Rotational action of the supraspinatus muscle on the shoulder joint

Biomechanical evaluation of augmentation of suture-bridge supraspinatus repair with additional anterior fixation

BACKGROUND

Studies have recently focused on evaluating the ability of the supraspinatus repair to withstand rotational loads. Other studies have focused on the importance of minimizing gap formation to avoid decreased healing and failure of repair. The objective of this study was to use a loading model that incorporates external rotation to biomechanically evaluate augmenting a suture-bridge technique for supraspinatus repair with an additional anterior fixation.

METHODS

Eight matched cadaveric shoulder pairs were randomized to 2 different types of repairs after a simulated supraspinatus tear. One group received a standard suture-bridge technique, and the other underwent a suture-bridge repair with an additional anterior fixation consisting of a 4.5-mm suture anchor. A custom apparatus was used to test all specimens, allowing for dynamic external rotation from 0° to 30° during loading. Cyclic loading was performed for 30 cycles from 0 to 90 N, followed by load to failure using a materials-testing machine.

RESULTS

No differences were found in linear stiffness, yield load, ultimate load, and energy absorbed for load to failure between the 2 groups (P > .05). There was a reduction in anterior gapping at ultimate load between the anterior augmentation repair group (6.4 ± 3.1 mm) and the standard suture bridge (9.4 ± 2.8 mm; P = .037).

CONCLUSION

There does not appear to be a biomechanical advantage with the addition of an anterior suture augmentation of a suture bridge for a supraspinatus repair. However, using an anterior augmentation for a suture bridge prevents gap formation at ultimate load in a biomechanical, dynamic external rotation model.

Current biomechanical concepts for rotator cuff repair

For the past few decades, the repair of rotator cuff tears has evolved significantly with advances in arthroscopy techniques, suture anchors and instrumentation. From the biomechanical perspective, the focus in arthroscopic repair has been on increasing fixation strength and restoration of the footprint contact characteristics to provide early rehabilitation and improve healing. To accomplish these objectives, various repair strategies and construct configurations have been developed for rotator cuff repair with the understanding that many factors contribute to the structural integrity of the repaired construct. These include repaired rotator cuff tendon-footprint motion, increased tendon-footprint contact area and pressure, and tissue quality of tendon and bone. In addition, the healing response may be compromised by intrinsic factors such as decreased vascularity, hypoxia, and fibrocartilaginous changes or aforementioned extrinsic compression factors. Furthermore, it is well documented that torn rotator cuff muscles have a tendency to atrophy and become subject to fatty infiltration which may affect the longevity of the repair. Despite all the aforementioned factors, initial fixation strength is an essential consideration in optimizing rotator cuff repair. Therefore, numerous biomechanical studies have focused on elucidating the strongest devices, knots, and repair configurations to improve contact characteristics for rotator cuff repair. In this review, the biomechanical concepts behind current rotator cuff repair techniques will be reviewed and discussed.

Muscle loading and activation of the shoulder joint during humeral external rotation by pulley and variable resistance

BACKGROUND

The aim of the study was to evaluate differences in the loading of glenohumeral joint muscles between a cable pulley machine (CP) and variable resistance machine (VR) during axial humeral external rotation.

METHODS

Eleven healthy male subjects took part in the study. Intramuscular electromyography from five muscles of the shoulder (medial deltoid, supraspinatus, infraspinatus and upper part of the trapezius), torque and power output was measured at different rotation angles and with different loads (10%, 50% and 100% of 1RM). Also the compressive and shear force in the glenohumeral joint was analyzed at the horizontal level at angles of rotation. External rotation was performed with a self-selected velocity on the scapular plane.

FINDINGS

In the CP the range of movement became narrower than in the VR with increasing workload (P<0.05). The activity of the infraspinatus did not grow in the CP after 50% load, while it did in the VR (P<0.01). The upper part of the trapezius was activated less in the CP than in the VR (P<0.01) machine when using 50% and 100% loads. In comparison with the CP, the shear forces that pull the head of the humerus in a posterior direction were more evenly distributed in the VR than in the CP at different angles of rotation (P<0.001).

