Biotribological performance of medical-grade UHMW polyethylene-based hybrid composite for joint replacement

Abstract

Ultra high molecular weight polyethylene (UHMWPE) is widely used for articulating surfaces in total hip and knee replacements. In the present work, the tribological properties of UHMWPE-based nano composites were studied in order to meet the demands of current bearing applications. UHMWPE matrix reinforced with 0.5, 1, and 2 weight percentage of alumina nano powder were fabricated by hot pressing. The dispersion and microstructure of composite material was established by X-ray diffraction (XRD) and scanning electron microscope (SEM) micrograph. The tests were carried out on a reciprocating sliding pin-on-disc tribometer at human body temperature (37±1^°C) under dry and human serum lubricating environments for a normal load of 46 N and 52 N, a constant sliding speed of 4 mm. Under these testing conditions, it has been observed that the wear behavior of the developed composites improved with increase in weight percentage of alumina nano powder. The results show that at 52 N load, the maximum value of wear rate was 7.9x10⁻⁷ mm³/Nm and the minimum value 1.6x10⁻⁷ mm³/Nm was obtained. SEM was used to examine the worn surface and it was observed that human serum adheres to the surface of the composite pins upon sliding, resulting in the formation of a film which results in better wear resistance of the composite pins under human serum lubrication than dry sliding. This study implies that the use of nano alumina power will reduce the wear of UHMWPE based composite under human serum lubrication.

Introduction

Cartilage repair and regeneration is one of the biggest challenges which is faced in the field of medicine. Bone and cartilage diseases are very common and have become one of the main reasons for lowering the quality of life ^1–3. The hip and knee joints being the complex joints in the body are at high risk of injuries and osteoarthritis. Cartilage degeneration is related to osteoarthritis, which is a joint disease. The degeneration of cartilage is due to the wear and degradation of the synovial fluid in these joints ^4–7. Some researchers suggest that these cartilage defects grow bigger with time and thus affect joint functioning ⁸ . At present, no method is available which can help in a complete restoration of these defects ⁹ . Thus, these defects are treated with the help of different surgical methods know as total joint replacement (TJR) which have shown satisfactory results. Medical-grade ultra high molecular weight polyethylene (M-UHMWPE) is used for TJR because of its outstanding properties such as low friction, biocompatibility, good mechanical properties, chemical resistance, and high wear resistance. M-UHMWPE is the material of interest to many researchers in the field of orthopedics. The desirable properties of M-UHMWPE make it the material of choice for joint replacement prostheses. ¹⁰

The main advantage of UHMWPE is in its clinical applications especially in the field of orthopedics. M-UHMWPE has a long, successful clinical track record in total hip replacements dating back to the 1960’s. Total hip replacement (THR) is a surgical procedure where UHMWPE is used as a liner for the articulation of the artificial joint. Similarly in total Knee Replacement (TKR) diseased or damaged surfaces of the knee joint are removed and replaced with artificial surfaces. A round ended implant is used for the femur, mimicking the natural shape of the bone. A flattened or slightly dished UHMWPE surface is then inserted, so that the weight is transferred from metal to plastic and not from metal to metal. In an elbow joint replacement, the prosthesis is made of metal with UHMWPE bushing at the articulation. Although total elbow arthroplasty has been successfully used for the treatment of relatively inactive patients with rheumatoid arthritis, implant survival has been limited when used for treatment of osteoarthritis or trauma. Like the knee, the axis of rotation of the elbow shifts in both position and orientation during flexion ^11–14.

Currently, researchers focus on improving the mechanical properties and wear resistance of M-UHMWPE. For this, it is necessary to reduce the debris generated from the M-UHMWPE bearings, which finally leads to osteolysis and results in prosthesis revision ^15,16. Alumina (Al₂O₃) is one of the most effective and widely used materials in the family of materials. Research indicates that this material exihibits good biotribological performance and the technical grades of this material are easily available. High purity alumina is a bio inert material and broadly used as total joint replacement material for its excellent wear, hardness, corrosion resistance and excellent biocompatibility. Alumina possesses high modulus of elasticity, compressive strength, and its bio inert behavior helps bone regeneration process and does not impair the cyto compatibility ^17,18. Aluminum oxide addition not only improves the mechanical properties but also good and sustained biocompatibility ^17,19.

M-UHMWPE is studied extensively as it is the material used for TJR ^20–22. The stability of M-UHMWPE implants is the biggest problem under different loading conditions when used in a human joint and retention of its tribological and mechanical properties is also a big concern area. In many research studies, the main aim was to increase the wear resistance and to achieve this, radiation crosslinking method was used ^23–25. However, this method resulted in oxidative-degradation of M-UHMWPE, which affected the mechanical properties ^26–29.

