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Functional outcomes of total hip arthroplasty in ankylosing and non-ankylosing hip conditions: A pilot study
*Corresponding author: Mohammad Zaim Chilmi, Department of Orthopedics and Traumatology, Dr. Soetomo General Academic Hospital, Surabaya, West Java, Indonesia. m-zaim-chilmi@fk.unair.ac.id
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Received: ,
Accepted: ,
How to cite this article: Warman FI, Santoso A, Sedar JA, Chilmi MZ. Functional outcomes of total hip arthroplasty in ankylosing and non-ankylosing hip conditions: A pilot study. J Musculoskelet Surg Res. doi: 10.25259/JMSR_235_2025
Abstract
Objectives:
Ankylosing spondylitis (AS) primarily affects the pelvis and spine, with peripheral arthritis and enthesitis being common manifestations. When debilitating hip degeneration occurs, total hip arthroplasty (THA) offers significant pain relief with functional improvement in patients with AS. This study aimed to compare the clinical outcomes following THA between patients with ankylosing hips and those with non-ankylosing hip conditions.
Methods:
We retrospectively reviewed data from January 2018 to June 2022 at Prof. Dr. R. Soeharso Orthopaedic Hospital, Surakarta. The Harris Hip Score (HHS) was used to assess outcomes in 23 patients with ankylosing hips and 23 patients with non-ankylosing hips undergoing THA. A modified HHS was applied, substituting two traditional components (range of motion and deformity) with assessments focused on return to professional activity and sexual function. Post-operative outcomes were assessed at 6 months to evaluate functional recovery.
Results:
At final follow-up, patients with non-ankylosing hips demonstrated significantly higher HHS compared to the ankylosing group. Functional improvement was observed in both groups postoperatively. The ankylosing group’s average HHS improved from 55.65 preoperatively to 76 postoperatively, while the non-ankylosing group improved from 55.65 to 87.95. The difference in post-operative scores between groups was statistically significant (P < 0.05).
Conclusion:
THA provides considerable pain relief and enhances hip mobility in patients with AS, yielding favorable functional outcomes as measured by the HHS. Nonetheless, patients without ankylosing hip pathology experience superior functional recovery post-THA.
Keywords
Ankylosing spondylitis
Arthritis
Hip joint
Range of motion
Total hip arthroplasty
INTRODUCTION
Ankylosing spondylitis (AS) is a chronic form of arthritis classified within the spondyloarthritis family. The pelvis and spine are the main areas affected, although enthesitis and peripheral arthritis may also manifest. AS typically manifests in younger adults and is related to an increased incidence of hip involvement. Performing total hip arthroplasty (THA) in AS patients poses unique challenges due to several factors. Nonetheless, THA has demonstrated effectiveness in alleviating pain and enhancing functional outcomes in patients with hip degeneration associated with AS.[1]
The reported prevalence of hip involvement among individuals with AS is in the range of 19–36%, with around 90% of these patients exhibiting bilateral symptoms. The pathogenesis of AS involves several processes, including synovitis, entheseal inflammation, medullary bone involvement, gradual joint degeneration, and the development of secondary osteoarthritis. AS typically affects the sacroiliac joints first and can progress to the spine and hips. Inflammation surrounding tendon insertions leads to fibrosis, which in turn leads to calcification, ultimately resulting in joint ankylosis.[2]
Despite this, relatively few studies have investigated the long-term outcome of THA in this patient group. Existing literature reveals variability in both clinical outcomes and duration of follow-ups. Overall, evidence suggests that THA in AS patients has a higher failure rate compared to those undergoing THA for osteoarthritis.[3]
This study aimed to compare the clinical outcomes of THA between individuals with ankylosing hip conditions and those with non-ankylosing hip conditions at a single medical center.
