Technical note: CT determination of the mineral density of dry bone specimens using the dipotassium phosphate phantom

Evaluation of local alterations in femoral bone mineral density measured via quantitative CT

The aim of this study was to investigate the accuracy of bone mineral density (BMD) determined by quantitative computed tomography (qCT) based on in situ and ex situ scans of cadavers of variable stature. The influence of surrounding tissue on the quantification of CT images of ex situ scanned femora was investigated in air and in water and compared with the in situ scanned femora. The study showed that the surrounding tissue has an impact on the grey value-based representation of the scanned object as well as on the calibration of BMD, influencing the determination of BMD. Local differences in BMD of up to 17.5% were observed, which might originate from beam hardening artifacts.

A comparison of the in vitro mineralisation and dentinogenic potential of mesenchymal stem cells derived from adipose tissue, bone marrow and dental pulp

Stem-cell-based therapies provide a biological basis for the regeneration of mineralised tissues. Stem cells isolated from adipose tissue (ADSCs), bone marrow (BMSCs) and dental pulp (DPSCs) have the capacity to form mineralised tissue. However, studies comparing the capacity of ADSCs with BMSCs and DPSCs for mineralised tissue engineering are lacking, and their ability to regenerate dental tissues has not been fully explored. Characterisation of the cells using fluorescence-activated cell sorting and semi-quantitative reverse transcription PCR for MSC markers indicated that they were immunophenotypically similar. Alizarin red (AR) staining and micro-computed tomography (µCT) analyses demonstrated that the osteogenic potential of DPSCs was significantly greater than that of BMSCs and ADSCs. Scanning electron microscopy and AR staining showed that the pattern of mineralisation in DPSC cultures differed from ADSCs and BMSCs, with DPSC cultures lacking defined mineralised nodules and instead forming a diffuse layer of low-density mineral. Dentine matrix components (DMCs) were used to promote dentinogenic differentiation. Their addition to cultures resulted in increased amounts of mineral deposited in all three cultures and significantly increased the density of mineral deposited in BMSC cultures, as determined by µCT analysis. Addition of DMCs also increased the relative gene expression levels of the dentinogenic markers dentine sialophosphoprotein and dentine matrix protein 1 in ADSC and BMSC cultures. In conclusion, DPSCs show the greatest potential to produce a comparatively high volume of mineralised matrix; however, both dentinogenesis and mineral volume was enhanced in ADSC and BMSC cultures by DMCs, suggesting that these cells show promise for regenerative dental therapies.

The use of preserved tissue in finite element modelling of fresh ovine vertebral behaviour

The aim of this study was to investigate whether the predicted finite element (FE) stiffness of vertebral bone is altered when using images of preserved rather than fresh tissue to generate specimen-specific FE models. Fresh ovine vertebrae were used to represent embalmed (n = 3) and macerated dry-bone (n = 3) specimens and treated accordingly. Specimens were scanned pre- and post-treatment using micro-computed tomography. A constant threshold level derived from these images was used to calculate the respective bone volume fraction (BV/TV) from which the conversion factor validated for fresh tissue was used to determine material properties that were assigned to corresponding FE models. Results showed a definite change in the BV/TV between the fresh and the preserved bone. However, the changes in the predicted FE stiffness were not generally greater than the variations expected from assignment of loading and boundary conditions. In conclusion, images of preserved tissue can be used to generate FE models that are representative of fresh tissue with a tolerable level of error.

Recovery of bone strength in young pigs from an induced short-term dietary calcium deficit followed by a calcium replete diet

This study investigated whether the deficits in bone strength of pre-pubertal pigs, induced by short-term deficits in dietary calcium can be recovered if followed by a calcium-fortified diet. Young pigs were divided into two groups based on diet: a marginal Ca diet (70% of established Ca requirements) or an excess Ca diet (150% of established Ca requirements) for 4 weeks. Each group was then randomly sub-divided into two groups and fed diets with either marginal or excess dietary Ca for 6 weeks in a cross-over design, resulting in four treatment groups: H150-H150, H150-L70, L70-H150, and L70-L70. Animals were DXA scanned at 2-week intervals during the 10-week period to obtain whole body bone mineral content (BMC) and density (BMD). After animals were euthanized, right femurs were collected for this study. Traits such as bone mineral density, mass, volume, area moment of inertia (MI) and the section modulus (SM) were computed from computed tomography (CT) data and failure load was measured from four-point bending tests. DXA results showed significant reduction in BMC (61.6%) and BMD (37.5%) in the (L70-L70) group compared to the (H150-H150) group. DXA results additionally showed that deficiencies induced by the 4-week marginal Ca diet in the (L70-H150) group were not recovered with a subsequent excess Ca diet. While mechanical test results also showed significant reduction (75%) in strength in the L70-L70 group, compared to the H150-H150 group, they revealed no differences between the failure loads of the (L70-H150) group and the (H150-H150) group. Similar results were also found for bone mineral mass and volume, indicating that recovery from a short-term dietary Ca deficiency is possible at the pre-pubertal stage. Furthermore, bone mineral content and bone volume calculated from CT data correlated highly with failure load (R(2)=0.78 and 0.84, respectively), while density, MI and SM only showed weak-to-moderate correlations (R(2)=0.40-0.56), implying that bone mineral mass and volume calculated from CT data are good non-invasive surrogates for strength of growing bones.

