Developing porosity of poly(propylene glycol-co-fumaric acid) bone graft substitutes and the effect on osteointegration: A preliminary histology study in rats

Abstract -Bioresorbable bone graft substitutes could eliminate disadvantages associated with the use of autografts, allografts and other synthetic materials. We investigated a bioresorbable bone graft substitute made from the unsaturated polyester poly(propylene fumarate) which is crosslinked in the presence of soluble and insoluble calcium filler salts. This compact bone graft substitute material develops porosity in vivo by leaching of the soluble filler salts. In attempt to develop materials whose in vivo porosity can be designed such that implant degradation would occur at a rate that remains supportive of the overall structural integrity of the repairing defect site, we studied the early tissue response upon implantation in a bony defect. Three grout formulations of varying solubilities using slightly soluble hydroxyapatite (HA) and soluble calcium acetate (CA) were evaluated in 3 mm holes made in the anteromedial tibial metaphysis of 200 g Sprague Dawley rats (n = 16 per formulation for a total of 48 animals). Grout formulations cured in situ. Animals from each formulation were sacrificed in groups of 8 at 4 days and 3 weeks postoperatively. Histologic analysis of the healing process revealed improved in vivo osteointegration of bone graft substitutes when a higher loading of calcium acetate was employed. All formulations maintained implant integrity and did not provoke sustained inflammatory responses. This study suggested that the presence of a soluble salt permits in vivo development of porosity of a poly(propylene fumarate) based bone graft substitute material.     

Bioresorbable bone graft substitutes of different osteoconductivities: a histologic evaluation of osteointegration of poly(propylene glycol-co-fumaric acid)-based cement implants in rats

Bioresorbable bone graft substitutes may significantly reduce the disadvantages associated with autografts, allografts and other synthetic materials currently used in bone graft procedures. We investigated the biocompatibility and osteointegration of a bioresorbable bone graft substitute made from the unsaturated polyester poly(propylene-glycol-co-fumaric acid), or simply poly(propylene fumarate), PPF, which is crosslinked in the presence of soluble and insoluble calcium filler salts. Four sets of animals each having three groups of 8 were evaluated by grouting bone graft substitutes of varying compositions into 3-mm holes that were made into the anteromedial tibial metaphysis of rats. Four different formulations varying as to the type of soluble salt filler employed were used: set 1--calcium acetate, set 2--calcium gluconate, set 3--calcium propionate, and set 4--control with hydroxapatite, HA, only. Animals of each of the three sets were sacrificed in groups of 8 at postoperative week 1, 3, and 7. Histologic analysis revealed that in vivo biocompatibility and osteointegration of bone graft substitutes was optimal when calcium acetate was employed as a soluble salt filler. Other formulations demonstrated implant surface erosion and disintegration which was ultimately accompanied by an inflammatory response. This study suggested that PPF-based bone graft substitutes can be designed to provide an osteoconductive pathway by which bone will grow in faster because of its capacity to develop controlled porosities in vivo. Immediate applicability of this bone graft substitute, the porosity of which can be tailored for the reconstruction of defects of varying size and quality of the recipient bed, is to defects caused by surgical debridement of infections, previous surgery, tumor removal, trauma, implant revisions and joint fusion. Clinical implications of the relation between developing porosity, resulting osteoconduction, and bone repair in vivo are discussed.     

Mechanical evaluation of a porous bone graft substitute based on poly(propylene glycol-co-fumaric acid)

A porous, resorbable polymer composite based on poly(propylene glycol-co-fumaric acid) (PPF) was mechanically evaluated in vitro for use as a bone graft substitute and fracture fixative. The test material created a dynamic system capable of initially providing mechanical integrity to bony voids and a degradative mechanism for ingrowth by native bone. The unsaturated polymer, PPF, was crosslinked in the presence of effervescent agents to yield a porous microstructure upon curing. An in vitro degradation study first assessed the temporal mechanical properties of the test material. This research was followed by an ex vivo study using a long-bone osteotomy model to characterize the mechanics of fixation. Results showed the initial compressive strength of the cross-linked PPF system was comparable to cancellous bone. The rate of strength loss was commensurate with the predicted mechanical recovery of healing bone with analogous results in a composite that comprised also 25% (by weight) autograft. Mechanical testing in the long-bone model demonstrated that PPF-based bone-graft substitute increased the flexural strength of K-wire stabilized osteotomies. These results suggest that this type of bone graft substitute may have clinical utility in the stabilization of complex tubular bone fractures.     

