Fig. 1
Sagittal T2-weighted image of a normal ACL reconstruction. The graft is of homogenous low signal intensity 18 months after surgery. Also note the graft and the tibial tunnel are located posterior to Blumensaat’s line (red line)
Hamstring tendon grafts demonstrate distinct imaging features due to the nature of the graft reconstruction. The two tendons are sutured together, folded onto one another, and subsequently sutured together again, resulting in a graft composed of four separate bundles folded twice (so-called M-technique). This unique configuration of the hamstring tendon graft therefore results in linear areas of mild to moderate increased signal intensity on fluid-sensitive sequences between the double-folded graft which is due to entrapped foci of fluid and granulation tissue between the graft bundles. With this particular graft reconstruction, fluid may also be observed within the femoral and tibial tunnel [5, 10].
Correct position of the femoral and tibial tunnels is crucial. Figure 2 demonstrates the accurate position of the tunnels and the graft. The graft typically demonstrates a straight, taut configuration and should lie dorsal to Blumensaat’s line. In order to assess accurate tunnel position, it is useful to divide the AP-diameter of the intercondylar notch and the tibial plateau each in four equal-length segments (see Fig. 2). On sagittal images, the femoral tunnel should lie at the intersection of the posterior femoral cortex and the posterior intercondylar roof (at or dorsal to segment 4 of the femur; see Fig. 2). In contrast, the tibial tunnel should be posterior to Blumensaat’s line with the centre of the tunnel situated in segment 2 of the tibia as demonstrated in Figs. 1 and 2 [2–4, 10]. On coronal images, the femoral tunnel should be at the posterosuperior corner of the intercondylar notch at 10–11 o’clock for the right knee and at 1–2 o’clock for the left knee. Similarly, the tibial tunnel on coronal images should be centred on the intercondylar eminence [2–4]. A far anterior femoral tunnel or a far posterior tibial tunnel position may cause instability of the graft. In contrast, a tibial tunnel position that is too anteriorly located may result in a more horizontal alignment of the graft and in possible graft impingement.
Fig. 2
The drawing shows the correct position of the femoral and tibial tunnels as well as the graft in relation to Blumensaat’s line after single-bundle reconstruction. Note that the graft is located posteror to Blumensaat’s line. (drawing adapted from: Woertler K (2014) Anterior cruciate ligament reconstruction. In: Woertler K, Waldt S (eds) Measurements and classifications in musculoskeletal radiology. Thieme, Stuttgart, New York, p 58
Complications of ACL Reconstruction
Complications after ACL-reconstruction include: graft rupture, graft stretching, graft impingement, arthrofibrosis, cyst formation and tunnel widening, mucoid degeneration of the graft, hardware failure and impingement due to metalwork, intra-articular loose bodies, infection and donor site pathology.
Graft Rupture
The ACL-graft is most susceptible to injury during the revascularisation process of the graft which occurs 4–8 months after surgery. However, graft ruptures typically occur after trauma and are clinically characterized by instability. Similar to the MRI-diagnosis of complete preoperative ACL-tears, MRI-findings of complete postoperative ACL-graft ruptures can be divided into direct and indirect signs. Direct signs of ACL-graft rupture are complete discontinuity or deficiency of the graft fibres and fluid-like increased signal intensity completely traversing the graft. Indirect signs of ACL-graft rupture are anterior tibial translation, buckling of the PCL, posterior displacement of the posterior horn of the lateral meniscus and new bone marrow oedema-like signal affecting the posterolateral tibial plateau and the lateral femoral condyle. The accuracy of conventional MRI in the diagnosis of ACL-rupture after graft reconstruction is however slightly lower than the diagnostic accuracy for ACL-tears in the preoperative knee. Furthermore, indirect signs of ACL-graft rupture are not specific for ACL-graft rupture and may also be present in cases of malposition of the ACL-graft. In contrast, MR-arthrography has been reported to have a sensitivity of 100% and a specificity of 89–100% in the diagnosis of postoperative ACL-graft tears [3–5, 10].
Graft Impingement
Graft impingement can result in restriction of knee joint extension and increases the risk of graft rupture. Graft impingement may be due to tunnel positioning or less commonly due to osteophyte formation or the result of protrusion of tunnel screws. The most common cause for graft impingement is a far anterior position of the tibial tunnel whilst a far lateral placement of the tibial tunnel is less common. In these cases, the ACL-graft abuts the roof or the side wall of the intercondylar notch during knee extension therefore resulting in graft shearing and fraying which ultimately may cause graft rupture [11]. On MRI, in addition to malposition of the graft, graft impingement demonstrates increased signal intensity within the fibres at the site of impingement on T1-and T2-weighted images and kinking of the graft at the anterior margin of the intercondylar notch (Fig. 3). Within the first 12 months after graft reconstruction, it can be difficult to differentiate if signal changes within the graft are due to impingement or due to revascularisation. Signal change limited to the distal 2/3 of the graft, kinking of the graft as well as persistence of the signal changes beyond 12 months after surgery favour the diagnosis of graft impingement [5, 10].
