Association Between Multimorbidity and Walking Independence After Total Knee Arthroplasty in Adults Aged ≥80 Years: A Retrospective Cohort Study

Abstract

Background

The demand for total knee arthroplasty, an established treatment for advanced knee osteoarthritis, is anticipated to increase among individuals aged ≥80 years. Given the high prevalence of multimorbidity among older adults in Japan, in this study, we aimed to elucidate the relationship between multimorbidity and post-operative walking independence in this demographic.

Methods

This retrospective cohort study included patients who underwent total knee arthroplasty for knee osteoarthritis between October 2023 and September 2024; those who underwent other surgeries or had significant mobility limitations were excluded. Patients aged ≥80 years with a Charlson Comorbidity Index of ≥2 were categorized into the “older patients with multimorbidity” group. The primary outcome was walking independence, which was assessed using the Functional Ambulation Category. Demographic and clinical variables were evaluated as potential confounders. Statistical tests included Student’s t-test, Mann–Whitney U test, chi-square test, and hierarchical multiple regression analysis; a p-value of <0.05 was considered significant.

Results

Thirteen and 42 patients were classified as “older patients with multimorbidity” and controls, respectively. Significant intergroup differences were observed in age, height, weight, and hemoglobin levels but not in the length of hospital stay and overall outcomes. Functional Ambulation Category scores declined postoperatively in both groups, with no significant intergroup difference in independent walking ability. Hierarchical multiple regression analysis of pooled data revealed no significant association between age or multimorbidity and walking independence at discharge.

Conclusion

The findings indicate that age and multimorbidity are not primary determinants of post-operative walking independence. Individualized rehabilitation may improve recovery outcomes, underscoring the potential benefits of standardizing postoperative care for older patients with multimorbidity who have undergone total knee arthroplasty.

Keywords

older adults multimorbidity rehabilitation total knee arthroplasty walking independence

Introduction

In Japan, 54.6% of individuals develop radiographic knee osteoarthritis,¹ which significantly impairs quality of life² and contributes to care needs under the long-term care insurance system.³ The demand for Total knee arthroplasty (TKA), an established treatment for advanced osteoarthritis, is expected to increase by 2030, particularly among older adults.⁴ Although age alone does not appear to negatively affect TKA outcomes and should not be considered a limiting factor,⁵ patients aged ≥80 years may experience longer hospital stays,⁶ higher postoperative complication rates,⁷ and delayed functional recovery, especially when preoperative decline is present.⁸ Nevertheless, careful preoperative evaluation and perioperative management can mitigate these risks, suggesting that TKA can remain a safe and effective treatment option for carefully selected older patients.^9,10

Multimorbidity is highly prevalent among older adults. In Japan, approximately 94.7% of individuals aged ≥80 years have multiple chronic conditions.¹¹ Previous studies have reported that multimorbidity can negatively influence improvements in health-related quality of life and is associated with functional limitations and dependence on walking aids after TKA.^12,13 However, while TKA has been shown to improve physical function even in patients with multiple comorbidities, data on walking independence post-TKA in this age group remain scarce.¹⁴

Therefore, this study aimed to clarify the relationship between multimorbidity and walking independence in patients aged ≥80 years undergoing TKA. We hypothesize that multimorbidity would be associated with reduced walking independence after surgery in this highly vulnerable population.

Materials and Methods

This retrospective cohort study was conducted at a single institution in Japan, including patients who underwent total knee arthroplasty (TKA) between October 2023 and September 2024. The study design followed the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) guidelines. The study was approved by the University Ethical Committee (Approval Number: E2024-0197). In accordance with ethical guidelines, an opt-out method was implemented, and all procedures adhered to the Declaration of Helsinki.

The predefined indications for TKA included advanced radiographic knee osteoarthritis (typically Kellgren–Lawrence grade 3 or 4) accompanied by severe joint pain and functional limitations that significantly impaired daily activities. Surgery was indicated only for patients who had failed to respond to at least 3 months of conservative management, such as oral analgesics, intra-articular injections, and physical therapy.

To ensure a homogenous study population, specific inclusion and exclusion criteria were established. The inclusion criteria were: (1) patients diagnosed with severe radiographic knee osteoarthritis (typically Kellgren–Lawrence grade 3 or 4) who failed to respond to adequate conservative treatment; and (2) patients who underwent primary total knee arthroplasty (TKA) between October 2023 and September 2024. The exclusion criteria were: (1) patients who underwent concurrent surgeries other than TKA; (2) patients who experienced severe difficulty walking during hospitalization or at the time of outcome assessment, precluding accurate functional evaluation; and (3) patients with incomplete medical or surgical records.

