Effect of insoles with different cushioning properties on knee impact after radial tear of posterior horn of medial meniscus: a finite element analysis

Abstract

The posterior horns of the medial menisci are more commonly injured during exercise. Cushioning insoles are one of the most widely used conservative interventions for preventing knee joint injuries. However, the effect of the cushion property of insoles on the knee joints with meniscal tears remain unknown. We investigated the effects of insoles with different cushioning properties on the knee joint following a radial tear of the posterior horn of the medial meniscus. Full extension landing with a heel-first strike pattern was performed by 12 participants randomly wearing ethylene vinyl acetate (EVA) and artificial cartilage foam (ACF) insoles, and the peak plantar pressures (pPP) were measured. The right knee of the participant whose pPP was closest to the median was loaded to a finite element model with radial tear. The pPP of EVA insoles (2386.42 ± 298.29 N) was higher than that of ACF (2174.16 ± 270.63 N) (t = 7.84, P < 0.001). The finite element analysis revealed that the peak von Mises Stress (pVMS) values of the medial and lateral menisci were 12.32 MPa and 7.90 MPa for injured joint with EVA, and 9.02 MPa and 6.84 MPa for injured joint with ACF. Hence, insoles with higher cushioning can decrease the vertical impact force and stress concentration of the knee joint after radial tear of the posterior horn of the medial meniscus.

Introduction

Menisci are essential structures in the complex biomechanics of the knee joint and play a fundamental role in shock absorption by attenuating axial loads on the articular surfaces of the opposed femur and tibia [1]. However, the meniscus is easily injured during athletic and daily living activities [2]. Once the meniscus is damaged, it leads to a disproportionate contact area and pressure within the knee, predisposing affected individuals to early onset osteoarthritis [3]. Meniscal tears lead to abnormal biomechanical changes, such as a greater vertical impact force or valgus/varus alignment [4]. Therefore, it is important to reduce the vertical impact force applied to the knee joint to protect the medial meniscus, especially during high vertical impact events.

Theoretically, reducing the vertical impact force on the ground may decrease the load to protect the tibiofemoral joint [5]. Cushioning insoles are commonly considered an effective intervention to attenuate the transient foot-ground impact forces generated during movement [6], such that less impact energy is transmitted to the tibiofemoral joint, which in turn reduces the intra-articular von Mises stress of the menisci and cartilages [7, 8]. The ethylene vinyl acetate (EVA) widely used to the manufacturing of cushioning insoles [9], but it was difficult to maintain excellent cushioning during long exercises because of EVA structural collapse [10]. Recently, a polyurethane insole named artificial cartilage foam (ACF) was tested to have higher cushioning properties than the EVA insoles in our previous study. It was found to be able to decrease some indicators such as knee adduction moment (KAM) and knee rotation moment in drop jump and provide better knee joint protection in people across different body mass index ranges [11]. However, the biomechanical effects of insoles with higher cushioning properties on the knee joints with meniscal tears remain unknown.

Statistically, the posterior horns of the medial menisci are more commonly affected by acute and chronic processes than the anterior two-thirds [1]. Therefore, this study aimed to investigate the effects of insoles with different cushioning properties on knee impact after a radial tear of the posterior horn of the medial meniscus by using a validated knee finite element model to analysis in a full-extension landing test with a heel-first strike pattern. We hypothesized that insoles with higher cushioning properties would decrease the vertical impact force and stress concentration of the knee joint with radial tear of the posterior horn of the medial meniscus compared with insoles with lower cushioning properties.

Materials and methods

Test of full extension landing with heel-first strike pattern

Twelve healthy men aged 21.67 ± 1.11 years, height 173.67 ± 8.53 cm, and weight 70.42 ± 6.36 kg, were recruited. All participants had a right-dominant leg and no history of lower-limb joint malformation, trauma surgery, or injury within the previous six months. The exclusion criteria included neuromuscular, psychological, and/or cardiopulmonary conditions that could substantially affect athletic ability. This study was approved by the Ethics Committee of Zhongshan Torch Development Zone People’s Hospital (no. 2022-0001). Each participant was given a full description of the study and signed an informed consent form.

