Three-Year Clinical Results of a Cruciate-Retaining Type of the Knee Prosthesis with Anatomical Geometry Developed in Japan

Arata Nakajima (  arata.nakajima@med.toho-u.ac.jp ) Toho University Sakura Medical Center Manabu Yamada Toho University Sakura Medical Center Masato Sonobe Toho University Sakura Medical Center Yorikazu Akatsu Toho University Sakura Medical Center Masahiko Saito Chiba Medical Center Keiichiro Yamamoto Toho University Sakura Medical Center Junya Saito Toho University Sakura Medical Center Masaki Norimoto Toho University Sakura Medical Center Keita Koyama Toho University Sakura Medical Center Hiroshi Takahashi University of Tsukuba Yasuchika Aoki Chiba University Toru Suguro Japan Research Institute of Arti cial Joint Koichi Nakagawa Toho University Sakura Medical Center

≧1 mm wide were detected in ve knees (4.1%). There were no major complications needing revision surgeries.
Conclusions: Patient-reported outcomes (PROs) for symptoms, pain and ADL after the CR-FINE TKA were generally improved, but those for sports, QOL and FJS were improved less. The incidence of radiolucent lines was rare but detected around the femoral components. Improvements of surgical technique or innovation of the implant design with mid-to long-term follow-up will be necessary to achieve better PROs from patients receiving the FINE knee.

Background
Although the outcomes of total knee arthroplasty (TKA) are generally acceptable, approximately 20% of patients have some complaints after TKA (1)(2)(3). The reasons for dissatisfaction after TKA remain poorly understood; however, failure of restoration of a physiological joint line has been suggested as a causative factor. In 2011, Bellemans introduced kinematically aligned (KA)-TKA as a surgical technique to realize a physiological joint line (4). With KA-TKA, the femoral and tibial components are implanted with mild varus limb alignment, relative to neutral alignment, in order to restore the physiological joint line to a prearthritic state. While good patient reported outcomes after KA-TKA have been reported (5)(6)(7)(8)(9), the longevity of polyethylene inserts and femoral and/or tibial components implanted with a varus alignment are a concern (10, 11).
The FINE total knee has been developed in Japan and used for approximately 20,000 TKAs of Japanese patients since 2001. It has unique design features, including an oblique 3 o femorotibial joint line (Fig. 1). This feature enables the implant to reproduce anatomical geometry and allows the osteotomy to be performed perpendicular to the mechanical axis. The FINE total knee is also designed to guide internal movements of the tibia via medial pivotal rotation, thus permitting deeper exion of the knee to better match the lifestyle needs of Japanese populations (12). The medial surface of the polyethylene insert has a convex curve which is designed to increase the rate of conformity to the femoral component, thereby enhancing internal rotation of the tibia. Conversely, the lateral surface has a at surface that has been designed to allow femoral rollback, enhancing internal rotation of the tibia via medial pivotal motion (12). Hence, the design concepts of the FINE total knee facilitate KA-TKA via conventional osteotomy.
Although 20 years have passed since this implant was clinically used for the rst time in Japan, a formal clinical evaluation including patient-reported and radiographic outcomes has not been undertaken. The aim of the present study was to evaluate the 3-year clinical results including patient-reported and radiographic outcomes in Japanese patients receiving a cruciate-retaining (CR) type of the FINE total knee.

Methods
Patients A total of 175 consecutive primary TKAs using a CR type of the FINE total knee (Teijin-Nakashima Medical Co. Ltd., Okayama, Japan) in 157 patients at our hospital between February 2015 and March 2017 were included in this study. One patient (one TKA) died from TKA-unrelated causes. None received a revision. Of the remaining 174 TKAs (156 patients), 122 knees (111 patients, 70%) were available for 3year follow-up data; the data for the Knee Injury and Osteoarthritis Outcome Score (KOOS) (13) except for a sports subscale and radiographs were available for all of those while data for KOOS-sports and the Forgotten Joint Score (FJS) (14) were available for 53 (30%) and 77 knees (44%), respectively (Fig. 2).
There were nine men and 102 women, with a mean age of 72.3 years (29-89) at the time of surgery. The mean body mass index was 27.2 kg/m 2 (16.7-39.6). One hundred twelve TKAs were performed for osteoarthritis, seven for rheumatoid arthritis (RA), and three for osteonecrosis (Table 1).
This study was approved by the institutional review board at our institution. All activities were performed in accordance with the ethical standards set forth in the Declaration of Helsinki, and informed consent was obtained from all patients who participated in this study.

