The human menisci are crescent-shaped fibrocartilaginous tissues within the knee joint that are responsible for load distribution, transmission, and dampening caused by everyday activities including walking and running. In fact, up to 75% of knee loads are borne by the menisci (Shrive et al., 1978; Walker et al., 2015). Accordingly, the primary role of this tissue is to protect the underlying articular cartilage against excessive stress concentration which can result in cartilage wear and, ultimately, the development of knee osteoarthritis (Lohmander et al., 2007).

The particular composition and structure of the meniscus allow the tissue to perform its primary function of dampening mechanical loads at the knee (Gaugler et al., 2015). The tissue is composed of approximately 70% interstitial fluids (Makris et al., 2011) with the remaining volume represented by a porous viscoelastic extracellular matrix (ECM) (Fithian et al., 1990; Morejon et al., 2021b; Norberg et al., 2021; De Rosa et al., 2022; Schwartz et al., 2022). The main component of the ECM is collagen fibers (75% of dry mass), which provide structural integrity (Williamson et al., 2001). Another component of the ECM is a network of negatively charged glycosaminoglycans (GAGs) which draw water into the tissue (Athanasiou and Sanchez-Adams, 2009). The intrinsic viscoelastic nature of the meniscus ECM causes energy dissipation during mechanical loading. However, energy dissipation is also caused by friction generated from fluid flow. When forces are applied to the meniscus, deformation of the ECM occurs creating a local increase of interstitial fluid pressure which forces water through the pores of the matrix. In turn, frictional drag between the fluid and porous matrix is created, which eventually leads to energy dissipation (Mow et al., 2005). Previous works have shown that the extent of energy dissipated by the meniscus is related to factors such as loading frequency, tissue water content, and meniscal region (Gaugler et al., 2015; Morejon et al., 2022).

Although knee loads often arise in the form of axial compression, they are transferred into hoop stresses via Poisson effects (Fox et al., 2012), resulting in significant tensile strains (Kolaczek et al., 2016). The meniscus core is highly organized with collagen fibers primarily aligned along the length of the tissue known as circumferential fibers. However, a much lower density of fibers is also aligned in the radial orientation (Skaggs et al., 1994; Petersen and Tillmann, 1998). Evidence of greater stiffness and strength along the circumferential orientation of the tissue shows that circumferential fibers enhance the mechanical properties to resist such hoop stresses (Proctor et al., 1989; Leroux and Setton, 2002; Li et al., 2005; Lakes et al., 2015; Peloquin et al., 2016). However, inhomogeneity exists in the macrostructure of the meniscus between the core and superficial layers (Choi et al., 2016; Andrews et al., 2017). Unlike the highly aligned core region, surface layers contain a mesh-like structure with collagen fibers randomly oriented in a 2-D network (Petersen and Tillmann, 1998; Sweigart and Athanasiou, 2005; Bryceland et al., 2017). Our previous work investigating meniscal shear properties demonstrated significant differences in complex modulus between the core and femoral (superior) and tibial (inferior) superficial layers (Schwartz et al., 2022). Whether similar findings occur between tissue layers in tensile loading is unknown.

Accordingly, we investigated the role of fiber orientation and tissue layer (i.e., femoral, tibial, and core) on tensile energy dissipation and mechanical properties of the meniscus. In addition, we hypothesize that the tissue’s capacity to dissipate energy is affected by its material characteristics including viscoelasticity and stiffness. Hence, the relationship between energy dissipation and tensile mechanical parameters was analyzed. Furthermore, the relationship between mechanical parameters and energy dissipation was examined. Amount of water contained in the tissue has been shown to correlate negatively with mechanical properties in compression and shear (Bursac et al., 2009a; Norberg et al., 2021; Schwartz et al., 2022). Accordingly, this study explored possible relationships between water content and tensile mechanical properties. It was hypothesized that the tensile behavior is significantly affected by the tissue’s orientation, layer structure, and water content. To test this hypothesis, tensile meniscal samples from different layers of the tissue were subjected to dynamic and quasi-static loading parallel and perpendicular to fiber orientation. Finally, samples were dried and weighed to relate their water content to mechanical properties.

Previous studies have characterized the tensile quasi-static response of the meniscus (Proctor et al., 1989; Leroux and Setton, 2002; Abdelgaied et al., 2015; Fischenich et al., 2015; Lakes et al., 2015; Warnecke et al., 2020). Herein, we extended this work by investigating both the quasi-static and dynamic tensile mechanical behavior of meniscal tissue across loading orientations, tissue layers, and compartments. Shear, compressive, and tensile mechanical properties have been previously investigated across different meniscal layers (Proctor et al., 1989; Tissakht and Ahmed, 1995; Moyer et al., 2012; Freutel et al., 2015; Choi et al., 2016; Maritz et al., 2021; Schwartz et al., 2022). However, to our knowledge, this is the first study to analyze the tensile dynamic properties and quantify energy dissipation in the superficial and core layers of the meniscus thereby moving closer to a clinical approximation of everyday joint and tissue loading.

