Kinesiology of the Shoulder Flashcards
What are the structures contributing to stability of the sternoclavicular joint?
- Articular surfaces
- Ligaments
- Joint capsule
- Muscles
What ligaments contribute to the stability of sternoclavicular joint?
- Anterior & posterior sternoclavicular joint ligaments
- Interclavicular ligament
- Costoclavicular ligament
How does the joint capsule contribute to stability of sternoclavicular joint?
Anterior disc
- Strengthens articulation
- Acts as shock absorber by inc. SA of joint contact
What are the sternoclavicular joint movements?
- Clavicular elevation & depression
- Clavicular protraction & retraction
- Clavicular posterior rotation
What is the purpose of sternoclavicular joint movements?
To place the scapula in an optimal position to accept head of humerus
Arthrokinematics of clavicular elevation & depression
Clavicular elevation = Convex articular surface rolls superiorly & simultaneously slides inferiorly on concavity of sternum
Clavicular depression = Convex articular surface rolls inferiorly & simultaneously slides superiorly on concavity of sternum
Arthrokinematics of clavicular protraction & retraction
Clavicular protraction = Concave articular surface at clavicle rolls & slides anteriorly on convex surface of sternum
Clavicular retraction = Concave articular surface at clavicle rolls & slides posteriorly on convex surface of sternum
What limits the extremes of clavicular protraction?
Excessive tightness in:
- Posterior bundle of costoclavicular ligament
- Posterior capsular ligament
- Scapular retractor muscles
All these limit the extremes of protraction of the clavicle
Which ligaments are taut and slack during clavicle elevation?
Taut = Costoclavicular ligament (stretched = helps limit + stabilize elevated position of clavicle)
Slack = Superior capsule + interclavicular ligament
Which ligaments are taut and slack during clavicle depression?
Taut = Superior capsule + interclavicular ligament (when fully depressed)
Slack = Costoclavicular ligament
Which ligaments are taut and slack during clavicle retraction?
Taut = Costoclavicular ligament + Anterior capsular ligament (elongates)
Slack = Posterior capsule ligament
What are the muscles contributing to the stability of the sternoclavicular joint?
- Sternocleidomastoid
- Sternothyroid
- Sternohyoid
- Subclavius
What are the structures contributing to stability in the acromioclavicular joint?
- Ligaments
- Articular Disc
- Muscles
What are the ligaments contributing to the stability of the acromioclavicular joint?
- Superior & inferior acromioclavicular joint ligaments (AKA superior & inferior capsular ligaments
- Coracoclavicular ligament = Conoid & trapezoid ligaments)
What does the acromioclavicular joint do?
Permits more subtle movements b/w scapula & lateral end of clavicle
What are the muscles contributing to the stability of the acromioclavicular joint?
- Deltoid
- Upper traps
Which ligament absorbs more energy at point of rupture (acromioclavicular joint)?
Coracoclavicular ligament = conoid ligament & trapezoid ligament
How is horizontal shear force (e.g. contact sports like rugby; impact on shoulder) resisted?
- Resisted primarily by superior & inferior AC joint ligaments
- Secondary resistance (if horizontal shear is severe) = coracoclavicular ligament
- If force applied to scapula exceeds tensile strength of ligament = rupture & subsequent dislocation of AC joint
What are the movements of the acromioclavicular joint?
- Scapular upward & downward rotation (abduction/flexion of shoulder)
- Scapular internal & external rotation
- Scapular anterior & posterior tilting
What is the scapulothoracic joint?
- Not a true joint = more of a point of contact b/w anterior surface of scapula & posterior-lateral thoracic wall
- No joint capsule; separated by muscles
- Scapula positioned b/w ribs 2-7 (anatomical pos)
What is the ‘resting’ posture of the scapula?
- 10° of anterior tilt
- 5-10° of upward rotation
- 30-40° of internal rotation
Scapulothoracic movements are combinations of _____________ and _____________ movements
Scapulothoracic movements are combinations of sternoclavicular and acromioclavicular movements
SC and AC movements in:
Elevation/Depression of the scapulo-thoracic joint
SC = Elevation/Depression
AC = Anterior/Posterior tilting (so that scapula is against the ribs)
SC and AC movements in:
Protraction/Retraction of the scapulo-thoracic joint
SC = Protraction/Retraction
AC = Internal/External rotation
SC and AC movements in:
Upward/Downward rotation of scapulo-thoracic joint
SC = Elevation/Depression
AC = Upward/Downward rotation
Functional importance full upward rotation of of scapulothoracic joint
- Projects glenoid fossa upward and antero-laterally to inc. further elevation
- Preserves optimal length-tension relationship of supraspinatus & middle deltoid muscles (relevant to abduction)
- Preserves volume of subacromial space = avoid impingement (e.g. of supraspinatus tendon)
Is the glenohumeral joint a true ball-and-socket joint?
