Knee valgus deformity and lateral bone defects affect the function of superficial medial collateral ligament: A finite element analysis

Abstract

Purpose

The superficial medial collateral ligament (sMCL) is the primary restraint to valgus laxity of the knee, which is one of the significant indicators of implant selection in valgus knee. Our purpose is to explore the influence of knee valgus deformity and lateral bone defects in the function of sMCL.

Methods

the right knee joint of a healthy male volunteer was subjected to CT and MRI scans. The scanned data were imported into Mimics, Geomagic, Solidworks and Ansys software to establish a three-dimensional finite element model of the human knee joint. Femorotibial angle (FTA)5°,10°,15°,20°,25°,30°,35° and lateral bone defect 0,0.5,1,1.5,2 cm are controlled in Solidworks. Tensile test in vitro of maximum load on sMCL was simulated in Ansys.

Results

The peak stress of sMCL is raising with valgus deformity while there is no lateral defect. Increasing lateral bone defect can lessen the augmentation of the stress of sMCL caused by the valgus deformity. The peak stress of sMCL when it is in maximum load is 35.252 MPa. While valgus 35°, the peak stress of sMCL exceeds the value, with or without bone defect; the same is true for the valgus 30° with 0, 0.5, 1 cm bone defect and valgus 25° without defect.

Conclusion

Our findings allow for preoperative evaluation of sMCL function in the valgus knee, which would play an instructive role to some extent for implant selection in total knee arthroplasty.

1. Introduction

A valgus deformity is present in around 10% of patients who need complete knee replacement surgery. Rheumatoid arthritis, posttraumatic arthritis, osteoarthritis, or metabolic bone disease are common causes of valgus deformity. The valgus deformity has two parts. The first is bone loss with metaphyseal remodeling, mainly in relation to the lateral femoral condyle and lateral tibial plateau. Secondarily, lateral soft tissue contracture in structures that have classically been felt to include the iliotibial (IT) band, lateral collateral ligament, popliteus tendon, posterolateral capsule, gastrocnemius, and hamstring muscle.1, 2, 3, 4

Implant selection is a challenge for surgeons in valgus knee because the stability of the knee joint is more severely damaged in valgus deformity.⁵ The sMCL is the largest structure of medial knee stabilizers and dominant for limiting valgus, playing a crucial role in the selection.⁶ When its function is impaired, the stability can be seriously disrupted, at which point the surgeon is forced to employ a constrained prosthesis.7, 8, 9 Three types of valgus deformity have been described, according to femorotibial angle (FTA). Mild: the FTA less than 15°. Moderate: the FTA ranges between 15° and 30°. Severe: the FTA is greater than 30°, more possibly to utilize a constrained or hinged total-knee design.^3,10

However, the choice of prosthesis is still debated. Some researches showed that even severe valgus deformity, a non-constrained implant could be employed to get a decent result.11, 12, 13, 14, 15 In addition to the valgus deformity, the function of sMCL may be affected by other factors, such as bone loss, age, primary disease etc. Moreover, in clinical practice, we observed that with a certain degree of lateral bone defects, non-constrained implant could be utilized in some severe valgus knee (FTA>30°) to get a benign outcome, which might be a prophetic element of sMCL function. Lateral bone defects include deficiencies from the lateral femoral condyle and the lateral tibia plateau. A paucity of research on the relations between the lateral bone defect and the function of sMCL in valgus knee. To mimic the condition, we developed a finite element model of the knee, controlling FTA for 5°,10°,15°,20°,25°,30°,35°valgus deformity and 0,0.5,1,1.5,2 cm lateral bone defect; the function of sMCL was sought to provide some evidence for implant selection.

