Clinical Trials in Orthopedic Disorders

: 2019  |  Volume : 4  |  Issue : 3  |  Page : 43--46

Incidence and risk factors for development of postoperative heterotopic ossification in a high-volume hip arthroscopy practice: A case series

Thomas Duncan Smith, Anjaneyulu Purnachandra Tejaswi Ravipati, Ivan H Wong 
 Division of Orthopaedics, Dalhousie University, Halifax, Canada

Correspondence Address:
Ivan H Wong
Division of Orthopaedics, Dalhousie University, Halifax


Background and objective: Heterotopic ossification (HO) is a known complication of hip arthroscopy and may significantly affect the outcomes of patients undergoing hip arthroscopy. Its incidence in the literature varies from 0–44% following hip arthroscopy without prophylaxis. Our primary goal was to find the local rate of HO after hip arthroscopy as determined by the Brooker Classification. Secondary outcomes included determining risk factors for development of HO by sex, age, body mass index, hip pathology and effects on outcome scores. Subjects and methods: Patients who underwent hip arthroscopy by a local high-volume surgeon from July 2012 to June 2015 were reviewed for the presence of pre- and postoperative HO retrospectively. Patients required at least 12 months of postoperative radiographic follow-up with anteroposterior pelvis radiographs. Patients were excluded if they had any other procedure performed at the time of hip arthroscopy or if medical records were not accessible. Radiographs were reviewed for HO according to the Brooker Classification. Demographic data including sex, age, body mass index, presence of labral tear and repair and outcome score were obtained. The incidence of HO and demographic averages were calculated and compared. This study received approval from the Nova Scotia Health Authority (NSHA) Research Ethics Board (REB) (approval No. 1021503) on August 8, 2016. Results: A total of 307 hips met inclusion criteria, 152 females and 155 males. The overall incidence of HO was 23.45% (72 hips). Fifty-six hips (78%) had Brooker class I HO, 13 (18%) hips had Brooker class II HO, and 3 (4%) hips had Brooker class III HO. No specific demographic variables or the pathology were seen to be associated significantly with development of HO. Outcome score differences were not significantly different between the HO and non-HO groups (P = 0.94). Conclusion: The incidence of HO in our series matched the range found in other studies of HO incidence in the literature. The incidence of HO dose not appear to be influenced by demographic variables including sex, age, and body mass index in our study. This data will add to the developing base of literature on HO incidence in hip arthroscopy and will serve as a comparison and benchmark for future study on HO prophylaxis.

How to cite this article:
Smith TD, Ravipati AP, Wong IH. Incidence and risk factors for development of postoperative heterotopic ossification in a high-volume hip arthroscopy practice: A case series.Clin Trials Orthop Disord 2019;4:43-46

How to cite this URL:
Smith TD, Ravipati AP, Wong IH. Incidence and risk factors for development of postoperative heterotopic ossification in a high-volume hip arthroscopy practice: A case series. Clin Trials Orthop Disord [serial online] 2019 [cited 2019 Sep 19 ];4:43-46
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Full Text


Heterotopic ossification (HO) is the formation of bone in abnormal locations including soft tissues and joints and is a known complication of hip arthroscopy.[1],[2],[3] In patients undergoing hip arthroscopy, reported incidence in the literature of postoperative HO development range widely during 0–44% in patients who do not receive prophylaxis.[4],[5],[6],[7],[8],[9],[10],[11]

HO is theorized to result from trauma inflicted upon bone and soft tissue around the hip joint during arthroscopy.[1],[4],[12] Although the definitive mechanism for HO has not been clearly understood, the initial traumatic and inflammatory process appear critical to the induction of HO. The underlying molecular mechanisms seem to be initiated by the bone morphogenic protein in interaction with the local peripheral nerves, the resulting inflammatory cascade is ultimately responsible for activation of locally derived stem cells, leading to formation, vascularisation, and innervation of newly formed bone.[12] Beckmann et al.[13], Bedi et al.[5] and Randelli et al.[9] in their studies mentioned that male gender is associated with higher incidence of HO. They also noted that capsular incision with femoral osteoplasty is a risk factor. However, Rath et al.[10] in their study found no difference in incidence of HO between males and females. They found no significant difference in patients who developed HO and those who did not.


Methods to reduce HO formation historically include post-operative radiation and NSAID therapy.[5] Bedi et al.[5] and Beckman et al.[13] have shown significant decrease in incidence of HO following administration of NSAID therapy in the postoperative period. Starting from 2015, we regularly used a COX-2 inhibitor for the first 6 weeks after the surgery.

