the tka has become the generally accepted treatment for osteoarthritis of the knee [ 1 , 4 , 6 , 9 , 12 , 13 , 28 , 30 ] . this posterior - stabilized prosthesis has stood the test of time and has had long - term survival rates between 93% and 98.7% after 9 to 18  years followup [ 9 , 12 , 13 , 28 , 30 ] . this posterior - stabilized prosthesis can be considered the gold standard today relative to which new designs are matched [ 9 , 12 , 13 , 28 , 30 ] . although numerous studies emphasize long - term survival , short - term outcomes also are important to patients . anterior knee pain is one of these issues : 4% to 49% of patients have anterior knee pain [ 2 , 3 , 23 , 25 , 29 ] . the pain reportedly does not decrease with time and restricts patients in climbing stairs , rising from a chair , cycling , or even normal walking [ 3 , 4 , 24 , 25 , 31 ] . apart from being bothersome and impairing quality of life , anterior knee pain is one of the main reasons for early revision [ 3 , 4 , 24 , 25 , 31 ] . there are a numerous theoretical benefits to the mobile - bearing tka , although few prospective , randomized studies have shown actual benefits . one theoretical advantage is the ability to self - align and therefore to accommodate small errors in component placement . if this is true , then better patellar tracking might decrease the incidence of anterior knee pain . we first asked whether the posterior - stabilized mobile - bearing ( psm ) knee prosthesis would lead to a lower incidence of anterior knee pain than the posterior - stabilized fixed - bearing ( ps ) knee prosthesis . we then asked whether one design would be superior to the other regarding overall pain , function , and quality of life . between november 2000 and july 2004 , we considered 113 consecutive patients with unilateral osteoarthritis of the knee , scheduled for primary tka , for participation in a prospective , double - blind , randomized clinical trial . all patients older than 21  years , in otherwise good health and good candidates for surgery , and able to participate in the followup program were invited to participate . we excluded patients with revision or unilateral knee arthroplasty , patellectomy , a fixed varus or valgus deformity greater than 20 , skeletal immaturity , charcot joints , unable or unwilling to cooperate in the followup program , life expectancy less than 5  years , or no signed informed consent . all 113 patients were willing to participate , but 10 were excluded owing to the previously mentioned exclusion criteria . block randomization was accomplished using a specially designed computer program developed by our institute s statistics department . the two patient groups were balanced based on age and , where applicable , previous operation , therefore ensuring approximately equally sized treatment groups . a power analysis was performed based on an estimated incidence of 25% of anterior knee pain [ 1 , 2 , 4 , 6 , 2326 , 29 , 31 ] . we presumed a reduction of at least 10% should be obtained to achieve a successful reduction , in which 10% was the lower limit of the reported incidence creating an effect size of 15% [ 1 , 2 , 4 , 6 , 2326 , 29 , 31 ] , stating a power of 0.8 with a 0.05 significance level resulting in a sample size of 100 patients per group . while performing an interim analysis , our study showed a difference in the incidence of anterior knee pain between the ps and psm groups . because our research question was answered , we stopped including patients in our study in july 2004 . after 1  year , 100 of the 103 ( 97% ) patients were available for followup . one patient died the first postoperative day as a result of a cardiac event ; one patient emigrated to the dutch antilles , and one patient withdrew from the study because of reasons not related to the prosthesis . the followup for all patients was 1  year (  2  months ) . the demographic data were similar between the 55 patients in the ps group ( 20 males , 35 females ) and the 48 patients in the psm group ( 17 males , 31 females ) . the number of patients who lived at home and who lived independently or with someone was equal in both groups . the age of the patients who had a ps knee ( mean , 68.9  years ; range , 2986  years ) was similar to the age of those who had a psm knee ( mean , 71.2  years ; range , 5387  years ) . body mass indices of the patients in the ps group and the psm group were comparable , with means of 29.1  kg / m ( range , 21.046.1  kg / m ) and 28.4  kg / m ( range , 18.456.6  kg / m ) , respectively . both treatment groups were comparable preoperatively for anterior knee pain ; 30 of the 48 patients with psm knees ( 62.5% ) and 28 of the 43 patients with ps knees ( 65.1% ) had anterior knee pain . the ps prosthesis was the nexgen complete knee solution legacy posterior stabilized fixed