title: Test bed for assessment of the kinematical determination of navigation systems for total knee arthroplasty. Does a limited range of motion of the hip joint influence the accuracy of the determination?
abstract: OBJECTIVE
  To locate the rotational center of the hip joint, CT-less navigation systems for artificial knee-joint replacement use movements of the femur with a rigid body attached. It cannot be assumed that the hip joint provides free mobility at all times. The purpose of the present study was: 1) To build a mechanical model to assess the system's accuracy in locating the rotational center of the hip by simulating a step-wise reduction of the range of motion (ROM) of the hip joint. 2) To determine the system's resolution by assessing a critical distance between two positions of the same femoral rigid body during the process of locating the rotational center of the hip. 3) To determine the sensitivity of the navigation system to the rotation of a femoral rigid body relative to the femoral bone while locating the rotational center of the hip joint.
  
  
  METHODS
  To assess the impact that a limited ROM of the hip joint has on the accuracy of determination of the hip joint's rotational center, a test bed was built. This enables validation of the algorithm used by a CT-less navigation system.
  
  
  RESULTS
  In the first part of the study, it was shown that a reduction of the ROM of the hip joint to 30% of its initial value had no evident influence on the accuracy of locating the rotational center of the joint. In the second part of the study, it was determined that the limit of resolution between two spatial points of the pivoting process is between 4.4 and 8.7 cm. The third part of the study showed that the examined system rejected the determination of the hip center even when the rigid body was only rotated through 22.5 degrees .
  
  
  CONCLUSIONS
  The results show that osteoarthritis of the hip with a limited ROM, for example, cannot be taken as a contraindication for the use of the evaluated CT-less navigation system. However, the surgeon should ensure that the pivoting of the femur is performed without hindrance within the free range of motion of the hip joint. In accordance with the vendor's recommendation, a minimum distance of 10 cm should be maintained between two spatial points. To ensure safe and unconstrained operation, the rigid body must be firmly attached to the bone and must not be dislocated.
authors.affiliation: Laboratory for Biomechanics and Experimental Orthopaedics, Department of Orthopaedic Surgery, University of Heidelberg, Faculty of Clinical Medicine Mannheim, Mannheim, Germany. markus.schwarz@ortho.ma.uni-heidelberg.de
authors.forename: M L R
authors.initials: ML
authors.lastname: Schwarz
authors.forename: P P
authors.initials: PP
authors.lastname: Pott
authors.forename: A
authors.initials: A
authors.lastname: Beck
authors.forename: S
authors.initials: S
authors.lastname: Heute
authors.forename: H-P
authors.initials: HP
authors.lastname: Scharf
mesh_terms: D019645:Arthroplasty, Replacement, Knee; D003198:Computer Simulation; D006621:Hip Joint / Q000502:physiology*; D008954:Models, Biological; D016059:Range of Motion, Articular; D025321:Surgery, Computer-Assisted