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AJSM PreView, published on August 10, 2012 as doi:10.1177/0363546512456195

Superior Capsule Reconstruction to Restore Superior Stability in Irreparable Rotator Cuff Tears A Biomechanical Cadaveric Study Teruhisa Mihata,*yz§ MD, PhD, Michelle H. McGarry,yz MS, Joseph M. Pirolo,yz MD, Mitsuo Kinoshita,§ MD, PhD, and Thay Q. Lee,yz PhD Investigation performed at the Orthopaedic Biomechanics Laboratory, VA Healthcare System, Long Beach, California 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. Keywords: patch graft; superior capsule; irreparable; rotator cuff; biomechanics

cuff tears are not reparable because of tendon retraction with inelasticity,4,30 muscle atrophy,4,8,17,18,29 and fatty infiltration.4,8,17,18,29,30 For the irreparable cases, patch graft surgery to the torn tendon has been proposed.|| However, a high rate of patch graft retears has been reported, although various materials have been used to close the defect including porcine small intestinal submucosa, porcine dermal collagen, and allografts.24,36,39 Radiographic studies showed that rotator cuff tears increase superior translation, causing subacromial impingement.5,6 In irreparable cases, the altered kinematics causes pathological osseous changes in the shoulder joint. Even after patch graft surgery to the torn tendon, the humeral head still shifts superiorly.24,36 Therefore,

Although rotator cuff tears can mostly be repaired with excellent results, some chronic large or massive rotator

*Address correspondence to Teruhisa Mihata, MD, PhD, Department of Orthopedic Surgery, Osaka Medical College, 2-7 Daigaku-machi, Takatsuki, Osaka 569-8686, Japan (e-mail: [email protected], tmihata @poh.osaka-med.ac.jp). y Orthopaedic Biomechanics Laboratory, VA Healthcare System, Long Beach, California. z University of California, Irvine, Irvine, California. § Department of Orthopedic Surgery, Osaka Medical College, Takatsuki, Japan. The authors declared that they have no conflicts of interest in the authorship and publication of this contribution. The American Journal of Sports Medicine, Vol. XX, No. X DOI: 10.1177/0363546512456195 Ó 2012 The Author(s)

||

References 3, 12, 13, 24, 25, 27, 31, 33-36, 39.

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the patch graft could be abraded and torn by subacromial impingement due to a lack of superior stability.24,36 Patients with irreparable rotator cuff tears have a defect of the superior capsule, which is located on the inferior surface of the supraspinatus and infraspinatus tendons. Although the shoulder capsule plays a role in stabilizing the glenohumeral joint,2,20,32,37 no studies, to date, have evaluated the potential benefit of treating the superior capsular defect for irreparable rotator cuff tears. Here, we hypothesized that the patch graft that is attached medially to the superior glenoid and laterally to the greater tuberosity to reconstruct the superior capsule would restore superior stability. Accordingly, the objective of this study was to compare superior stability among 3 types of patch grafting, including tendon reconstruction, capsule reconstruction, and both tendon and capsule reconstruction, for simulated irreparable rotator cuff tears.

MATERIALS AND METHODS Preparation of Specimens Eight fresh-frozen cadaveric shoulders from donors aged 69 to 76 years at the time of death were thawed overnight at room temperature before dissection and experimentation. Six of the specimens were from male donors, 2 were from female donors, and an equal number of right and left shoulders were tested. The shoulders were examined macroscopically for signs of abnormality. Each specimen was dissected free of skin, subcutaneous tissue, and muscles, whereas the capsule, coracoacromial ligament, all rotator cuff tendons, and humeral insertion of the pectoralis major, latissimus dorsi, and deltoid were preserved. The scapula was placed in a scapular box and surrounded with plaster of Paris; the axial orientation was preserved, as the glenoid was positioned parallel to the top of the scapular box. The humeral shaft was transected 2 cm distal to the deltoid tuberosity and fixed in a polyvinyl chloride (PVC) pipe by using screws and plaster of Paris. After the shoulder was securely affixed to the mounting device, it was attached to the shoulder testing system (Figure 1). All specimens were kept moist with 0.9% saline throughout the experiment.

