title: Minimally invasive registration for computer-assisted orthopedic surgery: combining tracked ultrasound and bone surface points via the P-IMLOP algorithm.
abstract: OBJECTIVE
  We present a registration method for computer-assisted total hip replacement (THR) surgery, which we demonstrate to improve the state of the art by both reducing the invasiveness of current methods and increasing registration accuracy. A critical element of computer-guided procedures is the determination of the spatial correspondence between the patient and a computational model of patient anatomy. The current method for establishing this correspondence in robot-assisted THR is to register points intraoperatively sampled by a tracked pointer from the exposed proximal femur and, via auxiliary incisions, from the distal femur.
  
  
  METHODS
  In this paper, we demonstrate a noninvasive technique for sampling points on the distal femur using tracked B-mode ultrasound imaging and present a new algorithm for registering these data called Projected Iterative Most-Likely Oriented Point (P-IMLOP). Points and normal orientations of the distal bone surface are segmented from ultrasound images and registered to the patient model along with points sampled from the exposed proximal femur via a tracked pointer.
  
  
  RESULTS
  The proposed approach is evaluated using a bone- and tissue-mimicking leg phantom constructed to enable accurate assessment of experimental registration accuracy with respect to a CT-image-based model of the phantom. These experiments demonstrate that localization of the femur shaft is greatly improved by tracked ultrasound. The experiments further demonstrate that, for ultrasound-based data, the P-IMLOP algorithm significantly improves registration accuracy compared to the standard ICP algorithm.
  
  
  CONCLUSIONS
  Registration via tracked ultrasound and the P-IMLOP algorithm has high potential to reduce the invasiveness and improve the registration accuracy of computer-assisted orthopedic procedures.
authors.affiliation: Department of Computer Science, Johns Hopkins University, Baltimore, MD, 21218, USA, sbillin3@jhu.edu.
authors.forename: Seth
authors.initials: S
authors.lastname: Billings
authors.forename: Hyun Jae
authors.initials: HJ
authors.lastname: Kang
authors.forename: Alexis
authors.initials: A
authors.lastname: Cheng
authors.forename: Emad
authors.initials: E
authors.lastname: Boctor
authors.forename: Peter
authors.initials: P
authors.lastname: Kazanzides
authors.forename: Russell
authors.initials: R
authors.lastname: Taylor
mesh_terms: D000465:Algorithms; D019644:Arthroplasty, Replacement, Hip / Q000379:methods*; D005269:Femur / Q000000981:diagnostic imaging / Q000601:surgery*; D006621:Hip Joint / Q000000981:diagnostic imaging / Q000601:surgery*; D006801:Humans; D021621:Imaging, Three-Dimensional / Q000379:methods*; D008953:Models, Anatomic; D019047:Phantoms, Imaging; D011859:Radiography; D025321:Surgery, Computer-Assisted / Q000379:methods*; D014463:Ultrasonography