Processing and properties of Na-doped porous calcium polyphosphates - Mechanical properties and in vitro degradation characteristics.

Porous calcium polyphosphate (CPP) is being investigated for use as a biodegradable bone substitute and for repair of osteochondral defects. The necessary requirements for these applications, particularly in load-bearing sites, include sufficient strength to withstand functional forces prior to bone ingrowth and substitution of the initial porous CPP template with new bone and cartilage (for osteochondral implants) in a timely and efficacious manner. The present study explored the effects of Na+ doping and processing to form porous structures of both higher strength and faster degradation than previously reported for 'pure' (non-doped) CPP structures of similar geometry. Compressive and tensile strengths were determined before and after 30-day in vitro degradation (PBS, pH 7.1 at 37 °C) and degradation rates assessed. Scanning electron microscopy (SEM), x-ray diffraction (XRD) and solid state nuclear magnetic resonance (31P SS NMR) were used to evaluate 'pure' and Na-doped CPP samples before and after degradation. The results indicated that the different processing protocols required to prepare samples of similar volume % porosity (a 2-step procedure with a Step-1 sintering temperatures equal to 575 °C being used with the Na-doped samples versus a 585 °C Step-1 treatment for 'pure' CPP) resulted in an approximate 1.5- to 2-fold increase in strength (tensile & compressive respectively) and 2-fold increase in degradation rate of Na-doped CPP compared with 'pure' CPP. This difference was attributed to the different Step-1 sintering temperatures used for sample processing.

Engineering of hyaline cartilage with a calcified zone using bone marrow stromal cells.

OBJECTIVE: In healthy joints, a zone of calcified cartilage (ZCC) provides the mechanical integration between articular cartilage and subchondral bone. Recapitulation of this architectural feature should serve to resist the constant shear force from the movement of the joint and prevent the delamination of tissue-engineered cartilage. Previous approaches to create the ZCC at the cartilage-substrate interface have relied on strategic use of exogenous scaffolds and adhesives, which are susceptible to failure by degradation and wear. In contrast, we report a successful scaffold-free engineering of ZCC to integrate tissue-engineered cartilage and a porous biodegradable bone substitute, using sheep bone marrow stromal cells (BMSCs) as the cell source for both cartilaginous zones. DESIGN: BMSCs were predifferentiated to chondrocytes, harvested and then grown on a porous calcium polyphosphate substrate in the presence of triiodothyronine (T3). T3 was withdrawn, and additional predifferentiated chondrocytes were placed on top of the construct and grown for 21 days. RESULTS: This protocol yielded two distinct zones: hyaline cartilage that accumulated proteoglycans and collagen type II, and calcified cartilage adjacent to the substrate that additionally accumulated mineral and collagen type X. Constructs with the calcified interface had comparable compressive strength to native sheep osteochondral tissue and higher interfacial shear strength compared to control without a calcified zone. CONCLUSION: This protocol improves on the existing scaffold-free approaches to cartilage tissue engineering by incorporating a calcified zone. Since this protocol employs no xenogeneic material, it will be appropriate for use in preclinical large-animal studies.

AP-1 DNA binding activity regulates the cartilage tissue remodeling process following cyclic compression in vitro.

Generating bioengineered cartilage yields tissue with physical qualities inferior to that of native tissue. Application of cyclic compression (30 min, 1 kPa, 1 Hz) to cartilage cells (chondrocytes) seeded on calcium polyphosphate substrates significantly increases the accumulation of collagens and proteoglycans by 24 hours, thus improving the tissue generated. The mechanism for this increase is not fully known but seems to follow a remodeling pathway of sequential catabolic and anabolic changes. The initial catabolic event involves increased transcription of matrix metalloproteinase (MMP)-3 and MMP-13 two hours after the end of cyclic compression. As MMP-3 and MMP-13 promoters contain activating protein-1 (AP-1) DNA binding sites, we investigated the effect of inhibiting DNA binding through the use of modified decoy oligodeoxynucleotides (ODN). Mechanical stimulation in the presence of the ODN blocked AP-1 DNA binding as detected by electrophoretic mobility shift assay and prevented the increased transcription of MMP-3 and MMP-13. As well the increased accumulation of collagens and proteoglycans by 24 hours in mechanically stimulated samples was prevented. The data suggests that the mechano-induction of MMP-3 and MMP-13 may be regulated at the AP-1 DNA binding site and that upregulation of these metalloproteases is a necessary component of the matrix remodeling initiated by cyclic compression.