MATERIALS AND METHODS
Study design
This retrospective cohort study was conducted by reviewing data from patients who underwent THA between January 2018 and June 2022 at Prof. Dr. R. Soeharso Orthopaedic Hospital, Surakarta. The study participants were divided into two groups: patients diagnosed with ankylosing hip disease and those with non-ankylosing hip disease. Ankylosing hip was defined based on clinical evaluation and radiographic criteria. Clinically, ankylosis was identified by a severely restricted or absent range of motion in the affected hip. Radiographically, ankylosis was characterized by joint space obliteration, periarticular ossification, and in some cases, bony fusion between the femoral head and the acetabulum. These features were confirmed through standard anteroposterior and lateral radiographs of the pelvis. No formal imaging scoring system was applied in this study, as the aim was to focus on clear radiographic evidence of advanced joint fusion consistent with surgical indications for THA.
In this study, participants were retrospectively matched in equal numbers (n = 23 each) based on the availability of complete clinical and radiographic data, to ensure a balanced comparison of outcomes between patients with ankylosing and non-ankylosing hip conditions.
Surgical and post-operative protocol
All patients in both the ankylosing and non-ankylosing groups underwent THA under general anesthesia. A standardized surgical technique was employed across all cases, involving a posterior approach and the use of cementless prostheses consisting of an acetabular cup and femoral stem. The same type of implant was consistently used in both groups. Due to inconsistencies in documentation, data on intraoperative blood loss and duration of surgery were not included in the analysis. Postoperatively, all patients followed a unified rehabilitation protocol that included a range of motion exercises, gait training, and progressive weight-bearing as tolerated, under the guidance of physiotherapy personnel. This standardized post-operative care aimed to minimize variability in functional recovery outcomes between the two cohorts.
Data collection
The hospital’s institutional database and electronic medical records were used to harvest patient data in past periods. Key variables included age, sex, etiology of the hip condition, and Harris Hip score (HHS) measured preoperatively and postoperatively. This study included patients undergoing primary THA for both ankylosing and non-ankylosing hip conditions, provided that their records contained complete clinical and radiographic information. Patients who had revision THA or incomplete data for the relevant variables were excluded.
Outcome measures
Each THA at our hospital was performed by qualified orthopedic surgeons, with surgical techniques tailored to each patient’s specific clinical characteristics. The primary outcome was the HHS, assessed at 6 months following surgery. A modified version of the HHS was employed in this study, substituting the original “range of motion” and “deformity” components with items assessing return to professional activity and sexual function.[4] However, this modified score has not undergone formal validation, which may affect the comparability and reliability of the results.
Analysis data
Data were analyzed using Statistical Package for the Social Sciences version 26.0 (IBM Corporation, New York, USA). Continuous variables were summarized as medians with interquartile ranges or means with standard deviations, depending on the data distribution. Categorical variables are reported as frequencies and percentages. The Chi-square test was applied to examine differences in clinical and demographic traits between groups for categorical data. To compare HHS between the ankylosing and nonankylosing groups, the Mann–Whitney U-test was applied for continuous variables that were not normally distributed. A P < 0.05 was considered statistically significant.
RESULTS
The ankylosing group consisted of 17 male and six female patients, while the non-ankylosing group included 12 males and 11 females. Within the ankylosing group, the underlying diagnoses were AS in 19 patients, trauma in 3 patients, and a history of chronic myeloid leukemia in 1 patient. The distribution of patients and their corresponding HHSs following THA across these categories is detailed in Table 1. None of the patients in the AS group was undergoing biological therapy at the time of surgery.
