title: Can we define envelope of laxity during navigated knee arthroplasty?
abstract: OBJECTIVE
  Functional outcomes after knee arthroplasty (TKA) remain poor. The ability to restore the soft tissue envelope intraoperatively may improve such outcomes. The aim of this study was to extend the scope of computer navigation as a tool to quantifying the envelope of laxity during subjective stress testing preoperatively and to quantify the effects of knee replacement and how it changes as a result of ligamentous failure.
  
  
  METHODS
  Loaded cadaveric legs were mounted on a purpose-built rig. Envelope of laxity was measured in 3 degrees of freedom using computer navigation. Knees were subjectively stressed in varus/valgus, internal/external rotation and anterior draw. This was performed preoperatively, during TKA and after sequential sectioning of ligaments. Real-time data were recorded at 0°, 30°, 60° and 90° of flexion. Mixed effect modelling was used to quantify the effects of intervention on degree of laxity.
  
  
  RESULTS
  In all cases, there was an increase in laxity with increasing flexion or ligament sectioning. Operator and movement cycle had no effect. Insertion of a TKA showed increased stability within the joint, especially in internal/external rotation and anterior drawer. Once the PCL and popliteus were cut, the implant only maintained some rotatory stability; thereafter, the soft tissue envelope failed.
  
  
  CONCLUSIONS
  This work has shown a novel way by which computer navigation can be used to analyse soft tissue behaviour during TKA beyond the coronal plane and throughout range of motion. Despite subjective stress testing, our results show reproducible patterns of soft tissue behaviour-in particular a wide range of mid-flexion excursion. It also quantifies the limits within which a cruciate-retaining TKR can maintain knee stability. This functionality may guide the surgeon in identifying and/or preventing soft tissue imbalances intra-operatively, improving functional results.
authors.affiliation: Newcastle Surgical Training Centre, Freeman Hospital, Freeman Rd., Newcastle Upon Tyne, NE7 7DN, UK, miltonghosh77@hotmail.com.
authors.forename: K M
authors.initials: KM
authors.lastname: Ghosh
authors.forename: A P
authors.initials: AP
authors.lastname: Blain
authors.forename: L
authors.initials: L
authors.lastname: Longstaff
authors.forename: S
authors.initials: S
authors.lastname: Rushton
authors.forename: A A
authors.initials: AA
authors.lastname: Amis
authors.forename: D J
authors.initials: DJ
authors.lastname: Deehan
mesh_terms: D019645:Arthroplasty, Replacement, Knee; D002102:Cadaver; D002358:Cartilage, Articular / Q000502:physiology; D006801:Humans; D017746:Joint Capsule / Q000502:physiology; D007593:Joint Instability / Q000175:diagnosis / Q000503:physiopathology*; D007717:Knee / Q000502:physiology*; D007719:Knee Joint / Q000502:physiology*; D008023:Ligaments, Articular / Q000502:physiology; D008953:Models, Anatomic; D016059:Range of Motion, Articular; D025321:Surgery, Computer-Assisted