This study was approved by the Ethics Committee of the Faculty of Medicine of the Munich University of Technology (ID number: 339/14).
Specimen preparation
Seven fresh-frozen human cadaveric shoulder specimens (4 right/3 left, 7 male, mean age 46.8 years) were thawed and dissected, removing all soft tissue below the deltoid tuberosity of the humerus and the inferior half of the scapula. Shoulders with macroscopic signs of osteoarthritis or other pathologic joint conditions were excluded from this study. The scapulae (in 10° forward inclination) and humeri (transected at 7 cm distal to the surgical neck) were then embedded in a polyurethane resin (RenCast® FC 53 Isocyanate / FC 53 Polyol, Huntsman, Belgium). Passive marker tracking tools were rigidly attached to the scapulae and humeri in order to record the position and orientation data during the testing procedure.
Preparation of Bankart lesion and Hill-Sachs defect
Since several studies described Hill-Sachs defects ≥25 % to be relevant for recurrent glenohumeral instability a 30 % defect size was chosen [2, 20]. The standardized Hill-Sachs defects were created using the method described by Sekiya et al. [20] An extended horizontal capsulotomy was performed to expose the antero-inferior rim of the glenoid and the posterior aspect of the humeral head. A line was drawn on the humeral head parallel to the anterior-inferior glenoid rim, representing the defect orientation. The humeral head diameter was measured with a caliper [mm]. A defect equivalent to 30 % of the postero-lateral humeral head was marked on the humeral head and created with an oscillating saw. The positions of suture anchors were marked in the valley of the defect, at the junctions of one-thirds of the defect length. The defect size, shape and suture anchor location were then duplicated on a template. Bankart lesions were created by sharp dissection of the labrum from the anterior-inferior glenoid rim. After defect creation the capsulotomy was anatomically closed without any over-tightening and/or overlap. Thus careful attention was paid to maintain the initial capsular tension. Finally subscapularis and teres minor muscles were tagged together.
Preparation of pressure-sensitive film
Using pressure-sensitive films (Prescale, Super Low Pressure Fuji Photo Film, Fuji Photo Film Co Ltd, Tokyo, Japan; pressure sensitivity range: 0.5 to 2.5 MPa), contact area between musculus infraspinatus tendon and Hill-Sachs defect was determined [%]. By use of individually prepared templates the films were cut to match the standardized Hill-Sachs defects. They were sealed and waterproofed within thin polyethylene sheets to be fixed on the bony surface of the Hill-Sachs defect using an adhesive agent. By doing so, correct positioning of the film was ensured throughout testing. Immediately after testing, the films were removed and digital images were created with a color scanner.
Surgical techniques
Bankart repair
Bankart repair was completed by placing 2 suture anchors (3.5 mm, Arthrex Inc., Naples, USA) at 4:30- and 6-o’clock positions on the glenoid and passing the sutures through the labrum using a horizontal mattress technique.
Remplissage
The distance between the suture anchors in the Hill-Sachs defect was measured [mm]. Two sites were marked on the infraspinatus tendon overlying the valley of the defect, with their location corresponding to the distance between the suture anchors. Sutures were passed through the same sites, ensuring that equal amount of tendon tissue was compressed.
Three remplissage techniques were used (see Fig. 1):
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Technique 1 (T1), as originally described by Purchase et al., was performed by inserting 2 double-loaded 5.5-mm anchors (Biocorkscrew, Arthrex Inc., Naples, USA) into the valley of the Hill-Sachs defect and passing horizontal mattress sutures through the infraspinatus tendon with surgical knots over the anchors [21].
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Technique 2 (T2) involved placing of the anchors and passing of the sutures as in T1; however, sutures were tied using the double pulley technique described by Koo and Burkhart [22]. For T1 and T2, the pressure sensitive film was placed after the insertion of both anchors and passage of sutures through the infraspinatus tendon. Sutures were then tied to effect compression of the infraspinatus tendon and posterior capsule.
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Technique 3 (T3) involved placement of two 4.75 mm knotless anchors (Swivel Lock, Arthrex Inc., Naples, USA) in the valley of the Hill-Sachs defect. The first knotless anchor was inserted with one end of a suture tape (Fibertape, Arthrex Inc., Naples, USA) and the tape was passed through the infraspinatus. The next knotless anchor was then inserted together with the tape through the infraspinatus tendon and into the valley of the Hill-Sachs defect. After pre-tensioning of the tape, by inserting the anchor according to the manufacturers recommendation, the tape was evenly tensioned. For T3 the pressure sensitive film was adhered to the defect before placement of the second knotless anchor. Care was taken to ensure that the second knotless anchor did not touch the pressure sensitive film. Subsequently the inferior capsule was repaired with sutures.