Surgical procedures
All TKAs were performed using the measured resection technique by anterior reference (15). Surgical approaches were chosen either mid-vastus or sub-vastus for varus knees, but the lateral parapatellar approach was used for valgus knees. A release of the deep bers of the medial collateral ligament (MCL) was routinely performed for varus knees. Surgeries were performed using conventional instruments; that is, distal femoral osteotomy was conducted perpendicular to the mechanical axis at a level 9-10 mm from the farthest point of the medial condyle, and the posterior condyle was osteotomized parallel to the surgical epicondylar axis (3°external to the posterior condylar line). A tibial osteotomy subsequently was conducted perpendicular to the anatomical axis of the tibia. The cutting level was set 8-10 mm distal to the convex of the lateral plateau. Following osteotomy, adjustments for soft tissue balancing were performed before the implants were xed to the bone with cement. Whether to replace the patellar component or not depended on the surgeons' decision; 80 knees received patellar replacement (66%, Table 1). Patients were discharged three weeks after surgery when they were medically stable, with pain controlled by oral analgesics and deemed by a physiotherapist to be mobilizing su ciently to function safely at home.

Radiographic examinations
Routine postoperative assessment included anteroposterior, lateral, and 60°skyline radiographs of the knee, and full-length standing radiographs of both lower limbs. The anatomical axis (the angle subtended by lines bisecting the medullary canals of the femur and the tibia) and the mechanical axis (the angle subtended by lines connecting the center of the femoral head and the center of the femoral condyles, and the center of the tibial plateau to the center of the talus) were measured from full-length standing radiographs. The alignment of the components was assessed on AP radiographs of the knee using the distal femoral valgus angle (DFVA, α) and proximal tibial varus angle (PTVA, β), while the femoral exion angle (FFA, γ) and tibial slope (TS, σ) were measured on lateral radiographs. The mechanical alignment was assessed by the hip-knee-ankle (HKA) angle based on the full-length standing radiographs with varus alignment designated as positive (Fig. 3). These measurements were performed using the protocol of Kilincoglu et al (16). Three independent observers (AN, MY, KY) examined radiographs for evidence of anterior notching of the femur, component failure or subsidence, lucent lines, osteolysis, and heterotopic ossi cation based on the standardized Knee Society radiological evaluation system (17).

Clinical evaluation
We used the Knee Society Score (KSS), which consists of a knee score (KSS-KS) and a function score (KSS-FS), as an objective evaluation of knee function (18). In addition to the KSS, we used the Japanese KOOS, an instrument of con rmed validity and reliability for patient-reported outcomes (PROs) based on its cross-cultural adaptation (19). The KOOS consists of a total of 42 knee-related items, and each item was scored from 0-4. Five KOOS subscales, including symptoms (KOOS-symptom), pain (KOOS-pain), ADL (KOOS-ADL), sports/recreation (KOOS-sports), and quality of life (KOOS-QOL) were converted to 100 points (13). Furthermore, we investigated the FJS for 77 TKAs (72 patients). The range of motion (ROM) was measured using a goniometer. Postoperative scores were compared with the preoperative scores. Both intraoperative and postoperative complications were noted from the medical records.

Statistical analysis
The reliability of each radiographic measurement was assessed using intraclass correlation coe cients. All radiographic measurements in this study showed good reliability (all values > 0.8), and discrepancies were discussed until consensus was achieved. The paired t-test was used to compare 3-year postoperative with preoperative scores. Results were expressed as the mean (standard deviation, SD). Correlations among postoperative KSS, KOOS and FJS were analyzed by Pearson's correlation coe cients. Data analyses were performed using SPSS software, version 21 (SPSS Inc., Chicago, IL, USA) and p-values of < 0.05 were considered statistically signi cant.
Mean postoperative exion angle (SD) was 124 (13.3)°. The postoperative exion angle increased signi cantly compared with the preoperative one (p < 0.001, Table 2). The mean improvement in ROM (SD) was 13 (19)° (Table 2). Seven knees had limited extension; six knees had an extension limitation between 10°and 20°and one knee had a limitation of 25°.

Correlations among postoperative KSS, KOOS and FJS
Correlations among postoperative KSS, KOOS and FJS were computed to investigate the relationships among these postoperative outcomes. FJS was correlated signi cantly with KSS-FS and all the subscales of KOOS, but not with postoperative exion angle or ROM (Table 4).
Radiographic analyses three years postoperatively revealed no instances of osteolysis or subsidence. Radiolucent lines ≧1 mm wide were detected in ve knees (4.1%), all of which occurred in zone 4 of the femoral components but were insigni cant clinically. There was one knee with heterotopic ossi cation in the quadriceps (0.8%), but it was asymptomatic (Table 5).
Complications One patient died due to a cause unrelated to TKA. One had a suspicious deep infection but joint uid culture was negative for bacteria and the knee was not revised. There were eight partial tears of the popliteal tendon intraoperatively, all of which were sutured using nylon thread. One patient had an intraoperative avulsion of the super cial bers of the MCL from its insertion to the tibia, which was reconstructed by suture and pull-out. One patient with RA had a medial subchondral fracture of the proximal tibia intraoperatively, which was xed using a cancellous screw. There was one lateral supracondylar fracture intraoperatively in an RA patient and one anterior nothing without a periprosthetic fracture, but no additional surgeries were required. One patient with a severely deformed valgus knee (preoperative femorotibial angle: 152°) had transient peroneal nerve palsy postoperatively but had recovered fully by the 3-year follow-up (Table 6).

