Abstract
Background:
An increasing number of total knee arthroplasties (TKAs) are performed in people with obesity, but TKAs in this population may come with increased risk of perioperative complications and decreased prosthetic survivorship. Given the lack of conclusive evidence on differences in functional outcomes, we aimed to use the Forgotten Joint Score-12 (FJS-12) to see how body mass index (BMI) affected functional outcomes after TKA.
Methods:
We recruited patients who underwent primary unilateral TKA because of osteoarthritic changes from January 2018 to November 2021. We collected the Forgotten Joint Score-12 (FJS-12) measure of functional outcomes and the Western Ontario and McMaster University Osteoarthritis Index (WOMAC) preoperatively and 6- and 12-months postoperatively. We also measured length of stay (LOS), readmission, and emergency department (ED) visits. We compared outcomes by BMI category using linear effects models.
Results:
We recruited 351 patients. No differences were found in LOS, readmissions, and ED visits by BMI category. Compared with the preoperative score, we observed significant differences by BMI category for the 6-month FJS-12 (β = −0.66, p = 0.007) and 12-month WOMAC (β = −0.34, p = 0.02) scores. At 6 months, patients with lower BMI showed a greater change in FJS-12 scores than those with higher BMI. However, by 12 months, all patients appeared to return to similar functional levels regardless of BMI.
Conclusion:
Despite a slower return to function, patients with elevated BMI were able to return to similar levels of function as those with a lower BMI by 12 months, with no significant differences in readmission, ED visits, or LOS. This similar return to function justifies candidacy for surgery.
Abstract
Contexte:
Les chirurgies pour prothèse totale du genou (PTG) sont de plus en plus nombreuses chez les personnes obèses. Or, chez cette population, la PTG peut s’accompagner d’un risque accru de complications périopératoires et d’un abrègement de la durée de vie de la prothèse. Étant donné l’absence de données concluantes quant à des différences de résultats fonctionnels, nous avons voulu utiliser le « score de l’articulation oubliée » (Forgotten Joint Score-12 [FJS-12]) afin de mesurer l’impact de l’indice de masse corporelle (IMC) sur le fonctionnement post-PTG.
Méthodes:
Nous avons recruté des personnes ayant subi une PTG primaire unilatérale pour arthrose entre janvier 2018 et novembre 2021. Nous avons noté le score de fonctionnement FJS-12 et l’indice de gravité des symptômes de l’arthrose WOMAC (Western Ontario and McMaster University Osteoarthritis Index) avant l’intervention, puis 6 et 12 mois plus tard. Nous avons aussi mesuré les durées de séjour (DDS), les réadmissions et les consultations au service des urgences (SU). Nous avons comparé les résultats par catégorie d’IMC au moyen de modèles à effets linéaires.
Résultats:
Nous avons recruté 351 personnes. Nous n’avons observé aucune différence aux plans des DDS, des réadmissions et des consultations au SU selon la catégorie d’IMC. En comparaison avec les scores préopératoires, nous avons constaté des différences significatives selon la catégorie d’IMC pour le score FJS-12 à 6 mois (β = −0,66, p = 0,007) et l’indice WOMAC à 12 mois (β = −0,34, p = 0,02). Après 6 mois, les personnes dont l’IMC était moins élevé présentaient une plus grande amélioration de leurs scores FJS-12 comparativement aux personnes dont l’IMC était plus élevé. Par contre, après 12 mois, toutes les personnes semblaient avoir récupéré un niveau de fonctionnement similaire, indépendamment de l’IMC.
Conclusion:
Malgré une évolution plus lente, au bout de 12 mois, les personnes ayant un IMC élevé ont pu retrouver un niveau similaire de fonctionnement comparativement aux personnes dont l’IMC était plus bas, sans différences significatives aux plans des réadmissions, des consultations au SU, ou des DDS. Cette amélioration similaire du fonctionnement est un argument en faveur de l’intervention chez cette population.