All TKAs were performed by the same orthopedic surgical team using standardized postoperative rest and rehabilitation protocols. A medial parapatellar approach was used for all patients, ensuring a homogenous surgical approach. Although the surgical approach was standardized, the type of implant used was individually selected based on intraoperative findings. Postoperative rehabilitation protocols included range-of-motion, weight-bearing, and walking exercises commencing on the first postoperative day, with patients allowed to walk independently in the ward after evaluation by a physiotherapist. Decisions regarding postoperative discharge to the patient’s home or transfer to another hospital were based on a comprehensive assessment of the patient’s social background (such as family support and home environment) and the need for continued rehabilitation.

The comorbidities of the selected participants were evaluated using the Charlson Comorbidity Index (CCI).¹⁵ The CCI is the most commonly used prognostic measure of the burden of illness in clinical research¹⁶ and predicts both short- and long-term mortality in acutely hospitalized older adult patients.¹⁷ Patients aged ≥80 years with a CCI of ≥2 were categorized as the “older patients with multimorbidity” group, while the remainder were considered the “control” group.

Walking status at admission and discharge was evaluated using the Functional Ambulation Category (FAC) as the primary outcome.¹⁸ The FAC is a 6-point categorical scale (0 to 5) that assesses ambulation based solely on the level of human assistance required, ranging from 0 (non-functional ambulation) to 5 (independent ambulation on all surfaces, including stairs). The FAC is a subjective clinical assessment and does not use specific objective cutoffs for walking distance or time. The score was determined using a subjective evaluation of movement stability and the need for physical assistance or supervision during basic mobility and gait training, as evaluated by physical therapists and confirmed by physicians and nurses. The FAC has demonstrated good interrater reliability (κ = 0.72),¹⁸ excellent test-retest reliability (κ = 0.950), and excellent concurrent validity.¹⁹ The number of days during hospitalization when FAC was ≥4.0 was also documented.

Patient baseline characteristics and potential confounders included age, Charlson Comorbidity Index (CCI), height, weight, preoperative blood parameters (albumin, hemoglobin, and red blood cell count), and the Geriatric Nutritional Risk Index (GNRI). These parameters were included because preoperative hemoglobin levels have been significantly associated with early mobilization following lower limb arthroplasty.²⁰ Similarly, nutritional status, evaluated using the GNRI, is directly related to postoperative gait function and recovery in older adults.²¹ Therefore, these variables were incorporated to account for their potential influence on walking independence. The GNRI was calculated using preoperative data based on the following established formula: GNRI = 14.89 × serum albumin (g/dL) + 41.7 × (actual body weight/ideal body weight). The ideal body weight was calculated using the Lorentz equations based on the patient’s height and sex.²²

Other recorded variables included dates of surgery, rehabilitation initiation, gait training initiation; timing of outcome assessment (defined as the number of days from surgery to discharge); care level under the long-term care insurance; and discharge destination (home or transfer to another medical facility). The follow-up period for this study was defined as the duration of the patient’s hospitalization, from the day of surgery until hospital discharge.

Statistical analyses were performed using SPSS Statistics version 30.0 (IBM Corp., Armonk, NY, USA). Because of the retrospective nature of this study, an a priori sample size calculation was not performed. However, for the hierarchical multiple regression analysis, a general rule of thumb recommends approximately 10 subjects per predictor variable. With a total pool of 52 patients and six independent variables (including confounders) entered the model, the sample size approached this recommended threshold, providing acceptable statistical power for the regression analysis.

Prior to statistical comparisons, the normality of data distribution for continuous variables was evaluated using the Shapiro–Wilk test, and the homogeneity of variances was assessed using Levene’s test. Normally distributed data were analyzed using Student’s t-test (for equal variances) or Welch’s t-test (for unequal variances), depending on the results of Levene’s test. The Mann–Whitney U test was used for non-normally distributed data. All statistical tests were two-sided. Effect sizes for baseline comparisons were calculated using Hedges’ g for normally distributed variables and r for non-normally distributed variables.

