Abduction moment arm of transposed subscapularis tendon

The moment arms of the muscles spanning the glenohumeral joint: a systematic review

The moment arm of a muscle represents its leverage or torque-producing capacity, and is indicative of the role of the muscle in joint actuation. The objective of this study was to undertake a systematic review of the moment arms of the major muscles spanning the glenohumeral joint during abduction, flexion and axial rotation. Moment arm data for the deltoid, pectoralis major, latissimus dorsi, teres major, supraspinatus, infraspinatus, subscapularis and teres minor were reported when measured using the geometric and tendon excursion methods. The anterior and middle sub-regions of the deltoid had the largest humeral elevator moment arm values of all muscles during coronal- and scapular-plane abduction, as well as during flexion. The pectoralis major, latissimus dorsi and teres major had the largest depressor moment arms, with each of these muscles exhibiting prominent leverage in shoulder adduction, and the latissimus dorsi and teres major also in extension. The rotator cuff muscles had the largest axial rotation moment arms regardless of the axial position of the humerus. The supraspinatus had the most prominent elevator moment arms during early abduction in both the coronal and scapular planes as well as in flexion. This systematic review shows that the rotator cuff muscles function as humeral rotators and weak humeral depressors or elevators, while the three sub-regions of the deltoid behave as substantial humeral elevators throughout the range of humeral motion. The pectoralis major, latissimus dorsi and teres major are significant shoulder depressors, particularly during abduction. This study provides muscle moment arm data on functionally relevant shoulder movements that are involved in tasks of daily living, including lifting and pushing. The results may be useful in quantifying shoulder muscle function during specific planes of movement, in designing and validating computational models of the shoulder, and in planning surgical procedures such as tendon transfer surgery.

Surgical Simulations Based on Limited Quantitative Data: Understanding How Musculoskeletal Models Can Be Used to Predict Moment Arms and Guide Experimental Design

The utility of biomechanical models and simulations to examine clinical problems is currently limited by the need for extensive amounts of experimental data describing how a given procedure or disease affects the musculoskeletal system. Methods capable of predicting how individual biomechanical parameters are altered by surgery are necessary for the efficient development of surgical simulations. In this study, we evaluate to what extent models based on limited amounts of quantitative data can be used to predict how surgery influences muscle moment arms, a critical parameter that defines how muscle force is transformed into joint torque. We specifically examine proximal row carpectomy and scaphoid-excision four-corner fusion, two common surgeries to treat wrist osteoarthritis. Using models of these surgeries, which are based on limited data and many assumptions, we perform simulations to formulate a hypothesis regarding how these wrist surgeries influence muscle moment arms. Importantly, the hypothesis is based on analysis of only the primary wrist muscles. We then test the simulation-based hypothesis using a cadaveric experiment that measures moment arms of both the primary wrist and extrinsic thumb muscles. The measured moment arms of the primary wrist muscles are used to verify the hypothesis, while those of the extrinsic thumb muscles are used as cross-validation to test whether the hypothesis is generalizable. The moment arms estimated by the models and measured in the cadaveric experiment both indicate that a critical difference between the surgeries is how they alter radial-ulnar deviation versus flexion-extension moment arms at the wrist. Thus, our results demonstrate that models based on limited quantitative data can provide novel insights. This work also highlights that synergistically utilizing simulation and experimental methods can aid the design of experiments and make it possible to test the predictive limits of current computer simulation techniques.

Differences in scapular orientation, subacromial space and shoulder pain between the full can and empty can tests

BACKGROUND

The empty and full can arm positions are used as diagnostic tests and in therapeutic exercise programs for patients with subacromial impingement syndrome. The adverse effects of these arm positions on the rotator cuff have not been fully described. The purpose of this investigation was to compare the acromio-humeral distance, three-dimensional scapular position, and shoulder pain during maximum isometric contractions in the empty and full can arm positions.

METHODS

Subjects with subacromial impingement syndrome (n=28) and a matched control group without shoulder pain (n=28) participated. Acromio-humeral distance, scapular/clavicular positions and shoulder pain were measured during maximal isometric contractions in each position.

FINDINGS

No difference was found in acromio-humeral distance (P=0.314) between the arm positions or between the groups (P=0.598). The empty can position resulted in greater scapular upward rotation (P<0.001, difference=4.9°), clavicular elevation (P<0.001, difference=2.7°), clavicular protraction (P=0.001, difference=2.5°) and less posterior tilt (P<0.001, difference=3.8°) than the full can position. No differences in the scapular positions were found between the groups. Positive correlations were seen between the scapular positions in the control and not in the subacromial impingement group.

