Shoulder function: the perfect compromise between mobility and stability

Three-dimensional reaching tasks: effect of reaching height and width on upper limb kinematics and muscle activity

In previous studies, upper limb coordination was usually analyzed during two-dimensional (2D) arm movements. Based on joint kinematics and muscle activity, it has been demonstrated that the shoulder joint controls the multi-joint movement. This study focused on three-dimensional (3D) reaching tasks and examined if the coordination strategies previously described in 2D can be transferred to 3D movements and if reaching to different locations in space has an effect on kinematic and upper limb muscle strategies. Ten healthy subjects reached to nine different targets in 3D space placed at arm length. Kinematic data of the shoulder and elbow and electrical activity of 10 upper limb muscles were registered. Differences in kinematics and EMG were compared between different reaching conditions. Activity of shoulder muscles increased earlier than elbow muscles inducing shoulder elevation prior to elbow extension. Reaching at different widths only influenced shoulder kinematics, whereas reaching at different heights influenced both shoulder and elbow joints. Modulation of reaching height induced an immediate adaptation of elbow flexion followed by an adaptation of shoulder elevation. As previously described in 2D, the shoulder joint leads the movement during 3D reaching tasks. Changing the 3D nature of a reaching task influenced the interaction between shoulder and elbow joint, with reaching height primarily affecting the elbow coordination strategy.

Upper limb joint dynamics during manual wheelchair propulsion

BACKGROUND

Inverse dynamic methods have been widely used to estimate joint loads during manual wheelchair propulsion. However, the interpretation of 3D net joint moments and powers is not always straightforward. It has been suggested to use joint coordinate systems (expression of joint moment on anatomical axes) and the 3D angle between joint moment and angular velocity vectors (propulsion, resistance or stabilization joint configuration) for a better understanding of joint dynamics.

METHODS

Nine spinal cord injured subjects equipped with reflective markers propelled in a wheelchair with an instrumented wheel. Inverse dynamic results were interpreted using joint coordinate systems, 3D joint power and the 3D angle between the joint moment and joint angular velocity vectors at the three upper limb joints. The 3D angle was used to determine if the joints were predominantly driven (angle close to 0 or 180 degrees) or stabilized (angle close to 90 degrees ).

FINDINGS

The wrist and elbow joints are mainly in a stabilization configuration (angle close to 90 degrees ) with a combination of extension and ulnar deviation moments and an adduction moment respectively. The shoulder is in a propulsion configuration, but close to stabilization (angle hardly below 60 degrees ) with a combination of flexion and internal rotation moments.

INTERPRETATION

Stabilization configuration at the joints could partly explain the low mechanical efficiency of manual wheelchair propulsion and could give insight about injury risk at the wrist, elbow and shoulder joints.

Teres major muscle activation relates to clinical outcome in tendon transfer surgery

BACKGROUND

In massive rotator cuff tears a teres major (TMj) tendon transfer to the insertion of the supraspinatus (SSp) reverses its adduction moment arm into abduction which is supposed to be an adequate salvage procedure. Analysis of muscle function to find biomechanical ground of such success is scarce.

METHODS

We compared pre- and postoperative clinical outcome of TMj transfer, i.e. Range of Motion, pain, Constant Shoulder scores and arm force. TMj activation was evaluated in 14 patients suffering massive cuff tears using activation ratios to describe the desired 'in-phase' and undesired 'out-of-phase' contribution to the external arm moment. Additionally, we analyzed activation of the latissimus dorsi (LD) and the medial part of the deltoids (DE). The activation ratios were compared to controls and TMj activation ratios were related to clinical outcome.

FINDINGS

TMj tendon transfer improved arm function. Pre-operatively, we observed 'out-of-phase'abduction activation of TMj and LD. After transfer patients activated TMj according to its new anatomical position. 'Out-of-phase' LD abduction activation persisted. The clinical improvements coincided with changes in activation ratio of TMj.

INTERPRETATION

'Out-of-phase' TMj adductor activation is associated with compromised arm function in patients with irreparable cuff tears. After transfer, TMj is activated in correspondence with its new anatomical function, which was supportive for the improved arm function.

