H. Seyyedhosseinzadeh, M. Kerdari, S. Rastegarramsheh
Rowan University, Orthopedic Research Group,
Keywords: femoral component, TKA, knee replacement, alignment, fat embolism
Summary:Total Knee Arthroplasty (TKA) is the current treatment for most of the end-stage knee diseases. Although it is among the successful surgeries, the success rate for this major surgery is only 80%. The primary reason for this relatively high failure rate is “Malalignment” of the implant and the resultant limb malalignment. Although the knee anatomy is well studied, we have not reached to a universally accepted “Landmark” to be used for aligning the components. The current methods for measuring the femoral component position and alignment are: Mechanical alignment, Kinematic Alignment, Patient-specific Instruments (PSI), Navigation system. The problems with the first two methods are they rely on already eroded joint surfaces for axial alignment and joint line level and on the femoral intramedullary guide for coronal and sagittal alignments. The major problem with the two last methods is their cost. Our technique is going to address these problems. Methods We used 1576 normal knee MRIs and 456 normal CT Scans from different centers all around the world to cover all possible races. We could find that the anatomic epicondylar axis (AEA) and anterior lateral cortex of the distal femoral shaft, together have all the above-mentioned criteria. Results The AEA is not affected by disease processes, has a fixed angle with distal femoral condyles articular surfaces to accurately determine the coronal alignment of the femoral component, and has a fixed angle with posterior femoral condyles articular surfaces to accurately determine the axial alignment of the femoral component. The length of AEA has a fixed relationship with its distance from the posterior femoral condyle articular surfaces, making the determination of the normal position of the posterior articular surface in an eroded condyle. The length of AEA also has a fixed relationship with its distance from the distal femoral condyle articular surfaces, making the determination of the normal position of the distal articular surface in an eroded condyle. Our findings show that the anterior lateral cortex of the distal femur, just above the trochlear articular surface, is always parallel to longitudinal femoral axis in the sagittal plane. It can be used as a reference for the sagittal alignment of the femoral component. Based on these findings, we designed a guiding system for preparing the distal femur to insert the final femoral component. This is a universal system that can be adjusted and used for any TKA implant. We used several 3D CAD files of normal femurs and tested our designed system for NRG TKA system (Stryker, USA). The reconstructed distal femurs with NRG implant were only ±0.5 mm off the preoperative anatomy. Discussion Using this method can result in more perfectly aligned femoral components. Besides, there is no need to open the femoral medullary canal for sagittal plane alignment, eliminating the risks for intraoperative fat embolism and postoperative bleeding and infection. Since this method does not rely on the diseased landmarks, the reconstructed prosthetic distal femurs will be closer to normal, pre-disease joints.