Enhancing the Tribological Properties of Bearing Surfaces in Hip Arthroplasty by Shot-Peening the Metal Surface

Abstract

Total hip arthroplasty (THA) is a surgical procedure for patients with pain and difficulty walking due to hip osteoarthritis. In primary THA, the acetabulum and femoral head are replaced by a prosthesis where the modular femoral head and inner liner of the acetabulum form the bearing surface. The most popular bearing surface used in the United States, metal-on-polyethylene, consists of a cobalt–chromium molybdenum (CoCrMo) alloy femoral head that articulates with a polyethylene acetabular liner, typically made of highly cross-linked polyethylene. While successful in most cases, THA sometimes fails, commonly from aseptic loosening due to the wear debris of polyethylene. Fine-particle shot peening (FPSP) is a simple method for enhancing the mechanical properties and surface properties of metal, including reducing friction and enhancing the lubrication properties of the metal surface. In this study, we applied FPSP to the CoCr in the femoral head of a hip prosthesis to improve its surface properties and conducted experiments with pin-on-disc tribometers using CoCr as a pin and highly cross-linked polyethylene as a disc to mimic the THA implant. The results show that FPSP significantly enhances the tribological properties of the CoCr surface, including lubrication; decreases the friction coefficient; and decreases the polyethylene wear volume.

1. Introduction

Total hip arthroplasty (THA) is a widely conducted surgical procedure globally, with an estimated 332,000 THA procedures performed in the United States in 2010 [1]. According to the United Kingdom’s National Joint Registry, 98,279 THA operations were performed in 2014 [2,3]. Osteoarthritis patients who develop pain and functional limitations of the hip are the common indications for total hip replacement [4,5].

THA is a common elective surgical procedure for patients who develop pain and experience difficulty in daily living due to osteoarthritis of the hip joint. THA has proven to be an effective procedure when a patient’s pain persists despite conservative treatments, leading to a decline in overall health, quality of life, or the ability to perform daily activities. Today, the percentage of THA procedures being performed on patients younger than age 60 is about 40% and is increasing steadily [6]. The longevity of the implant is therefore the most important factor for such patients. While THA is a successful procedure in most cases, failures are still reported. For late failures, which occur ten or more years after a surgery, the most common cause for revision with aseptic loosening is osteolysis due to the wear debris of polyethylene [7,8,9].

In a primary THA, the degenerative acetabular cartilage and the destroyed femoral head are both replaced by prostheses. A metal femoral stem is inserted into the femoral intramedullary canal and connected with a modular femoral head which can be made of a variety of materials, including ceramic or metal materials, and articulates with the acetabular liner component. There are two types of acetabular components: the monobloc polyethylene and the modular component. Monobloc polyethylene is commonly used with bone cement to ensure adherence to the bone. The modular acetabular component consists of a metal outer shell with an inner liner that can be made of a variety of materials, including polyethylene, ceramics, or metal. It is commonly attached to the bone without cement, using a porous surface that allows osteointegration from the host bone to the implant surface. In the THA system, the modular femoral head and the inner liner of the acetabulum form the bearing surface. The acetabular liner and femoral head serve as the motion surface and bear weight, creating the bearing surface. The bearing surfaces used in total hip arthroplasty (THA) can be composed of various material combinations, such as metal-on-polyethylene, ceramic-on-polyethylene, and ceramic-on-ceramic, serving as the motion surface [10]. In the United States, the most commonly utilized combination is metal-on-polyethylene [11], which involves a cobalt–chromium molybdenum (CoCrMo) alloy femoral head articulated with a polyethylene acetabular liner, usually constructed from highly cross-linked polyethylene.