Dynamic tracking of the scapula using skin-mounted markers

The shoulder complex is prone to numerous pathologies and instabilities due to its large range of motion. The extent of injury is assessed through a series of observations and physical examinations. It is hypothesized that objective kinematic analysis of the shoulder could yield useful functional insights to aid clinical practice. Non-invasive motion analysis techniques to monitor shoulder function have been developed using passive markers; however, accurate measurement of scapula kinematics is problematic because of overlying tissue. The scapula locator is the accepted standard by which alternative non-invasive techniques of scapula tracking are validated. In this study, the viability of using skin-mounted markers to measure dynamic scapula movement is determined. Complete kinematic descriptions of ten healthy shoulders were obtained. Elevations of the glenohumeral joint were similar with both techniques, indicating that the skin marker method is suitable for gathering functional glenohumeral data. The main differences of note are seen at the scapulothoracic articulation where the skin marker method underestimated lateral rotation by more than 50 degrees at maximum elevation. However, the correlation between the two approaches is greater than 0.7, suggesting that it may be possible to derive linear regression models to predict dynamic scapulothoracic lateral rotation accurately using skin-mounted scapula markers.

Numerical modelling of the shoulder for clinical applications

Research activity involving numerical models of the shoulder is dramatically increasing, driven by growing rates of injury and the need to better understand shoulder joint pathologies to develop therapeutic strategies. Based on the type of clinical question they can address, existing models can be broadly categorized into three groups: (i) rigid body models that can simulate kinematics, collisions between entities or wrapping of the muscles over the bones, and which have been used to investigate joint kinematics and ergonomics, and are often coupled with (ii) muscle force estimation techniques, consisting mainly of optimization methods and electromyography-driven models, to simulate muscular action and joint reaction forces to address issues in joint stability, muscular rehabilitation or muscle transfer, and (iii) deformable models that account for stress-strain distributions in the component structures to study articular degeneration, implant failure or muscle/tendon/bone integrity. The state of the art in numerical modelling of the shoulder is reviewed, and the advantages, limitations and potential clinical applications of these modelling approaches are critically discussed. This review concentrates primarily on muscle force estimation modelling, with emphasis on a novel muscle recruitment paradigm, compared with traditionally applied optimization methods. Finally, the necessary benchmarks for validating shoulder models, the emerging technologies that will enable further advances and the future challenges in the field are described.

Applicability of the coactivation method in assessing synergies of the scapular stabilizing muscles

HYPOTHESIS

This study investigated the feasibility of quantifying the coactivation levels in the evaluation of synergisms of the scapular stabilizing muscles during the elevation and lowering of the arms. This method, which quantifies the overlapping area of the normalized electromyographic (EMG) activity, could be applicable in assessing such muscular synergies.

MATERIALS AND METHODS

Both shoulders of 10 healthy volunteers were assessed. The coactivation of 3 synergic muscular groups (upper, middle and lower trapezius; upper trapezius and serratus anterior; middle trapezius and serratus anterior) and the isolated electromyographic activity of each muscle were analyzed.

RESULTS

Analysis of variance revealed that the coactivation values between the synergic groups were different from the isolated levels of each respective muscle for both movements (24.42 < F < 40.12; df=6; p < 0.001). The coactivation values of all groups during elevation were different from those during the lowering of the arms (13.31 <F < 959.92; df=1; p <or= 0.002), with progressive increases in EMG activity during the elevation and decreases during the lowering of the arms.

CONCLUSION

The significant differences in EMG activity between the isolated and the coactivation methods indicated that the coactivation method was adequate to assess the scapular muscular synergies.

Shoulder biomechanics: today's consensus and tomorrow's perspectives

Shoulder biomechanics is a fast growing field, which is progressively expanding its focus to include more applied research. The papers included in this Special Issue confirm this trend. After a classification of the papers as dealing with fundamental or applied research through theoretical or experimental methods, in this Editorial we tried to summarize the elements of consensus and the open issues discussed during the last International Shoulder Group meeting, held in Bologna (Italy) in 2008.

The biomechanics of reverse anatomy shoulder replacement--a modelling study

BACKGROUND

Reverse anatomy shoulder prostheses, in which a partial sphere is attached to the scapula and a socket to the humerus, have become popular for the treatment of arthritic shoulders with severe rotator cuff arthropathy. While they have been in relatively common use, their biomechanical aspects have not been fully investigated.

METHODS

This study uses an adaptation of a 3D biomechanical shoulder model to describe the DELTA reverse prosthetic shoulder geometry and to investigate its properties. The muscle configuration was modified to represent the pathology and joint contact forces were computed for standardised activities.The model also uses a contact detect algorithm to record the impingement of the prosthesis with the scapula.

FINDINGS

Results showed that the reverse design increases the deltoid function compensating for the dysfunctional rotator cuff muscles by providing sufficient moment arm (increased by 42%) to elevate the arm. It also restores joint stability by reversing the envelope of the joint contact forces and reacting to the increased shear forces. Despite these advantages, the model also confirms impingement and predicts bone notches from the contact of the prosthesis with the scapula border. Results indicate that optimised fixation and design alterations can reduce the problem but is difficult to eliminate it without compromising the joint stability.

