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Titel på arbejdetCalcium phosphate coatings for fixation of bone implants. Evaluated mechanically and histologically by stereological methods.
NavnSøren Overgaard
Afdeling / StedOrtopædkirurgisk afdeling, Århus Universitetshospital
UniversitetÅrhus Universitet
  • Hip and knee surgery
Abstract / Summary

The osteoconductive properties of HA coatings are well-documented. HA coating is able to enhance bone ingrowth and to reduce early migration of both hip and knee prostheses. Despite the clinical use of HA-coated prostheses several aspects relevant to HA coatings have not been elucidated. The optimum coating quality and surface texture is still a matter of debate. Moreover, the significance of coating resorption is controversial. It has been suggested that resorption disintegrates the coating and reduces the bonding strength between implant and bone and the strength of the coating-implant interface, which might lead to implant loosening, coating delamination and acceleration of third body wear processes. This thesis aimed to investigate the effects of Ca-P coating type, quality and surface texture on mechanical fixation, bone ingrowth and loss of coating in experimental models in dogs and man. Furthermore, the significance of systematic sampling in bone histomorphometry using the unbiased stereological vertical section method was analyzed. Results. The first group of studies showed that HA-coated implants with porous-coated surface demonstrated increased energy absorption compared with grit-blasted implants during both non weight-bearing and weight-bearing conditions with controlled micromotion of 500 mm. In addition, the HA coating delaminated on gritblasted implants during mechanical testing in contrast to porous-coated implants. Histomorphometry showed increased bone ingrowth to grit-blasted implants demonstrating that surface topology influenced surface activity. The next series of studies focused on the effects of Ca-P coating type, HA versus FA, during stable weight-bearing and non weight-bearing conditions. In dogs, no difference in mechanical fixation and bone ingrowth was demonstrated. However, in humans, HA-coated implants had significantly greater bone ongrowth than FA-coated implants after one year. The third group of studies evaluated the effects of HA coating crystallinity during controlled micromotion of 250 m m. After 16 weeks, low crystalline (50%) HA coating accelerated mechanical fixation and bone ingrowth compared with high crystalline HA (75%). High crystalline HA achieved significantly better anchorage from 16 to 32 weeks whereas mechanical fixation of low crystalline HA was unchanged. In all studies, loss of Ca-P coating was evaluated. It was demonstrated that the coatings were resorbed, partially, in vivo irrespective type and quality of the coating. HA coverage on porouscoated implants was significantly more reduced than on grit-blasted implants in dogs. No difference in overall resorption between HA and FA coatings was demonstrated. However, in humans, significantly less HA and FA coating was resorbed when bone was present on the coating surface compared with bone marrow or fibrous tissue. In addition, resorption of HA was greater than FA in the presence of bone marrow indicating that FA was more stable than HA. Low (50%) crystalline HA coating was significantly more reduced compared with high (75%) crystalline HA at both 16 and 32 weeks. However, no further coating loss was observed from 16 to 32 weeks suggesting two phases of coating resorption: Phase I (0–16 weeks) with rapid coating loss, and phase II (16– 32 weeks) with slow loss. Another important finding was that continuous loading and micromovements of 150 m m accelerated resorption in contrast to immobilization of the implant. In addition, unstable fibrous anchored implants had significantly more loss of HA coating as compared with bony anchored implants. In all studies, resorbed coating was partly replaced by bone in direct contact with the implant surface suggesting durable implant fixation. Sampling efficiency in the unbiased stereological vertical section method was analyzed in order to find an optimal sampling design for histomorphometric analyzes at different sampling levels (humans, sections, fields of view and number of counting items) with different sampling intensities. The analysis showed that only minor changes in variances were observed when the initial scheme of 14 sections from each implant was reduced to include only one of the two possible implant sides, every third field of view and half the probe density, reducing the total workload at the microscope to less than 10% on all sections. In addition, the number of sections for analysis could be reduced to every fourth section per implant (3–4 sections for evaluation) without significantly increase in variance. The study demonstrated that biological variation contributed to the majority of the total observed variance. Conclusion. The present series of investigations demonstrated that Ca-P coating type and quality and the underlying surface texture had significant influence on either mechanical fixation, bone in/ongrowth and loss of coating in dogs and man. In addition, the sampling design for histomorphometry could be optimized without reducing the quality of the data.

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