To increase the wear resistance and surface properties of metal surfaces, fine-particle shot peening (FPSP) has been widely used for surface modifications that can increase surface strengthening and surface tribological properties. The enhancement of the surface-strengthening layer results from the combined impact of compressive stress and alterations in the microstructure [12,13]. FPSP uses small spherical media made of metal, glass, or ceramic beads thrown against the surface materials. The benefit of this technique is the increased hardness and fatigue resistance of the material [14]. For the surface, FPSP can reduce the sharpness of the pit on the metal surface and produce a smoother surface that affects the coefficient of friction and the lubrication properties of the metal surface.

The FPSP process can influence the surface microstructure and material hardness, leading to the creation of micro-dimpling on the material’s surface. As a deliberate surface modification, micro-dimpling enhances tribological performance in various applications, such as joint prostheses [15,16,17,18]. The micro-dimpling on the material’s surface creates numerous small spaces in the fluid flow path, enhancing lubricant film formation. This improvement in tribological properties contributes to a reduction in the wear and tear process on the motion surface. The recent literature [19] has examined the influence of dimple shapes on lubricant film formation in total hip arthroplasty implants. This study found that when micro-dimples were applied to the CoCr femoral head of a total hip arthroplasty implant, the micro-dimpled prosthesis head significantly enhanced lubricant film thickness compared to the non-dimpled prosthesis head, particularly at the equilibrium position once the lubricant film thickness was fully developed.

The peening parameters, including particle size and particle type, can affect the surface properties after the surface is modified with the fine-particle shot peening process [20,21]. The hardness of particles is a crucial factor influencing the surface properties after surface modification. It is essential for the particle hardness to exceed that of the surface material in order to alter the surface microstructure effectively. Given the high hardness of CoCr alloy, particles used in the peening process for CoCr alloy must possess sufficient hardness to induce surface modification. Silica and ceramic particles, known for their higher hardness compared to CoCr alloy, are suitable materials for modifying CoCr surfaces.

In the recent literature, various methods of surface modification have been employed on metals to enhance the properties of metal surfaces; for example, the use of nitriding or electron beam treatment. A study on using electron beam treatment on CoCr metals to improve surface properties found that it could enhance surface roughness and corrosion resistance [22]. Additionally, the utilization of plasma nitriding for surface modification with AlxCoCrFeNi high-entropy alloys (HEAs) was shown to increase wear resistance [23]. Previous studies have shown that both nitriding and electron beam treatment can improve the wear resistance of metal surfaces, reducing surface abrasion. However, these studies focus on metal surfaces themselves, which differ from the bearing surfaces used in total hip arthroplasty that require the polyethylene surface to have less abrasion when in contact with CoCr metal.

The lifespan of an implant is crucial for patients undergoing THA. This is because a shorter implant lifespan may necessitate corrective surgery due to the wear or failure of the joint, leading to more complex and riskier procedures. Revision surgeries are associated with higher risks of complications compared to primary implant replacements, increasing patient risks and incurring higher surgical costs.

Past studies have focused on enhancing the surface properties and materials used in manufacturing the femoral head of THA implants to prolong their lifespan. For example, transitioning from using CoCr alloy to ceramics with higher surface hardness compared to CoCr has been explored. This change increases scratch resistance, reducing the likelihood of abrasion when in contact with other components, thereby decreasing abrasive wear on the bearing surface.

Ceramics also exhibit better lubrication properties and improved surface wettability compared to CoCr alloy, resulting in reduced friction on the motion surface [24]. However, ceramics have some disadvantages that may outweigh the benefits when compared to CoCr alloy. One drawback is the incidence of fractures in ceramic femoral heads [25,26]. Additionally, there are limitations in terms of femoral head size and trunnion length adjustment with ceramic materials, which may offer less flexibility compared to CoCr femoral heads. Furthermore, ceramic femoral heads tend to be more expensive, leading to potential healthcare cost concerns for certain patient groups.

