according to reports on large series of total hip arthroplasties ( tha ) and the percentage performed for osteonecrosis ( on ) , there are almost 20 000 new cases of on of the femoral head in the united states annually , which accounts for 5% to 10% of all primary tha in that country [ 15 ] . characteristically , on affects younger patients who typically refer to the orthopedic surgeon for the first time in their thirties to fifties , in the late stage of the disease . tha is a standard treatment option , but for most of the past 40 years it has had less success , specifically in patients with on and in those who are younger and more active [ 811 ] . femoral bone - conserving metal - on - metal ( mom ) hip resurfacing was introduced in the early 1990 s . it is a procedure that can be particularly useful in patients who may have outlived conventional total hip replacements . it is an attractive method of joint reconstruction , particularly in younger patients , as it preserves proximal femoral bone stock for further revisions , optimizes stress transfer to the proximal femur , and , due to the large diameter of the articulation , offers inherent stability and optimal range of motion [ 1416 ] . mom - associated problems , including migration of the femoral component , necrosis of the femoral head and fracture of the femoral neck or head , continue to be reported [ 7,1720 ] . mom hip resurfacing arthroplasty has recently emerged as an alternative to conventional tha in treating advanced stages of osteoarthritis of the hip [ 1417 ] and late ficat stage iii or iv of avascular necrosis . hip resurfacing has been attempted since the 1950 s . throughout the 1960 s and 1970 s clinical results were inferior to those of stemmed hip replacement , and the procedure was largely abandoned by the mid 1980 s . the rebirth of mom hip resurfacing can be attributed to advances in orthopedic implants manufacturing , as well as to the evolution of the surgical technique . a number of recent short- and mid - term reports described early successes of this procedure [ 1,14,2231 ] . since many reports have already been published regarding the successful outcome of resurfacing in advanced osteoarthritis , the purpose of this study was to analyze the clinical outcomes of mom total hip resurfacing arthroplasty in patients with osteonecrosis of the femoral head , and to compare them with the results from a matched group of patients with osteoarthritis . this historical cohort study consisted of a consecutive series of 52 patients managed by mom hip resurfacing for the treatment of end - stage osteonecrosis and osteoarthritis of the femoral head in a major orthopedic referral center from feb 2002 to may 2007 . the patients were divided into 2 groups : the first group included 28 patients with diagnosis of hip osteonecrosis of late ficat stage 3 and 4 ( figure 1 ) , and the second group consisted of 24 patients with grade 4 and 5 ( croft s grade ) of hip joint osteoarthritis ( figure 2 ) . there was no significant sex distribution difference between the 2 groups . in the avn group , 15 ( 53.6% ) patients were male and 13 ( 46.4% ) patients were female . in the oa group , 13 ( 54.2% ) were male and 11(45.8% ) were female ( p=0.251 ) . the mean age of patients was significantly different in the avn and oa groups ( 30.867.5 vs 47.8812.6 years ; p=0.003 ) . in the avn group , 8 ( 28.6% ) patients were in stage 3 and 20 ( 71.4% ) patients were in stage 4 of the disease . in the oa group , 10 ( 41.6% ) patients were in grade 4 and 14 ( 58.4% ) patients were in grade 5 of the disease . patients with stage 2 of avn , grade 3 of hip joint oa , infective osteoarthritis and trombophelebitis were excluded from the study . standard instruments for the mom surface replacement prosthesis ( cormet , corin medical ltd . ) the acetabular component was nearly hemispherical , with a titanium - plasma spray process creating a porous surface for cementless press - fit fixation . the smallest cap was never less than 44 in size and the cup size was always 8 numbers larger than its cap . figures 3 and 4 show x - rays of the patients with osteonecrosis and osteoarthritis operated on by the mom resurfacing procedure , respectively . thromboembolic prophylaxis with subcutaneous heparin or enoxaparin was started on the day of the operation , and was continued for 14 consecutive days . on the first postoperative day , all patients were allowed to stand and take a few steps , and physical therapy , including strengthening exercises , was initiated . patients were restricted to 