Nanotechnology approaches to improve dental implants.

The requirements imposed by the enormous scale and overall complexity of designing new implants or complete organ regeneration are well beyond the reach of present technology in many dimensions, including nanoscale, as researchers do not yet have the basic knowledge required to achieve these goals. The need for a synthetic implant to address multiple physical and biologic factors imposes tremendous constraints on the choice of suitable materials. There is a strong belief that nanoscale materials will produce a new generation of implant materials with high efficiency, low cost, and high volume. The nanoscale in materials processing is truly a new frontier. Metallic dental implants have been used successfully for decades, but they have serious shortcomings related to their osseointegration and the fact that their mechanical properties do not match those of bone. This paper reviews recent advances in the fabrication of novel coatings and nanopatterning of dental implants. It also provides a general summary of the state of the art in dental implant science and describes possible advantages of nanotechnology for future improvements. The ultimate goal is to produce materials and therapies that will bring state-of-the-art technology to the bedside and improve quality of life and current standards of care.

Effect of educational debt on career and quality of life among academic surgeons.

OBJECTIVE: The purpose of this study was to assess indebtedness among academic surgeons and its repercussions on personal finances, quality of life, and career choices. SUMMARY BACKGROUND DATA: The influence of educational debt on academic surgical career choices and quality of life is unknown. We hypothesized that educational debt affects professional choices and quality of life. METHODS: A web-based survey was designed to assess respondent demographics, educational and consumer indebtedness, and the influence of educational debt on career choices and quality of life among academic surgeons. RESULTS: Five hundred fifty-five surgeons responded (20.6% response rate). Two hundred seventy-four (66%) respondents finished postgraduate training with educational debt, 139 (34%) reported no debt, and 142 (26%) did not respond. Among those with educational debt, mean educational debt was $90,801 and mean noneducational consumer debt was $32,319. Individuals without educational debt reported a mean of $15,104 of noneducational consumer debt (P < 0.001) and had higher mean salaries (P = 0.017) versus those with educational debt. Eighty-seven percent of respondents with educational debt would make the same career choice again. However, 35% acknowledged it placed a strain on their relationship with their significant other, 48% felt it influenced the type of living accommodations they could afford, and 29% reported it forced their significant other to work. Alarmingly, 32% of academic surgeons would not recommend their career choice to their children or medical students. CONCLUSIONS: Many academic surgeons reported that their educational debt affected their academic productivity, career choices, and quality of life. Consequently, efforts to mitigate the impact of educational debt on academic surgeons are required to ensure medical students continue to pursue academic surgical careers.

Formation of hematopoietic territories and bone by transplanted human bone marrow stromal cells requires a critical cell density.

OBJECTIVE: Bone marrow stromal cells (BMSCs) include multipotent cells with the ability to form mature bone organs upon in vivo transplantation. Hematopoiesis in these bone organs has been ascribed to the action of skeletal stem cells, which are capable of differentiating towards bone and hematopoiesis-supporting stroma. Yet, the creation of hematopoietic territories may be in part a natural consequence of the formation of a sufficiently mature and large bone microenvironment. Here, we describe, for the first time, a relationship between BMSC numbers and the extent of bone/hematopoiesis formation in heterotopic transplants. METHODS: Human BMSCs were transplanted along with hydroxyapatite/tricalcium phosphate, utilizing a spectrum of dosages, into immunotolerant mice; the transplants were followed for up to 29 months. RESULTS: The extent of bone and hematopoiesis formation increased with increasing BMSC numbers; however, the relationship was sigmoid in character, and a threshold number of BMSCs was necessary for extensive bone formation or any hematopoiesis. Hematopoiesis only occurred in conjunction with extensive bone formation, and no hematopoiesis occurred where bone formation was poor. Consistent with our earlier studies of long-term BMSC transplantation, the transplants underwent a change in bone morphology but not bone content after 8 weeks. CONCLUSION: Our results have provided evidence that the formation of both hematopoiesis and a mature bone organ is as much a consequence of a sufficiently high local density of bone marrow stromal cells as it is the product of skeletal stem cell action.

In vivo formation of bone and haematopoietic territories by transplanted human bone marrow stromal cells generated in medium with and without osteogenic supplements.