Membrane type-1 matrix metalloproteinase is induced following cyclic compression of in vitro grown bovine chondrocytes.

OBJECTIVE: To determine if membrane type-1 matrix metalloproteinase (MT1-MMP) will respond to cyclic compression of chondrocytes grown in vitro and the regulatory mechanisms underlying this response. METHODS: Cyclic compression (30min, 1kPa, 1Hz) was applied to bovine chondrocytes (6-9-month-old animals) grown on top of a biodegradable substrate within 3 days of initiating culture. Luciferase assays using bovine articular chondrocytes were undertaken to demonstrate the mechanosensitivity of MT1-MMP. Semi-quantitative reverse-transcription polymerase chain reaction (RT-PCR) and western blot analysis were used to establish the time course of gene and protein upregulation in response to cyclic compression. The regulation of MT1-MMP was assessed by electrophoretic mobility shift assays, RT-PCR and western blot analysis. As well, an MT1-MMP decoy oligonucleotide and an extracellular signal-regulated kinase 1/2 (ERK1/2) pharmacological inhibitor were utilized to further characterize MT1-MMP regulation. RESULTS: After cyclic compression, MT1-MMP showed a rapid and transient increase in gene expression. Elevated protein levels were detected within 2h of stimulation which returned to baseline by 6h. During cyclic compression, phosphorylation of the mitogen activated protein kinase ERK1/2 increased significantly. This was followed by increased gene and protein expression of the transcription factor; early growth factor-1 (Egr-1) and Egr-1 binding to the MT1-MMP promoter. Blocking Egr-1 DNA binding with a decoy MT1-MMP oligonucleotide, downregulated MT1-MMP gene expression. The ERK1/2 inhibitor U0126 also reduced Egr-1 DNA binding activity to MT1-MMP promoter sequences and subsequent transcription of MT1-MMP. CONCLUSIONS: These data suggest that cyclic compression of chondrocytes in vitro upregulates MT1-MMP via ERK1/2 dependent activation of Egr-1 binding. Delineation of the regulatory pathways activated by mechanical stimulation will further our understating of the mechanisms influencing tissue remodeling.

Formation of biphasic constructs containing cartilage with a calcified zone interface.

The zone of calcified cartilage is the mineralized region of articular cartilage that anchors the hyaline cartilage to the subchondral bone and serves to disperse mechanical forces across this interface. In an attempt to mimic this zonal organization, we have developed the methodology to form biphasic constructs composed of cartilaginous tissue anchored to the top surface of a bone substitute (porous calcium polyphosphate, CPP) with a calcified interface. To accomplish this, chondrocytes were selectively isolated from the deep zone of bovine articular cartilage, placed on top of the CPP substrate, and grown in the presence of beta-glycerophosphate (10 mM, beta-GP). By 8 weeks, cartilage tissue had formed with two zones: a calcified region adjacent to the CPP substrate and a hyaline-like zone above. Little or no mineralization occurred in the absence of beta-GP. The mineral that formed in vitro was identified as hydroxyapatite, similar in composition and crystal size to that found in vivo. The tissue stiffness was seven times greater, and the interfacial shear properties at the cartilage-CPP interface were at least two times greater in the presence of this mineralized zone within the in vitro-formed cartilage than in tissue lacking a mineral zone. In conclusion, developing a biphasic construct with a calcified zone at the tissue-biomaterial interface resulted in significantly better cartilage load-bearing (compressive) properties and interfacial shear strength, emphasizing the importance of the presence of a mineralized zone in bioengineered cartilage. Because failure due to shear occurred at the cartilage-CPP interface instead of the tidemark, as occurs with osteochondral tissue, further study is required to optimize this system so that it more closely mimics the native tissue.

