Michelle H. McGarry

A cadaveric model of the throwing shoulder: a possible etiology of superior labrum anterior-to-posterior lesions.

BACKGROUND It has been speculated that a shift of the throwing arc commonly develops in athletes who perform overhead activities, resulting in greater external rotation and decreased internal rotation caused by anterior capsular laxity and posterior capsular contracture, respectively. Osseous adaptation in the form of increased humeral and glenoid retroversion may provide a protective function in the asymptomatic athlete but cannot explain the pathological changes seen in the shoulder of the throwing athlete. Therefore, the objective of the present study was to examine the biomechanical effects of capsular changes in a cadaveric model. METHODS Ten cadaveric shoulders were tested with a custom shoulder-testing device. Humeral rotational range of motion, the position of the humerus in maximum external rotation, and glenohumeral translations in the anterior, posterior, superior, and inferior directions were measured with the shoulder in 90 degrees of abduction. Translations were measured with the humerus secured in 90 degrees of external rotation. To simulate anterior laxity due to posterior capsular contracture, the capsule was nondestructively stretched 30% beyond maximum external rotation with the shoulder in 90 degrees of abduction. This was followed by the creation of a 10-mm posterior capsular contracture. Rotational, humeral shift, and translational tests were performed for the intact normal shoulder, after anterior capsular stretching, and after simulated posterior capsular contracture. RESULTS Nondestructive capsular stretching resulted in a significant increase in external rotation (average increase, 18.2 degrees 2.1 degrees ; p < 0.001), and subsequent simulated posterior capsular contracture resulted in a significant decrease in internal rotation (average decrease, 8.8 degrees +/- 2.3 degrees ; p = 0.02). There also was a significant increase in anterior translation with the application of a 20-N anterior translational force after nondestructive capsular stretching (average increase, 1.7 +/- 0.3 mm, p = 0.0006). The humeral head translated posteroinferiorly when the humerus was rotated from neutral to maximum external rotation. This did not change significantly in association with anterior capsular stretching. Following simulated posterior capsular contracture, there was a trend toward a more posterosuperior position of the humeral head with the humerus in maximum external rotation in comparison with the position in the stretched conditions, although these differences were not significant. CONCLUSIONS A posterior capsular contracture with decreased internal rotation does not allow the humerus to externally rotate into its normal posteroinferior position in the cocking phase of throwing. Instead, the humeral head is forced posterosuperiorly, which may explain the etiology of Type-II superior labrum anterior-to-posterior lesions in overhead athletes.

Excessive Humeral External Rotation Results in Increased Shoulder Laxity

Background The quantitative relationship between increased anterior shoulder laxity and increased humeral external rotation observed in throwers remains unclear. Hypothesis An elongated anterior capsule, especially the anterior band of the inferior glenohumeral ligament, produced by excessive humeral external rotation will result in increased anterior shoulder laxity and increased humeral external rotation. Study Design Controlled laboratory study. Methods Seven cadaveric shoulders were tested to measure the humeral rotational range of motion, glenohumeral translations, and length of the anterior band of the inferior glenohumeral ligament. Data were collected for the intact shoulders and after nondestructive stretching of 10%, 20%, and 30% beyond maximum humeral external rotation. Results Nondestructive excessive external rotational stretching resulted in a significant increase in superior (30%, 3.3 mm) and inferior (30%, 2.3 mm) length of the anterior band of the inferior glenohumeral ligament, external rotation (30%, 35°), and anterior (30%, 2.4 mm), inferior (30%, 2.2 mm), and anterior-posterior (30%, 5.1 mm) translations. There were significant positive linear correlations between the length of the anterior band of the inferior glenohumeral ligament, external rotation, and anterior translation. Conclusions Excessive humeral external rotation results in an elongation of the anterior band of the inferior glenohumeral ligament and an increase in anterior and inferior glenohumeral translations and humeral external rotation. Clinical Relevance Repetitive excessive humeral external rotation observed in throwers may be one of the biomechanical causes for increased shoulder laxity and increased humeral external rotation.

