Mobile vs fixedbearing total knee arthroplasty

Mobile-vs.Fixed-Bearing Total Knee ArthroplastyA Clinical and Radiologic StudySteven T.Woolson,MD,*and Gregory D.Northrop,MD†Abstract:A consecutive series of patients undergoing total knee arthroplasty(TKA)using either a mobile-bearing(MB)or afixed-bearing(FB)implant were studied.Forty-four patients received57MB rotating platforms,and40patients received45FB posterior stabilized implants.At an average41-month follow-up(range,2-6years),no significant differences were found between the groups with respect toKnee Society ratings or pain scores.Postoperativeflexion was not different betweenthe groups(116°for MB and118°for FB).Three MB knees were revised forimplant-related complications.There was no difference between these MB or FBknee implants clinically or radiographically at early follow-up.However,morepatients with a MB knee required early revision for failure of rotating patellar ortibial polyethylene implants.Key words:range of motion,mobile-bearing,fixed-bearing,total knee arthroplasty.©2004Elsevier Inc.All rights reserved.Mobile-bearing(MB)total knee design involves the use of matching femoral,patellar,and tibial articu-lar geometries to produce implants that provide congruent contact at these articular surfaces during the normal walking cycle.Highly congruent artic-ulating surfaces result in reduced stress in the poly-ethylene.Congruency of the femoral-tibial articular surface in the sagittal plane with elevated anterior and posterior polyethylene lips in MB rotating plat-form knees provides some constraint for the absent cruciate ligaments just as the post and cam mecha-nism provides constraint for the absent posterior cruciate ligament in mostfixed-bearing(FB)poste-rior cruciate–substituting knee designs.FB knees that have round femoral components articulating with a relativelyflat tibial polyethylene articular surface,which allow for some axial rotation,have higher contact stresses in the polyethylene than MB designs.Excellent clinical results have been reported for patients undergoing total knee arthroplasty(TKA) with implants that substitute for the posterior cru-ciate ligament with either FB[1–4]or MB[5–12] designs in medium-and long-term studies.How-ever,there are no previous studies that have di-rectly compared the clinical results or range of motion of MB and FB designs using a consecutive series of patients performed by one surgeon.Materials and MethodsThis study was designed to determine whether the outcome of patients undergoing primary TKA was different when receiving either a MB(LCS rotating platform,DePuy,Warsaw,IN)or a FB(Nex Gen PS,Zimmer,Warsaw,IN)prosthetic design by using data that were prospectively collected.A con-secutive series of patients undergoing primary tri-compartmental TKA by a single surgeon during aFrom*Stanford University Hospital and†Department of FunctionalRestoration,Stanford University Medical Center,Stanford,California.Submitted February28,2003;accepted October12,2003.No benefits or funds were received in support of this study.Reprint requests:Steven T.Woolson,MD,1101Welch Road,Suite C8,Palo Alto,CA94304.©2004Elsevier Inc.All rights reserved.0883-5403/04/1902-0001$30.00/0doi:10.1016/j.arth.2003.10.004The Journal of Arthroplasty Vol.19No.220041352-year period was randomized to1of these2im-plants,but informed consent from an institutional review board protocol was not obtained from the patients.As patients were scheduled for surgery by the senior author’s office staff,they were entered on a list of random numbers designated for one or the other implant.In this manner,the senior sur-geon was blinded to the randomization process. Patients who underwent bilateral knee arthro-plasty,whether staged or not,received1knee im-plant design.The MB LCS rotating platform design requires excision of both cruciate ligaments and has a rotat-ing,metal-backed patellar component.A standard rotating-platform polyethylene insert was used for all but3patients(5%)who had so-called“deep-dish”polyethylene inserts with higher lips as a result of instability during testing of trial compo-nents at surgery.In all patients who were younger than60years old and in some older patients who were found at surgery to have dense cancellous bone,both the femoral and patellar components were inserted in a cementless fashion,and the tibial component was cemented(so-called“hybridfixa-tion”)or cementlessfixation of all components was used.Thefixation surfaces of the cementless MB implants were porous-coated with chrome-cobalt beads,and all implants were press-fit without ad-junctivefixation such as screws.Cementfixation was used for all patients who were found to have osteoporotic cancellous bone with MB implants that had a grit-blastedfixation surface.The author chose to use cementless implants for young patients or patients with good bone stock,because there was extensive experience with these cementless MB implants in the literature with good results.The FB NexGen PS implant was the original de-sign in this series of implants.The modular FB tibial polyethylene spacer has a polyethylene post that substituted for the absent posterior cruciate liga-ment through a cam action with the femoral com-ponent.The patellar component was an all-poly-ethylene dome design.All of the FB implants were cemented,because the senior author was not aware of any reported experience with cementlessfixation for this knee implant.The same surgical technique and plan for component alignment was used for all patients in both