There is an increasing demand for total knee and hip arthroplasties as the population ages worldwide. Obesity has been associated with chronic conditions — including hypertension, type 2 diabetes, cardiovascular disease, certain types of cancer, and osteoarthritis — and is thought to play a substantial role in the increasing number of total knee arthroplasties (TKAs).1 With the prevalence of obesity increasing, it is projected that as many as 69% of TKAs could be performed in patients with obesity (body mass index [BMI] > 30) by 2029.2 Currently, there is some controversy as to the appropriateness of performing TKA in patients with obesity, given concerns of increased risk of medical perioperative complications and potentially decreased prosthetic survivorship.3–7 However, studies comparing patients with and without morbid obesity found no conclusive evidence to suggest significant differences in revision rates, time to discharge, disposition, functional independence measures, or discharge range of motion.7–11
The outcomes of TKA are assessed by various methods; however, there is considerable emphasis placed on postoperative functional outcomes. This is understandable, as the goal of TKA surgery is to alleviate pain and improve function. Therefore, several scoring systems have been devised to ascertain whether patients are subjectively satisfied with the functional outcomes of their arthroplasty. This can include clinician- and patient-reported scores. A widely accepted clinician-reported score is the Knee Society Score (KSS). Accepted patient-reported functional outcome measures include the Oxford Knee Score (OKS), the Western Ontario and McMaster University Osteoarthritis Index (WOMAC), and the Forgotten Joint Score-12 (FJS-12).
The FJS-12 is a 12-point questionnaire developed in 2012 as a powerful method to assess functional outcomes after total joint arthroplasty.12 This scoring system exhibits high validity and reliability scores.12–19 Furthermore, it is less susceptible to ceiling and floor effects, demonstrates higher internal consistency, and shows more responsiveness than previous functional outcome measures.12–19 It was shown to be particularly useful at 6-month and 1-year follow-up, and to maintain validity thereafter.18,19 The development of the FJS-12 has created a more sensitive measure of functional outcomes after a TKA. It is less susceptible to the pitfalls of previously used patient-reported function outcome measures.
Despite the increasing prevalence of obesity and TKAs, there is a still a lack of prospective literature in this area, especially using the FJS-12 scoring system. As such, we sought to use the FJS-12 to assess whether there was a significant difference in functional outcomes after TKA between patients with an elevated BMI and those with a lower BMI. In using this more sensitive scoring system, we hope to delineate the true effects of obesity on outcomes after TKA.
Methods
Study design and participants
From January 2018 to November 2021, we screened patients undergoing primary unilateral TKA because of osteoarthritic changes who attended preoperative physiotherapy education sessions. We excluded patients with previous history of TKA or total hip arthroplasty, those undergoing bilateral TKA, or those undergoing TKA for tumours or trauma. Baseline data collected included demographics (age, sex, weight, and height) and medical comorbidities. Surgical data and secondary outcome measures included length of stay and outcomes up to 1-year post-TKA, including readmission to hospital, visits to the emergency department (ED) pertaining to pain or postoperative complications, and infection requiring surgical intervention. Primary outcome measures used were the FJS-12 and WOMAC, measured before surgery, 6 months after surgery, and 1 year after surgery.
Sample size estimation
Using Stata MP 15, we conducted a series of power calculations for a linear regression slope test to determine the appropriate sample size for a minimal clinically important difference (MCID) in FJS-12 scores by BMI. At the time of study conception, we did not find any literature regarding the MCID when using FJS-12. Studies with comparable scales such as the OKS found an MCID of 8%.18 To ensure adequate power, we assumed an MCID of FJS-12 scores of at least 6%. The power calculations required us to estimate the population standard deviations (SDs) for BMI and the FJS-12. For both of these variables, we assumed an underlying normal distribution. We tested BMI and FJS-12 SDs of 4 or 5 and 10, 15, or 20 respectively. We set power (1−β), the probability of correctly rejecting the null hypothesis, at a minimum of 80% and required an α, the probability of incorrectly rejecting a true null hypothesis, of 0.05. Assuming an MCID of 6%, the optimal sample size ranged between 250 and 350 patients, dependent on the underlying sample BMI and FJS-12 parameters.
