Evaluation of the Contact Area in Total Knee Arthroplasty Designed for Deep Knee Flexion

Total knee arthroplasty (TKA) implants are becoming an interesting subject in implant design research and development activities due to their complexity. They should be able to facilitate knee movement while supporting body weight during daily usage. Meanwhile, incidents such as hyperflexion in TKA implants outside their designated configuration can lead to subluxation and dislocation in this study, a polyethylene component of a posterior-stabilized right knee joint implant was developed to facilitate a high range of motion (ROM). Finite element analysis (FEA) was used to analyze the contact area on the polyethylene component. FEA was used to simulate weight-bearing conditions at 0°, 30°, 60°, 90°, 120°, and 150° of knee flexion. The modified polyethylene component resulted in better performance in terms of contact area, especially at 120° of knee flexion. The two dominant contact areas on the polyethylene component were 733 mm² at 0° of knee flexion and 576 mm² at 120° of knee flexion. Furthermore, the current design of the polyethylene component can maintain a contact area of 65 mm² at 150° of knee flexion. The current design is expected to accommodate deep knee flexion movement in daily activities and reduce the possibility of subluxation and dislocation at the polyethylene component during deep knee flexion. In addition, a large contact area can reduce the potential wear on or fracture of the polyethylene component. Finally, the result of FEA was validated using a simulator of knee kinematic motion; there was no indication of subluxation and dislocation at any degree of knee flexion.

Dislocation; Finite element analysis; Hyperflexion; Polyethylene; Subluxation; Total knee arthroplasty

    Fiber- Knee replacement surgery or total knee arthroplasty (TKA) for the treatment of chronic degenerative pathologies of the knee has been a successful method for 60 years. During this period, the collaboration between surgeons and engineers has resulted in many developments in prosthesis design (Karczewski et al., 2021). For example, the first TKA allowed a single degree of freedom, but now TKA offers multiple degrees of freedom (Murray, 1928). Almost all TKAs performed in the United States and Europe have a range of motion (ROM) of 120° for knee flexion. This satisfies the inhabitants of the West, as it accommodates the range of motion required for most of their daily activities. However, this is not true for people in East Asia and the Middle East, whose sociocultural background varies greatly, as does their normal range of joint motion. This therapy is frequently refused because the resulting ROM is restricted (Villar et al., 1989). Squatting is used to perform activities on the toilet or just to rest, and it can easily be done for hours (Ahlberg et al., 1988). A cross-legged sitting posture is popular in many regions of Asia for eating on the floor as well as for informal activities such as chatting. Furthermore, kneeling is a popular practice among Muslims during prayer as well as among Japanese people during traditional rituals. For example, it has been reported that Saudi men have a knee flexion difference of more than 15° compared to Scandinavians (Hefzy et al., 1998). Most of the population in the Middle East routinely bend their knees to 165°. At prayer, most Muslims kneel with their limbs fully extended (between 150° and 165°) and with the shaft of the heel erect, reaching the posterior surface of the upper thigh. Since most commercial TKAs available today are not designed to achieve knee flexion of more than 120°, commercial TKAs do not meet the needs of patients in predominantly Muslim countries and Asian countries who practice traditional kneeling and sitting poses. In addition, differences in morphometry data between populations around the globe require a specific implant design for each population (Utomo et al., 2019). Hyperflexion outside the design configuration of implants are can lead to subluxations and dislocations (Li et al., 2004).

According to Thiele et al. (2015), some causes for revising the design of total knee arthroplasty are aseptic loosening (34.7%), instability (18.5%), and polyethylene wear (18.5%) (Thiele et al., 2015). These failure mechanisms have been associated with thin polyethylene (Pijls et al., 2012; Massin, 2016; Garceau et al., 2020; Presti et al., 2020; Crawford et al., 2021; Tzanetis et al., 2021). Polyethylene wear may result in osteolysis and the subsequent loosening of the components (Pijls et al., 2012; Massin, 2016). A significant amount of contact stress in the posterior post region might explain polyethylene wear and fracture in posterior-stabilized (PS) TKA (Nor Izmin et al., 2020; Garceau et al., 2020; Crawford et al., 2021; Tzanetis et al., 2021). Therefore, it is necessary to be prepared for patient management following possible knee implant failure.

