title: Frontal plane stability following UKA in a biomechanical study.
abstract: BACKGROUND
  Function and kinematics following unicondylar knee arthroplasty (UKA) have been reported to be close to the native knee. Gait, stair climbing and activities of daily living expose the knee joint to a combination of varus and valgus moments. Replacement of the medial compartment via UKA is likely to change the physiologic knee stability and its ability to respond to varus and valgus moments. It was hypothesized that UKA implantation would stiffen the knee and decrease range of motion in the frontal plane.
  
  
  METHODS
  Six fresh frozen cadaver knees were prepared and mounted in a six-degrees-of-freedom robot. An axial load of 200 N was applied with the knee in 15°, 45° and 90° of flexion. Varus and valgus moments were added, respectively, before and after implantation of medial UKA. Tests were than redone with a thicker polyethylene inlay to simulate overstuffing of the medial compartment. Range of motion in the frontal plane and the tibial response to moments were recorded via the industrial robot.
  
  
  RESULTS
  The range of motion in the frontal plane was decreased with both, balanced and overstuffed UKA and shifted towards valgus. When exposed to valgus moments, knees following UKA were stiffer in comparison with the native knee. The effect was even more pronounced with medial overstuffing.
  
  
  CONCLUSIONS
  In UKA, the compressive anatomy is replaced by much stiffer components. This lack of medial compression and relative overstuffing leads to a tighter medial collateral ligament. This drives the trend towards a stiffer joint as documented by a decrease in frontal plane range of motion. Overstuffing should strictly be avoided when performing UKA.
authors.affiliation: Department of Orthopedics and Rheumatology, University Hospital Marburg, Baldingerstrasse, 35043, Marburg, Germany, heyse@med.uni-marburg.de.
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: Heyse
authors.forename: Scott M
authors.initials: SM
authors.lastname: Tucker
authors.forename: Yogesh
authors.initials: Y
authors.lastname: Rajak
authors.forename: Mohammad
authors.initials: M
authors.lastname: Kia
authors.forename: Joseph D
authors.initials: JD
authors.lastname: Lipman
authors.forename: Carl W
authors.initials: CW
authors.lastname: Imhauser
authors.forename: Geoffrey H
authors.initials: GH
authors.lastname: Westrich
mesh_terms: D000293:Adolescent; D000328:Adult; D019645:Arthroplasty, Replacement, Knee / Q000379:methods*; D001696:Biomechanical Phenomena; D002102:Cadaver; D005260:Female; D006801:Humans; D007719:Knee Joint / Q000503:physiopathology* / Q000601:surgery; D008297:Male; D008875:Middle Aged; D020370:Osteoarthritis, Knee / Q000503:physiopathology / Q000601:surgery*; D016059:Range of Motion, Articular / Q000502:physiology*; D055815:Young Adult