Design of a double acting pneumatic cartilage loading device for magnetic resonance imaging

Studies of osteoarthritis initiation and progression that measure strain in cartilage require physiological loading levels. Many studies use magnetic resonance (MR) imaging, which necessitates a MR-compatible loading device. In this study, the design and validation of a new device, the cartilage compressive actuator (CCA), is presented. The CCA is designed for high-field (e.g., 9.4 T) small-bore MR scanners, and meets a number of design criteria. These criteria include capability for testing bone-cartilage samples, MR compatibility, constant load and incremental strain application, a water-tight specimen chamber, remote control, and real time displacement feedback. The mechanical components in the final design include an actuating piston, a connecting chamber, and a sealed specimen chamber. An electro-pneumatic system applies compression, and an optical Fibre-Bragg grating (FBG) sensor provides live displacement feedback. A logarithmic relationship was observed between force exerted by the CCA and pressure (R² = 0.99), with a peak output force of 653 ± 2 N. The relationship between FBG sensor wavelength and displacement was linear when calibrated both outside (R² = 0.99) and inside (R² = 0.98) the MR scanner. Average slope was similar between the two validation tests, with a slope of -4.2 nm/mm observed inside the MR scanner and -4.3 to -4.5 nm/mm observed outside the MR scanner. This device meets all design criteria and represents an improvement over published designs. Future work should incorporate a closed feedback loop to allow for cyclical loading of specimens.