Path of glenohumeral articulation throughout the rotational range of motion in a thrower's shoulder model.

Background Overhead-throwing athletes have increased external and diminished internal glenohumeral rotation that may alter glenohumeral kinematics. Purpose To quantify the kinematic changes present in a cadaveric model of a throwers shoulder. Study Design Controlled laboratory study. Methods In 8 fresh-frozen cadaveric shoulders, the rotator cuff and overlying muscles were removed, and the glenohumeral capsule, coracoacromial ligament, and coracohumeral ligament were left intact. The scapula was fixed, and the humerus was placed in 90° of shoulder abduction in a 6 degrees of freedom testing device. A compressive force of 44 N was applied. A throwers shoulder model was created, and sequential conditions were examined: intact, after anterior stretching, and after the addition of posterior-inferior capsular plication. Kinematic measurements were obtained through a complete range of glenohumeral rotation. Results Glenohumeral external rotation increased 16%, from 149° to 173° (P <. 001), after stretching in external rotation and remained increased by 11% to 166° (P <. 001) after posterior-inferior capsular plication. With the addition of the posterior-inferior capsular plication, internal rotation averaged 7°, which was not significantly different from the intact state (11°, P =. 55) or the stretched state (16°, P =. 07). The total glenohumeral rotation after stretching followed by posterior-inferior capsular plication did not differ significantly from intact state (P =. 25). At maximum external rotation, the humeral head apex was shifted posteriorly in the stretched (P =. 003) and plicated (P <. 001) states compared with the intact state. The humeral head apex was posteriorly displaced at 135° and 150° of external rotation compared with the intact condition (P =. 039 and. 049, respectively). In maximum internal rotation, anterior stretching had no significant effect on the humeral head apex position. However, after posterior-inferior capsular plication, the humeral head apex was significantly shifted inferiorly (P =. 005) and anteriorly (P =. 03) in maximum internal rotation compared with the intact state. Conclusion Significant changes in glenohumeral motion occur in this model during the simulated late-cocking and follow through phases of throwing. In this model, posterior capsular tightness alters the humeral head position most profoundly during the deceleration and follow-through phases of throwing.

Superior Capsule Reconstruction to Restore Superior Stability in Irreparable Rotator Cuff Tears A Biomechanical Cadaveric Study

Background: There have been many clinical reports of patch graft surgery for irreparable rotator cuff tears. However, the retear rate of the patch graft is relatively high because of the lack of superior stability, causing subacromial abrasions. Purpose: To compare superior stability among 3 types of patch grafting for simulated irreparable rotator cuff tears. Study Design: Controlled laboratory study. Methods: Eight cadaveric shoulders were tested in a custom shoulder testing system. Superior translation of the humerus, subacromial contact pressure, and glenohumeral joint force were quantified in the following 5 conditions: (1) when the rotator cuff was intact, (2) after cutting the supraspinatus tendon, (3) after the patch graft to reconstruct the supraspinatus tendon, (4) after the patch graft to reconstruct the superior capsule, and (5) after the patch graft to reconstruct both the supraspinatus tendon and superior capsule. While the graft was sutured to the torn tendon in condition 3, the graft was attached to the superior glenoid in condition 4. Results: Compared with values for intact rotator cuffs, cutting the supraspinatus tendon significantly increased superior translation (P < .05), significantly increased subacromial contact pressure (P < .05), and significantly decreased glenohumeral compression force (P < .05). Superior translation was restored partially after the supraspinatus tendon patch graft and restored fully after the superior capsule patch graft and after both patch grafts. All patch grafts fully restored the subacromial contact pressure (P < .05) but did not alter the glenohumeral joint force. Conclusion: When patch graft surgery is chosen for irreparable rotator cuff tears, the graft should be attached medially to the superior glenoid and laterally to the greater tuberosity to restore superior stability of the humeral head. Clinical Relevance: The superior capsule patch graft completely restored superior stability of the glenohumeral joint, while patch grafting to the supraspinatus tendon partially restored superior translation.