In this current study, the combined effect of anti-wear and anti-oxidant additives on the biotribological performance of M-UHMWPE has been observed. Therefore, nano-alumina (anti-wear) and Vitamin-C (anti-oxidant) fillers were incorporated in M-UHMWPE and their tribo-performance is investigated for a suitable use for cartilage replacement.

Materials and experiment

Materials

This study employs materials that were procured from Nanoshell USA. The base material was formed by UHMWPE powder (99.9% purity), having a density of 940 kg/m³. Alumina nano powder (99.9% purity) having a density of 3900 kg/m³ and Vitamin-C were used as fillers as shown in Figure 1. Ti6Al4V formed the counter-face for the wear tests. Tribological tests were carried out under dry and lubricated environments, with human serum as the lubricating medium. The human serum was obtained from a government hospital.

Figure 1.

SEM. Micrographs of (a) Vitamin-C, (b) alumina, and (c) UHMWPE powders as received.

Sample fabrication

Before sample fabrication, wet mixing of UHMWPE and alumina nano powders was carried out with ethanol as the wetting medium, using a homogenizer and magnetic stirrer. This solution was then filtered using filter paper and a homogenous mixture was obtained. To remove traces of the wetting medium, the mixture was then dried in an oven at 79^°C. The dried powder was mixed with Vitamin-C, as performed by Souza et al. ³⁰ The fabrication process of composite samples are shown in Figure 2. The samples were fabricated by compression technique using a hot press machine at a pressure of 200 bar. The powder mixture was heated to a temperature of 160^oC for 3 min while compressing, followed by a cooling period of 2 min. Cylindrical samples of 6 mm diameter were obtained. Molding temperature was kept above the melting temperature of UHMWPE to ensure that no unmelted particle remains in the composite matrix. Further tapering of the samples to 3 mm diameter was carried out to obtain pins of the composite for testing. These pins were subjected to polishing using emery papers and diamond pastes to attain a fine surface finish. Flat discs of Ti6Al4V, which act as the counter-face material, were also polished using emery papers of different grit sizes and diamond pastes till a mirror finish was attained. The composition of alumina nano powder was varied (0.5%, 1%, and 2%) and 2% of Vitamin-C was kept constant for each in the matrix to obtain three different composites.

Figure 2.

Schematic presentation of sample fabrication process.

Sample characterization

The XRD pattern of the composite material is presented in Figure 3. It determines the planes (110 and 200) of UHMWPE at two theta values of 21.68° and 24.12° ³¹ . The planes are assigned to (104, 110, 226, 113, 024 and 116) of nano alumina at two theta = 35.28°, 37.94°, 39.88°, 43.48°, 52.68, and 57.62° ³² . The planes (130, 211, 022, and 103) of Vitamin-C at two theta = 25.68°, 30.22°, 36.36°, and 47 are also clearly visible ³³ .

Figure 3.

XRD diffractogram of 2 wt. % alumina and Vitamin-C filled UHMWPE.

Figure 4 depicts the surface morphology of 2 wt. % alumina and Vitamin-C filled UHMWPE composite, which was examined using optical microscopy (Leica DM 6000M) and scanning electron microscopy. It can be discerned from the micrographs that uniform dispersion of the fillers has taken place which resulted in a homogenous composite.

Figure 4.

(a) Optical micrograph and (b) SEM micrograph of 2 wt. % of alumina and Vitamin–C-filled UHMWPE composite.

Experiment

A reciprocating tribometer (Pin-on-Disc) was used for the tribological experiments in dry and human serum lubrication environments at a human body temperature of 37±1^°C. The composite pins of 3 mm diameter were reciprocated against the Ti6Al4V disc specimens. These experiments were performed according to ASTM F732. A load of 46 N and 52 N was chosen in this study, the stroke was kept constant at 4 mm. The frequency was kept constant at 5 Hz for every experiment which was carried out for a duration of 60 min. The average value was reported after each test was repeated three times. The pin-on-disc tribometer, shown in Figure 5, is connected to the computer which is used to acquire the coefficient of friction data and the pin-on-disc configuration is shown in Figure 6.

Figure 5.

Reciprocating tribometer.

Figure 6.

Disc and Pin arrangement.

Wear rate was evaluated by using the following equation:

$$K=\frac{W_v}{Sd\times L}$$

(1)

where wear rate (mm³/Nm) is denoted by K, wear volume (mm³) is denoted by Wv, applied load (N) is denoted by L, and sliding distance (m) is indicated by Sd.

Scanning electron microscopy (SEM), Hitachi S-3600N was used for conducting detailed morphological examinations of the pin and disc surfaces.