INTERPRETATION

The study provides a deep understanding of the function of the reverse designs highlighting their advantages in irreparable rotator cuff arthropathy but also the associated problems that compromise their use. Despite the limitations results indicate that reverse designs can be optimised to provide maximum functionality.

Rapid muscle activation and force capacity in conditions of chronic musculoskeletal pain

BACKGROUND

The association between musculoskeletal pain and decreased maximal muscle strength capacity has been extensively studied, but knowledge about functional rapid force capacity in conditions of chronic musculoskeletal pain is lacking. The objective of this study is to investigate rapid muscle activation and force capacity of chronically painful muscles.

METHODS

Cross-sectional study with 42 women with chronic trapezius myalgia, and 20 healthy matched controls. Maximal capacity was determined as peak torque and peak EMG amplitude of the painful trapezius and painfree deltoid muscles during the stable high-force phase of maximal voluntary shoulder abduction, whereas rapid capacity was determined as the steepest slope of the torque-time and EMG-time curves, defined as rate of torque development and rate of EMG rise. Intensity of pain was registered prior to the test on a visual-analogue-scale.

FINDINGS

Peak torque was 18% lower at 115 degrees shoulder joint angle in women with myalgia compared with healthy controls (P<0.001), with a corresponding 29% lower level of peak EMG specifically of the painful trapezius muscle (P<0.001). Rate of torque development was 33-54% lower (P<0.001), with a corresponding 21-35% lower level of rate of EMG rise of both the painful trapezius and painfree deltoid (P<0.0001). Intensity of pain showed higher association with parameters of rapid capacity (R=-0.33 to -0.53, P<0.001-0.05) than with maximal capacity (R=-0.15 to -0.41, P<0.01-ns).

INTERPRETATION

In conditions of chronic musculoskeletal pain, the ability to rapidly activate painful and painfree synergistic muscles is more severely impaired than maximal muscle activation. These findings have clinical relevance for rehabilitation of chronically painful muscles.

Effect of physical training on function of chronically painful muscles: a randomized controlled trial

Pain and tenderness of the upper trapezius muscle is frequent in several occupational groups. The objective of this study is to investigate the effect of three contrasting interventions on muscle function and pain in women with trapezius myalgia. A group of employed women ( n = 42) with a clinical diagnosis of trapezius myalgia participated in a 10-wk randomized controlled intervention: specific strength training of the neck/shoulder muscles, general fitness training performed as leg bicycling, or a reference intervention without physical activity. Torque and electromyography (EMG) were recorded during maximal shoulder abductions in an isokinetic dynamometer at −60, 60, 0, and 180°/s. Furthermore, a submaximal reference contraction with only the load of the arms was performed. Significant changes were observed only in the specific strength training group. Pain decreased by 42–49% ( P < 0.01–0.05). Whereas the EMG activity of the unaffected deltoid remained unchanged during the maximal contractions, an increase in EMG amplitude (42–86%; P < 0.001–0.05) and median power frequency (19%; P < 0.001) were observed for the painful trapezius muscle. Correspondingly, torque increased by 18–53% ( P < 0.001–0.05). EMG during the reference contraction decreased significantly for both the trapezius and deltoid muscles ( P < 0.01). In conclusion, specific strength training relieves pain and increases maximal activity specifically of the painful trapezius muscle, leading to increased shoulder abduction strength in women with trapezius myalgia. Furthermore, decreased relative workload may indirectly augment pain reduction.

The range of passive arm circumduction in primates: do hominoids really have more mobile shoulders?

Hominoids and lorines are assumed to possess greater shoulder mobility than other primates. This assumption is based on morphological characteristics of the shoulder, rather than on empirical data. However, recent studies have shown that the glenohumeral joint of hominoids is not more mobile than that of other primates (Chan LK. 2007. Glenohumeral mobility in primates. Folia Primatol (Basel) 78(1):1-18), and the thoracic shape of hominoids does not necessarily promote shoulder mobility (Chan LK. 2007. Scapular position in primates. Folia Primatol (Basel) 78(1):19-35). Moreover, lorines differ significantly from hominoids in both these features, thus challenging the assumption that both hominoids and lorines have greater shoulder mobility. The present study aims to test this assumption by collecting empirical data on shoulder mobility in 17 primate species. Passive arm circumduction (a combination of glenohumeral and pectoral girdle movement) was performed on sedated subjects (except humans), and the range measured on the video images of the circumduction. The motion differed among primate species mostly in the craniodorsal directions, the directions most relevant to the animal's ability to brachiate and slow climb. Hylobatids possessed the highest craniodorsal mobility among all primate species studied. However, nonhylobatid hominoids did not have greater craniodorsal mobility than arboreal quadrupedal monkeys, and lorines did not have greater craniodorsal mobility than arboreal quadrupedal prosimians. Nonhylobatid hominoids and lorines had similar craniodorsal mobility, but this was due to a longer clavicle, more dorsal scapula, and lower glenohumeral mobility in the former, and a shorter clavicle, less dorsal scapula, and greater glenohumeral mobility in the latter. This study provides evidence for the reexamination of the brachiation, slow climbing, and vertical climbing hypotheses.

