Since the introduction of metal artifact reduction sequences (MARS), postoperative changes and abnormal findings after the implantation of orthopedic metallic hardware can be visualized using MR imaging with varying success.

While imaging after total hip replacement works quite fine imaging after total knee replacement remains a big challenge since the large metallic prosthesis components are surrounded by a relatively thin soft tissue layer and the structures of interest (e.g. collateral ligaments, cruciate ligaments, synovium, joint bodies) appear in close proximity to the metallic implants.

The purpose of this presentation is to provide an overview of the possibilities and restrictions of todays MARS MR imaging techniques in patients after total knee replacement based on a list of common postoperative problems. After following this presentation, the learners will understand the major clinical problems faced by orthopedists after total knee replacement, how MR imaging can contribute in these situations and where the limitations of technical possibilities are in a clinical setting. This presentation does not deal with technical considerations such as sequence optimization.

In general, a 1.5 T MR scanner should be preferred over 3.0 T since artifacts are considerably more pronounced with a higher field strength.

MR sequences can be optimized using:

- high-bandwidth radiofrequency pulses

- increased readout bandwidth

- in-plane distortion correction (view-angle tilting, VAT)

- through-plane distortion correction (slice-encoding metal artifact correction, SEMAC)

A possible imaging protocol consists of:

- a transverse and coronal STIR sequence,

- a coronal T1-weighted TSE sequence,

- and a sagittal PD-weighted TSE sequence.

Such a protocol can be acquired in approximately 17.5 minutes (shimming time not included).

After total knee replacement around 20-30% of the patients are not fully satisfied with the postoperative result. Apart from excessive preoperative expectations, the most common problems of patients after total knee replacement include pain and restricted range of motion. These problems are mainly caused by the following postoperative conditions:

- Prosthetic loosening due to prosthetic infection or due to aseptic loosening (e.g. inlay wear and pseudotumor/granuloma formation)

- Prosthetic instability due to a mismatch of prosthesis design and ability of active and passive stabilization of the patient (e.g. collateral ligament insufficiency)

- Malpositioning of the prosthesis, notably rotational misalignment (e.g. leading to patellar maltracking) - Prosthetic mismatch (e.g. overstuffing, lateral or medial protrusion)

- Pain in contralateral knee joint compartment after unilateral knee replacement or femoropatellar joint replacement

- Periprosthetic fracture

- Soft tissue pathologies around the knee joint

Prosthetic loosening presents as areas of bone resorption adjacent to the prosthesis and is caused either by infection or so called “aseptic loosening”. The latter is a kind of a foreign body reaction to polyethylene or metal particles and can be found as a result of pseudotumor/granuloma formation (e.g. caused by inlay wear, metallosis) or inappropriate biomechanical stress (e.g. due to misalignment of the prosthesis).

So far computed tomography was the modality of choice to show periprosthetic bone resorption. However, the changes may be hard to identify early if the periprosthetic lucencies are smaller than bean hardening artifacts around the prosthesis. Using MARS MRI prosthesis loosening can be diagnosed at a much earlier stage since even periprosthetic bone marrow edema-like changes can be detected. These changes represent early signs of bone remodeling and stress reaction – and appear much sooner than the formation of real periprosthetic bone lucencies. Periprosthetic bone resorption bands can be identified on T1- or T2-weighted MARS sequences. A reliable differentiation between septic and aseptic loosening is not possible so far.

The primary cause of prosthetic instability is a mismatch of prosthesis design and stabilization ability of the patient (e.g. collateral ligament insufficiency) and is mainly diagnosed clinically. Dynamic ultrasonography also allows for the evaluation of the stabilizing structures of the knee joint, but is user dependent and limited in evaluation of joint pathologies.

MARS MRI can help in the evaluation of the stabilizing ligaments (e.g. collateral ligaments, extensor apparatus) whereas ligament degeneration, partial or complete tears, or enthesopathies can be diagnosed. In patients with partial knee replacement, the cruciate ligaments remain important stabilizers of the knee joint. The cruciate ligaments as well as the structures of the remaining native joint compartment can be evaluated using MARS MRI.

