Navigation-assisted system for suture anchor insertion
A navigation-assisted system for rotator cuff repair was developed to guide anchor insertion at an angle close to the preset target angle (Fig. 1A). This system provides both arthroscopic and navigation views. Unlike the typical arthroscopic view, the position of a surgical tool in the navigation-assisted arthroscopic view is marked by an arrow, which turns red and shows a virtual instrument when a surgical tool is not captured in the arthroscopic image or green when the surgical tool is visible in the image. This augmented reality-based technique allows the operator to keep track of the direction of surgical tools even when they are invisible on the arthroscopy screen, thereby improving the hand–eye coordination of the operator during surgery. To implement this notification function, optical tracking markers (Passive Sphere Markers, Northern Digital Inc., Waterloo, Canada) are attached to the arthroscope and surgical tools to obtain information on their location and direction and then the arthroscope camera, hand-eye, and pivot are calibrated .
The navigation view developed displays a three-dimensional (3D) shoulder model and surgical tools from three perspectives; this improves the 3D spatial recognition of the relative positions of the patient’s shoulder and the tool and supplements the limited depth recognition of the two-dimensional arthroscopic image. The anchor insertion angle, defined by vertical and horizontal angles, is calculated as the angle between the humerus and the surgical tool, with the vertical and horizontal baseline angles as references, respectively (Fig. 1B), and displayed in real time for the operators. The vertical baseline is a straight line that connects the greater tuberosity and surgical neck, while the horizontal baseline is perpendicular to the vertical baseline, bisecting the lesser tuberosity in a plane that faces the glenoid in front. Defining the ideal target insertion angle against each baseline before anchor insertion enables the operators to insert anchors at an insertion angle close to the target insertion angle using the proposed navigation system.
To implement the navigation view, location-tracking markers were attached to a shoulder phantom or cadaver. Cadaver-to-image registration  was conducted to reproduce the relative positions of the shoulder and surgical tools in a 3D virtual space. The accuracy of anchor insertion using the navigation system is determined by patient-to-image registration. The fiducial and target registration errors were 1.68 and 1.34 mm, respectively, in the phantom model and 3.76 and 3.91 mm, respectively, in the cadaveric model.
Surgical performance evaluation using motion analysis
Surgical performance using the conventional arthroscopic system and proposed navigation-assisted system was evaluated with motion analysis (Prime 41; Natural Point, Inc., Corvallis, OR, USA; Fig. 2A). The conventional arthroscopic system (IM4000, IM4120; ConMed Linvatec, Utica, NY, USA) is a commercial 30° arthroscope measuring 4 mm in diameter with a viewing angle of 105°, whereas the proposed arthroscopic system with navigation technology (MGB Endoscopy, Seoul, South Korea) is a 0° arthroscope measuring 7 mm in diameter with a wide viewing angle of 150°. Both arthroscopic systems have an image resolution of 1920 ´ 1080 pixels, and the acquired images were displayed for the participating operators on a full-high-definition monitor on the experiment table.
Phantom models of the shoulder joint (Arthrex, Naples, FL, USA) were used in this study. All phantom models had rotator cuff tears of the same size. Similar experimental conditions and environment were used for all participants. The participants included three experts who were shoulder and elbow fellowship-trained orthopedic surgeons and two novices who were orthopedic residents with no experience with shoulder arthroscopy. All the five operators were right handed and handled the proposed system first and then the conventional arthroscopic system. Prior to the experiment, all participants were given instructions regarding the arthroscopic tasks and preset portals. The operators conducted a given task once with each of the two arthroscopic systems.
To ensure that the experimental environment for all participants was identical, the positions of the arthroscope, surgical tool, shoulder phantom, and both hands were preset using an arrangement tool (Fig. 2B). All participants began their tests by placing their hands on the palm contour of the arrangement tool and finished the tests by laying the arthroscope and surgical tool, as determined in advance. Two anchor insertion spots were marked, and an anchor was preinserted in one of the spots in the phantom model (Fig. 2C).
Two reflective markers were attached on the dorsal side of the third metacarpal of each operator’s hands. Four motion analysis cameras were set and calibrated prior to the experiment. The motion analysis system was capable of storing 3D (x, y, and z) location data with a resolution of up to 0.01 cm. Data obtained from the system were analyzed using MATLAB (R2012b; MathWorks, Torrance, CA, USA). The following surgical performance metrics were used to analyze the surgical skill of the operators: total path lengths (millimeter) of the arthroscope and surgical tool, number of movements, and time taken (seconds). The total path length refers to the sum of the distances of all 3D movements of the operator’s hands during surgery. The number of movements was defined as the number of occasions during which the instantaneous velocity exceeded the average velocity . The time taken was measured from the moment when the operator inserted an arthroscope in the portal to the moment when the operator placed it on the experiment table after the completion of the given experimental task.
The experimental tasks comprised anchor insertion and anchor suture retrieval. In the former, the operator observed the arthroscopy images to find the ideal anchor insertion angle and placed the surgical tools accordingly. For the phantom models, the ideal anchor insertion angle was preset to a single value (135°) in a vertical direction. This task was performed twice at both the anchor insertion spots. The anchor suture retrieval task required the operator to pull out the four sutures of the inserted anchor from the portal one by one using an arthroscopic retriever.
Comparison of suture anchor insertion angles in cadaveric model
In cadaveric experiments, the conventional and proposed navigation-assisted arthroscopic systems were compared for variation in anchor insertion angle (Fig. 3A, B). Anchor insertion was performed three times using each system. The conventional and proposed navigation-assisted arthroscopic systems used in cadaveric experiments were the same as those used in the phantom models. Three optical tracking markers were used—a patient reference marker attached to the humerus using Steinman pins, one marker attached to the arthroscope, and one marker attached to surgical tools. To implement the position and orientation between the surgical tool and the humerus in a virtual space, patient-to-image registration was performed using anatomical landmarks, similar to that performed in the phantom experiments.
In cadaveric experiments, three participants from the experts group first performed anchor insertion using the navigation system and then the conventional system. Anchor insertion was performed three times each at the supraspinatus and infraspinatus footprints on the greater tuberosity. The ideal anchor insertion angle was set at 45°–90° from the footprint cortex by an expert surgeon with shoulder arthroscopy experience [1,2]. Using our vertical and horizontal references, the anchor insertion angles in the vertical and horizontal directions were 140° and 90°, respectively, for the first anchor and 144° and 102°, respectively, for the second anchor. The participants performed the anchor insertion task with the goal of achieving the predetermined target angles.
As in the motion analysis experiments, the operators attempted to find the ideal anchor insertion angle and place the surgical tools accordingly. Once the operator determined the anchor insertion angle, the angle data were recorded.
Statistical analyses of the surgical performance and anchoring angle were performed using OriginPro ver. 9b software (OriginLab Corp., Northampton, MA, USA). The data homogeneity of the three surgical performance metrics and anchoring angle errors were evaluated using the Shapiro–Wilk normality test. As all the data were not normally distributed, the paired-sample Wilcoxon signed-rank test was used to determine the statistical significance of each measurement (total path length, number of movements, and time taken) using the proposed navigation system in comparison with the conventional arthroscopic system. In addition, the Mann–Whitney U test was used to confirm significant differences between the expert and novice groups. The level of significance was set at P < 0.05, with a single asterisk indicating P < 0.05 and double asterisks indicating P < 0.01. Lastly, post-experiment power analysis was performed at a 0.05 significance level to determine the power to detect a significant difference between the two systems.