The region-dependent biomechanical and biochemical properties of bovine cartilaginous endplate

Abstract

Regional biomechanical and biochemical properties of bovine cartilaginous endplate (CEP) and its role in disc mechanics and nutrition were determined. The equilibrium aggregate modulus and hydraulic permeability between the central and lateral regions were examined by confined compression testing. Biochemical assays were conducted to quantify the amount of water, collagen, and glycosaminoglycan (GAG). The equilibrium aggregate modulus of the CEP in the central region (0.23 ± 0.15 MPa) was significantly lower than for the lateral region (0.83 ± 0. 26 MPa). No significant regional difference was found for the permeability of the CEP (central region: 0.13 ± 0.07×10(-15)m(4)/Ns and lateral region: 0.09 ± 0.03 × 10(-15)m(4)/Ns). CEPs were an average of 75.6% water by wet weight, 41.1% collagen, and 20.4% GAG by dry weight in the central region, as well as an average of 70.2% water by wet weight, 73.8% collagen, and 11.7% GAG by dry weight in the lateral region. Regional differences observed for the equilibrium aggregate modulus were likely due to the regional variation in biochemical composition. The lateral bovine endplate is much stiffer and may share a greater portion of the load. Compared with the nucleus pulposus (NP) and annulus fibrosus (AF), a smaller hydraulic permeability was found for the CEP in both the central and lateral regions, which could be due to its lower water content and higher collagen content. Our results suggest that the CEP may block rapid fluid exchange and solute convection, allow pressurization of the interstitial fluid, and play a significant role in nutrient supply in response to loading.

Keywords: Aggregate modulus; Cartilaginous endplate; Hydraulic permeability; Intervertebral disc; Soft tissue mechanics.

Figure 1

(A) Disc motion segments in the bovine tail used in this study. (B) Schematic of specimen preparation. The region and size of test specimens are shown. (C) Schematic of the confined compression test chamber. (D) Schematic of the mechanical testing protocol.

Figure 2

Histological section of bovine C2-3 (original magnification ×10), showing (A) the nucleus pulposus (NP), the cartilage endplate (CEP), and the vertebral body (VB) and (B) the annulus fibrosus (AF), the CEP, and the VB.

Figure 3

Typical biphasic creep behavior of a bovine CEP. A good agreement is shown between the theoretical prediction and the experimental result.

Figure 4

Comparison of aggregate modulus H_A (A) and permeability k (B) of bovine CEP determined in this study (CEP-Central and CEP-Lateral) and bovine IVD tissue found in the literature (NP and AF) (Perie et al., 2005). For the aggregate modulus, a significant regional variation was detected in the CEP between the central and lateral regions (*p<0.0001; Central: n=14; Lateral: n=18). The aggregate modulus of the CEP in the central and lateral regions were comparable with that of the NP and AF, respectively (^[*] p < 0.05 (Perie et al., 2005)). For hydraulic permeability, no significant regional variations were detected in the CEP between the central and lateral regions (p=0.071). Compared with that in the NP and AF (Perie et al., 2005), the permeability in the CEP is found to be much smaller.

Figure 5

Results of biochemical assays measuring water content (A), total collagen content (B), and total GAG content (C) for each region of the bovine IVD. Significant differences were detected for water, collagen, and GAG contents between the NP, AF, and CEP. Regional differences were also found for water, collagen, and GAG contents in the CEP between the central and lateral regions (*p values are lower than 0.05 as shown in the biochemical composition section; n=8 each group).

Figure 5

Results of biochemical assays measuring water content (A), total collagen content (B), and total GAG content (C) for each region of the bovine IVD. Significant differences were detected for water, collagen, and GAG contents between the NP, AF, and CEP. Regional differences were also found for water, collagen, and GAG contents in the CEP between the central and lateral regions (*p values are lower than 0.05 as shown in the biochemical composition section; n=8 each group).

Figure 5

Results of biochemical assays measuring water content (A), total collagen content (B), and total GAG content (C) for each region of the bovine IVD. Significant differences were detected for water, collagen, and GAG contents between the NP, AF, and CEP. Regional differences were also found for water, collagen, and GAG contents in the CEP between the central and lateral regions (*p values are lower than 0.05 as shown in the biochemical composition section; n=8 each group).