20% weight - bearing ( with the aid of 2 crutches or a walker ) until the sixth postoperative week , and then were allowed to advance to 50% ( cane or crutch in the opposite hand ) . from the 12 week onwards , all patients were encouraged to bear full - weight as long as they could tolerate , and to continue hip - strengthening exercises 3 times per week . the patients were evaluated at 6 weeks , 6 months and 1 year postoperatively , and annually thereafter , and the data were only gathered at the final follow - up . pain , function , and deformity were evaluated with the use of the harris hip score ( hhs ) after the operation . a harris hip score of 90 points was defined as excellent ; 80 to 89 as good ; 70 to 79 as fair ; and < 70 as poor outcome . excellent and good results were classified as successful outcomes , whereas fair or poor results were classified as unsuccessful . the indications for revision to tha were notching , accidental neck fractures or acetabular cup failure , discovering large cysts in the neck and shortening of the neck where less than 2 cm of the neck remains during the operation . statistical analysis was performed with spss statistical software ( version 15.0 ; spss , chicago , illinois ) . the end point of survival was defined as revision ( removal or exchange of 1 or more components ) for any reason ( eg , aseptic loosening ) . statistical analysis was performed with spss statistical software ( version 15.0 ; spss , chicago , illinois ) . the end point of survival was defined as revision ( removal or exchange of 1 or more components ) for any reason ( eg , aseptic loosening ) . there was no significant difference between the mean hhs ( p=0.347 ) and hip joint range of motion ( p=0.346 ) between the 2 groups after surgery at the final follow - up ( table 1 . ) three ( 10.7% ) patients in the avn group and 3 ( 12.5% ) patients in the oa group encountered femoral neck fracture or acetabular cup failure ( p=0.251 ) that resulted in revision to standard tha . there was also no significant difference regarding mortality rate after surgery between avn and oa groups [ 1 ( 3.6% ) vs 2 ( 8.3% ) ; p=0.243 ] . the results of our study demonstrated that at a short - term follow - up ( a mean of 41 months ) there were high levels of function and activity . in 90% of our patients the results were satisfactory ; moreover , in this study the mom total hip resurfacing lead to similar results in patients with osteonecrosis of the femoral head and a matched group of patients with osteoarthritis . although the mean age of patients was significantly higher in the oa group , there was no difference regarding complications in the 2 groups . our result was are in agreement with mont et al . , who demonstrated that the mom procedure was satisfactory in 95% of patients ; moreover , 93% of osteonecrosis and 98% of oa patients were treated successfully with this procedure . revell et al found there was an overall survival rate of 93.2% at a mean of 6.1 years follow - up . they concluded that mom hip resurfacing for osteonecrosis can be considered a safe and effective method of surgery . amstutz et al . studied 85 hips with on and 915 hips with other etiologies ( 696 hips with osteoarthritis ) , demonstrating no difference in survivorship between the on group and the control group after mom resurfacing , even after adjusting for head size , body mass index , and defect size . they concluded that the etiology of on itself does not constitute a contraindication for resurfacing , and the risk factors for the procedure are similar to those of other etiologies . in an investigation by stalberg et al . , a modern hip resurfacing system was implanted in 1148 hips as part of a u. s. multicenter investigational device exemption study . of these , 116 hips had a preoperative diagnosis of osteonecrosis , compared to 1023 hips with osteoarthritis . survival rates were not significantly different ( 95.9% and 95.8% at 24 months for osteoarthritis and osteonecrosis , respectively ) . they concluded that resurfacing arthroplasty for patients with osteonecrosis appears to be a reasonable alternative , taking into consideration implant size , patient sex , and size of femoral deficiency . in another study , beckman et al . analyzed the bone quality and 3-dimensional microarchitecture of the femoral head and neck in patients with on of the femoral head , and compared them to a group of patients with primary osteoarthritis . they found no statistically significant differences in bone volume and bone volume fraction , connectivity density and the structure model index in patients with on of femoral head and those with oa . they concluded that hip resurfacing arthroplasty is a possible therapeutic option in the treatment of on of femoral head in young and active patients , and that an insufficient bone stock in on of the femoral head seems not to be the deciding factor for failure . lei et al .   