Contributions of the individual muscles of the shoulder to glenohumeral joint stability during abduction

The aim of this study was to determine the relative contributions of the deltoid and rotator cuff muscles to glenohumeral joint stability during arm abduction. A three-dimensional model of the upper limb was used to calculate the muscle and joint-contact forces at the shoulder for abduction in the scapular plane. The joints of the shoulder girdle-sternoclavicular joint, acromioclavicular joint, and glenohumeral joint-were each represented as an ideal three degree-of-freedom ball-and-socket joint. The articulation between the scapula and thorax was modeled using two kinematic constraints. Eighteen muscle bundles were used to represent the lines of action of 11 muscle groups spanning the glenohumeral joint. The three-dimensional positions of the clavicle, scapula, and humerus during abduction were measured using intracortical bone pins implanted into one subject. The measured bone positions were inputted into the model, and an optimization problem was solved to calculate the forces developed by the shoulder muscles for abduction in the scapular plane. The model calculations showed that the rotator cuff muscles (specifically, supraspinatus, subscapularis, and infraspinatus) by virtue of their lines of action are perfectly positioned to apply compressive load across the glenohumeral joint, and that these muscles contribute most significantly to shoulder joint stability during abduction. The middle deltoid provides most of the compressive force acting between the humeral head and the glenoid, but this muscle also creates most of the shear, and so its contribution to joint stability is less than that of any of the rotator cuff muscles.     

Identification of scapular kinematics using surface mapping: a validation study

The immediate goal of this study was to develop and validate a noninvasive, computational surface mapping approach for measuring scapular kinematics by using available motion capture technology in an innovative manner. The long-term goal is to facilitate clinical determination of the role of the scapula in children with brachial plexus birth palsy (BPBP). The population for this study consisted of fourteen healthy adults with prominent scapulae. Subject-specific scapular templates were created using the coordinates of five scapular landmarks obtained from palpation with subjects seated and arms relaxed in a neutral position. The scapular landmarks were re-palpated and their locations recorded in the six arm positions of the modified Mallet classification. The six Mallet positions were repeated with approximately 300 markers covering the scapula. The markers formed a surface map covering the tissue over the scapula. The scapular template created in the neutral position was iteratively fit to the surface map of each trial, providing an estimate of the orientation of the scapula. These estimates of scapular orientation were compared to the known scapular orientation determined from the scapular landmarks palpated in each Mallet position. The magnitude of the largest mean difference about an anatomical axis between the two measures of scapular orientation was 3.8° with an RMS error of 5.9°. This technique is practical for populations with visibly prominent scapulae (e.g., BPBP patients), for which it is a viable alternative to existing clinical methods with comparable accuracy.     

A comparison of the ontogeny of shape variation in the anthropoid scapula: functional and phylogenetic signal

This article compares ontogenetic shape variation in the scapula of 17 anthropoid species using three-dimensional landmark-based geometric morphometrics. These data are used to investigate functional and phylogenetic signal in the major components of scapular variation and to evaluate the degree to which postnatal growth contributes to interspecific differences in shape. Results indicate that the shape of the infant and adult scapula is primarily associated with positional behavior (e.g., orthograde suspensory nonquadrupeds versus pronograde quadrupeds), but within this functional structure there is phylogenetic signal, particularly at infant stages. Growth most closely correlates with infant/adult shape and locomotor function. These results suggest that the shape of the infant scapula drives the pattern of postnatal scapular growth and adult morphology. As such, variation in postnatal growth is not the primary source of interspecific variation in adult shape. Instead, interspecific differences in scapular morphology are hypothesized to be the result of selection for variation in embryonic developmental processes that affect shape.     

Convergences in Scapula Morphology among Small Cursorial Mammals: An Osteological Correlate for Locomotory Specialization

Small cursorial mammals, such as lagomorphs, elephant shrews, and the more cursorial caviomorph rodents, share both the similar locomotor gait of rapid half-bounding and a similar scapula anatomy of a long, slender, caudally projecting metacromion process. This scapular morphology is also present in some notoungulates (extinct endemic South American ungulates), in rabbit-like taxa such as Propachyruchos. In the rabbit Oryctolagus this elongated metacromion process serves to increase the moment arm of the acromiotrapezius and levator scapulae ventralis muscles, which we propose may aid in scapula stabilization and resisting ground reaction forces during the landing phase onto a single forelimb in half-bounding. A long, slender metacromion process is thus an osteological correlate of locomotor specialization, that of rapid half-bounding in small to medium-sized mammals.     

