The validation of a compression testing method for cancellous human jawbone by high-resolution finite element modeling.

PURPOSE: The aim of this study was to determine a reliable compression testing method for cancellous jawbone specimens and to validate it by high-resolution finite element (FE) modeling based on microcomputerized tomography (microCT) images of the specimens. MATERIALS AND METHODS: Three series of human femoral bone samples were tested to establish a compression protocol for human jawbone cores. A microCT scan of each bone sample was obtained. A simple destructive compression test was performed on the first series of 12 femoral bone samples (13 mm height and 6.1 mm diameter). The 5 femoral bone samples of the second series (13 mm height and 6.1 mm diameter) were constrained using end caps and subjected to 10 to 15 conditioning cycles before the destructive test from which the Young's modulus (Emeas) was determined. The third series of 5 smaller femoral samples (8 mm height and 5.5 mm diameter) and the series of 5 jaw bone samples (8 mm height and 5.7 mm diameter) underwent the same testing protocol. FE models were created based on the microCT images, and the simulated E-modulus (Ecalc) was calculated. RESULTS: The intraclass correlation between Emeas and Ecalc corresponded to 0.74 for the first series of femoral bone samples, 0.96 for the second series, and 0.51 for the third series. For the jawbone samples, the intraclass correlation coefficient equaled 0.88. CONCLUSION: Reliable results for compression testing of cancellous jawbone can be obtained with cylindric specimens with a diameter of 5.7 mm, a length:diameter ratio 1.4, and flat top and bottom surfaces. The recommended compression method is constrained compression with 10 to 15 conditioning cycles, followed by a destructive test.

Validation of microfocus computed tomography in the evaluation of bone implant specimens.

BACKGROUND: Microfocus computed tomography (muCT) is an emerging technique owing to its speed, full three-dimensional information, and nondestructive properties. PURPOSE: The aim of this study was to explore the efficacy of a muCT system (Philips HOMX 161, Philips Medical Systems GmbH, Hamburg, Germany) for visualization of the bone structure around screw-type titanium implants by comparing muCT images with their histologic homologues. MATERIALS AND METHODS: Eight screw-type titanium implants were placed in the femoral condyles of two goats. After the excised implant-bone specimens were embedded in resin, three-dimensional muCT of the excised implant and bone specimens was performed. Histologic sections were subsequently made. A total of 150 histologic sections were matched with muCT images. RESULTS: Bone trabeculae were clearly visible on the muCT scans. However, bone close to the implant or present in the apical surface features of the implant could not be detected. The overall matching between muCT scans (slices) and the histologic sections was 89%. CONCLUSION: Investigation of trabecular bone around titanium implants by muCT can be considered highly reliable for determining trabecular bone parameters, with the exception of measuring direct bone-to-implant contact.

The influence of Young's modulus of loaded implants on bone remodeling: an experimental and numerical study in the goat knee.

The aim of this study was to examine the influence of the Young's modulus of the implant material on the bone remodeling in a loaded condition. A combined animal experimental and computational study was set up. The animal experimental group comprised of 16 Saanen goats, each receiving one titanium implant (Young's modulus 110 GPa) and one high-density polyethylene (HDPE) implant (Young's modulus 1 GPa) in the left femoral condyle. Both types of implants received a titanium coating of 100 nm thickness. The implants protruded in the knee joint space and were directly weight bearing. The first group of eight goats was sacrificed after 6 weeks of loading and the second group of eight goats after 6 months of loading. The 16 femoral condyles with the 32 implants were prepared for microfocus computed tomography (micro-CT) scanning and histological sectioning. Three-dimensional trabecular bone parameters were calculated on the micro-CT images for the zones neck, middle, and apex of the implant. The percent of bone contact with the implant was measured on longitudinal histological sections. An axisymmetric finite element (FE) model was created to compare peri-implant bone strains and relative motion between a titanium and a HDPE implant for the experimental loading condition, and to assess the influence of different bone-implant interface (contact) conditions. From the statistical analysis of the 3D bone parameters, the difference between the titanium and HDPE implants was not significantly different (p > 0.05) between the zones (neck, middle, and apex) for both groups of goats. The implants could be considered in their entirety. After 6 weeks of loading, the PE implant presented lower connectivity and smaller marrow spaces in the circular region of 0-500 microm. In the region 500-1500 microm more bone volume was present for the PE implant. After 6 months, the PE implants showed more bone volume and thicker trabeculae than the titanium implants for the entire length of the implant. This effect was already present in the smallest region of interest, 0-500 microm. After 6 months more fibrous encapsulation was found around titanium implants. FE results demonstrated a substantial influence of the interface conditions on peri-implant strains and relative motion. For interface conditions that were representative for the early postoperative situation (involving press-fit and friction), differences in peri-implant bone strain distributions between titanium and HDPE could be related to the experimentally observed differences in amounts of bone and fibrous encapsulation. In contrast, differences in relative motion did not seem to play a role. Both the experimental and computational results suggest that implant stiffness can affect the peri-implant tissue response, which may be related to differences in peri-implant strains.

