After Anterior Cruciate Ligament (ACL) reconstruction, patients often wonder if the implants used are compatible with MRI scans. Modern ACL reconstruction typically uses implants designed to be safe for MRI. Generally, having an MRI after ACL reconstruction is possible and poses no safety risk with contemporary devices, allowing for continued non-invasive assessment of the knee.
Implant Materials in ACL Reconstruction
ACL reconstruction uses fixation devices like screws, buttons, and anchors to secure the new ligament graft within bone tunnels. These devices are made from metallic or non-metallic materials.
Metallic implants commonly include titanium or stainless steel, chosen for strength and stability. Titanium is weakly affected by magnetic fields. Stainless steel can vary in its magnetic properties.
Non-metallic options include bioabsorbable polymers and polyetheretherketone (PEEK). Bioabsorbable screws gradually break down and are replaced by bone. PEEK is known for its biocompatibility, strength, and radiolucency, meaning it does not interfere with imaging. The choice of material is important because it influences how the implant interacts with the magnetic fields present during an MRI.
MRI Safety and ACL Implants
Modern ACL implants are generally considered MRI safe or MRI conditional, meaning they pose no known hazard under specific conditions. The theoretical risks associated with metallic implants during an MRI, such as heating or displacement, are minimal with current ACL devices. This is because most contemporary implants are made from non-ferromagnetic or weakly ferromagnetic materials like titanium, which are not significantly attracted by strong magnetic fields.
Heating of implants during MRI scans has been studied, and while small temperature changes can occur, they are generally not clinically significant for orthopedic implants. Studies confirm that implants firmly fixed in bone are not affected by MRI-induced displacement forces. The robust design and material composition of modern ACL implants ensure that MRI procedures can be performed without causing harm.
Image Quality Considerations
Despite the safety of MRI with ACL implants, the presence of metallic components can affect the quality of the MRI images. This phenomenon is known as “artifact,” appearing as signal void or distortion in the images. Artifacts can make it challenging to visualize the tissues immediately surrounding the implant.
Metallic implants, even those considered MRI safe, can distort the main magnetic field, leading to areas of signal loss. Radiologists and technicians employ specialized MRI sequences and techniques to minimize these artifacts. These include Metal Artifact Reduction Sequences (MARS), Slice Encoding for Metal Artifact Correction (SEMAC), and Multi-Acquisition Variable-Resonance Image Combination (MAVRIC). These techniques use specific gradient and pulse sequences, such as fast spin-echo sequences with high bandwidths and thinner slices, to reduce image distortion.
Even with potential artifacts, MRI can still provide valuable diagnostic information for assessing soft tissues around the knee, including the graft, menisci, and cartilage. Bioabsorbable implants typically produce fewer artifacts, allowing for clearer post-operative MRI follow-up.
Preparing for Your MRI
Preparing for an MRI after ACL reconstruction involves a few important steps. Informing the MRI technologist and the referring physician about your ACL surgery. Providing details about the type of implant used, if known, can be helpful for the imaging team to select the most appropriate scanning protocols. Patients should bring any implant cards or surgical records, as these documents contain specific information about the materials used in the reconstruction.
Before entering the MRI room, you will be asked to remove all external metal objects, including jewelry, keys, watches, and items with magnetic strips. The powerful MRI magnet can interfere with external metal or pose a safety risk.
During the procedure, you will lie on a table that slides into the MRI machine. The machine will produce loud tapping or thumping noises during the scan, so earplugs or headphones are typically provided.