Shoulder injuries in athletes

The scapula serves many roles in order for proper shoulder function to occur. These roles include providing synchronous scapular rotation during humeral motion, serving as a stable base for rotator cuff activation and functioning as a link in the kinetic chain. Each role is vital to proper arm function and can only occur when the anatomy around the shoulder is uncompromised. The presence of bony and soft tissue injury as well as muscle weakness and infl exibility can alter the roles of the scapula and alter scapular resting position and/ or dynamic motion. This altered scapular position/ movement has been termed ‘scapular dyskinesis’. Although it occurs in a large number of shoulder injuries, it appears that scapular dyskinesis is a non-specifi c response to a painful condition in the shoulder rather than a specifi c response to certain glenohumeral pathology. The presence or absence of scapular dyskinesis needs to be determined during the clinical examination. An examination consisting of visual inspection of the scapular position at rest and during dynamic humeral movements, along with the performance of objective posture measurements and scapular corrective maneuvers, will help the clinician ascertain the extent to which the scapula is involved in the shoulder injury. Treatment of scapular dyskinesis should begin with optimised anatomy and then progress to the restoration of dynamic scapular stability by strengthening of the scapular stabilisers utilising kinetic chain-based rehabilitation protocols. Normal scapulohumeral rhythm, the coordinated movement of the scapula and humerus to achieve shoulder motion, is the key to effi cient shoulder function. Scapular position and motion are closely integrated with arm motion to accomplish most shoulder functions. Scapular movement is a composite of three motions—upward/downward rotation around a horizontal axis perpendicular to the plane of the scapula, internal/external rotation around a vertical axis through the plane of the scapula and anterior/posterior tilt around a horizontal axis in the plane of the scapula.1 The clavicle acts as a strut for the shoulder complex, connecting the scapula to the central portion of the body. This allows two translations to occur—upward/downward translation on the thoracic wall and retraction/protraction around the rounded thorax.1 When the humerus moves into elevation, clavicular elevation, retraction and posterior axial rotation occur at the sternoclavicular joint, while scapular internal rotation, upward rotation and posterior tilting occur at the acromioclavicular joint.2 Both the acromioclavicular and sternoclavicular joints contribute to scapular upward rotation with up to 31° of clavicular posterior rotation occurring at the sternoclavicular joint.2 The acromioclavicular joint is primarily responsible for the occurrence of scapular posterior tilting. Sternoclavicular joint retraction and acromioclavicular joint internal rotation are offsetting motions allowing scapular internal and external rotation to occur.2 The scapula plays several roles in normal shoulder function. Control of static position and control of the motions and translations allow the scapula to fulfi l these roles. In addition to upward rotation, the scapula must also posteriorly tilt and externally rotate to clear the acromion from the moving arm in forward elevation or abduction. Also, the scapula must synchronously internally/ externally rotate and posteriorly tilt to maintain the glenoid as a congruent socket for the moving arm and maximise concavity compression and ball and socket kinematics. The scapula must be dynamically stabilised in a position of relative retraction during arm use to maximise activation of all the muscles that originate on the scapula.3 4 Finally, it is a link in the kinetic chain of integrated segment motions that starts from the ground and ends at the hand. Because of the important but minimal bony stabilisation of the scapula by the clavicle, dynamic muscle function is the major method by which the scapula is stabilised and purposefully moved to accomplish its roles. Muscle activation is coordinated in taskspecifi c force couple patterns to allow stabilisation of position and control of dynamic coupled motion. Abnormal scapular motion and/or position have been collectively called ‘scapular winging’, ‘scapular dyskinesia’ and more appropriately ‘scapular dyskinesis’. Scapular winging refers to prominence of the medial border of the scapula, which is most often associated with long thoracic nerve palsy, and in some cases, overt scapular muscle weakness. ‘Winging’ describes a visual abnormality but it fails to indicate whether the abnormality is static, dynamic or both. Scapular dyskinesia by strict defi nition implies that a loss of voluntary motion has occurred. However, only the scapular translations (elevation/depression and retraction/protraction) can be performed voluntarily, whereas the scapular rotations are accessory in nature. Therefore, the term ‘dyskinesia’ is not always appropriate when describing abnormal scapular motion. Scapular dyskinesis (‘dys’—alteration of, ‘kinesis’—movement) is a collective term that refers to movement of the scapula that is dysfunctional. Scapular dyskinesis has been identifi ed by a group of experts as: (1) abnormal static scapular position and/or dynamic scapular motion characterised by medial border prominence; or (2) inferior angle prominence and/or early scapular elevation 02_sm058834&sm070805.indd 300 3/13/2010 3:15:37 PM Shoulder injuries in athletes Br J Sports Med 2010;44:300–305. doi:10.1136/bjsm.2009.058834 301 or shrugging on arm elevation; and/or (3) rapid downward rotation during arm lowering.5 However, static position and dynamic motion are two separate entities, so when describing the static appearance of the scapula and if an asymmetry is observed, it should be referred to as ‘altered scapular resting position’ rather than ‘scapular dyskinesis’. Scapular dyskinesis is a non-specifi c response to a painful condition in the shoulder rather than a specifi c response to certain glenohumeral pathology. Scapular dyskinesis has multiple causative factors, both proximally (muscle weakness/ imbalance, nerve injury) and distally (acromioclavicular joint injury, superior labral tears, rotator cuff injury) based. This dyskinesis can alter the roles of the scapula in the scapula– humeral rhythm.6 It can be due to alterations in the bony stabilisers, alterations in muscle activation patterns or strength in the dynamic muscle stabilisers. DYNAMIC SCAPULAR STABILITY Primary scapular stabilisation and motion on the thorax involves coupling of the upper and lower fi bres of the trapezius muscle with the serratus anterior and rhomboid muscles. Other muscles such as the pectoralis minor also play a role. Elevation of the scapula with arm elevation is accomplished through activation and coupling of the serratus anterior and lower trapezius with the upper trapezius and rhomboids.7 8 During this motion, the lower trapezius helps maintain the instant centre of rotation of the scapula through its attachment to the medial scapular spine. Its attachment to the scapular spine allows for a straight line of pull as the arm elevates and the scapula rotates upwardly, and creates a mechanical advantage to maintain this position. The lower trapezius has often been identifi ed as an upward rotator of the scapula because it maintains its long moment arm during the full range of upward rotation.8 However, it also plays a role as a scapular stabiliser when the arm is lowered from an elevated position. During the descent or return from upward elevation, the wellpositioned lower trapezius, when operating effi ciently, helps maintain the scapula against the thorax. The serratus anterior also plays a role as a stabiliser of the scapula. This muscle has been historically identifi ed as a protractor of the scapula due to high electromyographic activity elicited during various push-up maneuvers.9 Other evidence suggests the serratus muscle helps rotate the scapula upwardly. The serratus anterior is actually multifaceted in that it contributes to all components of three-dimensional motion of the scapula during arm elevation. The serratus anterior helps produce scapular upward rotation, posterior tilt and external rotation while stabilising the medial border and inferior angle, which prevents scapular winging.10 This is most likely caused by the variable fi bre orientation of the serratus anterior on the scapula and thorax. The highest level of serratus anterior activation occurs in the cocking phase of the throwing motion, and serratus anterior activation occurs in the earliest stages of arm elevation. It would appear that a prime role of the serratus in these activities is as an external rotator/stabiliser of the scapula with arm motion. The scapular position that allows optimal muscle activation of the shoulder joint muscles to occur is that of retraction and external rotation. Scapular retraction is an obligatory and integral part of a normal scapula–humeral rhythm in coupled shoulder motions and functions. It results from synergistic muscle activations in patterns from the hip and trunk through the scapula to the arm, which then facilitates maximal muscle activation of the muscles attached to the scapula. The retracted scapula can then act as a stable base for the origin of all the rotator cuff muscles. As these roles are key components of normal shoulder function, alterations in the roles may play a part in shoulder dysfunction. Research has demonstrated alterations of scapular motion and position in association with a wide variety of shoulder injuries. SCAPULAR DYSKINESIS AND SHOULDER INJURY