Testing apparatus
An industrial robot (Stäubli RX 90-B, Pfäffikon, Switzerland) was used to perform a defined motion of the humerus, while the scapula was in a fixed position (Fig. 2). A force torque sensor (FTS) (6 DoF JR3 Inc., Woodland, CA, USA) with a resolution of 0.1 N and 0.005 Nm was chosen to collect the force-moment data. A coordinate system associated with the scapula was used to define motion of the humerus with respect to the scapula, as previously described by Sekiya et al. implementing some minor modifications: The x-axis was defined as being perpendicular to the scapular plane and directed anteriorly. Rotation about the x-axis described abduction in the scapular plane [20]. The z-axis lies along the longitudinal axis of the humerus and rotation about the z-axis axis described internal-external rotation. The y-axis was defined as a cross product of the two already defined axes and following the right hand rule.
An optical tracking system (Polaris, Northern Digital Inc., Waterloo, Canada) was used to measure positions and movements of the humerus in respect to the scapula throughout testing.
To the knowledge of the authors, this navigated and force-moment controlled robotic setup represents a novel model for biomechanical evaluation of glenohumeral stability and range-of-motion.
Testing protocol
Reference trajectories for external rotation of the humerus were performed by hand and recorded by the optical tracking system. These data were converted into the coordinate system of the robot and used as position reference trajectory. Robotic parallel control was used in order to apply a humeral compressive force of 22 N to the scapula, minimizing forces in the orthogonal axis [18, 19]. At the same time, this allowed to perform external rotations maintaining a constant forward flexion and abduction angle and centered the humeral head within the glenoid. Furthermore, this method ensured that the humerus rotated about a biological and not a mathematical axis. The parallel control parameters were adjusted in a pre-test that the force control action (compressive force of 22 N) prevails over the position control action (external rotation replay of the reference trajectory).
The study protocol was designed to examine effects of rotational glenohumeral torque [Nm] with glenohumeral abduction angles of 0°, 30°, and 60° and external rotation angles of 0°, 30°, and 60°. Forward flexion was kept constant at 0° throughout all experiments. These positions were chosen, because of the final positions clinical relevance [2]. Most commonly antero-inferior glenohumeral instability is associated with a combined 90° abduction and 90° external rotation of the shoulder. Under laboratory conditions this was realized with 60° of glenohumeral abduction and 60° of glenohumeral external rotation of the specimens, given that the scapulothoracic joint accounts for another 30° of abduction and external rotation in vivo [2]. Therefore the final laboratory glenohumeral position corresponds to the above mentioned clinical relevant shoulder position.
Each specimen was preconditioned at 0° of abduction with 3 cycles of maximum external rotation. Measurements of the shoulder rotated through the arc of external rotation (≤60°) were taken at 0°, 30°, and 60° of abduction. The force sensor attached to the robot-arm recorded force-moment signals.
Glenohumeral engagement and dislocation testing
Prior to performing Bankart repairs and remplissage procedures a glenohumeral dislocation test was performed to confirm engagement and consecutive antero-inferior dislocation in all specimens. For this reason with 60° of glenohumeral abduction and external rotation a centralizing force of 22 N and an anterior-inferior directed 30 N force was applied by the robot. Glenohumeral engagement and dislocation was analyzed visually and by force profile change.
The same test was repeated after Bankart repairs and remplissage procedures have been performed. Glenohumeral integrity was quantified as the absence of anterior dislocation with engagement detected visually and by comparison to the force profile of the intact specimens.
Data collection pressure contribution
Digital scans of the pressure sensitive films were measured using Image-Pro Plus software (Media Cybernetics Inc., Rockville, MD, USA) and expressed in percentage of total film area vs. tendon coverage over defect area.
Statistical analysis
A power analysis was performed a priori. The analyses indicated that n = 7 specimens were required to ensure at least . Nine power for all outcome variables.
Statistical analysis was performed using SPSS software (version 13.0, SPSS, Chicago, Illinois). The level of statistical significance was defined as p < 0.05.
Data on ratio of area of tendon compression to the total area of the film is expressed as percentage value. Data for range of motion is represented my mean, standard deviation (SD), and 95 % confidence interval (95 % c.i.). Statistical analysis for differences between the groups was determined by one-way ANOVA [23]. Post-hoc test for differences of dependent variables was conducted using Tukey HSD.