Preoperative Physical Factors Predictive of Clinical Outcomes after Total Knee Arthroplasty

ABSTRACT

Objectives: Predicting postoperative clinical outcomes from preoperative physical factors of patients undergoing total knee arthroplasty (TKA) would be useful. This study aimed to investigate the association between preoperative physical factors and postoperative outcomes.

Methods: This study included 119 patients. The preoperative physical factors and 1-year postoperative clinical outcomes were collected and assessed. Physical factors included age, sex, body mass index, skeletal mass index, knee range of motion, 5-m walk time, and Timed Up-and-Go (TUG) test result. Postoperative outcomes were evaluated using a sum of symptoms, pain, activities of daily living, and quality of life subscales of the Knee injury Osteoarthritis Outcome Score (KOOS-4), and the Oxford Knee Score (OKS). Correlation between physical factors and postoperative outcomes was analyzed by Spearman’s rank correlation coefficient, and the association between physical factors and KOOS-4 or OKS was analyzed using multiple regression analysis. Receiver operating characteristic analysis was performed to calculate the cut-off value for the TUG test time associated with minimum postoperative OKS of 40.

Results: Among the preoperative physical factors, TUG test time showed significant correlation with OKS (ρ=−0.267), but none correlated with KOOS-4. Multiple regression analysis showed a significant association between TUG test time and OKS (95% confidence interval: −0.590 to −0.163) but not with KOOS-4. The cut-off value of preoperative TUG test time associated with minimum postoperative OKS of 40 was 12.96 s.

Conclusions: Among the preoperative physical factors of patients undergoing TKA, the TUG test time was associated with clinical outcomes at 1 year after surgery.

INTRODUCTION

Total knee arthroplasty (TKA) is performed for advanced-stage knee osteoarthritis, with approximately 80,000 surgeries performed annually in Japan.¹⁾ By 2030, this number of surgeries is expected to increase to 110,000–120,000.²⁾ Postoperative rehabilitation is important for the recovery of knee function; however, some patients have good postoperative outcomes, whereas others do not despite receiving the same rehabilitation program after TKA.

Previously, we investigated the factors associated with clinical outcomes 3 years after TKA and reported that younger age at surgery was associated with better knee function, which was defined as a Knee Society Functional Score above 80.³⁾ These results indicated that older age at surgery was associated with poorer postoperative knee function. Therefore, the recent trend toward older age at surgery^4,5,6,7) may lead to poorer postoperative outcomes. Particularly in elderly patients, early postoperative functional recovery is more important than long-term results. This background suggests a need consider the preoperative and postoperative approaches for elderly patients scheduled to undergo TKA and how these approaches may lead to early postoperative functional recovery.

For patients undergoing TKA, it would be useful to predict the extent to which knee function can be restored based on the patient’s preoperative physical factors. Younger age,⁸⁾ muscle strength,⁹⁾ walking ability, Timed Up-and-Go (TUG) test,¹⁰⁾ and Gait Efficacy Scale (GES) score¹¹⁾ were reported as predictors of postoperative outcomes. However, because of differences in patient backgrounds, types of knee prostheses, rehabilitation programs, and postoperative evaluation of clinical outcomes, there is no consensus on the preoperative physical factors predictive of postoperative outcomes.

Knee prostheses and surgical techniques are factors that significantly influence postoperative outcomes.^12,13,14) Therefore, to appropriately evaluate the preoperative physical factors associated with postoperative outcomes, it is desirable to use the same prosthesis, perform surgeries using the same technique, and train patients according to a uniform rehabilitation program. This study aimed to identify the preoperative physical factors that predict postoperative clinical outcomes after TKA in patients who received the same prosthesis and rehabilitation program.

MATERIALS AND METHODS

Patients

This study included 119 primary TKAs performed on patients with osteoarthritis, rheumatoid arthritis, or osteonecrosis. All patients underwent TKAs with the FINE total knee (Teijin-Nakashima Medical, Okayama, Japan) at our hospital between August 2017 and September 2019. The following exclusion criteria were used: bilateral simultaneous TKAs, flexion contracture of 30° or higher, deformed femur or tibia following trauma, and dementia upon recording clinical scores.

