total hip arthroplasty ( tha ) is widely performed for the purpose of alleviating pain and 
 improving activities of daily living ( adls ) .
it is important to repair the hip center and 
 lever arm that cause decreased mechanical efficiency due to hip deformity and to improve 
 mechanical efficiency when reconstructing hip joint function1 , 2 .
hip abductor muscle 
 strength after tha is known to be affected by the height of the hip center3,4,5 , femoral offset ( fo ) , and the gluteus medius 
 lever arm1 , 5 .
asayama and chamnnongkich5 examined osteoarthritis ( oa ) patients with at least 1.6  years of 
 postoperative follow - up after tha , and they demonstrated that the ratio of hip abductor 
 muscle strength of the reconstructed side to that of the unoperated side was strongly and 
 positively correlated with the ratio of fo to body weight lever arm and negatively 
 correlated with the height of the hip center . concerning tha and leg strength ,
it has been shown in patients scheduled for tha that 
 preoperative hip abductor muscle strength is 31% less than that in healthy individuals , and 
 that preoperative hip abductor muscle strength is not correlated with physical activity6
. additionally , hip abductor muscle strength 
 and total workload at 5 months after tha are 56% and 20% of the levels of community dwelling 
 elderly people , respectively , indicating a marked difference between the patients with tha 
 and elderly people7 .
moreover , patients 
 who have undergone tha have residual muscle atrophy of the quadriceps and do not show 
 recovery in quadriceps muscle strength8 . 

furthermore , it has also been reported that the hip abductor muscle strength on the 
 reconstructed side only recovers to about 80% of that on the unoperated side at both 6 
 months and 2  years postoperatively9 . in 
 patients with unilateral developmental dysplasia of the hip ( ddh ) scheduled to undergo tha , 
 liu and
wen10 reported that 
 cross - sectional area of the gluteus medius was significantly smaller , and that the fo and 
 gluteus medius lever arm were also significantly decreased on the reconstructed side vs. the 
 unoperated side ; they speculated that a greater length of time may be required for 
 postoperative muscle strength recovery when considering the changes in gluteus medius . 

clinically , even though the artificial joint placement site and the lever arm are properly 
 configured , many patients have a prolonged decrease in muscle strength .
previous 
 studies6,7,8,9,10 have demonstrated that hip 
 abductor muscle strength does not recover to the level of the unoperated side or of older 
 people in the geographic region at 5 to 6 months after tha surgery , and it has therefore 
 been postulated that multiple factors aside from mechanical factors are involved in the 
 prolonged decrease of muscle strength .
the purpose of this study was to compare the 
 artificial joint lever arm , which is a mechanical factor related to postoperative hip 
 abductor muscle strength , and other factors that are considered to potentially affect 
 postoperative hip abductor muscle strength , including preoperative hip abductor muscle 
 strength , the amount of subluxation of the hip , age11 , 12 , body mass index 
 ( bmi)11 , 12 , duration of disease13 , physical activity6 , 12 , compliance rate with exercise , and mental 
 state14 , 16 , and to elucidate the relative importance of factors affecting hip 
 abductor muscle strength during the first 6 months after surgery .
a total of 103 females with osteoarthritis scheduled for primary unilateral total hip 
 arthroplasty were eligible .
all surgeries were performed by , or under the supervision of , 
 one co - author ( kh ) .
the procedure takes 
 off 2025% of the gluteus medius and uses a shorter incision .
after setting the femoral and acetabular components , the gluteus medius and 
 the gluteus minimus were completely re - attached to the greater trochanter .
exclusion 
 criteria were as follows : patients with charnley classes b and c ; rheumatoid arthritis ; 
 osteonecrosis ; previous surgery on the affected hip ; disorders of the nervous system and 
 muscles ; depression ; or a schizophrenic disorder .
recruitment was conducted at the shonan 
 joint reconstruction center from may 3 , 2013 to february 6 , 2014 .
