title: Quantitative measurement of lower limb mechanical alignment and coronal knee laxity in early flexion.
abstract: BACKGROUND
  Non-invasive quantification of lower limb alignment using navigation technology is now possible throughout knee flexion owing to software developments. We report the precision and accuracy of a non-invasive system measuring mechanical alignment of the lower limb including coronal stress testing of the knee.
  
  
  METHODS
  Twelve cadaveric limbs were tested with a commercial invasive navigation system against the non-invasive system. Coronal mechanical femorotibial (MFT) alignment was measured with no stress, then 15 Nm varus and valgus applied moments. Measurements were recorded at 10° intervals from extension to 90° flexion. At each flexion interval, coefficient of repeatability (CR) tested precision within each system, and limits of agreement (LOA) tested agreement between the two systems. Limits for CR & LOA were set at 3° based on requirements for surgical planning and evaluation.
  
  
  RESULTS
  Precision was acceptable throughout flexion in all conditions of stress using the invasive system (CR ≤ 1.9°). Precision was acceptable using the non-invasive system from extension to 50° flexion (CR ≤ 2.4°), beyond which precision was unacceptable (> 3.4°). With no coronal stress applied, agreement remained acceptable from extension to 40° (LOA ≤ 2.4°), and when 15 Nm varus or valgus stress was applied agreement was acceptable from extension to 30° (LOA ≤ 2.9°). Higher angles of knee flexion had a negative impact on precision and accuracy.
  
  
  CONCLUSIONS
  The non-invasive system provides reliable quantitative data in-vitro on coronal MFT alignment and laxity in the range relevant to assessment of collateral ligament injury, pre-operative planning of arthroplasty and flexion instability following arthroplasty. In-vivo validation should be performed.
authors.affiliation: Faculty of Biomedical and Life Sciences, Thomson Building, University of Glasgow, University Avenue, Glasgow G12 8QQ, United Kingdom; Golden Jubilee National Hospital, Agamemnon Street, Clydebank, West Dunbartonshire G81 4DY, United Kingdom. Electronic address: davidrussell@doctors.org.uk.
authors.forename: David F
authors.initials: DF
authors.lastname: Russell
authors.affiliation: Golden Jubilee National Hospital, Agamemnon Street, Clydebank, West Dunbartonshire G81 4DY, United Kingdom.
authors.forename: Angela H
authors.initials: AH
authors.lastname: Deakin
authors.affiliation: William Hunter Lecturer in Anatomy, Faculty of Biomedical and Life Sciences, Thomson Building, University of Glasgow, University Avenue, Glasgow G12 8QQ, United Kingdom.
authors.forename: Quentin A
authors.initials: QA
authors.lastname: Fogg
authors.affiliation: Golden Jubilee National Hospital, Agamemnon Street, Clydebank, West Dunbartonshire G81 4DY, United Kingdom. Electronic address: frederic.picard@gjnh.scot.nhs.uk.
authors.forename: Frederic
authors.initials: F
authors.lastname: Picard
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D019645:Arthroplasty, Replacement, Knee; D001696:Biomechanical Phenomena; D002102:Cadaver; D005260:Female; D006801:Humans; D007593:Joint Instability / Q000503:physiopathology*; D007719:Knee Joint / Q000502:physiology*; D035002:Lower Extremity / Q000502:physiology*; D008297:Male; D016059:Range of Motion, Articular / Q000502:physiology*
keywords: Image-free navigation; Ligamentous laxity; Mechanical alignment; Non-invasive; Total knee arthroplasty