Commentary & Perspective by
Douglas A. Dennis, MD*,
Rocky Mountain Musculoskeletal Research Laboratory, Denver, Colorado
The value of and indications for mobile-bearing total knee arthroplasty are controversial. Advocates state that these designs allow for increased implant conformity and contact area, resulting in reduced polyethylene wear without a dramatic increase in transmitted stress to the fixation interface. Critics state that there is a need for a more exacting surgical technique, that bearing instability may occur, and that there is a risk of enhanced polyethylene wear resulting from the creation of a second articulating surface.
The present report is a five to twelve-year review of 528 rotating-platform total knee replacements (LCS Rotating Platform; Depuy, Warsaw, Indiana) that were implanted by a single surgeon with use of cementless fixation. It is unclear if this study included a consecutive series of patients. The authors reported an estimated survival rate of 89.5% at twelve years and an overall revision rate of 5.5%. These rates are comparable to those reported after cemented fixed-bearing total knee arthroplasty (Table E-1), and they are superior to those reported after cementless fixed-bearing total knee arthroplasty, the revision rates for which are reported to be 25% or higher1,2 The study suggests that the initial concerns that midterm follow-up would find unacceptable rates of bearing instability, increased polyethylene wear, osteolysis, and loosening secondary to increased implant conformity have not become clinically apparent.
The authors emphasize the importance of using an exacting surgical technique that requires obtaining balance and symmetry of both the flexion and extension gaps to avoid polyethylene-bearing instability. Only two knees (0.4%; two of 528) required revision for bearing instability, which is similar to the rate found in other reported studies of rotating-platform total knee arthroplasty3,4. Higher bearing instability rates5 (9.3%; four of forty-three) have been reported in early studies of mobile-bearing total knee arthroplasty that primarily involved the use of meniscal-bearing rather than rotating-platform designs. Advances in component design and surgical instrumentation, an increased understanding of the importance of obtaining balance of the flexion and extension gaps, and proper rotation of the femoral component have all decreased the prevalence of bearing instability following mobile-bearing total knee arthroplasty.
The low prevalence of osteolysis (one patient) in this report is related to many factors, including the duration of follow-up to midterm, the large extent of polyethylene contact area present during the flexion range of gait, the favorable gait kinematics, the low polyethylene stresses on the undersurface of the rotating bearing, and the reduction of polyethylene wear that was accomplished by decoupling the multidirectional motions present on the superior aspect of the polyethylene bearing (in fixed-bearing total knee arthroplasty) to more monodirectional motion patterns at two different (superior and inferior) interfaces, which thereby reduced cross-shear stresses and wear6. In his study of mobile-bearing total knee replacements, Greenwald7 demonstrated contact areas of approximately 400 to 800 mm2 during gait range, which kept contact stresses at 14 MPa or less. In vivo fluoroscopic kinematic studies of the present total knee replacement design have demonstrated reduced anteroposterior femorotibial translation during gait8, which is likely secondary to the increased sagittal conformity of the present design. This should limit polyethylene shear stresses and reduce polyethylene wear and the risk of subsequent osteolysis. Otto et al.9 reported contact areas greater than 700 mm2 on the undersurface of rotating-platform polyethylene bearings with low subsequent contact stresses (<8 MPa). In contrast to a purely rotating-platform total knee replacement design, however, additional mobile-bearing total knee arthroplasty systems exist which permit both rotation and anteroposterior translation to occur on the inferior aspect of the polyethylene bearing. In these designs, the inferior aspect of the polyethylene bearing is exposed to multidirectional motion patterns. Close follow-up evaluation of these types of mobile-bearing total knee replacements is merited to see if premature failure due to backside wear will occur secondary to multidirectional motion on the inferior aspect of the mobile polyethylene bearing.
In contrast to the high historical failure rates of metal-backed patellar components in fixed-bearing knees, no patellar fractures or patellar loosening was observed in the current report. This suggests that bearing mobility may play some role in dispersion of patellofemoral loads. However, some patellar polyethylene bearing exchanges were performed, although the exact number was not reported. Also, no information was provided regarding the prevalence of patellar subluxation. This information is critical to a full assessment of the value of mobile-bearing patellar components.
The average flexion reported twelve years postoperatively was only 105°. This is likely related to the increased sagittal conformity of the tibial polyethylene bearing, which limits posterior femoral rollback in deep flexion. Maximum flexion following total knee arthroplasty is determined more by condylar geometry than by the presence of bearing mobility.
The major weakness of this report is the lack of a fuller discussion about the complications that resulted in revision total knee arthroplasty. Were the seven polyethylene bearing exchanges due to instability, polyethylene failure, or other causes? What did the authors learn from these failures? Do they now recommend alterations in surgical technique? When do they perform a fixed-bearing compared with a mobile-bearing total knee arthroplasty?
In summary, this report demonstrates equivalent midterm results when compared with similar series of cemented, fixed-bearing total knee arthroplasties. Substantial bearing instability, accelerated polyethylene wear, osteolysis, and loosening were not observed. These results support the concept that this rotating-platform total knee replacement design can reduce the loads that are transmitted to the fixation interface while allowing low rates of polyethylene wear and osteolysis.
*The author did not receive grants or outside funding in support of their research or preparation of this manuscript. The author received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity (research support from DePuy and Zimmer; consultant for DePuy). No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the author is affiliated or associated.
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