Specimen size and porosity can introduce error into microCT-based tissue mineral density measurements

The accurate measurement of tissue mineral density, rho(m), in specimens of unequal size or quantities of bone mineral using polychromatic microCT systems is important, since studies often compare samples with a range of sizes and bone densities. We assessed the influence of object size on microCT measurements of rho(m) using (1) hydroxyapatite rods (HA), (2) precision-manufactured aluminum foams (AL) simulating trabecular bone structure, and (3) bovine cortical bone cubes (BCt). Two beam-hardening correction (BHC) algorithms, determined using a 200 and 1200 mg/cm(3) HA wedge phantom, were used to calculate rho(m) of the HA and BCt. The 200 mg/cm(3) and an aluminum BHC algorithm were used to calculate the linear attenuation coefficients of the AL foams. Equivalent rho(m) measurements of 500, 1000, and 1500 mg HA/cm(3) rods decreased (r(2)>0.96, p<0.05 for all) as HA rod diameter increased in the 200 mg/cm(3) BHC data. Errors averaged 8.2% across these samples and reached as high as 29.5%. Regression analyses suggested no size effects in the 1200 mg/cm(3) BHC data but differences between successive sizes still reached as high as 13%. The linear attenuation coefficients of the AL foams increased up to approximately 6% with increasing volume fractions (r(2)>0.81, p<0.05 for all) but the strength of the size-related error was also BHC dependent. Equivalent rho(m) values were inversely correlated with BCt cube size (r(2)>0.92, p<0.05). Use of the 1200 mg/cm(3) BHC ameliorated the size-related artifact compared to the 200 mg/cm(3) BHC but errors with this BHC were still significant and ranged between 5% and 12%. These results demonstrate that object size, structure, and BHC algorithm can influence microCT measurements of rho(m). Measurements of rho(m) of specimens of unequal size or quantities of bone mineral must be interpreted with caution unless appropriate steps are taken to minimize these potential artifacts.

Quantitative micro-computed tomography: a non-invasive method to assess equivalent bone mineral density

One of the many applications of micro computed tomography (microCT) is to accurately visualize and quantify cancellous bone microstructure. However, microCT based assessment of bone mineral density has yet to be thoroughly investigated. Specifically, the effects of varying imaging parameters, such as tube voltage (kVp), current (microA), integration time (ms), object to X-ray source distance (mm), projection number, detector array size and imaging media (surrounding the specimen), on the relationship between equivalent tissue density (rhoEQ) and its linear attenuation coefficient (micro) have received little attention. In this study, in house manufactured, hydrogen dipotassium phosphate liquid calibration phantoms (K2HPO4) were employed in addition to a resin embedded hydroxyapatite solid calibration phantoms supplied by Scanco Medical AG Company. Variations in current, integration time and projection number had no effect on the conversion relationship between micro and rhoEQ for the K2HPO4 and Scanco calibration phantoms [p>0.05 for all cases]. However, as expected, variations in scanning tube voltage, object to X-ray source distance, detector array size and imaging media (referring to the solution that surrounds the specimen in the imaging vial) significantly affected the conversion relationship between mu and rhoEQ for K2HPO4 and Scanco calibration phantoms [p<0.05 for all cases]. A multivariate linear regression approach was used to estimate rhoEQ based on attenuation coefficient, tube voltage, object to X-ray source distance, detector array size and imaging media for K2HPO4 liquid calibration phantoms, explaining 90% of the variation in rhoEQ. Furthermore, equivalent density values of bovine cortical bone (converted from attenuation coefficient to equivalent density using the K2HPO4 liquid calibration phantoms) samples highly correlated [R2=0.92] with the ash densities of the samples. In conclusion, Scanco calibration phantoms can be used to assess equivalent bone mineral density; however, they cannot be scanned with a specimen or submerged in a different imaging media. The K2HPO4 liquid calibration phantoms provide a cost effective, easy to prepare and convenient means to perform quantitative microCT analysis using any microCT system, with the ability to choose different imaging media according to study needs. However, as with any liquid calibration phantom, they are susceptible to degradation over time.

