title: Mobility of the rotating platform in low contact stress knee arthroplasty is durable.
abstract: OBJECTIVE
  The mobile bearing or rotating platform (RP) in total knee arthroplasty (TKA) is originally part of a low contact stress (LCS) concept, with bearing undersurface mobility compensating higher bearing upper-surface congruency. The in vivo range of axial femorotibial rotation in RP knees has been the subject of many studies, but always involving the performance of relatively low demanding task conditions. Hardly any study has addressed the maintenance of this rotation over time.
  
  
  METHODS
  Two consecutive series of patients with LCS RP knees were studied in a cross-sectional study of 1- and 5-year follow-up. They were assessed using optoelectronic movement analysis during gait and the performance of a sit-to-walk (STW) task with and without turning steps.
  
  
  RESULTS
  A mean range of rotation (SD) was found in the 1-year group of 13.4° (3.7) during gait, 17.8° (6.8) during STW straight, and 17.9° (6.9) during STW with turning. The range in the 5-year group was 11.2° (6.0) during gait, 18.5° (8.7) during STW straight, and 18.3° (8.3) during STW with turning. A so-called paradoxical axial rotation pattern during gait and STW straight occurred in both groups in a normal prevalence.
  
  
  CONCLUSIONS
  The amount and pattern of rotation in a LCS RP knee does not become impaired between 1 and 5 years postoperatively. The theoretical benefit of RP TKA has not been proven in any clinical study so far, and studies with suitable long-term follow-up need to prove whether this mobility also leads to improved prosthesis survival. However, our findings support the functioning of the rotating platform at a basal science level and illustrate the need for the use of more complex tasks in kinematic studies.
  
  
  METHODS
  Therapeutic study, Level III.
authors.affiliation: Department of Orthopedic Surgery, Diakonessen Hospital, Bosboomstraat 1, 3582 KE, Utrecht, The Netherlands. aw_zurcher@hotmail.com.
authors.forename: Arthur
authors.initials: A
authors.lastname: Zürcher
authors.affiliation: Department of Rehabilitation Medicine, VU University Medical Center, Amsterdam, The Netherlands.
authors.forename: Kim
authors.initials: K
authors.lastname: van Hutten
authors.affiliation: Department of Rehabilitation Medicine, VU University Medical Center, MOVE Research Institute Amsterdam, Amsterdam, The Netherlands.
authors.forename: Jaap
authors.initials: J
authors.lastname: Harlaar
authors.affiliation: Department of Orthopedic Surgery, Slotervaart Hospital and VU University Medical Center, Amsterdam, The Netherlands.
authors.forename: Ruud
authors.initials: R
authors.lastname: Pöll
mesh_terms: D000368:Aged; D019645:Arthroplasty, Replacement, Knee / Q000379:methods*; D001696:Biomechanical Phenomena; D003430:Cross-Sectional Studies; D005260:Female; D005500:Follow-Up Studies; D005684:Gait / Q000502:physiology; D006801:Humans; D007719:Knee Joint / Q000502:physiology*; D007720:Knee Prosthesis; D008297:Male; D008875:Middle Aged; D009068:Movement; D011475:Prosthesis Failure; D016059:Range of Motion, Articular; D012399:Rotation
keywords: Low contact stress; Mobile bearing; Rotating platform; Rotation; Sit-to-walk; TKA