subsets of 270 participants who visited our hospital for knee pain and met the american college of rheumatology criteria for symptomatic oa of the knee19 ) from october 2005 to september 2013 were evaluated .
we limited the subsets to women for avoiding the confounding variables and bias , because there were only a few male patients ( n = 8) .
all of the patients had a k - l grade  1 in the affected knee and dxa scan was taken of the ipsilateral proximal femur ( femoral neck , trochanter , intertrochanter , total hip - neck , trochanter , and ward 's triangle ) and lumbar spine ( l1 - 4 ) within 6 months of the first visit ( mean interval , 2.7 months ) . to avoid confounding in the statistical analysis , patients with valgus alignment of knees (  3 valgus about the mechanical axis ) , and other comorbidities ( e.g. , rheumatoid arthritis , septic arthritis ) that could also be the causes of knee pain and change in bone structure were excluded .
subjects who had unsatisfactory medial tibial plateau alignment in the radiographs were also excluded from the final analysis .
weight was measured to the nearest 0.1 kg using a single pair of electronic scales .
height was measured to the nearest 0.1 cm using a stadiometer . from these data ,
body mass index ( bmi ; weight / height , kg / m ) was calculated . mechanical axis in the scanogram ,
jsn in the anteroposterior ( ap ) view and the rosenberg view , and k - l grade were measured to assess the severity of oa .
thus , the relationship between the measured values of bmd and radiographic severity of osteoarthritic changes was analyzed considering the confounding variables .
standing ap films of the knee in full extension were obtained with the horizontal x - ray beam centered at the level of the superior patellae .
views were standardized with the back of the knees in contact with the cassette , the patella centralized over the lower end of the femur , and the beam centered 2.5 cm below the apex of the patella , with a tube - to - film distance of 100 cm .
also , the rosenberg view of the knee in 45 of flexion was obtained for accurate detection of jsn in the state of maximal load weight - bearing .
the rosenberg view is a 45 flexion , posteroanterior , weight - bearing view of the knee with the patellae touching the image receptor .
the x - ray tube is 100 cm away from the image receptor , centered at the patellae , and pointing caudad 10. the mechanical axis of the knee was measured from the angle formed by a line drawn from the center of the femoral head to the medial tibial spine and a line drawn from the medial tibial spine to the center of the ankle joint . as mentioned above ,
patients who having valgus alignment of knees were excluded.20 ) the radiographic determinations of knee alignment were made by two examiners who had adequate knowledge about the measurement techniques and the k - l grading scale .
all of the radiographs were digitized by the picture archiving and communication system software ( piviewstar , infinit technology , seoul , korea ) in our hospital , and all of the distances and angles were measured by using the calipers and goniometer provided in the software .
the software enables straight - line measurements with an accuracy of 0.01 mm and 0.01. jsn was defined as the narrowest interbone distance between the medial femoral condyle and the tibial plateau .
k - l grades were assessed using a standard atlas.21 ) the inter - observer reproducibility between two examiners was assessed in a randomly chosen subset of 30 radiographs .
icc for inter - observer variability was more than 0.94 and icc for intra - observer variability was more than 0.97 in all of the measurements .
partial correlation analysis was performed to analyze the relationship between the bmds ( femoral neck , femoral trochanter , femoral intertrochanter , total hip and spine ) and joint space distance in the ap and the rosenberg views , between the bmds and mechanical axis of the knee adjusted for demographic variables ( age , bmi ) .
analysis of covariance ( ancova ) was used to analyze the difference in bmd in the k - l grade category groups , which was also adjusted for covariates ( age , bmi ) .
we determined the sample size after conducting a pilot study with 50 cases . in the pilot study ,
we set the level of significance (  ) and power to 0.05 and 0.8 , respectively ; therefore , the result of sample size calculation was 193 and we ensured that a sufficient number of patients were included in the study .
subsets of 270 participants who visited our hospital for knee pain and met the american college of rheumatology criteria for symptomatic oa of the knee19 ) from october 2005 to september 2013 were evaluated .
we limited the subsets to women for avoiding the confounding variables and bias , because there were only a few male patients ( n = 8) .
all of the patients had a k - l grade  1 in the affected knee and dxa scan was taken of the ipsilateral proximal femur ( femoral neck , trochanter , intertrochanter , total hip - neck , trochanter , and ward 's triangle ) and lumbar spine ( l1 - 4 ) within 6 months of the first visit ( mean interval , 2.7 months ) . to avoid confounding in the statistical analysis , patients with valgus alignment of knees (  3 valgus about the mechanical axis ) , and other comorbidities ( e.g. , rheumatoid arthritis , septic arthritis ) that could also be the causes of knee pain and change in bone structure were excluded .
subjects who had unsatisfactory medial tibial plateau alignment in the radiographs were also excluded from the final analysis .
weight was measured to the nearest 0.1 kg using a single pair of electronic scales .
