1 Introduction

Total knee arthroplasty (TKA) is one of the most commonly performed orthopedic procedures, with more than one million cases conducted annually in the United States according to registry data.¹ Despite its widespread use, patient satisfaction following TKA remains relatively lower than that reported for total hip arthroplasty, with approximately 20 % of patients expressing dissatisfaction.² This highlights the need for continued refinement of surgical techniques and implant design to improve patient-reported outcomes and long-term implant survivorship.³

Among the various factors influencing TKA outcomes, implant design and size compatibility are critically important.⁴ Mismatch between the implant and patient anatomy has been associated with inferior clinical outcomes.^5–7 Most commercially available implants are based on Western anthropometric data; however, size mismatches have been reported even among Western populations.⁸ This issue is likely more pronounced in Asian patients, who generally exhibit smaller and morphologically distinct femoral and tibial geometries compared to their Western counterparts.^9,10

The FINE total knee prosthesis (Teijin Nakashima Medical, Okayama, Japan) was specifically developed using anatomical data from the Japanese population and has been in clinical use since 2001. Initially released as a cruciate-retaining (CR) type, a posterior-stabilized (PS) version was introduced in 2004. A notable feature of this implant is its asymmetric design incorporating a 3° tilt between the femoral component and tibial insert, aiming to replicate the physiological medial articular slope (Fig. 1). Additionally, given the association between medial pivot kinematics and improved patient satisfaction,¹¹ the insert is designed with high medial conformity and low lateral conformity to facilitate this motion. In the PS-type implant, the insert post is eccentrically located to further promote medial pivot mechanics. The inserts are manufactured from vitamin E-infused polyethylene to enhance wear resistance and durability.

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Fig. 1 Design of the FINE total knee prosthesis The design features a 3-degree tilt between the insert and femoral component to reproduce a physiological articular medial slope. The medial insert surface has high conformity, while the lateral surface has low conformity, to guide medial pivot motion. SEA, surgical transepicondylar axis.

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At our institution, the PS-type FINE prosthesis has been utilized in conjunction with a modified gap balancing technique since 2010. While clinical outcomes for the CR-type FINE implant have been previously reported,^12,13 to date, no study has specifically examined the mid-to long-term clinical and radiographic outcomes of the PS-type FINE prosthesis.

The purpose of this study was to evaluate the mid-to-long-term clinical and radiographic outcomes of the PS-type FINE total knee prosthesis in patients with a minimum postoperative follow-up of five years.

2 Materials and methods

This retrospective study included consecutive patients who underwent primary TKA using the PS type FINE prosthesis at our institution between March 2010 and October 2017, with a minimum postoperative follow-up of five years.

Clinical outcomes were assessed using the 1989 version of the Knee Society Score (KSS) and the Knee Injury and Osteoarthritis Outcome Score (KOOS). At the final follow-up, six independent orthopedic surgeons evaluated the KSS, KOOS, and passive range of motion (ROM) of the knee joint.

For radiographic assessment, the positions of the femoral and tibial components were measured on immediate postoperative plain radiographs, according to the method described by Ewald.¹⁴ The femorotibial angle (FTA) and hip-knee-ankle angle (HKAA) were measured preoperatively and at two weeks postoperatively on anteroposterior standing and full-length lower extremity radiographs.

Radiolucent lines were evaluated by zone at one year postoperatively and at the final follow-up. Zone classification was performed based on a modified version of Ewald's method,¹⁴ with minor adjustments made to the femoral zones to accommodate the two pegs of the FINE prosthesis (Fig. 2). Radiolucent lines measuring ≥2 mm were considered clinically significant.

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Fig. 2 Zone segmentation method of the femoral and tibial components for evaluating radiolucent lines Ewald's method was used for zone segmentation, with a slight modification on the femoral side to accommodate the two pegs of the FINE prosthesis. 2A radiolucent line of 2 mm or more was considered significant.

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Prosthesis survival was defined as the absence of revision surgery involving replacement of either the femoral or tibial component, or reoperation for any reason.

