Ex vivo degradation of a poly(propylene glycol-fumarate) biodegradable particulate composite bone cement

We have developed a biodegradable particulate composite bone cement consisting of a poly(propylene glycolfumarate)-(methylmethacrylate) matrix mixed with calcium carbonate and tricalcium phosphate particulates. Previous ex vivo studies suggest that this system provides sufficient strength for a number of potential clinical applications including structural reinforcement of osseous defects, internal fixation devices for age-related fractures, and delivery of antibiotics to treat osteomyelitis. As a first step toward investigating in vivo responses to this material, we studied the influence of varied concentrations of crosslinker, accelerator, and free radical on the mechanical properties of the cement. We then developed an ex vivo degradation assay and correlated the mechanical properties of degrading cement with the temporal changes in chemical properties of both the cement and the bathing medium. The optimal cement formulation was composed of one-third poly(propylene glycolfumarate)-(methylmethacrylate), one-third calcium carbonate, and one-third tricalcium phosphate, and provided initial compressive strengths of up to 30 MPa and compressive moduli of up to 300 MPa. Degradation rates, measured by a decline in mechanical properties, dissolution of calcium from the cement, and change in pH of the bathing medium, could be controlled by changing the concentration of reactants in the matrix. Specifically, an increase in methylmeth-acrylate or increase in both methylmethacrylate and benzoyl peroxide was inversely proportional to the rate of degradation and directly proportional to the initial mechanical properties. The degradation products and environmental changes appear to be compatible with physiologic remodeling and therefore justify examination of the in vivo response to implantation of this material.

Patellofemoral contact pressures exceed the compressive yield strength of UHMWPE in total knee arthroplasties

To address mechanisms involved in wear and permanent deformation of patellofemoral components in total knee arthroplasties, a previously reported knee joint loading model and pressure-sensitive film were used to measure patellofemoral contact areas and pressures in human cadaver knee joints after implantation with six different total knee joint designs. The joints were tested at three different Q angles (physiologic, -10 degrees, and +10 degrees) and four different flexion angles (30 degrees, 60 degrees, 90 degrees, and 120 degrees). Patellofemoral contact areas at normal Q angles ranged from 0.13 to 0.68 cm2 and increased with flexion angle up to 90 degrees. These contact areas differed significantly with flexion angle but not with Q angle. Variations in contact area with type of knee system were only marginally significant (P < .04), and post hoc tests showed no significant differences between individual knee designs. Contact pressures at normal Q angle also increased with flexion angle and ranged from 10 MPa to more than 49 MPa. Contact pressures at flexion angles greater than 60 degrees were, for all systems, well in excess of the compressive yield strength of ultrahigh-molecular-weight polyethylene and at least three to four times greater than the recommended maximum compressive stress level of 10 MPa.