To examine the independent effects of age and multimorbidity on postoperative walking independence, hierarchical multiple regression analysis was conducted using pooled patient data. The dependent variable was the FAC score at discharge. In the first step (Block 1), potential confounding factors—including preoperative hemoglobin levels, the Geriatric Nutritional Risk Index (GNRI), care level, and length of hospital stay—were entered into the model to adjust for baseline characteristics and physiological status. In the second step (Block 2), age and the Charlson Comorbidity Index (CCI) were entered as the primary predictor variables. Prior to interpreting the model, collinearity diagnostics were performed using the variance inflation factor (VIF) to assess the absence of multicollinearity among the independent variables. Statistical significance was set at p < 0.05.

Results

From the available records of patients with knee osteoarthritis, 86 who were hospitalized for treatment were identified, of whom 54 underwent TKA. Overall, 52 patients were included in this study; two patients who had difficulty walking at the time of the outcome assessment were excluded. Of the patients included, 12 and 40 were classified into the “older patients with multimorbidity” and control groups, respectively (Figure 1). None of the patients required additional treatment owing to adverse events such as serious postoperative complications, wound infections, or falls during hospitalization. Regarding the surgical details, the type of implant was individually selected based on the patient’s intraoperative ligament stability and bone condition. The distribution of implant types across the two groups is summarized in Table 1. The most frequently used implants were condylar stabilizing (CS, n = 29) and posterior-stabilized (PS, n = 15), followed by total stabilizer (TS, n = 4), cruciate-retaining (CR, n = 3), and condylar constrained knee (CCK, n = 1).

Figure 1.

Flow chart of patients

Table 1.

Patient Characteristics

Variable	All patients (n = 52)	Older patients with multimorbidity group (n = 12)	Control group (n = 40)	p-value	Effect size
Age (years)	77.0 (71.3 – 82.8)	82.5 (80.3 – 87.5)	74.0 (68.3 - 81.3)	< 0.001	0.498
Height (cm)	151.7 ± 10.0	152.9 ± 10.8	147.4 ± 5.6	0.093	0.556
Weight (kg)	61.4 ± 14.7	62.6 ± 15.5	57.2 ± 11.2	0.262	0.368
RBCs ( $\times$ 10⁶/μL)	4.31 ± 0.64	4.37 ± 0.64	4.12 ± 0.66	0.238	0.387
Hb (g/dL)	12.9 ± 1.8	13.2 ± 1.7	11.9 ± 1.8	0.023	0.763
Alb (g/dL)	4.1 (3.8 – 4.2)	4.1 (3.7 – 4.1)	4.1 (3.8 - 4.3)	0.844	0.027
GNRI	109.4 ± 11.7	109.7 ± 12.2	108.4 ± 10.6	0.742	0.107
Length of stay (days)	17.3 ± 2.9	17.9 ± 3.0	15.5 ± 1.9	0.012	0.846
Days from surgery to initiation of rehabilitation	1.5 (1.0 – 3.0)	1.0 (1.0 – 3.0)	2.0 (1.0 - 3.0)	0.323	0.137
Days from surgery to initiation of walking	3.0 (2.0 – 4.0)	1.0 (1.0 – 5.5)	3.0 (2.0 – 4.0)	0.724	0.049
Kellgren–Lawrence (3/4)	18/34	3/9	15/25	0.425
Implant type (CCK/CR/CS/PS/TS)	1/3/29/15/4	0/0/4/7/1	1/3/25/8/3	0.580
Care level (not eligible/support required/requiring long-term care)	38/8/6	5/3/4	33/5/2	0.009
Discharge destination (home/transfer to another medical facility)	13/39	1/11	12/28	0.128

Alb, albumin; GNRI, Geriatric Nutritional Risk Index; Hb, hemoglobin; RBC, red blood cell.

Statistical comparisons were performed using Student's t-test, Welch’s t-test, the Mann–Whitney U test, or the chi-square test, as appropriate. All statistical tests were two-sided. Effect sizes were calculated using Hedges’ g for normally distributed variables and r for non-normally distributed variables.

Regarding the patients’ basic characteristics, significant intergroup differences were observed in height and weight (p < 0.05, Table 1). Those in the “older patients with multimorbidity” group were significantly older (p < 0.001). Regarding blood test results, only hemoglobin levels were significantly lower in the “older patients with multimorbidity” group (p = 0.025, Table 1). The chi-square test showed a higher rate of receiving long-term care insurance in the “older patients with multimorbidity” group (p = 0.005, Table 1). Intergroup differences in the length of hospital stay significantly differences. In terms of FAC, only control groups showed a significant decrease in scores before and after surgery (Table 2). Additionally, there was no significant difference in the FAC scores at discharge between the two groups (p = 0.107, r = 0.223, Figure 2).