Fig. 3
Sagittal intermediate-weighted fat-saturated image of ACL-reconstruction demonstrating graft impingement. The graft is frayed, shows increased signal intensity within the distal 2/3 of the fibres and demonstrates kinking at the anterior margin of the intercondylar notch (arrow)
Arthrofibrosis
Patients suffering from arthrofibrosis clinically present with restricted movement and pain. The incidence of arthrofibrosis after ACL-graft reconstruction is 5–10% [12]. Two distinct forms of arthrofibrosis are described: localised anterior arthrofibrosis and diffuse arthrofibrosis [7, 10].
Localized Anterior Arthrofibrosis
Localised anterior arthrofibrosis (also called “cyclops lesion”) is characterised by a well-defined nodule immediately anterior to the ACL-graft at the level of the tibial plateau. On arthroscopy, it bears resemblance to the single eye of the giant “cyclops” in Greek mythology. Histologically, cyclops lesions largely consist of fibrous granulation tissue with some cases containing bony fragments and synovium [13]. Whilst cyclops lesions are observed in 1–10% of all patients after ACL-graft reconstruction, only a minority of patients are symptomatic (up to 2% of all patients after ACL-graft reconstruction) [13, 14]. Symptoms are thought to be due to impingement of the cyclops lesion between the femur and tibia which may result in an audible clunk. Whilst the aetiology of cyclops lesions remains uncertain, it has been postulated that they may originate from a residual tibial ACL stump, infrapatellar fat pad metaplasia, or the graft itself. On MRI, cyclops lesions appear as nodular lesions located immediately anterior to the distal aspect of the ACL-graft and are of low to intermediate signal intensity on all pulse sequences (Fig. 4) [13, 14]. The diagnostic accuracy of MRI in the detection of cyclops lesions is 85% and increases to more than 90% in lesions larger than 1 cm [13].
Fig. 4
Axial intermediate-weighted fat-saturated image after ACL-reconstruction demonstrating a cyclops lesion. A well-defined nodular lesion which is of intermediate signal intensity (long thin arrow) located immediately anterior to the ACL-graft (thick, small arrow) is in keeping with a cyclops lesion
Diffuse Arthrofibrosis
Diffuse arthrofibrosis is associated with significant restriction of movement which is due to extensive fibrosis located anterior to the ACL-graft extending into Hoffa’s fat pad, the suprapatellar recess and sometimes into the posterior aspect of the knee joint. On MRI, diffuse arthrofibrosis manifests as diffuse areas of low signal intensity on all pulse sequences which is more pronounced than the scar tissue observed after knee surgery [3, 4, 7, 15].
Tunnel Widening and Cyst Formation
In the early postoperative period, it is common to observe a small amount of fluid in the fixation tunnels which may moderately enlarge. However, cyst formation within the tunnel with subsequent tunnel widening exceeding 20 mm may be due to incomplete graft incorporation. The tibial tunnel is most commonly affected. Occasionally, cysts may extend into the pretibial region resulting in a clinically palpable swelling. Cyst formation is more common with hamstring grafts. Its association with graft failure remains uncertain. However, extensive tunnel expansion may complicate ACL graft revision surgery. On MRI, cyst formation results in increased tunnel diameter. Within the tunnel there is low signal intensity on T1-weighted images and increased signal intensity on T2-weighted images in keeping with fluid and/or granulation tissue [3, 16]. Furthermore, granulation tissue will demonstrate avid enhancement after gadolinium administration. Osteolysis is particularly well identified adjacent to bio-absorbable grafts due to minimal susceptibility artefacts [10].
Donor-Site Morbidity
Occasionally, complications may arise at the donor site. This particularly affects bone-patellar tendon-bone grafts where patellar tendinosis, patellar tendon rupture or patellar fracture may occur [4]. However increased signal intensity and thickening of the patellar tendon as well as a central gap at the site of tendon harvesting are normal postoperative findings within the first 2 years after surgery [3, 17]. It is therefore crucial to correlate the clinical and imaging findings.
Posterior Cruciate Ligament (PCL)
Posterior cruciate ligament injury is rare, and are mainly partial-thickness tears which may be treated conservatively. There has been increased interest in PCL-reconstructions in order to treat PCL instability and prevent premature osteoarthritis. Therefore, current indications for PCL-reconstruction are: acute or chronic PCL injuries with significant instability, associated avulsion fracture, and the presence of multiple ligamentous injuries [18].
