Sarandeep S. Huja

A Radiographic Evaluation of the Availability of Bone for Placement of Miniscrews

Monocortical screws are increasingly being used to enhance orthodontic anchorage. The most frequently cited clinical complication is soft tissue irritation. It is thus clinically advantageous for these miniscrews to be placed in attached mucosa. The purpose of this study was to (1) determine radiographically the most coronal interradicular sites for placement of miniscrews in orthodontic patients and (2) determine if orthodontic alignment increases the number of sites with adequate interradicular bone for placement of these screws. Sixty panoramic radiographs (n = 30 pretreatment, n = 30 posttreatment) of orthodontic patients were obtained from an archival database after Institutional Review Board approval. Selection criteria included minimal radiographic distortion and complete eruption of permanent second molars. Interradicular sites were examined with a digital caliper for presence of three and four mm of bone. If three or four mm of bone existed, then a vertical measurement from the cementoenamel junction (CEJ) to first measurement was made. In addition, the magnification error inherent in panoramic radiographs was estimated. Ninety-five percent confidence intervals were calculated for the vertical distances from the CEJ to the horizontal bone location. Bone stock for placement of screws was found to exist primarily in the maxillary (mesial to first molars) and mandibular (mesial and distal to first molars) posterior regions. Typically, adequate bone was located more than halfway down the root length, which is likely to be covered by movable mucosa. Inability to place miniscrews in attached gingiva may necessitate design modifications to decrease soft tissue irritation.

Bone remodeling in maxilla, mandible, and femur of young dogs.

Bone remodeling in the jaw is essential for metabolic needs, mechanical demands and for growth of the skeleton. Currently, there is no information on remodeling in the jaw of young dogs. Four ∼5‐month‐old male dogs were given a pair of calcein bone labels. After killing, bone sections were obtained from the maxilla, mandible, and femur. The jaw specimens were obtained from regions associated with erupting permanent teeth. Undecalcified specimens were prepared for examination by histomorphometric methods to evaluate mineral apposition rate (μm/d), mineralizing surface/bone surface (%), and bone formation rate (BFR, %/yr) in the bone supporting erupting teeth and in the femurs. Only intracortical secondary osteonal remodeling units were measured. There were significant (P < 0.05) differences in the BFR for the three sites examined, with the highest BFR (72%/yr) being in the femur. The mandible had a BFR twofold greater than the maxilla (51%/yr vs. 25.5%/yr). The rate of turnover in the jaw and femur of young dogs is distinct from a similar comparison between the jaw and appendicular skeleton of adult (∼1 yr old) dogs. Although BFR decreases with age in the femur, it remains elevated in the jaws. Anat Rec, 291:1–5, 2007.

Indentation Properties of Young and Old Osteons

The purpose of this study was to quantify differences in indentation modulus and microhardness between labeled osteons identified by epifluorescent microscopy and neighboring unlabeled osteons. In microradiographs and backscattered images, newly formed osteons appear more radiolucent (darker) than older osteons. This is ascribed to incomplete mineralization of the osteon. However, the mechanical properties of these young osteons are unknown. Nine femoral cross-sectional specimens were obtained from five skeletally mature dogs. Prior to death, the dogs received a pair of calcein bone labels. Labeled osteons were identified under an epiflourescent microscope. Bone specimens were transferred to a nanoindenter specimen holder, and the previous identified labeled osteons were located. Labeled (n = 102) and unlabeled (n = 101) osteons were examined by instrumented indentation testing. Indents were made to a depth of 500 nm at a loading rate of 10 nm/second. There were significant differences in the indentation modulus (P < 0.001) of labeled (10.02 ± 3.61 gigapascal (GPa), mean ± standard deviation) and unlabeled (15.11 ± 3.72 GPa) osteons. Similar differences existed in microhardness measurements. Newly formed osteons had lower modulus (34%) and hardness (41%) than older osteons found in femoral cross sections. These data provide information on the indentation moduli of osteons during an early phase of mineralization compared to osteons that have completed mineralization.

Effects of short-term zoledronic acid treatment on bone remodeling and healing at surgical sites in the maxilla and mandible of aged dogs.

