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ACE Report #7760

Mobile- versus fixed-bearing TKA: comparison of migration, bone loss, and clinical outcome

Study Type:Therapy
OE Level Evidence:2
Journal Level of Evidence:N/A

Mobile- vs. fixed-bearing total knee replacement

Acta Orthop. 2015 Apr;86(2):208-14

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63 patients with uni- or bilateral osteoarthritis were randomized to 2 groups that underwent mobile-bearing (MB) total knee arthroplasty (TKA) or fixed-bearing (FB) TKA. Over a 2 year follow-up period, total translation (TT) via radiostereometric analysis (RSA), bone mineral density (BMD), correlation between implant migration and BMD, and Oxford knee scores (OKS) were evaluated. Results indicated that FB tibial implants migrated a significantly greater degree than MB implants. Within both groups, the majority of migration occurred within the first 3 months after surgery. Both groups displayed similar loss of BMD, which did not correlate to implant migration. OKS was significantly increased in both groups, with no significant between-group differences observed.

Publication Funding Details +
Industry funded
DePuy International
None disclosed

Risk of Bias


Reporting Criteria


Fragility Index


Was the allocation sequence adequately generated?

Was allocation adequately concealed?

Blinding Treatment Providers: Was knowledge of the allocated interventions adequately prevented?

Blinding Outcome Assessors: Was knowledge of the allocated interventions adequately prevented?

Blinding Patients: Was knowledge of the allocated interventions adequately prevented?

Was loss to follow-up (missing outcome data) infrequent?

Are reports of the study free of suggestion of selective outcome reporting?

Were outcomes objective, patient-important and assessed in a manner to limit bias (ie. duplicate assessors, Independent assessors)?

Was the sample size sufficiently large to assure a balance of prognosis and sufficiently large number of outcome events?

Was investigator expertise/experience with both treatment and control techniques likely the same (ie.were criteria for surgeon participation/expertise provided)?

Yes = 1

Uncertain = 0.5

Not Relevant = 0

No = 0

The Reporting Criteria Assessment evaluates the transparency with which authors report the methodological and trial characteristics of the trial within the publication. The assessment is divided into five categories which are presented below.




Outcome Measurements


Inclusion / Exclusion


Therapy Description



Detsky AS, Naylor CD, O'Rourke K, McGeer AJ, L'Abbé KA. J Clin Epidemiol. 1992;45:255-65

The Fragility Index is a tool that aids in the interpretation of significant findings, providing a measure of strength for a result. The Fragility Index represents the number of consecutive events that need to be added to a dichotomous outcome to make the finding no longer significant. A small number represents a weaker finding and a large number represents a stronger finding.

Why was this study needed now?

Mobile-bearing (MB) total knee arthroplasty (TKA) devices have been developed with the theoretical advantages related to contact stress, wear, and loosening of components, although the expected advantages of MB TKA have not been established in prior trials. It has previously been indicated that MB and fixed-bearing (FB) implant designs perform equally well, and thus it is unclear whether MB TKA provides any significant benefit over FB designs. Hence, the purpose of this study was to compare MB and FB designs by investigating implant migration, periprosthetic bone mineral density (BMD) and patient reported outcomes at 2 years for the posterior cruciate ligament (PCL) retaining press-fit condylar (P.F.C) TKA.

What was the principal research question?

Do mobile-bearing designs improve tibial component fixation compared to fixed-bearing designs in total knee arthroplasty patients, assessed at 3, 6, 12, and 24-months follow-up?

Study Characteristics -
63 patients (50-75 years of age) with uni- or bilateral osteoarthritis (OA) and undergoing total knee arthroplasty. Inclusion criteria also included patients with less than 15 degrees of knee joint extension defect, without a weakened or missing PCL perioperatively. All surgeries were performed through a medial parapatellar approach with a tourniquet. (46 completed follow-up).
Mobile-bearing group: Participants received a P.F.C. Sigma cruciate-retaining TKA mobile-bearing tibial design (n=32, 23 completed follow-up, Mean age: 66 [54-75], M=9, F=15)
Fixed-bearing group: Participants received a P.F.C. Sigma cruciate-retaining TKA fixed-bearing tibial design; mid-line incision with a para-patellar approach to the knee joint, ACL was excised and the PCL was retained (n=31, 23 completed follow-up, Mean age: 66 [56-73], M=14, F=12)
The primary outcome was total translation (TT) assessed using radiostereometric analysis (RSA) at follow-up, using the postoperative radiograph as a reference. The point of measurement was the center of gravity of the computer-aided deisign (CAD) model in relation to the tibial bone markers as fixed body references. Bone mineral density was assessed using dual X-ray absorptiometry (DXA). BMD, and correlation between implant migration and BMD, were secondary outcomes. Oxford knee scores (OKS) before surgery and at 6, 12, and 24-month follow-up was reported by patients.
Prospective, RCT, Single-centered.
Follow-up was carried out at 3, 6, 12, and 24 months post-operatively with BMD determined at 3 days post-operatively and at 12 and 24-month follow-up.

What were the important findings?

  • Total translational migration (TT) was significantly higher in the FB group (24-months mean = 0.30 mm +/- 0.22) than the MB group (24-month mean = 0.17 mm +/- 0.09) at all 4 follow-up times (24 months, p= 0.04). Migration predominantly occurred within the first 3 months postoperatively in both groups.
  • Excessive migration (>0.2mm) between the 12 and 24 month follow-up periods was noted in two knees of each group.
  • Total rotational migration (TR) was similar between groups at all 4 follow-up times (24 months, p= 0.1)
  • The maximum total point motion (MTPM) was not significantly different between the FB group (24-month mean = 0.69 mm +/- 0.37) and the MB group (24-month mean = 0.55 mm +/- 0.28) at all follow-up times (p= 0.1)
  • Percent BMD loss on anterior/posterior images was a mean -8.8% at 12 months (p=0.04) and -0.8% at 24 months (p=0.4) in the MB group, and -2.3% (p=0.02) and -1.1% (p=0.4), respectively, in the FB group. Percent BMD loss on lateral images was a mean -11.8% at 12 months (p<0.01) and -6.2% at 24 months (p<0.01) in the MB group, and -11.5% (p<0.01) and -4.1% (p=0.1), respectively, in the FB group.
  • No correlation was noted between total translation and the change in BMD for either group at 24-month follow-up
  • Both groups demonstrated a significant increase in OKS scores between baseline and 6-month follow-up, and maintained through to 24 month follow-up. There were no significant between-group differences in OKS score at either the 6, 12, or 24-month follow-up periods.

What should I remember most?

In total knee arthroplasty, total translational migration of cruciate-retaining, fixed-bearing tibial implants was significantly greater than that of cruciate-retaining, mobile-bearing tibial implants, that majority of which occurred within the first 3 months postoperatively in both groups. Both groups displayed similar bone mineral density loss over 24 months, and no significant correlation between migration and BMD loss was observed. Both tibial implants led to significant, and similar, increase in clinical scores.

How will this affect the care of my patients?

Despite the difference between fixed- and mobile-bearing tibial implants in migration, total migration was low in both groups, and relatively unchanged past 3 months. Thus the decision between fixed- and mobile-bearing is still open for discussion, as the current results depict good fixation with similar clinical scores within the first 2 years after surgery. Future studies should analyze stability between implants over the longer term, providing a more accurate look at implant survivorship and longevity.

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