bearing knee , whereas the psm prosthesis was the nexgen complete knee solution legacy posterior stabilized mobile bearing knee ( both manufactured by zimmer , inc , warsaw , in ) . the mobile - bearing polyethylene component is more congruent than that of the fixed - bearing prosthesis . the tibial plate includes a rotational stop that helps prevent spinout of the articular surface . the surgery was performed by three experienced orthopaedic surgeons ( cnvd , grs , mus ) , each with a special interest in tka . the operations were performed using the same intraoperative protocol . general or spinal anesthesia and a preoperative dose of a second - generation cephalosporin ( zinacef ; glaxosmithkline , zeist , the netherlands ) were administered to all patients . a midline skin incision and a standard medial parapatellar approach were used in all patients . we first performed the horizontal tibial bone resection using an extramedullary alignment guide and then an intramedullary alignment guide was used for the femur . the desired angle was 7 valgus and we used the transepicondylar axis to obtain 3 external rotation relative to the posterior condyles . patients received a three - component cemented total knee prosthesis , ie , with a tibia component , femur component , and patella button . all patella buttons were identical with a three - pegged , polyethylene dome - shaped design . the patella buttons were cemented after the original thickness of the patella was restored or made slightly thinner . optimal patellar tracking was ensured by appropriate soft tissue balancing . in all patients with tightness of the lateral retinaculum , producing subluxation of the patella , the  no thumb rule was performed . when there was still doubt , the tourniquet was deflated and the patella tracking reassessed ; only then was release of the lateral retinaculum performed . lateral retinacular release was performed in four ( 7.7% ) of the ps knees and in four ( 8.3% ) of the psm knees . there was no difference in size of the polyethylene insert between the ps group ( mean , 11.4 , range , 917 ) and the psm group ( mean , 11.5 ; range , 1017 ) . all patients received the same postoperative treatment . on the second postoperative day , they began continuous passive motion . for prophylaxis , the patients were given a low - molecular - weight heparin ( fraxiparine ; sanofi - synthlabo , maassluis , the netherlands ) for 4  weeks . hospital discharge occurred when the patient was able to bend the knee actively in greater than 90 flexion and walk independently with crutches . after being discharged , physiotherapy was continued at home , improving function and independent walking . followup controls occurred at 6  weeks , 3  months , 6  months , and 12  months postoperatively , including standard radiographs and physical examinations . preoperative and 1-year postoperative , standardized subjective questionnaires were completed . at the time this study was instituted , rather , the patients were questioned ( by sjmb , dh , ma , rn ) regarding the location of any pain in the knee ; none of these evaluators were involved in the surgery and none were aware of the treatment . to determine the presence and severity of anterior knee pain , the following questions were used :  do you have anterior knee pain ?  or  does the pain of your knee get worse when standing up from a chair , climbing stairs , or riding a bicycle against the wind ?  the severity was classified by using the stair - climbing portion of the american knee society score as ( 1 ) none , ( 2 ) mild , ( 3 ) moderate , and ( 4 ) severe . in addition to the specific questions asked , we incorporated the sf-36 ( question 7 ) , the stair - climbing and pain portion of the american knee society score , and the oxford 12-item questionnaire concerning climbing stairs and standing up from a chair [ 11 , 15 ] . additionally , a thorough clinical evaluation was performed . at the preoperative intake and after 1  year (  2  months ) , patients completed questionnaires containing visual analog scores ( vas ) , the oxford 12-item questionnaire , and the sf-36 . we obtained vas scores , with a range of 0 to 100 , to assess current knee pain and the overall average knee pain during a 4-week period . the oxford 12-item questionnaire is a knee - specific questionnaire to determine knee function , with a range of 12 to 60 [ 11 , 15 ] . thirty - six questions reflect eight dimensions of functioning , with a range of 0 to 100 . we ( sjmb , dh , ma , rn ) performed a clinical evaluation using the american knee society score   to determine function , pain , range of motion ( rom ) , and stability of the knee ; none of these evaluators were involved in the surgery and none were aware of the treatment . this score ranges from 0 to 200 and can be divided into a knee score