A back-scattered electron microscopy (BSEM) study of the tight apposition between bone and hydroxyapatite coating

Abstract

IntroductionHydroxyapatite coating of orthopedic metallic implants by plasma-spray technique is presently a well assessed process. Hydroxyapatite (HA) favors the fixation of an orthopedic implant, like a hip prosthesis, by enhancing bone growth in tight apposition with the coating [1, 2]. We performed an electron microscopic analysis at the interface between newly formed bone and hydroxyapatite coating, in an experimental model in the rabbit. Samples were analyzed by scanning electron microscopy (SEM) to evaluate morphologic characteristics such as alignment of lamellae, location of haversian systems, and porosity of the hydroxyapatite coating. Then, on the same field of observation, we followed with a back-scattered electron microscopy (BSEM) analysis to evaluate the actual material distribution on the sample. With this technique, electrons are directed with a certain angle towards the surface of the sample and interact with the material so that they are reflected with a different energy; the final energy is detected and rendered graphically in gray tones [3] on a photomicrograph. BSEM is particularly suited to study the interface between bone and biomaterials because it allows the creation of a map of material distribution in which metallic substrate, hydroxyapatite coating with different densities, bone tissue in different stages of maturation, embedding media and artifactual detachments are easily discerned. AbstractWe performed a backscattered electron microscopy analysis of the interface between newly formed bone and hydroxyapatite coating, in an experimental rabbit model. Twenty cylinders made of Ti6Al4V and coated with hydroxyapatite at different crystallinity were implanted in the distal femural canal and retrieved at 4, 8, 26 and 34 weeks. Crystallinity of the coating varied from 90% to 60% and thickness varied between 50 and 100 µm. Osteocytes were detectable a few micrometers in proximity of the coating. They produced new bone which was so tightly apposed to the coating that high magnification BSEM did not resolve any discontinuity at the interface. This was not observed in uncoated implants. Degradation of the hydroxyapatite coating is not a simple hydrolytic process because newly formed bone is remodelled in areas were a tight apposition with hydroxyapatite is present. The coating itself is likely to be attacked by the resorptive action of multinucleated giant cells and osteoclasts. In conclusion, response to coated samples is morphologically characterized by tight apposition with bone. The substitution of areas of the coating by newly formed bone is possible.

50 years of scanning electron microscopy of bone—a comprehensive overview of the important discoveries made and insights gained into bone material properties in health, disease, and taphonomy

“…From Merolli et al Adapted by permission from Springer Nature. Copyright 2000 132 . c Local bisphosphonate delivery (BP; 16 µg/implant.…”

A Review of the Impact of Implant Biomaterials on Osteocytes

“…The inner wall of the osteocyte lacuna exhibits basophilic staining properties similar to the osteoid-like matrix interposed between mineralized bone and the implant surface ( Piattelli et al 1994 ). Bioactive coatings such as hydroxyapatite enhance the biological response to not only metal implants (Ti6Al4V; Merolli et al 2000 ) but also polymer implants (polyether ether ketone; Johansson et al 2016 ), giving rise to tightly interlocked lamellar bone, with osteocytes in close apposition to the coating. Also supporting the formation of osteocyte-containing mineralized tissue are thin (≈10 nm) graphene oxide coatings on cp-Ti ( Jung et al 2016 ); multilayer coatings consisting of chitosan and hydroxyapatite, with the capacity for controlled release of bone morphogenetic protein 2 ( Shah et al 2013 ); degradable particulates such as synthetically produced α-tricalcium phosphate/octacalcium phosphate ( Elgali et al 2014 ); deproteinized bovine bone ( Elgali et al 2014 ); and autogenous bone fragments generated during implant site preparation ( Shah and Palmquist 2017 ).…”

Fundamentals of Biomaterials

“…1.7 Osteoblasts can grow on artificial hydroxyapatite coating, improving significantly the possibility of obtaining a favorable response towards a coated metallic devices (a) (reproduced with permission from [14]). Reproduced with permission from [2] is made of, promoting an early favorable response from recipient bone towards the implant. Reproduced with permission from [2] is made of, promoting an early favorable response from recipient bone towards the implant.…”

“…1.2 The morphological character of tight appositionis present whenever osteocytes are found within a few micrometers of the material, and the newly formed bone, which they produced, interlocks with the material so tightly that even a high-magnification electron microscopic analysis cannot resolve any discontinuity at the interface. Reproduced with permission from [2] tightly that even high-magnification electron microscopic analysis cannot resolve any discontinuity at the interface [2]. HA, hydroxyapatite a b Fig.…”

“…a, reproduced with permission from [3]; b, reproduced with permission from [2]. 1.2) [2,3]. 1.1 A sample of hydroxyapatite-coated metallic substrate implanted in bone, when analyzed by scanning electron microscopy (a), allows evaluation of topographical characters such as: the alignment of lamellae, the location of Haversian systems, the porosity of the hydroxyapatite coating.…”