Statistical analysis
We examined mean BMI levels by patient sex (male or female), readmission (admitted v. not admitted during 1-yr follow-up period), postoperative ED visits (present or absent during 1-yr follow-up period), and length of hospital stay (< 1 d, 1 d, 2 d, 3 d, ≥ 4 d). We also reviewed the correlation between patient age and BMI.
To estimate the effect of BMI on functional outcomes over the study period, we used 2 linear fixed-effects models, 1 per measure of function (i.e., FJS-12 and WOMAC). Within each model, we included BMI (continuous), time (categorical; baseline, 6 mo, 12 mo), and the interaction of BMI by time. We also adjusted for patient age, sex, presence of diabetes, and presence of cardiovascular comorbidities. To aid with interpretation of the BMI coefficient (a continuous variable), we used midpoints to represent each of the World Health Organization BMI categories (Table 1).20
Table 1.
WHO BMI classification20
| BMI range | BMI classification | Representative midpoint |
|---|---|---|
| < 18.5 | Underweight | 18 |
| 18.5–24.9 | Normal | 21.75 |
| 25–29.9 | Pre-obese | 27.5 |
| 30–34.9 | Obese I | 32.5 |
| 35–40 | Obese II | 37.5 |
| > 40 | Obese III | 42.5 |
BMI = body mass index; WHO = World Health Organization.
Ethics approval
Ethics approval was received from the Thunder Bay Regional Health Sciences Centre Research Ethics Board (no. 100046).
Results
Cohort demographics
Of the 408 patients who were screened, 351 enrolled. Of these, 296 patients completed the 1-year follow-up and were included in the analysis. Participant BMI and FJS-12 SDs met our sample size estimates, resulting in adequate power. In total, 24 (8.1%) patients were categorized as having normal BMI, 69 (23.3%) were deemed pre-obese, and 203 were categorized as obese (n = 82 [27.7%] obese I, n = 59 [19.9%] obese II, n = 62 [21.0%] obese III). Mean BMI was similar between male and female patients. The mean age was 66.04 (SD 7.64) years and was found to have a moderate negative correlation with BMI (r = −0.195, p < 0.001). Cardiovascular issues and diabetes were the most frequently reported comorbidities (63.5% and 20.9%, respectively) with presence of either associated with higher BMI (diabetes: mean BMI difference = 3.9, p < 0.001; cardiovascular: mean BMI difference = 2.5, p = 0.002. More information on patient comorbidities can be found in Appendix 1, available at www.canjsurg.ca/lookup/doi/10.1503/cjs.008524/tab-related-content). Complete patient demographic data can be found in Table 2.
Table 2.
Demographic data for patients undergoing elective TKA
| Characteristic | No. (%) of patients | BMI, mean ± SD | p value |
|---|---|---|---|
| Sex | 0.5 | ||
| Male | 116 (39.2) | 33.8 ± 6.3 | |
| Female | 180 (60.8) | 34.3 ± 7.3 | |
| Smoking status | 0.2 | ||
| Current | 22 (8.0) | 32.2 ± 9.0 | |
| Former | 97 (35.4) | 35.0 ± 6.6 | |
| Nonsmoker | 177 (64.6) | 33.8 ± 6.8 | |
| Alcohol intake | 0.01 | ||
| Daily | 47 (19.3) | 31.9 ± 7.2 | |
| Occasionally (< 1/d) | 149 (61.1) | 33.9 ± 6.9 | |
| Never | 95 (38.9) | 35.5 ± 6.4 |
BMI = body mass index; SD = standard deviation; TKA = total knee arthroplasty.
Overall, 48 (16.2%) patients were readmitted; however, mean BMI was not significantly different between those readmitted and those not readmitted (Table 3). Similarly, although 108 (36.5%) patients were seen in ED visits with complications, there were no significant differences in mean BMI between those who did or did not visit the ED (Table 3). Most patients (38.9%) stayed in hospital for 2 days after their surgeries. Very few patients (1.7%) stayed less than 1 day. Of the other patients, 27.0% stayed 1 day, 22.3% stayed 3 days, and 10.1% stayed 4 or more days in hospital after surgery. There were no significant differences in mean BMI by length of stay after surgery (Table 3).
