title: Changes in the orientation of knee functional flexion axis during passive flexion and extension movements in navigated total knee arthroplasty.
abstract: OBJECTIVE
  Recently, the functional flexion axis has been considered to provide a proper rotational alignment of the femoral component in total knee arthroplasty. Several factors could influence the identification of the functional flexion axis. The purpose of this study was to analyse the estimation of the functional flexion axis by separately focusing on passive flexion and extension movements and specifically assessing its orientation compared to the transepicondylar axis, in both the axial plane and the frontal plane.
  
  
  METHODS
  Anatomical and kinematic acquisitions were performed using a commercial navigation system on 79 patients undergoing total knee arthroplasty with cruciate substituting prosthesis design. The functional flexion axis was estimated from passive movements, between 0° and 120° of flexion and back. Intra-observer agreement and reliability, internal-external rotation and the angle with the surgical transepicondylar axis, in axial and frontal planes, were separately analysed for flexion and extension, in pre- and post-implant conditions.
  
  
  RESULTS
  The analysis of reliability and agreement showed good results. The identification of the functional flexion axis showed statistically significant differences both in relation to flexion and extension and to pre- and post-implant conditions, both in frontal plane and in axial plane. The analysis of internal-external rotation confirmed these differences in kinematics (p < 0.05, between 25° and 35° of flexion).
  
  
  CONCLUSIONS
  The identification of the functional flexion axis changed in relation to passive flexion and extension movements, above all in frontal plane, while it resulted more stable and reliable in axial plane. These findings supported the possible clinical application of the functional flexion axis in the surgical practice by implementing navigated procedures. However, further analyses are required to better understand the factors affecting the identification of the functional flexion axis.
  
  
  METHODS
  IV.
authors.affiliation: Laboratorio di Biomeccanica e Innovazione Tecnologica, Istituto Ortopedico Rizzoli, Via Di Barbiano 1/10, 40136, Bologna, Italy.|Laboratorio di NanoBioteconologie (NaBi), Istituto Ortopedico Rizzoli, Via Di Barbiano 1/10, 40136, Bologna, Italy.|The Biorobotics Institute, Scuola Superiore Sant'Anna, Piazza Martiri della Libertà 33, 56127, Pisa, Italy.
authors.forename: Francesca
authors.initials: F
authors.lastname: Colle
authors.affiliation: Laboratorio di Biomeccanica e Innovazione Tecnologica, Istituto Ortopedico Rizzoli, Via Di Barbiano 1/10, 40136, Bologna, Italy.
authors.forename: Danilo
authors.initials: D
authors.lastname: Bruni
authors.affiliation: Laboratorio di Biomeccanica e Innovazione Tecnologica, Istituto Ortopedico Rizzoli, Via Di Barbiano 1/10, 40136, Bologna, Italy.
authors.forename: Francesco
authors.initials: F
authors.lastname: Iacono
authors.affiliation: Laboratorio di Biomeccanica e Innovazione Tecnologica, Istituto Ortopedico Rizzoli, Via Di Barbiano 1/10, 40136, Bologna, Italy.
authors.forename: Andrea
authors.initials: A
authors.lastname: Visani
authors.affiliation: Laboratorio di Biomeccanica e Innovazione Tecnologica, Istituto Ortopedico Rizzoli, Via Di Barbiano 1/10, 40136, Bologna, Italy.
authors.forename: Stefano
authors.initials: S
authors.lastname: Zaffagnini
authors.affiliation: Laboratorio di Biomeccanica e Innovazione Tecnologica, Istituto Ortopedico Rizzoli, Via Di Barbiano 1/10, 40136, Bologna, Italy.|Laboratorio di NanoBioteconologie (NaBi), Istituto Ortopedico Rizzoli, Via Di Barbiano 1/10, 40136, Bologna, Italy.
authors.forename: Maurilio
authors.initials: M
authors.lastname: Marcacci
authors.affiliation: Laboratorio di Biomeccanica e Innovazione Tecnologica, Istituto Ortopedico Rizzoli, Via Di Barbiano 1/10, 40136, Bologna, Italy. n.lopomo@biomec.ior.it.|Dipartimento di Ingegneria dell'Informazione, Università degli Studi di Brescia, Via Branze 38, 25123, Brescia, Italy. n.lopomo@biomec.ior.it.
authors.forename: Nicola
authors.initials: N
authors.lastname: Lopomo
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D019645:Arthroplasty, Replacement, Knee / Q000379:methods*; D001696:Biomechanical Phenomena; D005260:Female; D005269:Femur / Q000601:surgery; D006801:Humans; D007719:Knee Joint / Q000502:physiology* / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D009068:Movement; D011474:Prosthesis Design; D016059:Range of Motion, Articular / Q000502:physiology*; D015203:Reproducibility of Results; D012399:Rotation
keywords: Computer-aided surgery; Flexion–extension passive range of motion; Knee functional flexion axis; Knee kinematics; Total knee arthroplasty