Osteochondral defect repair using a novel tissue engineering approach: sheep model study.

Porous calcium polyphosphate (CPP) constructs of desired density were formed by sintering CPP powders. Articular cartilage was formed on these constructs in cell culture over an 8-week period with the resulting cartilage layer forming on the CPP surface and within the near surface pores thereby mechanically anchoring the cartilage to the CPP. The biphasic constructs so formed were implanted in sheep femoral condyle sites and left for short-term periods (3 to 4 months) or longer periods (9 months). Implant fixation within the condyle sites was achieved through bone ingrowth into the inferior CPP pores. The properties and characteristics of the as-in vitro-formed, short- and long-term implanted tissues were compared. The results indicated that such implants might be useful for repair of small subchondral defects.

Substrate porosity enhances chondrocyte attachment, spreading, and cartilage tissue formation in vitro.

Tissue engineering is being explored as a new approach to treat damaged cartilage. As the biomaterial used may influence tissue formation, the effects of substrate geometry on chondrocyte behavior in vitro were examined. Articular chondrocytes were isolated and cultured on the surface of smooth, rough, porous-coated, and fully porous Ti-6Al-4V substrates. The percentage of chondrocytes that attached to each substrate at 24 h was determined. After 24 and 72 h, chondrocytes were visualized by scanning electron microscopy and cell areas were measured. Collagen and proteoglycan accumulation within the first 24 h was determined by incorporation with [3H]-proline and [35S]-SO4, respectively. Chondrocyte attachment as well as matrix accumulation was enhanced as substrate surface area increased. Cell areas on the fully porous substrate were over four times greater than on any other substrate by 72 h in culture. After 8 weeks in culture, a continuous layer of cartilaginous tissue formed only on the surface of the fully porous substrate. This suggests that fully porous Ti-6Al-4V substrates provide the conditions that favor cartilage tissue formation by influencing cell attachment and extent of cell spreading. Understanding how substrate porosity influences chondrocyte behavior may help identify methods to further enhance cartilage tissue formation in vitro.

Cyclic compressive mechanical stimulation induces sequential catabolic and anabolic gene changes in chondrocytes resulting in increased extracellular matrix accumulation.

Overcoming the limited ability of articular cartilage to self-repair may be possible through tissue engineering. However, bioengineered cartilage formed using current methods does not match the physical properties of native cartilage. In previous studies we demonstrated that mechanical stimulation improved cartilage tissue formation. This study examines the mechanisms by which this occurs. Application of uniaxial, cyclic compression (1 kPa, 1 Hz, 30 min) significantly increased matrix metalloprotease (MMP)-3 and MMP-13 gene expression at 2 h compared to unstimulated cells. These returned to constitutive levels by 6 h. Increased MMP-13 protein levels, both pro- and active forms, were detected at 6 h and these decreased by 24 h. This was associated with tissue degradation as more proteoglycans and collagen had been released into the culture media at 6 h when compared to the unstimulated cells. This catabolic change was followed by a significant increase in type II collagen and aggrecan gene expression at 12 h post-stimulation and increased synthesis and accumulation of these matrix molecules at 24 h. Mechanical stimulation activated the MAP kinase pathway as there was increased phosphorylation of ERK1/2 and JNK as well as increased AP-1 binding. Mechanical stimulation in the presence of the JNK inhibitor, SP600125, blocked AP-1 binding preventing the increased gene expression of MMP-3 and -13 at 2 h and type II collagen and aggrecan at 12 h as well as the increased matrix synthesis and accumulation. Given the sequence of changes, cyclic compressive loading appears to initiate a remodelling effect involving MAPK and AP-1 signalling resulting in improved in vitro formation of cartilage.

A single application of cyclic loading can accelerate matrix deposition and enhance the properties of tissue-engineered cartilage.