There have been enhancements to femoral head prostheses by utilizing oxidized Zirconium coating on CoCr femoral heads. This approach combines the benefits of ceramic-like surfaces and the strong, fracture-resistant properties of CoCr in the production of femoral head implants. However, several studies [27,28] have indicated that the wear of polyethylene and corrosion issues do not significantly differ when compared to conventional CoCr femoral heads. Therefore, it can be concluded that oxidized Zirconium femoral heads do not contribute to reducing wear on the bearing surfaces used in total hip arthroplasty [29].

This research was conducted with the aim of improving the tribological properties of CoCr surfaces, potentially equaling or surpassing ceramics while retaining their strength and fracture resistance, unlike ceramic materials. By using the FPSP process to modify the material surface, this study aimed to increase the surface hardness to enhance scratch resistance. Previous research has utilized the FPSP process to modify metallic surfaces such as titanium alloy, but there has been limited exploration of using FPSP to adjust CoCr surfaces. Implementing the FPSP process can increase surface hardness, making the material more resistant to scratching, and create surface micro-dimples that act as reservoirs for lubricants, preventing them from being squeezed out during surface motion interactions.

This study is a continuation of our previous study [30] that revealed that the surface microstructure and micro-dimpling on the surface can affect the tribological properties of a motion surface. FPSP can change surface properties and enhance tribological properties by producing a smoother surface, increasing hardness to create more scratch resistance, and creating micro-dimpling to enhance lubrication properties. We applied FPSP to the CoCr used in the femoral head of a hip prosthesis to improve its surface properties and conducted experiments with pin-on-disc tribometers using CoCr as a pin and highly cross-linked polyethylene as a disc that mimic the total hip arthroplasty implant in the human body. This experiment compares an un-peened surface to surfaces peened with the various particle sizes and types used in the FPSP process.

2. Materials and Methods

This study is an experimental study.

2.1. Materials

2.1.1. Cobalt–Chromium Alloy

CoCr alloy is commonly utilized in the fabrication of medical implants, particularly for orthopedic applications [31,32]. CoCr alloy is extensively employed in medical implants due to its exceptional mechanical properties, which encompass high corrosion resistance and mechanical strength [33,34,35]. Titanium alloys are frequently utilized in medical implants due to their biocompatibility and mechanical properties. However, in comparisons between titanium-based alloys and CoCr alloys, it has been observed that CoCr alloys exhibit superior wear resistance and higher strength [36]. For joint replacement implants with a motion surface, CoCr alloys have demonstrated excellent tribological properties, making them suitable for use in the motion surfaces of joint arthroplasty implants. In total hip arthroplasty, the most common combination used for the bearing surfaces of implants is metal-on-highly cross-linked polyethylene. CoCr alloy is utilized in the femoral head part of the implant, articulating with a highly cross-linked polyethylene liner and serving as the motion surface in the total hip arthroplasty system. Therefore, studying CoCr alloys is crucial for optimizing their properties and understanding their behaviour in the bearing surfaces used in total hip arthroplasty applications.

The chemical composition of cobalt–chromium (CoCr) alloy typically includes cobalt (Co) as the primary element and chromium (Cr) as a significant alloying element. These alloys may also contain small amounts of other elements, such as molybdenum (Mo), carbon (C), iron (Fe), and trace elements like nickel, manganese, and silicon. The specific composition can vary based on the grade and intended application of the alloy, with adjustments made to achieve the desired mechanical, thermal, and corrosion resistance properties. CoCr alloys are commonly used in various applications, including medical implants, due to their excellent strength, corrosion resistance, and biocompatibility.

Cobalt–chromium (CoCr) alloy can be prepared using a variety of techniques. In this study, we used CoCr alloys prepared with a casting technique.

Regardless of the preparation method, it was important to ensure that the CoCr alloy was properly cleaned before the study to ensure that the surface was free from any contaminants that could interfere with the results (an ultrasonic bath was used for cleaning the surface in this study).

The materials used in this study were CoCr alloys prepared with a casting technique, with their chemical composition shown in

Table 1. The chemical composition of cobalt–chromium alloy.

