Higher stress load in the lateral compartment after over-corrected UKA compared with HTO: a 3d-finite-element analysis

doi:10.21203/rs.3.rs-3226974/v1

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Higher stress load in the lateral compartment after over-corrected UKA compared with HTO: a 3d-finite-element analysis

https://doi.org/10.21203/rs.3.rs-3226974/v1

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Background

The purpose of this study was to utilize a 3D finite-element(FE)model to analyze why there are differences in stress distribution in the knee joint between unicompartmental knee replacement༈UKA༉and high tibial osteotomy༈HTO༉ under the same force line.

Methods

The stress of the lateral meniscus, tibial plateau and inferior tibial plateau bone in healthy, OA, standard UKA, overcorrected UKA and HTO were assessed using FE models. In terms of stress load, standing is simulated by applying vertical static load, and then the stress distribution of knee joints in each group is compared.

Results

The results showed that the overcorrected UKA had significantly higher stresses in the lateral meniscus, lateral tibial plateau and below the plateau compared to the HTO. Also, the stress distribution in the knee joint after HTO is closer to that of a healthy knee due to the bridging effect of the lateral tibial Tomofix plate.

Conclusion

In general, when the lower limb force lines in UKA are corrected to the same level as in HTO, the stress distribution in the knee joint in HTO is close to physiological due to the bridging effect of the Tomofix plate. In contrast, the stresses on the lateral compartment of the UKA are significantly higher and more prone to lateral compartment OA.

UKA

HTO

stress distribution

Lateral compartment OA

Knee osteoarthritis (OA) is a disease based on the degeneration of articular cartilage caused by long-term chronic overload¹. Previous studies have found that about one third of patients with OA affect only one compartment, and 50% of the lesions occur in the medial compartment^{2; 3}. For patients with OA confined to the medial compartment, UKA and HTO are two commonly used surgical methods to alleviate symptoms and restore knee function⁴.

Although the principles of the two methods are different, there is no significant difference in the medium and long-term efficacy of the two methods in the treatment of medial compartment OA. HTO corrects knee osteovarus through high tibial osteotomy, and transfers the weight-bearing area from the medial injured compartment to the central or healthy lateral compartment, so as to alleviate symptoms and delay disease progression^{5; 6}. UKA aims to restore the normal function of the knee joint by replacing the worn medial compartment of the knee joint, correcting the force line of the lower limb to the midline or slight varus (fixed platform)^{7; 8}. Both of them adjusted the lower limb force line to a certain extent. The difference is that the force line after HTO is slightly valgus, while the force line after UKA is normal. A large number of clinical studies show that after accurate HTO treatment, patients have good medium and long-term clinical efficacy. After the biomechanical environment is improved, the wear of the medial compartment will not be intensified, and even new fibrocartilage will grow again at the worn part^{9; 10}. At the same time, the lateral compartment can still maintain a good state for a long time, and the progress of OA will not be accelerated^{11; 12}; However, if the correction of UKA force line is excessive and the lateral compartment shows high stress, the patient will have lateral knee pain early after operation, and can quickly progress to lateral knee compartment OA^{13; 14}. Therefore, we can infer that under the same mild valgus force line, the stress distribution of knee joint after HTO and UKA is different.

Finite element analysis (FEA) is a tool that can reconstruct complex tissues with irregular geometry. Applying finite element analysis to the simulation of OA related surgery can better understand the biomechanical information of postoperative knee. The aim of this study was to investigate the differences in the biomechanics of the overcorrected UKA and HTO posterior knee joints under equal valgus force lines. To better compare the differences, a total of five state knee models were involved in this study: healthy state, OA, standard UKA, overcorrected UKA and HTO posterior knee. The finite element models were assessed for stress distribution in the tibial plateau and lateral meniscus, strain in the lateral collateral ligaments and load distribution in the medial and lateral compartments.