Autologous transplantation of human bone marrow stromal cells (BMSCs) has been successfully used for bone reconstruction. However, in order to advance this approach into the mainstream of bone tissue engineering, the conditions for BMSC cultivation and transplantation must be optimized. In a recent report, cultivation with dexamethasone (Dex) significantly increased bone formation by human BMSCs in vivo. Based on this important conclusion, we analysed the data accumulated by our laboratory, where human BMSCs have been routinely generated using media both with and without a combination of two osteogenic supplements: Dex at 10(-8) m and ascorbic acid phosphate (AscP) at 10(-4) m. Our data demonstrate that for 22/24 donors, BMSC strains propagated with and without Dex/AscP formed similar amounts of bone in vivo. Thus, human BMSCs do not appear to need to be induced to osteogenic differentiation ex vivo prior to transplantation. Similarly, for 12/14 donors, BMSC strains cultured with and without Dex/AscP formed haematopoietic territories to a comparable extent. While Dex/AscP did not increase bone formation, they significantly stimulated BMSC in vitro proliferation without affecting the number of BMSC colonies formed by the colony-forming units-fibroblasts. We conclude that for the substantial majority of donors, Dex/AscP have no effect on the ability of BMSCs to form bone and myelosupportive stroma in vivo. However, due to increased BMSC proliferation, the total osteogenic population obtained from a single marrow sample is larger after cultivation with Dex/AscP than without them. Secondary to increased BMSC proliferation, Dex/AscP may stimulate bone formation if BMSCs and/or the transplantation system are less than optimal. Published 2011. This article is a U.S. Government work and is in the public domain in the USA.

Lamellar spacing in cuboid hydroxyapatite scaffolds regulates bone formation by human bone marrow stromal cells.

BACKGROUND: A major goal in bone engineering is the creation of large volume constructs (scaffolds and stem cells) that bear load. The scaffolds must satisfy two competing requirements--they need be sufficiently porous to allow nutrient flow to maintain cell viability, yet sufficiently dense to bear load. We studied the effect of scaffold macroporosity on bone formation and scaffold strength, for bone formed by human bone marrow stromal cells. METHODS: Rigid cubical hydroxyapatite/tricalcium phosphate scaffolds were produced by robo-casting. The ceramic line thickness was held constant, but the distance between adjacent lines was either 50, 100, 200, 500, or 1000 µm. Cultured human bone marrow stromal cells were combined with the scaffolds in vitro; transplants were placed into the subcutis of immunodeficient mice. Transplants were harvested 9, 18, 23, 38, or 50 weeks later. Bone formation and scaffold strength were analyzed using histology and compression testing. RESULTS: Sixty transplants were evaluated. Cortical bone increased with transplant age, and was greatest among 500 µm transplants. In contrast, maximum transplant strength was greatest among 200 µm transplants. CONCLUSIONS: Lamellar spacing within scaffolds regulates the extent of bone formation; 500 µm yields the most new bone, whereas 200 µm yields the strongest transplants.

Enumeration of the colony-forming units-fibroblast from mouse and human bone marrow in normal and pathological conditions.

Bone marrow stromal cell populations, containing a subset of multipotential skeletal stem cells, are increasingly contemplated for use in tissue engineering and stem cell therapy, whereas their involvement in the pathogenetic mechanisms of skeletal disorders is far less recognized. We compared the concentrations of stromal clonogenic cells, colony forming units-fibroblast (CFU-Fs), in norm and pathology. Initially, culture conditions were optimized by demonstrating that fetal bovine serum heat inactivation could significantly repress colony formation. Using non-heat-inactivated fetal bovine serum, the concentration of CFU-Fs (colony-forming efficiency, CFE) ranged from 3.5 +/- 1.0 to 11.5 +/- 4.0 per 1 x 10(5) nucleated cells in five inbred mouse strains. In four transgenic lines with profound bone involvement, CFE was either significantly reduced or increased compared to wild-type littermates. In normal human donors, CFE decreased slightly with age and averaged 52.2 +/- 4.1 for children and 32.3 +/- 3.0 for adults. CFE was significantly altered in patients with several skeletal, metabolic, and hematological disorders: reduced in congenital generalized lipodystrophy, achondroplasia (SADDAN), pseudoachondroplasia, and Paget disease of bone and elevated in alcaptonuria and sickle cell anemia. Our findings indicate that under appropriate culture conditions, CFE values may provide useful insights into bone/bone marrow pathophysiology.

Creation of new bone by the percutaneous injection of human bone marrow stromal cell and HA/TCP suspensions.