Feasibility of detecting trabecular bone around percutaneous titanium implants in rabbits by in vivo microfocus computed tomography.

OBJECTIVES: The goal of this study was to examine the feasibility of in vivo imaging of trabecular bone around titanium implants by means of microfocus computed tomography (micro-CT) and the use of rabbits for this purpose. MATERIALS AND METHODS: Ten male rabbits type Hollander, received a titanium implant (1.7 mm diameter and 10 mm length) in the trabecular bone of the left tibia. Seven weeks later a micro-CT scan was taken. Four rabbits were used to monitor potential harmful effects from X-ray absorption until 4 weeks after scanning. A second group of six rabbits was used for testing the hypothesis that a good correlation exists between in vivo micro-CT images and histological images of trabecular bone around titanium implants. The six rabbits were scanned and sacrificed immediately. The tibias were extracted and submitted to standard histological procedures. This resulted in a total of 12 histological sections and their corresponding 12 micro-CT images. Bone area measurements were performed at the left and right side of the implant in three regions: 0-500, 500-1000 and 1000-1500 microm distance from the implant interface. Intra-class correlations (ICC) were calculated between both techniques. RESULTS: The four rabbits did not show any sign of radiodermatitis 4 weeks after scanning. In the micro-CT images of the group of six rabbits, trabeculae are visible, but not well defined, due to the presence of noise in the image. The ICC for the right implant side were 0.44 for zone 0-500 microm, 0.48 for zone 500-1000 microm and 0.40 for zone 1000-1500 microm. The ICC for the left implant side could not be calculated. CONCLUSION: A low agreement was found between the bone measurements from histology and in vivo micro-CT images. The use of the in vivo micro-CT for trabecular bone imaging around metallic implants should be restricted to track tendencies in follow-up studies.

Structural and radiological parameters for the characterization of jawbone.

OBJECTIVES: The aim of this study was to determine the Hounsfield values of selected bone sites on a computed tomography (CT) scan of the jaw and to investigate the relationship between this radiological parameter and structural parameters. MATERIALS AND METHODS: A selection of 24 bone samples out of eight cadaver human jaws was made. The following parameters were measured: Hounsfield value in the jaw (HU1) determined by a first CT scan, Hounsfield value of the excised bone specimen (HU2) by a second CT scan, bone mineral density (BMD) by a dual-energy X-ray absorptiometry scan, bone volume (BV/TV) by the microfocus CT scan, first peak transmission time (T(US-1)) and first zero crossing transmission time (T(US-2)) by an ultrasound measurement and Young's modulus (E(MECH)) by a compression test. RESULTS: Thirteen specimens were composed of a mix of trabecular and a small amount of cortical bone, while another 11 specimens were composed of trabecular bone only. A good correlation was found between the HU value of the specimen (HU2) and BMD (rho = 0.99), BV/TV (rho = 0.97), T(US-1) (rho = -0.83), T(US-2) (rho = -0.87) and E(MECH) (rho = 0.83). For the pure trabecular bone specimens, the HU value of the excised bone specimen (HU2) was highly correlated (rho = 0.95) with the HU value of the total jaw scan (HU1). For mixed trabecular-cortical bone specimens, this relationship was weak (rho = 0.57). CONCLUSION: With the current CT scan technology, predictions of the mechanical properties of trabecular jaw bone based on Hounsfield values were only valid for jaws with a thin layer of cortical bone. For jaws with a thicker cortical layer, the prediction of the mechanical properties decreased significantly.