Cobalt (%)	Chromium (%)	Mo (%)	C (%)
60–62	29–31	5–6	0.55–0.65

.

The CoCr alloys used in this study had the following material properties:

-

A hardness of 400 HV;

-

A density of 8.3 g/cm³;

-

A melting point of 1300 degrees Celsius.

2.1.2. Highly Cross-Linked Polyethylene (HxPE)

In total hip replacement implants, a motion surface is used that moves when the patient move their hip joint. The motion surface of a total hip arthroplasty implant is an important part of the system that moves like a ball in the socket and consists of two parts: the femoral head (ball shape) and the acetabulum liner (socket part). The most frequently used material for the acetabulum liner component is polyethylene. HxPE has been noted for its superior wear resistance compared to conventional polyethylene. As a result, HxPE has become the preferred material for use as the acetabulum liner component.

In this study, medical-grade HxPE was used for the tribology test. The polyethylene manufactured from GUR 402 grade ultra-high-molecular-weight polyethylene involves a high degree of cross-linking via high-energy irradiation followed by thermal stabilization. To study the tribological properties by a pin-on-disc tribometer, highly cross-linked polyethylene was used to form the workpieces in a disc shape, and the CoCr was prepared in a pin shape that mimicked the bearing surfaces used in THA.

2.1.3. The Particles

Two distinct particle types, ceramic and silica, were employed in this study. The ceramic particles represent the higher-hardness material, and the silica particles represent the lower-hardness material. The ceramic particles were composed of Alumina–Zirconia composites, while the silica particles were made from silicon dioxide (SiO₂). In this study, three different particle sizes were utilized for each type of particle. The specific sizes of the particles are outlined in

Table 2. The sizes of the particles.

Particle Code	Particle Type	Particle Size (μm)
S63	Silica	63–106
S125	Silica	125–180
S250	Silica	250–355
C63	Ceramic	63–125
C125	Ceramic	125–250
C250	Ceramic	250–425

.

The ceramic particles possess material properties as follows: a hardness of 700 HV and a particle density of 3.85 g/cm³. On the other hand, the material properties of the silica particles include a hardness ranging from 500 to 550 HV and a density of 2.52 g/cm³.

2.2. Experimental Methods

2.2.1. Workpiece Preparation

For tribology testing, we used a pin-on-disc-type tribometer for evaluation of the coefficient of friction and wear of polyethylene. CoCr was prepared in a pin shape (Figure 1); to cleanse the surface material before performing the experimental study, we applied ultrasonic baths to all workpieces. Highly cross-linked polyethylene was used to form the workpiece in a disc shape for application as a disc in the pin-on-disc-type tribometer.

For wettability testing, CoCr was shaped cylindrically (Figure 2) with a diameter of 1 cm and a length of 1 cm. The surfaces of all workpieces were cleaned using ultrasonic baths before conducting the experimental study. Fine-particle shot peening was carried out on one side of the workpiece, and the wettability test was assessed by measuring the contact angle on the surface materials.

2.2.2. Surface Modification by Fine-Particle Shot Peening

A peening machine (Sinto brand, MY-30AP series, Nagoya, Japan) was utilized for the conventional peening process. The peening pressure was adjusted to 0.5 MPa, with a 20 cm distance between the nozzle and the material surface. A nozzle angle of 90 degrees was maintained during the process. The experiments were carried out under environmental control at room temperature (25 degrees Celsius) and atmospheric pressure.

Three different particle sizes were employed for each particle type (silica and ceramic particles), as indicated in Table 2, as mentioned previously.

2.2.3. Wettability

The wettability of both the un-peened and peened workpieces’ surface materials was assessed by measuring the contact angle. The camera was placed 10 cm away from the workpieces, and the macro mode was used for taking a picture. A droplet of normal saline solution was applied to the workpiece surface, and the contact angle was measured at the three-phase boundary where liquid, gas, and solid intersect. The pictures were used to measure the contact angle using the Image J programme on a laptop. This setup is shown in Figure 3.

