Does Scapular Dyskinesis Really Matter? A Narrative Review

Introduction

The glenohumeral joint (GH), acromioclavicular (AC), sternoclavicular (SC), and scapulothoracic joints are the four smaller joints that constitute the shoulder complex.^1,2 With its mismatched broad humeral head and small glenoid articular surface, the glenohumeral joint is designed for maximum mobility.³ The forelimb is critical for a variety of locomotor behaviors in nonhuman hominoids, from knuckle walking to suspension. The GH joint, which links the glenoid fossa of the scapula to the proximal humerus, is the most mobile joint in the body in hominoids and is the primary joint involved in arm movement.⁴ The shoulder joint is crucial to the operation of the arms and hands, which distinguishes humans from other mammals due to their dexterity. The glenohumeral, scapulothoracic, SC, and AC joints, are all coordinated in shoulder movement.^5-8 The scapula is closely concerned with shoulder function both anatomically and biomechanically. The scapula must move in tandem with the moving humerus to retain the ball and socket configuration. Scapular location changes at rest or with coupled arm motion are often linked to shoulder injuries that result in clinical dysfunction.⁹ Weakness of muscles, malfunctioning of neurologic system, or inhibition through intra-articular glenohumeral or subacromial processes can all influence scapular motion.^10-12 The correlation between scapular alterations and shoulder conditions is not clear, if it is a cause or a consequence.^13,14 Shoulder function is affected by altered scapular movements.¹⁵ Scapular dyskinesis is difficult to diagnose clinically, relying mainly on visual observation. Because of the emphasis on non-objective analysis and the examiners ‘expertise, this methodology has modest accuracy and sensitivity.¹⁶ Other identification methods have been studied using multidimensional kinematics with markings on bone surfaces over the skin with accordance to devices that capture movements to enable objective quantification of scapular motion. The lack of technical expertise and apparatus, minimal availability of control values, movement of the skin along the depth of scapula that is below the musculature and difficulty of multidimensional motion of scapula where three rotatory and two translatory motion values are taken into account restrict the use of kinematics.^17,18 As a result, the aim of this study was to identify the certainties and uncertainties associated with scapular dyskinesis.

Normal Osteokinematics

Scaption Abduction

The frontal or coronal plane shoulder joint abduction has been broadly researched, and it has been suggested that the Plane of Scapula (POS) in the shoulder joint is the typical plane of motion. True shoulder movements occur in the POS, which is characterized as shoulder elevation in a 30 to 50 degree anterior to frontal plane.^2,19-23 Scaption was titled after the tilting angle towards the center which attributes the sagittal planar tilt of scapula.^24,25 Scapula moves around the thoracic cage as the medial tilting angle rises. In this plane, the relation between optimal muscle length and tension of the shoulder abductors and rotators can be shown.⁸ The stability of joint is enhanced when joint congruence is more in the scaption plane.²¹ Flexion is a synchronized movement of the glenohumeral, acromioclavicular, sternoclavicular, and scapulothoracic joints that occurs in the sagittal plane between 162 and 180 degrees.²¹

Normal Arthrokinematics

The first phase of elevation occurs at GH joint, where there is 3 mm glide of humeral head superiorly. Superior shearing is caused by deltoid movement, and the rotator cuff muscle counteracts these forces and stabilizes the joint.²⁴ The subscapularis muscle’s primary role is to lower the humeral head, counterbalancing the deltoid’s superior migration power.²⁵ The critical stage of elevation is the middle part of the process. The deltoid muscle has a maximum shearing force.²⁶ The upper and lower trapezius, as well as the lower serratus anterior muscles, work together to increase scapular movement. On the glenoid, the humeral head glides 1.5 mm inferiorly and superiorly.²¹ Maximum glenohumeral joint movement is necessary in the final phase of elevation.²⁷ Strong extensibility of the teres major, minor, pectoralis major, and Latissimus dorsi and subscapularis muscles allows for greater and unrestricted humeral movement.²⁷

Dynamic Scapular Stability

The scapulohumeral, axiohumeral, and axioscapular muscles are the main ones which function on the GH and SC joints. The rotator cuff muscles are part of the scapulohumeral group of muscles, which arise on the scapula and have their insertion over the humerus.²⁸ The fibres of trapezius muscle along with the serratus anterior and rhomboid muscles have role in primary scapular stabilization and motion on the thorax. When the arm is lowered from an elevated position, the lower trapezius acts as an upward rotator of the scapula and a scapular stabilizer. A force couple is formed by the combined contractions of the subscapularis and infraspinatus that provides stability in the middle phase abduction, which is around 60 and 150 degrees.²⁹ Serratus anterior plays a role in the stabilization of the scapula by protracting the scapula. It prevents winging of the scapula by stabilizing the medial border and inferior angle of the scapula, allowing for upward rotation, posterior tilt, and lateral rotation.