PURPOSE It is unknown whether zoledronic acid (ZA) interferes with initial bone healing at extraction and implant sites. The goal of this study was to examine the effect of short-duration ZA on bone remodeling and healing after surgical insult in an aged dog model. MATERIALS AND METHODS Four 2- to 3-year-old male dogs were administered ZA (0.1 mg/kg per month for 4 months), and 3 age-matched untreated dogs received no drug. In both groups, after the ZA-treated group had completed receiving the drug, the third premolar was extracted unilaterally and 2 orthodontic mini-implants per jaw per dog were placed on the ipsilateral side. After a 6-week healing period, a pair of calcein bone labels were administered. Bone sections from the mandible, maxilla, rib, and femur were obtained. The percent necrosis in the alveolar and basal regions of tooth-supporting bone was assayed by lactate dehydrogenase, and dynamic histomorphometric parameters were quantified and analyzed by use of mixed models. RESULTS All extraction sites healed uneventfully, and no lesions resembling osteonecrosis were detected. The total percent necrosis was limited to less than 1% for all the bone sites examined. The ZA reduced bone remodeling at both surgical sites (extraction sites and mini-implant site) and nonsurgical sites. Although there was a significant (P < .05) increase in bone formation rate at the surgical sites in the untreated group, this increase was not significant (P = .3) in the ZA-treated group. CONCLUSIONS Bone remodeling occurs in ZA-treated animals at surgical sites. ZA dramatically reduced bone turnover, but no exposed lesions resembling osteonecrosis developed at extraction and mini-implant sites after the 4-month drug duration.

Bone Ingrowth and Initial Stability of Titanium and Porous Tantalum Dental Implants: A Pilot Canine Study

Purpose:To investigate if a dental implant system with a midsection covered by 3-dimensionally porous tantalum material would exhibit stability comparable with a traditional threaded titanium alloy implant system and whether bone would grow into the porous section. Methods:Three experimental and 3 control implants were placed in the individual mandibles of 8 dogs. Resonance frequency analysis assessed implant stability at 0, 2, 4, 8, and 12 weeks of healing. Histomorphometric and backscattered scanning electron microscopic analyses examined the presence of bone ingrowth into the experimental implant’s porous section and bone-to-implant contact along the titanium surfaces of both implants. Results:Implant stability did not significantly differ during 0 to 12 weeks of healing. Progressive tissue mineralization developed inside porous sections from weeks 2 to 12. Porous implants exhibited a combination of progressive osseointegration along their titanium surfaces and bone ingrowth inside their porous tantalum sections. Conclusions:Cortical and apical implant threads, combined with the porous section, were able to stabilize the experimental implant to the same degree as the fully threaded control implant.

Zoledronic acid decreases bone formation without causing osteocyte death in mice

Bisphosphonates have been associated with osteonecrosis of the jaw. The purpose of this study was to examine the effect of a potent bisphosphonate, zoledronic acid (ZA) on osteocyte viability and bone formation. Ten experimental C57BL/6 mice were administered ZA (0.1 mg/kg-i.p.) weekly for 9 weeks while four control mice did not receive the drug. A pair of calcein (30 mg/kg) labels was administered 10 and 3 days prior to sacrifice of the 34-week-old mice. Fresh mandibular and femoral sections were obtained to evaluate osteocyte viability using a lactate dehydrogenase (LDH) assay. In addition, sections from the femur, mandible and maxilla were prepared for standard histomorphometry. The operator was blinded for data collection to eliminate bias. Data on necrotic area/total bone area from the LDH sections were collected. In addition, standard histomorphometric variables including bone formation rate were calculated. Mixed models were used to analyse data. The osteocytes were overwhelmingly viable and no necrotic areas were detected in the mandible and femur of both groups. ZA was not directly cytotoxic to the mouse osteocytes. There was suppression in indices of bone formation at all skeletal sites of the ZA group compared to the control group. While ZA administration in mice does not produce necrotic osteocytes, it severely suppresses bone formation. Such reductions can have a profound effect on bone healing.

Relationships of Viscosity With Contact Hardness and Modulus of Bone Matrix Measured by Nanoindentation

Creep is an active form of time-dependent viscoelastic deformation that occurs in bone tissue during daily life. Recent findings indicate bone mineralization, which is involved in determining the elastic and plastic properties of bone matrix, can also contribute in controlling its viscoelastic property. Nanoindentation viscosity was used as a direct measure for the capacity of a material to resist viscous-like flow under loading. The objectives of this study were to examine (1) whether the nanoindentation viscosity obtained using the traditional viscoelastic Voigt model can describe creep response of bone matrix and (2) how the nanoindentation viscosity is related to contact hardness and elastic modulus. The Voigt model accurately described the creep behavior of bone matrix (r(2)>0.96, p<0.001). The nanoindentation viscosity had strong relationships with nanoindentation contact hardness (r(2)=0.94, p<0.001) and modulus (r(2)=0.83, p<0.001) independent of tissue ages of osteonal bone matrix. The strong positive relationships of nanoindentation viscosity with contact hardness and modulus can be interpreted as increases in the mineral portion of bone matrix may limit the interfibril motion of collagen while enhancing the mechanical stability of bone. We suggest that previous nanoindentation results can be reanalyzed to characterize the viscoelastic creep using the Voigt model.