A dynamic test battery showed that ED was inversely related to loading frequency in circumferentially oriented samples (p < 0.05). Previous studies reported a similar effect in cyclic compression (Morejon et al., 2021a; Morejon et al., 2022). Hence, the capacity of the meniscus to dissipate mechanical energy is time-dependent and reduced during high-frequency loading such as in the case of impact loading (Gaugler et al., 2015). This finding may result from the intrinsic viscoelasticity of the collagen fibers which rearrange in a time dependent manner (Han et al., 2018).

At all frequencies, ED was significantly larger along the direction parallel to the fibers (p < 0.05). After normalizing for load strain, energy dissipation was found to be in the range of previous studies under tensile loading in both orientations (Freutel et al., 2015; Lakes et al., 2015; Lakes et al., 2016). Dynamic tests measured ED of circumferential samples to average 44.5 kJ/m³ while radial samples averaged 5.0 kJ/m³ of energy dissipation. This implies that collagen fibers further enhance the tissue’s ability to dissipate mechanical loads. Nonetheless, in samples loaded perpendicular to the fibers, energy dissipation was ten-fold higher in tension than in compression (0.5 kJ/m³) at similar strains and frequencies (Morejon et al., 2021a; Morejon et al., 2022). The mechanism of energy dissipation in tension is believed to occur via time-dependent reorganization of the matrix fibers, while in compression, it occurs primarily by fluid flow friction (Park and Ateshian, 2006; Morejon et al., 2022). Hence, the results of this study show that the most efficient energy dissipation mechanism in the meniscus is associated with time-dependent reorganization of the matrix fibers (Gaugler et al., 2015).

Complex modulus, E* values increased slightly with load frequency but the effect was not statistically significant. A previous study by Bursac et al. on human menisci observed similar findings in tensile dynamic modulus ranging from 0.01 to 1 Hz (Bursac et al., 2009b). In the present study, average E* was significantly higher in circumferential (∼57 MPa) versus radial orientation (∼7 MPa). However, measurements of circumferential dynamic modulus by Bursac et al. ranged from 115 - 143 MPa, which are higher but within the same order of magnitude as the present study (Bursac et al., 2009b). This difference could be attributed to the species and region of the samples used. Bursac et al. used human samples from the outer third region of the meniscus as opposed to the present study which employed the central region of porcine menisci (Bursac et al., 2009b).

Phase shift, δ, significantly decreased with loading frequency (p < 0.05). This trend is consistent with previous studies in dynamic compression (Bursac et al., 2009a; Pereira et al., 2014; Coluccino et al., 2017), and shear (Norberg et al., 2021; De Rosa et al., 2022; Schwartz et al., 2022) at similar frequencies. The phase shift is commonly associated with mechanical energy dampening (Park and Ateshian, 2006; Bursac et al., 2009a; Pereira et al., 2014; Danso et al., 2015; Gaugler et al., 2015; Han et al., 2018). This study confirms that δ follows a similar frequency-dependent pattern to ED. Direction of loading had a significant effect on δ (radial: 8.2°, circumferential: 3.7°, p < 0.001), suggesting that the presence of collagen fibers reduced the viscous effect compared to the raw ground matrix.

Average E of meniscal samples in quasi-static loading was greater along the fiber direction (p < 0.001) and of similar magnitude to previous measurements on porcine menisci in circumferential and radial orientations (Abdelgaied et al., 2015; Lakes et al., 2015; Lakes et al., 2016; Maritz et al., 2021). Our findings confirm that circumferential fibers greatly enhance tensile stiffness to resist high hoop stresses produced by daily mechanical loading (Daszkiewicz and Łuczkiewicz, 2021). Also, mean UTS for circumferential samples, 23.9 MPa, was greater than the 1.7 MPa measured in radial specimens (p < 0.001). These values are similar to those from previous testing performed for both orientations on porcine menisci (Abdelgaied et al., 2015; Lakes et al., 2016). Therefore, collagen fibers may aid in meniscal tear prevention by reinforcing tissue strength in the circumferential orientation. ε _UTS averaged ∼32% which is comparable to values found in previous studies employing tensile loading (Freutel et al., 2015; Lakes et al., 2015; Lakes et al., 2016; Peloquin et al., 2016; Creechley et al., 2017; Peloquin et al., 2018). Differences in ε _UTS were discovered between medial and lateral meniscal compartments (p < 0.05). Similarly, in a previous study in human samples, maximum strain was also larger on average in the lateral compartment but differences were not statistically significant (Tissakht and Ahmed, 1995).