Table 2.

Functional Ambulation Category in Both Groups

	Older patients with multimorbidity group			Control group
FAC	Admission	Discharge	p-value(chi-square test)	Admission	Discharge	p-value(chi-square test)
5	8	1	0.116	35	8	0.002
4	2	5		3	22
3	2	5		1	8
2	0	1		1	2
1	0	0		0	0
0	0	0		0	0

FAC, Functional Ambulation Category.

Statistical comparisons between admission and discharge within each group were performed using the chi-square test. All statistical tests were two-sided.

Figure 2.

Functional ambulation category at discharge.

A hierarchical multiple regression analysis was conducted to examine the effects of age and multimorbidity (CCI) on walking independence (FAC score) at discharge. In the first block, potential confounding factors—including hemoglobin (Hb) levels, the Geriatric Nutritional Risk Index (GNRI), care level, and length of hospital stay (LOS) were entered. In the second block, age and CCI were entered as the primary predictors. Collinearity diagnostics confirmed no multicollinearity issues among the independent variables, as all variance inflation factors (VIFs) were below 5 (maximum VIF = 2.15). The first model was statistically significant (F (4, 47) = 2.930, p = 0.030, R² = 0.200). After adding age and CCI in the second block, the overall model showed borderline statistical significance (F (6, 45) = 2.311, p = 0.050, R² = 0.236). However, the inclusion of age and CCI did not significantly improve the explanatory power of the model (ΔR² = 0.036, p = 0.356). Hemoglobin was identified as a significant independent predictor of the FAC score at discharge (β = 0.453, p = 0.022), whereas GNRI showed a non-significant trend (β = −0.310, p = 0.075). Neither age (β = −0.133, p = 0.413) nor CCI (β = −0.184, p = 0.203) were significant predictors of the FAC score at discharge (Table 3).

Table 3.

Hierarchical Multiple Regression Analysis Predicting FAC Score at Discharge

Variables	B	SE	Standardized β	t-value	p-value	VIF
Block 1: Confounders
Hb	0.129	0.101	0.25	1.272	0.209	2.215
GNRI	-0.023	0.014	-0.293	-1.711	0.093	1.687
Care level	-0.042	0.215	-0.03	-0.196	0.846	1.376
Length of stay	0.021	0.041	0.079	0.502	0.618	1.431
Block 2: Main predictors
Age	-0.008	0.014	-0.095	-0.606	0.547	1.406
CCI	-0.084	0.099	-0.12	-0.853	0.398	1.144

R² = 0.147, Adjusted R² = 0.043, p = 0.231 for the final model. CCI, Charlson Comorbidity Index; GNRI, Geriatric Nutritional Risk Index; Hb, Hemoglobin; VIF, Variance Inflation Factor.

Discussion

In this study, we examined the extent of walking independence following TKA in older individuals with multimorbidity. Our findings indicated no significant difference in FAC scores between older patients with multimorbidity and those without, with both groups experiencing a decline in walking independence postoperatively. Additionally, even when the threshold was defined as the inability to walk on flat ground, neither multimorbidity nor age significantly affected FAC scores. Furthermore, there was no significant difference between the two groups in the onset of independent walking. This study demonstrated that walking independence after TKA is not influenced by comorbidities or age.

The present study demonstrated a postoperative decline in the FAC scores in both the “older patients with multimorbidity” and control groups. The predictors of walking independence on the third postoperative day include age, preoperative walking status, and pain level.²³ Although there are limited reports on walking independence at discharge, walking ability tends to decline postoperatively.²⁴ FAC, a measure of walking independence, significantly decreased among patients, irrespective of comorbidity status. It is important to note that this decline does not indicate surgical failure but rather reflects a short-term phenomenon resulting from acute surgical trauma, postoperative pain, and temporary muscle weakness. Because the follow-up in this study was limited to the time of hospital discharge, the FAC scores reflect this temporary functional nadir during the acute recovery phase. Patients typically regain and often surpass their preoperative mobility levels several weeks to months after TKA; however, assessing this long-term improvement was beyond the scope of the present study. However, the absence of a significant difference in the FAC scores at discharge between the two groups suggests that the “older patients with multimorbidity” group did not experience a disproportionate decline in physical strength following TKA. Furthermore, a uniform decrease in walking ability and independence was observed in all patients. This was corroborated by the results of the logistic regression analysis using a FAC score of 4.0, which is the standard for walking on flat ground. Consequently, in postoperative rehabilitation following TKA, it may be necessary to prioritize the patient’s postoperative condition (such as pain levels and ability to commence walking) over age and multimorbidity as primary risk factors.