Augmentation of osteoinduction with a biodegradable poly(propylene glycol-co-fumaric acid) bone graft extender. A histologic and histomorphometric study in rats

We investigated the feasibility of enhancing the regeneration of skeletal tissues by augmenting bone grafts with a composite biodegradable bone graft extender material based on the polymer poly(propylene fumarate), PPF. The material was mixed with autograft and allograft and placed directly into a cylindrical metaphyseal defect made in the rat tibia. These formulations were compared to defects without any graft material, autografts, allografts and PPF alone. Nine animals were included in each group. Animals were sacrificed at 1 and 4 weeks postoperatively. Implantation sites were then evaluated using histologic and histomorphometric methods. Results of this study showed that defects did not heal in sham operated animals. In the experimental groups, there was early new woven bone formation in the autograft group with near complete healing of the defect at four weeks. When PPF was used alone, gradual ingrowth of new bone was seen. Mixing of the PPF bone graft extender with either allograft or autograft material resulted in enhancement of new bone formation with both allo- and autograft. However, significantly more new bone formation than in the autograft group was only seen when the PPF bone graft extender was mixed with fresh autograft. Histomorphometry corroborated these findings. Results of this study suggest that a PPF-based material may be used to increase the volume of smaller amounts of bone grafts supporting the concept of "bone graft extenders" by application of engineered biodegradable porous scaffolds.     

Osteoconductivity of an injectable and bioresorbable poly(propylene glycol-co-fumaric acid) bone cement

We have investigated an injectable form of a resorbable bone cement based on in situ crosslinking of the unsaturated polyester, poly(propylene glycol-co-fumaric acid) (PPF). This material, filled with calcium gluconate/hydroxyapatite (CG/HA), cures to a hard cement degradable by hydrolysis. The purpose of this study was to evaluate the osteoconductive properties of this injectable cement. The cement was used as an adjunct to fixation with an intramedullary rod in the rat femoral osteotomy model. Ingrowth of new bone into the cement was examined in vivo. Negative and positive controls with rigid and loose internal fixation were included for comparison. Animals were evaluated histologically and histomorphometrically at 4 weeks postoperatively. Results of this study showed osteoblastic activity and new bone formation at the interface between the femoral bone and the cement in the experimental group. However, there was little bone remodeling at the endosteal surface in positive and negative controls. Histologic evaluation of the cement revealed the formation of cavitations, which likely resulted from leaching of the highly soluble calcium gluconate portion of the filler from the cement. These cavitations were sites of ingrowth of vascular and bony tissues. Intimate contact between the bone cement and the endosteal surface of the cortex was found. Quantitative histomorphometric analysis corroborated these observations. Findings of this study demonstrated the osteoconductivity of this type of injectable PPF-based bone cement.     

An injectable porous poly(propylene glycol-co-fumaric acid) bone repair material as an adjunct for intramedullary fixation

A bioresorbable bone repair material made from the unsaturated polyester poly(propylene glycol-co-fumaric acid), PPF, was investigated for its potential to act as an adjunct to alleviate the disadvantages associated with wire fixation. The PPF bone repair material is an injectable system that can be delivered to the intramedullary site and crosslinked in the presence of a hydroxylapatite filler and effervescent agents. To test the feasibility of using a bioabsorbable material as an adjunct in fracture fixation, femoral osteotomies were created in two groups of 10 Sprague-Dawley rats. Osteotomies were fixed with a threaded Kirschner wire or stabilized with a Kirshner wire augmented with the PPF bone repair material. The quantity of new bone across the osteotomy site was assessed at 4 weeks postoperatively. Histologic analysis of the healing process revealed enhanced osteoconduction across the osteotomy with the PPF bone repair material. These findings were corroborated by histomorphometric analysis of new bone formation. These findings imply suitability of the PPF bone repair material to act as an adjunct to wire fixation, such as techniques used in hand surgery.     

