A cartilage growth mixture model with collagen remodeling: validation protocols.

A cartilage growth mixture (CGM) model is proposed to address limitations of a model used in a previous study. New stress constitutive equations for the solid matrix are derived and collagen (COL) remodeling is incorporated into the CGM model by allowing the intrinsic COL material constants to evolve during growth. An analytical validation protocol based on experimental data from a recent in vitro growth study is developed. Available data included measurements of tissue volume, biochemical composition, and tensile modulus for bovine calf articular cartilage (AC) explants harvested at three depths and incubated for 13 days in 20% fetal borine serum (FBS) and 20% FBS+beta-aminopropionitrile. The proposed CGM model can match tissue biochemical content and volume exactly while predicting theoretical values of tensile moduli that do not significantly differ from experimental values. Also, theoretical values of a scalar COL remodeling factor are positively correlated with COL cross-link content, and mass growth functions are positively correlated with cell density. The results suggest that the CGM model may help us to guide in vitro growth protocols for AC tissue via the a priori prediction of geometric and biomechanical properties.

Tailoring secretion of proteoglycan 4 (PRG4) in tissue-engineered cartilage.

Articular cartilage provides a low-friction surface for joint articulation, with boundary lubrication facilitated by proteoglycan 4 (PRG4), which is secreted by chondrocytes of the superficial zone. Chondrocytes from different zones are phenotypically distinct, and their phenotypes in vitro are influenced by the system in which they are cultured. We hypothesized that culturing cells from the superficial (S) zone in two-dimensional monolayer or three-dimensional alginate would affect their synthesis of PRG4, and that subsequently seeding them atop alginate-recovered cells from the middle/ deep (M) zone in various proportions would result in tissue-engineered constructs with varying levels of PRG4 secretion and matrix accumulation. During monolayer culture, S cells retained their PRG4-secreting phenotype, whereas in alginate culture the percentage of cells secreting PRG4 decreased with time. Constructs formed with increasing percentages of S cells decreased in thickness and matrix accumulation, depending on both the culture conditions before construct formation and the S-cell density. PRG4-secreting cells were localized to the S-cell seeded construct surface, with secretion rates of 0.1-4 pg/cell/day or 0.1-1 pg/cell/day for constructs formed with monolayer-recovered or alginate-recovered S cells, respectively. Tailoring secretion of PRG4 in cartilage constructs may be useful for enhancing low-friction properties at the articular surface, while maintaining other surfaces free of PRG4 for enhancing integration with surrounding tissues.

Effect of hip hemiarthroplasty on articular cartilage and bone in a canine model.

The purpose of this study was to determine if acetabular articular cartilage damage occurs in the presence or absence of changes in subchondral plate thickness or porosity and trabecular bone architecture after hip hemiarthroplasty. Eight canines were sacrificed 6 months after receiving unilateral hemiarthroplasties in which a cobalt chrome alloy femoral head was used. The acetabular cartilage, subchondral plate, and trabecular bone were quantitatively evaluated. Although the articular cartilage in the treated hip showed gross and histologic degenerative changes, there were no differences in the treated and contralateral hips in any of the trabecular bone parameters or subchondral plate thickness. However, the subchondral plate porosity was increased 2.6-fold in the treated hip. Therefore, degradation of cartilage can occur in the absence of thickening of the subchondral plate or alterations in the supporting trabecular bone architecture. These observations provide a better understanding of the role that periarticular bone has in the degenerative process after hemiarthoplasty.

Differential effects of fibronectin fragment on proteoglycan metabolism by intervertebral disc cells: a comparison with articular chondrocytes.

STUDY DESIGN: This in vitro study used the alginate bead culture system to probe for differences in the effects of fibronectin fragment on cell proliferation and proteoglycan metabolism by different populations of intervertebral disc cells and articular chondrocytes. OBJECTIVE: To compare the effects of fibronectin fragment on cell proliferation, and proteoglycan synthesis and degradation by cells from the nucleus pulposus, the anulus fibrosus, and articular cartilage. SUMMARY OF BACKGROUND DATA: In articular cartilage, the administration of fibronectin fragment stimulates cartilage degeneration. Fibronectin fragment levels were increased in human intervertebral discs with increased disc degeneration. Fibronectin fragment injected into the central region of the rabbit intervertebral disc induced a progressive degeneration of that disc. METHODS: Bovine tails and metacarpophalangeal joints from 14- to 18-month-old animals were used. Alginate beads containing cells isolated from intervertebral discs and articular cartilage were cultured with (1-100 nmol/L) or without (control) fibronectin fragment in the presence of 10% fetal bovine serum. In these cultures, deoxyribonucleic acid and proteoglycan contents, as well as the rate of proteoglycan synthesis were determined. Proteoglycan degradation was measured in cultures with or without 10 nmol/L fibronectin fragment. RESULTS: In articular chondrocytes, fibronectin fragment strongly suppressed proteoglycan synthesis and stimulated proteoglycan degradation; the total proteoglycan content was diminished in a dose-dependent manner. Compared to articular chondrocytes, nucleus pulposus cells responded to fibronectin fragments in a similar, although less pronounced manner. On the other hand, anulus fibrosus cells treated with fibronectin fragment did not show any significant effects on proteoglycan degradation. A slight but statistically significant up-regulation of proteoglycan synthesis was observed at 10 nmol/L fibronectin fragment in outer anulus fibrosus cells. However, total proteoglycan content was decreased significantly at high concentrations of fibronectin fragment. CONCLUSIONS: Fibronectin fragment has different effects on cell proliferation, proteoglycan synthesis, degradation, and accumulation by articular chondrocytes and intervertebral disc cells. The different effects of fibronectin fragment in those different cell types suggest metabolic differences between these cells, and may further suggest differences in pathways of fibronectin fragment signaling as well as a differential need of these cells to be involved in tissue remodeling in which both anabolic and catabolic pathways might be altered.