| Objective:Total hip arthroplasty(THA)is an effective method to treat end-stage hip involvement in ankylosing spondylitis(AS).However,clinically,there are great differences in disease severity and postoperative hip function in AS patients with end-stage hip involvement.A full understanding of the clinical characteristics of end-stage hip disease in this special population and the related factors that may affect the postoperative functional recovery of patients will be helpful for joint surgeons to predict in advance and make corresponding management.The purpose of this study was:①To analyze the clinical characteristics of AS patients with end-stage hip involvement,including the study on gender differences in disease pattern and investigation on factors associated with bilateral THA;②To investigate the mid-term clinical results of primary THA in patients with AS and to identify factors related to postoperative poor flexion function;③ To study the pathological changes of muscle fibers and tendons in the periarticular muscle tissue in AS patients with hip involvement and their possible effects on the hip functional recovery after THA.Methods:(1)We retrospectively analyzed the 373 consecutive patients with AS who underwent primary THA due to end-stage hip involvement from 2012 to 2017.Research data on hip involvement in the patients were obtained from medical records and radiographs.The severity of radiographic hip involvement was evaluated by the Bath Ankylosing Spondylitis Radiology Hip Index(BASRI-hip)scoring system.① The patients were divided into two groups by gender with 340 men and 33 women.The data on clinical characteristics,radiographic hip involvement,and laboratory values were compared between the two groups.The comparison was performed again between the two groups after adjusting for the onset age and disease duration by propensity score matching(PSM).②The clinical characteristics and anti-tumor necrosis factor(TNF)therapy of the patients undergoing unilateral THA were compared with those with bilateral THA.Logistic regression was used to identify factors associated with bilateral THA.(2)The clinical outcomes of the patients after THA were followed up.① Excluding 47 patients who lost follow-up and 7 who died from non-surgical causes,319 patients with AS(544 hips)were included in this retrospective study.The mean age of the patients at surgery was 35±9 years(range,19～67 years).Survivorship of the implants,complications,postoperative hip function,and patient satisfaction were investigated.Functional outcomes were assessed by hip flexion range of motion(ROM),Harris hip score(HHS),and the Western Ontario and McMaster Universities Arthritis Index(WOMAC).According to the severity of hip involvement,the hips were divided into two groups:bony ankylosis hips and stiffness hips,and the postoperative hip function was compared between the two groups.②In addition to the above excluded 54 patients,6 patients(6 hips)were excluded due to lack of records on postoperative hip ROM.The postoperative hip function of the remaining 538 hips(313 patients)was analyzed retrospectively.The hips were divided into poor(≤90°)and good(>90°)hip flexion function groups based on the degree of hip flexion ROM recorded at the most recent follow-up.We grouped factors related to postoperative hip flexion function into three categories:preoperative,intraoperative,and postoperative factors.Multivariate logistic regression was performed to identify factors associated with postoperative poor hip flexion.(3)We prospectively enrolled 10 patients with AS and 5 patients with osteonecrosis of the femur head who received primary THA in our department from October 2020 to August 2021,and their pre-and post-operative hip function were recorded respectively.The patients were divided into three groups:group A and group B were the experimental groups(patients with AS),including 5 patients with bone ankylosis hips and 5 patients with stiffness hips,respectively;Group C was the control group,including 5 patients with osteonecrosis of the femur head matched by the demographic parameters of the experimental group.Specimens of the fascia lata tensor were obtained at the junction of muscle fibers and tendons before the incision was closed and then fixed in formalin solution.Muscle fibers were stained by Hematoxylin and Eosin(HE)staining and immunofluorescence double staining.Collagen