Preparation and characterization of calibration standards for bone density determination by micro-computed tomography

Phantoms for the calibration of local bone mineral densities by micro-computed tomography (microCT), consisting of lithium tetraborate (Li(2)B(4)O(7)) with increasing concentrations of hydroxyapatite [HAp, Ca(10)(PO(4))6(OH)2] have been prepared and characterized for homogeneity. Large-scale homogeneity and concentration of HAp in the phantom materials was determined using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS), while homogeneity on the micrometer scale was assessed through microCT. A series of standards was prepared by fusion of pure HAp with Li(2)B(4)O(7) in a concentration range between 0.12 and 0.74 g cm(-3). Furthermore, pressed and sintered pellets of pure HAp were prepared to extend the calibration range towards densities of up to 3.05 g cm(-3). A linear calibration curve was constructed using all individual standard materials and the slope of the curve was in good agreement with calculated absorption coefficients at the effective energy of the microCT scanner.

Subchondral bone density and cartilage degeneration patterns in osteoarthritic metacarpal condyles of horses

OBJECTIVE

To evaluate and correlate patterns of subchondral bone density and articular cartilage degeneration (derived by use of gross, histologic, and computed tomographic [CT] examinations) in equine third metacarpal condyles with and without osteoarthritis.

SAMPLE POPULATION

8 metacarpophalangeal (MCP) joints (n = 4 horses) without osteoarthritis and 6 osteoarthritis-affected MCP joints (4).

PROCEDURES

Horses were euthanized. The third metacarpal condyles of the joints were examined grossly and via CT (3 slice images/condyle). For 6 condylar zones, mean bone density and pattern of density distribution were determined. Data for osteoarthritis-affected and control joints were compared. Histomorphometric point count analyses identified areas of bone density for comparison with CT density measurements.

RESULTS

Osteoarthritis-affected condyles had heterogeneous subchondral bone with focal resorptive lesions and patterned sclerosis, whereas control condyles had symmetric bone density distribution. In osteoarthritis-affected condyles, bone density determined via gray scale image density analysis was greater (dorsal and medial pattern), compared with control condyles, and differed among zones because of resorption and sclerosis. With regard to bone density in osteoarthritis-affected condyles, histologic findings correlated with CT images, and bone lesions were significantly correlated with cartilage lesions.

CONCLUSIONS AND CLINICAL RELEVANCE

In horses, heterogeneous distribution and greater subchondral bone density were characteristic of osteoarthritis-affected condyles, compared with control condyles. Subchondral bone lesions correlated with overlying cartilage lesions in osteoarthritis-affected MCP joints. Identification of CT image characteristics appears to predict the presence of a cartilage lesion in MCP joints of horses with osteoarthritis.

Effects of a magnesium adhesive cement on bone stability and healing following a metatarsal osteotomy in horses

OBJECTIVE

To compare biodegradable magnesium phosphate cement (Mg-cement), calcium phosphate cement (Ca-cement), and no cement on bone repair, biocompatibility, and bone adhesive characteristics in vivo in horses.

ANIMALS

8 clinically normal adult horses.

PROCEDURES

Triangular fragments (1-cm-long arms) were created by Y-shaped osteotomy of the second and fourth metatarsal bones (MTII and MTIV, respectively). Fragments were replaced in pairs to compare Mg-cement (MTII, n = 8; MTIV, 8) with Ca-cement (MTIV, 8) or with no cement (MTII, 8). Clinical and radiographic evaluations were performed for 7 weeks, at which time osteotomy sites were harvested for computed tomographic measurement of bone density and callus amount, 3-point mechanical testing, and histologic evaluation of healing pattern and biodegradation.

RESULTS

All horses tolerated the procedure without clinical problems. Radiographically, Mg-cement secured fragments significantly closer to parent bone, compared with Ca-cement or no treatment. Callus amount and bone remodeling and healing were significantly greater with Mg-cement, compared with Ca-cement or no cement. Biomechanical testing results and callus density among treatments were not significantly different. Significantly greater woven bone was observed adjacent to the Mg-cement without foreign body reaction, compared with Ca-cement or no cement. The Mg-cement was not fully degraded and was still adhered to the fragment.

CONCLUSIONS AND CLINICAL RELEVANCE

Both bone cements were biocompatible in horses, and Mg-cement may assist fracture repair by osteogenesis and fragment stabilization. Further studies are warranted on other applications and to define degradation characteristics.

