Poster 140: The Effect of Posterior Tibial Slope and Flexion Angle on Posterior Medial Meniscal Root Forces

OBJECTIVES: Although the biomechanical and clinical consequences of posterior medial meniscal root (PMMR) tears have been previously reported, these have not been fully evaluated in terms of overall bony morphology. It is well documented that higher flexion angles lead to higher postero-medial pressure, with many groups proposing that PMMR tears are caused by elevated compression and shear forces at the root when the loaded knee is at high flexion angles. Additionally, several authors have advocated that increasing tibial plateau slope (PTS) is an anatomical risk factor for PMMR tears, due to the higher posterior shear forces at the root insertion site. Thus, the purposes of this study are to evaluate the forces across the PMMR utilizing a novel three-dimensional forces sensor with varying posterior tibial slopes and flexion angles. We hypothesized that an increased flexion angle and/or posterior tibial slope will result in increased posterior shear forces acting on the PMMR. METHODS: Ten fresh-frozen cadaveric knees (53.2 mean age, all male) were tested in all combinations of the three states of posterior tibial slope (5⁰, 10⁰, 15⁰) and the four states of varying flexion angles (0⁰, 30⁰, 60⁰, and 90⁰). A novel three-axis sensor that determines force measurements in three orthogonal directions was installed below the posterior tibial plateau, with the specimen being mounted to a load frame which applied a 500-N axial load. A 5-Nm internal rotational (IR) torque was then applied. After the IR torque, a 5-Nm external rotational torque was applied. The amount of compression-tension and shear forces acting on the PMMR were measured. RESULTS: Increased tibial slope significantly decreased tension and significantly increased compression of the PMMR (5°→10°: p = 0.0368, 5°→15°: p < 0.0001, 10°→15°: p < 0.0001) when the joint was loaded in compression. Increased tibial slope significantly increased anterior shear of the PMMR (5°→10°: p < 0.0001, 5°→15°: p < 0.0001, 10°→15°: p < 0.0001) when the joint was internally rotated. Increased tibial slope significantly decreased compression of the PMMR (5°→10°: p = 0.0188, 5°→15°: p < 0.0001) when the joint was externally rotated. Increased flexion angle significantly increased medial shear forces of the PMMR (0°→30°: p = 0.0362, 0°→60°: p = 0.0005, 0°→90°: p < 0.0001, 30°→90°: p = 0.0434) when the joint was loaded in compression. 90° of flexion significantly increased tension of the PMMR (0°→90°: p = 0.0438, 30°→90°: p < 0.0001, 60°→90°: p = 0.0005) when the joint was internally rotated. 30° of flexion angle significantly increased compression of the PMMR (0°→30°: p = 0.0004, 30°→60°: p = 0.0118, 30°→90°: p < 0.0001) when the joint was externally rotated. CONCLUSIONS: Increased PTS results in an increase in compression forces acting on the posterior horn of the medial meniscus when the knee joint is loaded. Increases in flexion angles displays an increase in medial shear forces seen at the PMMR under a load. This increase in force may place the PMMR at increased risk of stress and potential failure after repair. This study begins to provide clinicians with information to create safer protocols to decrease the forces experienced at the PMMR after injury or postoperatively.