height was measured to the nearest 0.1 cm using a stadiometer . from these data ,
body mass index ( bmi ; weight / height , kg / m ) was calculated . mechanical axis in the scanogram ,
jsn in the anteroposterior ( ap ) view and the rosenberg view , and k - l grade were measured to assess the severity of oa .
thus , the relationship between the measured values of bmd and radiographic severity of osteoarthritic changes was analyzed considering the confounding variables .
standing ap films of the knee in full extension were obtained with the horizontal x - ray beam centered at the level of the superior patellae .
views were standardized with the back of the knees in contact with the cassette , the patella centralized over the lower end of the femur , and the beam centered 2.5 cm below the apex of the patella , with a tube - to - film distance of 100 cm .
also , the rosenberg view of the knee in 45 of flexion was obtained for accurate detection of jsn in the state of maximal load weight - bearing .
the rosenberg view is a 45 flexion , posteroanterior , weight - bearing view of the knee with the patellae touching the image receptor .
the x - ray tube is 100 cm away from the image receptor , centered at the patellae , and pointing caudad 10. the mechanical axis of the knee was measured from the angle formed by a line drawn from the center of the femoral head to the medial tibial spine and a line drawn from the medial tibial spine to the center of the ankle joint . as mentioned above ,
patients who having valgus alignment of knees were excluded.20 ) the radiographic determinations of knee alignment were made by two examiners who had adequate knowledge about the measurement techniques and the k - l grading scale .
all of the radiographs were digitized by the picture archiving and communication system software ( piviewstar , infinit technology , seoul , korea ) in our hospital , and all of the distances and angles were measured by using the calipers and goniometer provided in the software .
the software enables straight - line measurements with an accuracy of 0.01 mm and 0.01. jsn was defined as the narrowest interbone distance between the medial femoral condyle and the tibial plateau .
the inter - observer reproducibility between two examiners was assessed in a randomly chosen subset of 30 radiographs .
icc for inter - observer variability was more than 0.94 and icc for intra - observer variability was more than 0.97 in all of the measurements .
12 ( spss inc . , chicago , il , usa ) was used for the statistical analysis .
partial correlation analysis was performed to analyze the relationship between the bmds ( femoral neck , femoral trochanter , femoral intertrochanter , total hip and spine ) and joint space distance in the ap and the rosenberg views , between the bmds and mechanical axis of the knee adjusted for demographic variables ( age , bmi ) .
analysis of covariance ( ancova ) was used to analyze the difference in bmd in the k - l grade category groups , which was also adjusted for covariates ( age , bmi ) .
we determined the sample size after conducting a pilot study with 50 cases . in the pilot study ,
we set the level of significance (  ) and power to 0.05 and 0.8 , respectively ; therefore , the result of sample size calculation was 193 and we ensured that a sufficient number of patients were included in the study .
the study population included 195 females who had radiographic features of knee oa ( k - l grade  1 ) .
the mean age of study participants was 68.5 years , with a range of 38 to 83 years .
the mean height , weight , and bmi were 151.58 cm ( range , 139.40 to 177.90 cm ) , 60.25 kg ( range , 39.10 to 96.00 kg ) , and 26.17 kg / m ( range , 18.39 to 39.33 kg / m ) , respectively .
mean jsn in the ap view was 2.00 mm , and mean jsn in the rosenberg view was 1.48 mm .
thirty participants were classified into k - l grade 1 , 22 into k - l grade 2 , 51 into k - l grade 3 , and 92 into k - l grade 4 .
there was a statistically significant relationship between mechanical axis of the knee and femoral neck , trochanter , intertrochanter , and total hip bmd ( p < 0.05 ) .
less severity of varus alignment was positively associated with the bmd of the proximal femur .
there was also a statistically significant difference in spine bmd in terms of the mechanical axis . in terms of the joint space distance
, there were statistically significant relationships between bmd of the proximal femur ( femoral neck , trochanter , intertrochanter , and total hip ) and joint space distance in the ap view ( p < 0.05 ) .
there were also statistically significant differences between bmd of the proximal femur and joint space distance in the rosenberg view ( p < 0.05 ) .
the bmd values of the proximal femur were positively associated with increased joint space distance .
joint space distance increased with greater total hip bmd both in the ap view and the rosenberg view ( in the ap view : femoral neck , r = 0.147 , p = 0.041 ; femoral trochanter , r = 0.251 , p = 0.000 ; femoral intertrochanter , r = 0.224 , p = 0.002 ; total hip , r = 0.233 , p = 0.001 and in the rosenberg view : femoral neck , r = 0.226 , p = 0.002 ; femoral trochanter , r = 0.289 , p = 0.000 ; femoral intertrochanter , r = 0.295 , p = 0.000 ; total hip , r = 0.307 , p = 0.000 ) .
spine bmd , however , was not significantly associated with joint space distance ( table 2 ) .
the mean values of bmd of the proximal femur had a tendency to decrease with increasing k - l grades , and there were significant differences in the bmd of the proximal femur ( femoral neck , trochanter , intertrochanter , total hip ) according to the k - l grade groups ( p = 0.028 , p = 0.004 , p = 0.002 , and p = 0.003 , respectively ) .
