Introduction

Total hip replacement (THR) is an operative treatment method for end-stage osteoarthritis, considered the most effective nowadays [1]. Due to the high prevalence of osteoarthritis among the population, the number of THR per year is constantly growing. It is estimated that about 10% of men and 13% of women over 60 years old suffer from symptomatic hip osteoarthritis [2]. These numbers are even higher in older people, making the disease typical for elderly patients and THR a typical operation. Data shows that in 2010 only in the US 2.5 million individuals (1.4 million women and 1.1 million men) underwent THR [3].
One of the typical complications that occur after this procedure is heterotopic ossification (HO). According to current studies, abnormal bone formation after trauma or hip replacement may occur in even 90% of cases [4]. Heterotopic ossifications bring about the following symptoms: pain, swelling, erythema, and warmth, along with joint immobility [5]. These factors could have an impact on maintaining the posture, especially in patients after THR.
It is well known since the 90s that the risk of falls among the population older than 65 is high. Data shows that over 30% of these people experience such an incident at least once a year [6]. The majority of them do not have further consequences; however, about 10% result in severe trauma to the head and musculoskeletal system [7]. Falls of people who underwent THR are sometimes connected with periprosthetic fractures, the treatment of which is much more complicated and associated with a higher risk of failure than treatment of common fractures of the lower limb. What may even worsen the situation is that older people often suffer from many diseases, including osteoporosis or neoplasm, which increase the risk of fracture. The rate of falls is even higher among patients hospitalised due to hip fractures. In the first month after returning home, about 14% of patients fall [8]. During the 6 months after hospitalisation, it reaches 53% [9]. So far, the influence of heterotopic ossifications on the risk of falls is unknown in the literature.

Aim of the study

The study’s primary aim was to assess whether patients who developed ectopic bone formation after total hip replacement have a higher risk of falls and inferior posture stability compared to the matched-cohort group without bone formation. The secondary aim was to assess whether there were any differences in patient-reported outcome measures.

Materials and methods

This study was conducted according to the STROBE (Strengthening the Reporting of Observational Studies in Epidemiology) Statement. The study protocol was designed as a retrospective matched-cohort observational study. This study was approved by Institutional Bioethics Committee (Number: KB/102/2007) and was registered on ClinicalTrials.gov (Registration number: NCT05218954).
Between January 2020 and December 2020, 312 patients suffering from primary hip osteoarthritis undergoing total hip replacement were observed. During the standard 6-weeks, 3-months, and 6-months follow-up visit, every patient had an AP pelvic weight-bearing x-ray. All patients were qualified and operated on by a fellowship-trained single surgeon. For the present analysis, the following demographic patient data were queried: sex, age at surgery (years), and BMI. 49 patients who developed ectopic bone formation were observed. A propensity score based on age, sex, and BMI were generated for these patients. Patients with HO were matched to patients without HO, operated by the same surgical team, using the same approach and implants with a 0.1 propensity score threshold, and priority given to exact matches.
All patients were operated through an anterolateral approach, lying on the healthy side with one-third of medium gluteal muscle dissection. Uncemented Taperloc/Allofit total hip implants (Zimmer Biomet, Warsaw, IN, USA) were used in each case (Fig. 1). Femoral neck dissection was performed after full joint dislocation between the tip of the greater trochanter to the point 10 mm proximal to the lesser trochanter. The acetabular cup was placed aiming 30-40 degrees of acetabular cup inclination and 10 degrees of anteversion. The femoral stem was aimed to be placed in the anatomical axis of the femur. The postoperative protocol included chemical and mechanical thromboprophylaxis unless specifically contraindicated. All patients received one dose of parenteral antibiotics at the induction of anaesthesia and two further doses post-operatively. No pre- or postoperative ectopic bone formation prophylaxis was performed. Flexion and extension exercises of the hip, knee, and ankle and isometric quadriceps contraction exercises were started on the first postoperative day, with full weight-bearing as tolerated. Mobilisation with a physiotherapist aimed to obtain a flexion-extension range of motion of the hip of 0-90°, mobilising and safely walking with crutches by the third postoperative day.
At the 6-month follow-up visit, radiographs analysis in terms of any ectopic bone formation was performed using Brooker’s classification [10].
All patients fulfilled WOMAC (The Western Ontario and McMaster Universities Arthritis Index) and Oxford questionnaires preoperatively on the 6-month follow-up visit.
Both cohorts underwent biomechanical assessment, performed by two independent blinded researchers (PC, KŻ) who were doctors and unaware if patients developed ectopic bone following total hip replacement. Every analysis was performed using the Biodex Balance System (Biodex, Inc, Shirley, NY), with participants feet standing steadily on the platform. Every participant was protected from falling with the use of support. The device screen was installed at the level of the patient’s sight to avoid taking an uncomfortable position that might have influenced the results.
Postural stability test was performed, containing the results from three measurements. Every measurement has been performed on the 12th level of the platform stability for 20 seconds with a 10-second break between every test. Results obtained from the participants were recorded as stability index (SI) values that represent deviations of platform position from the vertical in degrees. The higher the SI values, the more difficult it was for participants to maintain stability during the test.
Risk of fall test – a single test consisting of the results obtained from three measurements. Each measurement was performed with the Biodex Balance System platform set to constant instability at level 6 of the platform stability for 20 seconds with a 10-second rest time between attempts. The person conducting the test informed the patient in advance of how the test would proceed and then gave verbal instructions during each test. The patients were positioned in the centre of the platform, both feet set with their feet shoulder-width apart. The final result of each test was a computerised risk of fall assessment report with patient scores related to normative data.