Two different insoles were selected for this study: EVA (hardness of 35 Shore C, density of 0.11 g/cm³, JinXin Sports Production Technology, China) and ACF (hardness of 35 Shore C, density of 0.38 g/cm³, SoftValley Materials Laboratory Technology, China). The shapes of the two insoles were identical with a heel-to-toe drop of 2 mm, and both were made of a flat material. The cushioning performance of the two insoles was characterized via gravity-driven impact tests in accordance with the ASTM-F1976-13 standard, as reported in our previously published study [11]. The test results confirmed that ACF insoles had significantly higher instantaneous impact energy absorption capacity than EVA insoles, which was defined as “higher cushioning properties” in this study, providing an objective a priori justification for our study grouping and core hypothesis.

Each participant wore the same type of commercially available ordinary sports shoes without any special structure and the following characteristics: UK size 5.0–8.0, mass of 270–285 g, offset of 6 mm, and EVA midsole material. The EVA or ACF insoles were then placed in the sports shoes with an appropriate plantar pressure sensor (F-SCAN plantar scanning system, Tekscan Inc., Boston, MA, USA). The participants were tested with two insoles chosen in random order using a random number generator. To ensure that the participants remained blinded, a researcher was responsible for inserting the insoles into each shoe. Signed informed consent was obtained from all participants.

All participants were instructed in how to perform the full extension landing with a heel-first strike pattern: look ahead, open hands to the sides to maintain balance, lift right leg, keep the knee completely straight, dorsiflex the right foot, naturally move the weight forward and down, leave the left foot off the 10 cm platform, let the body fall upright naturally, and land on the right heel (Fig. 1). Before the formal experiment, all participants were given a 10 min warm-up and habituated to the insoles and shoes. The participants then began the test of full extension landing with a heel-first strike pattern, and peak plantar pressures (pPP) were collected using a calibration F-SCAN plantar scanning system with a frequency of 50 Hz. The participants completed three successful trials for each insole. They were given 10 min of recovery at the end of each fall, and 60 min of recovery was guaranteed between each type of insole.

Fig. 1.

Test of full extension landing with heel-first strike pattern on right leg from 10 cm high platform

Finite element analysis

A random participant was a 25-year-old healthy young man (height, 173 cm; weight, 65 kg), had his right knee joint scanned to obtain a set of slices using computed tomography (Philips Ingenuity 64CT, Philips Healthcare, Netherlands) and a 3.0 T MRI scanner (Intera Achiva, Philips, Netherlands). Image segmentation and three-dimensional (3D) reconstruction of the right knee joint were performed using medical image processing software (Mimics version 19.0, Materialize Mimics; Leuven, Belgium), which consisted of structures of the inferior femoral segment, superior tibia and fibula segments, and patella. This model was subsequently imported into reverse engineering software (Geomagic Wrap version 2017, 3D System Inc., Rock Hill, SC, USA) to convert the entire 3D geometric model of the right knee. Thereafter, the model was imported into computer-aided design software (SolidWorks version 2017, Dassault Systems, Concord, MA, USA) to create a radial tear in the posterior horn of the medial meniscus. According to the literature [12], the length of the tear was set to 66% of the meniscus width and radial tear was implemented by removing a small part of the solid volume of 0.1 mm in width from the meniscus. After the radial tear of the posterior horn of the medial meniscus was constructed in the knee FE model, we applied the measured peak plantar pressure from the in vivo plantar pressure test as the axial loading condition, to analyze the mechanical response of the meniscus and intra-articular structures under impact loading, with the intact healthy knee model as the control. The axial loading forces applied to the FE model were derived from the measured pPP of the participant whose pPP was closest to the median in in vivo full-extension landing test, to ensure that the loading conditions of the FE simulation were consistent with the actual impact conditions of the human body. The loading force was 2193.24 N for the EVA insole condition and 1960.51 N for the ACF insole.