and a function score . preoperative and postoperative radiographs of the knee were obtained in standing anteroposterior , lateral , and merchant views ( skyline of the patellofemoral joint ) . all radiographs of the knee were reviewed by one of the authors ( sjmb ) and a radiologist specialized in skeletal radiology ; both were blinded to outcome but could not be blinded to the type of prosthesis . the knee society total knee arthroplasty roentgenographic evaluation and scoring system was used postoperatively and at 1-year followup to determine the varus - valgus angle , signs of early loosening , or wear of the prosthesis . the fluoroscopic imaging in anteroposterior and lateral views was used to study component interfaces for an increase in radiolucent lines indicating loosening . the average preoperative femorotibial angle was 3.9 varus , with a range of 14 varus to 19 valgus . in the ps group , this was 3.8 varus , and in the psm group , this was 4.0. postoperatively , the average femorotibial angle was 6.9 valgus , with a range of 0.8 to 11.7 valgus . there were no differences between the ps group ( 6.7 ) and the psm group ( 7.0 ) . all complications were divided into intraoperative , general , and local complications and into early ( first 3  months ) and late ( 312  months ) complications . data were entered in a personal computer ( microsoft access 2000 ; microsoft corp , redmond , wa ) by an independent research assistant ( ins ) and subsequently were checked for inconsistencies and errors in data entry . as the continuous data did not have normal distributions ( kolmogorov - smirnov test , p  <  0.05 ) , we used nonparametric analyses . at baseline , differences between the ps and psm groups were assessed for age and body mass index using mann - whitney u tests , and categorical data , such as gender , independence , and anterior knee pain , were compared using chi square tests . preoperatively and postoperatively , comparisons between the groups were made for rom , american knee society knee score , american knee society function score , vas scores , oxford 12-items score , and sf-36 quality of life scores using mann - whitney u tests , and changes from baseline at 1  year followup were assessed using wilcoxon signed - ranks tests . to compare the prevalence of anterior knee pain between the ps and psm groups , differences between results of patients with and without anterior knee pain were determined for the vas scores , oxford 12-item score , american knee society function , and american knee society knee score using mann - whitney u tests . categorical variables , such as knee pain at rest , pain during walking , pain during walking up and down stairs , function walking , function walking stairs , and walking aids were compared using chi square tests . one year postoperatively , patients with psm prostheses had less ( p  =  0.03 ) mean anterior knee pain than patients with ps prostheses . two of the 47 patients ( 4.3% ) with psm prostheses and 10 of the 53 patients ( 18.9% ) with ps prostheses experienced persistent anterior knee pain ( fig .  1 ) . the severity of anterior knee pain experienced by the patients with psm prostheses ( one severe , one moderate ) and ps prostheses ( four severe , four moderate , two mild ) was comparable . there was no difference regarding obesity , gender , or patients who had anterior knee pain at baseline and who postoperatively reported anterior knee pain . patients with anterior knee pain scored lower on the american knee society score ( p  <  0.001 ) and oxford 12-item knee questionnaire ( p  =  0.04 ) than patients without anterior knee pain ( table  1 ) . 1a graph shows the number of patients who reported having no anterior knee pain ( negative ) or experiencing anterior knee pain ( positive ) in the two groups that had a posterior - stabilized fixed - bearing prosthesis ( ps ; n  =  53 ) or a posterior - stabilized mobile - bearing prosthesis ( psm ; n  =  47 ) implanted during tka.table  1comparison of patients with and without anterior knee paintestanterior knee pain ( n  =  12)no anterior knee pain ( n  =  88)p valuevas current42.215.3 < 0.001vas last month47.919.70.096oxford 12-item questionnaire32.123.60.044sf-36 q1 ( physical functioning)44.249.80.415sf-36 q7 ( pain)51.663.90.218american knee society function51.364.40.368american knee society knee score65.385.6 < 0.001knee pain at rest ( mild / severe pain)50%6.8% < 0.001knee pain walking ( mild / severe pain)58.4%7.1% < 0.001knee pain walking up and down stairs ( mild / severe pain)75%12.5% < 0.001function walking ( mild / severe)33.3%64.7% < 0.001function walking stairs ( normal or support walking down the stairs)16.6%31.8%0.002walking aids58.3%64.8%0.078significantly different at p  <  0.05 ; vas  =  visual analog scale . a graph shows the number of patients who