Stimulation of implant fixation by parathyroid hormone (1-34)-A histomorphometric comparison of PMMA cement and stainless steel

Whereas continuous exposure to PTH results in bone resorption, PTH administration at intermittent doses results in bone formation by increasing osteoblast number and activity. PTH leads to faster fracture repair and better fixation of orthopaedic implants in animal models. The present study evaluates if PTH is able to increase the contact surface between bone and implant and whether the effect of PTH is dependent on implant material characteristics. The implants were made as rods, either of stainless steel or Palacos® R bone-cement. The steel rods had a surface roughness of Ra 0.1 μm and the cement rods Ra 2.2 μm. In 40 adult male rats, one cement rod was inserted in the left tibia and one steel rod in the right tibia. After implantation, the rats were divided into groups by random. One group was injected three times a week with human PTH (1-34) at a dose of 60 μg/kg BW/injection. The second group was injected with the vehicle only. Both groups were then divided into groups for 2 and 4 weeks time till tibial harvest. The tibial segments around the hole of the rods were then prepared by standard histological techniques. The linear tissue surfaces, that had been in contact with the surface of the rod, were analyzed in a blind fashion. PTH increased the bone contact fraction compared with the vehicle in the steel group from 7.4 (SD 7.6) to 21.1 (SD 10.7) % after 2 weeks and from 9.8 (SD 8.1) to 47.1 (SD 13.3) % after 4 weeks. In the cement group PTH increased the contact index again compared with the vehicle from 7.8 (SD 10.2) to 53.6 (SD 26.3) % already after 2 weeks and from 14.3 (SD 15) to 65.6 (SD 15.7) % after 4 weeks. The bone trabeculae adjacent to the implant had become fewer and thicker after the treatment with PTH (1-34), with an increase of the bone mass in the area next to the bone-implant-interface. The earlier onset of PTH effects in the rougher cement group suggests that intermittent PTH treatment might lead to an increased micro-interlock between implant and bone, and might therefore be considered as a possible drug to enhance incorporation of orthopedic implants.