Table 3.
Postoperative outcomes of patients undergoing elective TKA
| Postoperative measure | No. (%) of patients | Mean BMI, mean ± SD | p value |
|---|---|---|---|
| Readmission within 1 year | 0.9 | ||
| Yes | 48 (16.2) | 34.0 ± 6.5 | |
| No | 248 (83.8) | 34.1 ± 7.0 | |
| Postoperative infection visit within 1 year | 0.9 | ||
| Yes | 108 (36.5) | 34.2 ± 6.9 | |
| No | 188 (63.5) | 34.0 ± 7.0 | |
| Length of stay post-TKA, d | 0.6 | ||
| < 1 | 5 (1.7) | 32.6 ± 3.7 | |
| 1 | 80 (27.0) | 33.2 ± 7.1 | |
| 2 | 115 (38.9) | 34.3 ± 7.1 | |
| 3 | 66 (22.3) | 34.3 ± 6.6 | |
| ≥ 4 | 30 (10.1) | 35.5 ± 6.9 |
BMI = body mass index; SD = standard deviation; TKA = total knee arthroplasty.
Across all BMI categories, there were significant increases in FJS-12 scores at 6-(β = 71.4, p < 0.001) and 12-month (β = 57.6, p < 0.001) follow-up compared with preoperative FJS-12 (Table 4 and Figure 1). Outcome scores differed significantly by patient BMI level at the 6-month follow-up (β = −0.85, p < 0.001) but not at either the preoperative (β = −0.18, p = 0.07) or 12-month (β = −0.36, p = 0.1) visit. Compared with preoperative FJS-12 function scores, BMI level was significantly associated with change at the 6-month visit (β = −0.66, p = 0.007) but not the 12-month visit (β = −0.17, p = 0.6). For example, we observed increases in FJS-12 scores of 46.5 and 53.1 at 6 months for patients categorized as obese II and pre-obese BMI levels, respectively, representing a mean difference in FJS-12 score of 6.6 (95% confidence interval [CI] 1.8 to 11.4). However, for the same BMI levels, reductions of 51.1 and 52.8 were seen at 12 months, respectively, representing a mean difference in FJS-12 score of 1.7 (95% CI –3.0 to 6.6). The association between BMI level and change in FJS-12 score between 6- and 12-month follow-up visits was not statistically significant (β = 0.49, p = 0.05). Although FJS-12 scores appeared to remain stable for patients with BMIs ranging within the underweight through obese I categories, patients with BMI levels of obese II or obese III appeared to benefit from further increases in FJS-12 score between the 6- and 12-month follow-up visits.
Table 4.
Mean pre- and postoperative FJS-12 score of patients undergoing elective TKA
| Representative BMI midpoint | Mean (95% CI) | |||
|---|---|---|---|---|
| Preoperative FJS-12 score | 6-month postoperative FJS-12 score | 12-month postoperative FJS-12 score | Difference in 6-month and 12-month postoperative FJS-12 score | |
| 18 | 16.4 (12.6 to 20.2) | 75.8 (67.6 to 84.0) | 70.9 (62.4 to 79.3) | −4.9 (−13.6 to 3.8) |
| 21.75 | 15.7 (12.5 to 18.9) | 72.6 (65.9 to 79.3) | 69.5 (62.6 to 76.4) | −3.1 (−10.1 to 3.9) |
| 27.5 | 14.6 (12.0 to 17.1) | 67.7 (63.0 to 72.4) | 67.4 (62.6 to 72.3) | −0.3 (−5.0 to 4.5) |
| 32.5 | 13.6 (11.3 to 16.0) | 63.5 (59.7 to 67.2) | 65.6 (61.8 to 69.5) | 2.2 (−1.4 to 5.7) |
| 37.5 | 12.7 (10.1 to 15.3) | 59.2 (55.1 to 63.3) | 63.8 (59.6 to 68.0) | 4.6 (0.8 to 8.5) |
| 42.5 | 11.8 (8.6 to 14.9) | 54.9 (49.4 to 60.4) | 62.0 (56.4 to 67.7) | 7.1 (1.7 to 12.5) |
BMI = body mass index; CI = confidence interval; FJS-12 = Forgotten Joint Score-12; TKA = total knee arthroplasty.
