Abstract
Objectives:
Posterior tibial slope (PTS) is a known anatomical factor in the biomechanical function of the anterior cruciate ligament. However, when PTS is typically measured on a lateral radiograph, this does not consider potential differences between the lateral and medial tibial plateau. The difference between medial posterior tibial slope (MPTS) and lateral posterior tibial slope (LPTS) in a knee is largely unknown. Generally, computed tomography (CT) is the gold standard for 3-dimensional osseus imaging but very few studies to date have attempted to quantify the medial and lateral PTS in the same knee using CT. Clinically, it is debated whether an elevated MPTS or the LPTS is critical for anterior cruciate ligament injury and reinjury. The objective of this study was to quantify the variation in the difference between MPTS and LPTS using full tibial CT scans.
Methods:
At our institution, CT-assisted preoperative planning is frequently used for osteotomies. Ninety-seven patients with clinically acquired CT scans went through the preoperative planning process (Bodycad, Quebec City, Canada) from Jan 2016 to Nov 2022. This includes patients undergoing coronal, sagittal or biplanar high tibial and distal femoral osteotomies. This process is semi-automated meaning that the user can intervene with the measurements and adjust the initial landmarks and best-fit. Distally, the software will find the center of the tibiotalar joint to provide a point for the tibial axis. The user may adjust that landmark if needed. To measure the tibial slope, the software finds the best fit tibial plateau in 3D and determines the slope angle medially and laterally. The software measures the MPTS and LPTS independently. The slope measurements were compiled and quantitatively analyzed including mean, median, range as well as the difference between the MPTS and LPTS. Patients were defined to have undergone a sagittal or coronal correction if there was more than 2 degrees of change in the sagittal plane or more than 5% change in the coronal plane weight bearing line. Patients were defined as having a biplanar correction when both conditions were met. The slopes were analyzed with respect to sex and laterality. Additionally, patients undergoing an osteotomy in conjunction with an ACL reconstruction (either single or two-staged) were compared to the other patients.
Results:
There are differences in the MPTS and LPTS within the same knee. When analyzing the whole group, the average absolute difference is 2.76⁰ with a standard deviation of 2.49. As shown in Table 1, 40% of knees had greater than a 2.5⁰ difference in MPTS and LPTS, 19% of knees had greater than a 5⁰ difference in MPTS and LPTS, 5% of knees had greater than a 7.5⁰ difference in MPTS and LPTS and 2% of knees had greater than a 10⁰ difference in MPTS and LPTS. The magnitude of the LPTS is greater than the MPTS 52% of the time. There is no significant difference in the average, standard deviation, mean or the 10th, 25th, 75th and 90th percentiles of the MPTS and LPTS as shown in Table 2. One hundred percent of the sagittal corrections and 84% of the biplanar corrections also underwent an ACL reconstruction in addition to an osteotomy. Characteristics of these groups of patients are shown in table 3. The sagittal and coronal osteotomy patients have a higher MPTS and LPTS than the patients undergoing a coronal osteotomy.
Conclusions:
The most important finding in this study is that the medial and lateral PTS can be very different within the same knee. There can be up to 11.62-degree difference in this population. Sixteen out of 97 patients had a difference of more than 5⁰ with 2 patients having a difference of greater than 10⁰. The data presented here has uncovered a description of the medial and lateral posterior tibial slope anatomy that was not evident with conventional lateral knee x-rays previously. We do not know the clinical significance of a large difference in the medial and lateral PTS and it brings forth an interesting set of clinical questions. 1) Which posterior tibial slope, medial or lateral, is most important for knee biomechanics, specifically regarding ACL stresses and function? 2) When performing a slope correcting osteotomy, is it the medial, lateral or average slope that should be the target for correction? 3) Should an intercondylar correction which only modifies one slope be considered? Further clinical studies should focus on investigating the relationship of differential PTS to native knee biomechanics as well as its implication for corrective osteotomy procedures and ACL reconstruction.