Our main objective of this study was to determine the local baseline incidence of HO following hip arthroscopy at our institution. Secondly, we wished to identify risk factors for development of HO including sex, age, body mass index (BMI), and primary pathology and finally we intended to assess the influence of severity of HO formation (based on Brooker staging) on patient outcomes.

 Materials and Methods

Design and study population

Radiographs from 307 patients that underwent hip arthroscopies between July 2012 and June 2015 with indications for hip arthroscopy being femoroacetabular impingement (FAI) or labral tear were included in the study. The primary surgeon was sports medicine fellowship trained with an experience of more than 8 years in hip arthroscopy. This study received approval from the Nova Scotia Health Authority (NSHA) Research Ethics Board (REB) (approval No. 1021503) on August 8, 2016. Given the retrospective nature of the study, a waiver for informed consent was approved by the local institutional review board. Images including anteroposterior pelvis radiographs were reviewed, looking for evidence of HO and required at least 6 months of radiographic follow up after the index procedure. These inclusion criteria were selected as it mirrored selection criteria used in similar studies by several other authors.[4],[5],[9],[10],[11],[13] Patients were excluded if they had non-arthroscopic procedures performed during the same operation, or if medical records were not accessible.

Anteroposterior pelvis radiographs

Preoperative anteroposterior pelvis radiographs were examined for the presence of HO and degree noted if present. Aanteroposterior pelvic radiographs were also obtained at 2 weeks, 6, 12, 24 months postoperatively to assess the development and progression of HO after hip arthroscopy. Hip arthroscopy was performed using standard portals, the anterolateral and anterior portals and included inter-portal capsulotomy, acetabular and femoral osteoplasty and psoas tenotomy. Postoperative anteroposterior pelvis radiographs were examined for the presence of HO based on the classification by Brooker et al.[14] For Brooker class I, islands of bone were identified within the soft tissues around the hip. In Brooker class II, bone spurs more prominent than in Brooker class I emanated from pelvis or proximal end of the femur with greater than 1 cm between opposing bone surfaces. Brooker class III presented similarity to Brooker class II, however the space between opposing bone surfaces was less than 1 cm. Brooker Class IV demonstrate ankylosed hips on radiographs.

Patient database

Patient data from medical records including sex, age, side of operation, BMI, hip pathology, and outcome scores (iHOT) were retrieved along with Brooker classification and inserted into a deidentified database. This database was used to determine the prevalence of the various demographic variables and HO.

Statistical analysis

Statistical software (SPSS version 24.0; IBM, Armonk, NY, USA) was used to analyze variables between patients who developed HO and those that did not. Unpaired two-sample t-tests and simple and multiple logistic regression analyses (at 95% confidence interval) were used to compare between groups as well as between Brooker classification.


A total of 307 hips were included for study including 152 female hips and 155 male hips. None of the patients had HO preoperatively. At final follow-up, 72 hips developed HO (incidence of 23.45%) while 235 had no HO. Of those with HO, 71% were male (n = 51), and 29% were female (n = 21). Age, BMI, duration of symptoms and follow-up duration were not statistically different between patients who developed HO and those who did not. Outcome score differences (iHOT) were not statistically significant when comparing patients who developed HO and those that did not (P = 0.94). Further, changes were not significant when comparing outcome scores between men and women. Similarly, pathology did not significantly differ between groups although alpha angles measured on pre-operative frog leg radiographs approached significance (0.079) [Table 1]. {Table 1}

Compared to females, males had an odds ratio to develop HO of 3.60 using simple logistic regression. However, this significance level was not maintained when adjusted for other variables (P = 0.11). Age, gender, body mass index, preoperative duration of symptoms and pathology (CAM/PINCER) were not found to be individual risk factors for development of HO ([Table 2].{Table 2}

At final follow-up (2-year), 56 hips had Brooker class I HO (78%), 13 hips had Brooker class II HO (18%) and 3 hips had Brooker class III HO (4%). No patient developed Brooker class IV HO. The earliest notable HO was at 2 weeks. The majority (50%) of patients developed HO at 6 months of follow-up. An additional 19 patients (26%) developed HO at 1 year of follow-up. Only six patients (8%) developed HO at 2 years of follow-up [Table 3]. Only one patient had revision surgery for debridement of HO.{Table 3}


HO can lead to poor outcomes due to pain and joint dysfunction and may require further intervention.[15],[16],[17] In this study, 72 hips had postoperative evidence of HO, giving an overall incidence of HO of any grade of 23.45%. Most of these were low grade with 56 hips with Brooker class I HO, 13 with Brooker class II HO, and 3 with Brooker class III HO. No hip at final follow-up had complete joint ankyloses or class IV HO. One patient underwent debridement for symptomatic HO.

