In Vivo Strain Measurements Collected Using Calcium Phosphate Ceramic-Bonded Strain Gauges

Abstract

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.

A Handheld Computer as Part of a Portable In Vivo Knee Joint Load-Monitoring System

“…The measurements were compared after loading the scaffolds at 1 Hz, 3 Hz and 10 Hz in order to evaluate the monitoring capabilities of the systems at load rates expected during normal gait, low impact slow exercises and high impact rapid activities such as cutting sports. The accuracy was also checked to determine consistency with results of previous experiments [18, 19, 20]. …”

“…They have provided accurate measurements of bone strain in animal [20, 21] and human models [22, 23]. However, these methods were suitable only for short-term, postoperative recordings due to the potential for infection and the discomfort caused by transcutaneous leads.…”

Long-term measurement of bone strainin vivo: The rat tibia

“…19,[21][22][23] The coating is applied to a conventional microminiature strain gauge and the gauge is placed adjacent to bone to allow bonding. We have shown that bond integrity can be maintained for up to 18 weeks in a dog model.…”

A comparison ofIn Vitro andIn Vivo degradation of two CPC strain gauge coatings

“…Partial bone bonding (up to 55%) using CPC coated strain gauges has been achieved in 3 weeks. 4 In addition, long-term bone bonding and good strain transfer can be maintained for 18 weeks or more, 2 but gauge/ CPC separation can prevent accurate strain sensing. 4,5 This delamination could result from a weak CPC-polysulfone or polysulfone-polyimide interface or from CPC particle dissolution.…”

“…Calcium phosphate ceramic (CPC) coated strain gauges have been developed in animal models to measure bone strain [1][2][3] resulting from physiological loading in order to gain a better understanding of the relationship between strain and bone growth, remodeling, and repair. Partial bone bonding (up to 55%) using CPC coated strain gauges has been achieved in 3 weeks.…”