Hyperelastic properties of human meniscal attachments

Abstract

Meniscal attachments are ligamentous tissues anchoring the menisci to the underlying subchondral bone. Currently little is known about the behavior of meniscal attachments, with only a few studies quantitatively documenting their properties. The objective of this study was to quantify and compare the tensile mechanical properties of human meniscal attachments in the transverse direction, curve fit experimental Cauchy stress-stretch data to evaluate the hyperelastic behavior, and couple these results with previously obtained longitudinal data to generate a more complete constitutive model. Meniscal attachment specimens were tested using a uniaxial tension test with the collagen fibers oriented perpendicular to the loading axis. Tests were run until failure and load-optical displacement data was recorded for each test. The medial posterior attachment was shown to have a significantly greater elastic modulus (6.42±0.78 MPa) and ultimate stress (1.73±0.32 MPa) when compared to the other three attachments. The Mooney-Rivlin material model was selected as the best fit for the transverse data and used in conjunction with the longitudinal data. A novel computational approach to determining the transition point between the toe and linear regions is presented for the hyperelastic stress-stretch curves. Results from piece-wise non-linear longitudinal curve fitting correlate well with previous linear elastic and SEM findings. These data can be used to advance the design of meniscal replacements and improve knee joint finite element models.

1

Schematic showing where specimens were excised from between the tibial plateau and meniscal body (Villegas, Hansen et al. 2008). 1-2mm sections were removed based on tissue availability.

2

Representative meniscal attachment A) prior to initiation of test, and B) immediately following mid-substance failure of specimen. White lines indicate where ink lines were placed to track strain. Failure of specimen can be seen tearing midsubstance with no appearance of lines placed just below the grip face, indicating no slippage during testing.

3

Representative tensile stress-strain curves in the transverse direction for human meniscal attachments using machine displacement to obtain ultimate stress and failure strain. Elastic moduli were computed from optical data (not shown) that was comparable to machine displacement. On average, the medial and lateral posterior attachments exhibited the greatest ultimate stress and failure strain, respectively.

4

Cauchy stress-stretch curve showing hyperelastic curve fits to the experimental data for the medial posterior attachment site. Solid line = Neo-Hookean, dashed line= Ogden, dotted line = Mooney-Rivlin, symbols = testing data points for each specimen. The latter two models fit the trend of the data well. Mooney-Rivlin tends to describe samples with a large toe region better than Ogden, which favors specimens that have less compliant linear regions.

5

Cauchy stress-stretch curve showing hyperelastic curve fits for the longitudinal data for the medial anterior attachment site. Symbols = test data, solid line = curve fit. The piece-wise function used to fit the data performs a good fit for all data sets considered.