INTERPRETATION

The VR seems to make a broader range of motion possible, lager activation the primary external rotators and evenly distributed shear forces than the CP. However, performing the exercise with VR and high load also activates the upper part of the trapezius.

RELEVANCE

These findings can be used in the development of exercise designs, methods and equipment for shoulder injury rehabilitation.

Influence of distinct anatomic subregions of the supraspinatus on humeral rotation

The supraspinatus, having distinct anterior and posterior subregions, is most commonly considered an abductor of the humerus, but it has also been shown to induce humeral rotation. The objective of this study was to quantify the magnitude and direction of humeral rotation that results from loading the distinct anterior and posterior subregions of the supraspinatus. Fourteen cadaver specimens were tested under four loading conditions based on physiological cross section area of the supraspinatus: (1) anterior only; (2) posterior only; (3) physiologic (each subregion loaded simultaneously); and (4) nonphysiologic (the tendon loaded as a whole). Each specimen was tested at 0, 15, 30, 45, and 60 degrees of glenohumeral abduction in the scapular plane and from 60 degrees of internal to 45 degrees of external rotation in 15 degrees increments. The humeral rotation that occurred with loading from the initial starting rotation position was measured using a rotary variable inductance transducer. In the scapular plane, the anterior subregion of the supraspinatus acts as both an internal and external rotator depending on the initial position of the humerus. The posterior subregion either acted as an external rotator or did not induce rotation. This study demonstrated a distinct functional difference between the anatomic subregions of the supraspinatus. This understanding will help to improve testing methods and the development of repair strategies of the supraspinatus.

The effect of abduction and rotation on footprint contact for single-row, double-row, and modified double-row rotator cuff repair techniques

BACKGROUND

An abduction pillow and abduction and rotation exercises are commonly used after rotator cuff repair. The effect of glenohumeral abduction and rotation on footprint contact has not been elucidated.

HYPOTHESIS

Abduction will decrease tendon-to-bone contact for all repairs. A modified double-row repair will maintain footprint contact more effectively at each position of humeral abduction and rotation than double- or single-row repairs.

STUDY DESIGN

Controlled laboratory study.

METHODS

In 6 fresh-frozen human shoulders, a modified double-row supraspinatus tendon repair was performed; a suture limb from each of 2 medial anchors was bridged over the tendon and fixed laterally. Double- and single-row repairs were performed sequentially; a total of 3 repairs were tested. For all repairs, a Tekscan pressure sensor was fixed at the tendon-footprint interface. The tendon was loaded with 30 N. The shoulders were tested at 0 degrees , 30 degrees , and 60 degrees of abduction with 0 degrees of rotation. For both dual-row repairs, 5 rotation positions were tested.

RESULTS

The greatest contact areas at neutral rotation were achieved at 0 degrees of abduction for the modified double-row, double-row, and single-row repairs (151.3 +/- 10.7 mm2, 80.7 +/- 30.0 mm2, and 61.3 +/- 26.1 mm2, respectively), with values decreasing as abduction increased. Each repair was significantly different from one another at each abduction angle (P < .05), except between single- and double-row repairs at 0 degrees of abduction. Mean interface pressure exerted over the footprint was greater for the modified double-row technique than for the other techniques at each abduction angle (P < .05). With respect to rotation, the modified double-row repair had significantly more footprint contact than did the double-row repair at each position tested (P < .05).

CONCLUSION

For a given repair, increasing abduction at neutral rotation reduced footprint contact. Internal rotation to 60 degrees provided among the highest contact measurements. The modified double-row technique provided the most contact.