Effect of porosity on the osteointegration and bone ingrowth of a weight-bearing nickel-titanium bone graft substitute

Porous nickel-titanium (NiTi) alloy is a promising new material for a bone graft substitute with good strength properties and an elastic modulus closer to that of bone than any other metallic material. The purpose of this study was to evaluate the effect of porosity on the osteointegration of NiTi implants in rat bone. The porosities (average void volume) and the mean pore size (MPS) were 66.1% and 259+/-30 microm (group 1, n=14), 59.2% and 272+/-17 microm (group 2, n=4) and 46.6% and 505+/-136 microm (group 3, n=15), respectively. The implants were implanted in the distal femoral metaphysis of the rats for 30 weeks. The proportional bone-implant contact was best in group 1 (51%) without a significant difference compared to group 3 (39%). Group 2 had lower contact values (29%) than group 1 (p=0.038). Fibrotic tissue within the porous implant was found more often in group 1 than in group 3 (p=0.021), in which 12 samples out of 15 showed no signs of fibrosis. In conclusion, porosity of 66.1% (MPS 259+/-30 microm) showed best bone contact (51%) of the porosities tested here. However, the porosity of 46.6% (MPS 505+/-136 microm) with bone contact of 39% was not significantly inferior in this respect and showed lower incidence of fibrosis within the porous implant.     

Quantitative measures of osteoinductivity of a porous poly(propylene fumarate) bone graft extender

Bioresorbable bone graft substitutes could alleviate disadvantages associated with the use of autografts, allografts, and other synthetic materials. However, little is known about the minimum autograft/extender ratio for a given material at which a sufficient osteoinductive effect is still seen. Therefore, we investigated a bioresorbable bone graft substitute made from the unsaturated polyester poly(propylene fumarate), PPF, at various mixing ratios with autograft. The bone graft extender is cross-linked in the presence of a hydroxylapatite filler and effervescent foaming agents citric acid and sodium bicarbonate. The porous bone graft extender material develops porosity in vivo by generating carbon dioxide during the effervescent reaction, resulting in foam formation and expansion with respective pore sizes of 50 to 1000 microm. In an attempt to determine how much cancellous autograft bone could be extended with the poly(propylene fumarate) material and at which ratio the autograft/extender combination remained supportive of the overall structural integrity of the repairing defect site, we studied the amount of new bone formation on implantation of the materials in 3-mm holes made in the anteromedial tibial metaphysis of Sprague-Dawley rats. The extender formulation was analyzed at high autograft/extender (75% autograft/25% extender) and low autograft/extender (25% autograft/75% extender) mixing ratios and compared with negative (extender alone) and positive (autograft alone) controls. Animals from each of the formulations were killed in groups of eight at 6 weeks postoperatively. Hence, a total of 32 animals were included in this study. Histologic analysis of the healing process revealed enhanced in vivo osteoinduction with the bone graft extender regardless of the autograft loading. Histomorphometry did not show any statistically significant difference between the high and low autograft/extender ratios. All formulations maintained implant integrity and did not provoke sustained inflammatory responses. This study suggested that the presence of even a small amount of autograft within the polymer-based bone graft extender results in significant enhancement of osteoinduction. This finding has immediate applicability to the development of bone graft extender formulations for clinical use.     

Enhanced bioactivity of a poly(propylene fumarate) bone graft substitute by augmentation with nano-hydroxyapatite

The bioactivity of a nano-hydroxyapatite-augmented, bioresorbable bone graft substitute made from the unsaturated polyester, poly(propylene fumarate), was analyzed by evaluating biocompatibility and osteointegration of implants placed into a rat tibial defect. Three groups of eight animals each were evaluated by grouting bone graft substitutes into 3-mm holes that were made into the anteromedial tibial metaphysis of rats. Thus, a total of 24 animals was included in this study. Two different formulations varying as to the type of hydroxyapatite were used: Group 1 - nano-hydroxyapatite, Group 2 - micron-hydroxyapatite, with a Group 3 control defect remaining unfilled. Animals of each of the three groups were sacrificed in groups of eight at postoperative week three. Histologic analysis revealed best superior biocompatibility and osteointegration of bone graft substitutes when nanohydroxyapatite was employed. At three weeks, there was more reactive new bone formation in this group when compared to the micron-hydroxyapatite group. The control group showed incomplete closure of the defect. This study suggested that nano-hydroxyapatite may improve upon the bioactivity of bone implant and repair materials. The model scaffold used in this study, poly(propylene fumarate), appeared to provide an osteoconductive pathway by which bone will grow in faster. Clinical implications of the use potential advantages of nano-hydroxyapatite on bone repair and orthopaedic implant design are discussed.     