A stochastic analysis of glenoid inclination angle and superior migration of the humeral head

BACKGROUND

Superior glenoid inclination, which is a relatively upward facing of the glenoid in the plane of the scapula, has been associated with rotator cuff pathology. Increased glenoid inclination may cause superior humeral head migration, which can cause impingement of the supraspinatus tendon. The purpose of this study was to test the hypothesis that inclination angle affects the probability of superior humeral head migration.

METHODS

A three-dimensional model of the glenohumeral joint was developed in which muscle forces were modeled as random variables. Monte Carlo simulation was used to compute the probability that the glenohumeral reaction force was directed such that superior humeral head migration should occur. An electromyogram-driven model was used to estimate shoulder muscle forces in healthy volunteers performing arm elevation.

FINDINGS

The model predicted that the probability of superior humeral head migration increased as glenoid inclination angle was increased. This finding was independent of the assumed shape of the muscle force probability distributions.

INTERPRETATION

The results support the theory that glenoid inclination may be a risk factor for rotator cuff pathology.

Proprioception of the shoulder after stroke

OBJECTIVE

To investigate position sense and kinesthesia of the shoulders of stroke patients.

DESIGN

Case-control study.

SETTING

A rehabilitation center.

PARTICIPANTS

A total of 22 inpatients with stroke and 10 healthy control subjects.

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

Angular displacement (in degrees) for threshold to detection of passive motion (TDPM) tests and absolute error (in degrees) for passive reproduction of joint position tests.

RESULTS

For patients, the TDPM for internal and external rotation was significantly higher for both the contralateral (paretic) side (internal, 7.92 degrees +/-7.19 degrees ; external, 8.46 degrees +/-8.87 degrees ) and the ipsilateral (nonparetic) side (internal, 4.86 degrees +/-5.03 degrees ; external, 6.09 degrees +/-9.15 degrees ) compared with the control group (internal, 1.83 degrees +/-1.09 degrees ; external, 1.71 degrees +/-.85 degrees ). Also, for internal rotation, TDPM was significantly higher for patients on the contralateral side compared with the ipsilateral side. For passive reproduction of joint position tests, no differences were found.

CONCLUSIONS

Both the contralateral and ipsilateral shoulders of stroke patients showed impaired TDPM. Passive reproduction of joint position does not seem to be affected as a result of a stroke. The control of the muscle spindles and central integration or processing problems of the afferent signals provided by muscle spindles might cause these effects.

Influence of component positioning on impingement in conventional total shoulder arthroplasty

BACKGROUND

Clinical experience suggests that component impingement can lead to eccentric implant loading and thereby cause glenoid loosening in conventional total shoulder arthroplasty. This study tests the hypothesis that certain implant component positioning configurations may lead to impingement within the physiological range of motion.

METHODS

A rigid-body model of the shoulder comprising the scapula and humerus was constructed. Within this 3D model, a commercially available total shoulder arthroplasty implant was positioned according to manufacturer guidelines. The configuration was modified around this default position to investigate the associated angle of inferior and superior impingement during glenohumeral elevation, as well as in lateral impingement during axial rotation at both 0 degrees and 60 degrees of glenohumeral elevation. Glenoid component size, version, inclination and inferior-superior offset as well as humeral component size, torsion, inclination, offset and height were examined. The influence of the humeral calcar anatomy was also investigated.

FINDINGS

Certain implant configurations caused component impingement in the physiological range of motion. The most sensitive parameters affecting impingement were: (1) the inclination of the glenoid component, (2) the inferior-superior position of the humeral component along the resection line and (3) the prominence of the humeral calcar. Glenoid offset and inclination and humeral head offset and height directly affected subacromial impingement.

INTERPRETATION

This study suggests that several intraoperatively adjustable implant positioning parameters can influence the likelihood of implant impingement in conventional total shoulder arthroplasty, and that the geometry of the humeral calcar should be taken into consideration when designing an operative strategy for shoulder joint replacement.