Interface strength studies of calcium phosphate ceramic coated strain gauges

Abstract

In vivo strain gauging has been used to understand physiological loading and bone remodeling. In early studies, a cyanoacrylate adhesive was used to bond gauges to bone, even though this adhesive is susceptible to biodegradation that results in rapid debonding. Calcium phosphate ceramic (CPC) coated gauges have been successfully bonded to bone for long periods. However, earlier studies noted occasional debonding of coatings from gauges. The goals of this project were to develop a technique to securely bond particles to gauge backings and develop an in vitro test and assess its accuracy in simulating in vivo degradation of this interface. Gauges were heated for different time intervals, roughened with carbide papers, and prepared using layered coatings of polysulfone and CPC particles that varied in size, shape, and crystallinity. They were soaked in solution or placed in muscle pouches of rats for up to 16 weeks. They were then epoxied to fixtures, mounted on an MTS machine, and loaded to failure. Heating and roughening gauge surfaces increased the interface strengths by up to 2000%. In vivo and in vitro testing showed an initial drop in the interface strength, which leveled off to approximately 7.0+/-2.0 MPa.

An instrumented scaffold can monitor loading in the knee joint

“…This configuration was selected based on bench top testing that showed consistent load determination when this configuration was used to evaluate mid‐stance loading 20. The exterior surfaces of scaffolds were coated with a blend of CPC particles (Biointerfaces, San Diego, CA), which have shown favorable bone‐to‐CPC bonding21 while providing excellent CPC‐to‐gauge interface strength 22. These coatings have previously been used in long‐term animal and clinical strain gauge studies 23, 24.…”

In vivo strain measurements from hardware and lamina during spine fusion

“…Calcium phosphate ceramic (CPC)–coated strain gauges, prepared using a previously published procedure,10, 11 which have been described for use in test animals,12–14 were implanted into a patient in conjunction with a subminiature remotely powered radio transmitter15, 16 to monitor bone strain changes. Gauges were placed on the lamina, based on evidence from a previous study16 and published measurements indicating peak strains occur near the pedicle 17–19.…”

“…Three FAE‐12‐100‐S6ET subminiature 1000‐ohm single‐element strain gauges (BLH Electronics, Canton MA) were wired to two conductor NMUF 2/32‐4046SJ (Cooner Wire, Chatsworth, CA) shielded cables. Blended CPC coatings consisting of 15% of an amorphous tricalcium phosphate (TCP) and 85% of a microcrystalline hydroxyapatite (HA), obtained from Biointerfaces (San Diego, CA), were attached to the gauges using a published technique 10–12. This technique involved coating the sensing surfaces of the gauges with medical grade polysulfone (Amoco, Huntington Beach, CA) dissolved in 1‐1‐2‐2 tetra‐chloroethane.…”

A novel biomimetic polymer scaffold design enhances bone ingrowth

“…Four additional scaffolds of each type were left unwired to allow μCT data to be collected and compared to histology measurements. All scaffolds were coated according to a published procedure27, 28, 46–48 in a mixture of CPC particles, previously designated CPC 6 and 7 30, 49. This mixture of CPC's has been shown to encourage rapid bone growth in a canine model as well as in patients 30, 49.…”

“…All scaffolds were coated according to a published procedure27, 28, 46–48 in a mixture of CPC particles, previously designated CPC 6 and 7 30, 49. This mixture of CPC's has been shown to encourage rapid bone growth in a canine model as well as in patients 30, 49. Wired scaffolds were then calibrated using a servo‐hydraulic materials testing system to create a unique load/strain calibration curve for each wired scaffold that could be used to interpret in vivo strain measurements as loads 27, 28, 31.…”