The Relationship of the Medial 1/3rd of the Tibial Tubercle to the Posterior Aspect of the Tibia

INTRODUCTION: Proper tibial rotation has been cited as an important prerequisite to optimal total knee replacement. The most commonly recognized rotational landmark is the medial 1/3 rd of the tibial tubercle. The purpose of this study was to quantify the amount of variability this structure has from a common reference as well as to understand the effects of component design when referencing this structure. METHODS: Subjects were prospectively scanned into a Virtual Bone Database (Stryker Orthopaedics, Mahwah, NJ), which is a collection of body CT scans from subjects collected globally. All CT scans displayed cropped bones were excluded. SOMA™ (Stryker) is a unique tool with the ability to take automated measurements of quantities such as distances and angles on a large number of pre-segmented bone samples which was then to perform calculations represented in this study. Demographic information for each subject was recorded were known. For the analysis, the mechanical axis of the tibia (MAT) was established by connecting the center of the proximal tibia to the center of the ankle. From the MAT, a perpendicular resection plane was made at a distance of 9 mm from the most proximal portion of the lateral condyle. This plane was then used as a virtual resection plane to establish the points for the remaining structures which was the medial 1/3 rd of the tibial tubercle and the posterior notch of the PCL insertion. The following axes were identified: 3TT (line between the medial 1/3 rd of the tibial tubercle and the posterior notch of the tibia); 3CTT (line between the medial 1/3 rd of the tibial tubercle and the center of the tibia); and the posterior axis of the tibia (line connecting the two most posterior points of the tibia at the virtual resection plane). Measurements made were the angle of the 3TT Line to the posterior axis and the angle of the 3CTT Line to the posterior axis. RESULTS: CT Scans of the Left Knees (n = 524), Right Knees (n = 527), and combined left/right knee (n = 1051) were collected for this study. The mean 3TT angle for the left knee was 74.6° ± 3.0° (Range: 60.2°–84.8°) and right knee was 74.5° ± 3.0° (Range: 65.1°– 85.1°). The combined (left/right) angle was 74.5° ± 3.0° (Range: 60.2°–85.1°). The mean 3CTT angle for the left knee was 71.2° ± 3.6° (Range: 57.6°–83.2°) and right knee was 71.1° ± 3.5° (Range: 61.4°–82.3°). The combined (left/right) angle was 71.1° ± 3.6° (Range: 57.6°–83.2°). The two methods resulted in a 3.4° difference, with the 3TT reference being more externally rotated. DISCUSSION: The tibial tubercle is a common landmark used to set the rotation of the tibial component and utilizing the posterior aspect of the tibia provides a common reference point to establish variations that could exist with this landmark. The amount of variation of the tibial tubercle can vary by over 25 degrees. Asymmetric baseplates will set rotation based on tibial coverage so variation from the tubercle is can not be accommodated if the surgeon routinely uses this as a landmark. Symmetric baseplates can provide more options for rotational placement.