Background
Hip osteoarthritis is one of the most frequent joint diseases, which develops in approximately one in four people over their lifetime [
1]. Patients with hip osteoarthritis suffer from pain, reduced muscle strength, function and balance as well as limited range of motion (ROM) in the affected hip joint [
2,
3]. First-line treatment of these impairments consists of conservative therapeutic interventions such as exercise therapy and physiotherapist-led treatments. However, if these treatment methods are ineffective and fail to provide improvements, total hip arthroplasty (THA) is required. Following THA, patients reported significant improvements for pain and hip function [
4,
5]. One year after the implantation of the artificial hip joint, the overall patient satisfaction with the operation result is high, ranging between 88 % [
6] and 93 % [
7]. However, several studies have shown that from 12 months post-THA onwards, the patients’ reported health-related quality of life (HRQoL) started to decrease over time [
8,
9], especially due to a decline in physical function [
10]. Comparing the physical function of patients to healthy, age-matched control subjects, deficits in muscle strength, balance and gait were observed one to two years after the surgery [
11‐
13]. The impairment of physical function and decreasing HRQoL have been associated with musculoskeletal comorbidities for patients after THA [
14]. Seven years post THA, one third of the patients suffered from low back pain and half with general musculoskeletal pain [
10]. Musculoskeletal pain can be induced by muscle asymmetries and muscle imbalances [
15]. In the first year after THA, asymmetries between the operated and non-operated side were found for the muscle strength of the lower extremities [
16]. Two years after THA, significant between-limb differences were still detected for the maximum isometric strength of the hip muscles [
17]. Concerning static standing, walking and sit-to-stand transition tasks, asymmetric limb loading was also demonstrated 1.5 years after unilateral hip replacement [
18].
However, most studies only monitored differences between the operated leg and the non-operated one for a follow-up period of one to two years. Data on inter-limb differences beyond two years post-THA are rare. Examining potential asymmetries beyond the two years is important as asymmetric limb relations and asymmetric joint loading may lead to overloading the non-operated limb inducing an early development or accelerated progression of osteoarthritis of the non-operated limb [
18,
19].
Therefore, the primary aim of the study was to include patients who had undergone THA four to five years ago and to investigate potential inter-limb differences in muscle strength of the hip, hip ROM, balance, and spatiotemporal gait parameters. The secondary aim was to compare the values of the operated leg of patients following THA to values of control subjects. This study had the purpose of an exploratory investigation in order to detect if inter-limb differences or deficits were still present at all in patients years after the surgery. We hypothesized that patients would show between-limb asymmetry in hip strength, hip ROM, balance and gait parameters four to five years after THA. Secondly, we hypothesized that the values of the operated leg would show deficits when compared to values of healthy, age-matched peers.
Discussion
Most studies have focused on investigating the clinical and functional outcome of patients following THA up to two years post-surgery. This study had the primary goal to include patients, who had undergone THA four to five years ago, and to investigate potential differences between the operated and non-operated side. Persisting deficits on side of the operated leg were found for single parameters in hip muscle strength, hip ROM and balance. In comparison with values of healthy subjects, the patients following THA demonstrated reduced hip muscle strength and hip ROM.
The isometric maximum strength analysis revealed that hip strength values were reduced on the operated side, but only the hip abductors demonstrated a significant inter-limb difference with an average deficit of 0.10 Nm/kg (9 %) on part of the operated side. Similar to our results, Rasch et al. showed that a significant strength deficit of the hip abductors (15 %) remained on the operated side two years after THA whereas the pre-operatively existing significant inter-limb strength asymmetries in hip extension, hip adduction and hip flexion had recovered within the two years [
17]. The strength difference of 9 % between the operated and the non-operated side seen in our study does not seem so high when comparing it to lower-limb strength asymmetries of 10 % reported for young asymptomatic healthy humans [
29]. However, regarding the age of the patients and the affected muscle group, this inter-limb difference might be clinical relevant. Hip abductors are known to be important for stabilizing pelvis during ambulation and unipedal tasks [
30]. Unilateral weakness of hip abductors has been shown to influence gait and balance [
30,
31] and therefore may affect many tasks of the everyday life. When comparing the hip strength values of the operated side to the values of control subjects, a general weakness of the hip muscles were detected for the patients following THA. Significant strength differences were seen for hip flexion, hip extension and hip abduction. Similar results were reported in the study of Bertocci et al. [
32]. A general weakness of the hip muscles, especially of the hip abductors, has been associated with poorer physical function [
33] and low back pain [
34]. Concerning the strength analysis, this study revealed persisting inter-limb asymmetry for the hip abductors as well as a persisting general hip strength deficit for patients four to five years after THA. This may partly explain the patients’ reported increasing impairment of physical function [
10] and decreasing HRQoL over the years [
8].
