Peak stresses shift from femoral tunnel aperture to tibial tunnel aperture in lateral tibial tunnel ACL reconstructions: a 3D graft-bending angle measurement and finite-element analysis

Can lateral tenodesis improve the rotational stability of the ACL reconstruction? A finite element analysis

One of the most common knee injuries is the Anterior Cruciate Ligament (ACL) rupture with severe implications on knee stability. The usual treatment is the ACL Reconstruction (ACLR) surgery where the surgeon replaces the torn ligament with a graft in an effort to restore knee kinematics. In case of excessive rotatory instability, Lateral Extra-Articular Tenodesis (LET) can be performed in combination with ACLR. Additionally, LET appears to reduce ACLR graft forces minimizing graft failure chances. However, there are concerns about overconstraining physiological rotation. To gain insight in this controversial topic, we developed an automatic, open-source tool to create a series of Finite Element (FE) models attempting to investigate the interactions of ACLR and LET through simulation. We started by creating a validated model of the healthy knee joint that served as reference for subsequent FE simulations. Then, we created FE models of standalone ACLR and combined ACLR-LET. Each model was assessed by applying a loading profile that resembles the reduction phase of the Pivot-Shift clinical exam. We measured the External Tibia Rotation (ETR), the Posterior Tibia Translation (PTT) of the lateral tibial compartment, and the ACLR graft stress developed around the femoral tunnel insertion site. We observed the following: a) LET reduces ETR and PTT compared to isolated ACLR, b) combined ACLR-LET is more sensitive to LET graft pretension with lower values showcasing performance closer to the healthy joint, c) LET reduces ACLR graft forces for the same pretension values, d) LET exhibits significant overconstraint for higher pretension values. In general, these findings are in agreement with relevant clinical studies and accentuate the potential of the developed framework as a tool that can assist orthopaedists during surgery planning. We provide open access for the FE models of this study to enhance research transparency, reproducibility and extensibility.

Dynamics analysis of the anterior cruciate ligament reconstruction surgery based on magnetic resonance imaging

In clinical practice, anterior cruciate ligament (ACL) rupture is always repaired by the single-beam reconstruction method. Before the surgery, the surgeon made the diagnosis based on medical images, such as CT (computerized tomography) and MR (magnetic resonance) images. However, little is known about how biomechanics governs the biological nature for femoral tunnel position. In the present study, three volunteers' motion trails were captured by six cameras when they were doing squat movement. The medical image can reconstruct the structure of the ligaments and bones and a left knee model was reconstructed by MIMICS by MRI data of DICOM format. Finally, the effects of different femoral tunnel positions on ACL biomechanics were characterized by the inverse dynamic analysis method. The results showed that there were significant differences in the direct mechanical effects of the anterior cruciate ligament at different locations of the femoral tunnel (p < 0.05), the peak stress of ACL in the low tension area was 1097.24 ± 25.55 N, and the peak stress of ACL in the distal femur was 356.81 ± 15.39 N, both of which were much higher than that in the direct fiber area (118.78 ± 20.68 N).

Finite Element Analysis and Experimental Validation of the Anterior Cruciate Ligament and Implications for the Injury Mechanism

This study aimed to establish a finite element model that vividly reflected the anterior cruciate ligament (ACL) geometry and investigated the ACL stress distribution under different loading conditions. The ACL’s three-dimensional finite element model was based on a human cadaveric knee. Simulations of three loading conditions (134 N anterior tibial load, 5 Nm external tibial torque, 5 Nm internal tibial torque) on the knee model were performed. Experiments were performed on a knee specimen using a robotic universal force/moment sensor testing system to validate the model. The simulation results of the established model were in good agreement with the experimental results. Under the anterior tibial load, the highest maximal principal stresses (14.884 MPa) were localized at the femoral insertion of the ACL. Under the external and internal tibial torque, the highest maximal principal stresses (0.815 MPa and 0.933 MPa, respectively) were mainly concentrated in the mid-substance of the ACL and near the tibial insertion site, respectively. Combining the location of maximum stress and the location of common clinical ACL rupture, the most dangerous load during ACL injury may be the anterior tibial load. ACL injuries were more frequently loaded by external tibial than internal tibial torque.

[Finite element analysis of the graft stresses after anterior cruciate ligament reconstruction]

OBJECTIVE

To explore the stress distribution characteristics of the graft after anterior cruciate ligament (ACL) reconstruction, so as to provide theoretical reference for the surgical plan of ACL reconstruction.

