Evaluation of Functional Outcomes, Survivorship and Complications of Hypoallergenic Fixed-Bearing Medial and Lateral Unicompartmental Knee Arthroplasty: A Minimum 2-Year Follow-Up

1. Introduction

Osteoarthritis (OA) of the knee is a degenerative joint disease that occurs when the cartilage in the knee joint gradually wears down. This can lead to pain, swelling, stiffness, and decreased mobility. It has been shown by recent studies that the prevalence of knee osteoarthritis worldwide is approximately 15% [1,2].

Its frequency increases with age, female sex, and other described characteristics. It represents a major cause of disability and source of societal costs in older adults [3].

Although OA is recognized as a tricompartmental disease of the entire knee [4], it is estimated that as many as 50% of the population experience unicompartmental disease. The medial tibio-femoral compartment is the most affected, with an incidence of 27%, followed by the patello-femoral compartment at 18% and lastly the lateral tibio-femoral compartment at 5% [5].

The vast majority of unicompartmental OA of the knee is located within the medial compartment, but about 10% is represented by osteoarthritis affecting the lateral compartment [6].

There is ongoing debate about the most appropriate treatment for patients with end-stage unicompartmental osteoarthritis, but unicompartmental knee arthroplasty (UKA) has proven to be a viable option for young and active individuals but also for older patients [7,8].

The Italian Arthroplasty Registry records a 22.2% (50,857) prevalence of primary unicompartmental knee arthroplasty (UKA) among all knee surgeries performed from 2007 to 2022. These findings demonstrate a rising trend in the utilization of unicompartmental knee replacements, which had a prevalence of approximately 15% in preceding years [9].

While UKA has gained popularity thanks to the development of new minimally invasive techniques, its rapid recovery after surgery and the bone stock preservation [10], careful patient selection is paramount to achieving successful outcomes [11].

Through rigorous patient selection and continuous refinement of surgical techniques, UKA has demonstrated excellent long-term clinical outcomes, with high survival rates and significant improvements in patients’ quality of life, with a mean 10-year survival rate of approximately 94% for medial UKAs [12,13,14].

Even for the lateral compartment, which is traditionally considered more challenging, excellent long-term results have been achieved thanks to implant manufacturing advancements and careful patient selection, with a mean 10-year survival rate of approximately 93% for lateral UKAs [15,16,17].

The literature consistently reports higher failure rates for UKA compared to Total Knee Arthroplasty (TKA). [18] [19] Failure etiologies vary between medial and lateral UKAs, with aseptic loosening, progressive osteoarthritis, polyethylene wear, and mobile dislocation being frequently observed [20,21].

Another cause of revision is metal hypersensitivity, which is extensively described in literature but still remains poorly regulated by guidelines [22,23,24].

A new fixed-bearing implant (Journey II UK Unicondylar Knee System or JUK, Smith & Nephew, Memphis, TN, US) has been launched in 2019. This implant achieves compatibility with a wide range of patients through the presence of components for both the medial and lateral compartments, a feature not always available in other implants, and an increased range of sizes, particularly smaller ones.

The introduction of a simpler instrument set allows to enhance and accelerate the timing of surgical technique due to the presence of a tibial guide that facilitates re-cutting and the ability to adjust the mediolateral position of the femoral component to allow for greater congruency with the tibial component.

The JUK II implant’s femoral component (Smith & Nephew, Memphis, TN, US) is composed of Oxinium™ and articulates against a cross-linked polyethylene surface via a round-on-flat bearing design, making it compatible with patients who have metal hypersensitivity.

Oxidized Zirconium (OxZr, Oxinium™, Smith & Nephew, Memphis, TN, US) is a biocompatible material that consists of a ceramic surface layer on an internal component of cobalt chrome. This process results in a material with enhanced properties, including minimized wear, increased strength and stability, and resistance to fretting and corrosion [25,26,27].

The previously cited combined characteristics have the potential to offer increased implant survivorship and improved clinical outcomes compared to currently available implants [28,29,30], owing to their greater patient-specific customization.

Following the introduction of a new implant, it is crucial to study its short-term performance to understand the implant’s survivorship, impact on clinical patient outcomes and to identify any potential weaknesses.