BACKGROUND: The in vivo transplantation assay has become a valuable tool for assessing the osteogenic potential of diverse cell populations. It has required that cells are cotransplanted with a matrix into recipient animals using large incisions and extensive dissections. Here, we demonstrate that transplants of an osteogenic cell population, bone marrow stromal cells (BMSCs), are capable of assembling into mature bone organs when injected as suspensions of cells and a particulate matrix. METHODS: Human BMSCs, along with hydroxyapatite/tricalcium phosphate (HA/TCP) particles, were placed either into the dorsal subcutaneous space or onto the calvarium of immunodeficient mice, either via injection or via a wide operative exposure. Transplants were harvested from 7 to 110 weeks later; their histologic and mechanical properties and their cellular origin were analyzed. RESULTS: A total of 43 transplants were evaluated. The extent of new bone and hematopoiesis, the bone's adherence to the underlying mouse calvarium, and the bone elastic modulus and hardness were comparable between the two groups. In situ hybridization confirmed a human origin of the new bone. CONCLUSIONS: Our data indicate that BMSCs and HA/TCP particles, when injected as a suspension, can assemble into mature bone organs, and that this bone has histologic and mechanical properties similar to bone formed in standard transplants delivered through a large incision. These results open the possibility for assessing the osteogenic capacities of cell populations, for modeling bone formation and repair and for treating bone deficits, all in the context of minimal surgical intervention or soft tissue disruption.

Long-term stable canine mandibular augmentation using autologous bone marrow stromal cells and hydroxyapatite/tricalcium phosphate.

Transplants of culture-expanded bone marrow stromal cells (BMSCs) combined with hydroxyapatite/tricalcium phosphate (HA/TCP) scaffolds successfully form cortico-cancellous bone to reconstruct the dog craniofacial skeleton. Yet, these transplants' long-term stability in large animal models has not been evaluated. This study's purpose was the evaluation of long-term BMSC transplant stability when used to augment the mandible. Here, autologous BMSC-HA/TCP transplants were introduced onto the unilateral dog mandible as onlay grafts, while contralateral control mandibles received HA/TCP onlays alone. Quantitative CT (qCT) scans were obtained both early and late after transplantation. Transplants were harvested up to 19 months later for histologic and mechanical analyses. In all dogs, BMSC transplants formed significantly greater amounts of bone over their control counterparts. The new bone formed an extensive union with the underlying mandible. BMSC transplants retained the majority of their initial volume, while control (HA/TCP only) transplants were nearly completely resorbed. By qCT, the extent of newly formed bone could be determined non-invasively. In summary, HA/TCP particles alone undergo a high degree of resorption, while autologous cultured BMSC-HA/TCP transplants provide long-term bony augmentation of the mandible.

Circulating connective tissue precursors: extreme rarity in humans and chondrogenic potential in guinea pigs.

Using a variety of cell separation techniques and cultivation conditions, circulating, adherent, connective tissue, clonogenic cells were found in just 3 donors out of 66, demonstrating that these precursors are extremely rare in postnatal human blood. Contrary to humans, guinea pig blood shows much more reproducible connective tissue colony formation; it was therefore chosen to study the differentiation potential of adherent blood-derived clonogenic cells. Out of 22 single colony-derived strains of various morphologies, only 5 spindle-shaped strains showed extensive proliferative capacity in vitro. None of these strains formed bone upon in vivo transplantation, whereas two strains formed cartilage in high-density pellet cultures in vitro. Both chondrogenic strains included cells expressing aggrecan, whereas nonchondrogenic strains did not. Out of four polyclonal strains studied, one formed both cartilage and abundant bone accompanied by hematopoiesis-supporting stroma. Evidently, there are cells in adult guinea pig blood capable of both extensive proliferation and differentiation toward cartilage: circulating chondrogenic precursors. Although some of these cells lack osteogenic potential and therefore represent committed chondrogenic precursors, others may be multipotential and consequently belong to the family of skeletal stem cells. This is the first demonstration of postnatal circulating chondrogenic precursors, as well as of precursor cells with chondrogenic but not osteogenic potential. Disclosure of potential conflicts of interest is found at the end of this article.

In vivo bone formation by human bone marrow stromal cells: reconstruction of the mouse calvarium and mandible.

