Introduction

Considering the significant load distribution and stability functions of the meniscus, it is unknown if load bearing activities such as cutting maneuvers have implications on tear length of small meniscal tears left in situ. Therefore, the first objective of this study was to quantify propagation of small vertical longitudinal tears in the posterior horn of the lateral meniscus during simulated cyclic cutting maneuvers on a robotic testing system. The second objective was to measure the resulting changes in knee kinematics, resultant forces in the lateral meniscus, in situ force in the ACL, and tibiofemoral bony contact forces. Because of knee loading during weight bearing activities, it was hypothesized that small vertical longitudinal tears in the posterior horn of the lateral meniscus would increase in length after cyclic loading, and, consequently, an increase in kinematics and forces would occur in intact and ACL-deficient knees.

Methods

Fourteen fresh-frozen cadaveric knees (mean age: 74.8 ± 14.4) were mounted onto a 6-degree-of-freedom robotic testing system (MJT Model FRS 2010, Japan). Knees with osteoarthritis greater than grade II according to the Kellgren-Lawrence grading scale were excluded. A small vertical longitudinal tear in the posterior horn of the lateral meniscus posterior to the popliteus tendon was created via a posterior arthrotomy. Tear length was measured with a custom-made device (accuracy 0.2 mm; inter-rater reliability 0.914). In six specimens, the ACL was resected arthroscopically, whereas in the other eight knees the ACL was left intact. A 10Nm valgus torque and 5Nm external rotation torque combined with a 250N of axial compression were applied while the knees were continuously flexed from 30º to 90° of flexion for 500 cycles. Meniscus tear length was measured after each 100 cycles as described. After the measurements, the posterior capsule and skin were closed in a standardized fashion.
Kinematic data was collected for the following knee states: 1) starting knee state (either intact or ACL deficient); 2) after posterior arthrotomy; 3) meniscus tear at baseline; 4) after 500 cycles of the applied loading condition at 30°, 60°, and 90°. The in situ force in the ACL, the resultant force in the lateral meniscus, and the tibiofemoral bony contact forces were calculated using the principle of superposition. A repeated measures ANOVA was used to compared the outcome parameters between knee states (p < 0.05).

Results

In ACL-intact knees, small vertical longitudinal lateral meniscus tears propagated significantly from baseline at 100, 200, 300, 400, and 500 cycles (all p < 0.001). This corresponded to a final tear propagation of 28.7% ± 5.4% greater than baseline. Similarly, in ACL-deficient knees, meniscal tear propagation was significant throughout all sets of cycles, corresponding to a tear propagation of 26.1% ± 8.3% greater than baseline. After meniscal tear propagation, external tibial rotation increased up to 45.5% and tibiofemoral bony contact force increased up to 69.6% in the intact knees (p < 0.05). Meanwhile, propagation of the meniscal tear led to an increase up to 116.5% in the resultant force in the lateral meniscus in the ACL-deficient knees (p < 0.05).

Discussion And Conclusion

The data of this study suggests that small vertical longitudinal lateral meniscus tears propagate significantly regardless of the integrity of the ACL and even after only 100 cycles of knee loading. The propagation of such tears altered kinematics and forces in the knee, with an increase in external tibial rotation, forces in the lateral meniscus, and tibiofemoral contact forces. The findings of the current robotic study are in contrast to recent clinical studies that found that small, vertical and stable tears in the posterior horn of the lateral meniscus could be left in situ. These propagated tears may have an inferior repair prognosis if they propagate into complex tears, propagate to the inner region of the meniscus, or are chronically addressed. Therefore, surgeons may consider tear propagation of the meniscus when deciding the most appropriate treatment option for small longitudinal tears.