The Center for MRCAS: HipNav

HipNav Overview

Dislocation of an artificial hip causes significant distress to patients and additional costs in order to relocate the hip. Impingement between the neck of the femoral implant (ball joint) and the rim of the acetabular component (socket) can lead to dislocations as well as advanced wear of a portion of the pelvis. The causes of impingement and dislocation are many. The most common cause of both impingement and dislocation is malposition of the acetabular component.

The Hip Navigation or HipNav system has been developed to permit accurate placement of the acetabular component during total hip replacement (THR) surgery. The system consists of three components: a preoperative planner, a range of motion simulator, and an intraoperative tracking and guidance system.

Preoperative Planning
The preoperative planner allows the surgeon to specify the alignment of the acetabular component within the pelvis, based upon preoperative computed tomography (CT) images.

ROM Simulation
A kinematic range of motion (ROM) simulator determines range of joint motion based upon the specific bone and implant geometry and alignment. The feedback provided by the simulator permits the surgeon to determine the optimal, patient-specific acetabular implant alignment for any implant system and determines an "envelope" of safe range of motion.

Intraoperative Tracking and Guidance
Several devices are used during surgery to allow the surgeon to accurately achieve the implant alignment specified in the preoperative plan. One device is an optical tracking camera which can track the position of special light emitting diodes (LED targets). The LED targets are attached to the pelvis and to conventional surgical tools to allow accurate (0.1 mm) and high-speed (100 Hz) tracking of these objects. Based upon these measurements, the location of the acetabular implant relative to the pelvis can be reliably determined. An interoperative computer displays to the surgeon the position and orientation of the implant with respect to the patient at the time of implant placement.

A cadaver trial has been undertaken to evaluate the accuracy of the HipNav System and to test the ability to use the surface geometry of bone to register the preoperative plan with the intraoperative position of the pelvis. Six cadaver specimens were scanned with fixed fiducial markers in place to provide the "ground truth". Preoperative planning, ROM simulation, and intraoperative data acquisition were performed using the HipNav system. In these trials, registration orientation errors ranged between roughly 0.5 degrees and 1.5 degrees. If there were no other error sources contributing to implant misalignment, these measurements suggest that HipNav could position the acetabular implant within a 1.5 degree (half-angle) cone centered at the desired orientation. In practice, insertion error may be larger due to other sources of inaccuracy such as tool calibration errors, deviation of implant alignment during the insertion process, and target sensing errors.

The goals of the HipNav system are to:

Reduce dislocations following total hip replacement due to acetabular malposition
Determine and potentially increase the "safe" range of motion
Reduce wear debris resulting from impingement of the implant's femoral neck with the acetabular rim
Track in real time the position of the pelvis and acetabulum during surgery

HipNav will also provide surgeons and researchers with a new tool for critically examining common assumptions concerning range of motion, bone motion, and "optimal" implant alignment. This information will help the surgeon achieve more reliable and accurate positioning of the acetabular cup and take into account specific anatomy for individual patients.