Structure-Function relationships of equine menisci

Abstract

Meniscal pathologies are among the most common injuries of the femorotibial joint in both human and equine patients. Pathological forces and ensuing injuries of the cranial horn of the equine medial meniscus are considered analogous to those observed in the human posterior medial horn. Biomechanical properties of human menisci are site- and depth- specific. However, the influence of equine meniscus topography and composition on its biomechanical properties is yet unknown. A better understanding of equine meniscus composition and biomechanics could advance not only veterinary therapies for meniscus degeneration or injuries, but also further substantiate the horse as suitable translational animal model for (human) meniscus tissue engineering. Therefore, the aim of this study was to investigate the composition and structure of the equine knee meniscus in a site- and age-specific manner and their relationship with potential site-specific biomechanical properties. The meniscus architecture was investigated histologically. Biomechanical testing included evaluation of the shore hardness (SH), stiffness and energy loss of the menisci. The SH was found to be subjected to both age and site-specific changes, with an overall higher SH of the tibial meniscus surface and increase in SH with age. Stiffness and energy loss showed neither site nor age related significant differences. The macroscopic and histologic similarities between equine and human menisci described in this study, support continued research in this field.

Fig 1. The equine meniscus.

Top Row: Macroscopic picture of a lateral and medial meniscus of a horse indicating the different anatomic regions tested (A: anterior horn, B: pars intermedia, C: posterior horn). Second Row: Cross sections of each anatomic region (t: tibial, f: femoral). Third Row: Different thickness of the superficial layer at the tibial and femoral meniscus surface shown in histologic sections stained with H&E (SL = superficial layer, OL = outer layer, IL = inner layer). Bottom Row: Collagen fiber orientation (van Gieson) and GAG composition (Alcian blue and Safranin O) in the respective layers (region B, femoral side).

Fig 2. Biomechanical testing device for determination of stiffness and energy loss.

Equine Meniscus mounted onto a custom made, curved jig to apply uniaxial compressive forces for determination of stiffness and energy loss.

Fig 3. Meniscus´ stiffness.

Meniscus´ stiffness was calculated as the slope of the load-deformation curve.

Fig 4. Meniscus´ energy loss.

Meniscus´ energy loss was calculated using the integral of the stress–strain curve during loading.

Fig 5. Age and topographic differences in GAG content.

Representative micrographs showing age related increase of GAG production (Alcian blue staining) in the SL, OL and IL (middle and abaxial zone) of a 1 year (y), 9y and 17y old horse (all pictures from lateral menisci, region B). Scale bars as depicted.

Fig 6. MicroCT analysis of the equine meniscus.

Meniscus samples were stained with phosphotungstic acid (PTA). The PTA stain allowed discriminating the superficial layer (SL) from the outer and inner deep layers of the meniscus based on lower staining intensity. Colour contours show the result of segmentation. Cross sectional area was largest for region C in the lateral and medial meniscus, while the thickness of the SL at both, the femoral and tibial surface, as well as the area of the axial tip of the SL was largest in region A for the lateral and medial meniscus. Yellow arrowheads and contour = SL at femoral surface; red arrowheads and contour = SL at tibial surface; green arrowheads and contour = outer meniscus contour; asterisk = axial tip of SL, double arrowheads = unstained regions of samples due to limited tissue penetration properties of PTA.