The findings revealed no significant differences in outcomes between the “older patients with multimorbidity” and control groups. Previous study showed that advanced age and multiple comorbidities affect home discharge rates after TKA.^25,26 However, this study observed no differences in outcomes between the “older patients with multimorbidity” and control groups. Although the proportion of patients returning home after TKA increased from 65.5% in 2011 to 94% in 2020,²⁷ 47% of patients were discharged home, with many opting for rehabilitation facilities owing to the preferences of patients, physicians, or caregivers, despite initiatives promoting home discharge.²⁸ Japan’s healthcare system has established a seamless coordination framework between acute and convalescent rehabilitation hospitals, encouraging patients to continue rehabilitation at specialized facilities rather than returning home.²⁹ Consequently, the low rate of home discharge observed in this study may reflect the influence of the Japanese healthcare system. However, given that the home discharge rate remains lower in Japan than in other countries, this may pose a future challenge in rehabilitation following TKA.

There are several limitations to this study that should be acknowledged. First, the study was conducted as a retrospective cohort study at a single institution, resulting in a relatively small sample size. Because an a priori power analysis was not performed, the possibility of a Type II error cannot be ruled out. This limitation should be considered when interpreting the findings, and future prospective studies with larger, pre-calculated sample sizes are needed to validate these results. Second, the Japanese healthcare system differs from those in other countries, leading to a lower rate of home discharge following TKA. Consequently, this study should not be considered a standard indicator, and its findings should not be over-generalized. Third, walking independence was assessed using subjective measures. Although pre-rehabilitation was not specifically evaluated in this retrospective cohort, its potential impact cannot be overlooked. Preoperative interventions aimed at optimizing physical and nutritional status may help mitigate postoperative functional decline, particularly in vulnerable populations such as older patients with multimorbidity. Future prospective studies are needed to investigate the efficacy of tailored pre-rehabilitation strategies in this population. Fourth, although our analysis focused on the impact of age and multimorbidity, postoperative mobility is influenced by multiple factors. Functional outcomes such as knee range of motion, postoperative pain, and overall patient satisfaction may affect a patient’s motivation and physical capacity to achieve walking independence. The inability to adjust for these potentially significant contributors represents a limitation of this study. Future prospective studies should incorporate these multidimensional assessments to better understand the determinants of mobility recovery after TKA. Finally, the follow-up period was limited to the duration of hospitalization. Consequently, the long-term effects of multimorbidity on walking ability and overall functional outcomes beyond 6 months post-TKA could not be evaluated. Future prospective studies with longer follow-up periods are needed to clarify these late-phase outcomes.

Conclusion

In conclusion, neither advanced age nor multimorbidity significantly affected walking independence at hospital discharge after total knee arthroplasty (TKA). These findings suggest that with appropriate preoperative evaluation and standardized rehabilitation, TKA can remain a safe and effective intervention for older patients with multiple comorbidities. Emphasizing early postoperative recovery factors, such as pain control and early mobilization, may be more important than chronological age or comorbidity burden in achieving short-term walking independence.

Footnotes

ORCID iD

Makoto Asaeda

Ethical Considerations

This study was approved by the Ethical Committee for Epidemiology of Hiroshima University (Approval Number: E2024-0197). In accordance with ethical guidelines, research information was publicly disclosed, and an opt-out method was used.

Consent to Participate

As this was a retrospective observational study, obtaining informed consent (opt-in) from all participants was not required. Approval was granted by the Ethical Committee for Epidemiology of Hiroshima University to implement an opt-out process by disclosing information about the study to participants.

Author Contributions

Makoto Asaeda designed the study, conducted the data collection, and drafted the manuscript. Atsuo Nakamae, Tomoyuki Nakasa, and Akinori Nekomoto supervised the clinical aspects and contributed to the interpretation of the results. Noriaki Maeda provided statistical expertise and assistance with data processing. Kiyo Ueda and Takeya Araki contributed to manuscript editing and critically reviewed the study methodology. Yukio Mikami assisted in data management and literature review. Nobuo Adachi supervised the entire study. All the authors have read and approved the final manuscript.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

Declaration of Conflicting Interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Data Availability Statement

The datasets generated and analyzed during the current study are available from the corresponding author upon reasonable request.*

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