3 Results

We reviewed 247 consecutive primary TKAs performed using the PS-type FINE prosthesis. Of these, 88 knees discontinued follow-up before 5 years, 21 had no available clinical scores, 13 were transferred to other hospitals, and 11 died from causes unrelated to TKA within 5 years. Ultimately, 114 knees (84 patients) with a minimum of 5-year follow-up data were eligible for the analysis (Fig. 3). The mean follow-up period was 112.2 ± 26.0 months (range, 59–166). The mean age at surgery was 71.9 ± 7.7 years (range, 48–90). The mean height was 152.3 ± 7.9 cm (range, 138.0–170.5). The mean weight was 62.0 ± 12.0 kg (range, 39–98.6). The mean body mass index was 26.6 ± 4.2 kg/m² (range, 18.0–37.7). The underlying diagnoses were osteoarthritis (104 knees), rheumatoid arthritis (six knees), osteonecrosis (three knees), and hemophilic arthropathy (one knee). The surgical side was right in 57 knees and left in 57 knees (Table 1).

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Fig. 3 Patient inclusion flowchart A total of 114 knees (84 patients) with at least 5 years of follow-up data were analyzed. TKA, total knee arthroplasty; PS, posterior type.

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Passive ROM significantly improved between the preoperative assessment and final follow-up, from −8.9° to −2.6° in extension and from 121.4° to 124.5° in flexion. The clinical outcomes at 5 years were favorable (Table 2). Radiographic parameters are presented in Table 3. Postoperative assessments confirmed that neutral alignment was generally achieved. The alignment of the prosthetic components was deemed acceptable, taking into account the 3° valgus inclination incorporated into the design of the femoral component. At 1 year postoperatively, only one knee (0.9 %) exhibited a radiolucent line in tibial zone 1. At the final follow-up, radiolucent lines were observed on the femoral side; however, on the tibial component, radiolucent lines were observed in two knees (1.8 %) in zone 1 and three knees (2.6 %) in zone 2.

No patients underwent revision surgery during the follow-up period. However, reoperation was performed in four knees due to hematoma (two knees), infection (one knee), and patellar clunk syndrome (one knee). The survival rate, with reoperation as the endpoint, was 98.3 % at 5 years and 95.4 % at 10 years (Fig. 4). Other complications included two superficial surgical site infections and one postoperative patellar fracture, all of which resolved with conservative treatment.

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Fig. 4 Kaplan–Meyer graph illustrating the survival of the FINE prosthesis with reoperation as endpoint The survival rate was 98.3 % at 5 years and 95.4 % at 10 years. None of the cases required revision surgery, resulting in a 100 % revision-free survival at 10 years.

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4 Discussion

Prior reports on the FINE knee prosthesis are scarce. Nakajima et al. reported clinical outcomes after 3 years of follow-up with the CR-type.¹² No studies have evaluated the PS-type. Furthermore, no reports on the mid-to-long-term outcomes have been published for either type.

A key characteristic of the FINE is the higher conformity on the medial insert surface and the lower conformity on the lateral insert surface, which facilitates the reproduction of medial pivot motion. This ball-and-socket shape on the medial side, sometimes referred to as a “medical pivot-type,” has been associated with favorable clinical outcomes¹⁵ and survival rates.¹⁶ Reproducing medial pivot kinematics, resembling the native knee, has been shown to improve postoperative patient satisfaction.¹⁷ Miyazaki et al. demonstrated that the CR-type FINE reproduces medial pivot kinematics postoperatively using an image-matching technique.¹⁸ Subsequently, Tomita et al. confirmed that the same kinematics can be achieved with the PS-type using a 2D/3D registration technique.¹⁹ Another characteristic of the FINE prosthesis is the incorporation of a 3-degree medial tilt between the insert and femoral component, which produces a joint inclination akin to that of the natural joint surface when using the mechanical alignment method. The kinematic alignment method, which aims to replicate the anatomical joint surface, has been reported to yield better clinical outcomes than mechanical alignment²⁰; this built-in inclination is, therefore, expected to improve clinical outcomes.

