Does total hip replacement impact on postural stability?

The hip joint plays an important role in maintaining an upright posture and body balance. The hip strategy is described as one of the two main strategies (together with the ankle strategy) responsible for maintaining a stable, vertical body posture [1–3]. Although most studies reported a predominant role of the ankle strategy during quiet standing, the research by Sasagawa et al. [4] suggests that the hip strategy may play a crucial role in realizing this task.

Chronic pain and limitations of physical activity in patients with osteoarthritis of the hip result in the worsening of proprioception, asymmetric limb loading and resulting dynamic balance disorders [5–7]. Additionally, deficits in muscle strength and the restricted range of movement in subjects with advanced hip arthritis contribute to gait disorders and increased risk of falls [8, 9]. Arnold and Faulkner [10] reported the occurrence of one fall over the previous year among as many as 45% of subjects with hip joint osteoarthritis [10]. Furthermore, it was shown that these disorders were still seen for at least one year after joint replacement [5]. The latest studies have shown impaired balance in the quiet standing position in patients with osteoarthritis of the hip [9]. It is known that total hip replacement (THR) results in the trauma of periarticular tissues [11] and the destruction or injury of the surrounding receptors responsible for sensing movement and joint position. It raises the question whether deficits of balance still occur after THR.

Several authors had compared the physical functionality of patients with hip joint osteoarthritis before and after THR and reported improvement [12, 13] which was associated with decreased pain intensity, improved lower limb functionality and significant improvement in the quality of life. Some long-term studies indicate the persistence of impairment and limitation after THR [14]. The opinions of researchers on the occurrence of static balance deficits after THR vary [11, 12, 15–20]. Some studies that evaluated balance focused on analyzing proprioception alone [7, 21–23]. Zati et al. [24] have shown that THR does not cause permanent changes in the perception of joint movement because renewed coxofemoral mechanics enables the strengthening of static and dynamic antigravitational reactions. Proske and Gandevia [7] also note that the sense of position and movement after the procedure has not changed.

Balance deficits may result not only from impaired proprioception but also from hip joint dysfunction which is caused by the decreased range of movement, stiffness, muscle weakness, contractures and resulting asymmetry of limb loading. This study presents the results of the analysis and evaluation of static balance in THR patients.

There was no significant difference in abductor muscle strength measured by the Lovett scale between the two groups (p = 0.649). Muscle strength was 4.75 (SD = 0.39) in the operated leg, 4.77 (SD = 0.37) in the non-operated leg in the THR group; and 4.85 (SD = 0.33) in the left leg, 4.80 (SD = 0.36) in the right leg in the control group.

Hip pain occurred in 4 patients after THR (2 patients reported pain in both legs, and 2 patients reported pain in the non-operated leg) and 2 subjects in the control group (in one leg). The pain level was below 3 points, and it did not have a significant effect on the results of the study.

Higher mean and median results for the displacement of the COP in the frontal and sagittal planes in the NS EO position were observed in the study group. However, the results of the mean velocity of COP displacement in the sagittal plane were statistically significant only. The median of the mean velocity of COP sway in the study group was 2.1 mm/s higher than that in the control group.

Similarly to previous reports, our results showed deficits in static balance in THR patients. In the NS EO and NS EC outcomes, we observed a significantly higher mean velocity of COP sway in the sagittal plane in the study group in comparison to the control group. Additionally, in the THR group during the NS EC test, we observed poorer results characterizing the spectrum. In the THR patients, we achieved a lower frequency of COP sway with a greater range of body sway in the sagittal plane. These two factors (measured in relation to 1 s) determine the interpretation of the spectrum parameter, and they indicate a slower postural response to body sway in the THR subjects. The results achieved in NS positions showed that deficits in static balance with abnormal postural control reactions may still occur in THR patients even a few months after the surgery.

Many studies have been performed within one year after surgery (usually 3, 6 and 12 months after surgery). Rougier et al. [33] also observed greater COP displacement in the sagittal plane in THR patients, but the study was conducted 12 days after the procedure. The authors suggested that the postural specificity of the patients appeared to be due to global sensorimotor impairment that altered the control of the loading-unloading mechanism at the hip level. Similarly, Szymanski et al. [15] assessed balance over a long period after the procedure and obtained poorer results in the THR group than in the control group. Nantel et al. [19] showed poorer stability among patients in a study conducted 6 months after THR; however, in this case, the instability was seen in the frontal plane. In all these studies the control group was composed of subjects without a hip endoprosthesis.

Another study has demonstrated the improvement of the parameters of static balance after THR [12]. However, the subjects were tested before and after THR, and the results were not compared to a control group. This comparison might explain why the authors suggested that suboptimal postural balance may improve after the procedure [12]. Similar results were reported by Truszczyńska et al. [34].

Balance appears to decrease when visual input is restricted [35]. Lin et al. [36] have concluded that the availability of visual information is important for maintaining standing balance in THR patients. In our investigation, we observed that during the NS EC tests the patients in both groups were less stable than during EO trials, which was demonstrated especially by the greater velocity of COP sway in the sagittal and frontal planes.

