The posterior horn of the medial and lateral meniscus both reduce the effective posterior tibial slope: a radiographic MRI study

Purpose: The purpose of this study was to quantify the posterior horn meniscal slope and determine its contribution to the reduction in posterior tibial slope. Methods: Patients aged between 16 and 60 years and had intact menisci with no evidence of previous injury or surgery were included. Patients with radiological evidence of osteoarthritis Grade II–IV, any acute or chronic meniscus injuries, fractures, and ligamentous injuries were excluded. The posterior bony slope (PTS) and the meniscus slope (MS) of the posterior horns were measured at 25, 50, and 75% from the medial and lateral borders of the tibial plateau. Results: 325 MR images (mean age 37.1 ± 10.9 years) were included. There were 194 males and 131 females, with 162 left and 163 right knees. The PTS in the medial compartment ranged from (−) 2.8° to 3.7° and from (−) 1.3° to 1.9° in the lateral compartment (p = 0.0001). The MS in the medial compartment ranged from 27.4° to 28.2°, and from 27.8° to 28.7° in the lateral compartment (p > 0.05). The differences between the medial and lateral knee compartment were statistically significant. At the 25% interval the p level was 0.037, at 50% p = 0.00001, and at 75% p = 0.0001. There were no significant between gender differences. Conclusions: The results of this study demonstrated a significant reduction in posterior tibial bone slope by the posterior horns of both the medial and lateral meniscus, from a mean of (−) 1° to 2° to a more horizontal anterior slope. The posterior bone slope was larger in the medial compartment by 1°, resulting in a smaller slope reduction in the lateral compartment.