Bone marrow stromal cells (BMSCs) contain a subset of multipotent cells with the potential to repair hard-tissue defects. Mouse BMSCs, combined with a collagen carrier, can close critical-sized homologous mouse calvarial defects, but this new bone has a poor union with the adjacent calvarium. When human BMSCs are transplanted for the purpose of engineering new bone, best results can be achieved if the cells are combined with hydroxyapatite/tricalcium phosphate (HA/TCP) particles. Here, we demonstrate that transplantation of cultured human BMSCs in conjunction with HA/TCP particles can be used successfully to close mouse craniofacial bone defects and that removal of the periosteum from the calvarium significantly enhances union with the transplant. Transplants were followed for up to 96 weeks and were found to change in morphology but not bone content after 8 weeks; this constitutes the first description of human BMSCs placed long-term to heal bone defects. New bone formation continued to occur in the oldest transplants, confirmed by tetracycline labeling. Additionally, the elastic modulus of this engineered bone resembled that of the normal mouse calvarium, and our use of atomic force microscopy (AFM)-based nanoindentation offered us the first opportunity to compare these small transplants against equally minute mouse bones. Our results provide insights into the long-term behavior of newly engineered orthotopic bone from human cells and have powerful implications for therapeutic human BMSC transplantation.

Canine cranial reconstruction using autologous bone marrow stromal cells.

Limited-sized transplants of culture-expanded autologous or allogeneic bone marrow stromal cells (BMSCs) form cortico-cancellous bone in rodent models. Initiation of clinical studies using autologous BMSC transplantation requires effective bone formation among sizable transplants in a large animal model as well as noninvasive techniques for evaluating transplant success. Here, we obtained bone marrow from the femurs of six dogs and expanded BMSCs in tissue culture. Autologous BMSC-hydroxyapatite/tricalcium phosphate (HA/TCP) transplants were introduced into critical-sized calvarial defects and contralateral control skull defects received HA/TCP vehicle alone. At intervals ranging from 2 to 20 months, transplants were biopsied or harvested for histological and mechanical analysis. Noninvasive studies, including quantitative computed tomography scans and ultrasound, were simultaneously obtained. In all animals, BMSC-containing transplants formed significantly more bone than their control counterparts. BMSC-associated bone possessed mechanical properties similar to the adjacent normal bone, confirmed by both ultrasound and ex vivo analysis. Evaluation by quantitative computed tomography confirmed that the extent of bone formation demonstrated by histology could be discerned through noninvasive means. These results show that autologous cultured BMSC transplantation is a feasible therapy in clinical-sized bone defects and that such transplants can be assessed noninvasively, suggesting that this technique has potential for use in patients with certain bone defects.

Bone formation in transplants of human bone marrow stromal cells and hydroxyapatite-tricalcium phosphate: prediction with quantitative CT in mice.

PURPOSE: To determine whether quantitative computed tomography (CT) can be used to estimate the extent of new bone formation in hydroxyapatite-tricalcium phosphate (HA-TCP)-based transplants. MATERIALS AND METHODS: Bone-forming transplants were generated by attaching cultured human bone marrow stromal cells to aliquots of HA-TCP particles and were placed in subcutaneous pockets in immunocompromised mice. After 8 weeks, the transplants were individually imaged; each scan included a phantom. Overall bone mineral density (BMD) of each transplant was obtained. Hematoxylin-eosin-stained sections of the same transplants were then examined histologically, which is the reference standard for assessing bone formation. The extent of bone in each transplant was scored on a semiquantitative scale ranging from 0 to 4 by three independent blinded observers; the bone score for each transplant was calculated by averaging the three observer scores. BMD was compared with the histologically determined bone score for each transplant. Statistical evaluations included (a) calculation of empiric receiver operating characteristic curves to determine optimum BMD thresholds and (b) determination of the relationship between BMD and bone score, including derivation of Pearson correlation coefficients. RESULTS: One hundred twenty transplants were evaluated. Average BMD of 600 mg/cm3 K2HPO4 or more was noted in transplants with appreciable bone formation (bone score > or = 3), while average BMD of less than 600 mg/cm3 K2HPO4 was seen in transplants with poor bone formation (bone score < 3) (P <.001). Among transplants with appreciable bone formation, the BMD was proportional to the extent of mineralized matrix present in the new bone. CONCLUSION: Use of quantitative CT offers a practical approach for the noninvasive determination of new bone formation in mineralizing bone marrow stromal cells and HA-TCP transplants.