This study was approved by the Ethics Committee of our institution (approval number: S24062). Written informed consent was obtained from the patients. All activities were performed in accordance with the ethical standards of the Declaration of Helsinki.

Preoperative Physical Factors of Patients

Besides age at surgery, body mass index (BMI), skeletal muscle index (SMI), knee range of motion (ROM) at the operated site, 5-m walk time (5-MWT),¹⁵⁾ and TUG test¹⁶⁾ were evaluated preoperatively. The 5-MWT and TUG test were performed twice, and the mean values were recorded.

Surgical Procedures

Multiple orthopedic surgeons performed TKAs using the measured resection technique through the mid-vastus approach. The surgeries aimed to achieve mechanically neutral alignment and were performed using the same prosthesis (FINE total knee). The cruciate-retaining (CR) type was chosen for patients with intact posterior cruciate ligament (PCL), whereas the posterior stabilized (PS) type was used for those with severely degenerated or deficient PCL. A distal femoral osteotomy was conducted perpendicular to the mechanical axis, and the posterior condyle was osteotomized parallel to the surgical epicondylar axis. A tibial osteotomy was subsequently conducted perpendicular to the anatomical axis of the tibia. To maintain surgical uniformity, these osteotomies were conducted using the same instruments. Following osteotomy, soft tissue balance was checked for extension and flexion. The prostheses were fixed to the bone using cement. We replaced the patellae, but not in patients without osteoarthritic changes in the patellofemoral joint or those with small patellae.

Postoperative Rehabilitation

From day 1 postoperatively, the patients received physical therapy for 40 min/day during hospitalization. ROM exercises and isometric training for the quadriceps were initiated. ROM exercises were performed manually and using continuous passive motion equipment. Walking exercise was initially started using parallel bars and changed to a walker or T-cane when the patients settled (approximately 1 week postoperatively). As the local swelling and pain were reduced, active quadriceps exercises were started with the patients sitting on a flat table. After daily rehabilitation, the affected areas, including the lower thigh and knee, were cooled in an ice bag. Patients were discharged 2 weeks postoperatively if they were medically stable, with pain controlled by oral nonsteroidal anti-inflammatory drugs, and deemed by a physiotherapist to mobilize sufficiently and function safely at home.

After discharge, patients continued rehabilitation at orthopedic clinics 2 days per week for 6 months. Rehabilitation protocols including ROM and muscle strength exercises were standardized across clinics. Instructions for self-training at home were not specifically provided. Patients were regularly followed up for 1 year after surgery, but outpatient rehabilitation was not provided at our hospital.

Clinical Evaluation

Clinical scores, including ROM, Knee Society Score (KSS), Knee Injury and Osteoarthritis Outcome Score (KOOS), and Oxford Knee Score (OKS), were recorded preoperatively and 1 year postoperatively. ROM was measured using a goniometer with the patient lying on a flat table. The KSS, which comprises a knee score (KS) and function score (FS), was used to objectively evaluate knee function.¹⁷⁾ Besides KSS, the Japanese KOOS, an instrument with confirmed validity and reliability for patient-reported outcomes based on its cross-cultural adaptation,¹⁸⁾ was used. The KOOS comprises five subscales: symptoms, pain, activities of daily living (ADL), sports, and quality of life (QOL).¹⁹⁾ For evaluation of the KOOS, a sum of four subscales, excluding sports, were scored as KOOS-4 (full score: 400).

Statistical Analyses

Preoperative and postoperative clinical scores were compared using the Wilcoxon signed rank test. Correlation between the preoperative physical factors and clinical outcomes was evaluated using Spearman’s rank correlation coefficients. Multiple regression analysis for the KOOS-4 or OKS was performed using a stepwise method to determine the predictive factors associated with clinical outcomes. Before the analysis, multicollinearity for the variables was validated and we confirmed that the variance inflation factor (VIF) of each variable was less than 10. Receiver operating characteristic (ROC) analysis was performed to calculate a cut-off value of TUG test time that distinguishes postoperative OKS of 40 or above from that less than 40.