A Comparison of Qualitative and Quantitative Methodological Approaches to Characterizing the Dorsal Side of the Scapula in Hominoidea and Its Relationship to Locomotion

Hominoidea have adapted to various forms of locomotion, each of which has specific requirements that are reflected in the shape of the scapula. We compared several qualitative and quantitative methods for characterizing the dorsal side of the scapula to detect morphological differences that reflect the adaptations of the scapula to locomotor behaviors. Our sample included 55 specimens of Hominoidea, representing five genera, including Homo, focusing specifically on the relative sizes of the scapular supraspinous and infranspinous fossae. In addition, we weighed the supraspinatus and infraspinatus muscles of 23 of the specimens to examine the feasibility of extrapolating muscle characteristics from osteological data. Our findings confirmed that the five genera exhibit significant differences in the relative size of the supraspinous and the infraspinous fossae that are related to their forms of locomotion. The supraspinous fossa was relatively small in Homo and Pongo but large in Pan, Gorilla, and Hylobates. The analysis of muscle weights showed that a substantial amount of information about soft tissues is lost in osteological analyses, leading us to recommend caution when drawing conclusions regarding forms of locomotion based only on osteological analyses.     

Difference between palpation and optoelectronics recording of scapular motion

The aim of this study is to determine the errors of scapular localisation due to skin relative to bone motion with an optoelectronic tracking system. We compared three-dimensional (3D) scapular positions obtained with skin markers to those obtained through palpation of three scapular anatomical landmarks. The scapular kinematics of nine subjects were collected. Static positions of the scapula were recorded with the right arm elevated at 0°, 40°, 80°, 120° and 160° in the sagittal plane. Palpation and subsequent digitisation of anatomical landmarks on scapula and thorax were done at the same positions. Scapular 3D orientation was also computed during 10 repeated movements of arm elevation between 0° and 180°. Significant differences in scapular kinematics were seen between static positions and palpation when considering anterior/posterior tilt and upward/downward rotation at angles over 120° of humeral elevation and only at 120° for internal/external rotation. There was no significant difference between positions computed during static positions and during the movement for the three scapular orientations. A rotation correction model is presented in order to reduce the errors between static position and palpation measurement.     

Geometric morphometrics of the scapula of South American caviomorph rodents (Rodentia: Hystricognathi): Form, function and phylogeny

The scapula of the ecomorphologically diverse South American caviomorph rodents was studied through geometric morphometric techniques, using landmarks and semilandmarks to capture the shape of this complex morphological structure. Representatives of 33 species from all caviomorph superfamilies, as well as Hystrix cristata for comparisons, were analyzed. Marked differences in scapular shape were found among the major caviomorph lineages analyzed, particularly in the shape and length of the scapular spine and development of the great scapular notch. Shape differences were not influenced by body size, and only partially influenced by locomotor mode. Thus, at this scale of analysis, phylogenetic history seems to be the strongest factor influencing scapular shape. The scapular shape of erethizontids, chinchillids and Cuniculus paca could represent the less specialized state with respect to the highly differentiated scapula of octodontoids and most cavioids. In this sense, the characteristic scapular morphologies of octodontoids and cavioids could reflect particular functional capabilities and constraints associated with the evolution of prevalent locomotor modes within each lineage.     

Tracking the scapula using the scapula locator with and without feedback from pressure-sensors: A comparative study

BackgroundThe scapula locator method has associated intra-observer and inter-observer errors caused by the dependency on the observer to locate the scapular landmarks. The potential effect of the pressures applied by the observer on the measured scapular kinematics when this method is used has also been overlooked so far. The aim of this study was to investigate the effect of using feedback on the pressures applied on the scapula using the locator on the intra-observer and inter-observer reliabilities of the method as well as on the kinematics obtained using this method.MethodsThree observers tracked the scapular motion of the dominant shoulder of each subject using the locator with no reference to pressure-feedback for three trials of bilateral elevation in the scapular plane and using the locator with pressure-feedback for three other trials. Variations between the measurements obtained were used to calculate the intra-observer errors and variations between the measurements obtained by the three observers for the same subject were used to calculate inter-observer errors. Repeated-measures ANOVA tests were used to look at differences between the two methods in terms of intra-observer and inter-observer errors and scapular kinematics.FindingsUsing pressure-feedback reduced the intra-observer errors but had no effect on the inter-observer errors. Different scapular kinematics was measured using the two methods.InterpretationsPressure-feedback improves the reliability of the scapula locator method. Differences in the scapular kinematics suggest that unregulated pressures have an effect on the physiological scapular motion.     