Evaluation of bone healing with eggshell-derived bone graft substitutes in rat calvaria: a pilot study

The purpose of this study was to investigate the potential effectiveness of a surface-modified natural calcium carbonate, hen eggshell (ES) as a bone graft substitute. The surface characteristics, cell viability on, and osteoconductivity of, particulated ES with and without hydrothermal treatment in phosphate solutions were evaluated. Hydrothermal treatment partially converted ES to calcium-deficient hydroxyapatite (HA) with surface microstructure. MTT assay indicated higher osteoblast viability on surface-modified ES compared with a commercially available bone substitute, anorganic bovine bone (Bio-Oss, BO) (p < 0.001). Histological and histomorphometric analysis showed significantly greater new bone formation and mineralized bone-to-graft contact of surface-modified ES, especially with hydrothermally treated ES, compared with BO in 5-mm diameter calvarial defects in rats at 4 and 8 weeks of healing (p < 0.01). Complete bony bridging was more frequently found with hydrothermally treated ES. The results of this pilot study indicate the potential efficacy of surface-modified particulated hen eggshell as an osteoconductive bone substitute in a rat calvarial defect model.     

In vivo investigations on composites made of resorbable ceramics and poly(lactide) used as bone graft substitutes.

Porous composites made of poly(L, DL-lactide) (PLA) and alpha-tricalcium phosphate (alpha-TCP) or the glass ceramic, GB14N, respectively, were investigated in a loaded implant model in sheep. Six, 12 and 24 months after implantation histological and biomechanical evaluation were performed and compared to autogenous bone transplants. No significant differences were observed between the composites. After 6 months, the interconnecting pores of the alpha-TCP-composite and the GB14N-composite were filled with newly formed bone (14 +/- 5% or 29 +/-15% of the implant, respectively) and soft tissue (30 +/-9% or 21 +/-12% of the implant, respectively). Only a mild inflammatory response was observed. The reaction was similar after 12 months. However, after 24 months a strong inflammatory reaction was seen. The newly formed bone was partly osteolytic. The adverse reaction occurred simultaneously to a significant reduction of the PLA component. The histological results were reflected by the biomechanical outcomes. Both composites showed compression strengths in the range of the autologous bone graft until 12 months of implantation. After 2 years, however, the strengths were significantly decreased. It is concluded that the new composites cannot yet be used for clinical application. An improvement in biocompatibility might be reached by a better coordination of the degradation times of the polymer and the ceramic component.     

Studies on poly(propylene fumarate-co-ethylene glycol) based bone cement

Poly(propylene fumarate-co-ethylene glycol) random (PPF-1) and block (PPF-2) copolymer oligomers were prepared. Comparing the setting characteristics of PPF-1 and PPF-2 with comonomer n-vinyl pyrrolidone (n-VP) and swelling characteristics of cured PPF-1 and PPF-2, lower setting temperature and setting time was observed with the former leading to higher swelling coefficient and lower cross link density in the cured PPF-1. Due to the high swelling coefficient and low setting exothermic temperature associated with PPF-1, the bone cement was prepared from PPF-1, n-VP and hydroxyapatite (HAP). The in vitro degradation studies reveal lesser weight loss and deformation of PPF-1/n-VP/HAP based cured resin in Ringer's solution and phosphate buffered saline in comparison with that of PPF-1/n-VP cured resin. Though the bone cement composite has adequate mechanical properties with HAP, the compressive strength and modulus of the composite aged in Ringer's solution and PBS reduced appreciably which is due to extensive hydration and plasticization by the PEG unit. However, the bone-binding and bond strength of the bone cement determined as the load for separation of bones was found to be similar to that of fast setting calcium phosphate-atelocollagen (5%) bone cement. The bone cement PPF-1/n-VP/HAP could be used as scaffold for correcting the bone defects.     

Effect of a poly(propylene fumarate) foaming cement on the healing of bone defects.

Regeneration of skeletal tissues has been recognized as a new means for reconstruction of skeletal defects. We investigated the feasibility of an injectable and expandable porous implant system for in situ regeneration of bone. Therefore, a composite biodegradable foaming cement based on poly(propylene fumarate) was injected into a critical size defect made in the rat tibia. Animals were divided into two groups comparing the foam in the experimental group against sham-operated animals having a drill hole but no implant in the control group. Eight animals were included in each group. Animals were sacrificed at 1, 3, and 7 weeks postoperatively. Implantation sites were then evaluated with histologic and histomorphometric methods. Results of this study showed that defects did not heal in sham-operated animals. In the experimental group, metaphyseal and cortical defects healed within the first postoperative week by formation of immature woven bone. At the site of the cortical drill hole defect, healing was noted to progress to complete closure by formation of mature bone. Histomorphometry corroborated these findings and showed that metaphyseal bone remodeling peaked at 1 week postoperatively and then decreased as healing of the cortical defect progressed. This suggests that near-complete restoration of the original state of the tibial bone occurred in this animal model supporting the concept of in situ bone regeneration by application of engineered biodegradable porous scaffolds. () ()     