Notably rotational misalignment (e.g. leading to patellar maltracking) is a problem. So far, computed tomography is the modality of choice to evaluate the exact position of the prosthesis components. The measurement technique is tricky and relies on comparison with the native contralateral side. There are attempts to measure rotational malpositioning after total knee replacement using MARS MRI.Prosthetic mismatch signifies a mismatch of prosthesis design or size of prosthesis components (e.g. overstuffing, lateral or medial protrusion). The significance of a prosthetic mismatch is still debated and discussed controversial in the orthopedic literature. Generally, a mismatch is suspected on plain films or computed tomography. So far, MR imaging has no role in the evaluation of these patients other than to rule out potential pain sources in the adjacent soft tissue.MARS MRI is the modality of choice in the evaluation of the untouched knee compartments after unilateral or femoropatellar knee joint replacement. Generally, the internal structures of the joint (cartilage, collateral ligament complex, cruciate ligaments, menisci and synovium) can be visualized. Computed tomography with intraarticular contrast media injection is an alternative with clearly inferior diagnostic power and should be reserved for patients with contraindications for MR imaging.CT is the modality of choice in the evaluation of traumatic fractures. However, insufficiency fractures can be reliably evaluated using MARS MRI. The presence of bone marrow edema-like patterns is often more sensitive than the appearance of fracture lines on computed tomography.Periarticular soft tissue changes are frequent after total knee replacement: Normal postoperative findings (e.g. scar, seroma and cyst formation) have to be distinguished from pathologic soft tissue changes that may occur in the postoperative course – like ganglion cysts, synovial cysts, deep vein thrombosis, enthesitis at the insertion site of the tendons (e.g. bursitis anserina, extensor apparatus insertions), pathologies to the proximal tibiofibular joint, arthrofibrosis, patellar clunk syndrome and tumors/metastases. Depending on the size of the residual metallic artifacts MARS MRI can be very helpful in the evaluation of these findings.There is a bunch of rare conditions leading to postoperative problems such as postoperative nerve damage, muscle changes (e.g. myositis ossificans, tumors) and others.MARS MRI opened a new spectrum of diagnostic possibilities after total knee replacement: This technique can detect signs of prosthesis loosening at a much earlier stage than computed tomography and is the modality of choice for evaluation of postoperative periarticular soft tissue problems. There is definitely a need for further technical improvements to reduce residual metal artifacts around the knee prosthesis.

· Sneag DB, Bogner EA, Potter HG. Magnetic resonance imaging evaluation of the painful total knee arthroplasty. Semin Musculoskelet Radiol. 2015;19:40-48

· Fritz J, Lurie B, Potter HG. MR Imaging of Knee Arthroplasty Implants. Radiographics. 2015;35:1483–1501.

· Hofmann S, Seitlinger G, Djahani O, Pietsch M. The painful knee after TKA: a diagnostic algorithm for failure analysis. Knee Surgery, Sports Traumatology, Arthroscopy. 2011;19:1442–1452.

· Cercek R, Bassett R, Myerthall S. Evaluation of the painful total knee arthroplasty. J Knee Surg. 2015;28:113–118.

· Hughes AJ, O'hEireamhoin S, Heffernan E, Hurson C. A Simple Approach to Assessment of a Total Knee Replacement's Rotationary Profile Using Computed Tomography. Orthop Surg. 2015;7:350–353.

· Agten CA, Del Grande F, Fucentese SF, Blatter S, Pfirrmann CWA, Sutter R. Unicompartmental knee arthroplasty MRI: impact of slice-encoding for metal artefact correction MRI on image quality, findings and therapy decision. Eur Radiol. 2015;25:2184–2193.

· Murakami AM, Hash TW, Hepinstall MS, Lyman S, Nestor BJ, Potter HG. MRI evaluation of rotational alignment and synovitis in patients with pain after total knee replacement. J Bone Joint Surg Br. British Editorial Society of Bone and Joint Surgery. 2012;94:1209–1215.

· Hirschmann MT, Konala P, Amsler F, Iranpour F, Friederich NF, Cobb JP. The position and orientation of total knee replacement components: a comparison of conventional radiographs, transverse 2D-CT slices and 3D-CT reconstruction. J Bone Joint Surg Br. 2011;93:629–633.