Research Papers

The Scaffold–Articular Cartilage Interface: A Combined In Vitro and In Silico Analysis Under Controlled Loading Conditions

[+] Author and Article Information
Tony Chen

Department of Biomechanics and
Orthopedic Soft Tissue Research Program,
Hospital for Special Surgery,
535 East 70th Street,
New York, NY 10021
e-mail: chento@hss.edu

Moira M. McCarthy

Sports Medicine and Shoulder Service,
Hospital for Special Surgery,
535 East 70th Street,
New York, NY 10021
e-mail: mccarthymo@hss.edu

Hongqiang Guo

Department of Biomechanics and
Orthopedic Soft Tissue Research Program,
Hospital for Special Surgery,
535 East 70th Street,
New York, NY 10021
e-mail: guoh@hss.edu

Russell Warren

Sports Medicine and Shoulder Service,
Hospital for Special Surgery,
535 East 70th Street,
New York, NY 10021
e-mail: warrenr@hss.edu

Suzanne A. Maher

Department of Biomechanics and
Orthopedic Soft Tissue Research Program,
Hospital for Special Surgery,
535 East 70th Street,
New York, NY 10021
e-mail: mahers@hss.edu

1Dr. Chen and Dr. McCarthy contributed equally to this manuscript.

2Corresponding author.

Manuscript received July 17, 2017; final manuscript received April 17, 2018; published online May 24, 2018. Assoc. Editor: David Corr.

J Biomech Eng 140(9), 091002 (May 24, 2018) (7 pages) Paper No: BIO-17-1313; doi: 10.1115/1.4040121 History: Received July 17, 2017; Revised April 17, 2018

The optimal method to integrate scaffolds with articular cartilage has not yet been identified, in part because of our lack of understanding about the mechanobiological conditions at the interface. Our objective was to quantify the effect of mechanical loading on integration between a scaffold and articular cartilage. We hypothesized that increased number of loading cycles would have a detrimental effect on interface integrity. The following models were developed: (i) an in vitro scaffold–cartilage explant system in which compressive sinusoidal loading cycles were applied for 14 days at 1 Hz, 5 days per week, for either 900, 1800, 3600, or 7200 cycles per day and (ii) an in silico inhomogeneous, biphasic finite element model (bFEM) of the scaffold–cartilage construct that was used to characterize interface micromotion, stress, and fluid flow under the prescribed loading conditions. In accordance with our hypothesis, mechanical loading significantly decreased scaffold–cartilage interface strength compared to unloaded controls regardless of the number of loading cycles. The decrease in interfacial strength can be attributed to abrupt changes in vertical displacement, fluid pressure, and compressive stresses along the interface, which reach steady-state after only 150 cycles of loading. The interfacial mechanical conditions are further complicated by the mismatch between the homogeneous properties of the scaffold and the depth-dependent properties of the articular cartilage. Finally, we suggest that mechanical conditions at the interface can be more readily modulated by increasing pre-incubation time before the load is applied, as opposed to varying the number of loading cycles.

Copyright © 2018 by ASME
Topics: Stress , Cycles , Cartilage
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Grahic Jump Location
Fig. 1

Schematic of the study design used to evaluate the effect of loading on scaffold–cartilage interface strength. Push-out testing was done at day 28 (before loading in the bioreactors started and used as an input to the bFEM) and at day 42 (post loading and used as an outcome metric for the experimental groups). Scaffold–cartilage interface strength was also quantified on day 0 for n = 6 samples (to allow for a supplementary assessment of incubation time on bFEM outcome).

Grahic Jump Location
Fig. 2

(a) A schematic of the two-dimensional axisymmetric bFEM of the macroporous PVA scaffolds–articular cartilage explant. (b) Graphic of the inhomogeneous material properties for articular cartilage: h = height of the samples, the z-axis is the direction of loading perpendicular to the nonporous platen on the base, where a value of zero is at the base of test system, r is the radius from the axis of symmetry.

Grahic Jump Location
Fig. 3

Scaffold–cartilage interfacial strength on days 0, 28, and 42. The interfacial strength of the scaffold–cartilage interface was measured by displacing the scaffold from the cartilage explant and measuring the stress until failure. Data are shown as mean±standard error. * denotes difference from day 42—0 cycles per day (p < 0.001).

Grahic Jump Location
Fig. 4

Biochemical quantification of s-GAG, collagen, and cellular content within the scaffolds. Biochemical quantification was performed for (a) s-GAG content, (b) collagen content (OHP assay), and (c) cell number (n = 8 per group). Bars are shown as mean±standard error. * denotes difference from unloaded group (p < 0.05).

Grahic Jump Location
Fig. 5

Vertical displacement of the loading platen varied with the number of loading cycles. After 100 loading cycles, the displacement of the loading platen varied less than 0.5% between cycles, indicating that the construct reached steady-state.

Grahic Jump Location
Fig. 6

(a) Distribution of vertical displacement through the scaffold, cartilage construct under peak load, (b) Distribution of vertical displacement at the horizontal midline (dotted line) of the model, (c) micromotion as a function of normalized tissue depth. Peak relative micromotion of 175 μm occurred beneath the top zone, (d) distribution of fluid pressure (kPa) in PVA and articular cartilage, and (e) distribution of compressive stress (kPa) in PVA and articular cartilage.



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