| No | Sex | Age | Etiology | Harris Hips score | |||
|---|---|---|---|---|---|---|---|
| Pre-Operative | P-value | Post-operative | P-value | ||||
| 1 | M | 28 | AS | 54.55 | <0.001 | 65.15 | <0.001 |
| 2 | M | 42 | AS | 49.55 | 77.15 | ||
| 3 | M | 39 | AS | 48.43 | 77.15 | ||
| 4 | M | 31 | AS | 48.43 | 76.75 | ||
| 5 | F | 24 | AS | 47.43 | 76.75 | ||
| 6 | F | 16 | AS | 52.55 | 78.75 | ||
| 7 | M | 49 | AS | 51.43 | 79.15 | ||
| 8 | M | 37 | TA | 55.95 | 79.15 | ||
| 9 | M | 29 | AS | 52.54 | 78.75 | ||
| 10 | M | 32 | AS+ Leukemia |
52.55 | 75.63 | ||
| 11 | M | 23 | AS | 62.95 | 75.63 | ||
| 12 | M | 38 | AS | 51.43 | 65.63 | ||
| 13 | M | 31 | AS | 50.75 | 75.5 | ||
| 14 | M | 23 | TA | 62.95 | 79.65 | ||
| 15 | M | 25 | AS | 34.43 | 66.5 | ||
| 16 | M | 33 | AS | 54.43 | 81.65 | ||
| 17 | M | 33 | TA | 55.75 | 81.65 | ||
| 18 | F | 35 | TA | 55.75 | 70 | ||
| 19 | M | 57 | AS | 42.75 | 79.15 | ||
| 20 | M | 37 | AS | 47.75 | 75.50 | ||
| 21 | F | 23 | TA | 50.75 | 77.05 | ||
| 22 | F | 26 | AS | 50.75 | 77.50 | ||
| 23 | M | 37 | TA | 50.75 | 75.50 | ||
AS: Ankylosing spondylitis, TA: Traumatic ankylosing, significance threshold: P < 0.05
In the ankylosing group, patient ages were categorized as follows: none were under 20 years old, 13 patients were between 20 and 40 years old, four patients were between 40 and 60 years old, and there were no patients over 60 years old. The mean age in this group was 33.17 years, with a range of 23–57 years. In contrast, the non-ankylosing group included no patients under 20 years old, 7 patients aged 20–40 years old, 9 patients aged 40–60 years old, and 7 patients aged over 60 [Table 2].
| No | Sex | Age | Etiology | Harris hips score | |||
|---|---|---|---|---|---|---|---|
| Pre-operative | P-value | Post-operative | P-value | ||||
| 1 | M | 29 | OA Hip | 46.75 | <0.001 | 94.50 | <0.001 |
| 2 | F | 59 | OA Hip | 50.35 | 95.50 | ||
| 3 | F | 55 | OA Hip | 51.25 | 76.55 | ||
| 4 | F | 65 | OA Hip | 50.20 | 81.70 | ||
| 5 | M | 21 | AVN head femur | 63.15 | 90.65 | ||
| 6 | M | 55 | AVN head femur | 63.15 | 91 | ||
| 7 | M | 68 | AVN head femur | 50.75 | 76.55 | ||
| 8 | F | 43 | OA hip | 75.80 | 96 | ||
| 9 | M | 52 | Traumatic OA hip | 55.60 | 90.8 | ||
| 10 | M | 54 | OA hip | 62.80 | 100 | ||
| 11 | M | 40 | AVN head femur | 56.80 | 88.65 | ||
| 12 | F | 50 | OA hip | 56.75 | 85.65 | ||
| 13 | M | 29 | AVN head femur | 65.75 | 96 | ||
| 14 | F | 43 | AVN head femur | 76.55 | 96 | ||
| 15 | M | 67 | AVN head femur | 65.25 | 100 | ||
| 16 | F | 22 | AVN head femur | 66.25 | 95.85 | ||
| 17 | F | 55 | AVN head femur | 61.25 | 92.00 | ||
| 18 | F | 62 | AVN head femur | 44.43 | 71.65 | ||
| 19 | M | 60 | AVN head femur | 40.90 | 76.55 | ||
| 20 | M | 25 | AVN head femur | 43.43 | 85.65 | ||
| 21 | F | 67 | OA hip | 55.90 | 88.75 | ||
| 22 | F | 62 | AVN head femur | 39.10 | 88.75 | ||
| 23 | M | 52 | AVN head femur | 37.68 | 76.50 | ||
OA: Osteoarthriti, AVN: Avascular necrosis, significance threshold: P < 0.05
The ankylosing group had an average pre-operative HHS of 50.75 (range: 34.43–62.95), which improved to an average post-operative score of 76.5 (range: 65.63–81.65). For the non-ankylosing group, the mean pre-operative HHS was 55.65 (range: 37.68–76.55), with a post-operative average of 90.08 (range: 71.65–100). Post-operative HHS was significantly higher in the non-ankylosing group compared to the ankylosing group (90.08 vs. 76.5), reflecting a superior functional outcome.
