At the Shadyside Hospital's Centre for Orthopaedic Research in Pittsburgh, a great deal of energy, devotion and grant funding is being invested in computer assisted orthopaedic surgery. In collaboration with the Centre for Medical Robotics and Computer Assisted Surgery and the Harvard Medical School, researchers have succeeded in designing a Hip Navigation system, referred to as HipNav, to assist surgeons before and during Total Hip Replacement (THR) operations, which are known to be extremely delicate. HipNav allows accurate and patient-specific placement of the acetabular implant in the pelvis with much less danger for dislocation or impingement of the implant's femoral neck or ball joint with the acetabular rim or socket afterwards.
It is highly unpleasant for both patient and surgeon to be forced to relocate an artificial hip, due to the initial malposition of the acetabular component. To try and avoid this kind of supplementary distress and additional costs, the HipNav solution offers an ingenious set of components, in the form of a pre-operative planner, a range of motion simulator, and an intra-operative tracking and guidance system. These three tools enable the surgeon to more properly align the acetabular cup with the specific pelvis anatomy of the individual patient, and to adapt the design and geometry of the implant. In this way, he might substantially reduce the degree of wear debris resulting from impingement, and even increase the safe range of joint motion through kinematic analysis.
Instead of using the traditional acetate templates and just a single anterior-posterior X-Ray of the pelvis, the surgeon feeds the pre-operative planner with Computed Tomography (CT) patient images, in order to position various cross sections of the acetabular implant upon orthogonal views of the pelvis. As a result, he can determine the exact implant size and location. Next, the range of motion simulator allows to predict the femoral positions, which might cause impingement for the selected implant design and placement. Several parameters are applied, such as extension, flexion, abduction, adduction, internal and external rotation. At this point, the surgeon can still decide to change the initial size and location for the final pre-operative plan.
In the operating theatre, the intra-operative tracking and guidance devices assist the doctor during the actual surgery, as to precisely execute the patient specific plan. The optical tracking camera "Optotrak" is able to determine the position of light emitting diodes (LEDs). These LEDs are attached to the pelvis and to the surgical tools, so that the surgeon during all phases of the operation, is fully conscious of the exact locations of both pelvis and acetabulum, especially when preparing the implantation. The most essential process is the registration of the pre-operative plan to the patient's position on the operating room table. Instead of the usual method, consisting of surgically implanted pins into the bone, the registration is performed by means of surface geometry.
This technique implies the use of a digitizing probe in order to sense multiple points on the bony surface of both pelvis and acetabulum. The data is then compared with the geometric details of the pre-operative CT images, in order to accurately determine the location of the pelvis. Consequently, a television monitor provides the surgeon with navigational feedback to finally perform the actual implantation. During the whole procedure, the position of both pelvis and acetabulum are constantly being tracked in real time by the HipNav system. The Pittsburgh research team has assessed this alternative technique by means of a cadaver trial, using six specimens, with hopeful and promising results.
The HipNav system should be able to reduce dislocation rates in primary as well as revising THR interventions. At the same time, it is bound to offer both clinicians and researchers a whole new category of "smart" tools for critical examination of common beliefs and assumptions with regard to "optimal" hip implant alignment. For a complete and fully illustrated description of the HipNav system, please check the Web site of the Centre for Orthopaedic Research.