by arthroscopic surgeons. Understanding the normal findings and complications can help a radiologist to make a proper assessment of the graft, which can assist the arthroscopic surgeon in better management of any arising complications.