the follow - up was 
 conducted 2 and 6 months after surgery from august 3 , 2013 to september 6 , 2014 .
the 
 tokushukai group ethics committee approved the study protocol ( i d : tge00304 - 115 ) .
the 
 intervention procedures were fully explained to all participants , and their written , 
 informed consent was obtained .
there were not any financial and personal relationships with 
 other people or organizations that could inappropriately influence or bias present work .
the patients were permitted full - weight bearing on the day of surgery , but compound motion 
 of the hip ( combined hip flexion 90 , hip adduction , and hip internal rotation ) was 
 contraindicated to prevent dislocation of the hip .
the standard physical therapy pathway 
 involved patients ambulating with a walker - cane on the morning after surgery , a single cane 
 on postoperative day ( pod ) 3 , and ascending and descending stairs on pod 5 . patients who 
 were able to walk with a single cane and ascend and descend stairs independently were 
 discharged home .
evaluations were conducted in the preoperative period and 2 and 6 months after surgery . 

investigators determined muscle strength ( isometric hip abductor strength ) , radiographic 
 analyses ( crowe class , postoperative femoral offset ( fo ) , height of the hip center ( hc ) ) , 
 frenchay activities index ( fai ) , compliance rate with home exercise , and the japanese 
 orthopaedic association hip - disease evaluation questionnaire ( jheq ) .
demographic data were 
 collected from clinical records and included : age , body mass index ( bmi ) , diagnosis , disease 
 duration , co - morbidity index , and the harris hip score .
isometric hip abductor strength measurements were performed in all patients using a 
 handheld dynamometer ( microfet2 , hoggan health industries , salt lake city , ut , usa ) in the 
 supine position .
a handheld dynamometer was placed lateral to the fibula 2.5  cm proximal to 
 the malleolus .
the torque and body weight ratio ( nm / kg ) were measured using the spina 
 malleolar distance and body weight .
three trials at maximum effort were performed , and the 
 highest value was used for the analysis .
anteroposterior radiographs of the pelvis were taken on the day of examination 
 preoperatively and 2 months after surgery .
the amount of hip subluxation in the preoperative 
 period was evaluated using the crowe class18 , and quantitative assessment was also performed ( crowe % ) . using 
 the inferior margin of each tear drop as the reference line ,
the ratio of the distance from 
 the reference line to the top of the femoral head on the unaffected side and the distance 
 from the reference line to the head neck junction on the affected side was calculated ( fig .
1.crowe class . using the inferior margin of each tear drop as the reference line , the 
 ratio of the distance from the reference line to the top of the femoral head on the 
 unaffected side ( a : solid bar ) and the distance from the reference line to the head 
 neck junction on the affected side ( b : stripe bar ) was calculated ( b / a100 ) . ) . for convenience ,
crowe class . using the inferior margin of each tear drop as the reference line ,
the 
 ratio of the distance from the reference line to the top of the femoral head on the 
 unaffected side ( a : solid bar ) and the distance from the reference line to the head 
 neck junction on the affected side ( b : stripe bar ) was calculated ( b / a100 ) .
fo was defined as the length of a perpendicular line from 
 the line of the axis of the femur to the center of the femoral head of the prosthesis . the 
 body - weight lever arm ( bwla )
was defined as the distance from the center of the femoral head 
 to the line of the center of gravity ( fig .
the ratio of fo ( white bar ) and bwla ( solid 
 bar ) was calculated ( fo % ) . ) .
the ratio of fo ( white bar ) and bwla ( solid 
 bar ) was calculated ( fo % ) .
hc was defined as the length of a perpendicular line from 
 the center of the femoral head of the prosthesis to the reference line connecting the 
 inferior margin of each tear drop .
the definition of a high hip center was 2  cm or more 
 above the anatomical hc19 .
physical activities were defined as 
 regular activities in the adl setting 3 months before the onset of this study .
the fai 
 evaluates the frequency and intensity of physical activities in the adl setting .