groups with the respective instru-ment systems.All of the patellae in both groups, however,were prepared for their respective im-plants by the same patellar reaming system(Zim-mer).All intraoperative and postoperative protocols were identical for each group of patients.A tourni-quet was used for all procedures and was not de-flated until after wound closure.All knees had a drain that was removed on thefirst postoperative day.Postoperatively,none of the patients had con-tinuous passive motion,but all received standard-ized physical therapy that began with full weight bearing on thefirst postoperative day whether the implant was cemented or cementless.Prophylaxis for deep venous thrombosis was performed using a combination of low-dose warfarin and intermittent pneumatic compression using thigh-high sequen-tial compression sleeves(Kendall,Mansfield,MA), and all patients wore thigh-high,graduated elastic stockings(Kendall)on both lower extremities from the time of surgery until thefirst outpatient visit. All patients had a preoperative and annual post-operative examination by the senior author,who recorded a Knee Society(KS)evaluation[13]at each visit for a minimum of2years.Two patients were not examined at the last follow-up.One pa-tient had moved from the area and was contacted by telephone,with a local orthopedic surgeon per-forming the knee examination.The other patient had been personally examined at11months,but had only a telephone interview at last follow-up. The latter patient’s knee examination was taken from the11-month examination for purposes of the KS scores.Active kneeflexion was measured with a goniometer.The patients were graded according to their level of functional disability by a modified Charnley clas-sification system[14].Class A included patients who had arthritis of only1knee.Patients who had bilateral knee arthritis or who had unilateral knee arthritis and a hip arthroplasty were placed in Class B.Patients in Class C were those who either had bilateral hip and knee arthritis,inflammatory poly-arthritis,or a medical condition that severely lim-ited their ability to walk,such as congestive heart failure or emphysema.An anteroposterior(AP)and a lateral view of the knee were obtained postoperatively within thefirst 2weeks and on an annual basis,but none of the radiographs were standardized for positioning pur-poses.One of the authors(G.D.N.),who was not involved with any of the surgical procedures,ex-amined all of the radiographs.Preoperative radio-graphs were measured only for tibial-femoral va-rus-valgus alignment.Immediate postoperative radiographs were measured for positioning of the individual femoral and tibial components and for tibial-femoral alignment.Thefinal postoperative radiographs were examined for bone-cement or bone-prosthesis radiolucent lines and for the pres-ence of osteolysis and component loosening.The136The Journal of Arthroplasty Vol.19No.2February2004KS radiographic evaluation protocol was used for all postoperative radiographs[15].The statistical methods used were an unpaired t-test and chi-square contingency calculation.ResultsBetween February1996and October1998,the senior author performed107consecutive primary tricompartmental TKAs on89patients,all of whom were included in the study.Forty-seven patients (60knees)were randomized to receive MB pros-theses,and42patients(47knees)were randomized to receive FB prostheses.Forty-five knees in36 patients had cementedfixation of MB implants,11 knees in7men(10knees)and1woman had hybridfixation,and3patients(4knees)had ce-mentless knee arthroplasty;these latter3patients included2men(3knees)and1woman.Five patients who had5knee arthroplasties(3 MB and2FB prostheses)died before the minimum 2-year follow-up and were excluded from the study.This left44patients(57knees)in the MB group and40patients(45knees)in the FB group who comprised thefinal study groups.Ten patients in the MB group and3in the FB group underwent 1-stage sequential bilateral knee arthroplasty under 1anesthetic.Three MB and2FB patients had staged bilateral knee arthroplasty.There was no significant difference between the2 groups with respect to their average age,height and weight,or to the gender,preoperative diagnoses,or modified Charnley classification(Table1).Preoper-atively,the groups were not different in their aver-age KS rating score or in their average KS func-tional score.However,there was a significant difference between the groups with respect to the average preoperative kneeflexion and for the av-erage preoperative arc of knee motion.There were no differences between the average preoperative or postoperative hematocrits and the average total blood replacement between groups, but the average operative time was less for the MB group(79vs85minutes,Pϭ.02),because26%of these MB patients had hybrid or cementlessfixation that required less surgical time than for cemented fixation.Seven MB(16%)and4FB(10%)patients had postoperative complications.These included confu-sion(2patients)and wound bleeding,pneumonia, a heel decubitus,urinary retention,and an arrhyth-mia(in1patient each)in the MB group and wound bleeding(2patients)and1patient each with pul-monary edema and a narcotic overdose in the FB group.None of the patients were found to have an acute postoperative venous thrombus on routine surveillance ultrasonography,but1MB patient had evidence of an old femoral vein thrombus.All of the patients had a minimum follow-up of2 years,with a maximum of6years,and there was no significant difference in the