METHODS

Based on 3D MRI and CT images, finite element models of the uninjured knee joint and knee joint after ACL reconstruction were established in this study. The uninjured knee model included femur, tibia, fibula, medial collateral ligament, lateral collateral ligament, ACL and posterior cruciate ligament. The ACL reconstruction knee model included femur, tibia, fibula, medial collateral ligament, lateral collateral ligament, ACL graft and posterior cruciate ligament. Linear elastic material properties were used for both the uninjured and ACL reconstruction models. The elastic modulus of bone tissue was set as 17 GPa and Poisson' s ratio was 0.36. The material properties of ligament tissue and graft were set as elastic modulus 390 MPa and Poisson's ratio 0.4. The femur was fixed as the boundary condition, and the tibia anterior tension of 134 N was applied as the loading condition. The stress states of the ACL of the intact joint and the ACL graft after reconstruction were solved and analyzed, including tension, pressure, shear force and von Mises stress.

RESULTS

The maximum compressive stress (6.34 MPa), von Mises stress (5.9 MPa) and shear stress (1.83 MPa) of the reconstructed ACL graft were all at the anterior femoral end. It was consistent with the position of maximum compressive stress (8.77 MPa), von Mises stress (8.88 MPa) and shear stress (3.44 MPa) in the ACL of the intact knee joint. The maximum tensile stress of the graft also appeared at the femoral end, but at the posterior side, which was consistent with the position of the maximum tensile stress of ACL of the uninjured knee joint. More-over, the maximum tensile stress of the graft was only 0.88 MPa, which was less than 2.56 MPa of ACL of the uninjured knee joint.

CONCLUSION

The maximum compressive stress, von Mises stress and shear stress of the ACL graft are located in the anterior femoral end, and the maximum tensile stress is located in the posterior femoral end, which is consistent with the position of the maximum tensile stress of the ACL of the uninjured knee joint. The anterior part of ACL and the graft bore higher stresses than the posterior part, which is consistent with the biomechanical characteristics of ACL.

A patient-specific finite element analysis of the anterior cruciate ligament under different flexion angles

BACKGROUND

The main responsibility of the anterior cruciate ligament (ACL) is to restore normal knee kinematics and kinetics. Although so far different research has been carried out to measure or quantify the stresses and strains in the ACL experimentally or numerically, there is still a paucity of knowledge in this regard under different flexion angles of the tibiofemoral knee joint.

OBJECTIVE

Understanding the stresses and strains within the ACL under various loading and boundary conditions may have a key asset for the development of an optimal surgical treatment of ACL injury that can better restore normal knee function. This study aimed to calculate the stresses and strains within the ACL under different flexion angles using a patient-specific finite element (FE) model of the human tibiofemoral knee joint.

METHODS

A patient-specific FE model of the human tibiofemoral knee joint was established using computed tomography/magnetic resonance imaging data to calculate the stresses and strains in the ACL under different flexion angles of 0, 10, 20, 30, and 45∘.

RESULTS

Although the role of the flexion angle in the induced stresses and strains of the ACL was insignificant, the highest stress and strain were observed at the flexion angle of 0∘. The concentration of the stresses and strains regardless of the flexion angles were also located at the proximal end of the ACL, where the clinical reports indicated that most ACL tearing occurs there at the femoral insertion site.

CONCLUSIONS

The results have implications not only for understanding the stresses and strains within the ACL under different flexion angles, but also for providing preliminary data for the biomechanical and medical experts in regard of the injuries which may occur to the ACL at relatively higher flexion angles.

Current Strategies and Future Directions to Optimize ACL Reconstruction in Adolescent Patients

The incidence of anterior cruciate ligament (ACL) injuries in the pediatric population has risen in recent years. These injuries have historically presented a management dilemma in skeletally immature patients with open physes and significant growth remaining at time of injury. While those nearing skeletal maturity may be treated with traditional, transphyseal adult techniques, these same procedures risk iatrogenic damage to the growth plates and resultant growth disturbances in younger patients with open physes. Moreover, conservative management is non-optimal as significant instabilities of the knee remain. Despite the development of physeal-sparing reconstructive techniques for younger patients, there remains debate over which procedure may be most suitable on a patient to patient basis. Meanwhile, the drivers behind clinical and functional outcomes following ACL reconstruction remain poorly understood. Therefore, current strategies are not yet capable of optimizing surgical ACL reconstruction on an individualized basis with absolute confidence. Instead, aims to improve surgical treatment of ACL tears in skeletally immature patients will rely on additional approaches in the near future. Namely, finite element models have emerged as a tool to model complex knee joint biomechanics. The inclusion of several individualized variables such as bone age, three dimensional geometries around the knee joint, tunnel positioning, and graft tension collectively present a possible means of better understanding and even predicting how to enhance surgical decision-making. Such a tool would serve surgeons in optimizing ACL reconstruction in the skeletally immature individuals, in order to improve clinical outcomes as well as reduce the rate of post-operative complications.