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. 2023 Apr;51(5):1136-1145.
doi: 10.1177/03635465231161071. Epub 2023 Mar 14.

Role of the Anterior Cruciate Ligament, Anterolateral Complex, and Lateral Meniscus Posterior Root in Anterolateral Rotatory Knee Instability: A Biomechanical Study

Lukas Willinger  1 , Kiron K Athwal  2 , Sander Holthof  2 , Andreas B Imhoff  1 , Andy Williams  3 , Andrew A Amis  2
Affiliations

Role of the Anterior Cruciate Ligament, Anterolateral Complex, and Lateral Meniscus Posterior Root in Anterolateral Rotatory Knee Instability: A Biomechanical Study

Lukas Willinger et al. Am J Sports Med. 2023 Apr.

Abstract

Background: Injuries to the anterior cruciate ligament (ACL), Kaplan fibers (KFs), anterolateral capsule/ligament (C/ALL), and lateral meniscus posterior root (LMPR) have been separately linked to anterolateral instability.

Purpose: To investigate the contributions of the ACL, KFs, C/ALL, and LMPR to knee stability and to measure instabilities resulting from their injury.

Study design: Controlled laboratory study.

Methods: Ten fresh-frozen human knees were tested robotically to determine restraints of knee laxity at 0° to 90° of flexion. An 88-N anterior-posterior force (anterior and posterior tibial translation), 5-N·m internal-external rotation, and 8-N·m valgus-varus torque were imposed and intact kinematics recorded. The kinematics were replayed after sequentially cutting the structures (order varied) to calculate their contributions to stability. Another 10 knees were tested in a kinematics rig with optical tracking to measure instabilities after sequentially cutting the structures across 0° to 100° of flexion. One- and 2-way repeated-measures analyses of variance with Bonferroni correction were used to find significance (P < .05) for the robotic and kinematics tests.

Results: The ACL was the primary restraint for anterior tibial translation; other structures were insignificant (<10% contribution). The KFs and C/ALL resisted internal rotation, reaching 44% ± 23% (mean ± SD; P < .01) and 14% ± 13% (P < .05) at 90°. The LMPR resisted valgus but not internal rotation. Anterior tibial translation increased after ACL transection (P < .001) and after cutting the lateral structures from 70° to 100° (P < .05). Pivot-shift loading increased anterolateral rotational instability after ACL transection from 0° to 40° (P < .05) and further after cutting the lateral structures from 0° to 100° (P < .01).

Conclusion: The anterolateral complex acts as a functional unit to provide rotatory stability. The ACL is the primary stabilizer for anterior tibial translation. The KFs are the most important internal rotation restraint >30° of flexion. Combined KFs + C/ALL injury substantially increased anterolateral rotational instability while isolated injury of either did not. LMPR deficiency did not cause significant instability with the ACL intact.

Clinical relevance: This study is a comprehensive biomechanical sectioning investigation of the knee stability contributions of the ACL, anterolateral complex, and LMPR and the instability after their transection. The ACL is significant in controlling internal rotation only in extension. In flexion, the KFs are dominant, synergistic with the C/ALL. LMPR tear has an insignificant effect with the ACL intact.

Keywords: Kaplan fibers; anterior cruciate ligament; anterolateral ligament; instability; kinematics; lateral meniscus root.

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Conflict of interest statement

One or more of the authors has declared the following potential conflict of interest or source of funding: This study was funded by a research grant from the German Society for Arthroscopy and Joint Surgery. It was also supported by a grant from Smith & Nephew Endoscopy Co, paid to a research account of Imperial College London. L.W. received funding from the German Research Foundation during his work at Imperial College London. Human samples used in this research project were obtained from the Imperial College Healthcare Tissue Bank (ICHTB). The ICHTB is supported by the National Institute for Health Research’s Biomedical Research Centre, based at Imperial College Healthcare NHS Trust and Imperial College London. The ICHTB is approved by Wales REC3 to release human material for research (17/WA/0161). AOSSM checks author disclosures against the Open Payments Database (OPD). AOSSM has not conducted an independent investigation on the OPD and disclaims any liability or responsibility relating thereto.

Figures

Figure 1.
Figure 1.
Anterior tibial translation laxity of the intact knee and instability after cutting the ACL and the anterolateral structures in response to 88-N anterior translation force in 6 degrees of freedom kinematics rig testing. The anterior translation was significantly increased after the ACL was cut at all flexion angles (P < .001). Cutting the anterolateral structures also significantly increased knee laxity from 70° to 100°. *P < .05. Data are presented as mean ± SD (n = 10). ACL, anterior cruciate ligament.
Figure 2.
Figure 2.
The contribution of the anterior cruciate ligament (ACL), Kaplan fibers, anterolateral capsule including the anterolateral ligament (C/ALL), and the lateral meniscus posterior root (LMPR) to resist internal rotation in robotic testing. #P < .05. *P < .01. Data are presented as mean ± SD (n = 10).
Figure 3.
Figure 3.
Changes in internal rotation after transecting the ACL and then the anterolateral structures in response to 5-N·m internal rotation torque in 6 degrees of freedom kinematics rig testing. Data are presented as mean ± SD (n = 10). *P < .05 (significant increase above ACL cut state). ACL, anterior cruciate ligament.
Figure 4.
Figure 4.
The contribution of the anterior cruciate ligament (ACL), Kaplan fibers, anterolateral capsule including the anterolateral ligament (C/ALL), and the lateral meniscus posterior root (LMPR) to resist valgus rotation in robotic testing. *P < .001. Data are presented as mean ± SD (n = 10, apart from the superficial medial collateral ligament [sMCL] when n = 5).
Figure 5.
Figure 5.
Changes in (A) anterior tibial translation and (B) internal rotation after cutting the anterior cruciate ligament (ACL) and the anterolateral structures in response to simulated pivot-shift load (combined 5-N·m internal torque and 8-N·m valgus torque) in 6 degrees of freedom kinematics rig testing. *P < .01. #P < .05 vs intact state. **P < .001 vs ACL cut state. Data are presented as mean ± SD (n = 10).
Figure 6.
Figure 6.
The resulting anterior tibial translation in response to a simulated pivot shift (combined 5-N·m internal torque and 8-N·m valgus torque) in 3 cutting orders: (A) ACL, C/ALL, KFs, LMPR (n = 3); (B) ACL, LMPR, C/ALL, KFs (n = 3); and (C) ACL, KFs, LMPR, C/ALL (n = 4). ACL, anterior cruciate ligament; C/ALL, anterolateral capsule and ligament; KFs, Kaplan fibers; LMPR, lateral meniscus posterior root.
Figure 7.
Figure 7.
The resulting tibial internal rotation in response to a simulated pivot shift (combined 5-N·m internal torque and 8-N·m valgus torque) in 3 cutting orders: (A) ACL, C/ALL, KFs, LMPR (n = 3); (B) ACL, LMPR, C/ALL, KFs (n = 3); and (C) ACL, KFs, LMPR, C/ALL (n = 4). ACL, anterior cruciate ligament; C/ALL anterolateral capsule and ligament; KFs, Kaplan fibers; LMPR, lateral meniscus posterior root.
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