The objective of this study is to assess the functional outcomes, evaluate the survivorship and identify the reasons for failure a hypoallergenic, fixed-bearing medial and lateral UKA implant, during a short-term follow-up. All surgeries were conducted by a single surgeon at one institution between 2020 and 2022. This series is a follow-up to a cohort previously described by our group [31].

2. Materials and Methods

2.1. Patient Selection

Patients’ data for this single-center retrospective study were extracted from the database of Humanitas San Pio X Hospital systems (Milan, Italy). Upon collection, the data were deposited in a restricted-access online storage.

Data from UKA procedures, using medial or lateral JUK II implants (Smith & Nephew, Memphis, TN, US) performed from December 2020 to December 2022, were analyzed.

The data collected from the institutional database included demographic information, the body mass index (BMI), American Society of Anesthesiologists (ASA) class, duration of the surgical procedure, duration of hospitalization, any surgical revisions, any intra-operative, peri-operative and post-operative complications.

This study adheres to the ethical principles outlined in the 2013 revision of the Declaration of Helsinki. Prior to data collection, informed consent was obtained from all subjects involved in the study.

2.2. Surgical Indication and Inclusion and Exclusion Criteria

From a clinical standpoint, surgical intervention was proposed to patients who met the Kozinn and Scott criteria [32], including a coronal plane deformity of less than 10 degrees of varus and 5 degrees of valgus, and a sagittal plane flexion deformity no greater than 15 degrees. Additionally, patients were required to have an intact anterior cruciate ligament, an unaffected contralateral compartment, and patellofemoral changes not exceeding grade II or III on the Ahlbäck classification [33]. Trochlear cartilage damage up to grade IV was permissible, provided it was centrally located. Patients with neurological diseases, ataxia, symptomatic osteoarthritis in two or more compartments, or those who did not meet the aforementioned deformity criterias, were excluded.

Furthermore, surgical intervention was only proposed to patients who radiographically presented with isolated medial or lateral compartment osteoarthritis with a cartilage wear loss of Kellgren-Lawrence grade of 3 or higher [34], or those with spontaneous femoral osteonecrosis visible on X-ray as a cartilage damage of Kellgren-Lawrence grade 3 or higher, or on MRI. Prior to surgical intervention, patients underwent a 3-month course of conservative treatment including physiotherapy, hyaluronic acid or corticosteroid injections, and rest from physical activity. Surgical intervention was proposed only if pain and disability persisted after this conservative treatment period

The study included patients who underwent a primary medial or lateral UKA using the JUK II (Smith &Nephew, Memphis, TN, US) implant and had a minimum 2-year follow-up.

Patients were excluded if they had undergone bilateral UKA, concomitant surgical procedures on the same knee (e.g., ACL reconstruction or osteotomy), had a concurrent inflammatory rheumatic disease, had not a minimum 2-year follow-up and refused to the informed consent.

2.3. Surgical Procedure

All surgical procedures were performed by a single, high-volume senior surgeon (F.D.A.).

All patients received prophylactic antibiotic somministration of 2g of Cefazolin intravenously 30 minutes prior to the procedure. If the patient is allergic or has a hypersensitivity to penicillins, 600 mg of Clarithromycin is administered intravenously, 30 minutes prior to incision. All surgical procedures were performed under spinal anesthesia, with the addition of an adductor canal block when deemed necessary. No tourniquet was used during any procedure.

While the technique for medial UKAs has been extensively described in a previous article [31], there are some differences in the lateral UKA technique. The skin incision in the latter case is in the lateral region of the knee, utilizing a parapatellar approach. The tibial cut was not made along the tibial epiphyseal axis but rather aimed for a cut perpendicular to the mechanical axis or slightly valgus, with a neutral slope [Figure 1]. The surgical steps followed the manufacturer’s recommendations. After performing the tibial cut, a trial 8mm spacer block was used to prevent flexion deformity, and the distal femoral cut was made using a specific template. If the 8mm trial spacer block does not fully fit within the joint, the instrumentation includes the use of a tibial re-cutting guide. The proximal femoral cut was then completed by aligning the component with the tibial component. Within this surgical instrument set, femoral cutting guides are categorized into groups ranging from 1 to 3, 4 to 7, and 8 to 10. Following the identification of the appropriate size based on the cartilaginous line, the femoral cutting component can be precisely adjusted medially or laterally by approximately 1 to 3 millimeters to accommodate individual anatomical variations. Once the trial components were positioned and the final sizes determined, the articular surface was roughened with multiple saw cuts to improve cement penetration. [31] Before cementation, the surface was washed with a pulsed lavage to remove debris, and the entire surgical team changed gloves. The prepared cement was applied to both the bone surface and the components. After implantation, excess cement was removed, paying particular attention to the posterior region, the patellofemoral joint, and the contralateral compartment. A trial polyethylene component was used to re-assess stability and limb alignment. If these characteristics were deemed satisfactory by the primary surgeon, the final polyethylene component was implanted. Following hemostasis, the wound is thoroughly irrigated and closed in layers. The deep fascia is sutured with a waterproof material, and the skin is closed primarily with staples or an intradermal suture. When clinically indicated, a 3-gram intra-articular injection of tranexamic acid was administered following closure of the articular capsule. Following surgery, postoperative radiographs are obtained to verify component placement.