Previous research has indicated that the FINE prosthesis provides better flexion ROM and pain relief than flat-surfaced prostheses designed for Asian populations.¹³ While our single-arm study design precludes comparison with other prostheses, our results are generally consistent with the mid-to-long-term clinical outcomes reported in previous studies.^16,21–23

Polyethylene wear has been reported to be lower in the medial pivot-type surface,²¹ suggesting enhanced durability compared to other insert types. In addition, the FINE insert contains vitamin E, which suppresses osteolysis by osteoclasts induced by wear particles.²⁴ These insert features are expected to improve implant survival, which was evident in this study; no revisions were required after a minimum follow-up of 5 years, the incidence of radiolucent lines was 0 % in the femur and only 2.6 % in the tibia, and the 10-year survival rate, with reoperation as the endpoint, was 95.4 %. Moreover, among the patients who required reoperation (4 out of 114 knees), two cases of hematoma and patellar clunk syndrome might have been preventable by meticulous intraoperative hemostasis and synovectomy.

This study had several limitations, including the low follow-up rate. This is primarily because many patients reside far from our facility as it is a core hospital in the prefecture, and patients find it increasingly difficult to travel as they age. Consequently, we cannot rule out the possibility of revisions or reoperations among the patients lost to follow-up. However, given the limited number of hospitals in those patients’ areas capable of managing complex surgical procedures, it is unlikely that any missed revisions or reoperations occurred. Second, because this was a single-center study, the generalizability of its findings is limited. Nevertheless, the inclusion of results from six surgeons offers greater generalizability compared to a single-surgeon study. Finally, this was a single-arm study. Because our institution exclusively uses a single type of prosthesis, except in a few special cases during the study period, comparison with other prostheses under the same conditions was not possible. Given that the results obtained in this study are comparable to the previously reported mid-to-long-term TKA results, the mid-to-long-term outcomes of the PS-type FINE prosthesis in Asian patients can be considered favorable.

5 Conclusions

The mid-to-long-term outcomes of the PS type FINE total knee prosthesis demonstrated favorable clinical and radiographic results, with high survivorship and low complication rates in a cohort of Asian patients undergoing primary TKA. Notably, no revision surgeries were required during the follow-up period, and the 10-year reoperation-free survival rate exceeded 95 %, indicating excellent implant durability. These findings suggest that the anatomical design characteristics of the FINE prosthesis—including its asymmetric articulation, high medial conformity, and 3° medial slope—are well-suited to the morphology and biomechanical demands of the Asian knee. Furthermore, the use of vitamin E-infused polyethylene likely contributed to the low incidence of radiolucent lines and absence of osteolysis, supporting the prosthesis’ long-term stability. Given the increasing global demand for TKA and the demographic shift toward aging populations, the development and evaluation of population-specific implants remain a critical component of personalized joint reconstruction. The results of this study underscore the clinical value of anatomically tailored implants and validate the PS-type FINE prosthesis as a reliable option for long-term joint function restoration in Asian patients. However, due to the retrospective design, absence of a control group, and single-center setting, further multicenter prospective studies with comparative cohorts and longer follow-up periods are warranted. Such investigations should aim to establish the generalizability of these findings and assess the relative performance of the FINE prosthesis against other widely used TKA systems in diverse populations. In conclusion, the PS-type FINE prosthesis offers durable, stable, and clinically effective outcomes in the long term. It represents a promising solution for surgeons seeking anatomically appropriate implants for Asian patients, with the potential to improve satisfaction and reduce the burden of revision TKA.

CRediT authorship contribution statement

Hiroya Akase: Design of the work, Data acquisition, Analysis interpretation of data, Drafting the work, Final approval. Masashi Miyazaki: Design of the work, Critical revision, Supervision, Final approval. Masashi Hirakawa: Analysis interpretation of data, Final approval. Yu Nagashima: Data acquisition, Final approval. Shougo Matsuda: Data acquisition, Final approval. Nobuhiro Kaku: Critical revision, Supervision, Final approval.

Consent to participate

All study participants or their legal guardians agreed to be part of the database and for the data collected to be used for research.

Guardian/patient's consent

The study protocol for this retrospective and noninvasive study was approved by the institutional review board of our hospital. Informed consent was obtained in the form of an opt-out, and the need to obtain informed consent from individual patients was waived by the ethics committee of our institution.

Ethics approval

All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards.

This study was approved by the Institutional Review Board of Oita University (approval number 1850). Informed consent was obtained from all patients using an opt-out method.

Availability of data and materials

The datasets used in this study are available after anonymization upon reasonable request.

Funding statement

This research received no specific grant from funding agencies in the public, commercial, or not-for-profit sectors.