Primary joint replacements generally function well with excellent clinical results. However, failure rates for young patients are still high and increasing in number. The longterm survival of an uncemented prosthesis is influenced by multiple factors depending on host physiology as well as properties of implanted material, initial mechanical stability, early osseointegration, and the surrounding bone. Parathyroid hormone is the principal regulator of calcium homeostasis and involved in the control of bone remodelling. Parathyroid hormone administered intermittently increases bone formation and mass by osteoblast stimulation. Early osseointegration and implant fixation could potentially be enhanced with adjuvant parathyroid hormone treatment. The aim of the studies in this PhD thesis was to determine if implant fixation of experimental implants can be improved with adjuvant intermittent administration of parathyroid hormone.
All studies used an experimental canine model of early implant fixation inserting porous coated titanium alloy implants with no weight bearing in a bed of cancellous bone. The study design was un-paired. Test animals were randomised to PTH (1-34) 5 μm/kg daily for 4 weeks. Implant fixation was defined by mechanical stability and osseointegration. Study I investigated the effect of parathyroid hormone on implant fixation of implants inserted press fit with surrounding bone in the proximal tibia of 20 canines. Histomorphometric analysis showed increased amount of new bone in contact with the implant. No improvement was observed in the surrounding bone. PTH did not increase mechanical fixation in pushout test. Study II investigated the effect of parathyroid hormone on implant fixation of implants surrounded by a critical 1 mm gap. Implants where inserted in the tibia of 20 canines. Bone density was increased in the inner gap and outer gap with PTH treatment. Bone at implant interface improved with PTH but did not achieve significance. Push-out testing showed that PTH Increased mechanical implant fixation in shear stiffness and total energy absorption. Shear strength was not significantly increased. Study III investigated the effect on implant fixation of implants surrounded by a 2.5 mm gap in which morsellised allograft was impacted. Implants were inserted in 20 Canines in the humerus. Histomorphometric analysis showed that PTH increased the amount of new bone within the gap, but not in contact the implant. There were no differences in amount of allograft. The push-out testing showed no differences in mechanical parameters.
The studies in this PhD thesis demonstrated that parathyroid hormone increases bone healing around implants in situations of insertion in press-fit or in more challenging environments of empty and grafted gaps. Early fixation was increased in implants with gaps, in which pure gap bone stimulation improved fixation. This warrants further preclinical studies.