Among those with HO, 51 hips (71%) were male. However, using regression analysis this was not found to be a significant factor.[18] Similarly in another study of HO following hip arthroscopy, Bedi et al.[5] did not find male sex to be a significant risk factor for HO despite most patients developing HO being male. They postulated that this finding was due to their low incidence of HO and resulting poor power. Rath et al.[10] also did not find difference with regards to gender in their study.

One other variable, alpha angle on pre-operative frog leg views, trended toward significance between those that developed HO and those that did not (P = 0.079) but this trend also disappeared with multiple logistic regression. Such an association makes sense considering what is known about the pathophysiology of HO development and has also been shown by Bedi et al.[5] to be statistically significant between those who did and did not develop HO. This association may be due to a lack of power within our study.

Outcome measures (iHOT) did not vary significantly between patients who did and did not develop HO, a finding demonstrated previously. For instance, Rath et al.[10] in their 2013 paper showed no difference in change in the Hip Outcome Score or the Modified Harris Hip Score between patients that did and did not develop HO. Applying these findings to clinical practice questions the benefit and need for HO prophylaxis. Given the seemingly benign effect of HO, prophylaxis, if effective in preventing its occurrence, may not change outcomes.

In contrast, literature from the arthroplasty field does show a negative effect of HO on outcomes and there are cases of HO following hip arthroscopy, such as in our case series, that require further intervention because of its development.[19] Again, perhaps we were unable to demonstrate the negative effects of HO on outcome measures as most of our cases had Brooker Class I HO. A further study involving more patients that develop higher class Brooker HO will undoubtedly show a correlation with worse outcomes.

Our study has strength in the number of variables we were able to harvest. We demonstrated trends and associations including pathology, and sex associated findings are in keeping with others in this field. However, some of our findings may be affected by our lack of power such as non-significant association of HO with sex, outcome scores and pathology. Additionally, as our study is retrospective in nature we did not have standardized, consistent radiographic follow-up. As a result, information such as the average time of presentation or development of HO postoperation could not accurately be determined, though the importance of this information is unknown.


We believe this study contributes to the understanding of the incidence of HO in a developing field of orthopedics. In this study, the overall incidence of HO development after hip arthroscopy was 23.4%, which is consistent with a range of 0–44% found in other series.[4],[5],[6],[7],[8],[9],[10],[11]

Further, we demonstrated a known risk factor for the development of HO (male sex) although this was not maintained when controlling for all assessed variables. Outcome differences between those with and without HO did not reach statistical significance. We suggest as others have done that this is the result of lower numbers of higher grade HO in our analysis. At our institution, we recently established a post-operative HO prophylactic regimen utilizing a COX-2 inhibitor. We hope to build on the findings from the present study to understand the effects of this intervention and whether its benefit warrants its use.

Additional file

Additional file 1: [SUPPORTING:1]Ethical approval documentation.

Author contributions

Concept of study, literature review, data collection, data analysis, and maniscript writing: TDS; manuscript revision and review: APTR; manuscript revision and review: IHW.

Conflicts of interest

The authors have no conflicts of interest to declare.

Financial support


Institutional review board statement

This study received approval from the Nova Scotia Health Authority (NSHA) Research Ethics Board (REB) (approval No. 1021503) on August 8, 2016.

Declaration of participant consent

The authors certify that they have obtained all appropriate patient consent forms. In the forms, 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.

Reporting statement

This study followed the Recommendations for the Conduct, Reporting, Editing and Publication of Scholarly Work in Medical Journals developed by the International Committee of Medical Journal Editors.

Biostatistics statement

The statistical methods of this study were reviewed by statisticians from Dalhousie University, Canada.

Copyright license agreement

The Copyright License Agreement has been signed by all authors before publication.

Data sharing statement

Individual participant data that underlie the results reported in this article, after deidentification (text, tables, figures, and appendices). Study protocol and informed consent form will be available immediately following publication, without end date. Results will be disseminated through presentations at scientific meetings and/or by publication in a peer-reviewed journal. Anonymized trial data will be available indefinitely at

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Peer review

Externally peer reviewed.

Open access statement

This is an open access journal, and articles are distributed under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 License, which allows others to remix, tweak, and build upon the work non-commercially, as long as appropriate credit is given and the new creations are licensed under the identical terms.

P-Reviewer: Xu G; C-Editors: Zhao M, Li JY; S-Editor: Li CH; L-Editor: Song LP; T-Editor: Jia Y


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