Abstract
Background:
While more than ½ of acetabular components placed during hip arthroplasty are significantly malpositioned, traditional surgical navigation and robotoics have not been widely adopted. This may be due to the additional time, expense, and complexity associated with this technology. As an alternative, smart mechanical navigation instruments, adjusted on a patient-specific basis, have been introduced to address the problem of cup malorientation. The current study assesses the accuracy of acetabular component alignment using a mechanical navigation instrument.
Patients and Methods:
The acetabular component was aligned in 58 consecutive hips in 58 patients using the HipSextant Mechanical Navigation System (Surgical Planning Associates, Inc. Boston, MA). The technique involves using a patient-specific plan and associated software. In planning for surgery, CT data are used to create a 3D model and to define the anterior pelvic plane (APP). A patient-specific HipSextant docking coordinate system is then determined by three points: one just behind the posterior acetabular rim, a second on the lateral side of the ASIS, and a third on the surface of the ilium (Figure 1). The HipSextant itself has two adjustable orthogonal protractors (in-plane and off-plane angle) and two adjustable arms so that the instrument is adjusted for each patient based on their specific anatomy. The instrument docks directly to the pelvis so the recommended orientation of the acetabular component is based on the actual position of the pelvis at the time of component implantation. A direction indicator points in the direction of the planned cup orientation (Figure 2). Cup alignment was further enhanced with the use of a parallel guide to improve parallel visualization (Figure 3). Postoperative cup orientation was measured using a validated two-dimensional/three-dimensional matching method [3,5].
Results:
Cup orientation measurements of the series of 58 THA's performed in the current study using the mechanical navigation instrument showed an error of inclination of 0.63° ± 2.3° [range, −5.3° to 6.0°] and an error of anteversion of 1.0° ± 2.7° [range, −5.3° to 7.0°]. There were no outliers in either anteversion or inclination.
Conclusion:
The current study demonstrates that the mechanical navigation instrument shows excellent cup alignment accuracy in a consecutive series of patients. This accuracy, compared to traditional navigation and robotic systems, may be due to the wide-based nature of the docking mechanism and the eliminate of the cumulative errors of registration and tracking inherent to more complex systems.
