Tension Distribution in Articular Surfaces of the Rotator Cable and Crescent: A Cadaveric Study.

BACKGROUND: The rotator cable functions as a stress and/or load transfer structure. Some studies suggested that a disruption of the cable negatively affects shoulder function and tendon integrity in patients with rotator cuff tears, while others found no functional impairment regardless of rotator cable tear severity. Although anatomical studies have identified distinct regions within the rotator cuff muscles, the strain distribution within the articular sides of the rotator cuff tendons that results from the tension in each region remains unknown. We hypothesized that the posterior region of the supraspinatus (SSP) muscle and the middle region of the infraspinatus (ISP) muscle, with their firm capsular attachments to the cable, transmit 3D strains, and thus tension, to the whole cable, leading to differences in tension within the cable. METHODS: The 3D strain distributions in the articular sides of the SSP and ISP tendons of 8 fresh-frozen cadaveric intact shoulders were determined when tension was applied to the various SSP and ISP muscle regions. RESULTS: Loading the anterior SSP muscle region yielded significantly higher strains in the anterior third of the cable compared with the posterior third (p < 0.05). Loading the posterior SSP muscle region yielded no significant differences among the cable and crescent regions. Loading the middle ISP muscle region yielded higher strains in the anterior and posterior thirds of the cable compared with the middle third (p < 0.01). Loading the superior ISP muscle region yielded no significant differences among the cable and crescent regions (p > 0.05). CONCLUSIONS: Tension generated from the posterior region of the SSP muscle and middle region of the ISP muscle was evenly distributed to the anterior and posterior attachments of the rotator cable, while the tension generated from other SSP and ISP muscle regions was locally transmitted to the respective attachment area. CLINICAL RELEVANCE: The rotator cable and crescent serve pivotal roles in transmitting tension generated from the deep regions of the rotator cuff muscles, i.e., the posterior SSP and middle ISP. These findings indicate that both the rotator cable and the rotator crescent play crucial roles as tension transmitters for the deep regions of the rotator cuff muscles. This information could have important implications for developing anatomically relevant repair techniques and enhancing rehabilitation protocols.

Muscle belly ratio is the most suitable estimate of the activity of the torn supraspinatus muscle.

Background: A torn rotator cuff muscle deteriorates over time leading with an increase in muscle atrophy and fatty infiltration. There are several clinical assessments for evaluating the atrophy of the torn supraspinatus muscle. However, it is unclear which approach can more accurately estimate the activity of the torn supraspinatus muscle. The purpose of this study was to determine which magnetic resonance imaging-based muscle atrophy imaging assessment currently implemented in the clinical setting accurately estimates the activity of the torn supraspinatus muscle. Methods: Forty patients who were diagnosed with a rotator cuff tear and were candidates for repairs were selected for this study. Cross-sectional area, occupation ratio, and tangent sign were analyzed on T1-weighted oblique sagittal plane magnetic resonance images in which the scapular spine leads to the Y-section. Muscle belly ratio of the supraspinatus muscle was analyzed by calculating the ratio of the width of the muscle belly to the distance from the greater tubercle to the proximal end of the muscle on T1-weighted coronal plane magnetic resonance imaging images. Fatty infiltration was evaluated using the Goutallier classification system. Tear size was obtained intraoperatively by measuring the width and length of the tear and classified based on the Cofield's classification. To assess activity of the torn supraspinatus muscle, participants were first instructed to sit on a chair with the affected arm resting on a table and the shoulder abducted to 60° in the scapular plane with neutral rotation. Elasticity of the supraspinatus muscle belly was then obtained at rest and during isometric contraction using with real-time tissue elastography. Muscle activity, a surrogate for contractility, was defined as the difference between the elasticities measured at rest and during isometric contraction. A stepwise multiple regression analysis was used to investigate independent factors, such as sex, tear width, cross-sectional area, occupation ratio, tangent sign, and muscle belly ratio, related to muscle activity. Results: Stepwise multiple regression analysis (R2 = 0.522, P < .001) revealed that supraspinatus muscle activity was significantly correlated with muscle belly ratio (ß = 0.306, P = .044) and Goutallier stage (ß = -0.490, P = .002). Conclusion: Estimations of muscle belly ratio are most suitable for assessing the activity of a torn supraspinatus muscle compared to other clinical measurements.