2. Material and methods

2.1. Healthy knee model

Consenting to the research, a healthy male (age:30 years; height: 1.70 m; body mass index (BMI):22.8kg/m2) without any deformity and bone defect of knee joint participated in this study. The volunteer's right knee joint underwent continuous spiral CT (SOMATOM Definition AS Siemens, Germany) with a 1 mm slice thickness. A total of 326 images in DICOM format (Digital Imaging and Communications in Medicine) were collected. The identical right knee joint received MR imaging using a Magnetom AVanto 1.5 T S, Germany; a total of 400 DICOM-format images were collected. The images were got through Mimics 21.0 and establish the original 3D model with many noise spots initially. Then, smooth and denoise the model from the Geomagic 2017 to perfect the solid model. Mimics 21.0 and Geomagic 2017 are software used to process the images collected from CT and MR scans of the volunteer's knee joint and to create the solid model for finite element analysis. And import it in Solidworks 2017 for assembly and cutting operations, so as to establish a complete finite element analysis model (Fig. 1). After providing a solid model foundation for the finite element analysis, the software Ansys 17.0 is finally imported to set the analysis parameter attributes and conditions for stress analysis. All models were assumed to behave with homogeneous, isotropic, and linear elastic behavior. Articular cartilage, ligaments, and meniscus were assigned a Young's modulus (E) of 5 MPa, 215.3 MPa, and 59 MPa, respectively, and the Poisson's ratios were 0.49, 0.4, and 0.46,16, 17, 18, 19 respectively. The models were remeshed using an interactive mesh of tetrahedron elements, a total of 115549 nodes and 71007 elements.

Fig. 1.

Healthy knee model in Solidworks. a Frontal view. b Posterior view. c Medial view. d Lateral view.

2.2. Load and boundary conditions

When a person stands and moves around, the knee joint is put under a particular degree of tension, and the sMCL receives the appropriate amount of stress. In order to simulate the stress on the knee joint, a certain stress is applied to the knee joint. The tibia is locked, and 646.8 N of force was applied in a downward, vertical orientation (648.8 N of force was converted based on the bodyweight of 66 kg in the demographic data). The superficial and deep layers of the medial collateral ligament and the starting and stopping points of each ligament with the bones were characterized as bonded contacts, while interactions at other sites were defined as no separation contacts. (Bonded contact: No penetration, no separation and no sliding between faces or edges. No separation contact: Similar to bonded, except frictionless sliding can occur along contacting faces.)

2.3. Model validation

The tibial anterior translation was observed to be 4.6004 mm applying a forward force of 134 N to the tibia in an extension position of the knee joint; the translation as reported to be 4.6–5.0 mm using the same load in previous studies. Thus, our results were consistent with the previously reported results on FEA studies, suggesting the effectiveness of our model.^18,20

2.4. Valgus deformity and lateral bone defects

It is clear that valgus deformity is an essential factor affecting superficial medial collateral ligament stress, as confirmed by previous studies.1, 2, 3, 4 Additionally, in the past, a restrictive prosthesis had been utilized for individuals who had severe knee valgus. Therefore different groupings were set up. Valgus deformity basically from the femur were made to the appropriate FTA degree in Solidworks, i. e, 5°,10°,15°,20°,25°,30°,35° (Fig. 2); Since the muscle and ligament cannot be extended accordingly, modified using Geomagic to match the corresponding anatomical locations. The rest of the bones, muscles and ligaments were untouched.

Fig. 2.

All sort of valgus deformity with different lateral bone defect (posterior view) in Solidworks. (FTA: 5°,10°,15°,20°,30°,35°. Lateral bone defect: 0 cm,0.5 cm,1.0 cm,2.0 cm).

There have been few prior investigations on the lateral bone defect, which may be a significant factor determining the strains in the sMCL. The lateral bone defect may lessen the tension on the sMCL that is mostly brought on by the valgus deformity. It most likely affects the choice of preoperative prosthesis. Thus, 0 cm lateral bone loss was defined as no any defect (including cartilage and meniscus), and 0.5, 1, 1.5, and 2 cm lateral bone defects were defined as mutual wear of the lateral femoral condyle and lateral tibial plateau. Seeing that the lateral defects is gradually worn between bones in reality, at different valgus, we employed the entity command to direct editing in Solidworks to make them move up and down a certain distance from each other; the distance is the amount of bone defect. And controlled it in 0, 0.5, 1, 1.5, 2 cm for different valgus deformities (Fig. 2).

