Most surgeons consider the orientation of suture anchor insertion for the repair of the subscapularis muscle as identical to that of the supraspinatus muscle. However, the footprints of both rotator cuff muscles are in different spatial orientations. The images provided by the arthroscope are intrinsically distorted, and the surgeons operate without any other visual feedback. Because surgeons are solely dependent on external anatomical appearance and arthroscope images, they must attempt optimal suture anchor insertion with limited information. Our findings suggest that the navigation system may help provide multiplanar visualization of the footprint, thereby helping surgeons perform these surgeries.
Under ideal conditions, the 45° angle of anchor fixation can be determined from the thickness of the tendon and distance from the suture site to the anchor. Theoretically, this can result in better pull-out strength and less stress on the suture. Recent improvements to the anchor, including a change in thread, can change the axis of fixation by 90° (Fig. 7). Therefore, the pull-out strength must be recalculated to determine the ideal anchor angle for rotator cuff repair. Placing the angle of the anchor parallel or almost parallel to the suture can maximize the pull-out strength of the anchor [8]. We hypothesized that the angle and exact location of the anchors could be determined using navigation-assisted arthroscopic anchor fixation.
In basic operating settings for subscapularis repair, the use of the navigation system improved the ability of surgeons to orient the angle of suture anchor placement. Real-time feedback from the navigation system with multiplanar situational awareness of the tool on the humerus rendered the overall procedure arthroscopically more accurate. The interobserver reliability of ICCs showed reduced angular errors, indicating that angular orientation is more difficult to achieve than location. Although viewing through the posterior portal allows the anchor angle to be easily measured in the coronal plane, sagittal movement of the anchor is difficult to discern because of the use of a single arthroscope, which cannot measure depth. Rotating the instrument in the intraarticular space can better determine the angle when the instrument is moved along a plane perpendicular to the arthroscope. In contrast, determining the angle of the instrument is difficult when the instrument is moved along a plane parallel to the arthroscope. The angle and location of the anchor and instrument can vary and be difficult to measure when viewed from the posterior or lateral portal. In contrast, viewing from the top of the cone allows the location to be easily selected, with the angle determined from the cone height and distance from the anchor tip to the center of the radius (Fig. 8). Thus, placing the anchor insertion portal close to the viewing portal (proxy-viewing portal) can determine the accurate orientation of the anchor. The navigation system can measure the angle from multiple reference points in real time. Based on the cone phenomenon, the proxy-viewing portal close to the anchor insertion hole can result in a more accurate anchor angle and location than the posterior viewing portal.
Arthroscopic subscapularis repair is a well-established technique [18, 19]. The surgeon is frequently dependent on the view provided by the arthroscope to orient the insertion angle. However, the view provided by the 30° arthroscope is inherently distorted, and the total area of the footprint visible through the posterior portal in the gleno-humeral view is limited. The limited information reduces the surgeon’s ability to accurately determine the optimal angle for insertion of the suture anchor to stabilize the repair.
The present study showed that the navigated approach enhanced reliable results for all glenoid positions. Although optimizing anchor placement should theoretically improve the biomechanical behavior of the repaired area, these clinical data were not generated from the present study. This study aimed to assess whether intraoperative multiplanar visualization could reduce the number of errors from optimal angles. Furthermore, by determining the anchor location and angle, this method can be useful for randomized control studies or follow-up examinations related to anchor location and angle. The inclusion of quantitative information regarding the accuracy and duration of the procedure can enable this method to be utilized to evaluate a surgeon’s skills in arthroscopic shoulder surgery or to determine training objectives [20, 21].
The use of the navigation system altered the understanding of the surgeon while inserting the anchor into the subscapularis and supraspinatus footprint site, by making the surgeon more aware of the instrument location, along with their angular trajectory and penetration location of the anchor. The navigation system also provides information regarding the surrounding cortical thickness if the surgeon follows the trajectory depicted in the navigation experiment. Determining the correct orientation of the desired angle for subscapularis repair is difficult with the conventional method. As the number of trials increase, achieving the target angle and location remain difficult. The targeted angle and location may vary widely even when a plastic model is used. The use of the navigation system can better achieve a direction close to the predesigned angle and location, particularly for determining the reference guide for orientation of the tool while making the pilot hole or inserting the anchor.
We are aware that the navigation system is developed to assist in practical surgical application involving real patients. In practical surgical application, there may be two possible ways to secure the reference marker of the tracker. One is to attaching the marker on the patient’s skin with an elastic strap, which has been widely used for brain applications in commercial systems. Another option is to have a small stab skin incision, which allows K-wire fixation to the bone. Later, reference markers can be attached to the end of the K-wire. A similar method has been widely used in robotic arm-assisted surgery in orthopedic field (MAKO, Stryker®).
This study is limited by our use of a plastic model in the operative setting. No subscapularis tendon is attached to the lesser tuberosity of the plastic models, such that the direction of the subscapularis tendon could not be determined. We used plastic models (Arthrex, USA), which are is used for arthroscopic training in a dry laboratory [10, 11] to minimize anatomic variations reported in the literature [22, 23]. The current study tested novices as the participating surgeon, therefore it limit the generalization of the result to the more experienced surgeons. Despite that limitation, we believe the result of the study may help to reduce the learning curve for trainees.