title: Balancing UKA: overstuffing leads to high medial collateral ligament strains.
abstract: OBJECTIVE
  Balancing unicondylar knee arthroplasty (UKA) is challenging. If not performed properly, it may lead to implant loosening or progression of osteoarthritis in the preserved compartment. This study was aimed to document the biomechanical effects of improper balancing. We hypothesised that overstuffing would lead to more valgus, higher strain in the medial collateral ligament (sMCL), and higher lateral contact force.
  
  
  METHODS
  Six fresh-frozen cadaver specimens were mounted in a kinematic rig. Three motion patterns were applied with the native knee and following medial UKA (passive motion, open-chain extension, and squatting), while infrared cameras recorded the trajectories of markers attached to femur and tibia. Three inlay thicknesses were tested (8, 9, 10 mm).
  
  
  RESULTS
  Overstuffed knees were in more valgus and showed less tibial rotation and higher strains in the sMCL (p < 0.05). Lateral contact forces were higher in some specimens and lower in others. Stiffening of the medial compartment by UKA, even well balanced, already leads to a knee more in valgus with a more stressed sMCL. Overstuffing increases these effects. Knees with a tight sMCL may even see lower lateral contact force. Biomechanics were closest to the native knee with understuffing.
  
  
  CONCLUSIONS
  The first two hypotheses were confirmed, but not the latter. This underlines the importance of optimal balancing. Overstuffing should certainly be avoided. Although kinematics is only slightly affected, contact forces and ligament strains are considerably changed and this might be of more clinical importance. It is advisable to use thinner inlays, if stability is not compromised.
authors.affiliation: Center for Orthopaedics and Trauma Surgery, University Hospital Marburg, Baldingerstrasse, 35043, Marburg, Germany. heyse@med.uni-marburg.de.
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: Heyse
authors.affiliation: Center for Orthopaedics and Trauma Surgery, University Hospital Marburg, Baldingerstrasse, 35043, Marburg, Germany.
authors.forename: Bilal F
authors.initials: BF
authors.lastname: El-Zayat
authors.affiliation: Smith & Nephew, Zaventem, Belgium.
authors.forename: Ronny
authors.initials: R
authors.lastname: De Corte
authors.affiliation: Department of Orthopaedics, Katholieke Universiteit Leuven, Louvain, Belgium.
authors.forename: Lennart
authors.initials: L
authors.lastname: Scheys
authors.affiliation: Klinikum Großhadern, Orthopädische Klinik und Poliklinik, Labor für Biomechanik und Experimentelle Orthopädie, Munich, Germany.
authors.forename: Yan
authors.initials: Y
authors.lastname: Chevalier
authors.affiliation: Center for Orthopaedics and Trauma Surgery, University Hospital Marburg, Baldingerstrasse, 35043, Marburg, Germany.
authors.forename: Susanne
authors.initials: S
authors.lastname: Fuchs-Winkelmann
authors.affiliation: Mechanical Engineering Technology TC, KU Leuven, Geel, Belgium.
authors.forename: Luc
authors.initials: L
authors.lastname: Labey
mesh_terms: D000368:Aged; D019645:Arthroplasty, Replacement, Knee / Q000009:adverse effects* / Q000379:methods*; D001696:Biomechanical Phenomena / Q000502:physiology; D002102:Cadaver; D005260:Female; D006801:Humans; D007719:Knee Joint / Q000601:surgery; D008297:Male; D017888:Medial Collateral Ligament, Knee / Q000503:physiopathology*; D013180:Sprains and Strains / Q000503:physiopathology*
keywords: Balancing; Contact mechanics; Knee kinematics; Overstuffing; Unicondylar knee arthroplasty