Patient-specific mechanical navigation system addresses hip replacement problems

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More than half of acetabular components placed are malpositioned, according to a recent study from Massachusetts General Hospital.

Component malposition is associated with the most common reasons for revision surgery including instability, wear and wear-associated loosening, and impingement.

The HipSextant Navigation System (Surgical Planning Associates Inc., Boston) is a mechanical instrument, adjusted on a patient-specific basis, designed to achieve appropriate component alignment. The method was invented and developed by Stephen B. Murphy, MD, of Boston.

Figure 1. The patient-specific 3-D plan for cup alignment using the mechanical navigation instrument is shown. The desired cup orientation is designated by the surgeon. The application calculates the instrument adjustments, such that, when the instrument is docked on the patient, the indicator points in the direction of desired cup orientation. Images: Chang W, Wellman SS, Tokunaga K

The system consists of a reusable mechanical instrument and a surgical planning software program. A low-dose CT study of the patient is obtained and uploaded to the product website where a patient-specific 3-D plan is developed and posted to the surgeon’s folder, also located on the website. During the planning process, the planner defines two coordinate systems: the anterior pelvic plane coordinate system and the patient-specific instrument docking coordinate system. The instrument docks with three legs, one behind the posterior rim of the acetabulum, a second on the lateral aspect of the anterior superior iliac spine (ASIS), and a third on the ilium and equidistant from the other two points. These point locations are unique for each patient.

Achieving a well-oriented cup

The system’s planning software then calculates the instrument adjustments to achieve a cup orientation goal in the familiar anterior pelvic plane coordinate system (Figure 1). The tilt of the pelvis is also calculated. Prior to surgery, the surgeon downloads, opens and checks the 3-D cup alignment plan.

Figure 2. The basepoint cannula with the calibrated extension in the infracotyloid notch and the tip of the drill guide cannula just outside the posterior rim indicates the basepoint location.

Figure 3. The cannulated leg of the instrument is slid over the basepoint pin, docked to the bone and secured by tightening the collet. The image depicts a right hip, viewed from the anterior side with the head to the right and the foot to the left.

The planned cup anteversion and/or inclination can be changed by the ordering surgeon either before or during surgery. The arms and protractors on the mechanical instrument are then set according to the calculations performed by the software application. In surgery, the instrument can be applied either before acetabular reaming or just prior to cup insertion. In the usual method, the surgeon performs acetabular reaming free-hand and then applies the instrument.

Once acetabular reaming has been completed, a calibrated drill guide is placed so the drill guide extension is placed in the infracotyloid notch and the tip of the drill guide itself is placed just outside the posterior rim of the acetabulum (Figure 2). This point is outside of the acetabular labrum, but inside the posterior capsule.

A pin is then inserted through the drill guide to identify this point, which is called the basepoint. Since this pin is used to define a point and not a line, the exact angle of the pin itself is not meant to be critical; however, the recommended angle of the drill guide in the coronal and transaxial planes is shown in the software application. The exact definitions of these angles are explained in the associated software application when the adjacent question mark button is pressed. In practice, small deviations from the ideal trajectory of this pin are accommodated by the flexibility of the pin once the entire instrument is docked.

Figure 4. The ASIS is palpated, and the sharp trocar is docked onto the lateral side of the ASIS by passing it through the anterior cannula.

Figure 5. With all three legs of the instrument docked to the bone, the cup insertion handle (the straight green part in this image) is aligned in three dimensions to be parallel with the direction indicator, controlling both anteversion and abduction.

Once this basepoint pin is inserted, the cup itself is placed provisionally within the prepared acetabulum. Next, the cannulated leg of the mechanical instrument is slid over the basepoint pin, docked onto the bone and secured by tightening the collet (Figure 3).

Next, the ASIS is palpated and a sharp trocar is passed through the anterior cannula of the instrument to dock onto the lateral side of the ASIS (Figure 4).

The position of the instrument is finalized by docking and inserting another sharp trocar through the proximal cannula at the apex of the instrument. With all three legs of the instrument contacting the bone, the direction indicator will then point in the direction of desired cup orientation. The cup insertion handle is then visually aligned in three dimensions with the direction indicator (Figure 5).

Figue 6. A parallel guide can be used to further fine-tune the visual alignment between the mechanical instrument and the cup impactor handle.

A parallel guide can then be used to visually fine-tune the cup alignment (Figure 6).

Trial reduction

Once the cup is inserted, a trial reduction can be performed either before or after removal of the instrument. Once the surgeon has ensured appropriate impingement-free hip joint stability, loosen the collet on the basepoint cannula, withdraw the basepoint pin and remove the instrument. Once the surgeon has developed some experience with use of the instrument, this technique will take approximately 3 minutes for routine procedures.

We recommend new users try the system on a sawbones model several times to get acquainted with the software application that runs the 3-D patient-specific plan, to get used to the intraoperative placement of the instrument and become familiar particularly with the calibrated drill-guide and basepoint pin insertion. Familiarity with those aspects of the method allows for greater ease of use in surgery.

The system is specifically designed for use with patients in the lateral position and is particularly optimized for posterior and lateral approaches to the hip. When using the anterolateral approach, the abductors and greater trochanter inhibit the use of a straight basepoint leg. Use of the system with the anterolateral exposure can be greatly facilitated with the use of a basepoint cannula adapter (Figure 7), which allows for placement of the basepoint leg in the same location while circumventing the greater trochanter and abductor muscles.

Figure 7. The basepoint adapter allows surgeons who use the anterolateral exposure to find the basepoint location while avoiding the abductors and greater trochanter.

When first using the system, you will notice the direction indicator may point differently than anticipated, either more horizontally, vertically, anteverted or less anteverted than expected. If the instrument is docked correctly, this is because the patient’s pelvis is significantly malpositioned on the operating table as the direction indicator points relative to the actual position of the pelvis. As users gain experience, they will also gain increasing confidence in the fact this method is capable of addressing the cup malposition problem with great simplicity and efficiency.

Current data suggest that use of the device may reduce the incidence of cup malposition from 50% to 0% or near 0% in just a few minutes per procedure.

References:

• Callanan MC, Jarrett B, Bragdon CR, et al. Risk factors for cup malpositioning. Quality improvement through a joint registry at a tertiary hospital. Clin Orthop Relat Res. 2011;469:319-329.
• Steppacher SD, Kowal JH, Murphy SB. Improving cup positioning using a mechanical navigation instrument. Clin Orthop Relat Res. 2011;469:423-428.
• www.hipsextant.com

• Winshih Chang, MD, can be reached at Somers Orthopaedic Surgery and Sports Med Group LLC, 664 Stoneleigh Ave., #300, Carmel, NY 10512-3990; 845-278-8400; email: winchangmd@gmail.com.
• Kunihiko Tokunaga, MD, director of the Niigata Hip Joint Center can be reached at Division of Orthopaedic Surgery, Department of Regenerative and Transplant Medicine, Niigata University Graduate School of Medical and Dental Sciences, Niigata City, Japan; email: ktokunagajp@yahoo.co.jp.
• Samuel S. Wellman, MD, can be reached in the Department of Orthopaedic Surgery, Duke University Medical Center, Duke Hospital, DUMC 3447, Durham, NC 27710; 919-684-3170; email: samuel.wellman@duke.edu.
• Disclosures: Chang, Wellman and Tokunaga have no relevant financial disclosures.