Biosynthetic response of cartilage explants to dynamic compression

Abstract

The biosynthetic response of calf articular cartilage explants to dynamic compression was examined over a wide range of amplitudes, waveforms, and frequencies. Glycosaminoglycan synthesis was assessed by 35 S‐sulfate incorporation, and amino acid uptake and protein synthesis were assessed by 3 H‐proline incorporation. Two culture chambers were designed to allow uniaxial radially unconfined compression and mechanical testing of cartilage disks: one chamber was used inside a standard incubator; the other was used with a mechanical spectrometer and allowed load and displacement to be monitored during compression. Dynamic stiffness measurements of 3‐mm diameter disks identified a characteristic frequency [0.001 Hz (cycles/sec)] that separated low‐ and high‐frequency regimes in which different flow and deformation phenomena predominated; e.g., at 0.0001–0.001 Hz, significant fluid was exuded from cartilage disks, whereas at 0.01–1 Hz, hydrostratic pressure increased within disks. At the higher frequencies, oscillatory strains of only ∼1–5% stimulated 3 H‐proline and 35 S‐sulfate incorporation by ∼20–40%. In contrast, at the lower frequencies (a) compressions of <5% had no effect, consistent with the dosimetry of biosynthetic inhibition by static compression (∼25% compression caused a ∼20% inhibition of radiolabel incorporation), and (b) higher amplitudes (cycling between disk thicknesses of 1.25 and 0.88–1.00 mm) stimulated 3 S‐sulfate incorporation by ∼20–40%, consistent with the kinetics of response to a single 2‐h compression and release. None of the compression protocols was associated with detectable alterations in (e.g., compression‐induced depletion of) total glycosaminoglycan content. This study provides a framework for identifying both the physical and biological mechanisms by which dynamic compression can modulate chondrocyte biosynthesis. In addition, the culture and compression methodology potentially allows in vitro evaluation of clinical strategies of continuous passive motion therapy to stimulate cartilage remodeling.