Biomechanical Analysis of Tibial Motion and ACL Graft Forces After ACLR with and without let at Varying Tibial Slopes

BACKGROUND: Lateral extra-articular tenodesis (LET) is being performed with increasing frequency in the setting of anterior cruciate ligament reconstruction (ACLR) as a way to decrease rotational instability and graft failure rates, especially in young, active patients. The degree of posterior tibial slope has been shown to affect ACL graft failure rates.(1) The effect of LET plus ACLRs on tibial motion and graft forces in the setting of varying tibial slopes has not been elucidated. HYPOTHESIS/PURPOSE: LET will decrease tibial translation, tibial rotation, and ACL graft forces compared to ACLR alone with increasing tibial slope throughout knee range of motion. METHODS: Twelve cadaveric knees (40.5 mean age, all female) were tested in 4 conditions (1: intact; 2: ACLR; 3: ACLR + LET; and 4: ACL cut) with varying posterior tibial slopes (5°, 10°, 15°, and 20°) at three flexion angles (0°, 30, ° and 60°). Specimens were mounted to a load frame which applied a 500-N axial load with 1 N-M of internal rotation torque (Figure 1). The amount of tibial translation was measured using a laser sensor and rotation was measured along the axis of the load frame. ACL graft forces were measured via a force transducer. RESULTS: No clinically relevant difference was found in graft force (all differences <6.8 N, except the 5° posterior tibial slope at 30° of flexion showing a 12.1 N reduction with LET), anterior tibial translation (all differences <2.4 mm), or internal rotation (all differences < 1.59°). Compared to the intact state, the cut ACL state had higher anterior tibial translation (maximum increase +9.1 mm at slope 20°, Flexion 60°) and internal rotation (maximum increase +4.3° at slope 5°, flexion 30°). Higher ACL graft forces were observed with increasing posterior tibial slope (ACLR graft force at 0° of flexion was 81.6 N at 5°, 97.8 N at 10°, 107.0 N at 15°, 113.9 N at 20° of slope). Figure 2 shows ACL graft forces in N, Figure 3 shows anterior tibial translation in mm and Figure 4 shows internal rotation in degrees. All figures present data at 30°of flexion. CONCLUSION: No clinically relevant difference was shown with the addition of LET in the setting of ACLR for patients with posterior tibial slopes from 5° to 20° at flexion angles of 0°, 30°, and 60° in regards to graft force, anterior tibial translation, and internal tibial rotation under a 500 N compressive load. REFERENCE: 1. Bernhardson AS, Aman ZS, Dornan GJ, et al. Tibial Slope and Its Effect on Force in Anterior Cruciate Ligament Grafts: Anterior Cruciate Ligament Force Increases Linearly as Posterior Tibial Slope Increases. Am J Sports Med. 2019;47(2):296-302. doi:10.1177/0363546518820302