the fai 
 score ( 045 points ) ranges from 0 points for a sedentary life style to 45 points for a very 
 active life style .
patients were told to do home 
 exercises and to complete the self - report sheet every day for 2 months after surgery .
they 
 were also asked to collect the self - report sheets at the end of the postoperative period ( 2 
 months after surgery ) .
the compliance rate with exercises was calculated based on the 
 performance of exercise sessions .
home exercises consisted of a 3-exercise menu : clamshell 
 exercise , hip raise exercise , and hip abduction exercise . each exercise session consisted of 
 1 set of 20 repetitions in the supine position .
psychological status measurements of all patients were performed using the jheq mental 
 score , a sub - scale of the jheq22 , which 
 is a self - administered questionnaire that can be useful in patients who frequently engage in 
 deep flexion of the hip joint due to lifestyle and culture .
the jheq mental score contains 
 these question items : anxiety , irritation , dissatisfaction , and difficulty participating in 
 activity or the local community .
statistical analyses were conducted by a co - investigator ( ja ) who was independent from the 
 recruitment , intervention , and data collection .
the minimal sample size for the stepwise 
 multiple regression analysis to examine significant factors ( =0.05 , power=0.95 , effect 
 size=0.4 ) was calculated . additionally , assuming a 1% dislocation rate after tha , a 1% 
 dropout rate was set ; 66 participants were required .
factors related to isometric hip abductor muscle strength 2 and 6 months after surgery were 
 examined by stepwise multiple regression analysis .
the dependent variable was isometric hip 
 abductor muscle strength 2 and 6 months after surgery .
independent variables were age , bmi , 
 disease duration , fo , crowe % , isometric hip abductor muscle strength in the preoperative 
 period , compliance rate with home exercise at 2 months after surgery , fai at 2 or 6 months 
 after surgery , and the jheq mental score at 2 or 6 months after surgery . for all statistical 
 tests ,
all data were analyzed using spss software ( version 21 , ibm , chicago , il , usa ) .
six participants were unable to complete the study after surgery because of alteration of 
 the surgical procedure ( n=1 ) , perioperative femoral fracture ( n=1 ) , other limb fracture 
 after surgery ( n=1 ) , depression ( n=2 ) , and did not attend follow - up examinations ( n=1 ) ; 
 thus , 97 participants were included in the analysis ( table 1table 1.demographic data of the patientsvariablemean  sdnumber of patients97age ( years)62.2  8.4body mass index at pre - operative period 
 ( kg / m)23.4  3.7diagnosis ( secondly oa due to hip dysplasia : 
 primary oa)94 : 3co - morbidity index ( score)0.3  0.6disease duration ( years)9.1  9.9crowe % ( amount of subluxation of the hip)21.1  2.1crowe class ( type i : ii : iii : iv)75 : 21 : 1 : 0oa : osteoarthritis ) .
femoral prostheses used included the m / l taper with kinectiv stem ( straight neck : 56 , 
 anteverted neck : 1 , retroverted neck : 2 ) ( zimmer warsaw , in , usa ) in 59 patients , a 
 trabecular metal primary hip prosthesis stem ( zimmer ) in 22 patients , and an sl - plus stem 
 ( smith & nephew andover , ma , usa ) in 16 patients .
femoral head diameter was 26  mm in 38 
 patients , 28  mm in 52 patients , 32  mm in 6 patients , and 36  mm in 1 patient .
the height of the hip center was within 1.2  cm of the anatomical hip 
 center in all patients .
chronological changes are shown in table 
 2table 2.chronological changes in the pre- and postoperative periodsevaluated measurespreoperative periodpostoperatively2 months postoperatively6 monthsharris hip score ( point)60.5  12.9 77.8  6.8 86.9  6.5hip abductor muscle strength ( nm / kg ) 0.61  0.18 0.72  0.19 0.83  0.26femoral offset % 43.9  7.2jheq mental score 10.4  6.2 16.9  6.6 20.0  6.1frenchay activities index 29.3  6.623.3  7.329.8  5.6compliance rate of home exercise86.8  18.9 .