The application of human bone marrow stromal cells and poly(dl-lactic acid) as a biological bone graft extender in impaction bone grafting

Concerns over disease transmission, high costs and limited supply have led to interest in synthetic grafts in the field of impaction bone grafting (IBG). Poly(DL-lactic acid) (PLA) grafts are attractive alternatives due to their biocompatibility, established safety and versatile manufacturing process. This study examined the potential of PLA scaffolds augmented with human bone marrow stromal cells (HBMSCs) in IBG. In vitro and in vivo studies were performed on impacted morsellised PLA seeded with HBMSC and compared to PLA alone. In vitro samples were incubated under osteogenic conditions and in vivo samples were implanted subcutaneously into severely compromised immunodeficient mice, for 4 weeks. Biochemical, histological, mechanical and 3D micro-computed tomography analyses were performed. HBMSC viability, biochemical activity and histological evidence of osteogenic cellular differentiation, post-impaction were observed in vitro and in vivo in PLA/HBMSC samples compared to impacted PLA alone. In vitro PLA/HBMSC samples demonstrated evidence of mechanical enhancement over PLA alone. In vivo studies showed a significant increase in new bone and blood vessel formation in the PLA/HBMSC constructs compared to PLA alone. With alternatives to allograft being sought, these studies have demonstrated PLA/HBMSC living composites, to be a potential prospect as a biological bone graft extender for future use in the field of IBG.     

Influence of chemical structure on glass transition temperatures of poly(propylene terephthalate) and poly(propylene isophthalate)

Poly(propylene terephthalate)

1 Systematic IUPAC name: poly(oxypropyleneoxyterephthaloyl).

(PPTP) and poly(propylene isophthalate)

2 Systematic IUPAC name: poly(oxypropyleneoxyisophthaloyl).

(PPIP) were prepared by melt polycondensation. Different fractions with number-average molar masses in the range 5 000 ? 20 000 g·mol^?1 were obtained and their respective glass transition temperatures (T_g) determined by calorimetry. The solubility parameters of the polymers were obtained by viscosity measurements in different solvents and were found to be 9,4 and 9,5 cal^1/2·cm^?3/2 (19,2·10³ and 19,4·10³ J^1/2·m^?3/2) for PPIP and PPTP, respectively. The glass transition temperatures were compared with those reported for analogous polyesters and it was shown that intramolecular interactions highly influence the T_g of these polyesters, whereas the effect of intermolecular interactions seems to be less important.     

Characterization of porous injectable poly-(propylene fumarate)-based bone graft substitute

The use of bone grafts for orthopedic applications have increased steadily over the past decade. With improvements in surgical technique, combined with an increasing aged population requiring orthopedic treatment, the need for bone grafts substitutes have also increased. To be useful clinically, the bone graft substitute must be biocompatible, bioabsorbable, and have convenient handling properties. In addition, it must possess a microarchitecture that allows cellular ingrowth and remodeling while simultaneously providing mechanical strength. Poly(propylene fumarate) (PPF) has been investigated as an injectable, biodegradable scaffold for orthopedic applications. Various methods to create a porous, interconnected polymer scaffold are available. The foaming technique is a convenient method to accomplish this task. Reactions between bicarbonate salts and weak acids generate CO(2) gas, causing a bubbling reaction during the polymerization process. This technique allows the porosity of the scaffold to be modulated. Image analysis and mechanical testing of porous PPF fabricated using the foaming technique shows that a highly porous, interconnected scaffold can be produced. At approximately 50% porosity, the scaffold has excellent handling properties, contains pore sizes ranging from 50 to 500 mum with an elastic modulus ranging from 20 to 40 MPa. The foaming technique provides an additional method by which clinically useful polymers can be fabricated for use in various bone tissue engineering applications.     

The effect of pH on the LCST of poly(N-isopropylacrylamide) and poly(N-isopropylacrylamide-co-acrylic acid)

Poly(N-isopropylacrylamide) (PNIPAAm) and poly(N-isopropylacrylamide-co-acrylic acid) (P(NIPAAm-co-AAc)) hydrogels are synthesized by irradiating the aqueous solutions of NIPAAm and NIPAAm/AAc with ⁶⁰Co γ-ray. The effects of pH on the swelling ratio and on the lower critical solution temperature (LCST) are studied by determining the dependence of swelling ratio on temperature in different pH butter solutions. Differential scanning calorimetry (DSC) is applied in determination of the LCST of the hydrogels. Fourier transform infrared (FT-IR) spectrometry is used in the comparison of hydrogels swelled in various pH conditions. As a result, PNIPAAm was found to be a pH-sensitive hydrogel and the LCST of the PNIPAAm and P(NIPAAm-co-AAc) hydrogels are influenced by pH.     

