Comparison of sit-to-stand and static standing balance ability between patients with total knee arthroplasty and elderly healthy subjects

Abstract

BACKGROUND:

Identifying the functional status of patients after total knee arthroplasty is important. This study aimed to examine the differences in sit-to-stand and static standing ability balance between patients who underwent total knee arthroplasty and healthy participants.

METHODS:

Twenty patients (mean age 70.05 years) who underwent total knee arthroplasty and 20 healthy participants (mean age 69.20 years) participated in this study. To assess the participants’ sit-to-stand, the Good Balance System (Metitur Ltd, Finland) was used to measure each participant’s mediolateral center of pressure (COP) displacement and time required to complete the sit-to-stand movement. To assess the participants’ static standing balance, the Good Balance System was used to measure each patient’s mediolateral COP displacement, anteroposterior COP displacement, and velocity moment when standing with eyes open and with eyes closed.

RESULTS:

A significant difference was found in the mediolateral COP displacement and the time required to complete the sit-to-stand movement in the two groups ( $P<$ 0.05). A significant difference was also found in the mediolateral and anteroposterior COP displacement and the velocity moment when standing with eyes open and with eyes closed in the two groups ( $P<$ 0.05).

CONCLUSIONS:

Patients who had total knee arthroplasty showed decreased sit-to-stand and static standing balance ability compared with the control group in the initial period after surgery.

Keywords

Balance center of pressure sit-to-stand total knee arthroplasty

1. Introduction

Osteoarthritis (OA) is a common disease accompanied by serious dysfunctions, including balance and gait disturbance. Total knee arthroplasty (TKA) is a type of surgery that is chosen when conservative treatment for OA is not sufficient, when the symptoms of a patient’s OA are serious, or when a patient with OA is in the final stage of OA [1]. TKA not only relieves the pain of patients but is also an effective method for enhancing mobility and correcting deformity to increase physical function in patients with OA [2].

After undergoing TKA, patients may find rising from a chair difficult because this process involves large movement amplitudes and requires the leg muscles to generate sufficient power to lift the body mass [3]. Balancing when standing or walking may also be difficult for these patients because of the change in sensation of their knee and motor functions [4]. Decreased balance ability compared with a patient’s condition before undergoing TKA has been reported following a TKA procedure [5]. Sit-to-stand movement and balance while standing are essential for daily tasks. When sit-to-stand and balance ability are reduced, daily tasks become difficult and the risk of a fall, which may cause serious injury, increases [6].

The process of standing from a sitting position is a critical prerequisite to standing and walking. However, difficulty with standing up or the inability to do so is common for many individuals with motor disability. Individuals who have undergone TKA show deficits in their ability to rise from a chair compared with healthy individuals [7].

The sensory system, which maintains balance, includes the visual, vestibular, and proprioceptive senses. Through interactions among these sensory organs, the motor system, and cognition, balance can be maintained effectively [8]. After undergoing TKA, an individual’s proprioceptive sense may not be able to recover to its previous condition even though improvements in the proprioceptive sense have been reported [9, 10]. Studies on recovery from TKA indicate that pain recovers within eight days, vertical knee force recovers within 17 days, and knee bending angle recovers to the baseline level by day 16 [11]. The operation may also affect an individual’s range of motion and leg muscle strength [12, 13]. Recently, advances in surgical techniques, early rehabilitation, and development of physical therapy have shown more improvement in the function of patients with TKA than in the past. However, whether such improvements of functional statuses are close to normal is not known.

Studies have reported the effects of TKA on sensory and motor functions [4, 14, 15]. Some studies comparing the standing balance ability using center of pressure (COP) displacement, analyzed the differences between the surgical methods [16, 17]. However, not much research has been conducted on comparing the balance ability of patients who underwent TKA with that of healthy participants. Additionally, studies on sit-to-stand movement have only examined stroke patients, elderly subjects, and patients with Parkinson’s disease [18, 19, 20].

