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.

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.

Strain distribution in the bursal rotator cuff based on whole-muscle and muscle subregion-specific loading: A cadaveric study.

Rotator cuff (RC) tears are common injuries leading to significant dysfunction of the shoulder. Rotator cuff tears alter tension and strain in muscles and tendons. Anatomical studies demonstrated that rotator cuff muscles are comprised of anatomical subregions. However, the strain distribution within the rotator cuff tendons generated from the tension from each anatomical subregion is unknown. We hypothesized that subregions would present distinct 3-dimensional (3D) strain distributions within the rotator cuff tendons, and that the anatomical insertion configuration of the supraspinatus (SSP) and infraspinatus (ISP) tendons might dictate strain, thus tension, transmission. 3D-strains in the bursal side of the SSP and ISP tendons of eight fresh-frozen cadaveric intact shoulders were obtained by applying tension on the whole SSP and ISP muscles, and on their subregions using an MTS system. Strains in the anterior region of the SSP tendon were higher than in the posterior region with whole-SSP anterior-region (p < 0.05) and whole-SSP muscle loading. Higher strains were observed in the inferior half of the ISP tendon with whole-ISP muscle (p < 0.05), middle-subregion (p < 0.01), and superior-subregion (p < 0.05) loading. Tension generating from the posterior-region of the SSP was primarily transmitted to the middle facet via an overlap between the SSP and ISP tendons insertions, while the anterior-region mainly distributed its tension into the superior facet. Tension generating from the middle and superior-regions of the ISP was distributed into the inferior portion of the ISP tendon. These results emphasize the importance of the distinct anatomical subregions of the SSP and ISP muscles in distributing the tension to the tendons.

Moment arms from the anatomical subregions of the rotator cuff muscles during flexion.

Rotator cuff (RC) muscles act as force couples to stabilize the glenohumeral joint and enable shoulder motion. We investigated the moment arms of anatomical subregions of the supraspinatus (SSP), infraspinatus (ISP), subscapularis (SSC), and the teres minor muscles during flexion. Eight fresh-frozen cadaveric shoulders were obtained and the anatomical subregions of the RC muscles were identified. Sutures were secured for each subregion at the musculotendinous junction and excursion during flexion from 30° to 90° at 10° increments was measured using a 3-D digitizing system. Kruskal-Wallis test followed by the Bonferroni post-hoc test was used to assess differences from subregions within a single muscle. There were significant differences in moment arms between the subregions from each RC muscle (P < 0.001). The anterior-superficial and -middle subregions of the SSP muscle presented positive (flexor) and decreasing moment arms with increasing flexion. The posterior-deep subregion showed moment arms with positive but decreasing values up to 65°, and negative (extensor) moment arms at larger angles. Subregions from the ISP showed positive and almost constant moment arms throughout range of motion, while the teres minor presented negative and almost unaltered moment arms. The superior and middle subregions of the SSC showed positive, but decreasing, moment arms with increasing angles up to 75° flexion, with negative moment arms towards end-range. The inferior subregion presented negative moment arms throughout flexion. Our results indicated that the posterior deep subregion of SSP muscle seems to act as a flexor at early range and as a stabilizer at mid-to-end range of flexion.

Stiffness of the infraspinatus and the teres minor muscles during shoulder external rotation: An in-vitro and in-vivo shear wave elastography study.

BACKGROUND: A better understanding of the morphological and functional differences in the anatomical sub-regions of the rotator cuff muscles is critical so that appropriate surgical and rehabilitation methodologies can be implemented in patients with shoulder-related injuries. The purpose of the current study was to develop a comprehensive imaging protocol using shear-wave elastography for the infraspinatus and teres minor muscles, and investigate differences in elastic properties of three distinct infraspinatus muscle sub-regions and of the teres minor muscle. METHODS: First, we developed a protocol for probe positioning for both muscles using three cadaveric shoulders. Second, we evaluated in-vivo elastic properties [passive and active stiffness (kPa)] and excursion (mm) outcomes from these muscles during shoulder external rotation. FINDINGS: Elastic properties were significantly different among the infraspinatus muscle sub-regions and teres minor muscle. Passive stiffness decreased with increasing rotation angles except for the middle sub-region of the infraspinatus muscle which showed a decreased up to mid-range followed by an increment towards the end-range. Overall, active stiffness of the infraspinatus muscle and teres minor muscle decreased with increasing rotation angles, while that of the middle sub-region increased up to mid-range, and decreased at the end-range. INTERPRETATION: Distinct characteristics of the infraspinatus and teres minor muscles, and more importantly, of the individual sub-regions within the infraspinatus muscle call for an in-depth analysis of their morphological and functional differences. Special attention should be put into these sub-regions when performing surgical and rehabilitation procedures for patients with shoulder-related injuries.

A quantitative alternative to the Goutallier classification system using Lava Flex and Ideal MRI techniques: volumetric intramuscular fatty infiltration of the supraspinatus muscle, a cadaveric study.

OBJECTIVE: The Goutallier classification system is the most commonly used method for grading intramuscular fatty infiltration in rotator cuff tears. This grading system presents low inter-observer reliability and an inability to provide quantitative and repeatable outcomes for intramuscular fat. We determined the correlation and reliability of two methods, the Lava Flex and Ideal IQ MRI techniques, in quantifying volumetric intramuscular fat, while also comparing to the Goutallier method. MATERIALS AND METHODS: The supraspinatus muscles of seventeen cadaveric shoulders were scanned using the Lava Flex and Ideal IQ MRI imaging protocols. Histological analysis was performed on the same muscles. Agreement, reliability, and correlation analyses were performed to compare all outcomes. RESULTS: The Lava Flex protocol took an average of ~ 4 min, while the Ideal IQ required about ~ 11 min to complete. Bland-Altman analysis showed good agreement between the Lava Flex and Ideal IQ [LOA (- 0.10 and 0.05)], and ICC analyses showed excellent reliability (ICC (1,1) 0.948; ICC (2,1) 0.947). There was a 91% correlation between the Lava Flex and Ideal IQ MR protocols. Weighted Kappa analysis between histology and the Goutallier classification showed fair-to-moderate agreement. DISCUSSION: The Lava Flex technique, taking about 30% of the acquisition time, may prevent motion artifacts in outcomes associated with the longer Ideal IQ technique. However, potential magnetic field inhomogeneities should be considered. The Lava Flex technique may be a faster and valid alternative to the Goutallier classification system.