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

Abstract

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.

Figure 1.

AP and Posteromedial views of the testing specimen mounted on load frame

Figure 2.

ACL Graft Force at 500N compression at Flexion 30° for ACLR (Red) vs ACLR+LET (Blue) states at varying posterior tibial slopes (5°, 10°, 15°, and 20°)

Figure 3.

Anterior Tibial Translation at Flexion 30° for all states (Intact (blue), ACLR+LET (red), ACLR (green), ACL cut (orange)) at varying posterior tibial slopes (5°, 10°, 15°, and 20°)

Figure 4.

Internal Tibial Rotation at Flexion 30° for all states (Intact (blue), ACLR+LET (red), ACLR (green), ACL cut (orange)) at varying posterior tibial slopes (5°, 10°, 15°, and 20°)

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