The Effect of Dynamic External Rotation Comparing 2 Footprint-Restoring Rotator Cuff Repair Techniques

Background Allowing for humeral external rotation while loading rotator cuff repairs has been shown to affect tendon biomechanics when compared with testing with the humerus fixed. Adding dynamic external rotation to a tendon-loading model using footprint-restoring repairs may improve our understanding of rotator cuff repair response to a common postoperative motion. Hypothesis A tendon suture-bridging repair will demonstrate better load sharing compared to a double-row repair, and there will be a differential gap formation between the anterior and posterior tendon regions. Study Design Controlled laboratory study. Methods In 6 fresh-frozen human cadaveric shoulders, a tendon suture-bridging rotator cuff repair was performed; a suture limb from each of 2 medial anchors was bridged over the tendon and fixed laterally with an interference screw. In 6 contralateral match-paired specimens, a double-row repair was performed. For all specimens, a custom jig was employed that allowed dynamic external rotation (0° to 30°) with loading. A materials testing machine was used to cyclically load each repair from 0 N to 90 N for 30 cycles; each repair was then loaded to failure. A deformation rate of 1 mm/s was employed for all tests. Gap formation between tendon edge and insertion was measured using video digitizing software. Results The yield load for the suture-bridging technique (161.88 ± 35.09 N) was significantly larger than the double-row technique (135.17 ± 24.03 N) (P = .026). The yield gap between tendon and lateral footprint was significantly greater anteriorly than posteriorly (1.62 ± 0.82 mm and 0.68 ± 0.47 mm, respectively) for the suture-bridging technique (P = .024) but not for the double-row technique (1.35 ± 0.52 mm and 1.05 ± 0.50 mm, respectively) (P = .34). There were no differences for gap formation, stiffness, ultimate load to failure, and energy absorbed to failure between the 2 repairs (P > .05). The anterior regions of the repair were the first to fail in all constructs. The suture-bridging repair remained interconnected for 5 of 6 repairs. Conclusions The tendon suture-bridging rotator cuff repair has a yield load that is higher than the double-row repair when allowing for external rotation during load testing. External rotation can accentuate gap formation anteriorly at a repaired rotator cuff footprint. Clinical Relevance Based on the tension of repair, there may be a role for reinforcing the repair anteriorly and limiting external rotation postoperatively.

A Biomechanical Comparison of 2 Technical Variations of Double-Row Rotator Cuff Fixation The Importance of Medial Row Knots

Background Previous studies have shown comparable biomechanical properties of double-row fixation versus double-row fixation with a knotless lateral row. SutureBridge is a construct that secures the cuff with medial row mattress suture anchors and knotless lateral row fixation of the medial suture ends. Recent completely knotless constructs may lead to lesser clinical outcomes if the construct properties are compromised from lack of suture knots. Hypothesis A completely knotless construct without medial row knots will compromise the biomechanical properties in both cyclic and failure-testing parameters. Study Design Controlled laboratory study. Methods Six matched pairs of cadaveric shoulders were randomized to 2 groups of double row fixation with SutureBridge: group 1 with medial row knots, and group 2 without medial row knots. The specimens were placed in a materials test system at 30° of abduction. Cyclic testing to 180 N at 1 mm/sec for 30 cycles was performed, followed by tensile testing to failure at 1 mm/sec. Results Data included cyclic and failure data from the materials test system and gap data using a video digitizing system. All data from paired specimens were compared using paired Student t tests. Group 1 had a statistically significant difference (P < .05) for gap formation for the 1st (3.47 vs 5.05 mm) and 30th cycle (4.22 vs 8.10 mm) and at yield load (5.2 vs 9.1 mm). In addition, there was a greater energy absorbed (2805 vs 1648 N-mm), yield load (233 vs 183.1 N), and ultimate load (352.9 vs 253.9 N) for group 1. The mode of failure for the majority (4/6) of group 2 was lateral row failure, whereas all group 1 specimens failed at the clamp. Conclusion Although lateral row knotless fixation has been shown not to sacrifice structural integrity of this construct, the addition of a knotless medial row compromises the construct leading to greater gapping and failure at lower loads. Clinical Relevance This may raise concerns regarding recently marketed completely knotless double row constructs.