Both groups demonstrated significant functional improvement following THA, as shown by the increase in HHS scores, from 50.75 to 76.5 in the ankylosing group, whereas the non-ankylosing group had higher scores, from 55.6 to 90.08, with greater functional improvement. Statistical analysis using the Mann–Whitney U-test confirmed that these pre- to post-operative improvements were significant in both groups (P < 0.05). No major complications were reported intraoperatively or postoperatively. Some patients experienced intraoperative joint tightness, which was addressed with soft-tissue release during surgery.
Furthermore, a Mann–Whitney U-test comparing the magnitude of functional improvement between the two groups revealed that patients without AS experienced significantly greater gains following THA (P < 0.001).
DISCUSSION
THA offers a significant reduction in pain and markedly improves hip range of motion in patients with AS. In special cases of ankylosing hip joints, where pain may be minimal or absent, THA is primarily performed to restore function.[5,6] The present study highlights the significant improvements in functional outcomes following THA in patients with AS. Our findings indicate a post-operative increase in the HHS from 50.75 to 76.5 in the AS group, reflecting a notable functional gain. This improvement is consistent with previous studies that have demonstrated the effectiveness of THA in alleviating pain and restoring hip function in AS patients, even in the presence of advanced joint degeneration. Similar to previous study results, which showed an average increase of 25.75 points in HHS, reinforcing the value of THA in improving joint function in AS patients.[7]
Comparatively, patients in the non-ankylosing group exhibited an even greater functional improvement, with their HHS rising from 55.65 to 90.08 postoperatively. This result aligns with the broader literature, which has consistently shown that non-ankylosing patients tend to experience superior functional outcomes following THA. Gupta et al. (2017) observed a similar trend in their study, where nonankylosing patients reported an increase in HHS from 49.74 to 85.12 after surgery.[8] Our study’s finding that the nonankylosing group had a significantly higher post-operative HHS (90.08 vs. 76.5) suggests that the absence of ankylosing conditions may facilitate a more favorable recovery, likely due to the absence of the structural and inflammatory challenges posed by AS. This difference in outcomes is supported by previous research, which indicates that nonankylosing patients often have better joint mobility and fewer complications during the post-operative phase.[8]
The findings of this study have important clinical implications for both pre-operative and post-operative management in patients with AS undergoing THA. Preoperatively, patients with AS often present with spinal stiffness, pelvic tilt abnormalities, and fused hips, which complicate implant positioning and intraoperative patient alignment.[2] These challenges necessitate comprehensive radiographic evaluation and precise surgical planning to avoid malpositioning or impingement. Furthermore, the intraoperative observation of joint tightness in several AS patients, requiring soft-tissue release, highlights the importance of anticipating restricted mobility during exposure and prosthesis insertion, as also reported in similar series.[7] Postoperatively, the application of a structured rehabilitation protocol was associated with meaningful functional improvements in both groups. Even in cases with pre-operative ankylosis, patients were able to achieve significant gains, indicating that THA remains a viable intervention for restoring mobility and quality of life when paired with consistent post-operative care.[9] Compared to larger multicenter studies that primarily emphasize long-term implant survival, our study adds value by focusing on early functional outcomes in a younger patient population. In addition, the use of a modified HHS incorporating domains such as return to professional activity and sexual function offers a more relevant assessment for working-age individuals.[10] These findings provide a foundation for further prospective studies that could inform tailored perioperative strategies for AS-related hip arthroplasty.
