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Apr 19, 2011

The use of a novel CBCT for implant surgery including pre-operative implant stability assessments.

Report from an ongoing prospective study
 
Lars Sennerby, DDS, PhD, Clinica Feltre, Feltre, Italy & University of Gothenburg,
Sweden
 
Prof Lars Sennerby graduated with a DDS degree from the University of Gothenburg in 1986 and defended a thesis on dental implants for a PhD degree in 1991. He is since 2000 professor in dental implantology. He worked in the Brånemark Clinic from 1989 to 2001 and is since then in private practice in Sweden and italy. Prof Sennerby is co-founder of Clinica Feltre together with Dr Peter Andersson. He is co-editor-in-chief of Clinical Implant Dentistry and Related Research.

Introduction
Firm primary stability is considered as one determinant for success of dental implants1. Primary implant stability is influenced by bone density, surgical technique and by the design of the implant2. For instance, higher primary stability is achieved (i) in dense rather than in soft bone, (ii) when reducing the final drill diameter and (iii) when using tapered implant designs. Subjective classification based on the surgeons’ perception during drilling has previously been used to assess bone density. Today, instrumental methods are also used to evaluate the mechanical features of the bone and subsequent implant stability at the implant sites. These techniques allow measuring of several parameters, which correlate with bone density, such as: Peak Insertion Torque (PIT)3, continuously measured Insertion Torque (IT)4 and Implant Stability Quotient (ISQ). 
The Challenge
By the introduction of computerized tomography (CT) and Hounsfield units (HU), it has been possible to obtain bone density information from reconstructed radiographic images during pre-perative therapy planning5. Previous investigations have shown a correlation between bone density, PIT and ISQ, when comparing HU values from preoperative CT examination with measurements of PIT and ISQ during and after implant placement6 , 7 . The investigations suggest that preoperative assessment of the bone density from CT examinations could be helpful if integrated in diagnostic software for implant planning. The possibility to assess both the volume and density of the bone structures by one tool would considerably improve treatment planning and facilitate the choice of surgical procedure and type of implant to use. A pilot study from our group found a statistically significant correlation between bone density assessments in CTs and
implant stability measurements using IT and ISQ8. In order to reduce irradiation and to simplify examination, a new generation of CTs has been developed, so called cone beam CTs (CBCT). However, modern CBCTs do not support the use of Hounsfield units.
 
Houndsfield units
A CT examination consists of a stack of digital images, each one representing an axial cross section of the scanned anatomical volume. The elements of a 2D digital image are called pixels, while the elements, which represent the cubic units of the anatomical volume, are called voxels. Hence, the stack of images represents a spatial discrimination of the scanned volume. Each pixel and voxel of the tomographic images represents a value, which is called a CT number. The CT numbers describe the average radiodensity of the tissues and corresponding voxels, which is quantitatively described by the linear attenuation coefficient. The Hounsfield scale was introduced to be able to quantify radiodensity in CT imaging based on the CT numbers.

The limits of present Cone Beam CTs
Because of the nature of the technique, CBCTs have so far not supported the use of Hounsfield Units (HU), since some factors negatively affect the accuracy and reproducibility of the assigned CT numbers. For example, scatter radiation increases the noise amplitude and reduces the spatial uniformity of the CT. This in turn, means that the CT numbers are not uniform and that the uncertainties may limit the reliability of the bone density evaluation based on HU values.
The novel BDE approach
To overcome the physical limits of using HUs with CBCT, the WhiteFox manufacturer ACTEON has proposed a novel approach called Bone Density Examination -BDE. This protocol was specifically designed to enhance the accuracy of bone density assessments. The BDE protocol includes scanning of the patient together with two reference specimens of known hydroxyapatite concentrations. The examination is calibrated using a simple procedure and bone density measurements can be made in the reconstructed images in a most accurate way.
A prospective study with the aim to validate the BDE protocol was initiated in 2010. The study includes consecutive patients with a need for replacement of one, several or all teeth with dental implants. At the end, at least 200 implants sites will have been included in the investigation. The study protocol was reviewed and approved by the ethical research committee at the Feltre hospital in Feltre, Italy.
The study protocol:
1) Presurgical CBCT examination of the patient using the WhiteFox CBCT with the BDE protocol and software for presurgical evaluation.
2) Implant surgery. Bone density and volume is registered using Lekholm & Zarb index, while Insertion Torque (IT) is continuously measured with a Elcomed SA200C drilling unit (W&H, Bürmoos, Austria) and analyzed in a specially designed software (Impdat, Kea Software GmbH, Poecking, Germany). The stability of the inserted implants is measured by a Mentor machine (Osstell AB, Gotheburg, Sweden) in ISQ unit in both buccal-lingual and mesial-distal directions.
3) Postoperative CBCT examination using the WhiteFox CBCT in order to find the exact position of the implants. Special software (Geomagic Studio 11, Research Triangle Park, NC, USA) is used to superimpose the position of the implants as extracted from the postoperative CBCT to the preoperative one. At the implant sites on the preoperative CBCT examination, the total sum of the CT numbers [HU] and the mean of the CT numbers [HU] are measured and converted to bone mineralization index (Hydroxyapatite concentration).
4) Statistics. The investigation will seek possible statistical correlations between the primary parameters: HU, Hydroxyapatite concentration, Insertion Torque, Implant Stability Quotient and Lekholm & Zarb index.