OBJECTIVE: Mechanical stimulation is a widely used method to enhance the formation and properties of tissue-engineered cartilage. While studies have evaluated the responsiveness of chondrocytes to mechanical stimuli, little is known about how much stimulation is actually required. Thus, the purpose of this study was to investigate the effect of a single application of cyclic loading to chondrocytes on the formation and properties of in vitro-formed tissue. DESIGN: Isolated bovine articular chondrocytes were seeded on ceramic substrates in 3D culture and subjected to a single application of compressive cyclic loading at 1, 8 or 15 days after seeding. Once the time at which the chondrocytes were most sensitive to mechanical loading was determined, the effect of a single application on the synthesis and accumulation of matrix molecules as well as the mechanical properties of the in vitro-formed cartilage tissue was evaluated. RESULTS: Chondrocytes were more responsive to cyclic loading applied early in culture. Cyclic forces applied 24 h after the cultures were established increased collagen and proteoglycan syntheses (48 +/- 11% and 49 +/- 11%, respectively). This single application of cyclic loading also increased the accumulation of collagen (stimulated: 207 +/- 20 microg, control: 173 +/- 9 microg) and proteoglycans (stimulated: 302 +/- 24 microg, control: 270 +/- 14 microg) as well as improved the mechanical properties of the in vitro-formed tissue (twofold increase in equilibrium stress and modulus) determined 4 weeks after the applied stimulus. CONCLUSIONS: A single application of cyclic loading to chondrocytes early in culture increased matrix accumulation and enhanced the mechanical properties of the in vitro-formed tissue. This suggests that mechanical forces do not have to be applied intermittently over long periods of time to accelerate in vitro tissue formation.

A targeted review of study outcomes with short (< or = 7 mm) endosseous dental implants placed in partially edentulous patients.

BACKGROUND: Generally, threaded root-form endosseous dental implants are thought to perform poorly in short lengths (i.e., < 10 mm). However, whether modifications in implant surface geometry will improve performance of short threaded implants is less clear. METHODS: The relationship between dental implant failure rates and their surface geometry, length, and location (maxilla versus mandible) was explored in the published literature. Using a MEDLINE search (1985 through 2001), studies were sought with the following criteria: 1) data suitable to calculate failure rates of implant lengths < or = 7 mm versus > 7 mm; 2) data separable into maxillary versus mandibular results; 3) criteria for "failure" clearly defined; and 4) minimal functional period of 2 years. RESULTS: Twelve papers were identified as follows: eight with machined threaded implants, two with acid-treated threaded implants, and two with sintered porous-surfaced press-fit implants. The following results were found: 1) machined surface implants experienced greater failure rates than textured surface implants; 2) with the exception of sintered porous-surfaced implants, 7 mm long dental implants appear to have higher failure rates than those > 7 mm length; and 3) with textured surface implants, higher failure rates were more likely in the maxilla than in the mandible, but with machined surface implants there were no differences in failure rates between maxilla and mandible. CONCLUSIONS: Dental implant surface geometry is a major determinant in how well these implants perform in short lengths, defined here as lengths of < or = 7 mm. While threaded implants show higher failure rates in short versus longer lengths, sintered porous-surfaced implants perform well in the defined "short" lengths. More studies are needed to better assess the performance of short, acid-washed threaded implants.

The effect of sol-gel-formed calcium phosphate coatings on bone ingrowth and osteoconductivity of porous-surfaced Ti alloy implants.

Ti-6Al-4V implants formed with a sintered porous surface for implant fixation by bone ingrowth were prepared with or without the addition of a thin surface layer of calcium phosphate (Ca-P) formed using a sol-gel coating technique over the porous surface. The implants were placed transversely across the tibiae of 17 rabbits. Implanted sites were allowed to heal for 2 weeks, after which specimens were retrieved for morphometric assessment using backscattered scanning electron microscopy and quantitative image analysis. Bone formation along the porous-structured implant surface, was measured in relation to the medial and lateral cortices as an indication of implant surface osteoconductivity. The Absolute Contact Length measurements of endosteal bone growth along the porous-surfaced zone were greater with the Ca-P-coated implants compared to the non-Ca-P-coated implants. The Ca-P-coated implants also displayed a trend towards a significant increase in the area of bone ingrowth (Bone Ingrowth Fraction). Finally, there was significantly greater bone-to-implant contact within the sinter neck regions of the Ca-P-coated implants.