We hypothesized that patients who had TKA would display decreased sit-to-stand and static standing balance ability. Therefore, this study sought to compare the sit-to-stand and static standing balance ability of patients with TKA and healthy participants.

2. Materials and methods

2.1 Participants

This study was conducted on 20 healthy subjects and 20 patients who had undergone TKA. Among the participants, 42 were female and 2 dropped out. Patients who had undergone TKA receive a hospital based rehabilitation exercise program. The inclusion criteria of the TKA group were unilateral TKA, patients who could rise from a chair independently, and those who could stand for 30 s without support. For all participants, the exclusion criteria were factors that could affect the study, such as nervous or musculoskeletal disorders and impairment of the vestibular or visual system. Table 1 presents the subjects’ characteristics.

Table 1
The general characteristics of the subjects ( $N=$ 40)

	TKA group	Healthy subjects	P-value
	( $n=$ 20)	group
		( $n=$ 20)
Age (years)	70.05 $\pm$ 4.15	69.20 $\pm$ 3.38	0.139
Height (cm)	156.75 $\pm$ 2.27	155.35 $\pm$ 3.83	0.829
Weight (kg)	60.70 $\pm$ 6.07	57.50 $\pm$ 7.02	0.424
MMSE	27.65 $\pm$ 2.07	28.15 $\pm$ 1.23	0.212
Operation side,	11/9
right/left
Duration after	16.56 $\pm$ 2.39
TKA (days)

Values are numbers or mean $\pm$ standard deviation. TKA, Total knee arthroplasty; MMSE, Mini-Mental State Examination.

2.2 Procedures

The Good Balance System (Metitur Ltd, Finland) was utilized to measure the sit-to-stand and the static standing balance of the participants. This equipment consists of regular triangular force platform (width 800 mm, height 70 mm, with strain gauge transducers at each corner of the platform) connected to a computer through a three-channel DC amplifier, an eight-channel 12-byte analogue-to-digital converter (sampling frequency used was 50 Hz) and a computer program installed in a notebook PC. The calibration of the force platforms was checked weekly. In addition, the force platform performed a self-test for the basic levels every time the computer program was opened.

This equipment can measure a subject’s mediolateral and anteroposterior COP displacement along the center of the pressure created on the force plate using the direction from the subject’s mass center to the gravitational direction. It can also add an average speed to the total moving distance of the subject’s mediolateral and anteroposterior COP displacement to calculate a subject’s velocity moment in mm ${}^{2}$ /s. In addition, this equipment can measure a subject’s mediolateral COP displacement and sit-to-stand duration during sit-to-stand movement.

To measure sit-to-stand, the participants’ starting position was standardized. The sit-to-stand protocol was used previously [21]. The participants were barefoot and seated on an armless, backless chair that was adjusted to the height of the subjects’ knee, which was determined as the distance from the lateral knee joint line to the floor.

The subjects placed their feet on the force plate with the medial border of their feet 10–15 cm apart. Each participant’s ankle was placed at about 10 ${}^{\circ}$ of dorsiflexion, and the knee angle was approximately 100 ${}^{\circ}$ –105 ${}^{\circ}$ of flexion. The participants were then instructed to stand at their usual self-paced, comfortable speed. After the command “ready, set, start” was given, the sit-to-stand movement was initiated with the word “start”. After about 3 s of standing, the participants were instructed to sit normally. The participants placed their hands at their waist and were not allowed to use their arms during rising.

To measure the participants’ static standing balance, the participants placed their feet on the force plate while standing and put their hands comfortably by their side. Each measurement was taken for 30 s while standing once with eyes open and once with eyes closed.

2.3 Statistical analysis

The collected data was analyzed using SPSS 21.0 for windows. Descriptive statistics were used to calculate the mean and standard deviation. For identifying the normal distribution of data, the Kolmogorov-Smirnov test has been done before the statistical analysis. Independent t-test was used for comparisons between groups in age, weight, height, and mini-mental state examination. Independent t-test was performed to compare the sit-to-stand and the static standing balance between two groups. The statistical significance level was set at 0.05.

