Abstract
Introduction
Half of all acetabular components placed using conventional methods are malpositioned1. The HipSextant™ Navigation System (Surgical Planning Associates, Boston, MA) is a mechanical navigation system, adjusted on a patient-specific basis, designed to achieve appropriate cup alignment as simply and rapidly as possible. The current study assesses the surgeon's ability to register and track the pelvis and align the cup using the system.
Methods
A bioskills model pelvis (Pacific Research Laboratories, Inc., Vashon, WA) was prepared by placing screws to mark the anterior pelvic plane points and by inserting a long cup alignment pin, simulating a cup insertion handle, into the acetabulum. The bone model was then scanned using CT. The HipSextantTM Navigation System Planning Application was then used to plan the use of the HipSextant for the surgery. This is accomplished by creating a 3D model, designating the AP plane (marked by the screws), and then determining the HipSextant docking points. One of these three points is behind the posterior wall of the acetabulum (the basepoint). The second of these three points is on the lateral aspect of the anterior superior iliac spine. The third point, the landing point, is located on the surface of the ilium and equally distant from the other two points (Figure 1). The two protractors on the HipSextant planning application were then adjusted to be parallel with the cup alignment pin on the bone model.
A surgeon and assistant were then asked to dock the HipSextant on the bone model and to visually align the direction indicator to be parallel with the cup alignment pin. The two protractor angles on the instrument were recorded. This allowed for calculation of error in operative anteversion and operative inclination between the plan and the actual alignment that was accomplished. Four pairs of surgeon and assistant each performed the docking and alignment procedure 10 times for a total of 40 measurements.
Results
The results of the 40 tests demonstrated a mean error of operative anteversion of −0.06 degrees with a standard deviation of 0.65 degrees and a range of 0 to 2 degrees. The mean error of operative inclination was −0.34 degrees with a standard deviation of 0.65 degrees and a range of 0 to 2 degrees (Figure 2). The ANOVA test demonstrated no statistical difference in the means of the four groups.
Conclusion
The current study demonstrates the ability of a pair of surgeons of varying experience to use the patient-specific mechanical navigation system to align the acetabular component with a high degree of accuracy with a very small range of error (maximum 2 degrees in both anteversion and inclination).