2.2.4. Tribology Test

The tribology tests, which included assessing the coefficient of friction and the friction force between two surfaces, were conducted using a CSM high-temperature tribometer (Anton Paar, TRB3, Graz, Austria)(Figure 4). The tribometer used was a pin-on-disc type with CoCr alloy (un-peened and peened in various particle types) in a pin shape, using HxPE for the disc. In the chamber between the motion surface, a 0.9% NaCl solution was applied for lubrication. Polyethylene was fixed in the chamber (Figure 5) that contained the lubricant. The tribometer was used under atmospheric pressure with the temperature of 25 degrees Celsius, and 10 N was loaded for 30,000 cycles. The speed was set at 20 cm/s. Finally, the coefficient of friction and friction force were measured.

2.2.5. Wear Volume Measurement

Wear volume was evaluated using a surface roughness tester (Keyence brand, VR-5000 series, Tokyo, Japan). The wear volumes were calculated from a wear track on the polyethylene surface after the tribological test. The average wear area under the surface materials was calculated using a computerized programme; the wear volume was then calculated using the following formula:

$$2\times 3.14 \times radius of wear track \times average area of wear track under polyethylene$$

Figure 6 displays the wear track on the polyethylene surface after the tribological test. The wear track was assessed using a surface roughness tester, and the wear volume was determined by the area under the surface materials.

3. Results and Discussion

3.1. Wettability Test

The concepts of contact angle and surface wettability, which represent the lubrication properties of the surface, were described by Thomas Young in 1805 [37]. Wettability is defined as the attraction of a liquid phase to a solid phase. Different intermolecular interactions between the liquid and the solid cause materials to have different wettability properties [38,39,40]. There are two different types of contact angles: static contact angles and dynamic contact angles [41]. Static contact angles are more commonly utilized due to their relatively easy measurability. A lower contact angle infers greater wettability, whereas a higher contact angle represents lower wettability (Figure 7). A motion surface needs greater wettability to improve the lubrication properties that can affect the wear of materials on the surface. Therefore, enhancing the wettability of CoCr material may reduce the wear and tear process on the bearing surfaces used in total hip arthroplasty, resulting in the increased longevity of the implant in the human body.

The wettability of the surface material before and after the peening process was assessed by measuring the contact angle. The results of using different contact angles are shown in Figure 8.

The contact angles obtained after the surface was modified by FPSP indicated that the contact angles of the peened workpieces were lower than those of the un-peened workpieces for both silica and ceramic particles (Figure 9). Thus, the FPSP process can be inferred to increase the hydrophilic properties of a surface to create more wettability, which improves the lubrication properties of the motion surface.

To explain how particle size affects the wettability of the surface material, in a recent study [30], it was demonstrated that different particle types can affect surface properties, including surface microstructure, hardness, and roughness. The findings indicated that the use of various types and sizes of peening media can significantly alter the surface morphology of the material. The study also revealed that proper peening parameters can enhance the surface hardness and reduce the surface roughness of CoCr alloy. In terms of surface morphology, ceramic particles were observed to create clearer dimples than silica particles. The higher hardness of ceramic particles, compared to silica particles, led to more pronounced changes in surface microstructure, hardness, and roughness. Consequently, the wettability of the surface material varies when employing different particle types in the fine-particle shot peening process, influencing surface microstructure, morphology, and roughness, along with the formation of distinct dimples on the surface.

In terms of the type of lubrication, we believe that this study primarily focuses on comparing the surfaces before and after surface modification. Therefore, the type of lubrication was not considered to be the main factor in this study. The normal saline used in this study was considered to be an isotonic solution.

