Design and Development of Composite Hip Prosthesis

S. Bag & S. Pal
School of BioSc & Engg., Jadavpur University , Kolkata – 700032, India
Corresponding Author E-mail: spalbme@yahoo.co.in

Introduction:

Hip joint is one of the most vulnerable joint in human body requiring replacement surgery. It has become the most widely accepted procedure for the treatment of disabling hip arthritis in modern orthopedic surgery (Beckenbaugh R D & Ilstrup D M,1978). Currently hip prostheses made of stainless steel (AISI-316L) are widely used for their good load bearing properties (Skinner H.B, 1988). The density of this metal is heavier by 6-7 times compared to bone, the density of which ranges between 0.8-2.1gm/cc. similarly the modulus of elasticity of surrounding bone (3-30 GPa) is nearly 10-20% that of stainless steel (stainless steel exhibits a modulus of elasticity of 200GPa) (Greesink R G T,1989; Ratner B D et. al., 1996). So the applied load is mostly transferred through the metal not to the bone (Karachalios T et. al., 2004). This results in, the bone being inadequately loaded and consequently it resorbs and also changes the biomechanical environment (Greesink, R.G.T, 1990). To make suitable hip prosthesis in terms of density and modulus of elasticity ceramic reinforced polymer composite material would be the ideal material as bone is a composite material (Farling G M et al, 1978; Bonfield W, 1988) consisting of collagen fiber matrix and embedded hydroxyapatite mineral. To mimic that we used UHMWPE and coated alumina ceramic as strengthening component in various weight percentage.

We produced a material very similar to bone and coated it with bioactive coating of hydroxyapatite using a novel hydrothermal technique. These materials are intended to provide both bone bonding ability and the desired mechanical properties including the ductility of the polymers and the stiffness of cortical bone Hip replacement is a common phenomenon in advanced orthopedic surgery. Since hip joint is a load bearing joint, so it is replaced only by the material having load bearing capacity. Generally hip prosthesis is made of stainless steel 316L because it has good load bearing property.

Hip prosthesis mould was designed in consultation with a local Industrial consultant and developed by us as per the standard size and shape of Austein Moore metallic hip prosthesis. The UHMWPE (GUR 4020) and UHMWPE-alumina ceramic composite hip prosthesis was prepared in the 3 piece hip mould using compression moulding technique.

Design and development of Hip prosthesis mould:

This work was supported by a research grant of the DRDO, to the 2nd author, School of Bioscience & Engineering, Jadavpur University.

Solid State Polymerization of Poly (lactic acid): Synthesis and Development of Kinetic Model via Functional Group Approach

1Vimal Katiyar, 2Hemant Nanavati*
Department of Chemical Engineering, Indian Institute of Technology Bombay, Powai, Mumbai-400076, India,
Email: 1 vkatiyar@iitb.ac.in and 2 hnanavati@che.iitb.ac.in 

Poly (L-lactic acid) (PLLA) belongs to the family of aliphatic polyesters. PLLA is a thermoplastic, high strength biodegradable polymer that can be produced from 100% renewable resources. It has acquired several biomedical applications such as sutures and drug delivery systems, as well as applications in packaging and in textiles. A non-hazardous and solvent-free technique to produce high molecular weight PLLA is solid-state polymerization (SSP). In this system, two main reactions occurring during SSP are esterification and depolymerization, and water and lactide are released as volatile byproducts. SSP is carried out by heating of spherical PLLA prepolymer of weight average molecular weight (Mw) of 16,000, at temperature T, Tg < T < Tm, under vacuum conditions, to achieve Mw > 300,000. Various analytical techniques such as IR spectral analysis, 1H, 13C and 19F NMR spectral analysis, Viscometric analysis, GPC analysis and DSC-TGA analysis have been performed to characterize the structural and bulk properties. Subsequently, a kinetic model has been proposed using a functional group (hydroxyl and carboxylic group) approach. A fringe micelle model is employed, where crystal regions are randomly distributed in an amorphous phase. The reactions are assumed to occur only in the amorphous region. The time-variation of crystallinity of the polymer is accounted for in the reaction rate expressions by using Avrami equation. The diffusion parameters are obtained using free volume theory for diffusion of gaseous molecules in polymer systems. The intrinsic reaction rate constants are modified to account for the translational mobility of chains and end group diffusion in the amorphous reason. The resulting set of coupled, unsteady state partial differential equations is solved to obtain the final number average molecular weight for the given experimental conditions. These values are validated against data generated by experiment. This model can be employed to determine reaction conditions required to obtain molecular weights suitable for the desired application.

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