reported having no anterior knee pain ( negative ) or experiencing anterior knee pain ( positive ) in the two groups that had a posterior - stabilized fixed - bearing prosthesis ( ps ; n  =  53 ) or a posterior - stabilized mobile - bearing prosthesis ( psm ; n  =  47 ) implanted during tka . comparison of patients with and without anterior knee pain significantly different at p  <  0.05 ; vas  =  visual analog scale . in both groups , the vas related to the current pain and overall average knee pain during a 4-week period in the affected knee improved ( p  <  0.001 ) postoperatively ( table  2 ) . however , at the 1-year followup , these improvements were similar ( p  =  0.78 ) . we observed no differences in postoperative pain at rest , pain with walking , and pain after climbing stairs , walking ability , the ability to climb or descend stairs , or the use of walking aids . table  2preoperative and postoperative comparisonsscoreps ( n  =  53)psm ( n  =  47)mean95% cimean95% cipreoperative  range of motion 11280140107.575140  american knee society knee score52.746.159.25346.959.2  american knee society function score41.233.4494333.952  vas current57.248.765.753.444.762.1  vas last month63.455.771.76355.271.0  oxford 12-item questionnaire36.332.639.738.434.442.6postoperative  range of motion111.7106.6116.8113.3108118.6  american knee society knee score82.77788.483.277.588.8  american knee society function score65567460.849.172.4  vas current21.913.22919.212.226.2  vas last month25.716.634.823.515.931.0  oxford 12-item questionnaire24.221.227.226.122.329.8  sf-36 q1 16.518.2  sf-36 q229.732.4  sf-36 q35.812.8  sf-36 q412.317.2  sf-36 q50.83.8  sf-36 q62.910.2  sf-36 q725.924.6  sf-36 q83.66.4preoperative and postoperative values are not significantly different at p  >  0.05 ; ps  =  posterior - stabilized fixed - bearing ; psm  =  posterior - stabilized mobile - bearing ; ci  =  confidence interval ; vas  =  visual analog scale ; q1  =  physical functioning ; q2  =  role limitation due to physical problems ; q3  =  role limitation due to emotional problems ; q4  =  social functioning ; q5  =  mental health ; q6  =  energy vitality ; q7  =  pain ; q8  =  general health perception . preoperative and postoperative comparisons preoperative and postoperative values are not significantly different at p  >  0.05 ; ps  =  posterior - stabilized fixed - bearing ; psm  =  posterior - stabilized mobile - bearing ; ci  =  confidence interval ; vas  =  visual analog scale ; q1  =  physical functioning ; q2  =  role limitation due to physical problems ; q3  =  role limitation due to emotional problems ; q4  =  social functioning ; q5  =  mental health ; q6  =  energy vitality ; q7  =  pain ; q8  =  general health perception . there was similar ( p  =  0.60 ) postoperative improvement in function for patients with the ps and psm prostheses . the rom had improved ( p  =  0.77 ) in both groups at 1  year postoperatively , but the improvement was similar in the two groups ( table  2 ) . the preoperative and postoperative clinical scores did not correlate ( r  =  0.27 , p  =  0.005 ) , reflecting independence of the responses before and after surgery . the quality of life measures were similar ( p value ranging from 0.28 to 0.88 ) between patients with psm and ps prostheses . patients with anterior knee pain reported lower levels of quality of life than patients without anterior knee pain ( table  1 ) . these patients were treated with a brace for 6  weeks resulting in a stable knee in both patients . three patients had a hematoma that was surgically dbrided ( one psm , two ps ) . six patients ( three in each group ) needed manipulation under anesthesia to achieve 90 flexion . after this procedure , another patient was treated with anticoagulation for 3  months for a nonfatal lung embolism . three patients were treated for urinary tract infections and one patient was treated for pneumonia . the presence of anterior knee pain is one of the major short - term complaints after tka [ 1 , 2 , 9 , 22 , 2426 , 29 , 31 ] . published reports show between 4% and 49.2% of all patients have anterior knee pain [ 1 , 2 , 9 , 22 , 2426 , 29 , 31 ] . since the introduction of the mobile - bearing prosthesis , numerous studies have been performed , each comparing different aspects of the hypothesized advantage of using a mobile - bearing prosthesis compared with a fixed - bearing prosthesis [ 1 , 5 , 6 , 9 , 10 , 14 , 17 , 1922 , 2427 , 31 , 32 ] . the theoretical advantage of the mobile - bearing prosthesis is the ability to self - align and therefore to accommodate small mismatches , which could lead to a decrease in the incidence of anterior knee pain [ 1 , 9 , 24 ] . our study was performed to determine if the psm could in fact decrease the incidence and severity of reported anterior knee pain . we followed our patients for a minimum of 1  year .  