Moment arms of the anatomical subregions of the rotator cuff muscles during shoulder rotation.

BACKGROUND: Rotator cuff muscles are responsible for humeral rotation. Moment arms of different regions of these muscles during humeral rotation were analyzed in neutral and abducted positions. METHODS: In eight cadaveric shoulders, subregions of the rotator cuff muscles were identified and their excursion during humeral rotation was measured in neutral and abducted positions from an internal rotation of 30° to an external rotation of 45°, with 15° increments, using a 3-D digitizing system. Statistical tests were used to assess differences between subregions within a single muscle. FINDINGS: The posterior-deep subregion of the supraspinatus muscle had greater moment arms compared to the anterior-superficial and anterior-middle subregions in both positions (p < 0.001). The middle and inferior subregions of the infraspinatus muscle and the teres minor muscle showed differences in moment arms compared to the superior region in an abducted position (p < 0.042). The superior subregion of the subscapularis muscle showed differences in moment arms compared to the middle and inferior subregions in an abducted position (p < 0.001). INTERPRETATION: The posterior-deep subregion of the supraspinatus muscle behaved similar to the infraspinatus muscle, as an external rotator. The anterior-superficial and anterior-middle subregions of the supraspinatus muscle showed a biphasic behavior during rotation at a neutral position, but acted as pure external rotators during rotation at an abducted position. Inferior subregions of the infraspinatus and subscapularis muscles showed the largest moment arms compared to superior subregions. These findings support distinct functional roles of the rotator cuff muscle subregions.

The functional role of the supraspinatus and infraspinatus muscle subregions during forward flexion: a shear wave elastography study.

Background: Knowledge of the morphological and functional differences in the anatomic subregions of the supraspinatus (SSP) and infraspinatus (ISP) muscles during forward flexion will provide useful information in the management of shoulder joint disorders. The purpose of this study was to investigate whether the SSP and ISP muscle subregions exhibit independent roles during forward flexion of the shoulder joint. Methods: Eight healthy male volunteers without any restriction in their shoulder joints were recruited for this study. Participants were instructed to sit on a chair with their back against the backrest. Shear modulus (kPa) was measured as a surrogate for muscle stiffness using shear wave elastography on the SSP and ISP muscle subregions. Active measurements of the nondominant arm were obtained during isometric contraction at a neutral position and every 15° intervals from 30° to 150° during forward flexion. Friedman test and Dunn's post hoc test were used to evaluate differences in measurement outcomes among angles during forward flexion in each muscle subregion. Results: Active stiffness outcomes of the anterior-middle subregion of the SSP muscle during forward flexion increased from 30° up to 45°, reaching a value of 182.4 ± 32.1 kPa (P < .001). Stiffness of the anterior-superficial subregion of the SSP muscle was highest at 30° (125.0 ± 20.6 kPa; P < .019) and linearly decreased up to 105° with increasing shoulder angle position. Stiffness of the superior, middle, and inferior subregions of ISP muscle presented a mountain-shaped trend, with peaks of 99.9 ± 23.5 kPa at 90° (P < .013), 144.2 ± 11.2 kPa at 90° (P < .013), and 122.9 ± 27.9 kPa at 105° (P < .007), respectively. Finally, the stiffness outcomes of the pectoralis major and anterior region of the deltoid muscles showed a mountain-shaped trend with peaks of 89.4 ± 23.5 kPa at 60° (P < .007) and 176.7 ± 22.9 kPa at 90° (P < .026), respectively. Conclusions: The SSP and ISP muscle subregions play a significant role during active forward flexion motion. While closely overlapped, the activity of the muscle subregions changed during the forward flexion motion range, starting with an active anterior-superficial subregion of the SSP muscle at the initial range of motion and an active inferior subregion of the ISP muscle toward midrange of motion. The SSP and ISP subregions did not demonstrate independent functional behavior during forward flexion.

Influence of fat infiltration, tear size, and post-operative tendon integrity on muscle contractility of repaired supraspinatus muscle.

