A New Designed Extramedullary Tibial Alignment Rod Based On Measurements Of Three-dimensional Reconstruction Model

Objective:In order to improve the precision of extramedullary tibial alignment rod in total knee arthroplasty (TKA), we are going to design a new tibial alignment rod, which uses anterior tibial cortex (ATC) as reference axis. The new designed extramedullary tibial alignment rod will provide a simple method in aiding surgeon to perform a high-precision tibial osteotomy in TKA.Methods:(1)75normal Chinese adults were examined of their lower extremities by CT scanning device. The scanning images of tibia were collected and handled with three dimensional (3D) reconstruction software systems. Identify the mechanical axis (MA) and three ATCs in tibial3D reconstruction model. The angle between ATCs and MA in coronal plane and sagittal plane are measured. The medial and lateral tibial plateau slope, which uses MA and ATCs as reference are also measured. The data of these parameters were recorded and grouped. Then the comparison between ATCs, genders and ages of these parameters were made by SPSS software system.(2) Base on the design philosophy and statistic results, design a new extramedullary tibial alignment rod for TKA. Apply the newly designed alignment rod in3D reconstruction system, to testify its validity.Results:(1) The angle between ATC and MA in coronal plane and the percentage of the angle smaller than3°is:ATC I1.63±0.75°,97.4%; ATC Ⅱ1.50±0.88°,97.4%; ATC Ⅲ1.47±0.83°,96.1%. The angle between ATC and MA in sagittal plane is:ATC Ⅰ4.12±0.73°; ATC Ⅱ4.15±0.74°; ATC Ⅲ4.22±0.72°. There is no significant difference between ATCs, genders and ages.(2) The medial tibial slope angle is:MA7.63±2.90°, ATC Ⅰ11.74±2.81°, ATC Ⅱ11.78±2.89°, ATC Ⅲ11.84±2.95°. The lateral tibial slope angle is:MA6.69±2.51°, ATC Ⅰ10.81±2.53°, ATC Ⅱ10.85±2.60°, ATC Ⅲ10.91±2.69°. There is no significant difference between genders and ages. (3) The new TKA tibial alignment rod is successfully designed. According to the adjustability of slope angle, the design can be divided into two sub-design:one with a fixed slope angle (11.8°), one with an adjustable slope angle. The varus and valgus angle of osteotomy with the use of the new designed alignment rod which has a fixed slope angle is0.06±0.92°, the difference between osteotomy slope angle and original slope angle is-0.35±2.17°.Conclusion:(1) It is feasible to make3D reconstruction of tibia on the basis of CT images. The measurements based on3D reconstruction gave more accurate results than the measurements in X-rays or CT images.(2) The angle between ATCs and MA is relatively stable among people. We can use ATC as substitute of MA as reference axis in TKA tibial osteotomy. The upper alignment node can be chosen from the plane beneath the joint space in a distance of9cm to11cm, and the lower node can be chosen from the plane in a distance of24cm to26cm.(3) ATC is almost parallel to MA in coronal plane. There is just a small varus angle. Move the end of alignment rod medially can help surgeon get a better coronal alignment. The newly designed alignment rod with a fixed slope angle (11.8°) can roughly help surgeon restore the physiological slope angle of tibia. The alignment rod with an adjustable slope angle can help surgeon restore a specific slope angle.(4) The new designed tibial alignment rod retains the merits of traditional extramedullary alignment rod, on the other hand, it improves alignment precision in TKA.