Finite element analysis (FEA) is a versatile tool for studying the contact area during the design of the polyethylene component (Ahmad et al., 2020). Evaluation by FEA of the contact area on the polyethylene component, particularly in the posterior region of the post, may assist in avoiding issues that develop after TKA. In previous studies (Ishikawa et al., 2015; Tanaka et al., 2016; Zhang et al., 2017; Azam et al., 2018; Kang et al., 2018; Tanaka et al., 2018), an FEA of weight-bearing deep knee flexion was performed for 0° to 120° of knee flexion. In this study, FEA was performed for 0° to 150° of knee flexion.

    In this study, the polyethylene component of posterior-stabilized right knee joint prostheses was developed from the benchmark product. Benchmarking is a systematic method or process of measuring product performance by comparing it with products from other companies that are considered the best in the same industry. Vanguard Posterior Stabilized Knee Zimmer Biomet was used as benchmark in this study. FEA was used to measure the contact area on the polyethylene component. We hypothesized that the geometric modification of the polyethylene component could improve the contact area and increase the distribution of contact stress; this may reduce subluxation and dislocation at the polyethylene during deep knee flexion and minimize the risk of implant failure. 

The current results were compared to the results of previous studies (Tanaka et al., 2016). For the FEA result using polyethylene, the largest contact areas were 329 mm² at 0° of knee flexion and 146 mm² at 120° with the knee bent (Tanaka et al., 2016).

Table 2 The contact areas (in mm²) on the polyethylene component during 0° to 150° of knee flexion from a comparable study by Tanaka et al. (2016) and the current study

In the current study, the largest contact areas were 733 mm² at 0° of knee flexion and 576 mm² at 120° with the knee bent. In previous studies, the contact area on the polyethylene was minimal at 60° of knee flexion. Meanwhile, in the current study, the contact area on the polyethylene was minimal at 150° of knee flexion. In the current study, the contact area was largest at 0° of knee flexion and decreased with increasing knee flexion. At approximately 90° of knee flexion, the posterior post region of the polyethylene began to come into contact with the femoral cam through the post-cam mechanism (Tanaka et al., 2016). This differs from the current results obtained, namely that the posterior post region of the polyethylene contacted the femoral cam at about 60° and that post-cam contact increases the contact area on knee flexion, as presented in Table 2.

According to Nakamura et al. (2015), In computer simulation research, the anteroposterior translation and geometric center of the femoral component were measured using the medial and lateral contact tool. Contact tools are important for evaluating the contact area and the contact stress (Nakamura et al., 2015) because a small contact area and a high contact stress are thought to induce significant problems such as polyethylene insert wear and fracture or severe post wear (Puloski, 2000; Reay et al., 2001; Mauerhan, 2003; Clarke et al., 2004; Casey et al., 2007).

In the current study, the contact area on the polyethylene was minimal at 150° knee flexion. This should be of particular concern because insufficient contact area might induce wear and fracture of polyethylene. The mean contact stress for arthroplasties is inversely proportional to the contact area. The contact area on the polyethylene component significantly increased during deep knee flexion. Many factors influence the site of the contact between the post and cam, including the form of the cam, the position of the attachment of the cam to the femoral component, and the curvature of the posterior femoral condyles (Utomo et al., 2020). The modification on polyethylene post feature is advantageous for reducing excessive tension at the bone-implant contact and preventing post fracture. In the current study, the design’s curved shape of the polyethylene post increased the contact area during deep flexion.

    The FEA was conducted under the assumption that all force was applied to the tibiofemoral articular surface and to the post’s polyethylene component. In our study, a relative force of 4 kN (Walker et al., 1997; Kuriyama et al., 2014; Azam et al., 2018) was applied. Despite this limitation, our research has provided vital information about the tibiofemoral joint and the post-cam mechanism. The design of the polyethylene component resulted in an increase in the contact area; this is expected to accommodate high flexion in daily activities and reduce the risk of subluxations and dislocations during deep knee flexion. 

    The author would like to thank the Indonesian Institute of Sciences (LIPI) for its financial support through a 2019 scientific scholarship (SK Number: B-4919/SU.3/HK.01/V/2019) and the TI-Bio Laboratory of the University of Indonesia, Depok, Indonesia, for providing all the facilities for this research. This research was funded by the University of Indonesia through the PUTI Saintekes 2020 program (NKB-4964/UN2.RST/HKP.05.00/2020). 

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