Does a Uni “Feel Better” than a Total Knee? Not Necessarily, When Using Modern Implant Designs

Abstract

Background: When comparing functional outcomes of patients with unicompartmental knee arthroplasty (UKA) versus total knee arthroplasty (TKA), studies often report the UKA as the preferred procedure; however, recent improvements in the design of modern TKA implants have aimed at narrowing this gap. Purpose: We sought to compare the “feel” of modern TKA implants to that of UKA, using the Forgotten Joint Score (FJS), a validated patient-reported outcome measure. Methods: We performed a retrospective review of patients who underwent TKA and UKA at 2 institutions between 2014 and 2017. All UKA procedures were robotic arm-assisted with a single implant, “traditional TKAs” were performed using traditional posterior-stabilized implants, and “modern TKAs” were performed using posterior-stabilized implants with a modern design. Differences in FJS were assessed using 1-way analysis of variance and independent 2-sample t tests. Results: A total of 600 patients were included in our study, with 200 patients in each surgical subcategory. Mean age was 62.8 ± 10.2 years and mean body mass index was 29.9 ± 4.9. Modern TKA and UKA had similar FJS at 1 year. While modern TKA had a significantly higher FJS than traditional TKA, UKA did not have a significantly higher FJS than traditional TKA. Conclusion: Our retrospective analysis found no significant differences in the FJS of patients who underwent UKA and TKA with a modern design; however, both had superior scores than traditional TKA designs. This finding suggests that modern TKA designs may have the potential to achieve the natural feeling that is typically associated with joint-conserving surgeries such as UKA, although longer follow-up is necessary.

Introduction

Osteoarthritis is the most common joint disease in the United States, with a lifetime incidence of approximately 50% [9,12]. If conservative treatments fail, patients may choose to undergo a knee replacement. Surgical options for knee replacement include total knee arthroplasty (TKA) and unicompartmental knee arthroplasty (UKA).

While TKA has traditionally been considered the gold standard for knee arthritis, advocates of UKA note that its advantages include less invasive surgery, ease of revision, faster recovery, superior range of motion and kinematics, and lower reported joint awareness [1,5–7,13]. Despite these benefits, UKA has several potential downsides compared with TKA, including higher revision and failure rates [8,10].

Assessing the “feeling” of a replaced joint after surgery has always been challenging, highlighting the weaknesses of standard outcome measures. The Forgotten Joint Score (FJS) assesses the patient’s ability to forget about the replaced joint and return to a feeling of normalcy [3]. It is a powerful outcome measure specifically for its ease of use, prior validation, and lack of a ceiling effect [2]. Given the continued debate over which surgical procedure provides patients with the most normal-feeling knee, we sought to compare the FJS among surgical groups at least 1 year after surgery. We hypothesized that modern TKA would result in better outcome measures than traditional TKA and more like those found with UKA.

Methods

We performed a retrospective review of patients who underwent TKA and UKA at 2 institutions between 2014 and 2017. Adult patients who underwent UKA, TKA using a modern design, or TKA using a traditional design were eligible for inclusion. Modern design was defined as a posterior-stabilized implant that sacrifices the anterior and posterior cruciate ligaments (ACL and PCL, respectively) with both an anterior and posterior cam-post mechanism, asymmetric femoral condyles, an oblique joint line, a medial pivot kinematic pattern, anatomic shaped tibia, and 1-mm tibial polyethylene inserts. Traditional design was defined as posterior-stabilized implant with symmetric condyles, a neutral joint line, no significant medial pivot kinematics, and a symmetrical tibia. Patients in each surgical group were similar across baseline variables for each surgical category. Patients undergoing TKA at either of the 2 institutions who had 1 of the 3 procedures and a minimum of 1 year of clinical follow-up were included in the study. All UKA procedures were of a single implant (Mako Restoris; Stryker), robotic arm–assisted, and on the medial side. The TKA procedures with a modern design were all a single implant (Journey II; Smith & Nephew), and those with a traditional design used a single implant (Vanguard PS; Zimmer Biomet), as well. All surgeries were performed by 1 of 4 knee surgeons with a volume of 300 or more knee procedures per year.

Demographic data including sex, body mass index (BMI), and age were collected at baseline. For each of the 3 surgical subcategories, a trained research assistant administered the FJS to patients at a minimum of 1 year following arthroplasty. Instructions were given to each patient to complete the questionnaire with respect to their recent knee surgery. Completed forms were scored using the standardized FJS scoring metric. This study was approved by the ethics committee at both participating institutions. Informed consent was obtained from each patient prior to enrollment in the study.

Statistical Analysis

Categorical variables were characterized using frequencies and percentages. Continuous variables were assessed using means and standard deviations. We used 1-way analysis of variance (ANOVA) testing to assess for differences in group means across the 3 surgical subcategories. Because ANOVA does not identify significant differences in individual comparisons, we also tested differences in each of the 3 scores across all 3 surgical subcategories using independent 2-sample t tests. Consideration was given to using a correction for multiple hypothesis testing. However, we deemed multiple hypothesis testing methods like Bonferroni unnecessary because the number of independent tests was fewer than 10, and corrections of multiple comparisons are often considered too conservative for limited numbers of hypotheses. To better understand the correlation between age and BMI with each score in the surgical subgroups, we also created linear models. All the statistical analyses were performed with the use of R software, version 3.5 (R Project for Statistical Computing).

