Functional evaluation of the TKA patient using the coordination and variability of rising

1. Introduction 

Total knee arthroplasties (=TKA) are implanted in patients with gonarthritis to reduce the amount of pain and increase the knee function of the patients. Thus far, the evaluation of the TKA is performed by the orthopaedic surgeon, using questionnaires (such as the WOMAC (=Western Ontario and MacMaster Universities osteoarthritis index)), X-ray and a physical examination [1]. However, this evaluation has many biases. Questionnaires are highly subjective, and biased by social desirability, culture, education, cognitive restrictions and language problems [3]. In addition, the patient and surgeon are usually focused on pain reduction and not specifically in improvement of knee function. However, with a patient population that is getting younger, functionality becomes more important in the TKA evaluation. Therefore, it is important to know how much improvement in knee function is gained by the TKA. In addition, there is a growing amount of different types of TKA prostheses with their own specific biomechanical kinematics. It is important that the functionality can be measured for the evaluation of these different prostheses. Thus far, however, there is not a specific and biomechanical method for measurement of knee function of TKA patients.

In this study, the knee function during “rising from a chair” movement has been studied. We choose this task, while it is an important daily life task and it is one of the most strenuous tasks for the knee. During rising from a chair, the knee torques are larger than during gait or stair-climbing [2], moreover, rising from a chair without aid of the arms produces knee joint forces up to seven times body weight [5].

To quantify the rising movement, the principles of the dynamic systems approach were used, which defines the relationships between body segments during movement [9], [15], [18]. These relationships are measured by the coordination of movement (=relative phase between body segments) and variability of movement (=standard deviation of the relative phase of a number of trials) [13], [17], [18]. In a normal rising movement, the knee and hip joints move in a well-coordinated way, and with small variability [4], [7]. The hip and knee joints need to work together, and it is assumed that a dysfunction in one of these joints can be seen in an altered coordination or variability of rising. Therefore, it can be expected that knee patients will rise in a less optimal coordinated manner, and patients will show a larger variability in the rising pattern compared to healthy subjects.

In this study, we measured the coordination and variability of rising of healthy subjects, pre-operative TKA-patients and TKA-patients one year post-operatively. We hypothesized, that knee patients show a less optimal coordinated manner of rising accompanied by a larger variability, compared to healthy subjects. Furthermore, we hypothesized that one year after surgery TKA patients will show a coordination pattern between hip and knee movement, and an amount of variability in rising pattern more similar to healthy subjects. If these hypotheses are correct, the coordination and variability can be used as parameters to measure improvements in knee function. Because orthopaedic surgeons are used to administer the WOMAC for TKA evaluation, we compared the coordination and variability of rising to the WOMAC score. The functional questions in the WOMAC questionnaires will be related to our functional method. However, due to the fact the WOMAC is a self-report questionnaire it will not be able to measure the biomechanical functioning. Therefore, we expected that an improvement in coordination and decrease in variability would only moderately correlate with an improvement in the WOMAC score.

2. Methods 

2.2. Materials 

Hip and knee angular displacements were measured bilaterally, and in the sagittal plane, with electrogoniometers (Biometrics, Newport Gwent, UK). The trochantor major and epicondylis lateralis were used as standardized reference points. A sample frequency of 1000Hz was used. Poly 5 software (© Inspector research systems, Amsterdam, The Netherlands) was used for data collection. Before measurement, the electrogoniometers were calibrated in sitting (90º knee and hip flexion) and standing position (full extension was defined as 0º knee and hip flexion). For the calibration procedure a manual goniometer was used for knee angle measurement.

A chair without arm rests and adjustable in height and depth was used (see Fig. 1).

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Fig. 1. This is a photograph of a patient with equipment sitting on the low chair.

2.5. Data analysis 

The data from the injured leg were used for the analysis. Matlab 6.0 (The Mathworks, Natick, MA) was used for all signal processing. After filtering (second order low pass Butterworth filter, cutoff frequency of 8Hz), the derivatives of knee and hip angular displacement signals were used for the knee and hip angular velocities. The start of movement was defined as the time when hip flexion velocity became larger than 10º/s, and the end was the point where hip extension velocity became smaller than 5º/s. This was done to exclude the back and forth rocking of the upper body in the analysis. All trials were unified in time scale by dividing the time axis in 1100 increments. Phase angles of hip and knee were derived from the normalized angle and angular velocities plots from hip and knee, respectively. Normalization of the angle and angular velocity amplitudes to 0 and 1 was performed to allow comparison between trials and subjects [11], [13]. The normalization was done for every trial separately, using Eqs. (1), (2)

(1)

where αnorm is the normalized joint angular displacement, α is the joint angular displacement, αmin is the minimum of the joint angular displacement during the trial, αmax is the maximum of the joint angular displacement during the trial and i is the time in increments.