NO
- Humeral head does not fit perfectly into glenoid fossa of scapula
- Allows for extensive mobility = humeral head can be pulled away from fossa a significant distance w/o pain/trauma
Glenoid fossa and humeral head in anatomical position
Genoid fossa: upward rotated & articular surface projects antero-laterally in scapular plane
Humeral head: projects medially, superiorly & posteriorly (due to retroversion)
What are the factors contributing to the mobility of the glenohumeral joint?
Articulating surfaces
- Not pure ball-and-socket joint
- Contribution from scapular movements
- Lined with articular cartilage
Joint capsule (fibrous)
- Vol. is about 2x humeral head size = can move alot (extensive mobility)
- Synovial mbn lines the inner wall of the capsule
- Isolates joint cavity from surrounding tissues
How much of the glenoid fossa is covered with articular surface?
About 1/3 of humeral head
- Allows only small part of the humerus to make contact w glenoid fossa at any given position
What are the factors contributing to stability of the glenohumeral joint?
- Ligaments
- Joint capsule (fibrous)
- Glenoid lubrum
- Muscles
- Biomechanics of scapulothoracic posture
What are the ligaments contributing to stability of the glenohumeral joint?
- Glenohumeral capsular ligaments
- Coracohumeral ligament
When does the glenohumeral capsular ligaments produce the greatest stabilizing tension?
Only when stretched at extreme motions
- Must be elongated/twisted to varying degrees
- Provides mechanical support + limit extremes of rotation & translation
What are the muscles contributing to stability of the glenohumeral joint?
- Rotator cuff (subscap, supraspin, infraspin, teres minor) = active forces produced by these muscles = active stabilizing tensions at any joint position
- Long head of biceps brachii = bc. cross superiorly over humeral head
What is the primary movement causing superior GH ligament to be taut?
- External rotation
- Inferior & anterior translation of humeral head
Slightly taut when in/near anatomical position
Slack when GH joint abducted beyond 35-45°
What is the primary movement causing middle GH ligament to be taut?
- External rotation
- Anterior translation of humeral head during 45-90° abduction
VV effective in limiting extremes of external rotation & vv slack during internal rotation
What is the primary movement causing inferior GH ligament to be taut?
Consists of: Anterior band, posterior band, connecting axillary pouch
- Axillary pouch = 90° abduction + anterior-posterior & inferior translations
- Anterior band = 90° abduction + full external rotation; anterior translation of humeral head
- Posterior band = 90° abduction + full ER
What is the primary movement causing coracohumeral ligament to be taut?
- Inferior translation of humeral head
- External rotation
What reinforces the rotator (cuff) interval?
The tendon of the long head of the biceps, the coracohumeral ligament & sup. & middle GH ligaments
Regions of the capsule that the rotator cuff cannot cover
What is the glenoid labrum?
- Fibrocartilage covering the rim of glenoid fossa
- Deepens the depth of glenoid fossa (attributing about 50% of depth) = Inc. contact area with humeral head & stabilizes joint
Why is the glenoid labrum prone to injury?
- Superior part loosely attached to rim of glenoid fossa (allows for overhead movement)
- 50% of fibres of biceps brachii long head tendon are direct extensors of superior glenoid labrum;
excessive tension (e.g. overhead movement) can tear superior glenoid labrum
What is usually the result of glenoid labrum injury?
SLAP
Superior Labrum Anterior and Posterior
- tear/lesions
- Tears of superior labrum near origin of long head of biceps were first noted in throwing atheletes
Which muscle protects the rotator cuff anteriorly?
Subscapularis
Which muscle protects the rotator cuff superiorly?
Supraspinatus
Which muscle protects the rotator cuff posteriorly?
Infraspinatus & teres minor
What does the rotator cuff muscles do?
Forms a cuff over humeral head
- Actively stabilizes GH joint during all dynamic activities
- tendons blend into joint capsule
What is scapulothoracic posture when standing at complete rest with arms at the side?
