0162-0134/00/$ - see front matter q2000 Elsevier Science S.A. All rights reserved. PII S0162-0134 ( 99 ) 00228-7 Friday Apr 07 10:51 AM StyleTag -- Journal: JIB (Journal of Inorganic Biochemistry) Article: 6322 www.elsevier.nl/locate/jinorgbio Journal of Inorganic Biochemistry 79 (2000) 97–102 Polymer-based composite hip prostheses R. De Santis, L. Ambrosio *, L. Nicolais Institute of Composite Materials Technology, National Research Council and Interdisciplinary Research Centre in Biomaterials, University of Naples ‘Federico II’, piazzale Tecchio 80, 80125 Naples, Italy Received 17 May 1999; received in revised form 7 October 1999; accepted 13 October 1999 Abstract A composite hip prosthesis (CHP) made from poly(ether-imide) reinforced with carbon and glass fibres was manufactured and charac- terized. The main objective of the study was to evaluate the effect of fibre organization on the mechanical properties of the composite femoral implant and compare with the bone. A stacking sequence of drop-off plies of carbon/glass fibres reinforcing poly(ether-imide)(PEI) constitutes a symmetrical and balanced CHP. The hip was manufactured according to the finite element modelling (FEM) design and using the compression moulding and water-jet technologies. The measured stress–strain data according to tensile, flexural and torsional tests showed agreement with the numerical calculation. Young’s modulus and the strength in tension are uniform along the stem axis (40 GPa and 600 MPa, respectively) while the elastic modulus in bending varies from 10 to 60 GPa in the tip–head direction. The composite stem showed a linear load–displacement relation up to 4500 N without breaking. Mechanical behaviour of the CHP is compared to that of a canine femur. Comparison with metal prostheses has also been undertaken. CHPs control stress–strain distributions, and hence the mechanical signals to bone, through a material-structure design. q2000 Elsevier Science S.A. All rights reserved. Keywords: Composite hip; Mechanical properties; Poly(ether-imide) 1. Introduction Hip prosthesis (HP) designs for total hip replacement (THR) present a stem which fits into the medullary canal passing through the proximal part of the femur (epiphysis). Thus, a HP mainly replaces the cancellous bone, occupies part of the medullary canal and substitutes part of the femur head tissue which is discontinued in its proximal part accord- ing to the type of injury or fracture [1]. Finite element modelling (FEM) and follow-up on implanted metallic hip prostheses (MHPs) show that long- term clinical stability is related to bone remodelling stress- shielding and excessive stresses cause necrosis/resorption of bone [2–6]. Cemented stems caused less bone resorption and lower interface stresses than uncemented stems made from the same materials. Bone cement or other interlayers act as mechanical buffers, withstanding the mechanical stresses due to the mis- match between metal and bone properties. Critical stresses in the Charnley prosthesis are the tensile and shear stresses across the bone/acrylic interface. However, it appears that it is not possible to alter the metal and acrylic properties in a * Corresponding author. Tel. q39-081-2425-9229; fax: q39-081-768- 2404; e-mail: ambrosio@unina.it way that will significantly improve the stress situation in the metal/bone/acrylic system [7–9]. An appropriate stem may produce stress reductions in the range of 30–70% in the cement at the cement interface [10,11]. Elastomer-coated prostheses (ECPs) have been developed in order to obtain the mechanical advantages of cemented THR [12,13]. However, linear elastic models of transversally isotropic bone and isotropic MHPs suggest resorptive phenomena around the stems [2,3,14]. Calcar bone resorption is always a consequence suggested of MHP designs; the loss of proximal bone is estimated to be around 40–60% in 5 years. The higher the stiffness of the prosthesis, the greater is the bone remodelling, and hence the greater the bone loss due to the stress-shielding effect [8]. The biomechanics of bone growth, absorption, fracture healing, etc., are related to materials properties, structural properties and bonding characteristics of the implant. Metal stems are homogeneous isotropic materials; thus, MHP design focuses on the geometry [8], while composite HPs (CHPs) are material-structure designs, providing many new options and possibilities in implant design [15]. Fibre-reinforced composite materials can offer strength comparable to that of metals, and also more flexibility than metals. Mechanical properties of a CHP can be tailored to meet the bone mechanical behaviour [16].