Time-dependent sensory nerve ingrowth into a bone conduction chamber

We studied time-dependent ingrowth of sensory nerve fibers into a bone defe ct in a rat bone conduction chamber model. In 10 male Sprague Dawley rats, a titanium chamber was implanted bilaterally in the proximal tibiae, repres enting an experimental bone defect. To mimic a clinical situation, the cham bers were filled with a fresh blood clot. After 1, 2, 4, 6 and 8 weeks,2 ra ts were fixed in vivo at each time before removal of specimens, and histolo gical and immunohistochemical analyses. We used antisera against protein ge ne product 9.5, neural growth-associated protein 43/B-50, calcitonin gene-r elated peptide, and substance P, to locate regenerating sensory nerve fiber s in the chamber. During bone defect healing, hematoxylin/eosin sections sh owed that new bone grew in through the ingrowth openings in the chamber, gr adually filling it and replacing the blood clot. At 1 and 2 weeks after imp lantation, no nerve fibers could be detected. At 4, 6 and 8 weeks, however, small numbers of nerve fibers were seen in 8 of 11 specimens. The nerve fi bers were located mainly in the dense fibrous tissue in close proximity to the new bone, and in some cases within the new forming bone. In this chambe r model, the periosteum is not in contact with the bone ingrowth openings, and all ingrowing nerve fibers thus originated from the cortical bone, endo steum or bone marrow. We speculated that these late ingrowing sensory nerve fibers may actively participate in bone repair.