Fig. 1.
Mean Forgotten Joint Score-12 (FJS-12) by body mass index (BMI) before and after total knee arthroplasty. CI = confidence interval.
Significant reductions were observed in WOMAC scores — representing an increase in function, irrespective of BMI level — at both 6-month (β = −33.5, p < 0.001) and 12-month (β = −26.7, p < 0.001) followup (Table 5 and Figure 2). Unlike the FJS-12 scores, the WOMAC scores at the preoperative visit of patients were significantly different (β = 0.31, p = 0.02) by BMI level but not at 6- (β = −0.22, p = 0.08) and 12-month (β = −0.03, p = 0.8) follow-up. Compared with preoperative WOMAC function scores, BMI level was significantly associated with change at 12 months (β = −0.34, p = 0.02) but not at 6 months (β = −0.08, p = 0.6). For example, reductions in WOMAC scores of 36.6 and 35.8 were observed at 6 months for patients with obese II (37.5) and pre-obese (27.5) BMI levels respectively, representing a mean difference in WOMAC score of 0.8 (95% CI −4.0 to 2.3). However, for the same BMI levels, reductions of 39.5 and 36.1, respectively were seen at 12 months, representing a mean difference in WOMAC score of 3.4 (95% CI 0.5 to 6.3). We observed a similar but less pronounced change in WOMAC scores from 6 to 12 months by BMI level (β = −0.26, p = 0.02).
Table 5.
Mean pre- and postoperative WOMAC score of patients undergoing elective TKA
| Representative BMI midpoint | Mean (95% CI) | |||
|---|---|---|---|---|
| Preoperative WOMAC score | 6-month postoperative WOMAC score | 12-month postoperative WOMAC score | Difference in 6-month and 12-month postoperative WOMAC score | |
| 18 | 44.5 (39.8 to 49.1) | 9.5 (5.0 to 13.9) | 11.6 (7.3 to 15.9) | 2.1 (−1.7 to 5.9) |
| 21.75 | 45.6 (41.7 to 49.5) | 10.3 (6.6 to 14.0) | 11.5 (7.9 to 15.0) | 1.2 (−1.9 to 4.2) |
| 27.5 | 47.4 (44.4 to 50.3) | 11.6 (8.8 to 14.4) | 11.3 (8.5 to 14.0) | −0.3 (−2.4 to 1.8) |
| 32.5 | 48.9(46.4 to 51.5) | 12.7 (10.2 to 15.2) | 11.1 (8.7 to 13.5) | −1.6 (−3.1 to −0.5) |
| 37.5 | 50.4 (47.7 to 53.2) | 13.8 (11.1 to 16.5) | 10.9 (8.3 to 13.6) | −2.9 (−4.6 to −1.2) |
| 42.5 | 52.0 (48.4 to 55.5) | 14.9 (11.5 to 18.3) | 10.8 (7.5 to 14.1) | −4.1 (−6.5 to −1.8) |
BMI = body mass index; CI = confidence interval; WOMAC = Western Ontario and McMaster University Osteoarthritis Index; TKA = total knee arthroplasty.
Fig. 2.
Mean Western Ontario and McMaster University Osteo-arthritis Index (WOMAC) score by body mass index (BMI) before and after total knee arthroplasty. CI = confidence interval.