Effect of humeral component version on impingement in reverse total shoulder arthroplasty

HYPOTHESIS Reverse shoulder arthroplasty is growing in popularity for patients with deficient rotator cuffs; however, the phenomenon of scapular notching continues to be a concern. This study examined the effects of humeral component version in the Aequalis Reversed Shoulder Prosthesis (Tornier, Edina, MN) on impingement of the humeral prosthesis against the scapula to test the hypothesis that the mechanical contact of the humeral component with the scapular neck is influenced by the version of the humeral component. MATERIALS AND METHODS Seven shoulders from deceased donors were tested after the Aequalis Reversed Shoulder was implanted. The deltoid, pectoralis major, and latissimus dorsi were loaded based on physiologic cross-sectional area. The degree of internal and external rotation when impingement, subluxation, or dislocation occurred was measured at 0°, 30°, and 60° glenohumeral abduction in the scapular plane. Testing was performed with the humeral component placed in 20° of anteversion, neutral version, 20° of retroversion, and 40° of retroversion. RESULTS Maximum external rotation at 0° abduction was -1° ± 4° at 20° anteversion, 15° ± 3° at neutral, 28° ± 4° at 20° retroversion, and 44° ± 5° at 40° retroversion (P < .05). Maximum internal rotation at 0° abduction was 128° ± 9° at 20° anteversion, 112° ± 9° at neutral, 99° ± 8° at 20° retroversion, and 83° ± 8° at 40° retroversion (P < .05). Maximum external rotation at 30° abduction was 70° ± 6° at 20° anteversion, 84° ± 7° at neutral, 97° ± 6° at 20° retroversion, and 110° ± 5° at 40° retroversion (P < .05). There was no limitation to internal rotation at 30° abduction. No impingement occurred at 60° abduction. DISCUSSION Version of the humeral component plays a role in range of motion and impingement in reverse total shoulder arthroplasty. Anteversion can significantly decrease the amount of external rotation achievable after reverse total shoulder surgery. CONCLUSION Placing the Aequalis Reversed Shoulder humeral component at between 20° and 40° of retroversion more closely restores a functional arc of motion without impingement.

The Effect of Recombinant Human Platelet-Derived Growth Factor BB–Coated Sutures on Rotator Cuff Healing in a Sheep Model

PURPOSE The purpose of this study was to determine whether suture could be coated with recombinant human platelet-derived growth factor BB (rhPDGF-BB) and whether the coated suture would improve histologic scores and biomechanical strength of sheep rotator cuff repairs. METHODS FiberWire sutures (Arthrex, Naples, FL) were dip coated in a collagen-rhPDGF-BB solution. Coating was confirmed by use of enzyme-linked immunosorbent assay. Rotator cuff tears were created in 18 sheep. The tendons were wrapped in Gortex (Gore Medical, Flagstaff, AZ) and allowed to scar for 2 weeks. Tendons were then repaired to bone by use of standard anchors loaded with either rhPDGF-BB-coated sutures or uncoated sutures. Gross examination, histologic analysis, and biomechanical testing were performed 6 weeks after repair. RESULTS Enzyme-linked immunosorbent assay confirmed successful loading of the growth factor onto the sutures. Gross examination showed well-healed tendon-to-bone interfaces in both rhPDGF-BB-augmented repairs and controls. Histologic analysis using a semiquantitative rating scale showed improved tendon-to-bone healing in the rhPDGF-BB-augmented repairs. There was no significant difference in the ultimate load to failure of rhPDGF-BB-augmented rotator cuff repairs compared with standard suture repairs at 6 weeks after repair. CONCLUSIONS We were able to coat No. 2 FiberWire with rhPDGF-BB. At short-term follow-up, rhPDGF-BB-coated sutures enhanced histologic scores of sheep rotator cuff repairs; however, ultimate load to failure was equivalent to standard suture repairs. CLINICAL RELEVANCE rhPDGF-BB-coated sutures seem to produce a more histologically normal tendon insertion.