aglietti et al . suggested that the performance of a mobile - bearing design may decline over time however , because anterior knee pain is relevant to patients in the short and long term , we do not believe this is a limitation . we did have relatively small sample sizes , but the data show a difference in the incidence of anterior knee pain between the ps and psm prostheses . we did not have a standardized , validated instrument specifically for anterior knee pain , but we presume our focused questions identified the presence and approximate severity of the pain . we studied only two specific devices from one manufacturer and do not presume they would be generalizable to other fixed- and mobile - bearing prostheses . the study was not designed primarily to detect functional differences and the power may have been too low to detect such a difference . we found less anterior knee pain in patients with the psm prosthesis than with the ps prosthesis . reported better american knee society scores and oxford 12-item questionnaire and pain scores for mobile - bearing prostheses 1  year after placing bilateral mobile - bearing and fixed - bearing prostheses in 40 patients . one study reported a mobile - bearing prosthesis resulted in a high rate of anterior knee pain ( 49.2% ) attributable to a suboptimal trochlear design . as reported by others , we found these subjective outcome scores improved from baseline for patients in both groups ; however , we observed no difference between the two types of prostheses ( table  1 ) . theoretically , the design of the psm prosthesis could lead to better rom during knee flexion activities . the patients in both groups reached a mean rom greater than 110 , which is comparable to reported ranges and is sufficient for normal daily functioning . anterior knee pain is known to cause difficulties in daily activities such as climbing stairs , rising from a chair , getting in and out of a car , or cycling , and therefore compromise these activities [ 2426 ] . we evaluated the postulated advantage of the psm compared with the ps knees but could not discern any difference except the decrease in anterior knee pain . as reported by others , we also found no difference in the short - term functional outcome between fixed- and mobile - bearing designs . the incidence of lateral retinacular release reportedly has decreased with the use of a mobile - bearing design ( from 10% for the fixed - bearing prosthesis to 0% for the mobile - bearing prosthesis ) ; however , in our study , the incidence was identical in both groups , confirming the findings in two other studies [ 24 , 27 ] . subjective questionnaires are essential instruments to evaluate our operative results [ 7 , 8 , 11 , 15 , 18 ] . quality of life can be influenced by many factors and not only the ones tested in our study . we detected no difference in quality of life between patients with psm and ps prostheses , but we could detect the influence of anterior knee pain on the general subjective health questionnaires and also on the knee - specific questionnaires ( table  1 ) . patients with anterior knee pain reported lower levels of quality of life than patients without anterior knee pain . our data support the notion that the psm prosthesis reduces the short - term incidence of anterior knee pain relative to the ps prosthesis .
anterior knee pain is one of the major short - term complaints after tka . since the introduction of the mobile - bearing tka 
 , numerous studies have attempted to confirm the theoretical advantages of a mobile - bearing tka over a fixed - bearing tka but most show little or no actual benefits . 
 the concept of self - alignment for the mobile bearing suggests the posterior - stabilized mobile - bearing tka would provide a lower incidence of anterior knee pain compared with a fixed - bearing tka . 
 we therefore asked whether the posterior - stabilized mobile - bearing knee would in fact reduce anterior knee pain . 
 we randomized 103 patients scheduled for cemented three - component tka for osteoarthrosis in a prospective , double - blind clinical trial . with a 1-year followup , 
 more patients experienced persistent anterior knee pain in the posterior - stabilized fixed - bearing group ( 10 of 53 , 18.9% ) than in the posterior - stabilized mobile - bearing group ( two of 47 , 4.3% ) . 
 no differences were observed for range of motion , visual analog scale for pain , oxford 12-item questionnaire , sf-36 , or the american knee society score . 
 the posterior - stabilized mobile - bearing knee therefore seems to provide a short - term advantage compared with the posterior - stabilized fixed - bearing knee.level of evidence : level i , therapeutic study . 
 see the guidelines for authors for a complete description of levels of evidence .