Does the Interval Slide Procedure Reduce Supraspinatus Tendon Repair Tension?: A Biomechanical Cadaveric Study

Specimen Preparation

A total of 40 Thiel-embalmed ³⁰ shoulders (donated to the Department of Anatomy of the Medical University of Graz [Graz, Austria] under the approval of the Anatomical Donation Program of the university) were used. Preparation and testing setup have previously been described by Porschke et al. ²⁵ Briefly, the scapula was fixed in a custom-made testing station using 3 bicortical screws and avoiding damage of rotator cuff or deltoid muscles. The supinated forearm was fixed at 30° of glenohumeral abduction (determined by using a goniometer) and neutral rotation (bicipital grove pointing anteriorly, perpendicular to the scapular plane) by another bicortical screw. The humerus, elbow, forearm, and hand were left intact (Figure 1). The humeral supraspinatus insertion was exposed by performing a deltoid split approach and careful resection of the overlying subacromial bursa. The remaining bursa that did not cover the supraspinatus footprint was preserved.

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Figure 1.

Schematic illustration of the test setup (right shoulder, anterior view). The shoulder was fixed at 30° of glenohumeral abduction. The sensor-enhanced arthroscopic grasper (1) lies aligned with the supraspinatus muscle (2), grasping the distal tendon stump of the supraspinatus. The sensor-enhanced arthroscopic grasper was lateralized in 1-mm steps via a slide (3) by using an isometric screw (4), thus reducing the tendon to the footprint. Load on the tendon was measured in real time.

At this time, shoulders were evaluated for exclusion criteria, which comprised full-thickness defects of subscapularis, supraspinatus, or infraspinatus tendons or any previous surgical procedure at the included shoulder. From 40 included shoulders, 9 were excluded owing to preexisting supraspinatus defects (n = 6) or previous surgical interventions (n = 3), leaving a total of 31 shoulders (mean age, 74 years; range, 68-84 years; 20 left and 11 right shoulders) for biomechanical analysis.

A full-thickness supraspinatus tendon tear was then created in all 31 specimens. Sharp detachment of the supraspinatus tendon insertion site was performed using a No. 15 blade starting from the anterior border and continuing 15 mm posteriorly. The supraspinatus tendon was then shortened to simulate a retracted tear by removing the distal 10 mm of the tendon. Tendon dimensions were measured using a digital caliper (myotendinous length [medial border of supraspinatus muscle insertion – lateral border of supraspinatus tendon] and the most distal tendon width after detachment and shortening of tendon). The specimens were then randomly assigned to an intervention group (n = 16) and a control group (n = 15).

Surgical Procedure (Intervention Group)

Figure 2 provides a schematic illustration of the IS procedure. For the AIS, an incision was created within the rotator cuff interval between the supraspinatus and subscapularis from lateral to medial past the articular surface of the glenoid, as described by Tauro. ²⁸ For the PIS, an incision between the supraspinatus and infraspinatus from lateral to medial up to the glenoid rim using the scapular spine as landmark was performed, as described by Lo and Burkhart. ²³ For the superior CR, a semicircular capsule incision was made between the rotator cuff and the glenoid from the AIS to the scapular spine. ^23,29

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Figure 2.

Schematic illustration of the surgical procedures (right shoulder, axial view): (1) anterior interval slide: incision between the subscapularis and supraspinatus; (2) posterior interval slide: incision between the supraspinatus and infraspinatus; (3) intra-articular capsular release: juxta-glenoidal capsular incision from 2- to 10-o’clock (dashed gray line). *Supraspinatus footprint. **Sensor-enhanced arthroscopic grasper.

Assessment of RT

RT was assessed by measuring the force that was needed to advance the supraspinatus tendon stump laterally. This was performed using a sensor-enhanced arthroscopic grasper, which has been used in previous research and has demonstrated validity. ²⁵ The sensor-enhanced arthroscopic grasper was visually adjusted to align with the tendon and muscle fibers of the supraspinatus. The device was then used to firmly grasp and hold the distal tendon stump, and stepwise lateralization (ie, reduction to the supraspinatus footprint) in 1-mm increments was performed. Load on the tendon was measured in real time (Figure 1).

To reduce the tension lost because of viscoelastic relaxation, the tendon was preconditioned by 20 cycles of lateralization to a maximum of 50 N (1 mm/5 s) ⁸ before definitive measurements. Then, the starting point was set after preloading to 0.5 N. The endpoint was defined for each shoulder as the point at which maximum tendon lateralization (in millimeters) before 50 N was reached, as preliminary testing had shown a high risk of tendon slippage at loads greater than 50 N. Next, the tendon was lateralized in 1-mm increments (0.1 mm/s) beginning at the starting point, with measurement of the loading force preceded by a 5-second equilibrium period. One test cycle was considered completed when the individually defined endpoint was reached. All specimens were tested 3 times, allowing a pause of 5 minutes between cycles. RT was defined as the mean load of the 3 test cycles at the endpoint.

Testing Protocol

In the intervention group, after RT evaluation of the native (retracted supraspinatus tendon tear) tendon (t₁), the AIS (t₂), PIS (t₃), and superior CR (t₄) were performed subsequently with RT testing after each procedure. The percentage change of RT was defined as changes in load relative to t₁. In the control group, the measurement of RT was performed at the same time points, without any interventions.

Statistical Analysis

For quantitative variables, mean and standard deviation were calculated. RT was compared using analysis of variance (ANOVA) for repeated measurements with post hoc testing according to Tukey-Kramer using the SAS procedure, proc mixed. Test results were considered statistically significant if the corresponding P value was less than .05. For assessment of the test-retest reliability, the intraclass correlation coefficient (ICC) was calculated for every lateralization step (3 repeated test cycles), each based on a 2-way ANOVA. Agreement strength was inferred from the ICC in accordance with the recommendations of Landis and Koch. ²⁰ ICC values were defined as follows: <0.20, poor; 0.21 to 0.40, fair; 0.41 to 0.60, moderate; 0.61 to 0.80, substantial; and >0.81, nearly perfect agreement. All statistical analyses were performed by a biostatistician of the Medical Faculty Mannheim at Heidelberg University, using the statistical software SAS (version 9.4; SAS Institute).

Limitations

Besides the lack of randomization regarding the order of procedures, further considerable limitations must be considered. Thiel-embalmed specimens were used in this investigation. The comparability of biomechanical properties of Thiel-embalmed specimens to that of native tissue was evaluated in few studies reporting heterogenic results. Some studies ^9,22 found altered elastic moduli of tendons compared with fresh-frozen cadavers, while one ² reported similar mechanical properties. The control group showed significantly lower changes in RT throughout the testing; therefore, the potential bias through altered biomechanical properties should at least be mitigated. The specimen had a high mean age of 74 years, and degenerative changes in tendon and muscle can be assumed. On the other hand, this also represents an age group in which rotator cuff tears are common. ^5,31 For this investigation, artificial tears of otherwise intact tendons were analyzed, which cannot fully simulate chronic rotator cuff tears, as it is known that biomechanical capacities change in chronic rotator cuff disease. ^6,7 This investigation is a point zero experiment that cannot take into account healing processes.