BACKGROUND: The purpose of this study was to evaluate the effect of fat infiltration, tear size, and post-operative tendon integrity, on post-operative contractility. METHODS: Thirty-five patients who underwent rotator cuff repair were included. The fat infiltration, tear size, and post-operative tendon integrity were evaluated by Goutallier stage, Cofield classification, and Sugaya classification, respectively. The muscle elasticity at rest and at contraction was assessed by real-time tissue elastography pre- and one-year post-operatively. We defined the difference in elasticity between at rest and at contraction as the activity value which reflects muscle contractility. RESULTS: The activity value in patients with Sugaya Type I tended to increase regardless of Cofield classification, whereas those with Sugaya Type III and IV tended to decrease. While the activity value in the patients classified as stage 1 and Type I tended to increase, patients classified as stage 2 showed decreased or constant in contractility even in those subjects classified as Type I. Stepwise multiple regression analysis showed both pre- (p = 0.004, r = -0.47) and post-operative activity values (p = 0.022, r = -0.39) to be significantly correlated only with the Goutallier stage. CONCLUSION: Multiple regression analysis indicated only the Goutallier stage was a significant independent factor for contractility of the supraspinatus muscle. Supraspinatus muscle contractility in patients classified as Types III and IV based on the Sugaya classification tended to decrease post-operatively, while patients whose contractility increased post-operatively were characterized by having a Type I tendon integrity.

Elastographic Region of Interest Determination for Muscle with Fat Infiltration.

PURPOSE: Ultrasound elastography has been used to evaluate the skeletal muscle stiffness as a biomarker for sarcopenia assessment. However, there is no consensus with respect to the size and location of the region of interest in assessing such fat infiltrated muscle. The objective of this study was to determine which cross-sectional area should be measured in torn disuse muscle with fat infiltration to accurately measure muscle activity using real-time tissue elastography (RTE). METHODS: Twenty-seven patients, whose rotator cuff muscle with torn tendon was successfully repaired, were followed by programmed rehabilitation. RTE measurements of the supraspinatus muscle were obtained during muscle contraction before and one-year after surgery so that the activity value was defined as the difference between elastography measurements at rest and elastography measurements during contraction. Given that the patients with successfully repaired and completed rehabilitation showed an increased activity value, the sensitivity for three regions of interest; posterior portion of the anterior-middle subregion (AM-p), anterior region (AR), and whole cross-sectional area of the supraspinatus (whole) were compared with the number of patients showing an increase in activity values as sensitivity analysis. RESULTS: The sensitivity showing an increase in activity values was 74.1% for the AM-p area, 70.4% for the AR area, and 81.5% for the whole area. Intraclass correlation coefficient1,3 was 0.87-0.97 for the AM-p area, 0.88-0.98 for the AR area and 0.92-0.99 for the whole area. CONCLUSION: The whole cross-sectional area is suitable to measure muscle activity in muscle with fat infiltration. The results in this study will provide some beneficial information when ultrasound elastography is used for the assessment of sarcopenia muscle with fat infiltration.

Shoulder scaption is dependent on the behavior of the different partitions of the infraspinatus muscle.

PURPOSE: The purpose of this study was to investigate if the three partitions (superior, middle, and inferior partitions) of the infraspinatus muscle previously described in anatomical studies will present different behavior during scapular plane abduction (scaption) as described using shear-wave elastography, especially during initial range of motion. METHODS: Eight volunteers held their arm against gravity 15° intervals from 30° to 150° in scaption. Shear-wave elastography was implemented at each position to measure shear modulus at rest and during muscle contraction, as a surrogate for muscle stiffness, of each partition. Muscle activity was defined as the difference in stiffness values between the resting positions and those during muscle contraction (ΔE = stiffness at contraction-stiffness at rest). RESULTS: The activity value for the middle partition was 25.1 ± 10.8 kPa at 30° and increased up to 105° (52.2 ± 10.8 kPa), with a subsequent decrease at larger angle positions (p < .001). The superior partition showed a flatter and constant behavior with smaller activity values except at higher angles (p < .001). Peak activity values for the superior partition were observed at 135° (23.0 ± 12.0 kPa). Increase activity for inferior partition began at 60° and showed a peak at 135° (p < .001; 32.9 ± 13.8 kPa). CONCLUSION: Stiffness measured using shear-wave elastography in each partition of the infraspinatus muscle demonstrated different behavior between these partitions during scaption. The middle partition generated force throughout scaption, while the superior and inferior partitions exerted force at end range.