average follow-up between groups(41months for the MB group and 43months for the FB group).Clinical ResultsAt thefinal follow-up,both groups of patients had excellent average KS rating scores that were above90points,but there were no significant dif-Table1.Preoperative Demographic and Knee DataMB FB Statistical Significance Patients,n4440Knees,n5745Average age-years(range)69.2(37–83)67.4(37–83)Pϭ.38 Average height(cm)171172Pϭ.65 Average weight(kg)(range)85(51–121)85(42–137)Pϭ.87 Diagnosis,no.of knees(%)OA44(77)38(84)Posttraumatic8(14)2(4)Inflammatory disease3(5)4(9)Osteonecrosis2(4)1(2)Male/Female20/2421/19Charnley Class,no.of patientsA1012Chi-squareB2318AϩB vs CC1110Pϭ.68 Averageflexion contracture,degrees(range)9.5(0–35)10.2(0–40)Pϭ.69 Averageflexion,degrees(range)119(85–140)112(75–135)Pϭ.005 Average KS rating score41.842.5Pϭ.83Mobile-vs.Fixed-Bearing TKA•Woolson and Northrop137ferences between the groups with regard to the average KS rating or functional scores(Table2). There were also no differences in the average knee flexion,arc of knee motion,or tibial-femoral align-ment between the groups.The clinical results of the MB patients that had cementedfixation(42of57knees)were compared with the subset that had hybrid and cementless fixation.The patients who had cemented knee re-placement were older,were smaller in stature and weighed less,and had lower average preoperative and postoperative hematocrits than those who had hybrid or cementlessfixation.This was expected, because cementlessfixation was used in more pa-tients who were young.The average surgical time was less for cementless procedures,as was ex-pected.There were no differences between the ce-mented and cementless MB patients with respect to the average preoperative KS scores or the period of follow-up.At last follow-up,both cemented and cementless patients had excellent KS scores,and each group had an average knee range of motion, which was almost identical.One MB patient and2FB patients had manipu-lation of their total knee postoperatively.One pa-tient in each group had a revision procedure at another hospital for treatment of arthrofibrosis;the MB patient had revision at65months and the FB patient at13months.Both of these patients had revision of the tibial components only with a reduc-tion in the thickness of each polyethylene insert. Two MB patients had failure of the metal-backed patellar component that required revision of that component only.One of these patients had severe wear of the patellar polyethylene as a result of tracking problems and had revision to an all-poly-ethylene component at44months.The other pa-tient who had a failure of the patellar polyethylene to rotate on the baseplate had revision of the patel-lar polyethylene insert only at25months.Both of these patients had severe metallosis of the knee joint resulting from wear of the patellar baseplate on the femoral prosthesis requiring a synovectomy, and the latter patient had exchange of the tibial polyethylene insert as well.One MB patient who had a diagnosis of rheumatoid arthritis had a tibial polyethylene insert dislocation so that the insert rotated90°and its bearing surface was not congru-ous with the femoral component.This so-called “polyethylene spinout”required urgent revision at 3weeks from a standard to a thicker deep-dish insert.One other MB patient who was bedridden because of multiple sclerosis sustained a peripros-thetic fracture of the distal femur during a fall at35 months that was treated nonoperatively.One FB patient who had had a primary diagnosis of osteo-arthritis following a proximal tibial osteotomy de-veloped patellar subluxation that required a soft-tissue realignment procedure.None of the implants in either group has been revised for loosening or osteolysis,and there were no early or late infec-tions.Radiographic ResultsThe preoperative tibial-femoral alignment of the knees was not significantly different between the groups.The same proportion of patients in each group had either a preoperative varus(Յ4°)or valgus deformity(Ͼ10°).The immediate postoperative radiographs dem-onstrated that the alignment of the tibial compo-nent for each implant was not significantly differ-ent.The average amount of posterior slope of the tibial components from the lateral radiographs was similar in both groups,but was less than the in-tended7°.The average radiographically measured tibial-femoral alignment of the knees in each group was not different;however,these radiologically deter-mined average values were slightly less than the average values that were measured clinically. Seven MB patients(12%)and1FB patient(2%) were found to have tibial radiolucent lines in zones under the tibial plateaus only at last follow-up.Two MB patients had radiolucencies in2zones,and2 MB patients had lines that were2mm in width, with the others measuring1mm.All radiolucent lines were in zones1and/or2on the AP or lateral views and did not involve the tibial post.The num-Table2.Clinical Results at Last Follow-upMB FB Statistical Significance Averageflexion,degrees(range)116(70–142)118(75–143)Pϭ.55 Average KS function score76.974.6Pϭ.60 Average KS rating score92.490.2Pϭ.34 Average knee alignment,degrees(range) 6.6(4–10) 6.5(3–10)Pϭ.87138The Journal of Arthroplasty Vol.19No.2February2004ber of patients in each group who had tibial radi-olucencies was not significantly different,although there was a trend toward a greater number in the MB group(Pϭ.06).Only2femoral radiolucent lines were found on the lateral radiographs that involved