The three-dimensional (3D) nonlinear model of the knee joint was constructed based on computed tomography (CT) and magnetic resonance imaging (MRI) images of a 25-year-old male (height 175cm, weight 65kg) with a left knee joint. Spiral CT was used for bone scanning (CT setting: 120 kV, 450 mA, slice thickness of 1.25 mm and a pitch of 0.5 mm/rev), and MRI was used for cartilage, meniscus and ligament scanning (MRI setting: TE: 32 ms, TR: 4450 ms, slice thickness: 2mm, flip angle 90°, NEX: 2 and FOV: 22 cm). The images obtained from CT and MRI scanning were imported into the image processing software Mimics for 3D modeling, and then imported into ABAQUS for FE analysis. At the same time, the past medical records of the subjects were checked, and no other related diseases and bone tissue problems were found. Therefore, it can be considered that the knee joint involved in the construction of the model is healthy.

The lower limb force line involved in this study is obtained by connecting the center point of the femoral head and the center point of the ankle in the three-dimensional model. The force line of the healthy group is the natural force line of the sample, which passes through the center point of the knee joint; OA group simulated the state of medial knee OA by cutting off part or all of the cartilage at the distal end of the medial femoral condyle of the knee and in front of the medial tibial plateau until the force line of the lower limb passed through the midpoint of the medial femoral condyle of the tibial plateau. The standard UKA group restores the force line to the natural force line by simulating UKA, that is, through the central point of the tibial plateau; In the overcorrected UKA group, the lower limb force line was adjusted to the lateral slope passing through the lateral tibial plateau by increasing the thickness of polyethylene gasket, that is, 62.5% of the tibial plateau (Fujisawa point); The setting of lower limb force line in HTO group was the same as that in over correction UKA group. The prosthesis used by UKA is the Oxford single condyle movable platform prosthesis, and the steel plate used by HTO is the Tomofix steel plate produced by Swiss Cindis company. They are modeled after physical scanning (Fig. 1).

We created five sets of three-dimensional linear models: Healthy, OA, Standard UKA, Overcorrected UKA and HTO. Each model includes bone structure, main ligaments, meniscus and articular cartilage. The bone tissue is composed of cortical bone and cancellous bone, and the thickness of cortical bone is 1mm. The femoral cartilage and the upper surface of the meniscus and the upper surface of the prosthesis are arranged in frictionless contact. In addition, bone and cartilage, bone and ligament, tibial cartilage and the lower surface of meniscus are set as binding contact. In the UKA model, the contact type between the liner of the Oxford prosthesis and the prosthesis platform is also set as frictionless contact. In the HTO model, the mesh of the superficial layer of the medial collateral ligament is deleted to simulate the complete removal of the superficial layer of the medial collateral ligament in the real HTO operation. The material properties of each material are listed in Table 1.

Table 1

Material properties for the FE model
Material	Modulus of elasticity(Mpa)	Poisson's ratio
Cortical bone	17000	0.30
Cancellous bone	350	0.25
Meniscus	59	0.49
Cartilage	13	0.475
ACL	169	0.45
PCL	177	0.45
MCL	332	0.45
LCL	345	0.45
Patellar ligament	169	0.45
PE material	940	0.46

In order to simulate the standing situation as truly as possible, the FE analysis of the five groups of models is divided into two analysis steps. The first step is the complete fixation of the tibia and the downward application of 1150N on the femoral head. The second step was complete fixation of the femur, and 134n was applied backward to the tibia¹⁵. Differences in the distribution of bone stress in the lateral meniscus, medial and lateral collateral ligaments, medial and lateral tibial plateaus, and below the plateau between the healthy, OA, standard UKA, overcorrected UKA, and HTO groups were compared.

1. Internal fixation in HTO bears most of the stress of the tibial plateau to protect the lateral platform

In the healthy group, the stress distribution on the inner and outer sides of the tibial plateau was similar, the stress was concentrated in the front. The maximum contact stress of medial tibial plateau in OA group was the highest, which was 8.6mpa, which was 258.3% higher than that in healthy group. The overcorrected UKA group and HTO group were close and increased compared with the healthy group; The healthy group was the lowest, and the over correction UKA group increased by 217.3% compared with the healthy group. The maximum contact stress at the lateral tibial plateau was not significantly increased in the HTO group, but was instead the lowest except in the healthy group. The maximum contact stress of the lateral tibial plateau in the excessive UKA group was 2.43 times that in the HTO group (Fig. 2 and Fig. 3).