The results of this study showed that after the surface had been modified through the fine-particle shot peening process, the wettability of the surface was improved when compared with the un-peened surface, and also showed that a smaller particle size could create a smaller contact angle than a larger particle size, but the type of particle (ceramic or silica particles) did not show a significant difference. This difference can affect the lubrication properties, resulting in a change in the wear and tear process on the motion surface when applied to the bearing surfaces used in THA.

3.2. Tribology Test

A pin-on-disc tribometer was widely used to quantify the friction force, coefficient of friction of the surface material, and the wear of the material couple under controlled environmental conditions. Many recent studies have performed pin-on-disc experiments attempting to determine the wear of polyethylene in the motion surfaces of human implants [42,43,44]. The tribometer (pin-on-disc type) can evaluate the tribological properties, including friction force, which can be calculated into the coefficient of friction (Figure 10). In THA implants, the bearing surface is the most important part that affects the longevity of the implant. Therefore, the enhancement of the tribological properties of the motion surface can improve the surgical outcomes in patients. The improvement of tribological properties through surface modification can be an important factor that may signal the development of new surface materials for bearing surfaces in hip joint replacements [45].

The tribology test, which involved evaluating the friction force between two surfaces, was conducted using a CSM high-temperature tribometer. The coefficient of friction was calculated based on the friction force and load acting on the motion surface. A pin-on-disc-type tribometer was used, using CoCr alloy in a pin shape and highly cross-linked polyethylene for the disc. In the chamber between the motion surfaces, a 0.9% NaCl solution was used as lubrication. The coefficient of friction was calculated every 2 s.

The result showed that FPSP can significantly reduce the friction coefficient of the surface material between CoCr and highly cross-link polyethylene for both types of particles (ceramic and silica). In the case of different particle types, the findings revealed that ceramic particles could significantly reduce the friction coefficient to a greater extent than silica particles, especially when compared at the same particle size (Figure 11). Additionally, concerning various particle sizes, larger particles were observed to have a more pronounced effect on reducing the friction coefficient between the motion surface compared to smaller particles.

A recent study [23] indicated that particles with higher hardness can more distinctly and profoundly alter the surface morphology, corroborating another previous study’s findings that ceramic particles, being higher in hardness, can produce clearer surface dimples compared to silica particles. Furthermore, in terms of particle size, both ceramic and silica particles of a larger size were observed to generate larger and clearer dimples on the CoCr surface, as opposed to smaller particles [30].

Micro-dimpling on a motion surface can improve the lubricant properties of the surface, leading to a lower friction force when the motion surface by the dimple is lubricated. The results from this study show the same direction as a recent study [30]; particles with higher hardness and larger particle sizes were found to have a significantly reduced average coefficient of friction compared to particles with a lower hardness and smaller particle sizes.

The use of micro-dimples on a surface material can improve the tribological properties of a motion surface by increasing the fluid-film formation between the coupled surface and enhancing the lubrication properties, resulting in a decrease in the friction force between the motion surface and a reduction in the wear process of surface materials. The recent literature has indicated that micro-dimpling on a ceramic bearing in total hip arthroplasty can reduce the coefficient of friction compared to a non-dimpled surface [15]. Other recent studies have also shown that appropriate micro-dimpling can enhance lubrication properties and decrease the friction coefficient, ultimately reducing wear and tear on the motion surface [16,17,18,19].

The hardness of the surface material is a crucial factor influencing its strength and wear process. Hardness is defined as the resistance of the material to permanent deformation, and it is indicative of its resistance to scratches that may occur during motion, leading to surface irregularities and increased sharpness. High-hardness materials exhibit greater resistance to scratching compared to materials with lower hardness.

The recent literature has demonstrated that the fine-particle shot peening process can elevate the surface hardness of materials [46]. Another study has reported similar findings, indicating a significant increase in the surface hardness of CoCr alloy after undergoing fine-particle shot peening. Moreover, the type and size of particles were found to influence the surface hardness of CoCr materials [30]. Consequently, materials with higher hardness, enhanced through fine-particle shot peening, may be a favourable choice for medical implants, especially those employed in the moving parts of the human body, such as joint arthroplasty implants, dental implants, and other orthopedic applications. This is attributed to their properties of wear and scratch resistance, which can contribute to the longevity of the implant in the patient.

