Evaluation of the reaction kinetics of CORTOSS, a thermoset cortical bone void filler

The primary objective of this research is to evaluate the reaction kinetics of CORTOSS(TM), a thermoset, Bis-GMA (2,2-bis[4-(2-hydroxymethacryloxypropyl) phenyl]propane) composite system as a function of time, material storage temperature and the temperature of the surrounding environment (site temperature). This study utilizes probability theory to predict the percentage of bifunctional monomers with 0,1 and 2 functional groups that have been reacted. This is a strong indicator of the potential for leaching unreacted components. A differential scanning calorimeter (DSC) was used to measure the isothermal enthalpy at varying site temperatures. After isothermal monitoring, the samples were dynamically heated from the respective isothermal temperature to 175 degrees C at 15 degrees C/min to measure the residual enthalpy from the unreacted functional groups. The experimental results indicate that the degree of conversion for this bifunctional system ranged from 76% to 86%. Applying probability theory it has been determined that approximately 95% of the bifunctional monomers are present with at least one double bond reacted and up to 5% of monomers remain unreacted. This is consistent with theoretical values postulated for various diffusion controlled thermoset systems (Macromolecules 32 (1999) 3913). Overall, curing under physiological conditions yielded a faster reaction rate and a significantly higher degree of conversion as compared to the lower site temperature conditions.

Comparison of a new bisphenol-a-glycidyl dimethacrylate-based cortical bone void filler with polymethyl methacrylate

A newly formulated and reinforced bisphenol-a-glycidyl dimethacrylate (bis-GMA) resin (Cortoss/Orthovita, Malvern, Pa.) was compared with Simplex P polymethyl methacrylate (Stryker Howmedica Osteonics, East Rutherford, N.J.) in rabbits for up to 52 weeks and in sheep for up to 78 weeks. As seen in scanning electron microscopy and histology examinations, both implant materials were surrounded by bone at late time periods, with fibrous layers of connective tissue seen in half the Simplex P specimens. No clinically significant safety differences between implant materials were apparent. Interfacial bond strengths between the implant and bone generally increased with time, but were 4.5-fold greater with Cortoss than Simplex P at 24 weeks, and 100-fold greater at 52 weeks. Forces required to displace 316SS rods held in place with Cortoss were consistently greater than forces to displace rods held in place with Simplex P. No statistically significant differences in displacement forces were found between rods held in place with Cortoss polymerized in situ and rods held with prepolymerized Cortoss. Interfacial bond strengths were greater for Simplex P that was polymerized in situ than for prepolymerized polymethyl methacrylate specimens. Cortoss synthetic cortical bone void filler is a good candidate material to fix implants in bone. It has characteristics consistent with long-term safety and has a better ability to bond to bone than Simplex P.

Potential of an ultraporous beta-tricalcium phosphate synthetic cancellous bone void filler and bone marrow aspirate composite graft

Autogenous cancellous bone is considered to be the best bone grafting material. Autogenous bone grafts provide scaffolding for osteoconduction, growth factors for osteoinduction, and progenitor stem cells for osteogenesis. However, the procurement morbidity, limited availability, and expense associated with the use of autogenous bone grafts are significant disadvantages. Allografts and xenografts lack the osteoinduction and osteogenesis properties of autogenous bone, and they introduce the potential for both transferring disease and triggering a host immune response. Synthetic bone grafts [hydroxyapatite or tricalcium phosphate (TCP)], while good platforms for osteoconduction, lack any intrinsic properties of osteoinduction and osteogenesis. A composite graft that combines synthetic scaffold with autogenous osteoprogenitor cells from bone marrow aspirate (BMA), a low-morbidity procedure, could potentially deliver the advantages of autogenous bone grafts without the disadvantages. A new ultraporous beta-TCP construct, engineered using solution-derived nano-particle technology, may prove to be an ideal carrier for BMA in such a composite. The unique, interconnected macroporosity, mesoporosity, and microporosity of this synthetic cancellous bone void filler allows it to wick in cells and nutrients via enhanced capillarity. Preliminary canine data support this expectation, demonstrating bone formation that suggests good penetration of cells and nutrients. These results suggest that BMA cells, absorbed into such a scaffold, may remain viable, thereby potentially making such a composite a true synthetic replacement for autogenous cancellous bone.

A multidisciplinary study of the healing of an intraarticular anterior cruciate ligament graft in a goat model

We evaluated knee function, tensile properties, and histologic appearance of a healing intraarticular bone-patellar tendon-bone autograft after anterior cruciate ligament reconstruction in a goat model. The patellar tendon graft was fixed such that both bone-to-bone (femoral tunnel) and bone-to-tendon (tibial tunnel) healing could be studied. The total anteroposterior translation significantly increased from 3 to 6 weeks, ranging from increases of 28.8% to 46.7%. In situ forces in the replacement graft decreased as much as 22.2% at 6 weeks. Conversely, tensile properties of the femur-anterior cruciate ligament graft-tibia complex did not change significantly from 3 to 6 weeks. However, the mode of failure changed from the graft pulling out of the tibial tunnel at 3 weeks to a mix of midsubstance failures (N = 2) and pullouts (N = 5) at 6 weeks. Histologic evaluations revealed progressive and complete incorporation of the bone block in the femoral tunnel, but only partial incorporation of the tendinous part of the graft in the tibial tunnel. The differences demonstrated at 3 and 6 weeks may be a result of the remodeling process of the midsubstance of the graft as the interfaces within the osseous tunnels mature.

Structure and function of the healing medial collateral ligament in a goat model

In this study knee joint function with a healing medial collateral ligament (MCL) at six weeks was examined with a robotic/universal force-moment sensor testing system during the application of two loading conditions: (1) 5 Nm valgus moment and (2) 67 N anterior load. Additionally the structural properties of the femur-MCL-tibia complex and the mechanical properties of the MCL substance were determined by uniaxial tensile tests. The histological appearance of the healing MCL was also observed. At 30 degrees and 60 degrees of knee flexion, valgus rotation of the healing knee was significantly increased compared to the sham. The in situ force in the healing MCL was significantly lower (34+/-17 N vs 54+/-12 N) at the same flexion angles (50+/-10 N vs 62+/-7 N). The anterior translation of the knee had returned to normal values at 30 degrees and 60 degrees of knee flexion. However, no differences could be found between the corresponding in situ forces in the healing MCL at all flexion angles examined during application of an anterior load. The stiffness of the healing group (52.5+/-19.4 N/mm) was significantly lower than the sham group (80.3+/-26.4 N/mm) (p<0.04). The modulus of the healing group was also significantly decreased (p<0.05). The findings suggest that the tensile properties of the healing goat MCL and valgus knee rotation have not returned to normal at six weeks after an isolated MCL rupture, however, anterior translation appeared to return to sham levels.