Improving cup positioning using a mechanical navigation instrument

Abstract

Background: Although surgical navigation reduces the rate of malpositioned acetabular cups in total hip arthroplasty (THA), its use has not been widely adopted. As a result of our perceived need for simple and efficient methods of navigation, we developed a mechanical navigation device for acetabular cup orientation.

Questions/purposes: We assessed accuracy of cup orientation (mean error of cup inclination and anteversion) of a novel mechanical navigation device, percentage of outliers, length of operation, and compared the results with a series of CT-based computer-assisted THAs.

Methods: Cup orientation of 70 THAs performed using the mechanical navigation device was compared with a historical control group of 146 THAs performed using CT-based computer navigation. Postoperative cup orientation was measured using a validated two-dimensional/three-dimensional matching method. An outlier was defined outside a range of ± 10° from the planned inclination and/or anteversion.

Results: Using the mechanical navigation device, we observed a decrease in the errors of inclination (1.3° ± 3.4° [range, -6.6° to 8.2°] versus 3.5° ± 4.2° [-12.7° to 6.9°]), errors of anteversion (1.0° ± 4.1° [-8.8° to 9.5°] versus 3.0° ± 5.8° [-11.8° to 19.6°]), percentages of outliers (0% versus 9.6%), and length of operation (112 ± 22 [78-184] minutes versus 132 ± 18 [90-197] minutes) compared with CT-based navigation.

Conclusions: Compared with CT-based surgical navigation, navigation of acetabular cup orientation using a mechanical device can be performed in less time, lower mean errors, and minimal equipment.

Fig. 1A−B

The mechanical navigation device (HipSextant; Surgical Planning Associates, Medford, MA) has two adjustable orthogonal protractors (in-plane and offplane angle), two adjustable arms, and a direction indicator pointing in the direction of the desired cup orientation. The HipSextant plane is defined by three points: the base point, the ASIS (anterior-superior iliac spine) point, and the landing point.

Fig. 2A–B

In planning surgery (A) the base point outside of the posterior wall of the acetabulum and 20 mm above the infracotyloid notch is defined using a distance guide. Intraoperatively (B) the base point is identified using a calibrated drill guide.

Fig. 3

Intraoperatively, the surgeon visually aligns the cup impactor handle or the alignment guide in angled instruments with the direction indicator to implant the cup in the desired orientation. Although this figure depicts an angled cup impactor, a straight cup impactor may be used as well.

Fig. 4

Scatterplot showing cup orientation using mechanical navigation and CT-based navigation. There were no outliers using mechanical navigation. The control group of CT-based navigation had a higher percentage (9.6%, p = 3.4%) of outliers.