Donald Deyoung

Strain transfer near hydroxyapatite (HA) coated canine hip implants was examined using simulated anatomical loading based on in vivo strain measurements. Strain changes near implants relative to intact control values were in excess of 100% for transverse and principal strains for zero time period (immediate postimplant) specimens. They were generally smaller (100% or less) for axial, transverse, and principal minimums in the same locations for 4 months postimplantation specimens. Cortical bone loss occurred in all implanted femora. The most extensive loss, up to 47%, occurred adjacent to the proximal section of the implant. Extensive trabecular bone formation, over 300% in some regions of each femur, was noted in all implanted femora. Backscattered electron imaging along the HA-coated sections of the implants showed extensive bone bonded to the coating. NOrmal light and UV light micrographs showed direct bone apposition to the implant surfaces and extensive bone formation in all tes...

Identification of the strains and the strain changes caused by implants is critical to the understanding of bone remodeling and can identify design changes needed to prevent bone loss near orthopedic implants. Calcium phosphate ceramic (CPC) coated strain gauges have been developed to allow long-term in vivo strain measurements. Previously used cyanoacrylate-bonded gauges have uncharacterizable sensing accuracy because the adhesive is resorbed from the instant it is placed in vivo. In this study CPC-coated strain gauges were used to measure physiologically "normal" bone strains collected from the proximal femora of dogs at a series of gait speeds and the postmortem sensing accuracy of the gauges was evaluated. Three male dogs were surgically implanted with up to six wired CPC-coated strain gauges placed around the circumference of their proximal femora. The dogs were trained to run on a treadmill, and in vivo strain measurements were collected following a 12-week period. The animals were tetracyline labeled and then euthanized and their femora explanted. Gauges were attached with cyanoacrylate to the intact contralateral control femora in the same position as the CPC-coated gauges on the test femora. Both femora were tested in cantilever bending to assess the functionality of the gauges and quality of the CPC-bone bond. After testing, all bones were embedded, sectioned, and ground. Sections from each femur were stained with mineralized bone stain and examined with transmitted and ultraviolet light to assess bone formation. Additional sections were examined with backscattered electron microscopy to confirm bone bonding to coatings. Wired gauges attached with the CPC coatings measured strain patterns during gait at several treadmill speeds. Patterns were similar and peak strains the same over a 2-week period. Mechanical testing showed bonding of CPC-coated gauges, and histologic examination showed intimate contact between gauge coatings and bone surfaces. Further development of CPC-bonded strain gauges is expected to result in a measurement system that provides ease of placement, and consistent longer term bone strain measurements with characterizable accuracy.

The aim of this study was to compare the bone-bonding rates of eight calcium phosphate ceramic (CPC) coatings attached to strain gauges, alone and in conjunction with an OP1 device (Creative BioMolecules, Hopkinton, MA) and autologous concentrated pericyte cells. These coatings were studied to develop faster bone bonding to long-term in vivo strain sensors. Characterization of the CPC powders using electron microscopy and X-ray diffraction showed that they had shapes ranging from spherical to rocklike and properties ranging from highly crystalline to amorphous. CPC coated gauges were placed on the femora of young male dogs during aseptic surgery and were initially held in place using resorbable sutures. Test groups were euthanized after 3, 9, and 12 weeks. Both femora of the dogs were explanted and cantilever loaded. Response of the implanted hydroxyapatite (HA) coated gauges were compared to the response of bench-top glued sets of gauges (controls) attached to the contralateral femur and reported as a percentage of the control values. One CPC coating type showed an average response of 30% of controls after 3 weeks, four showed average responses higher than 75% after 9 weeks, and three showed averages higher than 82% after 12 weeks in vivo. Amorphous CPC coatings bonded more quickly than crystalline ones and particle shape had less effect than crystal structure on bonding rates. When either OP1 or autologous concentrated pericyte cells were placed on selected CPC coated gauge surfaces, the CPC5 coated gauges bonded best after 3 weeks with a response of 59%. After the same time period in vivo, CPC3 and CPC7 provided responses of 40 and 16%, respectively. Comparison of a soluble calcium-coated CPC with an uncoated one that had identical crystal structure and similar particle shape indicated that the calcium coating slowed bone bonding substantially in the young dog model. Optical microscopy of stained undecalcified bone sections and backscattered electron imaging indicated bone formation at all bone-HA interfaces and an increase in the number of areas of bone remodeling adjacent to the gauge at all time periods. Gross bone remodeling due to strain gauge placement was only observed near the distalmost cell-seeded strain gauges. Selection of the type of coating and enhancement system can accelerate bone bonding to strain sensors but must be tailored to the bone of the model in which it is being used. Augmentation of CPC coatings with cells or OP1 resulted in variable enhancement of the bonding rate and depended on the CPC and the enhancement system.