fibers of the tendon were stained by Sirius red-staining.The staining results were observed microscopically to find differences between groups.Three photographs were randomly selected from the muscle fiber sections stained by immunofluorescence under the ×200 visual field.The proportion of type Ⅰ and type Ⅱ muscle fibers(%)and the ratio of the two(total amount of type Ⅰ fibers to that of type Ⅱ fibers,TA Ⅰ/Ⅱ)were calculated in this visual field,and the results were compared among the three groups.Three photographs were randomly selected from the muscle fiber sections stained by immunofluorescence under the ×400 visual field.Image-Pro Plus 6.0 software was used to calculate the cross-sectional area(CSA)of a single type Ⅰ muscle fiber and a single type Ⅱ muscle fiber in this visual field and the ratio of the two(CSA of a single type Ⅰ fiber to that of a single type Ⅱ fiber,CSA Ⅰ/Ⅱ),and the results were compared among the three groups.Three photographs were randomly selected from the Sirius red-stained tendon slices under the ×200 visual field.Image-Pro Plus 6.0 software was used to calculate the proportion of the collagen fiber area to the entire tissue area(%),and the results were compared among the three groups.Results:(1)The clinical characteristics of patients with end-stage hip involvement in AS are analyzed as follows.① Men underwent total hip arthroplasty(THA)earlier than women in the patients,with a median age of 31 years(range,19～67 years)vs 36 years(range,23～67 years),respectively(P<0.05).Hip involvement was found to be younger in men than that in women,with a median age of 18 years(range,7～56 years)vs 23 years(range,5～55 years)(P<0.05),and men with bilateral onset in hips had a higher frequency than women with that(66.2%vs 39.4%)(P<0.05).There was no gender difference in the proportion of bilateral advanced hip involvement(85.3%vs 72.7%)(P>0.05).The proportion of the patients who had spinal involvement(89.1%vs 69.7%),flexion contracture in the hip(43.8%vs 24.2%),hip range of motion=0°(53.5%vs 30.3%),and an elevated level of CRP(69.1%vs 51.5%)was significantly higher in men than that in women(P<0.05).After adjusting for the onset age and disease duration by PSM(1:1),men with bilateral onset in hips still had a higher frequency than women with that(76.7%vs 40.0%),and the proportion of the patients who had spinal involvement(90.0%vs 66.7%)and an elevated level of CRP(80.0%vs 53.3%)was significantly higher in men than that in women(P<0.05).②Bilateral THA was performed in 67.3%(n=251)of the patients.Male patients had a higher frequency of undergoing bilateral THA compared with female patients(P<0.05).The proportion of the patients who had bilateral onset in hips,flexion contracture in the hip,and a BASRI-hip score of 4 was significantly higher in patients with bilateral THA than in patients with unilateral THA,while that of the patients who had the administration of TNF inhibitors was significantly lower in patients with bilateral THA than in patients with unilateral THA(P<0.05).Patients with bilateral THA experienced a longer disease duration than those with unilateral THA(P<0.05).The result of the logistic regression showed that factors related to bilateral THA were bilateral onset in hips(odds ratio[OR]=10.177,95%confidence interval[CI],5.713 to 18.130),administration of TNF inhibitors(OR=0.346,95%CI,0.142 to 0.838),a BASRI-hip score of 4(OR=4.133,95%CI,2.308 to 7.400),and an ESR level(OR=1.018 for each unit increment,95%CI,1.001 to 1.035).(2)The mid-term clinical results of patients with AS undergoing primary THA and the factors related to postoperative poor flexion function are as follows.① Survivorship was 99.8%with a mean follow-up of 7 years(range,4-9 years).The main complications included dislocation(5 hips,0.9%),periprosthetic femoral fractures(15 hips,2.7%),infection(primary infection in 2 hips and periprosthetic joint infection in 1 hip,0.6%).The flexion-extension ROM improved significantly with a median from 0°(range,0～120°)to 100°(range,30～130°)after THA(P<0.001),and the mean HHS increased from 37±19 to 90±5(P<0.001).Very satisfied was recorded in 278(87%)patients and the main complaint about dissatisfaction in the remaining 41(23%)patients was poor hip flexion function(58.5%).Bony ankylosis hips had a significantly lower hip flexion ROM(median,Q1～Q3;100°,90～100° vs 110°,100～120°)and HHS(89±5 vs 92±5),and a significantly higher WOMAC score(43±20 vs 31±18)compared with stiffness hips(P<0.001).