2.5. Tensile test

A vitro tensile test is simulated to evaluate the stresses on sMCL as its function strictly disrupted, according to the literatures.^16,21 Loading condition: The proximal tibia was selected as the location for stretching, 534 N (the maximum load of sMCL) upward was applied to it. Boundary conditions: fixing the distal femur. Contact relations: all contacts with the tibia and femur are non-separated, others contacts are bounded (Fig. 3).

Fig. 3.

Maximum load on sMCL in tensile test a The fixed position of the femur b the orientation of applied load on tibia c: von Mises str ess distribution of the sMCL.

3. Result

3.1. Stress distribution of sMCL

The stress on various sMCL sections were calculated. Through the peak stress level of sMCL, the function of sMCL can be approximately evaluated, thus providing some reference for our preoperative prosthesis selection. In the study, Most of the peak stress of sMCL is concentrated at the bone attachment of ligament. The peak stress of sMCL was enhancing when valgus deformity was mounted with no defect, whereas the peak stress of sMCL was falling with lateral bone defect mounted, when valgus is fixed (Fig. 4, Fig. 5). The peak stress of sMCL mainly concentrated in the middle and upper part of the ligament when it is in maximum load is 35.252 MPa (Fig. 3). Increasing lateral bone defect can lessen the augmentation of the stress caused by the valgus deformity. While valgus 35°, the peak stress of sMCL exceeds the value, with or without bone defect; the same is true for the valgus 30° with 0,0.5,1 cm bone defect and valgus 25° without defect. And valgus 30° with 1.5 cm bone defect is very closed to it (Fig. 5). This implies that both valgus deformity and lateral bone defect are critical elements effecting peak stress of sMCL, which plays an instrumental role in preoperative prosthesis selection for TKA. Some patients with relatively severe valgus deformity with some lateral bone defect probably do not require a more restrictive grade of prosthesis.

Fig. 4.

Von Mises stress distribution of the sMCL with different valgus and lateral bone defect. (FTA: 5°,10°,15°,20°,30°,35°. Lateral bone defect: 0 cm,0.5 cm,1.0 cm,2.0 cm)

Fig. 5.

Changes in peak stress of sMCL with different valgus deformity and lateral bone defect. (FTA: 5°,10°,15°,20°,30°,35°; Lateral bone defect:0 cm,0.5 cm,1.0 cm,2.0 cm; Green plaid: peak stress of sMCL higher than the tensile test; Red plaid: peak stress of sMCL lower than the tensile test).

3.2. Multiple linear regression analysis

Obviously, knee valgus deformity and lateral bone defect have distinct effects on sMCL. Thus, statistics were carried out using multiple linear regression by SPSS 25.0. The adjusted R2 that measures how tightly the model fits the data is 0.882, indicating that the regression model had a higher degree of interpretation. The closer the adjusted R2 is to 1, the better the model is. The variance inflation factor and collinearity tolerance, Indicators to assess correlation or covariance, were smaller than 10, indicating no serious collinearity in regression model. The t value of valgus deformity and lateral bone defect was 14.59 and −5.171, P = 0.000 < 0.05. Finally, a regression equation was acquired: Y = 1.36X1-6.82X2+2.65 (X1: FTA, X2: lateral bone defect).

4. Discussion

The purpose of this study is to seek the effect of lateral bone loss and valgus deformity on the function of sMCL. The FEA related software was to utilized to build a complete knee joint model with ligament and meniscus, controlled the FTA and the lateral bone defects, and obtained the von stresses distribution of sMCL under different conditions. From the 0 cm–0.5 cm bone defect, it included wear of articular cartilage and meniscus. Then we simulated the tension test of maximum load in vitro and obtained the peak stress of sMCL, as 35.252Mpa, with the fibers in sMCL is severely damaged, loosen or broken. Our data shows increasing lateral bone defect can lessen the augmentation of the stress of sMCL caused by the valgus deformity, obviously it is also a variable affecting the function of sMCL. Valgus 35°, the stress of sMCL exceeds the value, with or without bone defect the same is true for the valgus 30° with 0,0.5,0.1 cm bone defect and valgus 25° without defect and valgus 30° with 1.5 cm bone defect is closed to it, the usage of sMCL would be strictly impaired at these circumstance, constrained implant may be more advantageous. Through the curve and equation, we can roughly get whether the peak stress of sMCL in a certain situation retained residual function. It showed that the knee joint retained stability in some severe valgus knee (like FTA 30°, defect 2 cm), which a non-constrained implant can be used if suitable release technique is utilized.