Bone damage associated with orthodontic placement of miniscrew implants in an animal model

INTRODUCTION The purposes of this study were to quantify bone damage associated with insertion of 2 types of miniscrew implants and to relate the amount of bone damage to monocortical plate thickness. METHODS Nondrilling (n = 28) and self-drilling (n = 28) miniscrew implants (6 × 1.6 mm, Dentaurum, Newtown, Pa), and pilot holes (n = 26) were placed bilaterally in the maxillae and the mandibles of 5 adult dogs immediately after death. Bone blocks were cut, bulk stained with 1% basic fuchsin, embedded in methyl methacrylate, sectioned, and mounted. Monocortical plate thickness was measured adjacent to the miniscrew implant insertion site. Damage amounts were quantified at distances of 0 to 0.5 mm (adjacent region) and 0.5 to 1 mm (distant region) from the bone-implant interface. Total fractional damaged area (%), fractional microcracked area (%), and fractional diffuse damaged area (%) were quantified by using standard histomorphometric methods. RESULTS The mean monocortical plate thickness of the specimens from the mandible (2.2 mm) was significantly (P <0.001) greater than that of the maxillary specimens (0.9 mm). In the mandible, the 3 damage parameters were greater with self-drilling miniscrew implants than with nondrilling miniscrew implants; however, there were no differences in the damage parameters in the maxilla. CONCLUSIONS Bone damage accumulation is related to the type of miniscrew implant and the thickness of the bone.

Indentation Modulus of the Alveolar Process in Dogs

One mechanism of bone adaptation is alteration in tissue level material properties. We hypothesized that alteration in the indentation modulus of the alveolar process is an adaptive response to the localized mechanical environment. Forty-eight specimens representing anterior and posterior regions of the maxilla and mandible were obtained from 6 mature male beagle dogs. The indentation properties of the alveolar bone proper and more distant osteonal cortical bone were estimated. The bone types were further divided into 3 regions (coronal, middle, and apical), with 27 indents being made in each region of tooth-supporting bone. There was a significant difference (p < 0.001) in the indentation moduli of the jaws (maxilla/mandible), location (anterior/posterior), and bone type (alveolar bone proper vs. cortical bone). However, statistical interactions exist which preclude the simple interpretation of results. The distribution of relative stiffness provides a better understanding of bone adaptations in the alveolar process.

Effect of estrogen deficiency on regional variation of a viscoelastic tissue property of bone

Estrogen deficiency changes the regional distribution of tissue mineral density leading to alteration of the mechanical properties of bone at the tissue level. Direct measurement of the regional variation of elastic modulus and viscosity, which is the capacity to resist time-dependent viscoelastic deformation, will aid in our understanding of how estrogen deficiency alters bone quality. It was observed that, compared to bone from other anatomical sites, the jaw bone is less sensitive to estrogen deficiency. Thus, the objective of this study was to examine the effect of estrogen deficiency on (1) the regional variations of tissue modulus and viscosity of bone using nanoindentation, and (2) the modulus-viscosity relationships in jaw and vertebral bones for comparison between different anatomical sites. Mandibular and vertebral bone specimens of sham surgery and ovariectomized (OVX) rat groups were subject to nanoindentation in hydration. Indentation modulus and viscosity were measured at relatively new (less mineralized) tissue regions and at the corresponding pre-existing old (more mineralized) tissue regions of mandibular and vertebral bones. In the mandibular bones, significant regional variations of indentation modulus and viscosity were observed (p<0.039) and OVX increased the indentation viscosity. While significant positive correlations were found between indentation modulus and viscosity (p<0.001), the correlation slopes for the mandibular and vertebral bones were significant different (p<0.001). The current results indicated that changes in viscoelastic property and its regional variation should be examined to obtain a better understanding of estrogen deficiency-dependent alteration of bone quality.