②There were 102 hips(19%)with a hip flexion ROM of no more than 90°.The result of multivariate regression showed that male sex(OR=9.42,95%CI,1.23 to 72.03),bony ankylosis(OR=3.02,95%CI,1.76 to 5.17),cup anteversion angle(OR=0.96,95%CI,0.93 to 0.98),cup inclination angle(OR=0.96,95%CI,0.93 to 0.99),the American Society of Anesthesiologists(ASA)class Ⅲ(OR=6.23,95%CI,1.83 to 21.70),knee involvement(OR=7.80,95%CI,2.75 to 22.16),and noise(OR=0.45,95%CI,0.25 to 0.81)were independent factors related to poor hip flexion after THA in patients with AS.(3)The pathological changes of muscle fibers and tendons in the periarticular muscle tissue in AS patients with hip involvement are as follows.No significant difference was shown in demographic data(P>0.05)and significant differences were shown in hip flexion-extension ROM,hip total ROM,and HHS among the three groups(P=0.006,0.003,and<0.001,respectively).There were significant differences in postoperative flexion ROM(median,range;100°,85～100° vs 110°,100～130° vs 130°,110～135°),HHS(86±3 vs 90±3 vs 92±2),and WOMAC(32±18 vs 14±12 vs 10±3)among the three groups(P=0.013,0.023,and 0.042,respectively).Microscopic observation showed that the proportion of type Ⅰ muscle fibers(green fluorescence)was higher and the proportion of collagen fibers of the tendon was lower in patients with AS than those in patients with osteonecrosis of the femur head.Under the ×200 field of muscle fiber slices stained by immunofluorescence,the proportion of type Ⅰmuscle fibers(%)in group A and B was significantly higher than that in group C(50.9±27.8,58.4 ± 16.6 vs 33.3 ± 25.9,P=0.019),and the proportion of type Ⅱ muscle fibers(%)in group A and B was significantly lower than that in group C(49.1±27.8,41.6±16.6 vs 66.7±25.9,P=0.019),but there was no significant difference between group A and group B(P>0.05).Under the ×400 field of muscle fiber slices stained by immunofluorescence,no significant differences were shown among the three groups in the CSA of a single type Ⅰmuscle fiber or a single type Ⅱ muscle fiber in this visual field and CSA Ⅰ/Ⅱ(P>0.05).Under the ×200 field of Sirius red-stained tendon slices,the proportion of collagen fibers(%)in group A and B was significantly lower than that in group C(41.2±34.0,30.4±16.7 vs 67.5±38.1,P=0.006),and there was no significant difference between group A and B(P>0.05).Conclusions:(1)① The disease pattern of hip involvement in AS has gender differences,with bilateral onset being the dominant pattern in men and unilateral onset being more common in women.The frequency of bilateral advanced hip involvement has no gender difference eventually.However,the severity of end-stage hip involvement in men with AS is more severe than that in women.The higher prevalence of spinal involvement in men with AS may be responsible for the more severe functional impairment compared with women.②Bilateral THA is more common in AS patients with end-stage hip involvement.Bilateral onset in hips,a BASRI-hip score of 4,and a higher level of ESR are risk factors associated with bilateral THA,while anti-TNF therapy is a protective factor reducing the progression of hip involvement to bilateral THA.(2)① Modern cementless THAs exhibit excellent mid-term clinical outcomes in patients with AS.Poor hip flexion function is the main complaint of patient dissatisfaction.Bony ankylosis has a significant adverse effect on postoperative hip function in patients with AS.For patients with juvenile onset AS,THA can be considered after adulthood on the premise of surgical evidence of hip involvement.② Nearly one out of the five hips have poor hip flexion function after THA.Properly increasing the anteversion and inclination angle of acetabular cup during THA can contribute to good postoperative hip flexion function.The optimum treatment strategy is that THA should be performed before ankylosis in patients with AS.(3)In patients with AS,the proportion of muscle fiber types changes in the hip periarticular muscle tissue,presenting as the proportion of type I muscle fibers increases while that of type Ⅱ muscle fibers decreases,and the collagen fibers content of the tendon also decreases.The pathological changes of tendon in the hip periarticular muscle tissue may affect the postoperative hip function after THA. |
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