Results were expressed as mean ± standard deviation (SD). Data analyses were performed using SPSS software version 28 (IBM, Armonk, NY, USA). Statistical significance was set at P <0.05.

RESULTS

Preoperative Physical Factors in Patients

The patient demographic data are shown in Table 1. There were 23 men and 96 women, with a mean age of 73.7 years (range 53–88 years) upon surgery. The mean BMI was 27.3 kg/m², and the mean SMI was 5.06 kg/m².

Table 1. Preoperative physical factors of patients included in this study

Characteristic	n=119
Age at surgery, years	73.7 ± 7.3
Male	23
Female	96
BMI, kg/m2	27.3 ± 3.8
Muscle mass of lower extremities, kg	5.94 ± 2.49
SMI, kg/m2	5.06 ± 2.09
ROM, degrees	107 ± 20
5-MWT, s	6.60 ± 2.19
TUG test time, s	14.6 ± 5.1

Data are expressed as mean ± standard deviation (SD) or as number.

Correlation between Preoperative Physical Factors and Clinical Outcomes

A total of 102 patients were given the CR-type prosthesis, and 17 were given the PS type. The preoperative and 1-year postoperative clinical scores are shown in Table 2. All of the postoperative scores including ROM, KSS (both KS and FS), KOOS subscales, KOOS-4, and OKS significantly improved compared with preoperative scores (P <0.001). The postoperative mean KOOS-4 was 324±41, and the postoperative mean OKS was 39.9±6.1.

Table 2. Preoperative and one-year postoperative clinical scores

Clinical score	Preoperative	1 year postoperative	95% CI of difference	P value
ROM, degrees	107 ± 20	123 ± 13	13.7–19.8	<0.001
KSS-KS	44.9 ± 16.9	96.3 ± 5.4	48.5–54.2	<0.001
KSS-FS	39.2 ± 18.5	74.6 ± 16.2	32.3–38.5	<0.001
KOOS-symptom	51.2 ± 18.5	84.1 ± 11.8	29.5–36.5	<0.001
KOOS-pain	48.2 ± 15.4	88.7 ± 11.1	37.5–43.9	<0.001
KOOS-ADL	58.1 ± 13.0	86.8 ± 10.7	26.1–31.4	<0.001
KOOS-QOL	25.8 ± 14.6	64.2 ± 19.1	34.3–42.4	<0.001
KOOS-4	183 ± 48	324 ± 41	130.3–151.2	<0.001
OKS	22.3 ± 7.5	39.9 ± 6.1	16.0–19.1	<0.001

Data are expressed as mean ± SD.

KOOS-4 (worst: 0, best: 400); OKS (worst: 0, best: 48).

Before the analysis, the Shapiro–Wilk test was performed to confirm whether the data for preoperative physical factors were normally distributed. Given that the data was not normally distributed, the correlation between preoperative physical factors and postoperative KOOS-4 or OKS scores was assessed using Spearman’s rank correlation coefficient. Among preoperative physical factors, including age, sex, BMI, SMI, ROM, 5-MWT, and the TUG test time, none was significantly correlated with KOOS-4; however, the TUG result was significantly correlated with the ADL subscale of KOOS (ρ= −0.212, P <0.05). In addition, the TUG result was significantly correlated with OKS (ρ= −0.267, P <0.01) (Table 3, Fig. 1).

Table 3. Correlation coefficients between preoperative physical factors and clinical scores

	KOOS-4			KOOS-ADL			OKS
	ρ	P value		ρ	P value		ρ	P value
Age	0.131	0.155		−0.001	0.992		−0.085	0.373
BMI	0.024	0.799		−0.101	0.273		−0.006	0.948
SMI	0.031	0.737		−0.048	0.605		0.009	0.923
ROM	0.038	0.683		−0.075	0.415		−0.067	0.48
5-MWT	0.064	0.488		−0.083	0.367		−0.124	0.192
TUG test	−0.04	0.664		−0.212	0.021*		−0.267	0.004**

*P <0.05; **P <0.01.

Fig. 1.

Correlations between TUG test time and KOOS-4, KOOS-ADL, and OKS. The TUG test time was significantly correlated with KOOS-ADL and OKS, but not with KOOS-4.

