Review question
We reviewed the evidence on benefits and harms of mobile bearing compared with fixed bearing implants for cruciate retaining total knee replacement in patients with osteoarthritis (OA) or rheumatoid arthritis (RA). We found 19 relevant studies.
Background
In some people, damage and pain in the knee from arthritis are so severe that surgery is required. In these people, the joint can be replaced by a knee implant. In total knee replacement surgery, the ends of the long bones of the leg are usually replaced with metal ends, and a plastic insert is placed between them. Like fixed-bearing implants, mobile-bearing implants use three components to provide a relatively natural joint. In a mobile-bearing knee, however, the polyethylene insert can rotate short distances inside the metal tibial tray. This design allows patients a few degrees of greater rotation to the medial and lateral sides of their knee. Compared with fixed bearing designs, mobile bearing knee implants require greater support from soft tissues, such as the ligaments surrounding the knee. If the soft tissues are not strong enough, mobile bearing knees are more likely to dislocate. They also may cost more than fixed bearing implants.
Study characteristics
On 27 February 2014, we found 19 studies that tested 1641 people with OA or RA. After surgery, these people were followed for at least six months. Most (98.5%) of the patients had OA. Seven out of 19 studies were funded by the prosthesis manufacturer; eight studies did not report their sources of funding.
Key results: at least six months after surgery
Knee pain (higher score means less pain)
• People in the fixed bearing group rated their pain as 0.09 points higher on the KSS scale of 0 to 50 than people in the mobile bearing group (absolute difference 2.4%).
• People in the mobile bearing group rated their pain at 41.4 points compared with 41.49 points in the fixed bearing group.
Clinical and functional scores (higher score means better function)
• People in the fixed bearing group rated their function as 0.10 points lower on the KSS scale of 0 to 100 than people in the mobile bearing group (absolute difference 0.1%).
• People in the mobile bearing group rated their function at 84.5 points compared with 84.4 points in the fixed bearing group.
Health-related quality of life (higher score means better quality of life)
• People in the fixed bearing group rated their physical quality of life as 1.96 points lower on the Short Form (SF)-12 scale of 0 to 100 than people in the mobile bearing group (absolute difference 1.96%).
• People in the mobile bearing group rated their physical quality of life to be 42.3 points compared with 40.34 points in the fixed bearing group.
Revision surgery
• 3 more knees per 1000 in the mobile bearing group needed further surgery than in the fixed bearing group. This may have happened by chance.
• 11 per 1000 knees in the fixed bearing group and 14 per 1000 knees in the mobile group needed further surgery to the knee (a revision).
Mortality
• 11 more people per 1000 in the mobile bearing group died than in the fixed bearing group. This may have happened by chance.
• 22 per 1000 people in the fixed bearing group and 33 per 1000 people in the mobile bearing group died.
Reoperation rate
• 12 per 1000 knees in both fixed bearing and mobile bearing groups needed a reoperation.
Other serious adverse events
• 1 more knee per 1000 in the fixed bearing group had another serious adverse event than in the mobile bearing group. This may have happened by chance.
• 7 per 1000 knees in the fixed bearing group and 6 per 1000 knees in the mobile bearing group had another serious adverse event.
Quality of the evidence
Mobile bearing implants probably cause little or no difference in pain compared with fixed bearing implants (moderate quality).
Mobile bearing implants may cause little or no difference in function, health-related quality of life, revision surgery, mortality, reoperation rates and serious adverse events compared with fixed bearing implants (low quality).
Moderate- to low-quality evidence suggests that mobile bearing prostheses may have similar effects on knee pain, clinical and functional scores, health-related quality of life, revision surgery, mortality, reoperation rate and other serious adverse events compared with fixed bearing prostheses in posterior cruciate retaining TKA. Therefore we cannot draw firm conclusions. Most (98.5%) participants had OA, so the findings primarily reflect results reported in participants with OA. Future studies should report in greater detail outcomes such as those presented in this systematic review, with sufficient follow-up time to allow gathering of high-quality evidence and to inform clinical practice. Large registry-based studies may have added value, but they are subject to treatment-by-indication bias. Therefore, this systematic review of RCTs can be viewed as the best available evidence.
It is unclear whether there are differences in benefits and harms between mobile and fixed prostheses for total knee arthroplasty (TKA). The previous Cochrane review published in 2004 included two articles. Many more trials have been performed since then; therefore an update is needed.
To assess the benefits and harms of mobile bearing compared with fixed bearing cruciate retaining total knee arthroplasty for functional and clinical outcomes in patients with osteoarthritis (OA) or rheumatoid arthritis (RA).