3. Results

A significant difference was found in the mediolateral COP displacement and the time required to complete the sit-to-stand movement in the two groups ( $P<$ 0.05) (Table 2). A significant difference was also found in the mediolateral and anteroposterior COP displacement and the velocity moment when standing with eyes open and with eyes closed in the two groups ( $P<$ 0.05) (Table 3).

Table 2
A comparison of sit-to-stand between TKA group and healthy subjects group ( $N=$ 40)

	TKA group	Healthy subjects	P-value
	( $n=$ 20)	group
		( $n=$ 20)
COP displacement	238.96 $\pm$ 115.93	92.39 $\pm$ 22.34	0.000 ${}^{*}$
in ML (mm)
Sit-to-stand	2.60 $\pm$ 0.93	1.85 $\pm$ 0.52	0.038 ${}^{*}$
duration (s)

Values are numbers or mean $\pm$ standard deviation. ${}^{*}p<$ 0.05, Statistically significant. THA, Total knee arthroplasty; COP, Center of pressure; ML, Mediolateral.

Table 3

A comparison of static standing balance between TKA group and healthy subjects group ( $N=$ 40)

	TKA group ( $n=$ 20)	Healthy subjects group ( $n=$ 20)	P-value
Eye open
COP displacement in ML (mm)	158.32 $\pm$ 43.20	105.79 $\pm$ 33.51	0.000 ${}^{*}$
COP displacement in AP (mm)	266.32 $\pm$ 79.93	189.19 $\pm$ 66.05	0.002 ${}^{*}$
Velocity moment (mm ${}^{2}$ /s)	26.37 $\pm$ 11.89	10.52 $\pm$ 5.48	0.000 ${}^{*}$
Eye closed
COP displacement in ML (mm)	237.32 $\pm$ 143.91	129.28 $\pm$ 48.11	0.004 ${}^{*}$
COP displacement in AP (mm)	471.32 $\pm$ 217.39	328.87 $\pm$ 115.14	0.015 ${}^{*}$
Velocity moment (mm ${}^{2}$ /s)	54.29 $\pm$ 59.41	19.67 $\pm$ 13.07	0.019 ${}^{*}$

Values are numbers or mean $\pm$ standard deviation. ${}^{*}p<$ 0.05, Statistically significant. THA, Total knee arthroplasty; COP, Center of pressure; ML, Mediolateral; AP, Anterioposterior.

4. Discussion

This study compared the sit-to-stand and static standing balance ability between patients who underwent TKA and healthy participants. An experimental assessments of balance use measures of COP excursion to quantify postural stability during standing, the greater COP excursion indicated the imbalance [22]. The much greater COP sway in mediolateral direction and more time to complete the sit-to-stand movement indicated poor dynamic postural stability [21]. Patients who underwent TKA showed increased mediolateral COP displacement and required more time to complete the sit-to-stand movement. Patients who underwent TKA exhibited increased mediolateral and anteroposterior COP displacement and velocity moment when standing with eyes open and with eyes closed compared with the healthy participants. The results of this study demonstrated that the TKA group had decreased sit-to-stand and static standing balance ability compared with the healthy participants.

Moving from a sitting to a standing position is among the most common activities in daily life. Body weight is usually distributed nearly symmetrically on the two legs when a healthy person rises or sits down [21]. Individuals who recovering from unilateral TKA often have asymmetrical lower extremity movement patterns, characterized by decreased weight bearing and knee extension moments on the surgical leg compared with the nonsurgical leg [23]. We demonstrated that patients who underwent TKA showed an increase in mediolateral COP displacement and required more time to complete the sit-to-stand movement than the control group. Studies have demonstrated that during the sit-to-stand movement, more load is applied to the non-operated leg than to the operated leg after surgery [3, 24]. This asymmetry may be a contributing factor to mediolateral COP displacement during the sit-to-stand movement. Furthermore, impaired sensory ability and muscle function may contribute to a postural sway abnormality in TKA patients.

