The success of the repair technique depends mostly on tunnel positioning, but there are also other factors that may contribute to the failure of the ACL reconstruction.

Here will be presented a normal postsurgical MRI appearance of ACL graft, and description of the most often imaging findings of graft failure and graft complications without failure. Complications are possible at the graft and donor site. We reviewed the imaging findings and clinical diagnoses performed during previous two years in our Center.

An ACL graft varies in signal as it matures. Early postoperative imaging shows a hypointense graft on all pulse sequences. During the remodeling period, the graft undergoes revascularization and resynovialization, which results in increased
signal intensity (Figure 3).

Fig. 3: 13-year-old girl who presented for routine orthopedic follow-up 3 months after ACL reconstruction using BPTB graft using patellar tendon allograft. Coronal PDw MR image shows continuity of graft fibers. There is high-signal-intensity edema in graft. This is a normal finding during remodeling phase, which occurs between 4 and 8 months after surgery.

The same patient, like in the previous figure, sagittal T1W image (Figure 4) presents the positon of the tibial tunnel.

Fig. 4: Sagittal T1W MR image shows a good position of the tibial bone tunnel. There is a mild amount of metallic artifact related to the tibial interference screw.

In Figure 5 and 6 is presented patient with complete posttraumatic tear of ACL graft.

Fig. 5: Sagittal T1w MR image shows a patient with complete post-traumatic tear of ACL graft. There are no intact graft fibres in the expected position of ACL graft (white arrow).

Fig. 6: Sagittal PDfs MR image shows the same patient like Fig.5, with complete post-traumatic tear of ACL graft. All fibres are completely torn. (Usually, because of the metallic artifact, we are using STIR sequence more than PDfs)

33-year-old male patient, 5 years after surgery, with insufficient rotational stability with a symptomatic pivot shift. In Figure 7 there is  enlargement of femoral tunnel, 15mm transverse diameter, and in Figures 8, the same patient, the tibial tunnel is too far back. This position of tibial tunnel is the main reason of instability.

Fig. 7: Sagittal PDw MR image shows enlargement of femoral tunnel, 15mm transverse diameter (red line).

Fig. 8: Sagittal PDw MR image, the same patient like Fig.7, shows the tibial tunnel is too far back and this is the main reason of joint instability. The oblique vertical red line represents Blumensaat line. The position of the tibial tunnel should be parallel but posterior to the slope of the Blumensaat line (intercondylar roof).

Fig. 9: Sagittal PDw image shows displaced bone plug of the graft in patient with laxity after ACL reconstruction.

In the next patient, 3 years after surgery, there are limited extension and chronic anterior knee pain. The reason for this was a vertical migration of the tibial screw, Figure 10, with some Hoffa's fat pad fibrosis and cartilage injury.

Fig. 10: Sagittal T2w image shows the vertical migration of the tibial screw (arrow) with fibrosis of the Hoffa’s fat pad and cartilage injury of the femoral condyle. The patient had limited extension and anterior knee pain

Extension loss or flexion contracture is a disabling complication following ACL reconstruction that is most commonly caused by graft impingement or localized anterior arthrofibrosis. Localized anterior arthrofibrosis also can cause anterior knee pain.

Fig. 11: Sagittal T1w MR image shows anterior arthrofibrosis - fibrosis of dorsal aspect of Hoffa’s fat pad (white arrow).

Focal arthrofibrosis (cyclops lesion) is the best to visualise on sagittal images (T2w) and it appears as a focal nodular or irregular area of T2 intermediate signal intensity just anterior to the distal aspect
of the ACL graft. In Figure 12 there is extensive anterior arthrofibrosis and cyclop lesion. Clinically, there is significant anterior knee pain and extension loss.

Fig. 12: Sagittal T2W MR image shows extensive anterior arthrofibrosis and cyclop lesion (white arrow).