Musculoskeletal scapular correlates of plantigrade and acrobatic positional activities in Papio cynocephalus anubis and Macaca fascicularis

This study investigates several musculoskeletal scapular correlates of quadrupedal behavior in predominantly plantigrade, Papio cynocephalus anubis and acrobatic, less plantigrade, Macaca fascicularis. Positional behaviors differ in these two primates reflecting the amount of time spent emphasizing one or the other of these captive behavioral regimes. Stresses associated with particular patterns of behavior result in different scapular morphologies. Papio exhibits a significantly broader scapula with a more dorsoventrally curved blade accompanying an enlarged teres major muscle. The supraspinous fossa is wider in Papio, while the infraspinous/teres major attachment is wider in Macaca. Angular measurements reflect the breadth dimension of the various fossae. These results utilize two species of primates to extend a methodology developed in previous studies and to suggest that a predictable interdependence occurs between scapular dimensions and certain behavioral activities.     

Load transfer across the scapula during humeral abduction

Stress analysis in the individual parts of the scapula under normal physiological conditions is necessary to understand the load transfer mechanism, its relation with morphology of bone and to analyse the deviations in stress patterns due to implantation of the glenoid. The purpose of this study was to obtain stress distribution in the scapula during abduction of the arm and to obtain a qualitative estimate of the function of coracoacromial ligament. An accurate three-dimensional (3D) finite element (FE) model of the natural scapula has been developed for this purpose, using computed tomography data and shell-solid modelling approach. The model was experimentally validated. A musculoskeletal shoulder model of forces that calculates all muscle, ligament and joint reaction forces, in six load cases (30-180 degrees) during unloaded humeral abduction was used as applied loading conditions for the 3D FE model. High tensile and compressive stresses (15-60 MPa) were generated in the thick bony ridges of the scapula, like the scapular spine, lateral border, glenoid and acromion. High compressive stresses (45-58 MPa) were evoked in the glenoid and at the connection of glenoid-scapular spine-infraspinous fossa. The stresses in the infraspinous fossa and supraspinous fossa were low (0.05-15 MPa). These results indicated that the transfer of major muscle and joint reaction take place predominantly through the thick bony ridges, whereas the fossa area act more as attachment sites of large muscles. During humeral abduction, coracoacromial ligament was stretched, and presumably will be under tension.     

Ontogenesis of the scapula in marsupial mammals, with special emphasis on perinatal stages of didelphids and remarks on the origin of the therian scapula

The development of the scapula was studied in embryonic and postnatal specimens of Monodelphis domestica and perinatal specimens of Philander opossum, Caluromys philander, and Sminthopsis virginiae using histological sections and 3D reconstructions. Additionally, macerated skeletons of postnatal M. domestica were examined. This study focused on the detachment of the scapulocoracoid from the sternum and on the acquisition of a supraspinous fossa, a supraspinatus muscle, and a scapular spine, all these events associated with the origin of the therian shoulder girdle. In none of the specimens is there a continuity of the cartilaginous scapulocoracoid with the sternum, even though the structures are in close proximity, especially in S. virginiae. At birth, the first rib laterally presents a pronounced boss that probably contacts the humerus during certain movements. Only the acromial portion of the scapular spine, which originates from the anterior margin of the scapular blade, is preformed in cartilage. The other portion is formed by appositional bone ("Zuwachsknochen"), which expands from the perichondral ossification of the scapula into an intermuscular aponeurosis between the supra- and infraspinous muscles. This intermuscular aponeurosis inserts more or less in the middle of the lateral surface of the developing scapula. Thus, the floor of the supraspinous fossa is present from the beginning of scapular development, simultaneously with the infraspinous fossa. The homology of the therian spine with the anterior border of the sauropsid and monotreme scapula is questioned. We consider the dorsal portion (as opposed to the ventral or acromial portion) of the scapular spine a neomorphic structure of therian mammals.     

Effects of scapular upward rotation exercises on alignment of scapula and clavicle and strength of scapular upward rotators in subjects with scapular downward rotation syndrome

The purpose of this study was to investigate the effects of a 6-week scapular upward rotation exercise (SURE) on scapular and clavicular alignment and scapular upward rotators strength in subjects with scapular downward rotation syndrome (SDRS). Seventeen volunteer subjects with SDRS were recruited from university populations. The alignment of the scapula and clavicle was measured using radiographic analysis and compared in subjects before and after a 6-week self-SURE program. A hand-held dynamometer was used to measure the strength of the scapular upward rotators. The subjects were instructed how to perform the self-SURE program at home. The 6-week self-SURE program was divided into two sections (the first section with non-resistive SURE during weeks 1–3, and the second section with resistive SURE using thera-band during weeks 4–6). The significance of the difference between pre- and post-program was assessed using a paired t-test, with the level of statistical significance set at p < 0.05. Significant differences between pre- and post-program were found for scapular and clavicular alignment (p < 0.05). Additionally, the comparison between pre- and post-program measurements of the strength of the scapular upward rotators showed significant differences (p < 0.05). The results of this study showed that a 6-week self-SURE program is effective for improving scapular and clavicular alignment and increasing the strength of scapular upward rotator muscles in subjects with SDRS.     