(2)

where is the normalized joint angular velocity, α′ is the joint angular velocity, is the minimum of the joint angular velocity during the trial, is the maximum of the joint angular velocity during the trial, and i is the time in increments.

Phase angles (ϕhip and ϕknee) were calculated as follows, for every trial separately:

(3)

where j is the joint (hip or knee), αnorm is the normalized joint angular displacement, is the normalized joint angular velocity and i is the time in 1100 increments [11], [16]. After that, the relative phase for every trial (=measure of coordination of movement) is calculated by:

(4)

The relative phase was averaged over the 10 trials to get a mean relative phase (MRP) for each subject and for each chair height, separately. This MRP is a measure of the coordination of rising. In addition, the relative phase of the control group (MRPcontrol) was calculated by averaging over the subjects within the control group for both chair heights, separately.

The variability of the relative phase (SRP) is measured by the standard deviation between corresponding points of the relative phases of the 10 rising trials, for each subject and chair height separately [16]. This SRP is a measure of the variability of rising. The SRPcontrol is calculated as the average SRP for the control group for both chair heights, separately. The individual MRP and SRP of the pre-TKA and post-TKA group were compared to the MRP and SRP of the control group, using Eqs. (5), (6):

(5)

(6)

where MRPdiff is the difference between average MRP plot of the control group and the individual MRP plots of the pre-TKA and post-TKA groups, SRPdiff is the difference between average SRP plot of the control group and individual SRP plots of the pre-TKA and post-TKA groups, h is the individual patients, k is the pre-TKA or post-TKA group, and i is the time in increments.

3. Results 

From the 12 patients who were included at the start, eight were measured one year after surgery. Ten pre-TKA patients were able to rise from the low chair, and all eight post-TKA patients could perform the low chair rise. Ten pre-operative TKA patients and all post-TKA patients performed the high chair rise. All subjects from the control group could rise from both chair heights.

In Fig. 2 the average relative phase and in Fig. 3 the average standard deviation of the relative phase are shown for the control, pre-TKA, and post-TKA groups, and for both chair heights, respectively. For the relative phase, it can be seen that the pre-TKA group showed a larger deviation from the relative phase plot of the control group than the post-TKA patients. These deviations were more pronounced between samples 400 and 800. The standard deviations were largest between samples 600 and 800. The pre-TKA group showed larger standard deviations compared to the control group, than the post-TKA group. These differences were largest between samples 200 and 700.

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Fig. 2. (a) The relative phase is shown for the control, pre-TKA and post-TKA groups, during rising from the low chair. (b) Shows the relative phase for the control, pre-TKA and post-TKA groups, during rising from the high chair.

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Fig. 3. (a) The standard deviation of the relative phase is shown for the control, pre-TKA and post-TKA groups, during rising from the low chair. (b) Shows the standard deviation of the relative phase for the control, pre-TKA and post-TKA groups, during rising from the high chair.

The results for coordination (MRPdiff (s.d.)) and variability (SRPdiff (s.d.)) are shown in Table 1 for the pre-TKA and post-TKA groups, for both chair heights. For the low chair, the coordination was not significantly different from the control group for both the pre-TKA (p=0.18) and post-TKA (p=0.27) groups. The variability was significantly different from the control group for the pre-TKA group (p=0.01), but not for the post-TKA group (p=0.24). For the high chair, the pre-operatively measured coordination and variability were significant different from the control group (p=0.03 and p=0.04, respectively). The coordination and variability of the post-TKA group were not different from the control group (p=0.73 and p=0.52, respectively).

Table 1. Difference in coordination (MRPdiff) and variability (SRPdiff) of rising from low and high chair (s.d. in brackets)

		Pre-TKA (n=12)	Post-TKA (n=8)
	MRPdiff-low	−1.99	1.28
		(s.d.=4.33)	(s.d.=3.03)
		p=0.18	p=0.27
	MRPdiff-high	−4.89*	0.31
		(s.d.=5.75)	(s.d.=2.45)
		p=0.03	p=0.73
	SRPdiff-low	4.69*	1.04
		(s.d.=4.75)	(s.d.=2.32)
		p=0.01	p=0.24
	SRPdiff-high	2.85*	0.40
		(s.d.=3.53)	(s.d.=1.69)
		p=0.04	p=0.52

Comparison is made between controls and pre-TKA and post-TKA groups. The p-values are given and significant differences are marked with *.