CLINICAL RELEVANCE

Results are consistent with the practice of immobilizing the shoulder with 30 degrees or less of abduction and up to 60 degrees of internal rotation to optimize footprint contact. A dual-row repair may maximize contact when initiating rehabilitation that involves abduction and rotation.

Changes in shoulder muscle function with humeral position: a graphical description

A graphical description of the change in the role played by each of the scapulohumeral muscles with respect to spatial joint position is presented. Moment arms were collected from a biomechanical model using the tendon travel method. Data cover elevation and flexion in a space between the frontal plane and a plane of elevation 60 degrees anterior to this. Segments of a given muscle were seen possibly to exhibit antagonistic moment components in relation to others, emphasizing the importance of muscle segmentation in biomechanical models. Graphical description of muscle function in conjunction with electromyographic studies enables a more complete assessment of active muscle function in relation to arm motion and position. In cases of attenuated muscular function, this also offers a means of detecting which muscle is involved and which other muscles possess compensating potential. Two examples illustrate the use of this data, particularly to clarify clinical issues.

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 degrees to 30 degrees ) 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.

The biomechanical effects of dynamic external rotation on rotator cuff repair compared to testing with the humerus fixed

BACKGROUND

Biomechanical testing without humeral motion is a standard method for evaluating rotator cuff repair constructs. This cannot elucidate the effects of dynamic external rotation on the repair, which is a common postoperative motion.

HYPOTHESIS

Biomechanical properties and gap formation of rotator cuff repairs will be different when dynamic external rotation is allowed to occur during loading.

STUDY DESIGN

Controlled laboratory study.

METHODS

In 6 matched pairs of human cadaveric shoulders, a commonly used single-row rotator cuff repair was performed. In 6 shoulders, a materials testing machine and a custom testing apparatus that permits cyclic rotation (0 degrees -30 degrees ) were employed (group 1). In contralateral shoulders, the apparatus was fixed to prevent humeral rotation (group 2). All repairs were cyclically loaded from 0 to 60 N at a displacement rate of 1 mm/s for 30 cycles. The constructs were then loaded to failure. Repair strength, gap formation, and strain were compared between groups.

RESULTS

Cyclic loading revealed no difference in linear stiffness between testing conditions. Hysteresis was significantly greater when dynamic external rotation was allowed to occur. With load to failure, there were no differences in yield or ultimate load. Anterior tendon gap formation was greater at end rotation (30 degrees of humeral external rotation) and at yield load, and strain on the posterior tendon was less with dynamic external rotation. With dynamic external rotation, gap formation and tendon strain were significantly greater in the anterior region of the supraspinatus tendon compared with the posterior region.

DISCUSSION

External rotation using postoperative physiologic loads affects gap formation and tendon strain between anterior and posterior supraspinatus tendon regions. Previous testing models without humeral rotation may underestimate gap formation and anterior tendon strain and overestimate posterior tendon strain.

CLINICAL RELEVANCE

Understanding regional differences with respect to these variables, depending on quality of repair, may provide the surgeon a framework from which to prescribe guidelines for postoperative rehabilitation.

Atypical shoulder muscle activation in multidirectional instability

OBJECTIVE

Surface and intramuscular electromyography was used to investigate shoulder muscle activity in subjects with multidirectional instability (MDI).

METHODS

Subjects (seven MDI, 11 control) performed repetitive shoulder abduction/adduction, flexion/extension and internal/external rotation movements on an isokinetic dynamometer. The activity of the deltoid, infraspinatus, supraspinatus, latissimus dorsi, and pectoralis major muscles were recorded using double-differential surface and intramuscular fine-wire electrodes. A repeated measures analysis of variance evaluated group differences in the amplitude, onset, termination and duration of the muscle activity.

RESULTS

Significant activation parameter differences for the supraspinatus, infraspinatus, posterior deltoid and pectoralis major muscles were found in the subjects with MDI. The rotator cuff and posterior deltoid muscles demonstrated abbreviated periods of activity when performing internal/external rotation, despite activation amplitudes that were similar to the controls. In contrast, the activation of the pectoralis major differed from the control group in both the amplitude and time domains when performing shoulder extension.