evaluated the short - term clinical outcomes of mom total hip resurfacing arthroplasty in treating osteonecrosis of the femoral head ( onfh ) in young and middle - aged patients , and compared them with hip osteoarthritis patients at the same period . they studied 33 patients ( 45 hips ) with onfh and 39 patients ( 45 hips ) with osteoarthritis . the patients were observed for 26 months in the onfh group and 28 months in the osteoarthritis group . femoral neck fracture occurred after 4 months in 1 patient in the osteoarthritis group , who received total hip arthroplasty ; no complication of prosthesis loosening , dislocation , incision infection , osteonecrosis , and bone absorption occurred in other patients . at last follow - up , the harris scores were 93.05.5 in the onfh group and 94.02.4 in the osteoarthritis group , showing no significant difference between the 2 groups ( p>0.05 ) ; but there were significant differences between pre- and post - operation ( p<0.01 ) . they concluded that the clinical short - term outcomes of metal - on - metal total hip resurfacing arthroplasty to treat onfh are satisfactory and can achieve similar outcomes to that of the osteoarthritis group . some previous studies reported that femoral resurfacing hemiarthroplasty can be a viable option for certain patients with late - stage osteonecrosis and minimal acetabular involvement [ 18,3537 ] . however , it is not a permanent solution , since failure eventually occurs as a result of acetabular cartilage erosion . resurfacing of both sides of the joint , as in mom resurfacing , may offer a more permanent solution than limited resurfacing of only the femoral side [ 3,18,24,34,3642 ] . in this study we used the hhs , which is a known classification for evaluation of the hip joint function . for example , pollar et al .   reported that by using the oxford hip score the result from the patients who underwent hybrid total hip arthroplasty and those who underwent mom resurfacing method were not different , but by using activity score and euro qol score the patients who underwent mom resurfacing arthroplasty achieved higher scores after 7 years of follow - up . this study was limited by the relatively small patient population and the limited duration of follow - up . we are waiting for long - term results to see if these early excellent results are maintained . on the basis of these initial findings , we recommend mom resurfacing as a viable treatment option for patients with advanced stages of osteonecrosis .
summarybackgroundcharacteristically , osteonecrosis affects younger patients who typically refer to the orthopedic surgeon for the first time in the third to fifth decades of life , in the late stages of the disease . 
 femoral metal - on - metal hip resurfacing is as an alternative to conventional total hip arthroplasty in treating osteoarthritis of the hip . 
 since there are already many reports regarding the successful outcome of resurfacing in advanced osteoarthritis , the purpose of this study was to analyze the clinical outcomes of this procedure in patients with osteonecrosis of the femoral head and to compare them with a matched group of patients with osteoarthritis.material/methodsthis retrospective cohort study evaluated a consecutive series of 52 patients with end - stage osteonecrosis ( 28 patients ) and osteoarthritis ( 24 patients ) of the femoral head , managed by metal - on - metal hip resurfacing in a referral orthopedic center from feb 2002 to may 2007 . 
 pain , function and deformity were evaluated with the use of the harris hip score after the operation . 
 patients were clinically followed for a mean of 41 months.resultsthe patients in the osteoarthritis group had a significantly higher mean age than those in the osteonecrosis group ( 47.8812.6 vs 30.867.5 , p=0.003).the clinical outcomes were similar for both groups . 
 there was no significant difference in mean harris hip score ( p=0.347 ) and hip joint range of motion ( p=0.346 ) between osteonecrosis and osteoarthritis groups after surgery.conclusionson the basis of these initial findings , we recommend mom resurfacing as a viable treatment option for patients with advanced stages of osteonecrosis .