Discussion
With the demand of TKA predicted to increase by up to 469% by 2060 in the United States alone, it is important that arthroplasty surgeons have a more concrete understanding of functional outcomes in patients.1 This is especially true for patients with high BMI who may be at a higher risk for needing a TKA. The literature surrounding the effect of BMI on outcomes after TKA is controversial. Many scoring systems have been used to tease out the relationship between BMI and TKA outcomes. Some studies have shown a negative effect of obesity on functional outcomes according to both OKS and WOMAC.22–24 Conversely, a study by Chen and colleagues25 measured outcomes with KSS and found a positive impact of obesity on functional outcomes. However, studies showing negative or positive effects often have small sample sizes or short follow-up periods. Overwhelmingly so, most studies have found no significant effects of obesity on functional outcomes after TKA; this is true for KSS, OKS, and WOMAC.26–38 Our study showed that, regardless of BMI, all groups had improvements in mean WOMAC scores within the 12-month follow-up period. Similar to previous studies, when comparing WOMAC scores of groups to the reference preobese BMI category, we found no significant differences at either the 6- or 12- month time point. We did, however, find a significant difference in the amount of change in WOMAC scores at 12 months, compared with preoperative scores. Specifically, those in the underweight and normal BMI categories had significantly less change in function than those in the obese I, II, or III BMI categories (p = 0.02 for all comparisons). These differences also existed between 12-month and 6-month post-operative WOMAC scores (p = 0.02 for all comparisons).
Compared with the WOMAC score and other measures of function, the FJS-12 was designed to be a more powerful method of measuring function after total joint arthroplasty and exhibits high validity and reliability.12–19 At the conception of this study, only a small number of studies had looked at the effect of BMI on functional outcomes post-TKA using the FJS-12, with 1 small study finding no significant effect and another study, which used multiple outcome measures, showing a negative effect of BMI.39,40 A more recent retrospective study with longer-term follow-up (2 yr), which compared FJS-12 scores of patients with level I, II, or III obesity, showed that although patients with higher BMI had lower mean FJS-12 scores, there were no statistical differences between groups. However, this study did not compare functional outcomes of patients with high BMI to those with normal or pre-obese BMI.41 There still exists a knowledge gap regarding the effect of BMI on functional outcomes after TKA, especially using a measure as sensitive as the FJS-12. Our study showed that all groups had improvements in mean FJS-12 over 12 months, with no statistical differences in groups at the preoperative stage. However, at 6 months, patients categorized in the lower BMI categories had significantly greater change in FJS-12 scores than those in the obese I, II, and III BMI categories.
With the increasing number of TKAs performed in patients with high BMI, it is important to be able to inform patients of the level of function they may be able to return to postoperatively. Our work shows that, despite a slower return to function, patients with elevated BMI were able to return to similar levels of function as those with lower BMI by the 12-month mark. Some literature points to higher complication rates after TKA among people with high BMI.3–7 Interestingly, we did not find any significant differences in readmission, postoperative ED visits, or length of stay. Despite the established increased risk of complications among people with high BMI in the literature, the similar return to function more than justifies candidacy for surgery.
Limitations
Limitations of this study include the use of a single centre in northwestern Ontario, which may inadequately capture the patient experience in Canada, let alone in an international setting. Furthermore, the recruitment methods relied on approaching patients during their preoperative physiotherapy education sessions. Although attendance at this session is strongly recommended before surgery, there was a notable drop in attendance from 2020 onward, presumably owing to hospital COVID-19 restrictions.
Conclusion
Despite a slower return to function, patients with elevated BMI were able to return to similar levels of function as those with a lower BMI by 12 months, with no significant differences in readmission, ED visits, or length of stay. This similar return to function justifies candidacy for surgery. Future directions for this research include a prospective study with longer follow-up to determine the potential differences in long-term functional outcomes, as well as other factors such as revision rate.
Supplementary Information
Footnotes
Competing interests: None declared.
Contributors: Mohammad Athar, Shalyn Littlefield, David Puskas, and Claude Cullinan contributed to the conception and design of the work. Simrun Chahal, Rabail Siddiqui, and Lahama Naeem contributed to data acquisition. Raghav Saini, Sacha Dubois, Kurt Droll, and Travis Marion contributed to data analysis and interpretation. Mohammad Athar, Simrun Chahal, Rabail Siddiqui, and Shalyn Littlefield drafted the manuscript. All of the authors revised it critically for important intellectual content, gave final approval of the version to be published, and agreed to be accountable for all aspects of the work.
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