Porous calcium polyphosphate scaffolds for bone substitute applications in vivo studies.

Porous rods (6 mm in length and 4 mm in diameter) of calcium polyphosphate (CPP) made by gravity sintering of particles in the size ranges of 45-105, 105-150. and 150-250 microm and with initial volume percent porosity in the range of 35-45% were implanted in the distal femur of New Zealand white rabbits. In an initial experiment, four rabbits implanted with rods made from coarse particles (150-250 microm) were sacrificed at each of the following time points: 2 days, 2 weeks, 6 weeks and 12 weeks. In a subsequent experiment, 10 rabbits were implanted with rods made by sintering 45-105 microm particles and another 10 were made by using particles of 105-150 microm. These rabbits were sacrificed at 6 weeks (five rabbits) and 1 year (five rabbits). No adverse reaction was found histologically at any time point in either experiment. These experiments show that CPP macroporous rods can support bone ingrowth and that between 12 weeks and 1 year, the amount of bones formed is equivalent to the natural bone volume found at similar sites. The degradation of the CPP material is inversely proportional to the original particle size and is rapid initially (within the first 6 weeks) and slows down thereafter. In conclusion, this material seems to promote rapid bone ingrowth and can be tailored to degrade at a given rate in vivo to some degree through appropriate selection of the starting particle size.

Managing the posterior mandible of partially edentulous patients with short, porous-surfaced dental implants: early data from a clinical trial.

Forty-eight Endopore dental implants were placed in the posterior mandibles of 24 partially edentulous patients. Seventeen of these implants replaced premolar teeth, while 31 replaced molars. Only 7-mm and 9-mm implants were used, and the majority of prosthetic restorations (83%) were single crowns. After a mean functional time of 32.6 months (range, 8.2 to 50.3 months), the implant survival rate was 100% and assessment of available radiographic data showed minimal to no crestal bone loss.

A prospective human clinical trial of Endopore dental implants in restoring the partially edentulous maxilla using fixed prostheses.

This is the first report of a group of 50 partially edentulous patients who received a total of 151 Endopore dental implants in the maxilla. A mean implant length of 8.7 mm was used, and 76.8% of implants were placed in the posterior maxilla. At re-entry, all implants appeared to be osseointegrated and were used to support fixed prostheses. Approximately half of the crowns (57%) in these prostheses were splinted to one another, while the remainder (43%) were not. At the time of this report, the mean functional time was 34.6 months and the cumulative survival rate was 97.3% (4 implants had failed). Analysis of carefully standardized sequential radiographs indicated no significant changes in mean crestal bone levels between baseline and any of the examination times (after 6 months, 1 year, and 2 years in function). There were no detectable correlations between crestal bone loss and the factors implant length (7, 9, or 12 mm); implant diameter (3.5, 4.1, or 5.0 mm); implant position anteriorly or posteriorly in the maxilla; or whether or not the implant-supported crowns were splinted.

Effect of material geometry on cartilagenous tissue formation in vitro.

The effect of material geometry, as defined by average pore size, on chondrocyte phenotype and cartilagenous tissue formation in vitro was examined. Bovine articular chondrocytes were plated on porous titanium alloy (Ti6Al4V) discs of different average pore sizes (13, 43, and 68 microm) and grown in culture for 4 weeks. Chondrocyte phenotype was maintained as indicated by the synthesis of large proteoglycans (Kav +/- SD: 13 microm = 0.28 +/- 0.01; 43 microm = 0.29 +/- 0.01; 68 microm = 0.27 +/- 0.02) and type II collagen. Light microscopical examination of histological sections of the composites showed that cartilagenous tissue had formed on all discs. The cartilagenous tissue on the discs of the smallest average pore size (13 microm) was significantly thicker than the tissue on the discs of larger average pore sizes and also had greater amounts of proteoglycan [mean glycosaminoglycan content +/- SD microg/disc): 13 microm = 246.9 +/- 7.8; 43 microm = 190.4 +/- 10.2; 68 microm = 156.6 +/- 25.8, p = 0.002] and DNA [mean DNA content +/- SD microg/disc): 13 microm = 12.5 +/- 0.6; 43 microm = 8.3 +/- 0.2; 68 microm = 9.3 +/- 0.9, p = 0.0008]. However, the amount of proteoglycan accumulated per cell was similar in the tissues generated on the discs of different average pore sizes. In contrast, the amount of collagen in the cartilagenous tissues showed no significant differences between the different pore sizes, but the amount of collagen accumulated per cell was less in the tissue formed on the smallest pore size disc (13 microm) as compared with the tissue formed on the discs of the larger pore sizes [mean hydroxyproline content/DNA (microg/microg) +/- SD: 13 microm = 1.56 +/- 0.2; 43 microm = 2.19 +/- 0.2; 68 microm = 2.3 +/- 0.3]. These results suggest that material geometry, as defined by pore size, can affect the amount and composition of the cartilagenous tissue that forms.