Although we did not study the position of the joint, the results obtained in the NS EO and NS EC evaluation may indicate that the lack of proprioceptors may affect maintaining balance. Our patients had anterolateral approach THR (between abductors and tensor fasciae latae). Most of them were treated in our hospital, where the procedure is to remove part of the hip capsule. The surgical approach in THR was identified as a factor that may affect surgical outcomes [37]. The Ruffini ending in the joint capsule, Golgi tendon organ in joint ligaments and muscle spindles were suggested as sensory receptors for joint movement. [11]. Opinions about the impairment of proprioception after surgery vary. Truszczyńska et al. [34] indicated that THR could cause receptor damage affecting postural stability and increasing the risk of falls, unlike Nallegowda et al. [38] who examined balance using dynamic posturography. The authors concluded that despite capsulectomy, patients with total hip replacement did not have any proprioceptive deficit and in the sensory organization tests with difficult tasks, the patients needed more sensory input from vision and vestibular sense. The coordination of all muscles seems to be the other condition that determines normal balance in patients after THR. For example, Chang et al. [17] assessed patients after minimally invasive THA. They suggested that though the surgery affected only the hip joint, it could have caused impairment of interjoint coordination (between the hip, knee, and ankle joints) necessary for bipedal stance postural control. The results for muscle strength further show that a single joint surgery also affects muscle strength around the remote joint in the lower leg, indicating that the muscular synergy for the control of stance postural stability might also be changed [17]. Inadequate muscle synergy might affect balance in the NS position.

We did not observe significant differences between the results in the study group and the control group obtained in tandem trials and the second variant of tandem trials. There are only a few studies in which patients were assessed with such feet placement. Despite the lack of statistically significant differences between the groups, we observed a tendency of increased postural sway in both planes when the width of the base of support was narrowed down (as foot placement changed from bilateral stance to tandem stance). These observations are in accordance with the findings of Chu-Ju-Chang et al. [17]. Additionally, in that study, the results of COP displacement both in the sagittal and frontal plane in the tandem position one year after the operation were higher than in the period before the surgery. However, that study did not use a control group (healthy subjects) [17].

Defining limb dysfunction caused by the surgery requires tests with a direct load on the THR limb. Bipedal loading, like NS or tandem, could not reveal the dysfunction of the operated limb because of greater compensatory loading of the non-operated limb. Therefore, the one-leg standing studies we conducted (see methodology) delivered additional information on postural control after THR. It is noted that when standing on one leg the stability of the pelvis is provided solely by the action of the ipsilateral abductors, the gluteus medius, the gluteus minimus and the tensor fasciae latae [39].

Similar to studies reported by Szymanski and Nantel [15, 19] not all participants in our research completed the test of standing on one leg on the balance platform. In our studies, as many as 8 subjects from the study group did not complete standing on the operated limb and an additional 5 on the non-operated limb. The results may generally suggest the occurrence of greater difficulty in keeping the standing posture on the operated limb by the THR patients. The results obtained from the remaining patients in the study group did not differ significantly from those in the control group. Larkin et al. [40] achieved different results showing significantly worse proprioception during standing on one leg in the THR group when compared with a control group. Both operative and non-operative sides showed worse results than in the control group. Rasch et al. [12] compared body sway on the day before surgery and after THR but compared the results from standing on the dysfunctional limb to the unaffected limb. They did not show significant differences between the lower limbs before and 2 years after the operation. Only in the 6 months after the procedure oscillations in the sagittal plane were greater on the operated side [12].

The results of the OLS test are consistent with these findings. The results in the THR group in the OLS-O trial are significantly poorer than those in the control group, which means that these subjects could stand shorter on the operated limb. Other authors also noted the problem with keeping balance in that position. In the Butler et al. study [41], the condition for passing the test in patients after joint arthroplasty was maintaining one leg balance for 10 s. Only 63% of the THR group passed that test. The results are in accordance with the observations of Dwyer et al. [42] who suggested the occurrence of muscle weakness in the lower limbs, especially weakness of the gluteus medius muscles in patients with hip osteoarthritis. Additionally, other authors [12] reported that the strength deficits of the abductor’s muscles occurred even 2 years after the operation. Similarly, Nantel et al. [43] and Tateuchi et al. [44] also observed a decrease in abductor femoris muscle strength among THR patients when compared to the control group.

When the findings by these authors are added to our results, we can conclude that the shorter timeframe of completing the test on the operated limb may point to prevailing insufficiency of the lower limb muscles after the operation, especially the gluteus medius muscles. The decreased performance of that muscle causes body balance disorders when standing on one leg. If these muscles are insufficient on the side supporting the pelvis, the pelvis is tilted to the opposite side and the trunk is bent toward the supported side [39]. As a result, such a posture diminishes body stability and prevents maintaining a one-leg standing position for a long time. When muscle strength was measured using the Lovett scale, no significant differences in abductor’s muscles were seen between the two groups and legs.

Measurement time and methodology only enabled the subjective assessment of muscle strength. It would be needed to use a more advanced method to assess muscle strength and its impact on static balance. To ultimately prove the effect of muscle dysfunction on body balance, the study should be complemented by dynamic and functional tests.

Our studies suggest that some deficits in body stability may persist even a few years after the procedure. That is why appropriate physiotherapy is a crucial factor when treating THR patients. Jogi et al. [45] observed a decreased area of the COP amplitude in the sagittal plane in patients who performed balance exercises added to the typical rehabilitation program after the operation. This suggests that physiotherapists may use these exercises for improving the patients’ body balance in the acute post-operative phase following THR. Additionally, Trudelle-Jackson et al. [46] concluded that a brief postsurgical rehabilitation program used by THR patients might not be sufficient. Therefore in the long term, it may be advisable to emphasize weight bearing and postural stability exercises. According to Judd et al. [47] targeted neuromuscular reeducation techniques after THR provided a positive effect on biomechanical outcomes, functional performance, and muscle strength.