Cartilage induction by controlled mechanical stimulation in vivo

To study mechanical control of tissue differentiation, we designed a new Ve rsion of the previously described bone conduction chamber. The bone conduct ion chamber consists of a cylindrical titanium chamber for implantation in the rat tibia. It has tissue ingrowth openings at one end, located subcorti cally, and the other end protrudes into the subcutis. The newly developed l oad chamber has a mobile piston so that an external compressive load can be transferred to the tissue within the chamber. Sprague-Dawley rats had a re gular bone conduction chamber implanted in one tibia and a load chamber imp lanted in the other. Mesenchymal tissue was allowed to grow into the chambe r for 3 weeks before the mechanical loading was started. Thereafter, twice a day, 20 cycles of compressive load were applied with a frequency of 0.17 Hz to the load chamber. This was estimated to produce a compressive hydrost atic stress of 2 MPa. The chambers, harvested after 7 weeks of loading, all contained newly formed bone. The bone ingrowth distance into the chamber w as decreased in the loaded specimens compared with the contralateral unload ed controls (p = 0.01). Instead, cartilage was found in the loaded chambers next to the piston. Beneath the cartilage was a dense bone plate under whi ch a marrow cavity had formed. No cartilage was found in the unloaded contr ols, but the architecture of the bone and marrow cavity was similar to that of the loaded specimens. We conclude that this model allows load to be tra nsmitted onto the ingrowing tissue and that the load parameters used cause this tissue to differentiate into cartilage close to the piston.