Limitations of using micro-computed tomography to predict bone-implant contact and mechanical fixation

• Published in: Journal of microscopy (Oxford) 2012-01, Vol.245 (1), p.34-42
• Main Authors: LIU, S., BROUCEK, J., VIRDI, A. S., SUMNER, D. R.
• Format: Article
• Language: English
• Subjects: Animals; Artefacts; Bone and Bones - physiology; Bone and Bones - ultrastructure; Bones; bSEM; Electron Microscope Tomography - methods; Fixation; implant fixation; Implants; Male; Mathematical models; metal-induced artefact; Metals; osseointegration; Prostheses and Implants; Rats; Rats, Sprague-Dawley; Surgical implants; Television; μCT
• ISSN: 0022-2720; 1365-2818
• DOI: 10.1111/j.1365-2818.2011.03541.x

Summary Fixation of metallic implants to bone through osseointegration is important in orthopaedics and dentistry. Model systems for studying this phenomenon would benefit from a non‐destructive imaging modality so that mechanical and morphological endpoints can more readily be examined in the same specimens. The purpose of this study was to assess the utility of an automated microcomputed tomography (μCT) program for predicting bone–implant contact (BIC) and mechanical fixation strength in a rat model. Femurs in which 1.5‐mm‐diameter titanium implants had been in place for 4 weeks were either embedded in polymethylmethacrylate (PMMA) for preparation of 1‐mm‐thick cross‐sectional slabs (16 femurs: 32 slabs) or were used for mechanical implant pull‐out testing (n= 18 femurs). All samples were scanned by μCT at 70 kVp with 16 μm voxels and assessed by the manufacturer's software for assessing ‘osseointegration volume per total volume’ (OV/TV). OV/TV measures bone volume per total volume (BV/TV) in a 3‐voxel‐thick ring that by default excludes the 3 voxels immediately adjacent to the implant to avoid metal‐induced artefacts. The plastic‐embedded samples were also analysed by backscatter scanning electron microscopy (bSEM) to provide a direct comparison of OV/TV with a well‐accepted technique for BIC. In μCT images in which the implant was directly embedded within PMMA, there was a zone of elevated attenuation (>50% of the attenuation value used to segment bone from marrow) which extended 48 μm away from the implant surface. Comparison of the bSEM and μCT images showed high correlations for BV/TV measurements in areas not affected by metal‐induced artefacts. In addition for bSEM images, we found that there were high correlations between peri‐implant BV/TV within 12 μm of the implant surface and BIC (correlation coefficients ≥0.8, p < 0.05). OV/TV as measured on μCT images was not significantly correlated with BIC as measured on the corresponding bSEM images. However, OV/TV was significantly, but weakly, correlated with implant pull‐out strength (r= 0.401, p= 0.049) and energy to failure (r= 0.435, p= 0.035). Thus, the need for the 48‐μm‐thick exclusion zone in the OV/TV program to avoid metal‐induced artefacts with the scanner used in this study means that it is not possible to make bone measurements sufficiently close to the implant surface to obtain an accurate assessment of BIC. Current generation laboratory‐based μCT scanners typically have voxel sizes of