In Table 2, the group results of the WOMAC categories are given. The control group showed the highest scores for all sub-categories, while the pre-TKA group showed the lowest values. The differences in WOMAC scores between pre-TKA and control group were all significant (p=0.001), whereas the post-TKA was not significantly different from the control group (p>0.05).

Table 2. The WOMAC scores are given for total, pain, stiffness and function (s.d. between brackets)

		WOMACtotal	WOMACpain	WOMACstiffness	WOMACfunction
	Control group	91.7	96.7	84.2	91.2
		(s.d.=8.9)	(s.d.=5.6)	(s.d.=12.9)	(s.d.=10.5)
	Pre-TKA	48.2*	47*	48.8*	48.5*
		(s.d.=20.7)	(s.d.=20.0)	(s.d.=22.4)	(s.d.=22.4)
	Post-TKA	82.0	90.6	67.2	81.3
		(s.d.=12.4)	(s.d.=12.1)	(s.d.=24.9)	(s.d.=12.6)

The scores are given for control, pre-TKA and post-TKA groups. The scores are given as a percentage of the maximal score of 100. The significant differences compared to the control group are marked with *.

The correlation coefficients between the WOMAC categories and the coordination (MRPdiff) and variability (SRPdiff) are shown in Table 3 for both chair heights. The correlation coefficients between variability and the WOMAC were negative, which means that a larger amount of variability is accompanied by a lower WOMAC score. The correlation coefficients between the coordination and WOMAC were positive, which indicates that knee and hip move more like the control group at a higher WOMAC score. The correlation coefficients were moderately high, except between WOMACstiffness and SRPdiff-low (r=−0.23). The highest coefficients were found between MRPdiff-high and WOMACtotal (r=0.63) and WOMACfunction (r=0.82).

Table 3. The correlations between the WOMAC scores and coordination (MRPdiff) and variability (SRPdiff) are shown for both low and high chair

		WOMACtotal	WOMACpain	WOMACstiffness	WOMACfunction
	MRPdiff-low	0.53	0.56	0.47	0.48
		(p=0.02)	(p=0.02)	(p=0.05)	(p=0.04)
	MRPdiff-high	0.63	0.58	0.58	0.61
		(p=0.007)	(p=0.01)	(p=0.01)	(p=0.009)
	SRPdiff-low	−0.54	−0.52	−0.23	−0.54
		(p=0.02)	(p=0.03)	(p=0.37)	(p=0.02)
	SRPdiff-high	−0.55	−0.48	−0.54	−0.55
		(p=0.02)	(p=0.05)	(p=0.02)	(p=0.02)

4. Discussion 

In the clinic, orthopaedic surgeons will usually not quantify the functioning of the knee of patients with total knee prostheses. Subjective measurements, such as the WOMAC questionnaire and the surgeon’s impression are still the standards used for evaluation of the TKA. Recently, it has been demonstrated that the WOMAC physical function subscale can be unsuccessful in detecting change in function, due to the overlap in questions with the WOMAC pain subscale [14]. In addition, low correlations between questionnaires and physical performance tests are found [8], [19]. A few performance tests are available, such as the self-paced-walk test, the stair test and the timed-up-and-go test [12], [19], which all measure the time or speed component as a measure of function. These tests are valid and are able to measure changes over time, and can be used for functional evaluation. However, when rehabilitation programs or different types of prostheses have to be evaluated, it is important to know how the movement is performed, and not just how fast. Especially, when testing older subjects speed should not be the factor to be measured, because the result of the knee function can be biased by disturbing factors such as balance problems. Therefore, we believe that a kinematic analysis gives additional insight into the biomechanical functioning of the TKA patient. In this study, we analyzed rising from a chair and measured the coordination between knee and hip movement and the variability in coordination of rising as possible function related parameters.

