David D. Hile

Enhanced peripheral nerve regeneration through a poled bioresorbable poly(lactic-co-glycolic acid) guidance channel

In this study we investigated the effects of materials prepared with electrical poling on neurite outgrowth in vitro and nerve regeneration in vivo. Neuro-2a cells were seeded on poled and unpoled poly(lactic-co-glycolic) (PLGA) films and observed at time periods 24, 48 and 72 h post-seeding. The percentage of cells with neurites and the neurites per cell were quantified using light microscopy. At 48 and 72 h post-seeding, both the number of cells with neurites and the neurites per cell were significantly increased on the poled films compared to those on unpoled films. An established rat sciatic nerve model was used for in vivo studies to assess the effects of PLGA guides, poled for two different periods, on peripheral nerve regeneration. Guides were inserted in rats to bridge a 1.0 cm gap created in the right sciatic nerve. After four weeks, nerves regenerated through poled guides displayed a significant increase in conduction velocity and significantly increased numbers of axons across the guides, as compared to nerves regenerating through an unpoled guidance channel. Electrical poling was shown to promote neurite growth, axon regeneration and the conduction rate of the repaired nerve. We concluded that guides prepared with electrical poling enhance peripheral nerve regeneration.

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.

Porous Poly(propylene Fumarate) Foam Coating of Orthotopic Cortical Bone Grafts for Improved Osteoconduction

A porous biodegradable scaffold coating for perforated and demineralized cortical bone allografts could maintain immediate structural recovery and subsequently allow normal healing and remodeling by promoting bony ingrowth and avoiding accelerated graft resorption. This new type of osteoconductive surface modification should improve allograft incorporation by promoting new bone growth throughout the biodegradable scaffold, hence encasing the graft with the recipients own bone. We investigated the feasibility of augmenting orthotopically transplanted cortical bone grafts with osteoconductive biodegradable polymeric scaffold coatings. Five types of bone grafts were prepared: type I, untreated fresh-frozen cortical bone grafts (negative control); type II, perforated and partially demineralized cortical bone grafts without additional coating (positive control); type III, perforated and partially demineralized cortical bone coated with a low-porosity poly(propylene fumarate) (PPF) foam; type IV, perforated and partially demineralized cortical bone coated with a medium-porosity PPF foam; and type V, perforated and partially demineralized cortical bone coated with a high-porosity PPF foam. Grafts were implanted into the rat tibial diaphysis. Fixation was achieved with an intramedullary threaded K-wire. Two sets of animals were operated on. Animals were killed in groups of eight with one set being killed 12 weeks, and the other 16 weeks, postoperatively. Radiographic, histologic, and histomorphometric analyses of grafts showed that the amount of new bone forming around the foam-coated grafts was significantly higher than that in the type I control group (uncoated) or that in type II group (perforated and partially demineralized cortical bone grafts). Although all foam formulations appeared initially equally osteoconductive, histologic evaluation of medium-porosity PPF foam-based coatings appeared to result in a sustained response 16 weeks postoperatively. Significant resorption was present in perforated and partially demineralized cortical bone graft allografts, with some accompanying new bone formation occurring primarily within the laser holes. Therefore, PPF foam-coated cortical bone grafts appeared to be better protected from excessive bone resorption, as frequently seen with invasion of fibrovascular tissue. Biomechanical analysis of the PPF foam-coated grafts corroborated findings of the morphometric analysis in that the failure strength at the allograft-host bone junction sites of all PPF-coated cortical bone grafts was higher than in the uncoated controls.

Composite poly(lactide)/hydroxylapatite screws for fixation of osteochondral osteotomies. A morphometric, histologic and radiographic study in sheep

The aim of this study was to evaluate the healing of an osteochondral fragment created in the distal sheep femur in response to fixation with a biodegradable polylactide/hydroxylapatite composite screw. Poly(L-lactide) screws were used for comparison. At follow-up times of 4 and 8 weeks, the specimens were examined with standard radiography and computed tomography, as well as with macro- and micro-histomorphometry. The intact contralateral femur served as a control. Only minimal signs of degradation of the polymer could be seen in the histologic specimens. At 8 weeks, nearly all osteotomies had healed completely and an association between implant type and delayed osteotomy healing was found. The width of the repair tissue at the tissue–implant interface was 250 ± 48 μm, representing a clear transition zone of newly formed trabecular bone separating the implant from the surrounding plexiform bone. This study showed that large polylactide implants which are buffered with hydroxylapatite show benign tissue responses and good implant osteointegration. The osteotomy healing in a weight-bearing osteochondral fragment model in sheep utilizing a composite polylactide/hydroxylapatite screw was equivalent to a similar polylactide screw implant, indicating that hydroxylapatite-buffered screw implants could be used for similar indications in current clinical use.

Dimensional Stability of the Alveolar Ridge After Implantation of a Bioabsorbable Bone Graft Substitute: A Radiographic and Histomorphometric Study in Rats

This study evaluated reconstruction of the alveolar ridge after molar extraction in rats with bioabsorbable bone repair scaffolds. The material was prepared from the unsaturated polyester poly(propylene glycol-co-fumaric acid) (PPF), which may be cured in situ to form a porous scaffold. The intention is to use this material either as a stand-alone bone graft substitute or as an extender to autograft harvested from mandibular reconstruction sites. The bioactivity of the graft substitute was investigated in a rat residual ridge resorption model. PPF bone repair material was injected into the defect site, where it cross-linked in situ in the presence of a hydroxyapatite (HA) filler and effervescent agents. The PPF-based material develops porosity during an in situ cure by generating carbon dioxide during the effervescent reaction of citric acid and sodium bicarbonate. The incorporation of HA promotes osteoconduction within the bone repair scaffold. In this study, bioactivity of the porous scaffold was evaluated as a function of HA particle size (micrometer-sized vs nanometer-sized particles). The maxillary or mandibular molars on the right side were extracted from 96 adult Sprague-Dawley rats. A 2-mm round bur was used to create a uniform trench defect measuring 2 mm in diameter, 2 mm in depth, and 4 mm in length at each extraction site. The defect site was (1) treated with PPF bone repair material containing nanometer-sized HA, (2) treated with PPF material containing micrometer-sized HA, (3) treated with demineralized freeze-dried bone allograft, or (4) left untreated. Rats were sacrificed at 2, 4, 7, and 12 weeks postoperative. Resorption of the residual alveolar ridge was assessed by radiographic outcomes. Bone ingrowth through the defect site was measured by histomorphometric outcomes. Mandibular and maxillary ridge heights increased for all treatments used in this study. There were no clinical indications that addition of either of the PPF bone repair materials retarded hard- or soft-tissue healing of the extraction sites. Although not statistically significant, the mandibular defects treated with PPF containing nanometer-sized HA healed at a faster rate as determined by ridge height and new bone formation measurements when compared with the other treatments. These findings suggest the feasibility of using PPF bone graft substitutes for oral-maxillofacial applications.