The assembled models were then subjected to engineering simulation software (ANSYS version 17.0, ANSYS, Inc., Canonsburg, PA, USA) for meshing and subsequent establishment of finite element (FE) models (Fig. 2). Tetrahedral elements have been utilized as unit type [13, 14]. The material properties are listed in Table 1. The element size was determined to be approximately 1 × 1 mm in the ligaments, cartilage, and bones, and 0.2–0.8 mm in the menisci [19]. The entire model contained approximately 73,299 nodes and 61,923 elements. Regarding the boundary conditions, the femoral cartilage–meniscus and femoral cartilage–tibial cartilage contacts and interactions were considered frictionless, while the tibia–tibial cartilage and femur–patellar cartilage contacts and interactions were set to have no separation, and other contacts and interactions were set as bonded. The meshed FE model image along with the schematic of boundary conditions was shown in Fig. 3.

Fig. 2.

(a) Front and (b) back views of knee FE model and (c) medial meniscus radial tear

Table 1.

Material properties assigned in FE model

Component	Material model	Young’s modulus (MPa)	Poisson’s ratio
Femur	Isotropic elastic [15]	17,000 [15]	0.3 [15]
Tibia	Isotropic elastic [15]	13,000 [15]	0.3 [15]
Patella	Homogenous isotropic linear elastic [16]	15,000 [16]	0.3 [16]
Articular cartilages	Isotropic elastic [17]	20 [17]	0.49 [17]
Menisci	Isotropic elastic [18]	120 [18]	0.45 [18]

Fig. 3.

Stress patterns of knee joint at 10 cm full extension landing with heel-first strike pattern and EVA or ACF insoles

The loading forces were acquired from the peak plantar pressure of the young man whose right knee joint was used to build the finite element model. Of these, 60% of the selected force was attributed to the medial tibial plateau and 40% to the lateral tibial plateau [20].

To verify this FE model, a reference vertical compressive load of 1150 N (two body weights) was applied to the center of the femoral segment, where the tibiofibular segments were fixed and the rotation and displacement of the femoral segment were unconstrained [20, 21]. The peak von Mises Stress (pVMS) of the menisci, femoral cartilage, and tibial cartilage in the knee FE model was evaluated and compared with the biomechanical test of Yuefu et al., [22].

Analysis

All statistical analyses were performed using SPSS 26.0 (IBM Corp., Armonk, NY, USA). The normality of the pPP data was verified using the Shapiro-Wilk test. A paired-samples t-test was conducted to compare the pPP between EVA and ACF insoles, with the significance level set at α = 0.05.

Results

Peak plantar pressures

The pPP values of ACF (2174.16 ± 270.63 N) and EVA (2386.42 ± 298.29 N) insoles were statistically significantly different (t = 7.84, P < 0.001). The pPP of the participant whose pPP was closest to the median were 2193.24 ± 74.88 N with EVA insoles and 1960.51 ± 46.21 N with ACF insoles.

Finite element model validation

In the FE model, the pVMS of the menisci, femoral cartilage, and tibial cartilage were 3.74 MPa, 3.18 MPa, and 2.98 MPa, respectively, being consistent with the results of the FEA in [23], in which the knee joint with meniscal tears was applied an axial load of 1150 N (Table 2). Hence, our FE model was deemed valid.

Table 2.

Validation of knee FE model

	pVMS in this study (MPa)	pVMS in [1] (MPa)
Menisci	3.74	3.00
Femoral cartilage	3.18	3.20
Tibial cartilage	2.98	2.75

Finite element analysis

In the condition of a healthy knee joint subjected to 2193.24 N (with EVA insole), the pVMS of the medial meniscus was 8.23 MPa, lateral meniscus was 7.44 MPa, femoral cartilage was 6.35 MPa, and tibial cartilage was 6.10 MPa. The maximum contact stress between the cartilage and meniscus was 251.00 N; the maximum velocity of the tibia was 2.95 m/s; and the maximum acceleration of the tibia was 1.83 mm/ms². Higher stress areas were distributed in the medial margin of the medial meniscus, medial margin and posterior horn of the lateral meniscus, and medial contact area of the femoral and tibial cartilages (Table 3; Fig. 3).

Table 3.

