Risk Factors Of Heterotopic Ossification Following Total Hip Arthroplasty In Patients With Ankylosing Spondylitis

ObjectiveThis study was to identify the risk factors of heterotopic ossification(HO)after total hip arthroplasty(THA)in ankylosing spondylitis.MethodsThe clinical data were retrospectively analyzed,from 87 patients(132 hips)with ankylosing spondylitis undergoing primary total hip arthroplasty between June 2011 and December 2012.Hips were inserted in 76 men(115 hips)and 11 women(17 hips).The average age of all patients is 34.8 years old(18?80).The average follow-up time is 28.2 months(6?65).Analysis and comparison the clinical and imaging data of all patients.The clinical data is age,gender,duration of symptoms,preoperative hip ankylosis,preoperative ESR and CRP,duration of surgery,intraoperative blood loss,type of anesthesia and type of implant fixation.And the image information is hip radiographs of preoperative and postoperative.The presence of HO were evaluated on the anteroposterior radiographs at a minimum of 6 months follow-up and graded according to the Brooker classification.Functional outcome before surgery and last follow up was measured using the Harris hip score(HHS).ResultsOf the 132 hips,43 hips(32.6%)were found to have developed HO.Brooker grade I HO was seen in 17 hips(12.9%),grade ? in 17(12.9%),grade III in 8(6.1%)and grade IV in 1 hip(0.7%).The incidence of HO were 42 in males vs 1 in females.The average HHS improved from 36.7(range:0-62.0)before operation to 82.9 ?(range:69.0-99.0)at last follow up.The risk factors were divided into two categories:modifiable and nonmodifiable.Male gender(P = 0.045),preoperative ankylosed hip(P<0.001)were nonmodifiable risk factors which increased the prevalence of HO.Of the various modifiable risk factors,prolonged duration of surgery(P = 0.044)and general anesthesia(P = 0.009)were associated with increased occurrence of HO.Age,preoperative ESR and CRP,intraoperative blood loss,and type of implant fixation have no obvious correlation with HO.ConclusionDuration of surgery,Soft tissue injury and anesthesia type were associated with increased occurrence of HO.To prevent the formation of HO following THA in AS,efforts to reduce the duration of surgery to minimize soft tissue trauma,and avoid general anesthesia in the operation should be considered.