Testing Conditions, Position, and Muscle Loading All biomechanical testing was performed under each of the 5 following conditions (Figure 2): condition 1: intact rotator cuff tendon (Figure 3A); condition 2: after cutting the supraspinatus tendon and superior capsule because Nimura et al28 have shown that it is hard to separate the supraspinatus tendon and superior capsule on the greater tuberosity (Figure 3B); condition 3: after patch grafting to reconstruct the supraspinatus tendon (Figure 3C); condition 4: after patch grafting to reconstruct the superior capsule (Figure 3D); and condition 5: after patch grafting to reconstruct both the supraspinatus tendon and superior capsule, which we termed a double-layer patch graft (Figure 3E). In condition 2, the supraspinatus tendon insertion on the greater tuberosity was cut sharply, after which the

Figure 1. Custom shoulder testing system, which allows 6 degrees of freedom for positioning the glenohumeral joint. Condition 1: Intact rotator cuff tendon The supraspinatus tendon and superior capsule were cut Condition 2: Simulated the irreparable rotator cuff tear The medial side of the patch graft was attached to the lateral edge of the simulated supraspinatus tendon tear with No. 2 FiberWire, and the lateral side of the graft was attached to the greater tuberosity with suture anchors Condition 3: Patch grafting to reconstruct the supraspinatus tendon The sutures between the patch graft and the supraspinatus tendon were removed. The medial side of the patch graft was then attached to the superior glenoid by using 2 suture anchors. Condition 4: Patch grafting to reconstruct the superior capsule An additional patch graft was attached to the lateral edge of the simulated supraspinatus tendon tear, and to the greater tuberosity Condition 5: Patch grafting to reconstruct both the supraspinatus tendon and superior capsule

Figure 2. Experiment design. supraspinatus tendon and superior capsule were cut medially to the glenoid along the anterior and posterior edges of the supraspinatus tendon. To simulate the irreparable rotator cuff tear, the lateral edges of the supraspinatus

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Figure 3. (A) Intact rotator cuff tendon of a human cadaveric shoulder in the superior view (left) and posterior view (right). (B) Simulated irreparable rotator cuff tear of a human cadaveric shoulder in the superior view (left) and posterior view (right). (C) Patch graft of the supraspinatus tendon of a human cadaveric shoulder in the superior view (left) and posterior view (right) as well as a schematic drawing (middle). The medial side of the patch graft was attached to the lateral edge of the simulated supraspinatus tendon tear, and the lateral side of the graft was attached to the greater tuberosity. (D) Superior capsule reconstruction of a human cadaveric shoulder in the superior view (left) and posterior view (right), together with a schematic drawing (middle). The medial side of the patch graft was attached to the superior glenoid by using 2 suture anchors, which were inserted into the superior glenoid at the 11-o’clock and 12-o’clock positions (black arrows) on the glenoid in the right shoulder. (E) Patch graft of both the supraspinatus tendon and superior capsule of a human cadaveric shoulder in the posterior view (right), together with a schematic drawing (left). A second patch graft was attached to the lateral edge of the simulated supraspinatus tendon tear on the medial side and to the greater tuberosity on the lateral side. AC, acromion; B, biceps long head; C, coracoid process; G, glenoid; GT, greater tuberosity; H, humeral head; ISP, infraspinatus; SSP, supraspinatus; Sub, subscapularis; Gr, graft.

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tendon and superior capsule were retracted to the glenoid by applying muscle forces. In condition 3, the medial side of the patch graft was attached to the lateral edge of the simulated supraspinatus tendon tear by using No. 2 FiberWire (Arthrex Inc, Naples, Florida), and the lateral side of the graft was attached to the greater tuberosity by use of a single-row repair with two 5.0-mm metal corkscrew anchors and No. 2 FiberWire, which were inserted at the lateral edge of the greater tuberosity. The graft was also sutured to the subscapularis and infraspinatus tendons with a side-to-side stitch using No. 2 FiberWire. In condition 4, once all of the measurements were taken, the sutures between the patch graft and the supraspinatus tendon were removed. The medial side of the patch graft was then attached to the superior glenoid by using two 5.0-mm metal corkscrew anchors with No. 2 FiberWire, which were inserted into the superior glenoid at the 11-o’clock and 12-o’clock positions on the glenoid of the right shoulder (1-o’clock and 12-o’clock positions of the left shoulder) to reconstruct the superior capsule. In condition 5, an additional patch graft was attached to the lateral edge of the simulated supraspinatus tendon tear by using No. 2 FiberWire on the medial side and to the greater tuberosity by using 2 more 5.0-mm metal corkscrew anchors with No. 2 FiberWire, which were inserted 5 mm lateral from the lateral edge of the greater tuberosity. All of the patch grafts from conditions 3 to 5 were attached at 45° of shoulder abduction. This abduction angle was selected because our pilot study showed it to maintain appropriate tension at 90° of shoulder abduction and to prevent graft tears at 0° of shoulder abduction. Fascia lata of allografts was used for all patch grafts in this study. A 5-mm-thick graft was made by folding the fascia lata 2 or 3 times and suturing around the edge of the folds. Anterior-posterior widths and medial-lateral lengths of the grafts were matched to the width of the supraspinatus tendon and to the distance between the greater tuberosity and the superior glenoid at 45° of shoulder abduction. All measurements were performed at 0°, 45°, and 90° of shoulder abduction, which were achieved by 0° of scapular upward rotation and 0° of glenohumeral abduction, 15° of scapular upward rotation and 30° of glenohumeral abduction, and 30° of scapular upward rotation and 60° of glenohumeral abduction, respectively. The humeral rotation angle was determined to be 0° (0° abduction [abd]/0° external rotation [ER]) and 30° of external rotation (0° abd/30° ER) at 0° shoulder abduction and 30° (45° abd/30° ER, 90° abd/30° ER) and 60° of external rotation (45° abd/60° ER, 90° abd/60° ER) at 45° and 90° of shoulder abduction for measurements of the superior translation of the humerus, subacromial contact pressure, and glenohumeral joint force. We used 2 different loading conditions for each testing position based on data of the cross-sectional area of each muscle1,40: loading condition 1: 40 N, deltoid; 20 N, pectoralis major; 20 N, latissimus dorsi; 10 N, supraspinatus; 10 N, infraspinatus 1 teres minor; 10 N, subscapularis; and loading condition 2: 80 N, deltoid; 10 N, supraspinatus; 10 N, infraspinatus 1 teres minor; 10 N, subscapularis. Loading condition 1 was used to simulate a balanced load that allowed the humeral head to be placed at the center

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of the glenoid, whereas loading condition 2 was used to apply a superior force. The supraspinatus was unloaded after cutting of the supraspinatus tendon, after patch grafting of the supraspinatus tendon, after patch grafting of the superior capsule, and after patch grafting of the supraspinatus tendon and superior capsule to simulate the lack of physiological muscle tension that follows an irreparable supraspinatus tendon tear, where muscle is severely atrophied clinically.4,8,17,18,29 Muscle forces were applied by using cables sutured to the tendons. We used polyester sutures with a long-chain polyethylene core (No. 2 FiberWire) in a Krackow locked running stitch. Adjustable pulleys enabled simulated force vectors to approximate those of anatomic muscle loading.

Measurements The location of the humeral head relative to the acromion was recorded with a 3-dimensional digitizing system (accuracy, 0.3 mm) (MicroScribe 3DLX, Immersion Corp, San Jose, California) under loading conditions 1 and 2. The shoulder position under loading condition 1 was defined as the initial position in this study because our pilot study confirmed that the humeral head was placed at the center of the glenoid. Under loading condition 2, the superior translation force was achieved by increasing the deltoid force and unloading the pectoralis major and latissimus dorsi. To obtain the reproducible position of the proximal humerus and acromion, the proximal part of the bicipital groove and the anterior-lateral edge of the acromion were marked with small screws before testing. To evaluate superior shoulder stability, superior translation of the humerus was calculated by comparing the distance between 2 small screws in the superior-inferior direction under loading condition 1 with that under loading condition 2.2,14,19-22,32,37 Subacromial contact pressures were recorded by using a Tekscan pressure sensor (saturation pressure, 10.3 MPa) (model 4000, Tekscan, Boston, Massachusetts) under loading condition 2.19,21 Little contact pressure has been detected under condition 1. The sensor was placed under the acromion. The Tekscan sensor pads were calibrated by using 2 points, 10- to 20-N calibration, before the experiment with the Instron 4411 load cell (Instron, Norwood, Massachusetts). The calibration protocol was selected based on the results of our pilot studies of accuracy and reproducibility for this specific experiment. Humeral rotational range of motion was measured by use of a 360° goniometer attached to the testing system2,20,22,32,37 to evaluate capsular tightness. Humeral rotation was measured only under condition 1 because the rotation torque applied with the humeral head migrated superiorly might cause graft tears, resulting in inconsistent data. The goniometer degree measurements were inscribed in 2.5° increments on a stationary circular plate. The specimens were preconditioned with ten 5-second cycles of 1.1 Nm of torque in external and internal rotations. The maximal rotation was then measured with 2.2 Nm of torque.2,20,32 Total rotational range of motion was calculated by adding external and internal rotational ranges of motion.

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TABLE 1 Superior Translation of the Humerusa Supraspinatus Tear

Intact

Tendon Patch

Superior Capsule Reconstruction

Double-Layer Patch

Translation, Translation, Translation, Translation, Translation, Translation, Translation, Translation, Translation, Translation, mm % mm % mm % mm % mm % 0° abduction 0° external rotation 30° external rotation 45° abduction 30° external rotation 60° external rotation 90° abduction 30° external rotation 60° external rotation

3.7 6 0.9 3.0 6 0.6

100 100

5.4 6 0.7b 6.4 6 0.7b

146 213

4.6 6 0.4b,c 4.8 6 0.6b,c

124 160

3.0 6 1.9c,d 2.9 6 0.7c,d

81 97

3.7 6 0.9c 2.8 6 0.5c,d

100 93

3.3 6 0.8 2.5 6 0.5

100 100

5.9 6 1.2b 5.2 6 0.9b

179 208

4.8 6 0.9b,c 3.8 6 0.7b,c

145 152

3.7 6 0.8c,d 3.2 6 0.6c,d

112 128

3.2 6 0.9c,d 2.2 6 0.5c,d

97 88

2.4 6 0.6 1.5 6 0.3

100 100

2.8 6 0.7 1.9 6 0.4

117 127

2.4 6 0.6 1.8 6 0.4

100 120

2.1 6 0.5c,d 1.6 6 0.5c

88 107

1.9 6 0.6c,d 1.2 6 0.3c

79 80

a

Values are given as mean 6 standard deviation. Superior translation percentage was calculated by dividing each datum by intact data at the same position. A significant difference compared to the intact states (P \ .05). c A significant difference compared to the simulated supraspinatus tear (P \ .05). d A significant difference compared to the tendon patch (P \ .05). b

TABLE 2 Subacromial Contact Pressurea Supraspinatus Tear

Intact

0° abduction 0° external rotation 30° external rotation 45° abduction 30° external rotation 60° external rotation 90° abduction 30° external rotation 60° external rotation

Tendon Patch

Superior Capsule Reconstruction

Double-Layer Patch

Pressure, MPa

Pressure, %

Pressure, MPa

Pressure, %

Pressure, MPa

Pressure, %

Pressure, MPa

Pressure, %

Pressure, MPa

Pressure, %

0.05 6 0.01 0.06 6 0.01

100 100

0.07 6 0.04 0.11 6 0.03b

140 183

0.04 6 0.01 0.06 6 0.01c

80 100

0.01 6 0.01c 0.01 6 0.01c

20 17

0.03 6 0.01c 0.05 6 0.01c

60 83

0.07 6 0.01 0.06 6 0.01

100 100

0.14 6 0.03b 0.13 6 0.03b

200 217

0.08 6 0.01c 0.08 6 0.01c

114 133

0.07 6 0.01c 0.07 6 0.01c

100 117

0.07 6 0.01c 0.07 6 0.01c

100 117

0.08 6 0.01 0.11 6 0.02

100 100

0.10 6 0.02b 0.13 6 0.01

125 118

0.10 6 0.01 0.11 6 0.02

125 100

0.09 6 0.01c 0.10 6 0.02c

113 91

0.08 6 0.01c 0.09 6 0.02c

100 82

a Values are given as mean 6 standard deviation. Subacromial contact pressure percentage was calculated by dividing each datum by intact data at the same position. b A significant difference compared to the intact states (P \ .05). c A significant difference compared to the simulated supraspinatus tear (P \ .05).

In each of these positions, the glenohumeral joint force was measured by using a 6 degrees of freedom load cell (Assurance Technologies Inc, Garner, North Carolina) connected to a personal computer.10,15,16,38 Glenohumeral joint forces were measured in 3 planes: anterior-posterior, superiorinferior, and medial (compressive)–lateral under loading condition 2, which led to superior migration of the humeral head.

are shown as means 6 standard deviations of the mean. All P values \.05 were considered significant. Based on the mean and standard deviation of the first 3 specimens, 4 specimens were determined as needed to reach 80% power based on differences in superior translation of the humerus, subacromial contact pressure, and glenohumeral joint force, and 8 specimens were needed based on differences in rotational range of motion. According to these considerations, 8 specimens were tested.

Statistical Analysis All measurements were performed twice and the averages calculated. Statistical analyses were performed by using a repeated-measures analysis of variance followed by a Tukey post hoc test. Superior translation of the humerus, subacromial contact area and pressure, glenohumeral total rotational range of motion, and glenohumeral joint force were compared between 5 testing conditions. The data

RESULTS Superior Translation of the Humerus After the supraspinatus tendon was cut under condition 2, superior translation of the humerus significantly increased by 1.7 mm at 0° abd/0° ER (P \ .05), 3.4 mm at 0° abd/30° ER (P \ .0001), 2.6 mm at 45° abd/30° ER (P \ .001), and

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TABLE 3 Total Rotational Range of Motiona Total Rotational Range of Motion, mm Intact 0° abduction 45° abduction 90° abduction

Supraspinatus Tear

159 6 8 178 6 6 158 6 13

Tendon Patch

166 6 10 182 6 7 167 6 15

Superior Capsule Reconstruction b,c,d

167 6 10 182 6 8 164 6 14

143 6 13 158 6 12b,c,d 153 6 14b,c,d

Double-Layer Patch 143 6 12b,c,d 157 6 13b,c,d 143 6 14b,c,d

a

Values are given as mean 6 standard deviation. A significant difference compared to the intact states (P \ .05). c A significant difference compared to the simulated supraspinatus tear (P \ .05). d A significant difference compared to the tendon patch (P \ .05). b

TABLE 4 Glenohumeral Joint Forcea Glenohumeral Compression Force, N

0° abduction 0° external rotation 30° external rotation 45° abduction 30° external rotation 60° external rotation 90° abduction 30° external rotation 60° external rotation

Intact

Supraspinatus Tear

Tendon Patch

Superior Capsule Reconstruction

Double-Layer Patch

27.1 6 1.8 31.8 6 1.7

18.6 6 1.8b 21.2 6 1.6b

19.6 6 2.2b 21.5 6 2.1b

19.8 6 2.2b 22.1 6 1.9b

20.1 6 2.2b 22.3 6 1.8b

53.9 6 2.6 55.9 6 3.0

46.3 6 2.6b 48.3 6 2.5b

47.8 6 2.0b 49.3 6 2.6b

47.0 6 2.4b 48.6 6 2.5b

45.4 6 1.9b 48.8 6 2.6b

76.6 6 3.0 77.0 6 3.4

72.9 6 4.0b 73.4 6 3.6b

71.1 6 3.2b 70.9 6 3.1b

70.8 6 3.2b 70.9 6 3.8b

70.8 6 2.8b 70.6 6 3.8b

a

Values are given as mean 6 standard deviation. A significant difference compared to the intact states (P \ .05).

b

2.7 mm at 45° abd/60° ER (P \ .0001). After the patch graft of the supraspinatus tendon under condition 3, superior translation of the humerus significantly decreased; however, superior translation after the tendon patch was significantly greater than the intact condition. Superior translations after the superior capsule reconstruction and the double-layer patch graft were not significantly different from that in the intact condition. The difference between the superior capsule reconstruction and the double-layer patch graft was not significant (Table 1).

Subacromial Contact Pressure The cutting of the supraspinatus tendon resulted in a significant increase in subacromial contact pressure by 0.05 MPa at 0° abd/30° ER (P \ .05), 0.07 MPa at 45° abd/30° ER (P \ .001), 0.07 MPa at 45° abd/60° ER (P \ .001), and 0.02 MPa at 90° abd/30° ER (P \ .05). There was no significant difference in subacromial contact pressure between the intact condition and all patch grafts in conditions 3, 4, and 5 (Table 2).

Total Rotational Range of Motion After the supraspinatus tendon was cut, the average total rotational range of motion increased by 7°, 4°, and 9° at 0°,

45°, and 90° of shoulder abduction, respectively, although these increases were not statistically significant. The patch graft of the supraspinatus tendon did not affect the total rotational range of motion compared with the intact condition. Superior capsule reconstruction significantly decreased the total rotational range of motion by 16° (P \ .05), 20° (P \ .01), and 5° (P \ .05) compared with the intact condition at 0°, 45°, and 90° of shoulder abduction, respectively. There was no significant difference in the total rotational range of motion achieved between the superior capsule reconstruction and the double-layer patch graft (Table 3).

Glenohumeral Joint Force The cutting of the supraspinatus tendon significantly decreased the glenohumeral compression force by 8.5 N at 0° abd/0° ER (P \ .0001), 10.6 N at 0° abd/30° ER (P \ .0001), 7.6 N at 45° abd/30° ER (P \ .0001), 7.6 N at 45° abd/60° ER (P \ .0001), 3.7 N at 90° abd/30° ER (P \ .05), and 3.6 N at 90° abd/60° ER (P \ .01) (Table 4). In comparison with the supraspinatus cut condition, none of the patch grafts in conditions 3, 4, and 5 significantly changed the glenohumeral compression force. The glenohumeral joint force in the anterior-posterior and

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superior-inferior directions did not change significantly under any of the 5 conditions.

DISCUSSION For irreparable rotator cuff tears, patch graft surgery with many kinds of materials has been reported.{ However, a high rate of patch graft retears has been reported because the patch graft could be abraded and torn by subacromial impingement due to a lack of superior stability.24,36,39 In this study, we compared superior stability among 3 types of patch grafting, including tendon reconstruction, capsule reconstruction, and both tendon and capsule reconstruction, for simulated irreparable rotator cuff tears. In this study, the patch graft to reconstruct the supraspinatus tendon significantly decreased superior translation but did not completely restore it to that of the intact rotator cuff condition. This result suggests that retears of conventional patch grafts to the torn tendon may result from some residual superior instability, leading to abrasion and retears after surgery. On the other hand, the patch graft to reconstruct the superior capsule, which is attached medially to the superior glenoid and laterally to the greater tuberosity,28 completely restored superior translation to that of the intact rotator cuff condition, indicating that it is less likely for the reconstructed superior capsule to be abraded. The survival rate of the superior capsule reconstruction may, therefore, be higher than that of patch grafts of torn tendons. The cutting of the supraspinatus tendon resulted in a significant increase in subacromial contact pressure, which may cause shoulder pain by subacromial impingement.9,26,41,42 Then, all 3 types of patch grafting fully restored the subacromial contact pressure to the intact level. This result suggests that any type of patch grafting could relieve pain due to subacromial impingement. The patch graft to reconstruct the supraspinatus tendon did not change the total rotational range of motion, whereas the superior capsule reconstruction significantly decreased the total rotational range of motion by 16°, 20°, and 5° compared with the intact condition at 0°, 45°, and 90° of shoulder abduction, respectively. This result suggests that shoulder stiffness may occur after superior capsule reconstruction. In some shoulder scoring systems, including those of the University of California, Los Angeles (UCLA)7 and the Japanese Orthopaedic Association,11 150° of postoperative elevation, which corresponds to a 17% decrease relative to 180° of elevation, is generally considered excellent. In this study, the percentage decrease in total rotational range of motion after the superior capsule reconstruction was 10.2%, 11.0%, and 3.2% relative to the total rotational range of motion in the intact condition at 0°, 45°, and 90° of shoulder abduction, respectively. Therefore, the limited range of motion after superior capsule reconstruction may not be severe and may be acceptable. The cutting of the supraspinatus tendon significantly decreased the glenohumeral compression force at all of the testing positions, whereas none of the patch grafts {

References 3, 12, 13, 24, 25, 27, 31, 33-36, 39.

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significantly changed the glenohumeral joint force. This result implies that the compression force mainly contributes to the supraspinatus tendon force and that superior capsule reconstruction may restore shoulder stability without increasing glenohumeral compression force. We also investigated the effect of a double-layer patch graft for reconstruction of both the superior capsule and the torn tendon. Superior translation of the humerus, subacromial contact pressure, and glenohumeral joint force did not differ significantly between the superior capsule reconstruction alone and the double-layer patch graft. Therefore, reconstruction of the superior capsule may be sufficient to restore superior stability of the glenohumeral joint in irreparable rotator cuff tears. This cadaveric study design provides certain strengths to the study. Only a cadaveric study can directly measure subacromial contact pressure using a pressure sensor. In addition, the exact location of the humerus and scapula can be measured because the bone surface is exposed; thus, the results obtained using this approach may be more accurate than those from imaging studies: our study was accurate to 0.3 mm because it was only dependent on the MicroScribe (Immersion Corp) measuring tool. Moreover, our study was consistent because the exact same force could be applied in the same direction for all specimens. Our study has some weaknesses. First, muscle loading was static rather than dynamic, low loading and subphysiological because of the cadaveric study design and testing system. In our testing system using static testing, we used the glenohumeral position as the independent variable and measured the change in superior translation of the humerus, subacromial contact pressure, range of motion, and glenohumeral joint force for each condition. Second, cadaveric specimens do not account for biological healing potential. Third, this experiment has been performed only in 2 muscle loading conditions. Despite these limitations, we believe that our results are of value because our tested muscle loads were determined based on the cross-sectional area of each muscle. A fourth limitation, also related to the cadaveric study design, is that our results represent the biomechanics at time zero after surgery. Scarring might affect the result. Our pilot study showed that no graft retears in 10 patients were found on postoperative magnetic resonance imaging during a follow-up (12-18 months) after arthroscopic superior capsule reconstruction.23 In the future, we will investigate the long-term follow-up of the superior capsule reconstruction for irreparable rotator cuff tears. Fifth, each loading trial may cause microtears of the capsular ligaments and rotator cuff tendons. However, we believe that it is not significant because the muscle forces based on data of the cross-sectional area of each muscle should be physiological. In conclusion, superior capsule reconstruction completely restored superior stability, whereas patch grafts to the torn tendon only partially restored this stability. When patch graft surgery is chosen for irreparable rotator cuff tears, superior capsule reconstruction in which the graft is attached medially to the superior glenoid and laterally to the greater tuberosity may be better to restore superior stability of the humeral head.

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The American Journal of Sports Medicine

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