In order to explore the difference of stress distribution in the bone under the tibial plateau in each group, the tibial stress distribution map of 10mm around the center point of the knee joint was intercepted. In the healthy group, the distribution of bone stress under the platform was more uniform, and the internal and external stresses were roughly the same; In the OA group, the stress was concentrated in the medial plateau, and the increase was most pronounced in the anteromedial part of the medial plateau; The overall distribution of standard UKA group was more uniform, but increased than that of healthy group; In contrast to the healthy group, the bone stress under the lateral tibial plateau in the overcorrected UKA group was significantly higher than that in the medial tibial plateau. The distribution of bone stress under the platform in HTO group was similar to that in healthy group, but the local stress at the plate and screw fixation increased significantly (Fig. 4).

2. Under the same stress line, the contact stress of the lateral meniscus of UKA increased significantly

The maximum contact stress of the lateral meniscus of each FE model was measured under the simulated standing condition. Of the five groups, contact stresses in the lateral meniscus were greatest in the overcorrected UKA group and least in the HTO group. The maximum contact stress of the lateral meniscus increased by 52% when the healthy knee progressed to OA. The maximum contact stress of lateral meniscus in standard UKA group and HTO group was close to that in healthy group. The overcorrected UKA group increased by 98.7% compared with the healthy group (Fig. 5 and Fig. 6).

HTO and UKA have been more and more widely used in the treatment of OA in recent years because of their minimally invasive and knee preserving characteristics, and both have achieved excellent clinical efficacy^{4; 16; 17}. The surgical principles of the two are distinct, and the management of lower extremity force lines varies considerably. HTO precisely adjusts the Mikulicz line by open osteotomy of the proximal tibia to transfer most of the stress from the medial side of the knee joint to the middle or lateral compartment, so as to alleviate the pain of the medial compartment^{6; 14}. With the popularization of standard HTO surgery technology and the application of Tomofix plate specially used for osteotomy, HTO surgery has achieved clinical efficacy and survival rate comparable to TKA. Codie et al. collected the data of 556 patients with HTO after operation from the London Health Science Center and found that only 5% of the patients needed TKA treatment within 5 years, and about 79% of the patients maintained a good state of the lateral compartment after operation for up to 10 years without the progression of OA¹⁸. UKA usually requires the postoperative force line to pass through the center of the knee joint or slightly inward. However, if the UKA force line is corrected to be the same as HTO, there will be residual lateral knee pain after operation, and even the lateral compartment will rapidly progress to OA in a short time.

The stress distribution and difference of tibial plateau, lateral meniscus, ligament and bone under tibial plateau of over corrected UKA and HTO are the focus of this study. In this study, we found that when the UKA force line was corrected to pass through the Fujisawa point, the maximum contact stress of the lateral meniscus and the lateral tibial plateau increased by 98.7% and 217.3% respectively compared with the healthy group. Meniscus is an important fibrocartilage structure in the knee joint. It can effectively increase the contact area between femur and tibia, buffer and absorb vibration, increase lubrication, maintain the stability of knee joint, and support and protect the articular cartilage covered by it^{19; 20}. The marked increase in the maximum contact stress of the lateral meniscus increases its probability of injury or even tear²¹. Whereas damage to the lateral meniscus accelerates the progression of OA in the lateral compartment^{22; 23}; Furthermore, the maximum contact stress of the lateral platform in the overcorrected UKA group also increased more than twice that in the healthy group, and stress and mechanical wear are the direct factors of OA²⁴. Heyse et al. found that inappropriate intraoperative osteotomies or thick spacers resulted in increased postoperative stress loads on the lateral compartment, as well as higher valgus stress and higher tension on the superficial medial collateral ligament, leading to pain in the early postoperative period after UKA²⁵. Overall, overcorrected UKA is more like artificially caused OA of the lateral compartment of the knee.