INTERPRETATION

Our results did not show a difference in acromio-humeral distance between the arm positions or groups, indicating that the kinematic differences between the positions are not associated with altered acromio-humeral distance. The increased pain in the EC position might be due to the lack of an association amongst the scapular positions rather than the deficiency of a single scapular motion.

Biomechanical effectiveness of different types of tendon transfers to the shoulder for external rotation

BACKGROUND

Our purpose was to determine and compare the external rotation moment arm (ERMA) of the latissimus dorsi (LD), teres major (TM), and lower trapezius (LT) when transferred to selected locations on the proximal humerus. We hypothesize that the LT transfer has a higher ERMA compared with LD or TM.

MATERIALS AND METHODS

Six fresh frozen cadaveric hemithoraces were used in a novel experimental design. The tendon and joint displacement method was used to calculate ERMA for 6 transfer pairs: LD to superolateral humeral head (SHH), LD to proximal-lateral humeral diaphysis (LHD), TM to SHH, TM to LHD, LT to infraspinatus insertion (ISI), and LT to teres minor insertion (TMI).

RESULTS

Tendon transfer pair had a significant effect on ERMA (P < .001), with a significant interaction effect between tendon transfer and position of the humerus (P < .0001). With the humerus at 0° abduction, the ERMAs of the LT-ISI (28.1 mm) or LT-TMI (22.3 mm) transfers were significantly higher than the ERMAs of LD-SHH (10.6 mm; P = .0001, P = .04) or LD-LHD (6.5 mm; P < .0001, P < .001). Also, ERMAs of LT-ISI and LT-TMI transfers were significantly higher than ERMA of TM-LHD (10.4 mm; P = .0001, P = .03).

CONCLUSIONS

Shoulder external rotation tendon transfers differ in effectiveness and may be affected by arm position. LT potentially results in superior restoration of shoulder external rotation with the arm at the side compared with LD and should be considered as a potential tendon transfer to restore external rotation in selected patients.

Latissimus dorsi transfer to restore external rotation with reverse shoulder arthroplasty: a biomechanical study

In patients with pseudoparesis of the shoulder resulting from irreparable rotator cuff tears, reverse shoulder arthroplasty (RSA) can restore active elevation, but external rotation remains less predictable. Latissimus dorsi transfer (LDT) has been shown to be effective in restoring external rotation in patients with posterosuperior tears of the rotator cuff. The aim of this study is to determine the capacity of the LDT to restore external rotation in combination with RSA and to investigate the mechanical advantage produced by 3 different insertion sites. A biomechanical model was created using a reverse total shoulder prosthesis with 3 different transfer insertions. Moment arms were measured for 2 static positions and 1 motion of the humerus. The moment arm analysis showed that LDT can improve active external rotation in the setting of a reverse prosthesis. An insertion site on the posterior side of the greater tuberosity (adjacent to the teres minor insertion) produced a greater external rotation moment arm.

Effects of rotator cuff tears on muscle moment arms: a computational study

Rotator cuff tears cause morphologic changes to cuff tendons and muscles, which can alter muscle architecture and moment arm. The effects of these alterations on shoulder mechanical performance in terms of muscle force and joint strength are not well understood. The purpose of this study was to develop a three-dimensional explicit finite element model for investigating morphological changes to rotator cuff tendons following cuff tear. The subsequent objectives were to validate the model by comparing model-predicted moment arms to empirical data, and to use the model to investigate the hypothesis that rotator cuff muscle moment arms are reduced when tendons are divided along the force-bearing direction of the tendon. The model was constructed by extracting tendon, cartilage, and bone geometry from the male Visible Human data set. Infraspinatus and teres minor muscle and tendon paths were identified relative to the humerus and scapula. Kinetic and kinematic boundary conditions in the model replicated experimental protocols, which rotated the humerus from 45 degrees internal to 45 degrees external rotation with constant loads on the tendons. External rotation moment arms were calculated for two conditions of the cuff tendons: intact normal and divided tendon. Predicted moment arms were within the 1-99% confidence intervals of experimental data for nearly all joint angles and tendon sub-regions. In agreement with the experimental findings, when compared to the intact condition, predicted moment arms were reduced for the divided tendon condition. The results of this study provide evidence that one potential mechanism for the reduction in strength observed with cuff tear is reduction of muscle moment arms. The model provides a platform for future studies addressing mechanisms responsible for reduced muscle force and joint strength including changes to muscle length-tension operating range due to altered muscle and tendon excursions, and the effects of cuff tear size and location on moment arms and muscle forces.

Tendon excursion and gliding: Clinical impacts from humble concepts

As integral components of the musculoskeletal system, the primary function is transmission of muscle forces to the skeletal system. Proper excursion and gliding of the tendon determine the efficiency of this function. Studies of the tendon excursion and gliding based on two simple mechanical concepts have resulted in several significant clinical implications.