Mechanical regulation of localized and appositional bone formation around bone-interfacing implants.

The local mechanical environment around bone-interfacing implants determines, in large part, whether bone formation leading to functional osseointegration will occur. Previous attempts to relate local peri-implant tissue strains to tissue formation have not accounted for implant surface geometry, which has been shown to influence early tissue healing in vivo. Furthermore, the process by which mechanically regulated peri-implant bone formation occurs has not been considered previously. In the current study, we used a unit cell approach and the finite element method to predict the local tissue strains around porous-surfaced and plasma-sprayed implants, and compared the predictions to patterns of bone formation reported in earlier in vivo experiments. Based on the finite element predictions, we determined that appositional bone formation occurred when the magnitudes of the strain components at the tissue-host bone interface were <8%. Localized, de novo bone formation occurred when the distortional tissue strains were less than approximately 3%. Based on these threshold tissue strains, we propose a mechanoregulatory model to relate local tissue strains to the process of peri-implant bone formation. The mechanoregulatory model is novel in that it predicts both appositional and localized bone formation and its predictions are dependent on implant surface geometry. The model provides initial criteria with which the osseointegration potential of bone-interfacing implants may be evaluated, particularly under conditions of immediate or early loading.

Fabrication of porous calcium polyphosphate implants by solid freeform fabrication: a study of processing parameters and in vitro degradation characteristics.

Solid freeform fabrication (SFF) involves the creation of a solid 3-D object of desired shape by successively adding raw materials in particles or layers. Its use in fabricating surgical implants is being explored. The objective of this study was to determine the feasibility of using SFF to build porous parts of calcium polyphosphate (CPP), a linear condensed phosphate that has been suggested as a material for forming bioresorbable skeletal replacement implants. CPP powders (<25 microm in particle size) were added to an UV curable monomer (SOMOS 6110) at a solids loading of 25 vol %, with the addition of a commercial dispersant to prevent particle agglomeration and settling. Viscosity and cure depth measurements were performed to insure that CPP suspension met the requirements deemed necessary for use in SFF. The CPP suspension was bulk cured and sintered in molds in order to assess binder removal and sintering parameters. Using a three-point bend test, the ultimate bending strength and energy-to-fracture of sintered CPP samples simulating parts to be formed by this strategy were characterized. In vitro degradation studies using 0.1M of tris-buffered solution were performed to assess the effect of aging on mechanical properties of the samples as a function of the processing route and resulting structures. The polymer binder successfully was removed from the cured ceramic suspension by developing a procedure that combined slow heating rates with low temperature dwells. Sintering CPP at 585 degrees C for 1 h produced amorphous material samples with average porosity of 27.7 +/- 2.0%. Sintering CPP at 600 degrees C for 1 h produced a crystalline material with samples having an average porosity of 22.9 +/- 1.3%. Crystalline CPP was found to exhibit superior bend strength and toughness compared with amorphous CPP. Both samples experienced a decline in mechanical properties during in vitro degradation; however, the effects were more pronounced with the amorphous CPP samples. Amorphous CPP was found to degrade four times faster than crystalline CPP, as shown by high levels of phosphate present in the degradation solution and a noticeable increase in the porosity of the samples. Crystalline CPP was more resistant to attack as dissolution was limited to surface features of the sintered particles.