the contrast test was also conducted , and the k - l grade 1 group showed a statistically significant difference in the bmd of the proximal femur compared to k - l grade 2 , 3 , and 4 groups , and k - l grade 1 , 2 , and 3 groups showed statistically significant differences in the bmd of the proximal femur compared to k - l grade 4 group .
there was no significant difference in spine bmd among the k - l grade groups ( p = 0.072 ) ( table 3 ) .
in this study , we focused on the relationship between the bmd of the proximal femur and severity of radiological knee oa in the ipsilateral side based on the cross - sectional data in a korean population especially among women , while studying the relationship between spine bmd and the severity of radiological knee oa at the same time .
the overall results demonstrated that after adjusting for demographic variables ( age , bmi ) , low bmd of the proximal femur ( femoral neck , trochanter , intertrochanter and total hip ) was positively correlated with joint space width .
also , higher k - l grade was associated with lower bmd of the proximal femur . on the other hand ,
a number of studies about the relationship between bmd and oa have been reported and the inverse relationship between op and oa has been shown in the cross - sectional data.1,2,3,4,5,6 ) in the framingham study , mean femoral bmd at the 3 proximal femur sites was 5%-9% higher in patients with either grade 1 , grade 2 , or grade 3 knee oa compared with those with no knee oa.3 ) another cohort study reported that higher bone density was associated with an increased prevalence of knee oa and an increased risk of incident knee oa , but higher bmd was associated with a decreased risk of progressive knee oa.11 ) other cross - sectional studies showed that oa subjects had a 6%- 9% increase in lumbar spine bmd4 ) and patients who had hip oa were found to have 8%-10% higher spine and hip bmd.6 ) our data are obtained from patients who had both symptomatic and radiological knee oa .
also , we made an effort to minimize the difference in the time between the assessment of knee oa and the bmd measurement (  6 months ) for collecting the cross - sectional data .
development of oa is associated with changes in the subchondral bone22,23 ) and crosstalk between the subchondral bone and the cartilage contributes to progression towards different stages of oa.24 ) as the initiation of oa precedes the appearance of radiographic features,23 ) it may be more difficult to analyze the actual relationship between bmd and oa .
severity of radiological oa , as demonstrated by a narrower joint space or a higher k - l grade , had a negative correlation with the bmd of the ipsilateral proximal femur .
also , contrary to the result of a previous study,25 ) there was no correlation between the spine bmd and knee oa ( in terms of joint space distance and the k - l grade ) .
pain and disuse of the affected limb may have caused a local decrease in bmd of the proximal femur on the ipsilateral side , while spine bmd was relatively unaffected by these effects .
so in our setting , it can be suggested that local factors affected the interaction between oa and bone density in addition to systemic factors .
our data , obtained from a korean population , especially from women , may reflect a different lifestyle compared with that of caucasian population3,6,9,10,11 ) as the patients with pain due to oa mostly avoid physical activities and exercises .
therefore , we think that these factors led to the obtainment of results , which are the different from those in previous studies . as a corollary , while studying the relationship between oa and op , various factors should be taken into consideration .
only females were included in the study population ; hence , it did not accurately reflect the korean population .
although we made an effort to adjust for physical variables , we were not able to control for individual lifestyle and physical activity of knee oa patients . also , we only included symptomatic patients with knee oa ; therefore , we could not assess the relationship between the bmd and radiographic knee oa in healthy groups and could not analyze the association between bmd and the incidence of knee oa .
however , we believe that this report provides useful information about the relationship between oa and op in asian patients .
 backgroundthe relationship between osteoarthritis ( oa ) and osteoporosis ( op ) is complicated and it may differ according to the site or stage of disease . the purpose of this cross - sectional study is to examine the relationship between the severity of radiological knee oa and the degree of op in the ipsilateral proximal femur as denoted by bone mineral density ( bmd ) in a korean population , especially among women.methods one hundred ninety - five female patients who had knee pain and radiological knee oa were investigated with respect to the relationship of knee oa severity with bmd . 
 the bmd of the proximal femur and spine was measured by dual energy x - ray absorptiometry , and the severity of knee oa was evaluated based on kellgren - lawrence ( k - l ) radiographic criteria , joint space narrowing ( jsn ) and mechanical axis of knee alignment . partial correlation analysis and ancova adjusted for confounding factors ( age and body mass index ) 
 were performed to assess the relationship.resultsthere was a statistically significant relationship between the bmd of the proximal femur and jsn , and the bmd of the proximal femur was positively associated with increased joint space width . 
 there was a lack of association between the spine bmd and jsn . 
 the bmd of the proximal femur was also significantly lower in patients who had a higher k - l grade.conclusionsthe radiographic finding of severe oa in the knee is associated with decreased bmd of the ipsilateral proximal femur including the femoral neck , trochanter , intertrochanter , and region of the entire hip ( neck , trochanter , and ward 's triangle ) . 
METHODS
Study Design
Radiological Measurements
Reproducibility
Statistical Analysis
RESULTS
DISCUSSION