Background
As hydroxyapatite (HA) has good osteoconductive properties, HA is used as coating material for the implants in cementless
arthroplasty. However, its effect is not sufficient for osteoporotic bone. Parathyroid hormone (PTH) is known to have anabolic
effects on bone formation. Intermittent administration of PTH increases both cancellous and cortical bone mass. The aim of
this study was to confirm the effect of the fixation strength of HA-coated implants in the osteoporotic condition with a mechanical
test and a bone histomorphometric method.
Methods
Female Sprague-Dawley rats were used for this study. Four weeks after ovariectomy (OVX) or sham surgery, HA-coated titanium
rods were inserted into the distal femoral canal (Sham+HA group and OVX+HA group). PTH was administered immediately after
the implantation of the HAcoated rods (OVX+HA+P group). We measured the shear strength at the bone-implant interface by a
push-out test and the newly formed bone volume on the implant (BV.Im) by bone histomorphometry at 2 and 4 weeks after implantation.
Results
The bone-implant shear strength in the OVX+HA group was significantly lower than that in the Sham+HA group at 2 weeks after
implantation of the rods. In the OVX+HA+P group, the strength was significantly higher than that in the other groups. Similarly,
at 4 weeks, statistically significant differences were confirmed in the bone-implant shear strength among the Sham+HA group,
the OVX+HA group, and the OVX+HA+P group. BV.Im in the OVX+HA group was significantly lower than that in the Sham+HA group
at 2 weeks after implantation. BV.Im was significantly higher in the OVX+HA+P group than that in the OVX+HA group. However,
there was no difference in BV.Im between the Sham+HA group and the OVX+HA+P group. At 4 weeks after implantation, BV.Im was
significantly lower in the OVX+HA group than that in the other groups, but no difference was found between the Sham+HA group
and the OVX+HA+P group.
Conclusions
Intermittent administration of PTH has an effect to increase new bone formation on the surface of HA-coated implants in the
osteoporotic condition. This finding suggests that PTH administration is useful to improve the initial fixation of HA-coated
implants even in osteoporotic patients.