The relative phase between hip and knee movement is a measure of the coordination [15], [16], [17]. This coordination of rising is also referred to as rising strategy or pattern [6]. In this study, we used the difference in relative phase between TKA patients and a control group (MRPdiff) as measure of coordination. For the low chair, it was found that for the pre-TKA and post-TKA groups the MRPdiff were not significantly different from zero. This means that the TKA patients performed the rising movement with a similar coordination pattern between their hips and knees as the control group. For the high chair, however, the MRPdiff was significantly different from zero, which means that the pre-operative patients showed a different coordination pattern compared to the control group. The post-TKA group did not show significant differences in MRP with the control group, for both chair heights. Apparently, the pre-TKA patients favor a different rising strategy, as shown during the easier rise from the high chair. Patients can afford a rising strategy that is less coordinated, but obviously easier and maybe less painful for the injured leg. However, the rising movement from the low chair is much more difficult and a change in strategy would therefore result in a failure of rising. A year after the surgery, the TKA patients do not favor a different rising strategy during rising from a high chair anymore, and can rise in a normal coordinated manner.

The standard deviation of the relative phase is a measure of the variability of rising. In this study, we used the difference in variability between the pre-TKA and post-TKA groups and control group (SRPdiff) as function related parameter. For both chair heights, the variability of rising was larger for the pre-TKA group than for the control group (SRPdiff-low>0). The variability of rising of the post-TKA group showed no difference with the control group, for both chair heights.

Hence, the pre-TKA patients showed a more variable rising pattern for both chair heights, whereas the post-TKA group had a more stable rising strategy for both chair heights. This suggests, that prior to surgery, the patients were impaired and had more problems with finding the most favorable rising pattern.

In conclusion, it is found that the improvement in coordination was only seen for the high chair and the decrease in variability was shown for both chair heights.

Thus far, the WOMAC questionnaire has been frequently used for the evaluation of the TKA patient. To give orthopaedic surgeons more insight in the valuable additional use of the coordination and variability parameters, we correlated them with the WOMAC score. The correlation coefficients between the WOMAC categories and the coordination parameter (MRPdiff) were all positive, which means that a higher WOMAC score is accompanied by a coordination pattern more similar to control subjects. Most interesting are the correlation coefficients between WOMACfunction and MRPdiff-low and MRPdiff-high. These correlations were 0.48 and 0.61, respectively. The correlation coefficients between the WOMAC categories and the variability of rising were negative (a high WOMAC score is accompanied by little variability). The relevant correlation coefficients between WOMACfunction and SRPdiff-low and SRPdiff-high were −0.54 and −0.55, respectively. These coefficients were smaller than for the MRP. The correlation coefficients between MRPdiff and SRPdiff and the WOMAC were moderately high, if we take into account that the WOMAC is a self-report measure, whereas coordination and variability measure the in vivo knee function. McDowell and Newell showed in their study, that correlation coefficients between physical performance tests and questionnaires are usually not higher than between r=0.20 and r=0.60 [10]. The moderately high correlation coefficients found between the WOMAC questionnaire and coordination and variability indicate that both methods measure function. Kennedy et al. [8] suggested that for a complete assessment of the knee function both self-reports and physical performance tests should be used. They argued that the power of questionnaires is the ability to evaluate multiple aspects of function, however, errors in judgment, impaired cognition and inaccuracy in answering affect the validity of self-report measure. The strength of physical performance tests and functional tests is that they measure objectively and have the ability to measure change over time. The disadvantage is the assessment of usually only one parameter. Therefore, the combination of the WOMAC and our functional test gives an adequate evaluation of the TKA patient.

The combination showed that the MRPdiff-high discriminated between control group and pre-TKA group, supported by a high correlation coefficient with the WOMAC. Therefore, we conclude that it is a good measure for evaluation of the knee function. The MRPdiff-low did not discriminate between the pre-TKA group and control group, and can thus not be used as functional parameter. Therefore, the coordination parameter can be used as measurement of function, only during rising from the high chair.

The SRPdiff-low and SRPdiff-high are good measures for knee evaluation, because of the ability to discriminate between control subjects and pre-TKA patients, and because of the high correlation coefficients with the WOMAC. Therefore, the variability of rising can be used as a measure of function for rising from both chair heights. However, whereas the altered coordination between hip and knee movement is only seen during rising from the high chair, the measurements can be reduced to this height only.

We conclude that pre-operative TKA patients indeed show an altered rising movement due to knee dysfunction. One year after surgery, the TKA patients have normalized their rising movement. In addition to the commonly used questionnaires, the MRPdiff-high, SRPdiff-low and SRPdiff-high give insight into the rehabilitation of the TKA patient.