CONCLUSIONS

MDI is associated with atypical patterns of muscle activity that occur even when highly constrained movements are used to elicit the activity.

SIGNIFICANCE

In addition to glenohumeral hyperlaxity, the results suggest that dysfunctional neuromuscular control of the rotator cuff is also a contributing factor to the pathoetiology of MDI.

Dynamic contributions to superior shoulder stability

It has been suggested that superior decentralization of the humeral head is a mechanical factor in the etiology of degenerative rotator cuff tears. This superior decentralization may be caused by muscular imbalance. The objective of this study was to investigate the contribution of individual shoulder muscles to superior stability of the glenohumeral joint. In 10 fresh frozen cadaver shoulders the tendons of the rotator cuff, teres major, latissimus, pectoralis major, deltoid and biceps were prepared. The shoulders were tested in a shoulder-loading device in 0 degrees, 30degrees, 60 degrees and 90 degrees of glenohumeral abduction. A constant superior force of 20 N was applied to the humerus. Tensile loads were applied sequentially to the tendons in proportion to their cross-sectional areas and translations of the humeral head relative to the glenoid were recorded with a 3Space Fastrak system. Depression of the humeral head was most effectively achieved by the latissimus (5.6 +/- 2.2 mm) and the teres major (5.1 +/- 2.0 mm). Further studies should elucidate their possible in vivo role in the frontal plane force couple to counter balance the deltoid. The infraspinatus (4.6 +/- 2.0 mm) and subscapularis (4.7 +/- 1.9 mm) showed similar effects while the supraspinatus (2.0 +/- 1.4 mm) was less effective in depression. Therefore, the infraspinatus and subscapularis should be surgically repaired whenever possible. The supraspinatus may be of less importance for superior stability than previously assumed.

Dynamic inferior stabilizers of the shoulder joint

BACKGROUND

The glenohumeral joint is soft-tissue balanced. However, few studies have focused on its dynamic inferior stabilizers.

OBJECTIVE

The objective of this study was to investigate the dynamic contributions of five shoulder muscles to inferior stability of the glenohumeral articulation in four joint positions.

METHODS

The anterior, lateral and posterior deltoid, supraspinatus, short head of biceps, coracobrachialis and long head of triceps from ten cadaveric shoulders were tested in 0 degrees, 30 degrees, 60 degrees and 90 degrees of glenohumeral abduction. A constant inferior force of 15 N was applied to the humerus. The tendons were loaded sequentially in proportion to their respective muscle's cross-sectional area. Translations of the humeral head on the glenoid were recorded with a 3-Space tracking device.

RESULTS

The lateral deltoid (8.2 mm, SD 4.8 mm) was potentially most effective in superior translation of the humeral head followed by the posterior deltoid (7.7 mm, SD 4.8 mm). The coracobrachialis and short head of biceps had considerable capability to translate the humeral head superiorly (2.8 mm, SD 1.3 mm) while the supraspinatus showed the weakest effects (1.3 mm, SD 0.5 mm).

RELEVANCE

Strengthening exercises of the deltoid may be useful in the treatment of inferior glenohumeral instability, while the supraspinatus seems to be less important for inferior glenohumeral stability than previously assumed.

Functional stability of the glenohumeral joint

Normal shoulder function is determined by the stability provided by the passive, active and control subsystems of the joint complex. Given the complexity of the shoulder, it is not surprising that it is one of the most common joints presenting with pathology. Knowledge and understanding of the anatomy and the intricate relationships of each of the subsystems is essential for successful assessment and treatment. This paper presents a review of the anatomy, biomechanics and the integrated function of the glenohumeral joint which are essential for motion. The principles of rehabilitation of dynamic control of the glenohumeral joint are introduced.