Wang et al. (2014) demonstrated that non-cemented THA techniques yield favorable outcomes over long-term follow-ups of 73–170 months.[7] According to their study, prosthesis survival rates were 100% at 5 years and 92.3% at 10 years, with a mean HHS increasing significantly from 22.1 preoperatively to 91.7 after an average follow-up of 128.4 months. These results are in good agreement with those of AS cohorts published by Tang and Chiu, who observed a 95.5% prosthesis survival rate at 5 and 10 years, followed by a sharp reduction to 63.6% at 11 years.[11] Another study reported that 12 patients had 24 ankylosed hips converted to THA, with three hips showing osteolysis and two showing loosening.[12] According to Joshi et al.(2002), a large series of 181 hips from patients with AS with a 10-year follow-up showed an implant survival rate of 71% at 27 years.[13] In comparison, long-term survivorship for THA without ankylosing conditions has been reported as 89.4% after 15 years, 70.2% over 20 years, and 57.9% over 25 years.[14]
Functional hip outcomes, measured by the HHS, showed a significant improvement, with an average increase of 60.6 points.[10] Oommen et al. reported HHS improvements from 48 to 73 and from 53 to 82 in historical and more recent data within the same patient series.[9] Similarly, Gupta et al. found that in patients with non-ankylosing hips, THA improved the mean HHS from 49.74 to 85.12 points.[8]
This study has several limitations, including a relatively small sample size and a single-center, retrospective design, which may limit the generalizability of the findings to broader patient populations. Second, the equal group sizes resulted from purposive sampling based on data completeness rather than the natural case distribution, an approach chosen to minimize imbalance and allow for more meaningful statistical comparisons, which may introduce selection bias. Third, the follow-up period of 6 months may be insufficient to fully capture long-term outcomes such as prosthesis survival, late post-operative complications, or sustained functional improvement, particularly in the context of chronic conditions like AS. Baseline characteristics such as body mass index, comorbidities, and socioeconomic status were not included in the analysis, which may represent potential confounding variables affecting the outcomes. In addition, the use of a modified HHS, substituting range of motion and deformity components with assessments of return to professional activity and sexual function, lacks formal validation, potentially limiting the reliability and comparability of our functional outcome data.
CONCLUSION
THA is a well-established and safe surgical approach for managing ankylosed hips. It provides substantial pain relief and markedly enhances the hip range of motion in AS patients. The functional outcomes for ankylosing hips improve significantly, as reflected by increases in HHS.
Acknowledgments:
We sincerely thank the medical staff and administrative team for their invaluable support and assistance in data collection. Special appreciation goes to the patients who participated, as their cooperation was essential to the success of this study.
Author Contributions:
FIW, AS, JAS, and MZC contribute to the conceptualization of the study. FIW, AS, and JAS designed the methodology, data collection, and formal data analysis. AS and JAS prepared the material. MZC and AS as a supervisor. All authors have critically reviewed and approved the final draft and are responsible for the manuscript’s content and similarity index.
Ethical approval:
The study protocol was evaluated and approved in compliance with the principles of the Declaration of Helsinki and the International Conference on Harmonisation’s Good Clinical Practice (ICH-GCP) standards under approval number PP.01.03/D. XXV.2.3/1893/2025, dated February 24, 2025
Declaration of patient’s consent:
The authors certify that they have obtained all appropriate patient consent forms. In the form, the patients have given their consent for their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.
Use of artificial intelligence (AI)-assisted technology for manuscript preparation:
Artificial intelligence (AI) tools, including language models, were utilized to enhance the grammar, clarity, and formatting of the manuscript. All content, analyses, and interpretations were solely developed and validated by the authors, who take full responsibility for the integrity and accuracy of the work.
Conflicts of interest:
There are no conflicting relationships or activities
Financial support and sponsorship: This study did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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