PCL-Reconstruction
PCL-reconstruction largely utilize the same grafts as those used for ACL-reconstruction. The fixation of the graft may be directly onto the tibia (so-called “inlay technique”) or through the tibial tunnel (also called “transtibial tunnel technique”), the latter being associated with impingement of the graft at the so-called “killer turn” point which occurs at the opening of the tibial tunnel (Fig. 5). Both single-bundle and double-bundle techniques are utilized. The single-bundle technique reconstructs the anterolateral bundle of the PCL and requires the creation of one femoral and one tibial tunnel. The double-bundle technique reconstructs both the anterolateral and the posteromedial bundle of the PCL and requires the drilling of two femoral tunnels and one single tibial tunnel. The double-bundle technique is technically more challenging and has not been proven to be superior [18, 19].
Fig. 5
Sagittal T2-weighted image of a normal PCL reconstruction using the transtibial tunnel technique. The PCL graft is thickened however it is of homogenous low signal intensity. Note the “killer turn” in the opening of the tibial tunnel (arrow), which is associated with impingement
Normal Postoperative MRI-Findings After PCL-Reconstruction
The reconstructed PCL-graft undergoes the same signal changes as the ACL-graft. Thickening of the PCL-graft is frequently observed (Fig. 5) [20]. The position of the femoral tunnel in the single-bundle technique should be in the anterior half of the insertion site of the native PCL at 11 o’clock in the right knee or at 1 o’clock in the left knee on coronal images and it should be 8–10 mm away from the articular margin. The tibial fixation site on the sagittal images should be located in the middle of the posterior half of the retrospinal surface, 8–15 mm distal to the articular surface. On axial images, it should be immediately medial to the articular midline. A too anterior location of the tibial tunnel, a too posteriorly located or excessively high located femoral tunnel may result in instability. Bone marrow adjacent to the tunnels and the fixation sites may be present within the first 12 months after surgery [18].
Complications of PCL Reconstruction
Complications of PCL and ACL graft reconstruction are similar and include: graft rupture, graft laxity and impingement, arthrofibrosis, cyst formation and tunnel widening. Graft impingement is a noteworthy complication as it may occur due to the “killer turn” position in the transtibial technique or due to erroneous location of the tunnels [18, 20, 21]. On MRI, thickening and increased signal intensity on T1- and T2WI may be seen within the graft, which persists or worsens over time [18].
Medial Collateral Ligament (MCL)
Although isolated MCL injuries are usually treated conservatively, a complete tear of the MCL in combination with other ligamentous injuries or an isolated full-thickness tear which has failed conservative treatment may require surgery [22]. The MCL may be repaired using staples or sutures. On MRI after surgery, the MCL is typically thickened and of increased signal intensity on T2WI, occasionally demonstrating new bone formation. Later, the repair site may diminish in thickness and shows a decrease in signal intensity within the ligament in keeping with scar tissue formation [3].
Disruption of the MCL repair will appear as discontinuity of the ligament with fluid at the site.
Medial Patellofemoral Ligament (MPFL)
MPFL Surgery
Lateral patellar dislocation may result in medial patellofemoral ligament laxity or tear which in turn may lead to recurrent patellar dislocation. In these patients, MPFL repair or reconstruction is indicated if conservative treatment has failed. The concept of MPFL reconstruction is relatively new. However, the technique has gained popularity recently. The goal is to restore patellofemoral stability.
Various techniques have been described, including primary repair with or without augmentation, reconstruction using autologous tendon, allografts, and synthetic grafts. In MPFL reconstruction, a single- or double-bundle technique is used to fix the graft to the medial distal femur and the medial patella. Patellar fixation may be achieved by placement of bone anchors, creation of a patellar tunnel or by using a quadriceps autograft tendon. Femoral fixation can be performed by placement of a bone anchor, tunnel formation and screw placement, post and washer fixation and suturing [23, 24].
Normal Postoperative MRI-Findings of MPFL Reconstruction
Complications of MPFL Reconstruction
Postoperative complications include patellar fracture, quadriceps avulsion fracture, protrusion of bone anchors or screws, graft laxity, persistent patellar instability and recurrent patellar dislocation which may lead to graft failure. On MRI, graft failure demonstrates fluid-like increased signal intensity within the graft, with partial or complete disruption of graft fibres [23, 25].
Meniscus
Meniscal Surgery
The aim of meniscal surgery is to improve patient symptoms and to maintain meniscal tissue. Loss of meniscal tissue is associated with an increased risk of osteoarthritis. The three forms of meniscal surgery are: meniscal repair, partial (or rarely total) meniscectomy and meniscal transplantation. Possible causes for persistent or recurrent postoperative pain are residual meniscal tears after partial meniscectomy, non-healing meniscal sutures, dislocated meniscal fragments, meniscal re-tear after new trauma, cartilage defects and the development of osteoarthritis.
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