Result and Discussion

Tribological performance

Pin-on-disc configuration was employed to conduct wear studies of composite materials reciprocating against Ti6Al4V under dry and human serum lubricated environments. Figures 7(a) and (b) depict the variation of wear rate with an increase in alumina weight percentage in UHMWPE under dry and lubricated conditions for 46 N and 52 N, respectively. As the reinforcement of filler is increased, a general trend of reduction in wear rate is observed for the composite materials when sliding against Ti6Al4V under dry as well as lubricated conditions. It was observed that a higher wear rate value is obtained for composite tribo-pair under dry conditions. The wear rate of the composite pins when sliding with Ti6Al4V discs was obtained for each test condition which was calculated from the weight loss of the pin. Equation (1) was used to calculate the wear rate of composite pins under the dry sliding condition as a function of load.

Figure 7.

Wear of composites in a dry and lubricating environment under a load of (a) 46 N and (b) 52 N

It was observed that the wear rate exhibits a decreasing trend as the composition of alumina is increased from 0.5% to 2%. The least value of the wear rate is attained for composite material when reinforced with two weight percentage of alumina. For the test conducted at 46 N load, maximum wear rate is attained for 0.5 wt. % composite under dry sliding (9.5x10⁻⁷ mm³/Nm) and minimum value is obtained for 2 wt. % composite under human serum lubrication (2.3x10⁻⁷ mm³/Nm). Similar behavior was observed at 52 N load, where maximum value of wear rate was 7.9x10⁻⁷ mm³/Nm and the minimum value 1.6x10⁻⁷ mm³/Nm. This is due to the proper bonding and distribution of alumina in the UHMWPE matrix material, causing it to bear the main load on the sample and thus increase the wear resistance. The inclusion of alumina results in the increase in hardness of the ultra high molecular weight polyethylene due to its reinforcing effect.

Wear surface characterization

Scanning electron microscopy (SEM) is used to analyze the surface morphologies of medical-grade composite pins after tribological experiments. Figure 8 depicts the morphology of the composite pins with 0.5 wt. %, 1 wt. %, and 2 wt. % alumina after testing under a dry sliding and human serum lubrication at 52 N load. Figure 8 (a, c, e) depicts the surface morphology of the wear track for the test conducted at a load of 52 N in a dry environment. It is evident from figure (e) that only minor scratches are visible along the sliding direction on the composite pin with 2 wt. % alumina. The surface morphology of the wear tracks for the test conducted at a load of 52 N under human serum lubrication is depicted in Figure 8 (b, d, f). Formation of the wear protective serum film was observed along the sliding direction, which resulted in enhancement of wear resistance of the composite material with 0.5 wt. %, 1 wt. %, and 2 wt. % alumina. The reduction in wear rate, as observed in Figure 7, can be attributed to the formation of this serum film on the pin’s surface. Hussain et al. have also reported similar film-like observations ¹⁰ .

Figure 8.

SEM micrographs of the wear track of composite pins at a load of 52 N with (a) 0.5 wt. %, (c) 1 wt. %, and (e) 2 wt. % alumina composite under dry sliding and (b) 0.5 wt. %, (d) 1 wt. %, and (f) 2 wt. % alumina composite under human serum lubrication.

Conclusion

Composites of UHMWPE were fabricated with a fixed composition (2 wt. %) of Vitamin-C and varying composition (0.5, 1, and 2 wt. %) of alumina. These composites were then subjected to tribological tests under dry and human serum lubrication at loads of 46 N and 52 N. The following observations were made:

1. Wear rate revealed a decreasing trend with the increase in the composition of alumina for tests carried out at 46 N and 52 N under both dry and human serum environments.

2. The maximum value of wear rate (9.5 × 10⁻⁷ mm³/Nm) was observed at a load of 46 N for the composite pin with 0.5 wt. % alumina, under dry sliding conditions.

3. The minimum value of wear rate (1.6 × 10⁻⁷ mm³/Nm) was attained at a load of 52 N for the composite pin with 2 wt. % alumina under human serum lubrication.

4. SEM analysis of the wear tracks of composite pins with 2 wt. % alumina sliding at a load of 52 N under a dry environment revealed minor abrasive wear with a slight accumulation of wear debris.

5. For tests under human serum lubrication, the SEM micrographs revealed the presence of wear protective serum film under the load of 52 N.

6. The reduction in the wear rate with the addition of alumina can be attributed to the high hardness and compressive strength of alumina, which also acts as a load-bearing component.

7. Besides, the formation of the wear protective serum film, as confirmed by SEM micrographs, contributes to the overall reduction in wear rate under human serum lubrication.