Introduction

The overall incidence of anterior cruciate ligament (ACL) tears reaches on average 0,8 per 100'000 patients. Even if ACL reconstruction does not prevent the early onset of osteoarthritis over the long term, it can improve knee kinematics and reduce the risk of secondary damage to the cartilage or meniscal tissue.
Nowadays, ACL reconstruction techniques aim to recreate the anatomic rather than the isometric footprints. Theoretically, intraoperative fluoroscopy is thought to increase the tunnel placement's accuracy and reproducibility. Therefore, patient-specific mapping of the anatomical footprints would possibly improve the proper pin placement on an anteroposterior and lateral view. Using specifically developed MRI sequences, a precise definition of the anatomic footprint location on formatted anteroposterior and lateral views can be generated. Those MRI sequences would mimic standard X-Ray images, which can be used as a reference template for the intraoperative fluoroscopy.

This study's main objective is to assess the feasibility of processing the MRI information with identified ACL-footprints into 2D-images similar to a conventional anteroposterior and lateral X-Ray image of the knee. The secondary aim is to conduct specific measurements to assess the reliability and reproducibility. This study is a proof of concept of this technique. 

Material & Methods:
Five anonymized MRIs of a right knee were analyzed using an Optima MR360 1.5T Advance, GE Healthcare Scan. A senior orthopedic knee surgeon performed the footprints identification using the multiplanar reformation tool (Materialise Mimics® 17.0 research). An ad-hoc software allowed a volumetric 3D image projection on a 2D anteroposterior and lateral view. The previously defined anatomical femoral and tibial footprints were precisely identified on these views. This specific MRI sequence was named according to the acronym « CLASS » for "Compressed Lateral and anteroposterior Anatomical Systematic Sequences".

Two orthopedic surgeons measured the following parameters: coronal ratio of tibial footprint medial to lateral, sagittal ratio of tibial footprint anterior to posterior, sagittal ratio of femoral footprint high to low, sagittal ratio of femoral footprint deep to shallow, femoral intercondylar notch roof angle, tibial slope, sagittal articular surface and Blumensaat's line-angle, coronal articular surface and ACL-angle and sagittal articular surface and ACL-angle.

The intraclass correlation coefficient (ICCs), including 95% confidence intervals (CIs), has been calculated to assess intraobserver reproducibility and interobserver reliability.

Results

Five MRI scans of a right knee have been assessed (three females, two males, mean age of 30.8 years old). Five 2D-"CLASS" have been created.
The measured parameters showed a "substantial" to "almost perfect" reproducibility and an "almost perfect" reliability.

Conclusion

This study confirmed the possibility of generating 2D-compressed images ("CLASS") with the localized centroid of the femoral and tibial ACL footprints from a 3D volumetric model. The “CLASS” also showed that these footprints were easily identified on standard anteroposterior and lateral X-Ray views of the same patient, thus allowing an individual identification of the anatomical femoral and tibial ACL's footprints.