In contrast, the maximum contact stress of lateral meniscus and lateral tibial plateau in HTO group was significantly less than that in over corrected UKA group, which was close to that in healthy group. This is different from our expectation, because when the force line is corrected to the lateral Fujisawa point, the lateral compartment should have higher stress. Through the analysis, we found that the local stress of plate and screw fixation was significantly increased in the HTO group, while the stress was higher in the proximal tibial cortical bone and lower in the cancellous part of the middle, especially in the cancellous bone near the osteotomy line, forming an obvious area of low stress range. Therefore, we believe that the steel plate and screw bear part of the stress load of the tibial plateau after HTO, and play a bridging role, so that part of the stress is transmitted to the screw and surrounding bone below the osteotomy line through the steel plate, so as to reduce the contact stress of the inner and outer compartments of the knee joint, and form a stress shielding effect near the osteotomy line. This is similar to a study by Kyong et al., in which a stress shielding effect occurred in tibiae after HTO due to the high steel properties of the Tomofix plate using locking screws and its own, ie, the stress load the plate bears increases and the tissue around the osteotomy line decreases²⁶. Therefore, comparing the over correction UKA and HTO of the same force line, because the internal fixation of the latter bears part of the lower limb stress load, offsets the high contact stress in the lateral compartment caused by the valgus of the force line to a certain extent, and the postoperative knee joint is closer to the normal physiological state. This also explains why the lateral bone compartment of accurate HTO patients can remain in good condition for a long time without progression of OA.

Our study also evaluated the knee stress distribution of healthy knee, OA and standard UKA. The results show that the stress distribution in the medial and lateral compartments of the healthy knee joint is relatively balanced because the force line is in the middle position. The stress is concentrated in the front because the knee is in an extended position and the anterior cruciate ligament is intact^{27; 28}. Because of the varus of force lines, the stress load of the medial gap was increased in the OA group, and in particular, the maximum contact stress of the medial tibial plateau was increased by 117% compared with that in the healthy group. At the same time, the stress is concentrated in the front of the medial compartment of the knee joint, which corresponds to the cartilage wear situation seen in the clinically naturally progressing form of medial OA ^{29; 30}. After performing a standard UKA, the stress distribution of the medial and lateral compartments of the knee joint is restored to near normal physiology as the worn medial compartment is displaced while the lower force lines return to the natural force lines. Therefore, most of the long-term follow-up regarding medial UKA in the clinic has achieved good clinical outcomes and patient satisfaction^{14; 31; 32}.

This study has certain limitations. First, a FE model was constructed based on the imaging data for only one healthy male. Choosing a single individual simulation can eliminate some of the variables under study, such as weight, height, bone shape, and other differences. More subjects will be selected for evaluation in future studies, but the main conclusions drawn from the study should not change; Second, this FE analysis only employed vertical static load, which is different from the complex stress situation of the knee joint in daily life and work. The next step requires simulating more movement, such as sitting, going up and down stairs, and squatting; Finally, we only compared the distribution of knee stress when the UKA and HTO lower limb force lines passed through the Fujisawa point, but even the HTO, force line correction varied between individuals and was not restricted to the Fujisawa point³³.

Overall, both the lateral meniscus and tibial plateau sustained significantly more stress load on the overcorrected UKA compared with the HTO; Because of the bridging effect of internal fixation, the stress distribution of the latter knee is closer to the physiological condition.

Data Availability

All data analyzed during this study are included in this article.

Ethical Approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Availability of data and materials

Data will be available upon request by the first author GP.

Authors' contributions

Study design: X.Y. and W.W. Data collection/validation: W. W and P.Z. Data analysis: H.L. and G.P. Result interpretation: X.Y. and W.W. Reporting and editing: W.W. Project guarantor: X.Y. and W.W.

Conflicts of Interest

The authors declare that they have no competing interests.

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No competing interests reported.

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Higher stress load in the lateral compartment after over-corrected UKA compared with HTO: a 3d-finite-element analysis

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