Discussion
The most characteristic point of the FINE knee is the design that reproduces the anatomical geometry, that is, a 3°obliquity built into the medial femorotibial surface in both coronal and axial planes. Here we showed that the postoperative mean DFVA was 98 o , slightly more than that of the conventional prostheses. These features allow surgeons to perform KA-TKA without the femoral component implanted slightly in valgus and the tibial component implanted slightly in varus. Furthermore, the FINE knee adopts an ultra-high molecular weight polyethylene insert including vitamin E with antioxidant properties. As there is an increased risk of revision in KA-TKA using conventional prostheses (20,21), these characteristic designs of the FINE knee are expected to show superior longevity to the conventional prostheses implanted in kinematic alignment.
The second characteristic point of the FINE knee is the polyethylene insert, which is dish-shaped medially and has a at-surface laterally. This structure allows natural internal rotation of the tibia and roll-back of the lateral femoral condyle during exion, leading to deep exion. Here, the mean postoperative exion angle (SD) and the improvement in ROM were 124 (13)° and 13°, respectively ( Table 2). The correlation analyses among postoperative KSS, KOOS and FJS showed signi cant correlations between exion angle and KSS-KS, and ROM and KSS-KS, while exion angle and ROM did not show any signi cant correlations with KOOS subscales or FJS (Table 4). These results suggest that postoperative exion angle does not have a signi cant impact on PROs if it reaches over 120°. Rather, stability during all ranges of motion may be a more important factor to achieve better PROs.
For all patients included in this study, we performed TKAs using a measured resection (MR) technique. van Lieshout et al. showed that the joint line was elevated after TKA using the MR technique (22). In addition, Luyckx et al., using cadaver knees, demonstrated that despite a well-balanced knee in full extension and at 90°of exion, increased mid-exion instability was evident in knees in which the joint line was raised (23). Because we cut femoral posterior condyles by anterior reference and used a CR-type for the patients who participated in this study, we might have implanted femoral components that were smaller than the anatomical anteroposterior length of the femoral condyles, and this may have caused shortening of the posterior condylar offsets. This may also raise the joint line, which in turn, causes midexion instability.
Recently, the FJS has been used to evaluate top-performing TKAs since it has a diminished ceiling effect (14). Parratte  To obtain better PROs from the FINE knee, we are now performing TKAs using the pre-cut technique developed by Kaneyama et al (27), which allows the amount of cutting of the femoral posterior condyles to the extension gap to be adjusted. These improvements in surgical technique are expected to avoid shortening of the posterior condylar offset, leading to better ROM and stability in exion after TKA (28)(29)(30). Although we could not show better PROs for the CR-FINE knee than other top-performing prostheses developed in the US or Western Europe, these improvements in the surgical technique will raise the PROs after the CR-FINE TKA.
Radiographic analyses for the CR-FINE TKA three years postoperatively demonstrated radiolucent lines more than 1 mm wide in ve knees (4.1%) ( Table 6). All radiolucent lines were detected in zone 4 on the femoral component side. The possible reason for the radiolucent line in this location may be the application of less or no bone cement in that zone to avoid spill-over after implantation of the femoral components. Although no clinical problems occurred over three years postoperatively, continuous attention should be paid to whether these radiolucent lines will spread or not. It should be noted that no subsidence of the tibial components occurred during the 3-year follow-up period. Tibial base plates of the FINE knee adopt an asymmetric design between medial and lateral sides, which provides good coverage on the cut surface of the tibia. These features probably contribute to the absence of subsidence of the tibial components.
There are some limitations to this study. First, the sample size was relatively small and the patients were recruited from a single institution. Second, 30% of the knees were not available for analysis of 3-year KOOS data. Moreover, only 30% of knees were available for a KOOS-sports subscale, and 44% were available for FJS. Nevertheless, our results show that the FINE knee is widely acceptable for Japanese patients with knee deformities, considering the good PROs comparable to other top-performing knee prostheses.

Conclusions
We showed 3-year clinical results of a CR-type of the FINE total knee that was implanted through the MR technique. Postoperative KOOS-symptom, -pain and -ADL were > 85 points, but KOOS-sports, -QOL and FJS were less satisfactory. There were no major complications needing revision surgeries. Incidence of radiolucent lines was rare around the femoral components. There were no instances of osteolysis or subsidence. Improvements of surgical technique or innovation of the implant design with mid-to longterm follow-up will be necessary to achieve better PROs from patients receiving the FINE knee.