Variation of Friction Factor Through the Gait Cycle in an UHMWPE-CoCrMo Hip Endoprosthesis

* email: [email protected] John Charnley was the first who introduced low friction arthroplasty concept. In November 1962, he implanted a hip prosthesis using a high density polyethylene (HDPE) cup, which articulated with a stainless steel femoral head of 22,25 mm diameter. The principle of the low friction arthroplasty has remained the same ever since. Because HDPE have not a good wear resistance, ultra-high molecular weight polyethylene (UHMWPE) is introduced. Its molecular weight is approximately ten times that of HDPE. UHMWPE has highest impact strength of any plastic [1]. This biomaterial is choice for load-bearing articular components used in total joint arthroplasty, for prostheses of the hip, knee and elbow. It is highly resistant to wear and has low coefficient of friction. Under the articulation of artificial joints, UHMWPE acts as a bearing surface under the lubrication of synovial fluid containing various lipoproteins [2]. UHMWPE have a crystalline structure with a crystalline lamellae (10-50 μm length, 10-50 nm thickness placed at 10-50 nm distance), disposed in an amorphous matrix [3]. The degree of crystallinity C (lower than HDPE), of UHMWPE is known to strongly influence several of its tensile mechanical properties such as Young’s modulus, yield stress and ultimate tensile properties. Young’s modulus is proportional with C and yield stress with (0,1 + C)2 [4]. There are two basic types of UHMWPE: standard and cross-linked (x-UHMWPE). Cross-linking of UHMWPE macromolecules has been performed using cross-linking agents such as peroxides, and through gamma or electronbeam irradiation. These process leads to a decreasing of crystalinity degree of UHMWPE and consecutively to decrease of Young modulus and yield stress, but this structure improve wear-resistance [4]. UHMWPE have a visco-elasto-plastic behaviour. A bidimensional impulse generation applied to polymers (like UHMWPE) show that the viscoelastic behaviour of polymers has consequence of impulse propagation [5]. Function of UHMWPE type (fabrication process), values of Young’s modulus are: for UHMWPE E = 1,3÷1,7GPa and for x-UHMWPE E = 0,6÷1,1GPa [3, 6]. Plastic deformation of UHMWPE is occurring in: amorphous layers by interlamellar shear, interlamellar separation, lamella stack rotation, and in crystalline regions by chain slip, transverse slip and dislocation generation. Yield stress of UHMWPE is overcome between 20MPa and 24MPa [4]. Hip prosthesis includes a prosthetic nail that is implanted in femur, a femoral head, an acetabular cup (made by injection moulding [6]) that is inserted in coxal bone (fig. 1). A great deal of studies, regarding the friction factor variation was performed in different testing conditions, like pin on disk, ball on plane, ball on socket, in dry regime or lubricated with different lubricants (distilled water, physiologic serum, plasma, etc.). The friction and wear behaviour of UHMWPE samples were studied by using ball sliding on UHMWPE disc under plasma lubrication and friction coefficient result in the lowest value around 0,075 [7]. In another study, the friction and wear properties of UHMWPE rubbing against the modified alloys under lubrication of distilled water were investigated using a pinon-disc tribometer [8] where the value of friction coefficient was fond approximately 0,082. There are, also, studies who respect the physiological conditions where UHMWPE acetabular cups were tested against CoCrMo femoral heads in a hip joint simulator run for 2,5 million cycles in bovine calf serum [9]. No one of these studies established the friction factor variation for one gait cycle, complying to geometrical, loading and surface parameters. This variation being able to predict the medium value of the friction factor after the prosthesis was implanted in the femur.