Results

Each of the 3 surgical subgroups—UKA, modern TKA, and traditional TKA—included 200 patients from the database. While the 3 groups were similar across age, sex, and BMI, there were some minor discrepancies between groups (Table 1). The group undergoing modern TKA was younger (58.1 ± 9.8 years) and had a higher proportion of women (59%) than the UKA or traditional TKA patient groups. The 600 patients in this study had a mean age of 62.8 ± 10.2 years and a mean BMI of 29.9 ± 4.9 kg/m². Overall, 51% of patients were female.

Table 1.

Baseline characteristics.

	UKA (n = 200)	Modern TKA (n = 200)	Traditional TKA (n = 200)
Sex
Female	87 (44%)	117 (59%)	99 (50%)
Male	113 (57%)	83 (42%)	101 (51%)
Age, y
Mean (SD)	64.5 (9.9)	58.1 (9.8)	65.7 (9.3)
BMI, kg/m2
Mean (SD)	28.6 (4.7)	30.8 (5.0)	30.2 (4.7)

UKA unicompartmental knee arthroplasty, TKA total knee arthroplasty, BMI body mass index.

Average follow-up was 1.6 years (range 1.0–1.9 years) and there was no significant difference in mean follow-up between groups (P = .67). There were no missing data for FJS for any of the 3 prosthetic types. The mean FJS values for UKA, modern TKA, and traditional TKA were 73.7 ± 25.7, 77.3 ± 19.3, and 72.0 ± 19.8, respectively. The ANOVA analyses revealed statistically significant differences among the 3 surgical groups (P = .048). For the FJS, there was no difference noted between UKA and modern TKA (P = .11). Modern TKA, on the contrary, was associated with a significantly higher FJS (P = .007), relative to traditional TKA. The UKA did not have a significantly higher FJS than traditional TKA (P = .47).

When examining the relationship between BMI and reported outcome measures, the most striking difference was seen for the association of BMI with FJS (Figs. 1 and 2). The FJS was similar among all 3 surgical subgroups for patients with low BMI. However, patients with higher BMIs reported superior FJS scores for the modern TKA prosthetic, relative to UKA and traditional TKA.

Fig. 1.

Shown are the Forgotten Joint Score (FJS), plotted against age, for each prosthetic subtype. The shaded regions indicate 95% confidence intervals.

UKA unicompartmental knee arthroplasty, TKA total knee arthroplasty.

Fig. 2.

Shown are the Forgotten Joint Score (FJS), plotted against body mass index (BMI) for each prosthetic subtype. The shaded regions indicate 95% confidence intervals.

UKA unicompartmental knee arthroplasty, TKA total knee arthroplasty.

Discussion

To our knowledge, this is the first study to assess differences in how UKA, modern TKA, and traditional TKA patients “feel” their knees. We found that, consistent with our hypothesis, patients who underwent modern TKA were more likely to forget about their joint than traditional TKA patients and were more comparable to patients who had undergone UKA.

There were several limitations to this study. First, patients were not randomized to the type of arthroplasty, and differences in confounders at baseline could have altered our results, even though baseline values were relatively similar across the 3 groups. A large, randomized controlled trial could help definitively determine the superior prosthetic for each patient subpopulation, although such trials are difficult to perform in orthopedics, especially for surgical treatments. Second, the difference between postoperative and preoperative scores is likely a more statistically meaningful measure than the absolute postoperative FJS. The difference in scores helps account for baseline morbidity and allows for a better understanding of improvement in relevant measures following surgery. Third, postoperative measures were obtained at a single date, at least 1 year after surgery. Trending postoperative measures over several years would improve assessments of long-term outcomes but could be confounded by natural declines in some patient-reported outcome measures (PROMs) over time [3]. Finally, this was a retrospective analysis and is therefore subject to the risk of biases inherent in observational designs.

A recent prospective study comparing patients with UKA and traditional TKA found that patients with medial UKA reported superior results, relative to traditional TKA, across a range of outcomes, including awareness of the prosthesis, functional outcome, clinical outcome, and quality of life [9]. The authors hypothesize that the conservation of soft tissue and bone with UKA could improve patients’ ability to forget about the joint. However, the authors do note several limitations, including a short follow-up time and group allocation based on preference. In our study, the improved ability of patients with modern TKA to forget about their joint may be due to the potentially improved kinematics of a modern design prosthesis [4].

A systematic review and meta-analysis published in 2019 sought to present a clear and comprehensive summary of data on UKA and TKA [11]. The review found that both TKA and UKA are viable options for knee arthroplasty in a broad range of patients, but UKA outperformed TKA in several outcome domains including shorter operating times, improved range of movement, and fewer early complications (including myocardial infarction, cerebrovascular events, deep tissue infection, and venous thromboembolism). Revision rates were significantly better overall for TKA. Unlike our study, the systematic review did not analyze PROMs as assessed by FJS.

In conclusion, our retrospective analysis of how patients feel after knee surgery using a validated outcome measure suggests that implant design could have more effect than previously considered. In the coming years, large, randomized controlled trials should assess arthroplasty procedures across a range of outcomes, to determine whether certain prostheses may be more effective for different patient subpopulations and clinical scenarios.