Concerning the inter-limb examination of balance parameters in this study, a significantly increased COP length on the operated side in the single-leg stance was observed for the patients following THA. In previous studies, increased COP variables were seen as increased body sway and interpreted as a decreased performance of the postural system [
35,
36]. Trudelle-Jackson et al. also investigated inter-limb differences in the single-leg stance in patients following THA and showed significant lower measures of postural stability on the side of the operated hip one year after the surgery [
37]. The increased COP sway on the operated side seen in this study may be due to the detected abductor weakness on the operated leg of the patients following THA. It may have been harder for the patients to stabilize the pelvis in the horizontal plane on this leg causing greater sway. This can also be seen in the trend of a greater mediolateral displacement of the COP on the operated side when compared to the non-operated one. Besides influencing postural control, unilateral hip abductor weakness has been shown to affect gait pattern. In our study, no inter-limb differences between the operated and non-operated leg were detected for the spatiotemporal gait parameters implying symmetric gait for patients four years post-THA. This is in line with most studies, which demonstrated a recovery of asymmetric gait of patients following THA within one to two years [
17,
24].
The active ROM analysis of the hip joint, however, revealed a significant inter-limb difference for patients following THA. Patients showed a significantly reduced hip flexion angle on the operated side with an average deficit of 11° compared to the non-operated one. Similar results were obtained in the study of Häkkinen et al. One year after hip resurfacing the patients showed a 6° lower flexion angle on the side of the operated hip [
38]. When comparing the hip flexion angle of the operated side to controls, the deficit in hip flexion on part of the patients following THA became more evident. A significant difference of 18° were observed for hip flexion between groups. Significantly reduced hip angles were also detected for hip extension and hip abduction on part of the patients following THA. Restoring hip ROM is just as important as restoring hip strength for the patients following THA as low ROMs were associated with high levels of disability [
39]. Inter-limb differences in lower-limb joint ROM and strength may also lead to asymmetric joint loading which may result in the development of disorders in contralateral and adjacent joints. Therefore, symmetric inter-limb relations of muscle strength and ROM should always be pursued in order to prevent overloading one side.
This study showed that four to five years after THA, significant asymmetries between the operated and the non-operated leg were still present for single parameters partly confirming our hypothesis on persisting inter-limb differences years after THA. Compared to the values of control subjects, significantly reduced values for hip strength and hip ROM were found for the operated leg. These findings confirm our hypothesis on persisting deficits of the operated leg years after THA.
Some limitations have to be addressed in our study. First, no data were collected on the operation method. Different operation approaches may be associated with different muscle and tissue damages [
40], which might have had an influence on the results of our isometric maximum strength analysis. In future studies, patients following THA should also be controlled for osteoarthritis in other joints as this could affect the isometric strength as well. The short data acquisition time in the single leg stance also needs to be mentioned as a limiting factor. According to Scoppa et al. [
41], collection time for COP-related balance data should not be less than 25 seconds. As participants were only capable of holding the single leg stance for a short time, alternative test conditions for measuring inter-limb differences in balance should be considered.
Last, the small sample size of patients following THA is the major limitation of this study. This study had more of an exploratory character to detect if any inter-limb differences were still present at all in patients years after the surgery. As this study indicated persisting asymmetries and deficits, studies with larger numbers of participants should be conducted to confirm the significance of these results.
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