Multiple Regression Analysis for Clinical Outcomes

The independent variables were age, sex, BMI, SMI, ROM, 5-MWT, and TUG test time. We confirmed the normal distribution of the residuals through the use of Q-Q plots. Among the variables, none was associated with KOOS-4; however, the TUG test result was significantly associated with postoperative OKS (95% confidence interval [CI] −0.590 to −0.163, P <0.001) (Table 4).

Table 4. Multiple regression analysis for OKS at 1 year after surgery

Independent variable	Unstandardized coefficient (B)	Standardized coefficient (β)	95% CI for B	P value
(Constant)	45.251		41.980, 48.523	<0.001
TUG test	−0.377	−0.316	–0.590, –0.163	<0.001

ROC Curve for TUG Associated with Postoperative OKS

Because the TUG result was associated with OKS, efforts were made to establish the cut-off value for predicting better clinical outcomes. Based on the fact that the mean postoperative OKS for all cases was 39.9±6.1, patients with OKS of 40 or greater were defined as better patient-reported outcomes, and we created an ROC curve for the TUG test result associated with postoperative OKS of 40 or greater. The area under the curve (AUC) was 0.614, and the cut-off value was 12.96 s (sensitivity, 0.632; specificity, 0.636) (Fig. 2).

Fig. 2.

ROC curve for TUG test time associated with postoperative OKS of 40 or higher. The AUC was 0.614, and the TUG cut-off value was 12.96 s (sensitivity, 0.632; specificity, 0.636) (shown by asterisk).

DISCUSSION

The main result of this study was that the recorded time on the TUG test was identified as a preoperative physical factor associated with better postoperative outcomes (OKS ≥40). The TUG test time represents the time in which patients not only walk 3 m, but also stand up from a chair, turn, and sit down on the chair.¹⁶⁾ These results suggest that it is important to train patients to perform these movements as fast and smoothly as possible preoperatively.

Previous studies have reported that the TUG test result is related to the postoperative outcomes of TKA. Givens et al.²⁰⁾ reported that the TUG test result was the best predictor of PROMIS CAT (patient-reported outcome measurement information system computerized adaptive test) physical function. Ferreira et al.¹⁰⁾ demonstrated that a TUG test result of 19.3 s or lower and age less than 62 years were preoperative predictors of better scores in the physical function subscale of the Western Ontario and McMaster Universities Arthritis Index (WOMAC). Ko et al.²¹⁾ found that the 6-min walk distance was significantly associated with the 30-min walk distance at 1 year postoperatively; however, the TUG test result was most strongly associated with WOMAC and physical function. Furthermore, Kuwakado et al.²²⁾ stated that the TUG test result at transfer was a factor affecting the length of stay in a convalescent hospital for patients who underwent THA and TKA in an acute care hospital. Therefore, the TUG test result may be a factor associated with physical function and walking ability preoperatively and during the early postoperative period. Notably, this is the first study to report that TUG test time is a predictor of better clinical outcomes when postoperative outcomes are evaluated using the OKS. In this study, there was a significant correlation between TUG test time and OKS, but it was a weak correlation (ρ= −0.267). Abujaber et al.²³⁾ demonstrated that KOS-ADLS (Knee Outcome Survey-Activities of Daily Living Scale) was weakly associated with TUG test time (r=−0.301) in patients with end-stage knee osteoarthritis, which is largely consistent with our results. This weak correlation may be attributed to differences in evaluation methods where the TUG test primarily assesses objective mobility, whereas OKS includes subjective functional assessment. Combining the TUG test result with other factors (e.g., muscle strength, pain, psychological factors) might provide a more comprehensive prediction of postoperative outcomes.

The OKS is a 12-item questionnaire that assesses knee pain and ADLs during the past 4 weeks and is used as a simple method to determine knee function.²⁴⁾ In this study, the median OKS at 1 year postoperatively was 39.9; therefore, we defined better outcomes as OKS of 40 or greater. Considering that the full score of OKS is 48, an OKS of 40 corresponds to a scoring rate of 83.3%, which may be a difficult standard to meet the cut-off value of 12.96 s for the TUG test result, as shown by the ROC analysis. However, it may be worthwhile to use this cut-off value as a goal for preoperative rehabilitation.