Materials and Methods

Data sources and search strategy

Two independent researchers screened the electronic databases including Google Scholar, Medline, and Scopus. Search strategy consisted of MeSH terms such as “Scapular Dyskinesis,” “Scapular Abnormalities,” “Shoulder,” “Biomechanics,” “Scapula,” “Rotator Cuff,” and “Shoulder pain”, as well as combination of these terms. Papers published between 2010 and 2020 were included. Studies were required to be published in the English language.

Study selection process

To find the articles that met the inclusion criteria, the primary researcher first read through the titles and abstracts. Two authors independently reviewed the full-text materials for relevance after reading the abstracts.

Furthermore, if there were any differences, the reviewers discussed them until they were satisfied with the final inclusion.

Eligibility criteria The studies were selected based on the following criteria:

Inclusion criteria

(a) Full-text available articles (b) Studies published from 2010 to 2020 (c) Studies involving scapular dyskinesis and shoulder pathologies, as well as scapular dyskinesis as a sport-specific functional adaptation, were included in the study.

Exclusion criteria

(a) Studies published in non - English (b) Studies that did not report the results adequately, were excluded.

Study selection

Two reviewers worked independently on a two-stage screening process. Stage 1 included checking the titles and abstracts of studies published between 2010 and 2020 for applicability, and resolving conflicts of interest with the third reviewer. In Stage 2, eligible studies were independently reviewed by two reviewers, and the final selection was made after viewing the full-text studies in accordance with the eligibility criteria.

Scapular Dyskinesis (SD)

Subacromial Pain Syndrome (SAPS) has been related to changes in scapular biomechanics, and the most common causative mechanisms, such as inflexibility (tightness) and changes in the periscapular muscles, usually include soft tissue. Changes in scapular biomechanics have been linked to SAPS, and the most prevalent causal mechanisms, such as inflexibility (tightness) and abnormalities in the periscapular muscles, typically involve soft tissue changes.^30-33 Scapular dyskinesis is characterized by changes in the orientation of the scapula and patterns of scapular motion in relation to the thoracic cage.⁹ The word does not imply a cause or determine characteristics that are associated with different shoulder injuries.⁹ Previous cross-sectional studies showed that, it is impossible to say if scapular dyskinesis caused or attributed to shoulder pain.²

Epidemiology

SD occurs often in patients with shoulder diseases such as Rotator Cuff diseases, GH dysfunction, impingement syndrome, and labral tears, according to published reports.^34-41 Scapular dyskinesis is seen in 68 percent of rotator cuff abnormalities patients, 94 percent of labral tears patients, and 100 percent of glenohumeral instability patients. The incidence of non-overhead athletes was 33.3 percent, while overhead athletes had 61 percent. SD was found in elite young swimmers, with a 8.5 percent frequency in a sample of 661 athletes who were asymptomatic.⁴²

Classification of Scapular Dyskinesis

Type 1: Prominence of the inferior medial scapular border and its unusual rotation around a transverse axis.

Type 2: The entire medial edge of the scapula is prominent, and it rotates abnormally around a vertical axis.

Type 3: Superior translation of the entire scapula, as well as prominence of the scapula’s superior medial border.⁹

Etiology

It is possible to identify proximal and distal causative factors in SD. Scapular muscle, lower trapezius, and serratus anterior weakness are proximal components, while joint internal imbalances such tear of labrum, reduced GH stability, and gap in acromioclavicular joint are distal components.³⁴ Thoracic kyphosis and an increase in cervical lordosis causes excessive scapular protraction and acromial decompression, which may lead to subacromial impingement.^9,43,44 Long thoracic nerve damage causes winging of the scapula’s medial boundary, which affects normal scapular kinematics.^45-48 Abnormal shoulder motion is most often caused by changes in soft tissue, such as muscle stiffness and flexibility.^45,47,48 Stiffness of the pectoralis minor helps to reduce the external rotation and posterior tilt of the scapula during arm elevation.^47,49-51 Fatigue of the stabilizer muscles reduces rotator cuff power and worsens scapular dyskinesis symptoms.^9,47 Scapular kinematics are altered by a shorter clavicle, which causes the scapula to be anteriorly tilted, backward, and internally rotated.³⁴ Impingement syndrome, glenohumeral dysfunction, clavicular fracture, rotator cuff disorder, superior labrum damage, and AC joint pathology are all possible causes of scapular dyskinesis.^52,53,55-57