Differences in osseointegration rate due to implant surface geometry can be explained by local tissue strains.

Experimental evidence indicates that the surface geometry of bone-interfacing implants influences the nature and rate of tissues formed around implants. In a previously reported animal model study, we showed that non-functional, press-fitted porous-surfaced implants placed in rabbit femoral condyle sites osseointegrated more rapidly than plasma-sprayed implants. We hypothesized that the accelerated osseointegration observed with the porous-surfaced design was the result of this design providing a local mechanical environment that was more favourable for bone formation. In the present study, we tested this hypothesis using finite element analysis and homogenization methods to predict the local strains in the pre-mineralized tissues formed around porous-surfaced and plasma-sprayed implants. We found that, for loading perpendicular to the implant interface, the porous surface structure provided a large region that experienced low distortional and volumetric strains, whereas the plasma-sprayed implant provided little local strain protection to the healing tissue. The strain protected region, which was within the pores of the sintered porous surface layer. corresponded to the region where the difference in the amount of mineralization between the two implant designs was the greatest. Low distortional and volumetric strains are believed to favour osteogenesis, and therefore the model results provide initial support for the hypothesis that the porous-surfaced geometry provides a local mechanical environment that favours more rapid bone formation in certain situations.

Porous calcium polyphosphate scaffolds for bone substitute applications -- in vitro characterization.

Porous structures were formed by gravity sintering calcium polyphosphate (CPP) particles of either 106-150 or 150-250 microm size to form samples with 30-45 vol% porosity with pore sizes in the range of 100 microm (40-140 microm). Tensile strength of the samples assessed by diametral compression testing indicated relatively high values for porous ceramics with a maximum strength of 24.1 MPa for samples made using the finer particles (106-150 microm). X-ray diffraction studies of the sintered samples indicated the formation of beta-CPP from the starting amorphous powders. In vitro aging in 0.1 M tris-buffered solution (pH 7.4) or 0.05 M potassium hydrogen phthalate buffered solution (pH 4.0) at 37 degreesC for periods up to 30d indicated an initial rapid loss of strength and P elution by 1 d followed by a more gradual continuing strength and P loss resulting in strengths at 30d equal to about one-third the initial value. The observed structures, strengths and in vitro degradation characteristics of the porous CPP samples suggested their potential usefulness as bone substitute materials pending subsequent in vivo behaviour assessment.

Fracture resistance of dentin-composite interfaces using different adhesive resin layers.

OBJECTIVES: The objective of this study was to study the effect of different adhesive layers on the interfacial fracture toughness (K(ICi)) of dentin-resin composite interfaces. METHODS: Miniature short-rod fracture toughness specimens containing a chevron-shaped dentin-bonded interface along their midplane were used for testing. Each interface zone contained a thinned (one coat of unfilled adhesive resin, air-thinned), one-layer (one coat of unfilled adhesive resin, brush-thinned), two-layer (two coats of unfilled adhesive resin, brush-thinned), 10% filled or 45% filled adhesive resin layer. After storage in distilled water at 37 degrees C for 24h, the fracture toughness test specimens were loaded in tension at an extension rate of 0.5mm/min until fracture and the K(ICi) were determined. The results were analysed by ANOVA and Fisher's LSD test (p<0.05). Scanning electron microscopy was used to examine representative fracture surfaces. RESULTS: There were no significant differences in mean K(ICi) among the different unfilled adhesive resin layer groups. SEM examination of these specimens showed that fracture generally occurred between the resin-infiltrated layer and adhesive resin layer during interfacial fracture toughness testing. The mean K(ICi) for the 10% filled groups was not significantly different from the unfilled groups. The 45% filled group, however, demonstrated the highest K(ICi) values, the thickest adhesive resin layer under SEM examination, and a fracture path through the adhesive resin layer. CONCLUSIONS: There were no significant differences in K(ICi) when the unfilled adhesive resin was used despite different application methods. The 45% filled adhesive resin improved the properties of the dentin-composite interface with respect to both interfacial fracture resistance and dentinal seal after fracture.