MRI of the Postoperative Knee- Interpretations and Diagnostic Criteria

Introduction:

The knee is a complex joint that connects the femur bone, tibia, and patella. The knee contains bones, ligaments (medial and lateral collateral; anterior and posterior cruciate ligament), cartilage (medial and lateral meniscus; articular cartilage), tendons, muscles, the joint capsule, and synovial fluid. The most common surgical procedures for knee injuries are partial meniscectomy, anterior cruciate ligament (ACL) reconstruction, meniscal repair, and cartilage repair procedures.

What Are the Postoperative Knee Interpretations of MRI?

1) Anterior Cruciate Ligament (ACL) Reconstruction:

• Nowadays, ACL reconstruction is performed using an autograft (graft taken from the patient) rather than an allograft. Therefore, it can be achieved by using the bone-tendon-bone (BTB) technique and the semitendinosus or gracilis tendons (STGT) technique.

• BTB reconstruction is performed with the patellar tendon along with two bone fragments. On postoperative MRI, it appears as an area of defect on the central third of the patellar tendon after the surgery. STGT reconstruction uses the semitendinosus and gracilis tendons as autografts. Postoperative MRI shows tendon healing and no visible defects. BTB and STGT are intra-articular reconstructions.

• On a sagittal view of MRI, the placement of the femoral tunnel aperture is at the junction of the posterior femoral cortex and the line drawn along the roof of the intercondylar notch. The tibial tunnel aperture is placed at the front aspect of the middle of the tibial epiphysis and posterior to the line drawn along the roof of the intercondylar notch.

• In the front view, the femoral tunnel aperture is placed at the superolateral part of the intercondylar notch. And the tibial tunnel is placed at the projection of the tibial spine.

• In recent days, the double-bundle reconstruction technique has been in use. This technique uses two tibial tunnels or two femoral tunnels and two tendon grafts.

2) Normal Appearance of ACL Reconstructions on MRI-

• In T1 weighted images and MRI fluid-sensitive sequences, the reconstruction area appears to have low signal intensity up to two months after surgery.

• From two months to about 1.5 to two years after surgery, the process of ligamentization occurs in the area of reconstruction. It appears as a high signal intensity area on T1 weighted images, and on fluid-sensitive sequences, it seems as an edema-like high signal intensity area.

• After 1.5 to 2 years of the surgery, the reconstruction area appears as a low signal intensity area on T1 weighted images and fluid-sensitive images. Sometimes the intermediate signal intensity is seen on T1 weighted and fluid-sensitive images.

• After four years of the reconstruction surgery, the area appears to have an intermediate signal in some areas of the graft.

3) Postoperative Meniscus Imaging:

• Meniscectomy- Meniscectomy is performed as a complete and partial procedure. Total meniscectomy is rarely performed nowadays as it results in osteoarthritis. Partial meniscectomy involves the removal of part of the meniscus and restores the meniscus stability to prevent the risk of osteoarthritis. MRI images of partial meniscectomy are small, irregular meniscus and truncated meniscus free edges, along with areas of degeneration on the meniscal surface. There should not be any remaining meniscal fragments. There might be a risk of radial meniscus tears after meniscectomy.

• Meniscus Repair- Though meniscus repair requires less meniscus removal, it has more postoperative restrictions. It is indicated in cases of recent peripheral tears and meniscus detachment from its capsule below 40 years old. In PD and T1 weighted MRI, meniscal repair appears as an area of high signal intensity on intermediate or short echo sequences.

• Meniscal Transplantation- Meniscal transplantation involves the use of allograft and is performed in patients below the age of 40 suffering from pain after meniscectomy. This procedure mainly focuses on pain relief.

• Postoperative Cartilage Imaging: Cartilage repair is usually performed after osteochondral fractures, trauma-induced cartilage defects, and osteochondritis. The surgical procedures performed for cartilage defects are the fixation of the osteochondral fracture, osteochondral graft transplantation, and subchondral bone marrow stimulation.

• Subchondral Bone Marrow Stimulation- Microfracture is the most commonly performed stimulation technique. This procedure involves the stimulation of clots and migration of bone marrow cells resulting in the formation of fibrocartilaginous tissue. This is indicated only when the lesion is smaller than two to three centimeters. Subchondral bone modifications after surgery are shown as an edema-like signal which disappears after one year. About 25 to 50 percent of postoperative cases have an overgrowth of subchondral bone on MRI without the clinical sign. On T1 weighted images, subchondral edema-like signal decreases, and irregular subchondral bone plate persists after one year.

4) Osteochondral Autograft-

• This procedure is performed by filling the cartilage defect using the osteochondral plugs obtained from the lateral and medial parts of the femoral trochlea (non-weight-bearing areas). This cartilage usually contains about 80 percent hyaline cartilage and 20 percent fibrocartilage.

• Edema, like high signal intensity, appears on fluid-sensitive sequences of MRI up to six months after the surgery.

• Between six months and one year, the signal becomes normal with defects on the cartilage surface.

What Are the Diagnostic Criteria Used to Detect Postoperative Meniscal Tears?

• Conventional MRI classifies the meniscus into three types: normal, new tear, and doubtful.

• The new tears are shown as a displaced meniscal fragrant on MRI. A parameniscal cyst usually appears from a horizontal tear due to the contact between the joint and the meniscus.

• If less than 25 percent of the meniscus is resected during the surgery, the new tear appears as fluid-like or not high signal intensity visible on at least two 3 mm slices on fluid-sensitive sequences.

• If more than 25 percent of the meniscus is resected during the surgery, the new tear appears as fluid-like high signal intensity visible on at least two 3 mm slices on fluid-sensitive sequences.

• Meniscal tear is doubtful when the edema or fluid-like high signal intensity is visible only on one 3 mm slice on fluid-sensitive sequences.

Conclusion:

Parameniscal cysts are diagnosed with conventional MRI as they are usually not enhanced on CT arthrography or T1 weighted images of MR arthrography. For postoperative evaluation of the knee injury, a fat-suppressed proton density (PD)-weighted sequence is usually performed. Sometimes, non-fat suppressed T2 weighted sequences are helpful as they are less prone to metal artifacts. STIR (short tau inversion recovery) which is used to suppress signals from fat tissues, or Dixon sequences (a magnetic resonance imaging sequence to get equal fat suppression) have the advantage of less susceptible metal artifacts, and they can visualize bone edema.