2.4. Postoperative Care

Adhering to a fast-track protocol, patients are mobilized on the same day and commence physiotherapy under the guidance of a physiotherapist with a partial progressive weight-bearing. As thromboprophylaxis, all patients receive 4000 IU of low-molecular-weight heparin starting 12 hours post-surgery and continuing for 5 weeks. Post-operative follow-up includes weight-bearing anteroposterior and lateral radiographs at 1, 3, 6, 12, and 24 months.

Radiographic assessment of the knee joint was conducted utilizing the Knee Society Roentgenographic Evaluation system [35]. This system enabled the evaluation of the femoro-tibial angle and the identification of any radiolucent areas within the femur and tibia on both anteroposterior and lateral radiographic projections.

The decision to revise the surgical intervention was informed by a comprehensive assessment encompassing patient symptomatology, documented medical history, diagnostic imaging results, and intraoperative observations.

2.5. Clinical Outcomes

Outcomes Patient-reported outcome measures (PROMS) were analyzed at baseline, 12, and 24 months. These included the Knee Society Knee Score (KSKS), the Knee Society Function Score (KSFS) [36], the Oxford Knee Score (OKS) [37], and the University of California, Los Angeles Activity Score (UCLA) [38].

Physical Component Summary (PCS) and the Mental Component Summary (MCS) of the Short Form 36 Health Survey (SF-36) [39] and the Forgotten Joint Score-12 (FJS-12) [40] were analyzed too. This last score was collected at 12 and 24-month follow-up. Patient satisfaction (rated from 1, not satisfied, to 10, completely satisfied) was collected at the last follow-up visit.

2.6. Statistical Analysis

All analyses were carried out using R (version 4.4.2). The Shapiro–Wilk test was employed to assess whether the data exhibited a non-parametric distribution. Calculated mean values were provided for all continuous data, while percentage frequencies were used for qualitative variables. Baseline and postoperative clinical scores were compared using the non-parametric Friedman test, a method for repeated measures analysis. This test was applied to evaluate differences in the KSKS, KSFS, OKS, UCLA, PCS, and MCS scores between baseline and the next two time points (12 and 24 months). The test was also applied to evaluate the FJS-12 at 12 and 24 months. A p-value < 0.05 was considered statistically significant.

3. Results

3.1. Demographics

The demographics data are reported in Table 1.

3.2. Clinical Outcomes

The clinical and functional outcomes of the patients and the result of the statistical analyses are reported in Table 2.

3.3. Complications and Revisions

Analysis of 257 cases revealed a 99.61% implant survival rate at 24 months using re-surgery for any reason as an endpoint. There was a single adverse event related to aseptic loosening of the tibial component. The patient underwent a conversion to TKA 12 months after the UKA.

No additional intraoperative or rehabilitation-related complications were encountered.

4. Discussion

The new hypoallergenic fixed-bearing medial and lateral UKA implant provided excellent clinical results with low rate of complications. Statistical analysis demonstrated highly significant improvements in all clinical outcome scores (p < 0.05) between preoperative and postoperative assessments.

By preserving a larger portion of the articular surface, UKA allows for more natural and fluid knee motion [4][43]. This translates into an improved body image and greater patient satisfaction, as demonstrated by the result of the patients’ satisfaction with a mean of 9.17 ± 1.16 at the last follow-up.