significant factors related to isometric hip abductor muscle strength at 2 
 months after surgery were , in extraction order : 1 .
isometric hip abductor muscle strength in 
 the preoperative period ( =0.467 , p<0.05 ) ; 2 .
jheq 
 mental score at 2 months after surgery ( =0.212 , p<0.05 ) ( table 3table 3.factors related to hip abductor muscle strength at 2 months and 6 months after 
 surgeryindependent variablestandard partial regression coefficient ( )rrincluded factors ( 2 months after surgery)hip abductor muscle strength at preoperative period0.4670.5500.303 * bmi0.2910.6070.369 * jheq mental score at postoperatively 2 months0.2120.6430.413 * included factors ( 6 months after surgery)hip abductor muscle strength at preoperative period0.4430.5320.283 * bmi0.2860.6040.364 * jheq mental score at postoperatively 6 months0.1980.6350.403 * * p<0.05 ) .
significant factors related to isometric hip abductor muscle strength at 6 
 months after surgery were , in extraction order : 1 .
isometric hip abductor muscle strength in 
 the preoperative period ( =0.443 , p<0.05 ) ; 2 .
jheq 
 mental score at 6 months after surgery ( =0.198 , p<0.05 ) ( table 3 ) .
in addition , delp and wixson19 reported that a 2-cm superior positioning of the hip center , 
 without lateral placement , does not have major adverse effects on hip abductor muscle 
 strength .
accordingly , this study did not examine the effect of hc on hip abductor muscle 
 strength .
previous reports on predicting hip abductor muscle strength after tha can be roughly 
 classified into two types : reports related to lever arm , which is at the core of 
 reconstructing hip function1,2,3,4,5 , and reports concerning 
 preoperative physical function or demographic data6 , 
 11,12,13,14,15,16 . 

while there are many reports on factors involved in hip abductor muscle strength , there are 
 no reports on the relationships among these factors , and in order to prevent the 
 prolongation of decreased hip abductor muscle strength , it is necessary to elucidate which 
 factors are involved in such prolongation and to preferentially address the issues 
 pertaining to these factors . in the present study ,
the same factors were related to hip abductor muscle strength at 2 
 months and at 6 months after surgery , and the order in which they were extracted was also 
 the same .
preoperative hip abductor muscle strength was extracted as the most significant 
 factor , while the postoperative compliance rate with exercise was not a significant factor . 

jan and hung23 conducted a 3-month study 
 concerning postoperative voluntary training primarily involving antigravity exercise and 
 walking , and they reported that functional improvements can be expected even at 18 months 
 post - tha , as long as the compliance rate with exercise is 50% and the hip abductor muscle 
 strength has improved .
the compliance rate with exercise in the present study was high , at 
 86.8  18.9% at 2 months postoperatively ; however , this was not extracted as a factor , 
 possibly because exercise tasks were lower in load compared to previous studies .
the present 
 results also indicated that it is important to focus on muscle strengthening exercises from 
 the preoperative stage in order to efficiently achieve postoperative hip abductor muscle 
 strength .
however , rooks and huang24 
 conducted a 6-week , pre - tha exercise therapy regimen primarily using muscle strength 
 training machines ( ~10 rm : 10 times repetition maximum ) and pool exercises , and they 
 reported that , while the physical functioning score on the sf-36 improved , gross extensor 
 muscle strength did not improve .
further investigations are necessary to identify a method 
 to efficiently improve preoperative hip abductor muscle strength .
obese patients needed higher 
 torque of hip abductor muscle strength compared with thin patients because hip abductor 
 muscle strength : torque and the body weight ratio were measured using the spina malleolar 
 distance and body weight .
this indicates that obese patients are at a disadvantage for 
 gaining appropriate muscle strength .
however , it may not be considered effective to 
 encourage patients to lose weight before surgery .
it has been 
 reported that depression after tha is involved in increased perioperative adverse 
 events25 and adl limitations at two 
 years postoperatively26 , and it increases 
 the risk of early revision surgery27 .
it 
 is postulated that the jheq mental score not only affects postoperative hip abductor muscle 
 strength , but it also has a mutual relationship , in that a prolonged decrease in muscle 
 strength affects the patients mental state , such as self - efficacy28 . to intervene for the patients mental state29 , 30 ,
it is important to encourage the patients self - efficacy for 
 activity and social participation through improving hip abductor muscle strength . 