Some other studies show similar conclusion.^13,15 In 2014 Zhou et al. reported a retrospective review with some severe valgus knees who underwent primary TKA, it concluded that not only constrained implants can be successfully used, if proper ligament balancing techniques were used and adequate ligament balance was attained, the knee may not require a more constrained component.²² In recent years, many researches also advocated that misuse of more constrained implant should be shunned.²³

As valgus deformity mounted, sMCL would be stretched, the stress on it would be ascended, and its function will be gradually damaged, matching our result. The lateral bone defect is mainly from the lateral femoral condyles and the lateral tibial plateau, caused by the dysplasia of the bone and the wear from valgus. As early as 1991, Rand elaborated on bone defects and reported relevant treatment.²⁴ Our data suggested that the stress on sMCL tapered with mounting of lateral bone defect in the valgus knee. We hypothesized that it is probably due to the reducing of the medial joint space, or it may be due to changes of the rotational alignment of the knee joint. However, as the bone defect aggravated, a certain number of inflammatory factors might be concentrated, playing deleterious impact on sMCL. This relies on further researches.

There might be a solid grasp of variables with the function of sMCL, except for valgus deformity and lateral bone loss. Woo et al. pointed that age might be a factor of MCL feature. Significant ascents in the linear stiffness, ultimate load and energy absorbed at failure of the MCL were noted in rabbits during skeletal maturation, but the MCL of the older rabbits began to show a weakness in these properties.²⁵ Daniel et al. found that the dilatory response of MCL to ACH, bradykinin, histamine, and substance P were abolished in rabbits knee osteoarthritis model; Response to shear stress was also attenuated. This shadows that a difference in the primary disease may also lead to an impact on the function of the medial collateral ligament.²⁶ Previous studies were conducted on MCL directly, whereas sMCL, the largest structure in MCL, was similarly affected.

There are some limitations in the study. Finite element analysis may not fully simulate the morphological changes in sMCL, because the material properties of sMCL are more intricate in vivo environment.²⁷ Some discrepancies might be in the fabrication of the bone defect with reality. Moreover, the lateral bone defect and valgus deformity were separated in our analysis to explore specific situations. But whether the lateral bone defect synchronized with the formation of knee valgus or not is unsettled. If valgus precedes the defect, the function of the ligament would be severely impaired even if the stress on the ligament is reduced with the bone defect processing.

In conclusion, increasing lateral bone defect can lessen the augmentation of the stress of sMCL caused by the valgus deformity. In clinical practice, we can get a more accurate preoperative prosthesis planning by using radiological examination (X-ray or CT) based on our regression equation or stress curve to evaluate the function of sMCL. Not all patients with severe valgus deformity require a constrained prosthesis with higher possibility of revision and osteolysis.

Funding

This study was funded by The National Key R&D Program of China (No.2021YFA1102600); National Natural Science Foundation of China (No. 82002293, 82272443); Science and Technology Planning Project of Guangzhou City, China (Grant No. 202201020495, 202201020481); Guangdong Basic and Applied Basic Research Foundation (Grant No. 2019A1515011647, 2021A1515010693, 2021A1515010294, 2022A1515010256, 2023A1515010501).

Author contributions

Junming Huang and Hao Sun contributed equally to this work. Junming Huang, Hao Sun, contributed to the conception of the study. Deng Li acquired the data and performed data analysis. Junming Huang and Hao Sun contributed significantly to manuscript preparation. Yimin Wang, Jie Xu and Ruofan Ma revised the work critically for important intellectual content. All authors read and approved the final manuscript.

Ethics approval

This study was ruled exempt from formal review by the Ethical Committee of Sun Yat-sen memorial hospital, given the patient provided written informed consent.

Consent to participate

None declared.

Consent to publish

None declared.

Code availability

Not applicable.

Availability of data and materials

All data generated or analyzed during this study are included in this published article.

Declaration of competing interest

No potential conflict of interest was reported by the authors.