Methodology of Statistical Analysis
Statistical analysis of results was performed. The t-student test or the U Mann-Whitney test was used for quantitative variables, according to the normality of distribution examined with the Shapiro-Wilk test. For qualitative variables, the chi-square test was performed.
All statistical analyses were performed using the STATISTICA 8.0 PL package (StatSoft, Inc. 2008). Auxiliary calculations, other charts and tables will be made in the MS Excel 2003 spreadsheet (Microsoft Corporation).

Results

49 (15.7%) patients with HO were observed. A total of 39 patients from no HO matched-cohort control group and 46 from the HO group completed the full assessment at the final 6-month follow-up. 3 patients were excluded from the final evaluation due to the absence at the follow-up, malleolus fracture and undergoing a total hip replacement in the contralateral hip 3 months after the first one.
The baseline characteristics of participants are depicted in table 1.
There were statistically significant (p value <0.05) differences between the group of patients with HO and those without HO in the results of the postural stability test, the antero-posterior stability index, the medio-lateral stability index and the result of the risk of fall test. There were no statistically significant (p>0.05) differences in the results of the biomechanical tests during the comparative analysis between groups of patients with different classes of ossification, except for the groups with stage I and stage IV heterotopic ossification – statistically significant differences were obtained (p <0.05) in all performed tests. All of these tests were performed at the 6-month follow-up visit. Results are presented in Table 2 and Table 3.
No statistically significant differences were observed in the results of the WOMAC and Oxford questionnaires. Results are depicted in Table 4.
Discussion

