Original study - ZZI 04/2009

Bony integration of an alloplastic bone substitute material (NanoBone) after maxillary sinus augmentation –
Potential of ultrastructural and histomorphometric analysis*

M. O. Klein1, H. Götz2, H. Duschner2, W. Wagner1

Modern bone substitute materials (BSM) have to meet numerous structural and biological requirements. Accordingly, morphological in vitro analysis and histomorphometric ex vivo investigations are of great significance to estimate BSM biocompatibility. Aim of the study was a respective evaluation of a modern alloplastic BSM (NanoNone).

Structural in vitro analysis of the native BSM was carried out by electron microscopy and microcomputed tomography (µ-CT) with special regard to porosity. 14 months after maxillary sinus augmentation with NanoBone and collected autologous bone particles in one individual patient case, a representative trephine biopsy out of the augmentation volume was histomorphometrically analysed employing conventional histology and µ-CT. Volume ratios of newly formed bone and remaining BSM particles were calculated via assessment of 2D-phase distribution of tissue density.

In vitro investigation of the BSM showed a chiselled macrostructure with a total porosity of > 65 % as well as a high ratio of pores > 250 µm, which were almost exclusively localized interparticulary. Histomorphometric analysis of the trephine biopsy revealed a good bony integration of the BSM with evidence of BSM resorption and replacement by vital bone tissue after 14 months. The volume ratio of newly formed bone was 37 %.

The presented methods for pre-clinical and clinical evaluation of modern BSM complement one another in a reasonable manner.

Keywords: bone substitute material, ultrastructural analysis, histomorphometry, microcomputed tomography, maxillary sinus augmentation

Introduction

The indications for alloplastic bone substitute materials in the jaw region extend from the filling of small periodontal lesions to extensive reconstruction of large jaw defects. In implantology, they have proven useful both in augmentation prior to insertion of endosseous dental implants and to optimize the implant site in single-stage procedures. Particularly in sinus lift operations, use of suitable bone substitute materials to augment the sinus floor usually allows avoidance of autologous bone harvesting with its associated morbidity. Numerous studies and also systematic reviews underpin the equivalence of alloplastic substitutes for this indication of maxillary sinus augmentation [27, 6, 7].

On the other hand, because of the “foreign body“ character of the bone substitute materials, both patients and clinicians have reservations regarding their unrestricted, though scientifically well-founded use, so that autologous bone is still often regarded as the substitute material of first choice. Xenogeneic bovine bone substitutes in particular are still not universally accepted by patients because of the feared risk of infection with bovine spongiform encephalopathy (BSE), although such an infection has not been found to date and the risk can accordingly be classified as extremely small [22].

Modern bone substitute materials have to meet numerous requirements. Along with their (temporary) spacer function and stabilization of blood clot, biocompatibility and at least circumscribed biodegradability are obligatory preconditions for vascular ingrowth, osteoconduction and thus functional healing. The relatively uniform requirements of the substitutes are in contrast with the enormous variety of commercial pro-ducts with extremely varied chemical and structural characteristics. Their chemical composition has a critical influence on the resorption of the augmentation material; it ranges from biological apatites through synthetic monophasic calcium phosphate compounds (α-, β-tricalcium phosphate, hydroxyapatite) [23] and silicates to multiphasis mixed ceramics. By structural characteristics is meant the particle size and geometry together with the intra- and inter-particle pore dimensions; these range from small (≈ 50 µm), round and solid particles to large particles (> 2 mm), highly porous materials with a complex structure [18].

There are numerous methods for imaging and providing a structural description of bone substitutes and their interaction with bone tissue. In vitro, methods based on electron microscopy (EM) are used widely [3, 19]. The high resolution in the nanometer range allows conclusions about the microporosity of individual particles. Histological examination, e. g based on the cutting and grinding technique, is a standard method for documenting the bony integration of alloplastic bone substitutes. However, assessment of the clinically relevant three-dimensional positional relationships of multiple particles using EM and histological methods is limited.

Microcomputed tomography (µ-CT) is a suitable direct and tissue-sparing method for the structural analysis of hard biological tissues such as bone [21, 15, 2]. Moreover, both native alloplastic bone substitutes and biopsies obtained after augmentation can be examined to assess the healing behavior (bony integration) [8, 20, 16, 18].

In this article, the examination methods described above are presented using the example of an innovative synthetic multiphasic bone substitute material (NanoBone) both native and following sinus floor augmentation by obtaining a representative trephine biopsy.

Material employed, case description and analysis methods

1. Bone substitute material, electron microscopy

The alloplastic, all-synthetic bone substitute NanoBone (Artoss GmbH, Rostock) was used. Table 1 provides an overview of the manufacturer's data on the material. Imaging of the individual particles with electron microscopy was performed in high vacuum mode (Quanta 200 FEG, FEI, Eindhoven, Netherlands, Fig. 1).

2. Case description

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