Therefore, the improvement of lubrication properties and surface hardness are the crucial factors that can reduce the average friction coefficient after performing fine-particle shot peening on a CoCr surface. Similar to a recent study in the extant literature [19], the result also showed that micro-dimpling on the motion surface of an artificial hip joint can enhance the lubricant film formation when compared with a non-dimpled surface. FPSP can create a small dimple on the surface material that can enhance the lubrication properties via the dimple acting as a pool for the lubrication and decrease the friction force between the motion surfaces. The results showed that FPSP can decrease the coefficient of friction between the motion surfaces that can be applied to artificial implants in the human body using the CoCr femoral head on highly cross-linked polyethylene, with synovial fluid in the hip joint acting as the lubrication.

3.3. Wear Volume

Wear volume was measured by the surface roughness tester by calculating the wear track on the surface of highly cross-linked polyethylene after 30,000 cycles of the tribological test. The results are shown in Figure 12.

In addition, the results showed that FPSP can significantly reduce the wear volume of highly cross-linked polyethylene in both ceramic and silica particles. Yet, the results for different particle types also showed that ceramic particles can reduce the wear volume more than silica particles when comparing particles of the same size. When different particle sizes were compared, the results showed that a larger particle size can reduce the wear volume of a highly cross-linked polyethylene surface more than a smaller particle size.

The wear of polyethylene is a serious problem in artificial joints when implanted into the human body. For instance, the wear debris of polyethylene can induce the inflammatory process and increase the osteoclast activity that destroys the bone around the implant, resulting in debonding between the bone and implant interface (loosening of the implant). In THA patients, the loosening of the implant is the most important cause of revision surgery [7,8,9].

By stimulating the function of an artificial joint that uses a CoCr femoral head and highly cross-linked polyethylene, this study showed that a lower wear volume of polyethylene can be achieved using a better motion surface. The results of this study thus suggest that applying FPSP to the CoCr surface using various particle types can significantly reduce the wear of polyethylene.

4. Conclusions

The main objective of this study is to enhance the surface of CoCr to reduce abrasion on the articulating surface when used with bearing surfaces used in total hip arthroplasty, which are made of highly cross-linked polyethylene. The focus is on developing the metal femoral head section without modifying the polyethylene, enabling it to be utilized alongside the existing polyethylene available in the market.

The results from this study showed that using the FPSP process on CoCr material can significantly enhance the tribological properties, including the coefficient of friction, and decrease the wear volume of highly cross-linked polyethylene. Developing the properties of the surface of CoCr, which is a material used in the femoral head parts of total hip arthroplasty, can be applied to implants for human use.

FPSP can address this problem by creating micro-dimpling on the surface material and producing a smoother surface that can reduce the friction force and enhance the lubrication properties between the motion surface. FPSP may reduce the wear process by three factors:

• FSPS can increase the surface hardness in particles of any size [28], which may affect the tribological properties by enhancing the scratch resistance of the metal surface, thereby reducing the abrasive wear on the moving surface.
• FPSP can produce a smoother surface and reduce the sharpness of pits on the metal surface, thereby decreasing the coefficient of friction between the moving surfaces and reducing the wear process on polyethylene.
• FPSP can create small micro-dimples that act as pools of lubricant, enhancing the wettability of the metal surface and improving its lubrication properties.

Notably, these results may apply to THA bearing surfaces made of the same material combination used in this study. As a future research direction, we will apply this knowledge to artificial hip joints and perform simulation tests to study the wear of polyethylene in real situations mimicking the human body (ball and socket motion). A reduction in the wear process can increase the longevity of implants in the human body and reduce the incidence of revision surgery in patients.