Scapular Dyskinesis as a Functional Adaptation

Scapular dyskinesis has been described as a sport-specific adaptation in both patients with shoulder pathology and healthy overhead athletes.^11,54 The demands and exposure in sports can cause scapular orientation changes in the serve dominant arm due to changes in neuromuscular control strategies. In adult overhead throwers, increased scapular upward rotation during glenohumeral elevation was interpreted as positive alteration to unique game needs.^58,59

Clinical Examination

The role of scapular upward rotation during upper extremity elevation has been addressed by a variety of sources. Due to the difficulty of monitoring scapular motions in multiple planes, a predictable method to diagnose SD clinically is yet to be established. In pre-participation screening, a simple field-based test that measures winging and scapular asymmetry has a higher reliability rate.¹¹ The interrater reliability of the Scapular Assistance Test (SAT) was kappa=0.53, 77 percent, and the Scapular Reposition Test (SRT) was p=0.61, 95 percent.^11,55 The posterior displacement of the medial border of the scapula is tested using a sitting hand press-up test. An international panel of researchers and clinicians recently reached a consensus on the use of the dynamic scapular dyskinesis test.⁶⁰ The action is classified as Dyskinesis, with ‘yes’ (existence of bilateral alteration or dysrhythmia/asymmetry) or ‘no’ (absence of bilateral alteration or dysrhythmia/asymmetry). This approach has been demonstrated effective in the eyes of observers and to be of reasonable clinical usefulness.^57,34,61

Results

Certainties

According to one study, among people with shoulder pain, there was a clear and statistical association between pectoralis minor and levator scapulae duration. When shoulder pathology causes downward rotation and anterior tilt of the scapula, the length of the pectoralis minor and levator scapulae is reduced. In EMG studies, individuals with shoulder pain showed increased upper trapezius action. Excessive clavicular elevation is mechanically related to unwanted scapular anterior tilting motion. Excess scapular internal rotation, manifested as a prominent medial border of the scapula, has been linked to delayed activation of the middle trapezius. Excessive internal rotation of the scapula is caused by delayed activation of the lower trapezius.³ According to one study, asymptomatic athletes with scapular dyskinesis are at an increased risk of developing shoulder pain by 43 percent.²

Uncertainties in studies

In one study, the Pectoralis Minor Index (PMI) and Levator Scapula Index (LSI) had lower values in acute shoulder pain, but the Shoulder Pain and Disability (SPADI) scores were high, and this is still unknown. Improvement in the Pectoralis minor index and the Levator Scapula Index, which may be clinically and functionally important for people with shoulder disorders, has less evidence. In the absence of symptoms that suggest the onset of shoulder dysfunction, there is still no evidence of a connection between PMI and LSI. Scapular upward rotation, PMI, and LSI were evaluated in static rather than dynamic positions. These parameters can be analyzed at the start of a traumatic arc and in both diseased and stable individuals. The link between scapular muscle length and shoulder dysfunction needs to be investigated.³ Randomized control trials are required to establish causation between SD changes and specific results of shoulder pathologies with altered scapular kinematics.⁶¹ The relationship between scapular position alterations and shoulder conditions is not clear; if it is a cause or a consequence.⁵⁶ It is unclear if the existence of scapular dyskinesis that is unrelated to asymmetry is a shoulder pain-related disorder or just natural movement variability.⁵⁷ There is still no conclusion on whether deficits in the scapula’s dynamic three-dimensional motion are the most significant factors in shoulder dysfunction.³⁴

Discussion

After reviewing few studies, it was discovered that there is a strong connection between muscle length and scapular deviations, with a reduction in pectoralis minor and levator scapulae length leading to downward rotation and anterior tilt of the scapula when shoulder pathology exists. Scapular dyskinesis greatly increases the likelihood of pain in the shoulder by 43%, implying that scapular dyskinesis is more likely to be the cause of shoulder pain than the victim in a few cases of shoulder pain. The evidence for a cause-and-effect link between scapular dyskinesis and shoulder pathology is limited. The relationship between PMI and LSI, as well as their relationship with scapular dyskinesis, is yet to be investigated. There is lack of evidence to say that threedimensional scapular motion anomalies may lead to shoulder dysfunction.

Limitations

Despite reviewing a wide range of literature from 2010 to 2020, only a few studies of high methodological quality were found. There were only a few high-quality randomized controlled trials available for review.

Conclusion

The relationship between scapular dyskinesia (position or movement) and risk of rotator cuff, glenoid labrum, subacromial bursa, coracoacromial ligament or biceps long head injury is still unclear. Understanding scapular dysfunctions by analyzing the scapula in a dynamic position demands further research. More research is required to arbitrate if scapular dyskinesis is a natural variance in movement or a shoulder pain-related disability. More randomized controlled trials are required to better understand how scapular kinematics alters in various shoulder pathologies and scapular dyskinesis variants. As a whole, the review’s purpose has been achieved.

Declaration of Interest

None