Double calibration: an accurate, reliable and easy-to-use method for 3D scapular motion analysis

The most recent non-invasive methods for the recording of scapular motion are based on an acromion marker (AM) set and a single calibration (SC) of the scapula in a resting position. However, this method fails to accurately measure scapular kinematics above 90° of arm elevation, due to soft tissue artifacts of the skin and muscles covering the acromion. The aim of this study was to evaluate the accuracy, and inter-trial and inter-session repeatability of a double calibration method (DC) in comparison with SC. The SC and DC data were measured with an optoelectronic system during arm flexion and abduction at different angles of elevation (0-180°). They were compared with palpation of the scapula using a scapula locator. DC data was not significantly different from palpation for 5/6 axes of rotation tested (Y, X, and Z in abduction and flexion), where as SC showed significant differences for 5/6 axes. The root mean square errors ranged from 2.96° to 4.48° for DC and from 6° to 9.19° for SC. The inter-trial repeatability was good to excellent for SC and DC. The inter-session repeatability was moderate to excellent for SC and moderate to good for DC. Coupling AM and DC is an easy-to-use method, which yields accurate and reliable measurements of scapular kinematics for the complete range of arm motion. It can be applied to the measurement of shoulder motion in many fields (sports, orthopaedics, and rehabilitation), especially when large ranges of arm motion are required.     

Functional and historical determinants of shape in the scapula of Xenarthran mammals: Evolution of a complex morphological structure

The mammalian scapula is a complex morphological structure, composed of two ossification plates that fuse into a single structure. Most studies on morphological differentiation in the scapula have considered it to be a simple, spatially integrated structure, primarily influenced by the important locomotor function presented by this element. We used recently developed geometric morphometric techniques to test and quantify functional and phylogenetic influences on scapular shape variation in fossil and extant xenarthran mammals. The order Xenarthra is well represented in the fossil record and presents a stable phylogenetic hypothesis for its genealogical history. In addition, its species present a large variety of locomotor habits. Our results show that approximately half of the shape variation in the scapula is due to phylogenetic heritage. This is contrary to the view that the scapula is influenced only by functional demands. There are large‐scale shape transformations that provide biomechanical adaptation for the several habits (arboreality, terrestriality, and digging), and small scale‐shape transformations (mostly related to the coracoid process) that are not influenced by function. A nonlinear relationship between morphometric and phylogenetic distances indicates the presence of a complex mixture of evolutionary processes acting on shape differentiation of the scapula. J. Morphol. 241:251–263, 1999. © 1999 Wiley‐Liss, Inc.     

Scapular orientation following repetitive prone rowing: Implications for potential subacromial impingement mechanisms

While fatigue of the rotator cuff demonstrably causes superior humeral head migration and concomitant risk of impingement, the relationship between specific muscular fatigue, scapular dyskinesis and impingement risk is less clear. The purpose of this study was to examine changes in scapular orientation following a simulated prone rowing fatiguing protocol that targeted the scapula stabilizing muscles while attempting to alleviate rotator cuff muscular demands. Scapular orientation and muscle activity were collected from participants before and immediately after the fatiguing task. This task fatigued both the stabilizing (upper and middle trapezius, and latissimus dorsi) and rotator cuff (supraspinatus, and infraspinatus) muscles. The upper extremity muscle fatigue pattern caused by the protocol did not elicit any significantly changes in three-dimensional scapular position with all post-fatigue changes being ?1° (p = 0.17–0.58). These results indicated that scapular reorientation is likely not the dominant mechanism of fatigue-induced subacromial impingement development. However, the substantial variability present in the kinematics prevents complete exclusion of scapular dyskinesis as a secondary causal mechanism of impingement.     