To the best of our knowledge, no literature is available to analyse the risk of falls among patients after total hip replacement and heterotopic ossifications. It is one of the most important facts due to a lack of knowledge in this field of orthopaedics.
Some articles discuss the influence of HO on the clinical status of patients. Rudiger et al. [11] emphasise in their study that Brooker grade IV ossifications significantly influence poor scores of patient-reported outcomes after THR. In another study by Pohl et al. [12], the authors point out that patients who developed Brooker III or IV ossifications had a poorer range of motion and did not improve postoperative hip joint mobility compared to patients with Brooker grade 0, I or II. The presented study does not seem to confirm these observations, and appropriate analyses have not been conducted to confirm or deny these observations.
Some studies analyse the balance and the risk of falls in patients before and after THR [13,14,15]. These studies examine dynamic [14], static [15] or both dynamic and static balance [13] of patients pre-and postoperatively. Additionally, these studies have a different follow-up period – from 4 months to 3 years. It was observed that the risk of falls after THR is reduced due to the improvement in patient balance; however, they had residual balance deficits, which may still affect the risk of falls, although to a lesser extent than preoperative balance.
Comparing total hip replacement with total knee replacement (TKR), there is quite a significant difference in the risk of falls between patients after TKR and THR. The latter group shows twice the risk of falls [16].
A few recent papers described the incidence of heterotopic ossifications after total hip replacement [17,18,19]. In the presented study, the group of patients with HO accounted for 15.7% of the entire study group. As is observed in the studies mentioned above, the numbers of patients affected by HO vary significantly, which was also confirmed in the review article by Zhu et al. [20] – the incidence of HO varied from 5.2 to 87 %. Such a variation probably depends on many factors, such as age, gender, preoperative diagnosis, treatment method, surgical approach or choice of implants. Some of these factors were analysed by Pavlou et al. [21]. In the study by Okano et al. [19], only female patients were included. Perhaps it was the reason for a small number of heterotopic ossification cases because the male gender is considered a predisposing factor [18, 20, 21]. This study does not confirm this observation. However, Pavlou et al. [21] also listed two other significant risk factors besides the male gender, namely lateral approach to the hip during surgery and the use of bone cement for acetabular and stem seating. All patients in the presented study were operated on an anterolateral approach; therefore, it cannot be clearly stated that it had any influence on the increased risk of heterotopic ossification development. Taking into account the second predisposing factor mentioned by Pavlou et al. means that the use of bone cement and press-fit implants in the group of patients analysed in this study could affect such a small number of HO cases.
Previous papers show that falls after total hip replacement are quite common, and over 50% of falls occur in the first year after surgery [22]. To find the reason for such worrisome data, many analyses were performed. So far, it has been proved that sex, drug use or comorbidities do not have a statistically significant impact on the risk of falls, and the only factor that seems to play a role is patients’ age [23]. Other papers show the essential role of preoperative strength of hip abductors and fall history [24]. Since both factors seem to have something in common, it could be a point of interest for studies. Hip abductor muscles show a decrease in strength with age [25]. Moreover, despite careful stitching of the muscles, performing THR through an anterolateral or direct lateral approach causes damage to them, which also affects the functioning of hip abductors and may increase the risk of falls. What is more, this decrease may even worsen with a shortening of the leg because the tip of the greater trochanter and the acetabulum are closer, and the tension of gluteus medius and gluteus minimus diminishes which is something commonly seen in arthritic hip joints. If one side is affected more, it can give signs of leg discrepancy – distance is even more shortened due to pelvic rotation.
This study shows a significant effect of heterotopic ossification on an increased risk of falls, especially in the group with Brooker grade III and IV, compared to the group of patients who did not develop HO postoperatively. Similar conclusions can be drawn by analysing the results of the postural stability tests – statistically significant differences were obtained in the results between the cohorts with and without ossification and between the group with ossification grade I and grade IV according to the Brooker scale. The presented results show a significant impact of ossification on postural stability and an increased risk of falls.
The results observed in this study are not so surprising after analysis of the biomechanics of human posture, which is far more complicated than it appears to be. Upright posture is often compared to a pendulum turned upside down with the axis of rotation situated in the ankle joint [26, 27]. In case of balance disturbances, the mechanism is changed into a double-pendular [28] with a secondary counter-phase pendulum with an axis of rotation in the hip joint. Both these models are generated with the contribution of sensory-motor control mechanisms to prevent any physical manifestation of disbalance. That model seems to stay contrary to the previous one, which only takes into account a one-pendular model [29].

Conclusion

Our study proved that heterotopic ossifications that appear in hip abductor muscles could impact balance and risk of falls in patients after total hip replacement. That supports our previous hypothesis. Heterotopic ossification is not only a complication that can result in pain, swelling, erythema, and warmth, along with joint immobility but may also cause more serious problems such as falls. After total hip replacement in the elderly, it can lead to severe consequences, such as a periprosthetic fracture or head trauma.
Based on our study, the group of patients that develops heterotopic bone tissue after total hip replacement is not very numerous. However, taking this cohort into account, it is worth considering the rationale for prophylaxis against heterotopic bone formations and adding, for example, indomethacin prophylaxis to the standard postoperative treatment protocol that, till now, includes thromboprophylaxis and analgesic treatment.

Acknowledgements
This research received no external or internal funding. None of the authors is professionally or financially affiliated with any company.

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