Excessive Glenohumeral Horizontal Abduction as Occurs During the Late Cocking Phase of the Throwing Motion Can Be Critical for Internal Impingement

Background The objective of this study was to determine the effects of increased horizontal abduction with maximum external rotation, as occurs during the late cocking phase of throwing motion, on shoulder internal impingement. Hypothesis An increase in glenohumeral horizontal abduction will cause overlap of the rotator cuff insertion with respect to the glenoid and increase pressure between the supraspinatus and infraspinatus tendon insertions on the greater tuberosity and the glenoid. Study Design Controlled laboratory study. Methods Eight cadaveric shoulders were tested with a custom shoulder testing system with the specimens in 60° of glenohumeral abduction and maximum external rotation. The amount of internal impingement was evaluated by assessing the location of the supraspinatus and infraspinatus articular insertions on the greater tuberosity relative to the glenoid using a MicroScribe 3DLX. Pressure in the posterior-superior quadrant of the glenoid was measured using Fuji prescale film. Data were obtained with the humerus in the scapular plane and 15°, 30°, and 45° of horizontal abduction from the scapular plane. Results At 30° and 45° of horizontal abduction, the articular margin of the supraspinatus and infraspinatus tendons was anterior to the posterior edge of the glenoid and less than 2 mm from the glenoid rim in the lateral direction; the contact pressure was also greater than that found in the scapular plane and 15° of horizontal abduction. Conclusion Horizontal abduction beyond the coronal plane increased the amount of overlap and contact pressure between the supraspinatus and infraspinatus tendons and glenoid. Clinical Relevance Excessive glenohumeral horizontal abduction beyond the coronal plane may cause internal impingement, which may lead to rotator cuff tears and superior labral anterior to posterior (SLAP) lesions.

Effects of Capsular Plication and Rotator Interval Closure in Simulated Multidirectional Shoulder Instability

BACKGROUND Arthroscopic treatment of multidirectional shoulder instability with use of capsular plication and rotator interval closure has been shown to be effective in several clinical studies; however, the biomechanical effects of these procedures have not been elucidated. The purpose of this study was to assess biomechanically the effect of arthroscopic capsular plication combined with rotator interval closure on rotational range of motion, humeral head position throughout rotation, and glenohumeral translation. METHODS Seven cadaveric shoulders were stretched to 10% beyond the maximum range of motion in 60 degrees and 0 degrees of glenohumeral abduction. Testing was performed for the intact and stretched conditions and following three sequential capsular repairs: anterior plication, posterior plication, and rotator interval closure. Rotational range of motion, humeral head position throughout the range of motion, and glenohumeral translations were measured in both positions. RESULTS Stretching increased the total rotational range of motion in 60 degrees and 0 degrees of abduction. After anterior plication alone, total rotation decreased significantly (p < 0.05) in both positions and was restored to the intact state. Total translation with a 20-N load increased significantly in the 60 degrees of abduction position after stretching (p = 0.03). Anterior-posterior translation decreased significantly compared with the stretched state only after all components of the repair were completed in 60 degrees of abduction (p = 0.0003 with a 15-N load and p = 0.0001 with a 20-N load). This decrease was also found to be significantly less than the intact condition (p = 0.008 with a 15-N load and p = 0.001 with a 20-N load). A similar trend in results was found with superior-inferior translations in the 0 degrees of abduction position. CONCLUSIONS Capsular plication alone reduces range of motion to the intact state. Reductions in translation, however, may require the addition of rotator interval closure. Changes in translation and rotation after repair are dependent on arm position. In some positions, the addition of rotator interval closure may also result in overtightening.