ED was positively correlated with E*, E, and UTS (R ² > 0.35). This result implies that the capability of energy dissipation by the tissue depends largely on its ECM’s mechanical stiffness and strength. Collagen fibers are not only associated with providing high tensile strength, since they are also intrinsically viscoelastic (Gautieri et al., 2012; Sopakayang et al., 2012). Therefore, besides strength and stiffness, collagen fibers may also increase the capacity of the meniscus to dissipate energy. Average δ had a negative correlation with ED (R ² = 0.3–0.6) suggesting that the association of δ with ED may not be direct when comparing across tissue samples. However, it remains useful for estimating the effect of frequency on energy dissipation when measuring phase shift (Park and Ateshian, 2006; Bursac et al., 2009a; Pereira et al., 2014; Danso et al., 2015; Gaugler et al., 2015).

No significant differences existed between the three tissue layers in ED, E*, δ, E, UTS, or ε _UTS . Previous studies similarly observed no significant differences in circumferential tensile mechanical parameters between superficial and core tissue layers (Tissakht and Ahmed, 1995; Freutel et al., 2015). Suggesting that the mesh-like organization of the surface layer fibers yields a similar tensile response to the core along its fiber-aligned orientation. However, other studies have reported conflicting evidence about the tensile properties of the core in the circumferential orientation: one study found it to be more compliant (Maritz et al., 2021), while another found it to be stiffer (Proctor et al., 1989) than superficial layers. The tensile properties reported in the literature may be affected by sample dimension, species, and specific region of collection (Lechner et al., 2000). Therefore the discrepancies seen between the present study and others may be a result of variations between the aforementioned factors (Proctor et al., 1989; Maritz et al., 2021). Further investigation is needed to verify if the structural differences between the three layers lead to differences in tensile mechanical properties.

Samples evaluated in the current study had an average φ _w of 73.5% and this was not statistically different across meniscal compartment and layer. Degree of water content for our samples is comparable with previous measurements of porcine menisci (Schwartz et al., 2022). Also, φ _w had a weak negative correlation with ED (R ² = 0.101). A similar relationship was seen in compressive energy dissipation measurements under similar loading strains and frequencies (Morejon et al., 2021a; Morejon et al., 2022). Weak negative correlations were also obtained between φ _w and E*, E, and UTS of circumferential samples from all layers (p < 0.01). A similar trend was noted in previous studies wherein water content correlated with reduced stiffness parameters and energy dissipation in compression and shear (Joshi et al., 1995; Bursac et al., 2009a; Seitz et al., 2013; Norberg et al., 2021; De Rosa et al., 2022; Schwartz et al., 2022). A possible explanation for this trend could be that samples with a higher water content contain less ECM mass which is known to provide the tissue with its mechanical integrity (Athanasiou and Sanchez-Adams, 2009). Therefore, samples with lower ECM to water ratios tend to be mechanically weaker. Finally, δ had a positive correlation with φ _w in circumferential samples (R ² = 0.330). This indicates that the tissue’s intrinsic viscoelastic properties tend to increase with tissue hydration.

There are some limitations in the present study. First, the tissues were derived from porcine as opposed to human menisci. However, porcine tissue has been widely used as an animal model approximating human tissue whereby similar magnitudes and trends in mechanical behavior have been found between the two species (Joshi et al., 1995; Sweigart et al., 2004; Norberg et al., 2021). Secondly, testing frequency did not exceed 1 Hz due to limitations in accurate data collection at higher frequencies. Importantly, higher loading rates may be experienced by the meniscus during running or jumping. Also, only central regions of the menisci were investigated in the present study. The differences in mechanical properties between central and horn regions have been extensively investigated with some major differences noted (Kleinhans and Jackson, 2018; Seitz et al., 2021; Morejon et al., 2022). Finally, samples employed were cut in the radial and circumferential orientations of the meniscus but not in the vertical or axial direction. Variations in the material parameters have been previously observed in compression studies (Berni et al., 2021). However, since both radial and axial directions are perpendicular to the circumferential fibers, we have limited our samples to only one of these. Hence, future studies should focus on addressing the aforementioned limitations by sampling all regions and orientations of the tissue and/or testing at higher loading frequencies.

The main finding of this study was that tensile energy dissipation and mechanical properties of meniscal tissue are highly dependent on loading direction. Importantly, circumferential fibers stiffen, strengthen, and increase the energy dissipating capability of the meniscus. Conversely, surface layers were not mechanically different from tissue core evaluated in the circumferential orientation. Moreover, energy dissipation was positively correlated to its stiffness and negatively related to water content. Finally, water content was weakly associated with tensile mechanical properties of samples loaded parallel to fibers. Overall, this study improves our understanding of the tensile mechanical behavior of meniscal tissue. To effectively replicate the mechanical behavior of native meniscus, replacement surrogates should focus primarily on mimicking the fiber-orientation. The large discrepancy in the tensile properties between the radial and circumferential orientations play a critical role in the functioning of the tissue.