title: Patient-related factors influence stiffness of the soft tissue complex during intraoperative gap balancing in cruciate-retaining total knee arthroplasty.
abstract: OBJECTIVE
  How much force is needed to pre-tension the ligaments during total knee arthroplasty? The goal of this study was to determine this force for extension and flexion, and for both compartments, and to identify predicting patient-related factors.
  
  
  METHODS
  Eighty patients [55 females, mean age 71 (SD 9.7)] were recruited and had a navigated cruciate-retaining total knee arthroplasty. Distraction of the medial and lateral compartments of the extension and flexion gap (90°) with an instrumented bi-compartmental double-spring tensioner took place after finishing the bone cuts. Applied forces and resulting gap distances were recorded by the navigation system, resulting in a force-elongation curve. Lines were fitted with the intersection defined as the stiffness transition point. The slopes (N/mm) represented the stiffness of the ligamentous complex. Linear multiple regression analysis was performed to identify predicting factors.
  
  
  RESULTS
  The amount of force at the stiffness transition point was on average 52.3 (CI9550.7-53.9), 54.5 (CI9552.7-56.3), 48.3 (CI9546.2-50.2), and 59.3 (CI9557.0-61.6) N for the medial and lateral extension and flexion gap, respectively, and varied considerably between patients. The force at the stiffness transition point was significantly different between extension and flexion and both compartments (P < 0.05). Stiffness of the ligaments statistically significantly helped to predict the amount of force at the stiffness transition point, as well as body mass index, gender, and varus-valgus alignment.
  
  
  CONCLUSIONS
  The amount of force at the stiffness transition point varies between 48 and 59 N, depending on flexion/extension and compartment. Patient-related factors influence the stiffness transition point and can help predict the stiffness transition point. When forces higher than 60 N are used for gap distraction, the ligamentous sleeve of the knee might be over-tensioned.
  
  
  METHODS
  Prognostic study, Level I-high-quality prospective cohort study with >80 % follow-up, and all patients enrolled at same time point in disease.
authors.affiliation: Sint Maartenskliniek Research, Nijmegen, The Netherlands. p.heesterbeek@maartenskliniek.nl.
authors.forename: P J C
authors.initials: PJC
authors.lastname: Heesterbeek
authors.affiliation: Orthopedic hospital Gersthof, Vienna, Austria.
authors.forename: N
authors.initials: N
authors.lastname: Haffner
authors.affiliation: Sint Maartenskliniek Orthopaedics, Nijmegen, The Netherlands.
authors.forename: A B
authors.initials: AB
authors.lastname: Wymenga
authors.affiliation: Medivation, Windisch, Switzerland.
authors.forename: J
authors.initials: J
authors.lastname: Stifter
authors.affiliation: Orthopedic hospital Gersthof, Vienna, Austria.
authors.forename: P
authors.initials: P
authors.lastname: Ritschl
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D019645:Arthroplasty, Replacement, Knee / Q000379:methods*; D001696:Biomechanical Phenomena; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D007719:Knee Joint / Q000503:physiopathology* / Q000601:surgery; D008023:Ligaments, Articular / Q000503:physiopathology* / Q000601:surgery; D016014:Linear Models; D008297:Male; D008875:Middle Aged; D020370:Osteoarthritis, Knee / Q000503:physiopathology / Q000601:surgery*; D011379:Prognosis; D011446:Prospective Studies; D016059:Range of Motion, Articular
keywords: Computer navigation; Cruciate-retaining total knee arthroplasty; Gap balancing; Soft tissue complex; Total knee arthroplasty