The study included patients treated at the Outpatient Rehabilitation Ward of Wiktor Dega's Orthopaedic and Rehabilitation Clinical Hospital at Poznan University of Medical Sciences, Poland. Patients from both groups were treated in the clinic for four weeks. The assessment of balance was performed only once at the beginning of the rehabilitation process.
The THR group included 30 individuals (25 women and 5 men) who had undergone unilateral total hip replacement following advanced osteoarthritis. The patients had unilateral anterolateral THR. The average time of the postoperative period was 5.3 years (SD=4.0). The control group comprised 30 people (25 women and 5 men) without hip endoprosthesis who did not report any pain or limited range of motion in joints. Due to the age of patients, those qualified for the group showed Iº radiological changes in hip joints as measured on the Kellgren-Lawrence scale. The THR group and the control group were age and sex matched. Average age of a person with endoprosthesis was 69.4 ± 6.2, whereas for healthy individuals it was 68.8 ± 5.9. No statistically significant differences in age, height, weight and the BMI index were found between the study and the control group (Table 1). There was also no significant difference in abductor muscle strength measured with the Manual Muscle Test between the two groups (p=0.649).
Table 1 near here
The inclusion criteria for both groups were as follows: moving without crutches, age above 55 years and scoring at least 9 points on the AMTS scale (Abbreviated Mental Test Score). The applied AMTS scale  was supposed to help identify and exclude from the study individuals with reduced cognitive functions and decreased ability to concentrate. The criteria for exclusion for both groups were diseases affecting the balance such as: vestibular problems, neurological conditions (hemiplegia, peripheral neuropathy, stroke, Parkinson's disease, SM), muscles diseases, rheumatic diseases (rheumatoid arthritis, ankylosing spondylitis, psoriatic arthritis), sciatica, operations in the area of the spine and lower limbs (knee arthroscopy with ACL reconstruction or meniscectomy, total knee replacement, total hip replacement in both extremities, osteotomy or arthrodesis). All subjects expressed written consent to participate in the study. The study was conducted in compliance with the Declaration of Helsinki and with the approval of the Bioethics Committee of Poznan University of Medical Sciences (reference number 949/14). Data were anonymised before analysis.
Experimental procedures and instruments
Dynamic balance and functional mobility were assessed with Metitur’s Good Balance force platform and the following tests: Timed Up and Go test, 3m walk test, Functional Reach Test, 30s Chair Stand Test, Step Test and Berg Balance Scale.
Dynamic test on force platform
The dynamic test on the force platform was based on the principle of biofeedback. It was performed on 1 board (including "path”) and with different sensitivity of platform (B100 and B60). The board showed “paths” for displacement of centre of feet pressure (Figure 1). Patients could observe certain position of the centre of feet pressure (COP) on the screen. It was visualized as a cursor and the tasks were to achieve the targets successively displayed on the screen during displacement of the body. The subject’s position during dynamic tests was upright with feet placed parallel and 20 cm apart . The B100 scale was characterized by a greater distance to designated targets than the B60 scale. Before measurement, volunteers were informed that velocity (time) on the B100 board and accuracy (distance) on the B60 board were equally important.
In both cases values were recorded for task performance time (s), distance of COP displacement (mm), COP displacement distance in the saggital plane (Y) and COP displacement distance in the frontal plane (X).
Figure 1. B100 board on a force platform.
Timed Up and Go (TUG)
The TUG test is recommended as a routine screening tool for risk of falls by the American Geriatric Society and British Geriatric Society . During the test, patients were to rise from a chair, walk a distance of 3 meters, make a turn of 180 ° having crossed a designated line and return to the chair. Recording the time of performing the task was initiated by the “start” command and stopped the moment a patient returned to the sitting position with the back resting against the chair. Patients were instructed to do the task as quickly as possible, but at the maximum speed at which the patient could walk safely without running [27, 28]. Each participant completed three trials. An average value from all three trials was recorded as study data for further analyses.
3m walk test (3m)
The test was conducted to asses gait speed . Slow gait is associated with risk of falling . Patients were instructed to stand with their toes touching the start line and walk fast beyond the taped finish line. The time from the moment their foot crossed the start line to the moment their both feet crossed the stop line was measured. Each participant completed three trials. An average result from all three trials was recorded as study data for further analyses.
Functional Reach Test (FRT)
The FRT is a popular method of measuring the dynamic balance of the elderly and is useful as a predictor of the risk of falling and a decline in function of the elderly. During test, the patient was standing by the wall with their feet shoulder-width apart,one shoulder flexed at 90° and the other arm on the side. A ruler was attached to the wall. Patients were instructed to reach their maximal distance (in centimetres) without moving their feet or losing balance and come back to the standing position. If subjects raised a heel or took a step during testing, the trial was repeated. Each participant completed three trials.An average distance for all three trials was recorded as study data for further analyses .
30s Chair Stand Test (CST)
Low levels of body strength are the primary cause of both balance problems and falls in the elderly population. CST measures lower body strength and relates it to the most demanding daily life activities (e.g., climbing stairs, getting out of a chair or bath tub or rising from a horizontal position) . The test consists of standing up and sitting down from a chair as many times as possible within 30 seconds. Initially, subjects were seated on the chair with their arms folded across the chest and with a back in an upright position. They performed only one trial and started it after a command. A standard chair with armrests was used. All patients performed the test using the same chair [32, 33].
The Step Test (ST)
The step test developed as assessment of dynamic standing balance after stroke. However, we have incorporated it into our study, since in our clinical practise the test is often applied when THR patients learn to walk on stairs. When tested, subjects were instructed to place one foot onto a 7.5 cm high step and then take it back down to the floor repeatedly as fast as possible. The score is the number of steps completed in the 15-second period for each lower extremity . Both sides were tested two times, with the THR group completing the test first with the operated leg (ST O) and then the non operated leg (ST N). Scores for each lower extremity were recorded separately. Better results were recorded as study data for further analyses. We calculated a reference value in the control group (ST C), which is a mean representing all tasks conducted with the left and right limb.
Berg Balance Scale (BBS)
The Berg Balance Scale (BBS) is currently the most commonly used clinical test for assessment of functional balance in older adults, which matches our study group. Subjects did 14 different tasks including static tests with different feet positions and functional balance control tasks including transfer, getting up and sitting on a chair, reach, turning and stepping. The BBS tests performance on levels from 0 (can't perform) to 4 (normal perform) and can give a total score of 56. Higher scores on the BBS indicate better balance. A total score obtained in 14 trials was recorded for further analyses .
Data were analysed with the Statistica version 13.1. Demographic data and clinical characteristics are presented as means, standard deviations (SD), median, minimum (min) and maximum (max). The Shapiro-Wilk test was used to assess the normality of the distributions in the test score. To compare the differences between the THR group and the control group, we applied the independent t-test, the Welch test or the non-parametric U Mann-Whitney test depending on which test assumptions were met. P-values less than 0.05 were considered statistically significant.