title: Novel computer-assisted method for revision arthroplasty of the knee.
abstract: OBJECTIVE
  To introduce the navigation system of software and instruments designed specifically for revision total knee arthroplasty (TKA).
  
  
  METHODS
  We present an imageless navigation system for revision TKA, with optical point and tracker identification to assess kinematic and anatomical landmarks. The system automatically positions the cutting guides with a motorized cutting unit. The cutting unit is placed on the distal femur with a femoral clamp and acts as a rigid body and the base for all femoral cuts. The surgical technique for using the navigation system for revision TKA is based on the technique used in primary TKA. However, there are some important differences. The most notable are: (1) differences in estimation of the position of the primary implant relative to the bone and the mechanical axes; (2) the specific possibilities the revision navigation software offers in terms of optimal joint level positioning; and (3) the suggested "best fit" position, in which the clock position, stem position and offset, femoral component size, and mediolateral position of the femoral component are taken into account to find the optimal femoral component position. We assessed the surgical technique, and accompanying software procedural steps, of the system, identifying any advantages or disadvantages that they present.
  
  
  RESULTS
  The system aims to visualize critical steps of the procedure and is intended as a tool to support the surgeon in surgical decision-making. Combining a computer-assisted cutting device with navigation makes it possible to carry out precise cuts without pinning. Furthermore, the femoral clamp provides a stable fixation mechanism for the motorized cutting unit. A stable clamp is paramount in the presence of periarticular bony defects. The system allows the position of the primary implant relative to the bone and mechanical axes to be estimated, at which point any malalignments can be corrected. It also offers an optimal joint level position for implantation, and suggests a "best fit" position, in which the clock position, stem position and offset, femoral component size, and mediolateral position of the femoral component are considered. The surgeon can therefore make decisions intraoperatively to maximise alignment and, hence, outcomes. Based on the intraoperative findings of joint stability, the surgeon can modify the preoperative plan and switch from a constrained condylar system to a hinged version, or vice versa.
  
  
  CONCLUSIONS
  The system is flexible and easy to learn and allows improvements in workflow during TKA.
authors.affiliation: Hanns-Edgar Hoffart, Kreisklinik Jugenheim, 64342 Seeheim-Jugenheim, Germany.
authors.forename: Hanns-Edgar
authors.initials: HE
authors.lastname: Hoffart
authors.affiliation: Hanns-Edgar Hoffart, Kreisklinik Jugenheim, 64342 Seeheim-Jugenheim, Germany.
authors.forename: Harald
authors.initials: H
authors.lastname: Dinges
authors.affiliation: Hanns-Edgar Hoffart, Kreisklinik Jugenheim, 64342 Seeheim-Jugenheim, Germany.
authors.forename: Stefan
authors.initials: S
authors.lastname: Kolbeck
authors.affiliation: Hanns-Edgar Hoffart, Kreisklinik Jugenheim, 64342 Seeheim-Jugenheim, Germany.
authors.forename: Peter
authors.initials: P
authors.lastname: Ritschl
authors.affiliation: Hanns-Edgar Hoffart, Kreisklinik Jugenheim, 64342 Seeheim-Jugenheim, Germany.
authors.forename: Hagen
authors.initials: H
authors.lastname: Hommel
keywords: Computer-assisted surgery; Knee; Navigation system; Revision total knee arthroplasty; Surgical technique