Regarding clinical outcomes, the OKS was significantly associated with only TUG test time. This may be attributed to the fact that the OKS is an evaluation method focusing on pain and ADL, whereas the KOOS-4 is the sum of subscales of symptoms, pain, ADL, and QOL. In particular, symptoms and QOL include items for which patients have high demand, making it difficult to find correlations with the preoperative physical factors. We investigated the correlation between preoperative physical factors and subscales of KOOS and found a significant correlation between the TUG test time and the ADL subscale of KOOS (ρ=−0.212, P <0.05). This suggests that the TUG test is a strong surrogate marker, especially for ADL, in the patient-reported outcomes.

Given that the TUG test is a predictor of postoperative outcomes, rehabilitation to shorten the time on the TUG test is important, even preoperatively. Iwata et al.²⁵⁾ investigated the factors associated with gait speed and TUG test result preoperatively and at 2 and 3 weeks postoperatively; they found that knee extension velocity was the strongest predictor at all time points. This observation suggests that an aggressive preoperative rehabilitation program focusing on knee extension velocity is necessary to restore physical function in the early postoperative period. They also showed that quadriceps strength on the non-operated side was related to preoperative and postoperative gait speed; therefore, quadriceps training on the non-operated side may be important for the early postoperative recovery of gait function.

Mawarikado et al.¹¹⁾ examined the preoperative factors affecting health-related QOL in the early postoperative period after TKA; they reported that the modified Gait Efficacy Scale (mGES) score was associated with the KOOS subscale scores at 3 and 12 months postoperatively. The GES was originally developed as a measure of confidence in one’s ability to safely perform ambulatory activities.²⁶⁾ Newell et al.²⁷⁾ modified the GES to make it more relevant to daily living environments. In the case of TKA, which is often performed in elderly patients, it is important to reduce patients’ anxiety about walking and increase their confidence preoperatively to achieve a high level of postoperative satisfaction.^28,29)

In a systematic review on the importance of preoperative rehabilitation for TKA, Vervullens et al.³⁰⁾ found that pain at 6 months and function at 12 months were significantly better in the group that received preoperative rehabilitation than in the group that did not. There is another report stating that prehabilitation may slightly improve early postoperative pain and function, but the effects remain too small and too short-term to be considered clinically important.³¹⁾ Although further research is needed on the importance of preoperative rehabilitation, based on the results of the current study, it is reasonable to recommend a rehabilitation program for gait and muscle strength training with a TUG test goal of less than12.96 s.

There are some limitations in this study. First, this was a single-center study and the number of cases was small. Further studies with increased numbers of cases are needed to establish our findings. Second, multiple surgeons were involved in the surgeries; therefore, surgical uniformity might have been lost among surgeons. Third, radiographic evaluation for the prosthesis placement was not investigated. Given that the prothesis placement may have impacts on postoperative gait and physical function, further investigation is necessary. Fourth, although the postoperative rehabilitation program was uniform for all patients regarding ROM exercise, lower limb muscle strength, gait training, and other basic programs, some patients could not receive sufficient training during the acute rehabilitation period (approximately 2 weeks postoperatively) because of pain, decreased motivation, or other reasons. Finally, rehabilitation after discharge from the acute care hospital varied, including transfer to a convalescent hospital and home discharge with or without outpatient rehabilitation, which may have affected the postoperative outcomes.

CONCLUSION

This study investigated the association between preoperative physical factors and patient-reported outcomes 1 year after TKA. The results showed that the preoperative TUG test time was significantly associated with the postoperative OKS. The cut-off value for the TUG test time associated with OKS of 40 or higher was 12.96 s. To achieve better postoperative clinical outcomes, it may be worthwhile to use this cut-off value as a goal for preoperative rehabilitation for patients undergoing TKA.

ACKNOWLEDGMENTS

This work was partially supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (grant number JP23K08619 to A.N.).

CONFLICTS OF INTEREST

A.N. and K.N. received a scholarship donation from Teijin-Nakashima Medical. The remaining authors declare no conflict of interest.

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