additionally , for patients who experience anxiety or frustration about postoperative living , 
 it is necessary to establish initiatives to encourage self - efficacy , including peer 
 counseling by patients who have previously undergone tha and do not experience problems in 
 everyday life31 .
contrary to the authors expectations , fo was not extracted as a factor affecting 
 postoperative hip abductor muscle strength .
tezuka and inaba32 reported that increasing fo together with shifting the hip center 
 medially and distally is effective in hip abductor muscle strength recovery .
in contrast , 
 bjrdal and bjrgul33 reported that 
 increased fo did not affect hip function , consistent with the present findings . at 2 months 
 after surgery in the present study ,
the standard deviation of the mean was smaller for fo 
 than for hip abductor muscle strength ( 16.4% vs. 26.4% ) . from this perspective , it is 
 postulated that fo was not extracted as an influencing factor because fo was constructed 
 with consideration of its effects on hip abductor muscle strength , resulting in such effects 
 becoming homogeneous
first , preoperative femoral neck anteversion 
 was not measured . however , of the study patients , 77.3% had crowe class i , 21.6% had type 
 ii , and only one patient had type iii .
the kinectiv was used in patients with dysplasia , a 
 condition in which anteversion anomaly is a concern .
a retroverted neck was used in patients 
 with excessive anteversion , only 2 cases ( 2/59 ) , and a straight neck was used in the 
 majority of patients . based on this information , the differences between each patient in 
 neck anteversion after tha probably had a minimal effect on postoperative hip abductor 
 muscle strength .
second , compliance rates with home exercise were checked by the self - report 
 sheet ; postoperative exercises after discharge were not performed under the supervision of a 
 physical therapist in the present study . as conclusion ,
the effects of fo , as well as preoperative and postoperative factors , on hip 
 abductor muscle strength were investigated during the first 6 months after tha .
the same 
 factors were related to hip abductor muscle strength at 2 months and 6 months after surgery , 
 and the order of extraction was : 1 .
fo was not extracted as a significant factor related to postoperative 
 isometric hip abductor strength .
 [ purpose ] the importance and effect of hip joint geometry on hip abductor muscle strength 
 are well known . 
 in addition , other perioperative factors are also known to affect hip 
 abductor muscle strength . 
 this study examined the relative importance of factors affecting 
 hip abductor muscle strength after total hip arthroplasty . 
 [ subjects and methods ] the 
 subjects were 97 females with osteoarthritis scheduled for primary unilateral tha . 
 the 
 following variables were assessed preoperatively and 2 and 6 months after surgery : 
 isometric hip abductor strength , radiographic analysis ( crowe class , postoperative femoral 
 offset ( fo ) ) , frenchay activities index , compliance rate with home exercise , japanese 
 orthopaedic association hip - disease evaluation questionnaire ( jheq ) , and demographic data . 
 
 factors related to isometric hip abductor muscle strength 2 and 6 months after surgery 
 were examined . 
 [ results ] significant factors related to isometric hip abductor muscle 
 strength at 2 and 6 months after surgery were , in extraction order : 1 . 
 isometric hip 
 abductor muscle strength in the preoperative period ; 2 . 
 bmi ; and 3 . 
 the jheq mental score 
 at 2 and 6 months after surgery . 
 [ conclusion ] preoperative factors and postoperative 
 mental status were related to postoperative isometric hip abductor strength . 
 fo was not 
 extracted as a significant factor related to postoperative isomeric hip abductor 
 strength . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION