Biomechanics and Clinical Functionof the Rotator Cuff


Chapter 3

Biomechanics and Clinical Functionof the Rotator Cuff



Nam Su Cho, Woong Kyo Jeong, Michelle H. McGarry, and Thay Q. Lee

Introduction


The shoulder is a complex anatomic structure due to its multiple passive and active structures working together for stability and joint motion. The shoulder’s ability for multiple degrees of motion is based on the interaction of multiple structures that react to mechanical stimuli and adjust accordingly. The rotator cuff, a group of muscles surrounding the glenohumeral joint, not only acts as a dynamic stabilizer further supporting static stabilizing structures but also adds to the passive stability of the shoulder by bulk and barrier effects due to their location and orientation around the glenohumeral joint. It also helps provide concavity compression and the torque needed for motion in multiple planes. The biomechanics of the shoulder are intimately associated with the rotator cuff, as altered biomechanics are often the result of rotator cuff pathology and vice versa. Rotator cuff tears often result in loss of function, strength, and stability. In this chapter, the biomechanics and clinical function of the rotator cuff and rotator cuff pathology are discussed in depth.

Normal Function of Rotator Cuff


1. Glenohumeral Joint Stability

The shoulder has the greatest range of motion of any joint in the human body and has six degrees of freedom of motion (anterior-posterior/superior-inferior/medial-lateral translation and flexion-extension/external-internal rotation/abduction-adduction). Due to this large range of motion, however, it is predisposed to instability. There are both static and dynamic stabilizing mechanisms in the shoulder, including soft tissue, bony geometry, and shoulder muscles. The rotator cuff muscles help provide shoulder function and act as a stabilizer in both passive and active states.

The subscapularis muscle provides a barrier to resist anterior dislocation of the humeral head. Early studies of shoulder stabilization involved sectioning of various anatomic components in cadavers. At 0 degrees of abduction, the subscapularis muscle plays the primary stabilizing role; at 45 degrees of abduction, the subscapularis, middle glenohumeral ligament, and a portion of the inferior glenohumeral ligament provide stability; and nearing 90 degrees of abduction, the inferior glenohumeral ligament prevents dislocation. Biomechanical studies also offer additional information regarding directional stability. The long head of the biceps tendon (LHBT) provides more than 30 N of anterior stabilization in neutral rotation, with the subscapularis providing the majority of the stabilization in external rotation. Ligaments play a bigger role in stability, as they become loaded at higher displacements. The supraspinatus and biceps muscles are important active stabilizers in inferior stabilization, with the inferior glenohumeral ligament passively stabilizing in external rotation. Posterior stability is also provided by passive muscle tension of the rotator cuff. A cadaveric study by Ovesen et al. demonstrated that both the supraspinatus and infraspinatus/teres minor were important in stabilizing the shoulder posteriorly. Posteriorly, the subscapularis muscle resists subluxation, with the coracohumeral (CH) ligament contributing in neutral rotation. Release of the coracoacromial (CA) ligament to treat impingement causes glenohumeral laxity both anteriorly and inferiorly, indicating its importance as a static restraint.

Intricate coordination and anatomical positioning of the rotator cuff muscles and the LHBT creates an ideal configuration to actively compress the humeral head into the cavity of the glenoid. Shoulder anatomy also provides the rotator cuff muscles with short lever arms, establishing a stable and dynamic fulcrum during abduction.

2. Shoulder Motion


3. Force Couples About the Glenohumeral Joint

Force couples occur when two opposing muscle groups create a given moment around a fulcrum. The rotator cuff creates a force couple around the glenohumeral joint, with coordinated activation and inactivation of agonist and antagonist muscles. Two major force couples act synergistically on the glenohumeral joint: one in the coronal plane and the other in the transverse plane.

Inman et al. first described the force couple acting in the coronal plane between the deltoid and inferior rotator cuff (infraspinatus, teres minor, and subscapularis) (Fig. 3.1). The deltoid moment lies above the center of rotation, while the inferior cuff moment acts parallel to the lateral border of the scapular below the center of rotation. This force couple is important in producing stable glenohumeral abduction. More recently, the pectoralis major and latissimus dorsi have been included in this force couple. The depressor moment produced by the inferior rotator cuff may be too weak to counterbalance the strong deltoid moment. However, the pectoralis major and latissimus dorsi have similar depressor moments as the deltoid and therefore are now thought to work along with the inferior rotator cuff in the coronal plane force couple. Recent biomechanics studies have replicated the moments produced by the pectoralis major and latissimus dorsi to create a more anatomic shoulder construct.

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FIG. 3.1 The coronal plane force couple with the deltoid acting superiorly and the inferior rotator cuff (subscapularis, infraspinatus, teres minor), pectoralis major, and latissimus dorsi acting inferiorly.

The transverse force couple comprises moments produced by the subscapularis anteriorly and infraspinatus and teres minor posteriorly (Fig. 3.2). Cadaver studies determined that glenohumeral joint motion is not affected as long as this force balance is intact. Quantitative analysis confirmed that the direction and magnitude of joint reaction forces were most affected by the integrity of the anterior-posterior force balance, with no significant change after incomplete or complete tear of the supraspinatus tendon. This dynamic relationship is an important aspect of understanding normal shoulder motion and how a disrupted force balance can play a role in shoulder pathologies. Inability to maintain a balanced transverse force couple can lead to anterior or posterior translation of the humeral head. For example, in the setting of a massive rotator cuff tear involving the infraspinatus and teres minor (or posterior moment), the larger moment produced by the subscapularis can lead to anterior translation of the humeral head. This uncoupling between forces leads to an unstable fulcrum for glenohumeral motion.

A cadaveric study by Mihata et al. investigated the effects of a simulated weakened subscapularis as seen in overhand throwers on glenohumeral kinematics and contact pressures. In their model, they used a custom shoulder testing system that replicated multiple lines of pull for the rotator cuff, deltoid, pectoralis major, and latissimus dorsi. Therefore the authors used all muscles in the transverse and coronal force couples. They found that less force on the subscapularis lead to a significant increase in external rotation and posterosuperior glenohumeral contact pressure. This is likely due to the disrupted transverse force couple leading to an imbalance between anterior and posterior forces. The strong moment arm of the infraspinatus, usually restrained by the subscapularis, now has a stronger moment arm relative to the subscapularis moment arm on the humerus leading to posterosuperior translation. These results give insight to the pathophysiology behind internal impingement and partial-thickness articular-sided rotator cuff tears seen in throwing athletes.

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FIG. 3.2 The transverse plane force couple comprising the infraspinatus and teres minor acting posteriorly and the subscapularis acting anteriorly.

4. Biomechanical Aspects of Clinical Examination for Rotator Cuff Disorders

Understanding biomechanics of the shoulder is critical for the clinical examination of rotator cuff disorders. For example, it is critical to assess passive range of motion to eliminate a concurrent frozen shoulder, a condition that precludes immediate repair. Excessive external rotation relative to the normal shoulder may signify a torn subscapularis tendon. Active range of motion generally will gauge the functional limitations.

Motor testing can isolate the muscles involved by understanding the anatomy, function, and biomechanics of specific rotator cuff muscles. Resisted forward elevation in the scapular plane can help assess supraspinatus involvement, although a negative test does not mean a tear is not present, especially when the tear is only partial, leaving a tear that is functional on examination. In addition to elevation, the supraspinatus rotates the humerus internally and externally. Weakness to external rotation at the side generally suggests supra- and infraspinatus involvement; this test can be the most specific way to rule in a supraspinatus tear. However, a negative test does not preclude a supraspinatus tear, although the tear may be full-thickness, as it may be only partially torn in the anterior-posterior dimension.

The belly-press, lift-off, and bear-hug tests can help measure subscapularis pathology. The belly-press test is performed with the hand pressing maximally into the abdomen and the elbow in line with the trunk in the sagittal plane—a positive test is manifested by a relative weakness compared with the normal side or the elbow dropping posteriorly in the sagittal plane. The lift-off test is performed by placing the dorsum of the hand against the midlumbar spine (L2–L4); a positive test is apparent when the patient is unable to internally rotate and lift the hand away from the back. The bear-hug test is performed with hand placed on the contralateral shoulder over the acromioclavicular joint; a positive test results when the examiner can externally rotate the hand away from the initial position. Lag signs are ominous for large to massive tears, such as the hornblower’s test for the teres minor, and the external-rotation-at-the-side lag sign for the infraspinatus (usually also signifying supraspinatus involvement).

Neer and Hawkins tests are typically used to assess external impingement underneath the coracoacromial ligament arch. Pain posteriorly with abduction and external rotation, with or without instability, indicates the possibility of internal impingement, particularly in an overhead throwing athlete. Other provocative tests such as the Speed test, an anterior apprehension maneuver, Yergason test, O’Brien test, Jobe relocation test, and the crank test can be used to assess for nontendinous concurrent pathologies, including labral tears, which can dictate the type of imaging study that is obtained, and the surgical approach to be used.

Pathologic Condition of Rotator Cuff


1. Changes in Joint Kinematics With Rotator Cuff Pathology


The rotator cuff plays a role in multiple functions, including abduction, rotation, and glenohumeral concavity compression. Biomechanical studies have characterized and quantified rotator cuff function in the context of various tear patterns and repair techniques. These variables have been shown to affect various biomechanical parameters, such as glenohumeral kinematics (e.g., translation and rotation), footprint contact, tendon strain, gap formation, tendon motion, fluid extravasation, energy absorbed to failure, and load to failure.

The maximum contractile force of a muscle is based on many factors including sarcomere length, cross-sectional area, and initial position. Most muscles have their maximum contractile force in the midrange of muscle length, and their force is diminished at the extremes of length. Muscle force is based on strong bony connections to produce a strong fulcrum for movement. Patients with rotator cuff tears frequently show loss of active motion and strength of the shoulder. Tendon defects change the amount of torque that can be generated by the rotator cuff for motion.

As reviewed earlier, the rotator cuff musculature provides balanced forces that impart mobility and stability to the glenohumeral joint. Disruption of this innate force couple results in abnormal joint kinematics, as the stable fulcrum for rotation of the humeral head in the glenoid is lost. Such force changes are dependent on tear size and location. Anterior tears of the supraspinatus insertion are more likely to be symptomatic and progress as a result of increased regional strain patterns due to joint force imbalance, potentially causing additional pain and requiring surgical intervention. However, completely repairing a rotator cuff after a chronic multitendon tear can be challenging due to tendon retraction and stiffening. Instead, restoring the balance of the anterior-posterior forces by repairing only the infraspinatus in a supraspinatus-infraspinatus tear may be sufficient to restore shoulder function.

Disruption of the force balance and normal shoulder stability and motion after loss of tendon function initiates changes in almost all adjacent tissues. Rotator cuff tears are often accompanied by tears in the glenoid labrum. Superior humeral head translation and loading of the LHBT due to decreased stabilization cause displacement of the labrum and increased labral tissue strain. A number of studies utilizing the rat model further investigated these and other changes. Without repair, rotator cuff tears can cause cartilage degeneration in the labrum, putting it at risk for injury. Returning to a high level of activity increases the severity of this damage, significantly decreasing the expression of cartilage matrix proteins such as type II collagen and aggrecan in the glenoid. Articular cartilage of the humeral head also shows surface irregularities, loss of proteoglycans (PGs), and clonal chondrocyte formation 12 weeks after rotator cuff transection. Mechanical properties of adjacent untorn tendons, including the LHBT and the subscapularis, also deteriorate, becoming stiffer at both the insertion and the midsubstance. Functional impairment of muscles associated with torn tendons decreases their potential to produce normal forces after repair, largely due to atrophy and fatty infiltration. Moreover, chronic fibrosis increases muscle stiffness, increasing tension at the repair site and impeding the repair process. Adjacent muscles react in a compensatory way, becoming hypertrophic. Clearly, normal shoulder stability and motion defined by the rotator cuff are crucial in maintaining the health of the entire shoulder.

2. Partial-thickness Rotator Cuff Tears (PTRCTs)


PTRCTs are a common cause of shoulder pain and are more common than full-thickness rotator cuff tears. Cadaveric shoulders have shown the incidence of PTRCTs to be between 13% and 32%. They typically involve younger patients, and they most commonly involve the supraspinatus tendon. Articular-sided PTRCTs are two to three times more common than bursal-sided tears.

Biomechanical studies have shown that a PTRCT increases the strain distribution on the intact rotator cuff. Mazzocca et al. showed there is an increase in strain with greater than 50% involvement of the supraspinatus in PTRCTs. They also showed that the strain returned to the intact state after repair. The depth of a supraspinatus PTRCT has been shown to be directly associated with strain in the intact tendon. Yang et al. showed that as the depth of an anterior supraspinatus PTRCT approaches 50%, a nonlinear relationship exists between strain and thickness. This implied that PTRCTs greater than 50% should warrant greater concern for the surgeon, and the chance for tear propagation was greater. Bey et al. used magnetic resonance imaging to assess intratendinous strains in cadaveric shoulders in different angles of glenohumeral abduction. An articular-sided PTRCT was created in each specimen, and results showed that the tear increased intratendinous strain at all abduction angles greater than 15 degrees. With the articular side of the cuff and humeral head in contact with each other at lower abduction angles, the authors postulate that this contact could play a role in load sharing between the two.

Using a finite element analysis model, Sano et al. showed that all types of PTRCT (bursal-sided, articular-sided, and intrasubstance) increased stress concentration on the articular surface around the tear. An increased abduction angle also produced increased stress concentration around the tear site. In the intact tendon model as well as all three tear models, there was an increase in stress seen on the articular side compared with the bursal side.

While the surgical principles of repair of PTRCTs are still debated, there is no doubt that a PTRCT increases the strain around the surrounding intact cuff. Whether the increase in strain is enough to cause further tear propagation is dependent on the size and location of the tear. The findings from these studies led the authors to conclude that many PTRCTs may eventually lead to a full-thickness tear, which is consistent with other clinical studies.

Kim et al. analyzed in vivo intratendinous strain of the supraspinatus tendon, the superficial, middle, and deep regions of 15 shoulders were marked with speckles using 2D speckle tracking echocardiography (2D STE). The displacement and the strain of each speckle during isotonic and isometric shoulder motion were evaluated. A different strain was found between the superficial and deep regions within the intact live supraspinatus tendon. The strain and displacement patterns vary according to isometric versus isotonic shoulder motions. On the basis of their observations, they suggested that the delaminated tear of the rotator cuff tendon must be repaired separately layer by layer to resist the inhomogeneous strain after the repair.

3. Full-Thickness Rotator Cuff Tears


Full-thickness rotator cuff tears are less frequent than PTRCTs and usually occur in an older population. In a cadaveric study by Fukuda et al. there was a prevalence of 7% for full-thickness tears and 13% for partial-thickness tears. Other studies have reported a prevalence of full-thickness tears ranging from 8% to 26%. The location, size, and shape of the rotator cuff tear ultimately define the biomechanical changes that occur. A cadaveric study by Andarawis-Puri et al. showed that increasing the width of a full-thickness supraspinatus tear beyond 33% caused an increase in maximum and a decrease in minimum strain in the infraspinatus tendon. There was no difference in strain in the infraspinatus between an intact supraspinatus tendon and a tear with a width less than 33%. This implies a strain-shielding effect of the infraspinatus tendon, which may prevent tear propagation in the supraspinatus, likely due to overlapping tendinous insertions. These results also show a reciprocal strain-shielding effect of the supraspinatus on the infraspinatus. They also found that anterior one-third tears of the supraspinatus do not affect the strain pattern of the infraspinatus. Mura et al. showed the infraspinatus is an important producer of abduction torque, and rotator cuff tears that extend into the infraspinatus significantly decrease abduction torque. These tears were also shown to lead to superior migration of the humeral head.

Rotator cuff tears commonly propagate from anterior to posterior; however, it is unknown when the biomechanical environment is altered. A rotator cuff tear progression model has been developed to primarily elucidate a critical tear size that significantly affects glenohumeral joint biomechanics. In a study by Oh et al. it was shown that a tear of the entire supraspinatus significantly increased rotational range of motion and decreased abduction capability. Further tear progression to the infraspinatus significantly shifted the humeral head posteriorly at the midrange of rotation and superiorly and laterally at maximum internal rotation. This is consistent with the traditional concept of “force-couple” between the subscapularis and infraspinatus muscles; with relatively more force generated anteriorly through the subscapularis, the resultant humeral head kinematics can be predicted. This study also evaluated the effects of pectoralis major and latissimus dorsi muscle loading and concluded that the pectoralis major and latissimus dorsi played an important role to depress and stabilize humeral head migration. This is in agreement with previous studies showing the importance of the pectoralis major and latissimus dorsi in the coronal plane force couple to stabilize the glenohumeral joint. These findings suggest that once a rotator cuff tear has progressed to include the entire supraspinatus, surgical intervention may be necessary to restore normal glenohumeral joint biomechanics and prevent further tear progression. Also, to stabilize the glenohumeral joint with a rotator cuff tear, exercises should emphasize strengthening the remaining intact musculature including the pectoralis major and latissimus dorsi.

Regarding the subscapularis, biomechanical characterization of 3D footprint morphology and tear characteristics has been described. The bony footprint can be divided into 4 “facets,” facet 1 (Fig. 3.3) being the most superior. Footprint area measured, from facet 1 to 4: 3 mm2 (34% of footprint), 145.8 mm2 (28%), 115.7 mm2 (22%), and 77.0 mm2 (15%), respectively. Based on a strain gradient, full-thickness tears involving the first facet had a higher propensity for propagation than partial-thickness tears, suggesting an indication for repair. A more medial single anchor repair was shown to have a higher ultimate load to failure compared with a more lateral position. Regarding function, based on the morphology characterizations, the most superior facet of the subscapularis tendon was found to be generally oriented in line with the supraspinatus tendon, thus suggesting the possibility that this portion of the subscapularis contributes relatively more to abduction than internal rotation. This may help explain a functional tear of the supraspinatus.

4. Massive Rotator Cuff Tears


Massive rotator cuff tears (>5 cm) generally involve complete rupture of the supraspinatus tendon and partial or complete rupture of one or more of the subscapularis, infraspinatus, and teres minor tendons. Some rotator cuff tears are asymptomatic and are only identified incidentally. Additionally, some individuals with massive rotator cuff tears maintain active shoulder abduction, and some maintain good postoperative active range of motion despite high rates of repeat tears after repair.

These observations illustrate that our understanding of shoulder function in the presence of a substantial tear is not complete. One hypothesis is that contraction of the deltoid muscle causes the humeral head to be captured underneath the coracoacromial arch, with subsequent pivoting of the humerus about this abnormal center of rotation. Good function can be compatible with massive avulsion of the rotator cuff, provided that balance between the deltoid muscle and the remaining intact portion of the rotator cuff is not severely impaired. Burkhart et al., stressing the importance of the transverse force couple, stated that unrepaired rotator cuff tears with intact force couples and an intact rotator cable may be compatible with normal function if the patient can obtain satisfactory pain relief, although these tears may be quite large. These statements seem to agree with the observation that the factors more likely to result in satisfactory postoperative range of motion and overhead function were intact subscapularis and teres minor tendons with absence of muscular atrophy.

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FIG. 3.3 Bursal view of subscapularis tendon insertion, so called 1st facet view. The comma tissue or the cable between subscapularis and supraspinatus by rotator interval tissue was cut to show the 1st facet better. This facet is in average 14mm x 14mm roughly. (Courtesy Jae Chul Yoo, MD, unpublished figure, Sungkyunkwan University School of Medicine, Samsung Medical Center, Seoul, Korea.)

Hansen et al. measured the increases in residual muscle forces necessary to maintain active shoulder motion with rotator cuff tears of various sizes. They reported the results associated with a custom cadaver shoulder controller that utilized position and orientation closed-loop feedback control. In the presence of a massive rotator cuff tear, stable glenohumeral abduction without excessive superior humeral head translation requires significantly higher forces in the remaining intact portion of the rotator cuff. These force increases are within the physiologic range of rotator cuff muscles for 6-cm tears and most 7-cm tears. Increases in deltoid force requirements occur in early abduction; however, greater relative increases are required of the rotator cuff, especially in the presence of larger rotator cuff tears. In summary, rotator cuff tear size, rotator cuff muscle force, and deltoid force are important determinants of shoulder function in the presence of massive rotator cuff tears. The findings of this study provide guidance for the rehabilitation of patients with rotator cuff tears and those who have had rotator cuff repair and may provide justification for early surgical repair of rotator cuff tears.

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FIG. 3.4 The anatomy of the rotator cable (RCb). (A, B) As seen, it is a thickened band that inserts posteriorly at the inferior edge of the infraspinatus tendon and inserts anteriorly just posterior to the bicipital groove. It encloses the thinner region of the rotator cuff known as the rotator crescent (RCr). SS, supraspinatus; BT, biceps tendon; IS, infraspinatus; TM, teres minor.

A biomechanical study by Campbell et al. evaluated the role of the pectoralis major and latissimus dorsi muscles on humeral head translation, glenohumeral joint forces, and acromiohumeral contact pressure in the setting of a massive rotator cuff tear. The pectoralis major and latissimus dorsi muscles in massive rotator cuff tear were effective in improving glenohumeral kinematics and reducing acromiohumeral pressures.

