Introduction Considerable debate persists whether mobile bearings can be considered as an effective alternative to fixed bearings in total knee arthroplasty (TKA) [4, 5, 29]. Early good re- sults [6, 13, 16] have encouraged numerous designs of mobile bearing prosthesis. The main aim is to guarantee less constraint to motion at the replaced joint while maintain- ing larger congruity at the prosthetic articular surfaces. Moreover, the transmission of shear forces between the bone-implanted components is expected to be minimised, with an associated smaller risk of tibial component loos- ening but also with a necessarily higher resistance by the ligaments. The associated possible risk of double-side wear has recently been questioned by retrieval studies [16, 18]. The numerous designs differ in terms of the degree of conformity between the femoral and the bearing compo- nents and of the degree of constraint of the bearing on the tibial base-plate. The advantage of self-alignment is also claimed [14] in mobile bearing arthroplasty, but not yet confirmed experimentally. The rationale was that a mobile bearing knee is enabled to find a most appropriate relative position of the three components about which axial rota- tion can occur with a larger congruency at the articulating surfaces. However, the degree of conformity changes with Abstract To determine the tibio- femoral and bearing motions in well functioning mobile bearing total knee arthroplasties, a quantitative analysis by means of fluoroscopy was carried out during stair ascend- ing, chair rising-sitting and step up- down motor tasks. Femoral and tibial component positions were obtained using a CAD-model-based shape matching technique. Direct measure- ment of mobile bearing motion was carried out by tracking previously in- serted tantalum beads. A relatively small motion of the bearing was ob- served. The advantage of self-align- ment claimed for the mobile bearing arthroplasty seems to be confirmed by subject-specific ranges of motion exhibited by these patients. It has also been demonstrated that these to- tal knee arthroplasties, which are not fully conforming in flexion, provide little constraint to antero-posterior motion, leading to a kinematic pat- tern very similar to an ACL deficient knee. Keywords Total knee arthroplasty · Mobile bearing tracking · Fluoroscopy · Prosthesis component kinematics · Activities of daily living KNEE Knee Surg Sports Traumatol Arthrosc (2004) 12 : 144–151 DOI 10.1007/s00167-003-0384-5 S. Fantozzi A. Leardini S. A. Banks M. Marcacci S. Giannini F. Catani Dynamic in-vivo tibio-femoral and bearing motions in mobile bearing knee arthroplasty Received: 2 November 2002 Accepted: 6 March 2003 Published online: 9 July 2003 © Springer-Verlag 2003 S. Fantozzi (✉) · A. Leardini · S. Giannini · F. Catani Movement Analysis Laboratory, Istituti Ortopedici Rizzoli, Via di Barbiano 1/10, 40136 Bologna, Italy Tel.: +39-051-6366570, Fax: +39-051-6366561, e-mail: fantozzi@ior.it S. Fantozzi Department of Electronics, Computer Science and Systems, University of Bologna, Viale Risorgimento 2, 40136 Bologna, Italy S. A. Banks The Biomotion Foundation, 1411 N. Flagler Drive, Suite 9800, West Palm Beach, FL 33401, USA M. Marcacci · S. Giannini · F. Catani Department of Orthopaedic Surgery, Istituti Ortopedici Rizzoli, Via Pupilli 1, 40136 Bologna, Italy