the anterior and posterior implant inter-faces,and were1mm in thickness in both MB patients.None of the tibial or femoral radiolucent lines were continuous,and all developed at the bone-cement interface.None of the knees in either group demonstrated evidence of component migra-tion or loosening.All of the cementless implants appeared to have solid bone-ingrowthfixation at last follow-up,because there were no radiolucent prosthesis-bone gaps that developed over time.One MB patient who had cementedfixation was found to have a minimal amount of osteolysis under the anteriorflange of the femoral component on the lateral view that was1mm in width and16mm in length.DiscussionFB and MB prosthetic designs have been used since the1970s for treatment of arthritis of the knee.There have been no previous studies of the results of a consecutive series of patients who re-ceived one or the other of these2different knee designs performed by1surgeon.Kim et al.[16] compared the results of a MB design(LCS meniscal-bearing knee,DePuy)to a FB total knee design (AMK,DePuy),both of which were posterior cru-ciate–sparing prostheses,by performing bilateral 1-stage knee arthroplasty using both implants in the same patient.However,the functional differ-ences between implants could not be determined in this study,because each patient was his or her own control.Studies of the rotating-platform design of the MB knee have shown excellent outcomes and survivorship[5,6,8–11].However,in studies in which the range of motion was reported,the aver-age kneeflexion that was achieved by the LCS rotating-platform design in all but1of these studies was limited to between94°and 108°[5,8,11,12,17,18].Most cohort studies of FB posterior cruciate–substituting knees report knee flexionϾ110°.Only1report of the FB prosthesis that was used in this study has been published to date,that being Tanzer et al[19],who reported that the average range of motion of20NexGen PS knees at2years was111°.Despite excellent results re-ported by various authors for separate cohorts of patients who have received either the LCS MB or the NexGen or Insall-Burstein II FB knees,a com-parison of the results of different knee designs is impossible without a more scientific study.The clinical results of this study were very similar with respect to postoperative pain ratings and the ability to climb stairs.Both groups had excellent pain relief and KS rating scores.More MB patients required support aids postoperatively(16%vs4%) than FB patients;however,this difference was not significant(Pϭ.066).Thefinal average kneeflex-ion and average arc of motion of both groups were the same;however,the FB patients had a greater average improvement in the arc of kneeflexion (11.8°vs4.1°,Pϭ.06)and a slightly greater aver-age kneeflexion contracture(4.5°vs2.2°,Pϭ.05).A negative anterior drawer sign was present in more MB patients than FB patients(90%vs67%, Pϭ.047),perhaps because of the greater congruity of the femoral component to the polyethylene in-sert inherent with the low–contact stress design. However,the better AP stability of the MB implant did not translate into a better functional result that could have been proven byfinding that more of the MB patients were able to climb or descend stairs without assistance.Because of the absence of differences between the clinical results of patients having either ce-mented or cementless procedures,we feel that the MB patients can be considered as a homogeneous group regardless of implantfixation. Radiographically,more FB knees were in varus (tibial-femoral alignmentϽ5°)than MB knees (73%vs51%,Pϭ.01),but the average tibial-femoral angle was not different(3.7°for the MB group vs3.3°for the FB group,Pϭ.54).The tibial instrumentation of both systems produced good implant positioning(average tibial component an-gle on APfilm was89°for each implant),but the FB femoral implants were in less valgus than MB knees on the APfilms(3.4°vs4.9°,Pϭ.002).Four MB patients underwent revision surgery for implant-related failure or arthrofibrosis compared with2FB patients who had revision for patellar instability or arthrofibrosis(chi-square,Pϭ.58). Although there was no statistically significant dif-ference between groups in the revision rate,3MB patients(5%)had a patellar or tibial polyethylene insert failure that required revision before4years of follow-up compared with none of the FB patients. The use of a deep-dish tibial polyethylene insert in all patients would eliminate the possibility of tibial polyethylene dislocation or so-called“spinout,”but the effect of the use of this more constrained im-plant on the postoperative range of motion is un-known.Although knee joint dislocation has been reported as high as3%(3of103patients)in1studyMobile-vs.Fixed-Bearing TKA•Woolson and Northrop139of a FB posterior cruciate–substituting prosthesis (Insall-Burstein)[2]that has a post and cam mech-anism,this complication has not been reported to date for the NexGen prosthesis.There is concern about the2MB metal-backed patellar components(3.5%),which required early revision.Other authors have documented an LCS MB patellar failure rate of0to2.2%[5,8,10,11,20]. The use of an all-polyethylene patellar implant would eliminate these complications,as reported by Callaghan et al.[8].Because this study reported only the early results of these implants,the data on bone-cement radi-olucent lines and osteolysis or loosening were not expected to reveal a difference between implants. 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