The effect of pH on the LCST of poly(N-isopropylacrylamide) and poly(N-isopropylacrylamide-co-acrylic acid)

Poly(N-isopropylacrylamide) (PNIPAAm) and poly(N-isopropylacrylamide-co-acrylic acid) (P(NIPAAm-co-AAc)) hydrogels are synthesized by irradiating the aqueous solutions of NIPAAm and NIPAAm/AAc with 60Co gamma-ray. The effects of pH on the swelling ratio and on the lower critical solution temperature (LCST) are studied by determining the dependence of swelling ratio on temperature in different pH butter solutions. Differential scanning calorimetry (DSC) is applied in determination of the LCST of the hydrogels. Fourier transform infrared (FT-IR) spectrometry is used in the comparison of hydrogels swelled in various pH conditions. As a result, PNIPAAm was found to be a pH-sensitive hydrogel and the LCST of the PNIPAAm and P(NIPAAm-co-AAc) hydrogels are influenced by pH.     

A preliminary study on the enhancement of the osteointegration of a novel synthetic hydroxyapatite scaffold in vivo

Synthetic hydroxyapatite, a bioactive calcium phosphate, is clinically used as a bone replacement bioceramic because of its similarity in composition to bone mineral, biocompatibility, and osteoconductive nature. The aim of this study was to evaluate the bioactivity of a novel synthetic porous hydroxyapatite (PHA) in vivo in rabbit and to investigate the enhancement of its bioactivity and osteointegration. In the investigation reported here, insulin-like growth factor-I (IGF-I) has been used to enhance the bioactivity of PHA. Cylindrical PHA implants with or without IGF-I were implanted bilaterally in rabbit femurs. Fluorochrome bone markers were administered at 7-day intervals. The implants with the attached bone were retrieved at postmortem, 1 and 3 weeks after implantation, for histological and histomorphometric analysis. Undecalcified sections stained with toluidine blue showed new bone formation. Mineralization of the new bone formed in the interface, surrounding trabecular bone, and within the pores of the implants was studied. Lamellar bone mineral apposition rate (MAR) was assessed and compared among treatment groups, sham, PHA alone, and PHA with IGF-I (500 ng/implant), by fluorochrome label incorporation using UVL microscopy. We report for the first time, that the supplementation of PHA implants with IGF-I significantly increased new bone formation and MAR (6.58 +/- 0.08 microm/day) compared with implantation of PHA alone (4.08 +/- 0.05 microm/day) or sham operation (3.11 +/- 0.12 microm/day). These results suggest that synthetic PHA might provide a delivery system for bioactive agents to accelerate bone healing in orthopedic procedures.     

In vitro degradation of porous poly(propylene fumarate)/poly(DL-lactic-co-glycolic acid) composite scaffolds

This study investigated the in vitro degradation of porous poly(propylene fumarate) (PPF-based) composites incorporating microparticles of blends of poly(DL-lactic-co-glycolic acid) (PLGA) and poly(ethylene glycol) (PEG) during a 26-week period in pH 7.4 phosphate-buffered saline at 37 degrees C. Using a fractional factorial design, four formulations of composite scaffolds were fabricated with varying PEG content of the microparticles, microparticle mass fraction of the composite material, and initial leachable porogen content of the scaffold formulations. PPF scaffolds without microparticles were fabricated with varying leachable porogen content for use as controls. The effects of including PLGA/PEG microparticles in PPF scaffolds and the influence of alterations in the composite formulation on scaffold mass, geometry, water absorption, mechanical properties and porosity were examined for cylindrical specimens with lengths of 13 mm and diameters of 6.5 mm. The composite scaffold composition affected the extent of loss of polymer mass, scaffold length, and diameter, with the greatest loss of polymer mass equal to 15+/-5% over 26 weeks. No formulation, however, exhibited any variation in compressive modulus or peak compressive strength over time. Additionally, sample porosity, as determined by both mercury porosimetry and micro-computed tomography did not change during the period of this study. These results demonstrate that microparticle carriers can be incorporated into PPF scaffolds for localized delivery of bioactive molecules without altering scaffold mechanical or structural properties up to 26 weeks in vitro.