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FIG. 3.5 Completed repair in a left shoulder with margin convergence to bone technique that uses a suture anchor placed just posterior to the bicipital groove to reconstruct the anterior attachment of the rotator cable. 
(From Nguyen ML, Quigley RJ, Galle SE, et al. Margin convergence anchorage to bone for reconstruction of the anterior attachment of the rotator cable. Arthroscopy. 2012;28(9):1237–1245.)

5. Rotator Cable and Crescent


Burkhart et al. first coined the terms “rotator crescent” and “rotator cable” after repeatedly noticing a thick bundle of fibers on the articular surface of the rotator cuff on arthroscopy. They called the thick cable-like structure the “rotator cable” and the thinner tissue from the cable to the enthesis was named the “rotator crescent.” Multiple radiographic, anatomic, and histologic studies have confirmed the presence of the rotator cable and crescent. The rotator cable is an extension of the coracohumeral ligament that runs perpendicular to the supraspinatus from just posterior to the bicipital groove to the posterior border of the infraspinatus (Fig. 3.4). It is thought that the rotator cable protects the thinner, avascular rotator crescent. Clinically, a study by Kim et al. showed that rotator cuff tears involving the anterior attachment of the rotator cable have a higher incidence of fatty degeneration, and Nguyen et al. showed that in massive rotator cuff tears involving this anterior attachment, reconstruction using a suture anchor decreases gap formation throughout the entire repair. They named this surgical technique “margin convergence to bone,” and it utilizes a suture anchor just posterior to the bicipital groove to reconstruct the anterior attachment. One limb of the double-loaded suture is placed in the anterior supraspinatus and the other through the rotator interval to provide both footprint reconstruction and margin convergence (Fig. 3.5). This technique significantly decreased gap formation throughout the entire repair when compared with soft-tissue margin convergence. This study highlighted the importance of anatomic repair of the rotator cuff and the role of the rotator cable to stress-shield the rotator crescent.

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FIG. 3.6 The single-row rotator cuff repair with two anchors placed at the lateral edge of the tendon footprint and simple sutures passed through the torn tendon edge. 
(From Park MC, McGarry MH, Gunzenhauser RC, Benefiel MK, Park CJ, Lee TQ. Does transosseous-equivalent rotator cuff repair biomechanically provide a “self-reinforcement” effect compared with single-row repair? J Shoulder Elbow Surg. 2014;23(12):1813–1821.)

Biomechanics Related Rotator Cuff Repair


1. Different Rotator Cuff Repairs and Their Biomechanical Effects


Given its line of action, the supraspinatus force vector functions to abduct the glenohumeral joint in the scapular plane. The insertion for the rotator cuff has been described in multiple studies; rotator cuff footprint dimensions can represent an area approximately 12 by 24 mm (medial-lateral by anterior-posterior). The first-generation arthroscopic rotator cuff repair techniques involved a single row of suture anchors placed at the lateral footprint. The suture was then passed through the torn tendon edge and the tendon was tied down to the anchor (Fig. 3.6). A limitation of this repair is that it relies on the lateral tendon edge for fixation, which can be compromised in chronic tears. Also, a single-row repair only allows for footprint reconstruction and contact pressure at the insertion point of the suture anchor. Concern exists regarding the inability to establish the medial-to-lateral footprint with a single-row construct, which may account for the suboptimal healing rates and high retear rates. The double-row fixation concept was developed to optimize the biomechanical properties of rotator cuff reconstructions. This gives medial row and lateral row fixation to increase the contact area of the repair (Fig. 3.7). The relationship between abduction and footprint contact has been described based on repair type: single versus double row. In general, increasing abduction led to decreasing footprint contact in a single-row repair (Fig. 3.8). The “transosseous-equivalent” repair (Fig. 3.9A), which provides obligatory tendon suture-bridging compression over this footprint (Fig. 3.9B), was shown to have better footprint contact dimensions compared with the other techniques (single row and “traditional” double row) at each level of humeral abduction (0 degrees, 30 degrees, 60 degrees). Importantly, abducting the humerus has been shown to decrease repair tension, and more tension has been shown to adversely affect healing. Therefore there appear to be competing variables with abduction; less repair tension at the expense of less footprint contact.

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Mar 28, 2020 | Posted by in ORTHOPEDIC | Comments Off on Biomechanics and Clinical Functionof the Rotator Cuff

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