We searched The Cochrane Library, PubMed, EMBASE, CINAHL and Web of Science up to 27 February 2014, and the trial registers ClinicalTrials.gov, Multiregister, Current Controlled Trials and the World Health Organization (WHO) International Clinical Trials Registry Platform for data from unpublished trials, up to 11 February 2014. We also screened the reference lists of selected articles.
We selected randomised controlled trials comparing mobile bearing with fixed bearing prostheses in cruciate retaining TKA among patients with osteoarthritis or rheumatoid arthritis, using functional or clinical outcome measures and follow-up of at least six months.
We used standard methodological procedures as expected by The Cochrane Collaboration.
We found 19 studies with 1641 participants (1616 with OA (98.5%) and 25 with RA (1.5%)) and 2247 knees. Seventeen new studies were included in this update.
Quality of the evidence ranged from moderate (knee pain) to low (other outcomes). Most studies had unclear risk of bias for allocation concealment, blinding of participants and personnel, blinding of outcome assessment and selective reporting, and high risk of bias for incomplete outcome data and other bias.
Knee pain
We calculated the standardised mean difference (SMD) for pain, using the Knee Society Score (KSS) and visual analogue scale (VAS) in 11 studies (58%) and 1531 knees (68%). No statistically significant differences between groups were reported (SMD 0.09, 95% confidence interval (CI) -0.03 to 0.22, P value 0.15). This represents an absolute risk difference of 2.4% points higher (95% CI 0.8% lower to 5.9% higher) on the KSS pain scale and a relative percent change of 0.22% (95% CI 0.07% lower to 0.53% higher). The results were homogeneous.
Clinical and functional scores
The KSS clinical score did not differ statistically significantly between groups (14 studies (74%) and 1845 knees (82%)) with a mean difference (MD) of -1.06 points (95% CI -2.87 to 0.74, P value 0.25) and heterogeneous results. KSS function was reported in 14 studies (74%) with 1845 knees (82%) as an MD of -0.10 point (95% CI -1.93 to 1.73, P value 0.91) and homogeneous results. In two studies (11%), the KSS total score was favourable for mobile bearing (159 vs 132 for fixed bearing), with MD of -26.52 points (95% CI -45.03 to -8.01, P value 0.005), but with a wide 95% confidence interval indicating uncertainty about the estimate.
Other reported scoring systems did not show statistically significant differences: Hospital for Special Surgery (HSS) score (seven studies (37%) in 1021 knees (45%)) with an MD of -1.36 (95% CI -4.18 to 1.46, P value 0.35); Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) total score (two studies (11%), 167 knees (7%)) with an MD of -4.46 (95% CI -16.26 to 7.34, P value 0.46); and Oxford total (five studies (26%), 647 knees (29%) with an MD of -0.25 (95% CI -1.41 to 0.91, P value 0.67).
Health-related quality of life
Three studies (16%) with 498 knees (22%) reported on health-related quality of life, and no statistically significant differences were noted between the mobile bearing and fixed bearing groups. The Short Form (SF)-12 Physical Component Summary had an MD of -1.96 (95% CI -4.55 to 0.63, P value 0.14) and heterogeneous results.
Revision surgery
Twenty seven revisions (1.3%) were performed in 17 studies (89%) with 2065 knees (92%). In all, 13 knees were revised in the fixed bearing group and 14 knees in the mobile bearing group. No statistically significant differences were found (risk difference 0.00, 95% CI -0.01 to 0.01, P value 0.58), and homogeneous results were reported.
Mortality
In seven out of 19 studies, 13 participants (37%) died. Two of these participants had undergone bilateral surgery, and for seven participants, it was unclear which prosthesis they had received; therefore they were excluded from the analyses. Thus our analysis included four out of 191 participants (2.1%) who had died: one in the fixed bearing group and three in the mobile bearing group. No statistically significant differences were found. The risk difference was -0.02 (95% CI -0.06 to 0.03, P value 0.49) and results were homogeneous.
Reoperation rates
Thirty reoperations were performed in 17 studies (89%) with 2065 knees (92%): 18 knees in the fixed bearing group (of the 1031 knees) and 12 knees in the mobile group (of the 1034 knees). No statistically significant differences were found. The risk difference was -0.01 (95% CI -0.01 to 0.01, P value 0.99) with homogeneous results.
Other serious adverse events
Sixteen studies (84%) reported nine other serious adverse events in 1735 knees (77%): four in the fixed bearing group (of the 862 knees) and five in the mobile bearing group (of the 873 knees). No statistically significant differences were found (risk difference 0.00, 95% CI -0.01 to 0.01, P value 0.88), and results were homogeneous.