 A tear in the knee meniscus can cause this shock-absorbing structure to shift from its normal position. A new study on pygmies suggests that even a small degree of meniscal displacement can have a big impact on meniscal load transmission, increasing stress to the knee joint. The findings are published in the American Journal of Sports Medicine. Tears of the lateral meniscus, or LM, are common sports injuries. Associated meniscal displacement is sometimes visible on MRI, which can aid in diagnosis of a tear. However, it's not clear how the amount of displacement is related to the severity of the tear, or to the degree of load transmission impairment. To help clarify these relationships, researchers used a robotic system to assess eight pygmies before and after radial LM tears were created. The system applied a 300-Newton axial load to each knee at 30° and 60° of flexion. A sensor attached to the tibia measured the three-dimensional forces on the knee, while cameras tracked LM displacement according to the relative positions of reflective markers. Each knee was tested under a series of four LM conditions, with an intact LM, and with a 30%, 60%, and 90% radial tear. The LM was then removed, and the robotic system measured the knee forces over the same exact trajectories from the previous conditions, but without the meniscus present. The resultant force through the LM was calculated based on the forces in all of the tested conditions. This resultant force was used to evaluate the effect of each degree of tear on the load transmission function of the LM. The 30% and 60% tears did not cause meniscal displacement, or affect the force through the LM under axial loading. However, the 90% tear caused significant anteroposterior displacement under loading at both knee flexion angles due to tear separation. Because of this displacement, the resultant force on the LM was markedly reduced, indicating that load transmission through the LM was impaired. Importantly, the LM dynamics inside the joint and the potential effects on the medial meniscus weren't evaluated, and acute versus chronic effects weren't considered. In addition, the robot was only able to apply a standing force, running and jumping forces weren't tested. There may also have been tissue or sensor differences between trials, since the same knees were tested repeatedly. Although more work is needed to translate the findings to humans, the study indicates that even one millimeter of displacement after a radial LM tear can significantly reduce load transmission, suggesting that repair surgery is worth considering for human patients with displaced torn LMs.