FEA knee joint parameters at 10 cm full extension landing with heel-first strike pattern

	Healthy meniscus+ EVA insole	Radial-tear meniscus + EVA insole	Radial-tear meniscus + ACF insole
pVMS of medial meniscus (MPa)	8.23	12.32	9.02
pVMS of lateral meniscus (MPa)	7.44	7.90	6.84
pVMS of femoral cartilage (MPa)	6.35	6.97	6.09
pVMS of tibial cartilage (MPa)	6.10	6.72	5.95
Contact force (N)	251.00	251.00	183.00
Maximum velocity of tibia (m/s)	2.95	2.95	2.10
Maximum acceleration of tibia (mm/ms2)	1.83	1.83	1.59

In the condition of a posterior-horn-radial-tear knee joint subjected to 2193.24 N load (full extension landing with heel-first strike pattern with EVA insole), the pVMS was 12.32 MPa for the medial meniscus, 7.90 MPa for the lateral meniscus, 6.97 MPa for the femoral cartilage, and 6.72 MPa for the tibial cartilage. The maximum contact force between the cartilage and meniscus was 251.00 N; the maximum velocity of the tibia was 2.95 m/s; and the maximum acceleration of the tibia was 1.83 mm/ms². Higher stress areas were greater compared with those of the healthy knee joint with EVA insoles (Table 3; Fig. 3).

In the condition of a posterior-horn-radical-tear knee joint subjected to 1960.51 N (full extension landing with heel-first strike pattern with ACF insole), the pVMS of the medial meniscus was 9.02 MPa, lateral meniscus was 6.84 MPa, femoral cartilage was 6.09 MPa, and tibial cartilage was 5.95 MPa. The maximum contact stress between cartilage and meniscus was 183.00 N; the maximum velocity of tibia was 2.10 m/s; and the maximum acceleration of the tibia was 1.59 mm/ms². The higher stress areas were reduced compared to those in the posterior-horn-radical-tear knee joint with the EVA insole but were comparable to those in the healthy knee joint with the EVA insole (Table 3; Fig. 3).

Discussion

In this study, we investigated the effects of insoles with different cushioning properties (EVA or ACF) on knee impact after a radial tear of the posterior horn of the medial meniscus, using a full-extension landing test with a heel-first strike pattern and finite element analysis. The results showed that the insoles made from ACF significantly decreased the pPP and pVMS of the medial/lateral menisci and femoral/tibial cartilages, when compared with the EVA insoles, thus confirming the superior cushioning properties of ACF on knee structures.

Recently, numerous studies have confirmed that the material properties of cushioning insoles are key factors affecting knee biomechanical indicators associated with joint injury risk [24, 25]. Theoretically, reducing vertical ground impact force can optimize the intra-articular mechanical environment of the tibiofemoral joint, which is of particular clinical relevance for knees with existing meniscal tears. In this study, two different cushion materials were used to compensate for the adverse biomechanical effects of meniscal tears on the tibiofemoral joint. The first was EVA, which is commonly used as a traditional cushioning material in insole manufacturing [26]. However, EVA material is prone to fatigue-induced structural collapse after long-term repeated cyclic loading, making it difficult to maintain stable cushioning performance during long-term and frequent use [10]. The other is ACF, a novel mixed cellular material and biomimetic metamaterial with a three-dimensional ultrastructure similar to that of human cartilaginous tissue [27]. Our previous gravity-driven impact test, based on ASTM-F1976-13, found that the maximum displacement, maximum impact acceleration, and absorption energy of ACF insoles were better than those of EVA insoles, showing that the cushioning property of ACF was significantly higher than that of EVA [11].

Physical performance tests are important for assessing knee joint function. During knee flexion during walking or running, the muscular system of the knee joint acts as a primary active absorption mechanism [28]. In contrast, during full extension landing with a heel-first strike pattern, the insufficient energy dissipation produced by the lower extremity muscles is coupled with an excessive ground reaction force, which increases the impact stress and aggravates the risk of injury [29]. Therefore, a full extension landing with a heel-first strike pattern at 0.1 m height was selected in this study, which gave an impact velocity of 1.4 m/s as fast walking [30]. The pPP of ACF insoles was significantly lower than that of EVA insoles, further confirming that ACF insoles have higher cushioning properties than EVA insoles.