Curvature, length, and cross-sectional geometry of the femur and humerus in anthropoid primates

The aims of this study were to describe the curvature of anthropoid limb bones quantitatively, to determine how limb bone curvature scales with body mass, and to discuss how bone curvature influences static measures of bone strength. Femora and humeri in six anthropoid genera of Old World monkeys, New World monkeys, and gibbons were used. Bone length, curvature, and cross-sectional properties were incorporated into the analysis. These variables were obtained by a new method using three-dimensional morphological data reconstructed from consecutive CT images. This method revealed the patterns of curvature of anthropoid limb bones. Log-transformed scaling analyses of the characters revealed that bone length and especially bone curvature strongly reflected taxonomic/locomotor differences. As compared with Old World monkeys, New World monkeys and gibbons in particular have a proportionally long and less curved femur and humerus relative to body mass. It is also revealed that the section modulus relative to body mass varies less between taxonomic/locomotor groups in anthropoids. Calculation of theoretical bending strengths implied that Old World monkeys achieve near-constant bending strength in accordance with the tendency observed in general terrestrial mammals. Relatively shorter bone length and larger A-P curvature of Old World monkeys largely contribute to this uniformity. Bending strengths in New World monkeys and gibbons were, however, a little lower under lateral loading and extremely stronger and more variable under axial loading as compared with Old World monkeys, due to their relative elongated and weakly curved femora and humeri. These results suggest that arboreal locomotion, including quadrupedalism and suspension, requires functional demands quite dissimilar to those required in terrestrial quadrupedalism.

Computed tomographic osteoabsorptiometry of the elbow joint in clinically normal dogs

OBJECTIVE

To evaluate subchondral bone density patterns in elbow joints of clinically normal dogs by use of computed tomographic (CT) osteoabsorptiometry.

SAMPLE POPULATION

20 cadaver forelimbs from 10 clinically normal dogs.

PROCEDURE

Each elbow joint was imaged in parasagittal and transverse planes of 1.5-mm thickness. Slice data were converted to dipotassium phosphate equivalent density (PPED) values. Sagittal, parasagittal, and transverse medial coronoid process topographic maps were constructed. Defined zones were created for each of the 3 CT planes, and confluence and peak PPED values were determined.

RESULTS

The lowest PPED value was 340 mg/ml (articular and subchondral confluence), and the highest was 1780 mg/ml (peak subchondral density). Detectable effects of joint laterality were not found in the confluence or peak PPED measurements or in the peak-to-confluence PPED ratio for all 3 CT planes. Significant differences were found among zones in all 3 planes for confluence and peak PPED measurements and between sagittal and transverse planes for peak-to-confluence PPED ratios. Subjectively, the pattern of density distribution among dogs was fairly consistent for the sagittal and parasagittal slices. Three specific patterns of density distribution were apparent on the transverse topographic maps of the medial coronoid process that corresponded to conformational differences.

CONCLUSIONS AND CLINICAL RELEVANCE

The use of CT osteoabsorptiometry provides a repeatable technique that can be used to noninvasively examine bone density and the effects of stress acting on joints in vivo. Variability in density values for any of the CT planes was not identified among clinically normal dogs.

The influence of alveolar structures on the torsional strain field in a gorilla corporeal cross-section

Anthropologists have often used mandibular torsional properties to make inferences about primate dietary adaptations. Most of the methods employed are based on assumptions related to periodontal and alveolar properties. This study uses the finite element method to evaluate some of these assumptions with a cross-section through the third molar of a gorilla. Results indicate that the properties of alveolar bone play an important role in determining the strain field. In comparison, the exact stiffness values of the periodontal ligaments seem to have a much smaller impact. Replacing the dental roots and periodontal ligaments with alveolar bone, however, has a significant influence on the strain field. It underestimates the maximum shear strain by about 28% along its periosteal aspect when alveoli are modeled as cortical bone. It overestimates the strain by a smaller amount when alveoli are modeled as trabecular bone. This study supports the assumption that primate mandibles behave like a closed-section under torsion under the limiting condition that the alveolar bone stiffness is more than half of the value of cortical bone; alveolar bone can then be modeled as cortical bone with a minimal loss of accuracy. In addition, this study suggests that the minimum cortical thickness should be considered for torsional strength. Finally, modeling accuracy can be significantly increased if both dental and periodontal structures can be realistically incorporated into mandibular biomechanical models. However, this may not be always feasible in studies of fossil mandibles. This is due mainly to the difficulties involved in estimating alveolar bone densities and in distinguishing boundaries between cortical bone, alveolar bone, periodontal ligaments, and dental roots in fossil specimens.