title: Unicompartmental knee arthroplasty cannot restore the functional flexion axis of a living knee to normal.
abstract: OBJECTIVE
  The purpose of this study was to investigate the hypothesis that a medial unicompartmental knee arthroplasty might restore the functional flexion axis of a knee to normal. The flexion axis can be indirectly identified by tracking the vertical translation of anatomic landmarks that basically move around the flexion axis during a knee motion. If a unicompartmental knee could help restore the normal flexion axis, the anatomic landmarks after the arthroplasty would show the vertical translation similar to those of normal knees during a knee flexion.
  
  
  METHODS
  While performing a squatting motion, the kinematics of 17 knees were determined before and after a medial unicompartmental arthroplasty to calculate the vertical translation of a clinical epicondylar axis, using a three- to two-dimensional registration technique through a single-plane fluoroscopic system incorporating a biplanar static radiography. The results were compared with a normal data, and a statistical analysis including a two-way repeated-measured analysis of variance was performed.
  
  
  RESULTS
  For the medial end, from 10° to 100° knee flexion, normal, osteoarthritic, and unicompartmental knees had the average superior vertical translation of 7.3 ± 4.2, 4.3 ± 7.2, and 2.4 ± 3.1 mm, respectively, with statistical significance between normal and unicompartmental knees (p < 0.001). The vertical translation did not return to normal post-implantation.
  
  
  CONCLUSIONS
  A unicompartmental knee could not reproduce the normal flexion axis. As for clinical relevance, the changes of the implant design and surgical procedure may be necessary to obtain the normal flexion axis reproducing a normal motion.
  
  
  METHODS
  IV.
authors.affiliation: Department of Orthopaedic Surgery, Niigata Medical Center, Niigata City, Niigata, Japan. tommochi121710@gmail.com.|Department of Orthopedic Surgery, Department of Regenerative and Transplant Medicine, Niigata University Graduate School of Medical and Dental Science, Niigata City, Niigata, Japan. tommochi121710@gmail.com.|Department of Orthopaedic Surgery, Niigata Prefectural Central Hospital, Joetsu City, Niigata, Japan. tommochi121710@gmail.com.
authors.forename: Tomoharu
authors.initials: T
authors.lastname: Mochizuki
authors.affiliation: Department of Orthopaedic Surgery, Niigata Medical Center, Niigata City, Niigata, Japan.
authors.forename: Takashi
authors.initials: T
authors.lastname: Sato
authors.affiliation: Department of Orthopedic Surgery, Department of Regenerative and Transplant Medicine, Niigata University Graduate School of Medical and Dental Science, Niigata City, Niigata, Japan.
authors.forename: Osamu
authors.initials: O
authors.lastname: Tanifuji
authors.affiliation: Department of Health Sciences, Niigata University School of Medicine, Niigata City, Niigata, Japan.
authors.forename: Koichi
authors.initials: K
authors.lastname: Kobayashi
authors.affiliation: Department of Orthopedic Surgery, Department of Regenerative and Transplant Medicine, Niigata University Graduate School of Medical and Dental Science, Niigata City, Niigata, Japan.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Yamagiwa
authors.affiliation: Department of Orthopaedic Surgery, Niigata Medical Center, Niigata City, Niigata, Japan.
authors.forename: Satoshi
authors.initials: S
authors.lastname: Watanabe
authors.affiliation: Department of Orthopaedic Surgery, Niigata Medical Center, Niigata City, Niigata, Japan.
authors.forename: Yoshio
authors.initials: Y
authors.lastname: Koga
authors.affiliation: Department of Health and Sports, Niigata University of Health and Welfare, Niigata City, Niigata, Japan.
authors.forename: Go
authors.initials: G
authors.lastname: Omori
authors.affiliation: Department of Orthopedic Surgery, Department of Regenerative and Transplant Medicine, Niigata University Graduate School of Medical and Dental Science, Niigata City, Niigata, Japan.
authors.forename: Naoto
authors.initials: N
authors.lastname: Endo
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D019645:Arthroplasty, Replacement, Knee; D001696:Biomechanical Phenomena; D006801:Humans; D007719:Knee Joint / Q000601:surgery*; D008875:Middle Aged; D020370:Osteoarthritis, Knee / Q000503:physiopathology / Q000601:surgery*; D016059:Range of Motion, Articular
keywords: 3D to 2D registration; Clinical epicondylar axis; Functional flexion axis; Osteoarthritis and normal knees; Unicompartmental knee arthroplasty; Vertical translation