We compared the FJS-12 at 12 months and at the last follow-up of 24 months demonstrating a statistically significant improvement from 81.64 ± 16.77 to 85.47 ± 15.38 (p < 0.05) showing patient’s successful adaptation to life with the artificial joint. The data obtained in our study are superior to those found in literature [44,45].

The preservation of bone stock and the reproduction of a native-like knee biomechanics results in higher UKAs FJS-12 scores when compared to TKA [46]. Of the 257 patients, only one case of aseptic loosening of the medial tibial component was observed after 1 year from surgery, with no intraoperative complications reported on the other patients. The data obtained thus far suggest that our results are at least on par with, and potentially better than, those previously published [47,48,49,50].

Following a comprehensive literature review, only one additional study was identified that evaluated the outcomes of both medial and lateral JUKII [51]. Similar to our study, only primary medial and lateral UKAs performed by high-volume surgeons were retrospectively analyzed. The total cohort comprised 944 UKAs, with 370 reaching at least 1.75 years of follow-up, comparable to our cohort. Using re-operation for any reason as an endpoint, 2-year survival was 98.2%. Of the 9 re-operations, however, only 3 required conversion to TKA. Clinical outcomes demonstrated progressive improvements; however, only 81 patients completed all KOOS assessment stages. While the KOOS showed a positive effect in this subgroup, no statistically significant difference was found compared to patients who did not complete all follow-up stages [51].

It is well-established that engaging in any level of physical activity, even at a low intensity, is a protective factor against many diseases, particularly cardiovascular conditions. Moreover, maintaining adequate muscle tone helps to reduce the incidence of musculoskeletal injuries. In patients with knee osteoarthritis, the role of physical activity is even more pronounced. Given the increasing number of patients undergoing UKA, including younger individuals, the proportion of patients who return to sport (RTS) is a crucial indicator of the procedure’s success, as it also leads to improved PROMs.

A meta-analysis conducted in 2022 revealed that, with a mean follow-up of 48 months, over 90% of patients returned to sports, a finding consistent with our study [52]. RTS was linked to improved PROMs, despite being undertaken at reduced intensities compared to pre-surgery levels [52].

The mean preoperative UCLA score of our patients improved from 4.53 ± 1.6 to 7.3 ± 1.65 after 24 months of follow-up. Similarly, a 2021 study by Panzram et al. demonstrated a statistically significant increase in UCLA scores from a pre-operative mean of 2.9 ± 1.7 to a post-operative mean of 6.3 ± 1.4 at a minimum 2-year follow-up [53]. Furthermore, it is noteworthy that the proportion of high-impact sports activities post-surgery is not substantial, and elevated BMI levels negatively impact these characteristics [53].

A cohort of 85 patients with a minimum follow-up of 2 years, found no significant difference in RTS (amateur level) between medial and lateral UKA, aligning with our findings [54]. Consequently, both procedures can be considered for younger patients with high functional demands.

Our cohort consisted exclusively of patients receiving hypoallergenic implants. These findings align with the existing literature indicating that the selection of hypoallergenic implants does not compromise return to sports (RTS) compared to traditional chromium-cobalt implants or with other materials, even in patients with metal hypersensitivity [55,56].

All hypoallergenic implants used in this study had an Oxinium™ component. The study of D’Ambrosi et al. in 2022, does not demonstrate any inferiority of this component compared to other materials used in other UKAs [57].

Despite the positive outcomes observed with Oxinium™ in our cohort and other studies previously cited, it is important to recognize that the scientific literature also describes adverse clinical cases characterized by complications like metallosis and polyethylene dislocations both in UKAs and TKAs [58,59,60,61].

This study has several limitations. Firstly, conducted at a high-volume tertiary referral center, its findings may not be generalizable to institutions with lower UKA volumes. Secondly, the retrospective design, the lack of a control group and the inclusion of both medial and lateral UKAs within the same study introduce potential biases. Thirdly, the relatively short 2-year follow-up period is a limitation.

5. Conclusions

Our study of the new Oxinium™ metal-backed fixed-bearing medial and lateral UKA implant demonstrated favorable clinical outcomes, along with excellent 2-year survival rates, successful return to sports activity, and high patient satisfaction. However, randomized controlled trials with extended follow-up periods are necessary to validate these findings with a higher level of evidence.