The results of our study demonstrated that patients who underwent TKA required a longer time to rise from a chair than the control group. Impairment in muscle function was evident in patients who underwent TKA and showed a lower mean peak torque and total work for knee flexion and extension compared with age- and gender-matched control subjects [1]. Patients with TKA could not generate enough knee angular velocity during rising compared with the healthy elderly subjects [25]. Having normal quadriceps strength is important for maintaining dynamic stability during the common activity of arising from a chair [7]. Impaired muscle function, including muscle strength and control, may contribute to individuals’ need for more time to stabilize their sway when completing a standing-up motion.

Balance is controlled by the central nervous system after it receives information from the visual, vestibular, and proprioceptive senses [26]. The proprioceptive sense may change because of pain, effusion, trauma, and fatigue, which may cause balance disorders and other clinical symptoms [27]. OA may cause knee joint inflammation, decreased joint space, reduced mechanoreceptors, and reduced proprioceptive sense due to decreased daily activities. TKA can recover joint space and soft tissue tension and can reduce pain and inflammation [16, 17]. Therefore, these changes may improve the proprioceptive sense and affect balance ability.

Many studies have reported that TKA improves the proprioceptive sense of TKA patients [9, 10, 17], but some have reported that the effect is minimal [28, 29]. As the results of such studies vary in the effects of TKA on the proprioceptive sense, further studies are needed to examine these effects. Studies on muscle strength changes have reported that the muscle strength of quadriceps and the hamstring become weaker after TKA [12]. Many studies have reported that functional changes in the legs are observed after TKA, but limited studies have been conducted on how the balance ability of TKA patients integrated these changes compared with healthy subjects.

When maintaining standing balance on a firm ground, dependence on the proprioceptive sense is important [30]. However, TKA patients have a worse proprioceptive sense than healthy subjects [31]. This study was performed on a firm solid force plate that required the proprioceptive sense more than the other senses to maintain balance. As TKA patients might not deliver the correct proprioceptive sensations to their central nervous system, their mediolateral and anteroposterior COP displacement and velocity moment were higher than those of the control group. These findings are consistent with those of previous research [32].

Velocity moment increases with age. A sharp increase in velocity moment and a decrease in balance ability were reported in subjects over 60 years of age [33]. Given this increase in velocity moment after TKA, the balance ability of patients who underwent TKA was worse than that of the control group.

Stan et al. measured COP displacement in a standing position with eyes open and with eyes closed and found that mediolateral and anteroposterior COP displacement increased after surgery [5]. Kim et al. reported no significant difference in the anteroposterior sway on the center of gravity when standing on a fixed force plate with eyes closed before a TKA and up to six months after TKA [16]. The factors that affected balance ability varied after TKA, and the proprioceptive sense of TKA patients gradually recovered. In the current study, patients who had TKA were measured on an average of 16.56 days after surgery, and their mediolateral and anteroposterior COP displacement and velocity moment were higher than those of the control group.

As demonstrated in the results of this study, patients who had TKA were exposed to increased risk of falling because of sit-to-stand and standing problems, which could generate dramatic consequences. They need to undergo training such as strength, range of motion, coordination, proprioception, sit to stand, and balance.

This study conducted experiments on patients undergoing physical therapy after TKA in one medical institution. Additional research is needed to determine the difference in the sit-to-stand movement and static standing balance ability based on physical therapy after TKA.

5. Conclusions

This study aimed to compare the differences in the sit-to-stand and static standing balance ability between patients who underwent a TKA and healthy participants. The results demonstrated that patients who had TKA exhibited decreased sit-to-stand and static standing balance ability compared with the control group in the initial period after surgery. Clinicians should consider incorporating sit-to-stand and static standing balance training during the initial rehabilitation phase following TKA.

Footnotes

Conflict of interest

The authors declare that they have no competing interests.

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