In the present study, we investigated the effects of insoles with different cushioning properties on knee impact after a radial tear in the posterior horn of the medial meniscus. By comparing the groups of healthy menisci with EVA insoles and radical-tear meniscus with EVA insoles, we found that the stress of different structures of the injured knee joint was higher than that of the healthy knee joint. In particular, the peak stress of the medial meniscus increased the most (49.69%), and the stress was concentrated in the torn area. In addition, the high-stress area spread from the inner edge to the body and outer edge, and stress concentration also appeared in the front horn. Although the increase in peak stress of the lateral meniscus was the smallest at only 6.18%, a high stress concentration occurred in the entire lateral meniscus, which is consistent with the clinical situation in which lateral meniscus injury is easily complicated after medial meniscus injury [31]. By comparing the results of the radical-tear meniscus with EVA and ACF insoles, the pVMS of all structures of the knee joint decreased with ACF insoles, and the pVMS of the medial meniscus decreased the most to 26.79%, compared with those of the radical-tear meniscus with EVA insoles. The stress concentration distribution area was significantly reduced. Moreover, the stress concentration distribution area of each structure of the knee joint of the radical-tear meniscus with ACF insoles was equivalent to or lower than that of a healthy meniscus with EVA insoles. These results suggest that insoles with higher cushioning properties may reduce joint loading and stress concentration during full-extension landing following a radial tear of the posterior horn of the medial meniscus.

Limitations

First, all recruited volunteers were male. The lack of female volunteers may have limited the generalizability of our study, and caution is necessary when extrapolating these results to patients of both sexes. Second, this study included healthy participants rather than patients with radical tears of the posterior horn of the medial meniscus. Third, the radial tear of the posterior horn of the medial meniscus in this study was traumatic and the degenerative tear will be explored in the future. Fourth, the linear-elastic models were used to the menisci that are viscoelastic materials which may affect the results [32]. Fifth, the plantar pressure of full-extension landing with a heel-first strike pattern was measured, which was somewhat different from actual motion, such as walking and running. Additionally, this study only assessed the instantaneous cushioning performance of the insoles under a single impact condition, and did not evaluate their long-term structural stability, fatigue resistance, or sustained energy absorption capacity under repeated cyclic loading; our results are limited to this specific landing task and cannot be generalized to other untested movement scenarios. Finally, a virtual calculation (FEA) was performed, and its findings require further clinical verification.

Conclusion

We investigated the effects of insoles with different cushioning properties (EVA or ACF) on knee impact after a radial tear of the posterior horn of the medial meniscus, using a full-extension landing test with a heel-first strike pattern and finite element analysis. Insoles with higher cushioning properties may reduce knee joint loading (pPP and pVMS) under a laboratory-based full-extension landing condition following a radial tear of the posterior horn of the medial meniscus.

Authors’ contributions

All authors contributed to the conception and design of this study. Rui Jia, Jun Wang, Tiancheng Fan, Zhihua Cai, and Hong Hong performed material preparation, data collection, and analysis. Hongtao Zhang and Xiuming He recruited and directed the participants. The first draft of the manuscript was written by Rui Jia and Jianyi Li, and all authors commented on previous versions of the manuscript. All the authors have read and approved the final version of the manuscript.

Funding

This study was supported by the National Key Research and Development Program of China [grant number: 2022YFF1202600], the Basic and Applied Basic Research Foundation of Guangdong Province, China [grant number: 2022A1515011604], the Basic and Applied Basic Research Foundation of Guangdong Province, China [grant number: 2025A1515010663], and the Space Medical Experiment Project of CMSP [grant number: HYZHXMH01007].

Data availability

The datasets generated and/or analysed during the current study are not publicly available due to institutional restrictions but are available from the corresponding author on reasonable request.

Declarations

Ethics approval and consent to participate

Approval was obtained from the Ethics Committee of Zhongshan Torch Development Zone People’s Hospital. The study procedures were conducted in accordance with the tenets of the Declaration of Helsinki.

Consent for publication

All individual participants included in the study signed written informed consent regarding publishing their data and photographs.

Competing interests

The authors declare no competing interests.