New characters for the functional interpretation of primate scapulae and proximal humeri

The study of muscle function in nonhuman primates through the technique of electromyography (EMG) has facilitated the identification of specific functional roles for muscles in particular behaviors. This has led to a more complete understanding of the biomechanics of certain regions of the musculoskeletal system, and should facilitate our ability to identify morphological features useful in the functional interpretation of fossil material. The current paper represents one such investigation of a new set of morphometric characters of the scapula and proximal humerus suggested by EMG analyses of shoulder muscle function. A set of new metric variables were examined on the scapulae and proximal humeri of 25 species of extant anthropoid primates, as well as on casts of scapulae and humeri of three fossil primate taxa. The variables are primarily related to the line of action and attachments of the rotator cuff muscles. The position of the scapular spine, the degree of lateral expansion of the subscapular fossa, the size and shape of the subscapularis insertion facet on the lesser tubercle, and the orientation of the infraspinatus insertion facet on the greater tubercle all appear to successfully sort the extant taxa into locomotor groups. Their appearance on the fossil specimens generally supports previous functional interpretations of each taxon's locomotor abilities based on a variety of other characters, suggesting that these traits are equally applicable to fossil material. © Wiley-Liss, Inc.     

Measurement of scapular kinematics with the moiré fringe projection technique

The aim of this study was to investigate the applicability of the moiré fringe projection technique (MFPT) to quantify the scapular motions relative to the thorax. This system was composed of a LCD projector, a digital photographic camera, and a microcomputer. To automatically obtain the scapular profiles, the phase shifting method was combined with the MFPT. Four fringes were projected on the scapula and four on the reference planes. By the simple subtraction of the reference values from the scapular phase maps, the map due to the moiré fringes could be digitally obtained. After the phase decoding, the tridimensional (3D) profiles were obtained without prior information about the samples and the calculations of the scapular kinematics were carried out using dedicated software. On average, the movements of lateral rotation ranged from ?1.8±6.1° to ?26.5±3.5°; the protraction from 28.4±4.7° to 27.7±6.8°, and the posterior tilt from ?6.4±7.8° to ?21.7±6.1°, during the arm elevation in the scapular plane performed by six healthy subjects. For the test–retest reliability, the intra-class correlation coefficients ranged from 0.92 to 0.997 and the maximum estimated error was 0.8%. The MFPT allowed the scapular 3D measurements to be obtained in a digital and non-invasive manner. The main advantages compared with other existing systems were its ease in implementation, the use of standard optical components, and its possible clinical applications.     

Physiotherapy vs. capsular shift and physiotherapy in multidirectional shoulder joint instability

PurposeThe aim of the study was to compare the kinematic parameters and the on–off pattern of the muscles of patients with multidirectional instability (MDI) treated by physiotherapy or by capsular shift and postoperative physiotherapy before and after treatment during elevation in the scapular plane.ScopeThe study was carried out on 32 patients with MDI of the shoulder treated with physiotherapy, 19 patients with MDI of the shoulder treated by capsular shift and postoperative physiotherapy, and 25 healthy subjects. The motion of skeletal elements was modeled by the range of humeral elevation, scapulothoracic angle and glenohumeral angle, scapulothoracic (ST) and glenohumeral (GH) rhythms, and relative displacement between the rotation centers of the humerus and scapula. The muscle pattern was modeled by the on–off pattern of muscles around the shoulder, which summarizes the activity duration of the investigated muscles.ResultsThe different ST and GH rhythms and the increased relative displacement between the rotation centers of the scapula and the humerus were observed in MDI patients. The physiotherapy strengthened the rotator cuff, biceps brachii, triceps brachii, deltoid muscles, and increase the neuromuscular control of the shoulder joints. Capsular shift and physiotherapy enabled bilinear ST and GH rhythms and the normal relative displacement between the rotation centers of the scapula and humerus to be restored. After surgery and physiotherapy, the duration of muscular activity was almost normal.ConclusionThe significant alteration in shoulder kinematics observed in MDI patients cannot be restored by physiotherapy only. After the capsular shift and postoperative physiotherapy angulation at 60° of ST and GH rhythms, the relative displacement between the rotation centers of the scapula and humerus and the duration of muscular activity were restored.     

Objective classification of scapular kinematics in participants with movement faults of the scapula on clinical assessment

The aim of this study was to assess the potential of employing a classification tool to objectively classify participants with clinically assessed movement faults (MFs) of the scapula. Six participants with a history of shoulder pain with MFs of the scapula and 12 healthy participants with no movement faults (NMFs) performed a flexion movement control test of the scapula, while scapular kinematic data were collected. Principal component scores and discrete kinematic variables were used as input into a classifier. Five out of the six participants with a history of pain were successfully classified as having scapular MFs with an accuracy of 72%. Variables related to the upward rotation of the scapula had the most influence on the classification. The results of the study demonstrate the potential of adopting a multivariate approach in objective classification of participants with altered scapular kinematics in pathological groups.