Head of humerus remains stable against glenoid fossa = static stability
- Scapula is upwardly rotated (held in place by muscular force)
- Static stability controlled by resultant force vector of superior capsular structures (sup. capsular ligament, coracohumeral ligament, supraspinatus tendon)
- Resultant capsular force with force vector due to gravity = compressive locking force at right angle to glenoid fossa surface (compress humerus head against glenoid fossa)
What is scapulothoracic posture when there is load at wrist level?
Normal scapulothoracic posture mechanics (upward rotated scapula + compressive locking force due to sup. capsular structures & gravity)
- Rotator cuff muscles contract isometrically in direction parallel to the horizontal vector of superior capsular structures
What happens when there is a loss of upward rotation position of the scapula (scapulothoracic posture)?
There would be a change in angle between the superior capsular structure and gravity vectors = reduces the magnitude of compression force across the GH joint = can result in plastic deformation in superior capsular structure
Muscles & ligaments for abduction of glenohumeral joint
- Supraspinatus contract = to direct the roll of humeral head
- Inferior capsular ligament taut = support head of humerus
- Superior capsular ligament relatively taut (bc. pulled from attached contracting supraspinatus) = prevent impingement b/w humeral head & undersurface of acromion process
Arthrokinematics of abduction of glenohumeral joint
Convex head of humerus rolling superiorly while simultaneously sliding inferiorly
+ scapular movement = min height maintained = prevent undesired compression of contents within the space
Arthrokinematics of external rotation of glenohumeral joint
Simultaneously rolls posteriorly & slides anteriorly along transverse diameter
Arthrokinematics of flexion & extension, internal & external rotation in 90° abduction
Primarily a spin b/w humeral head & glenoid fossa (surrounding capsular structures taut)
What is the coracoacromial arch?
Functional ‘roof’ of GH joint
- Formed by coracoacromial ligament (attaches b/w anterior margin of acromion & lateral border of coracoid process) & acromion process
What is in the subacromial space?
- Subacromial bursa (2 bursa sacs = superior to humeral head)
- Supraspinatus muscle & tendon
- Biceps brachii long head
- Superior capsule
What are the two bursa sacs in the subacromial space?
Subdeltoid bursa and Subacromial bursa
What is the function of the subdeltoid bursa?
Limit friction b/w deltoid & supraspinatus tendon & humeral head
What is the function of the subacromial bursa?
Protects the supraspinatus muscle & tendon from undersurface of acromion
(Lies above supraspinatus & below acromion)
What happens if there if non-ideal roll & slide arthrokinematics (e.g. adhesive capsulitis = excessive thickening/stiffness of inferior capsular ligament) for abduction?
Inferior slide of humeral head during abd. is limited
- w/o concurrent inf. slide = superiorly rolling humeral head = jamming of head against coracoacromial arch
- Excessive sup. migration of humeral head
= humeral head forced upward & against tissues in subacromial space (excessive stress)
= repeat compression/abrasion of spraspinatus tendon, subacromial bursa, long head of biceps tendon, sup. parts of capsule
What is the scapulohumeral rhythm?
- Synchronous & simultaneous upward rotation of the scapula with humeral flexion/abduction
- Generally, glenohumeral joint : scapulothoracic joint = 2 : 1
- BOTH GH & ST joints contribute significantly to the overall motion of flexion & abduction of shoulder
What are the 6 kinematic principles of shoulder abduction?
- Active shoulder abduction of 180° occurs as result of simult. 120° GH abd & 60° ST upward rotation (based on 2:1 scapulohumeral rhythm)
- 60° scapulothoracic upward rotation is result of simult. elevation at SC joint & upward rotation at AC joint
- Clavicle retracts at SC joint, if abduction is in front plan
- Upwardly rotating scapula tilts posteriorly (+ sometimes ext. rotates slightly) at AC joint
- Clavicle posteriorly rotates around its own axis
- GH joint externally rotates if abduction is in frontal plane
What differences are there when doing shoulder abduction in
frontal plane vs scapular plane ?
Scapular plane:
- Allow greater elevation of humerus
- More natural movement
- Less mechanically coupled to obligatory external rotation of humerus
- Places apex of greater tubercle under relatively high point of coracoacromial arch
(for full frontal plane abd., ext. rotation of humerus must be combined with abd. = so greater tubercle clears posterior edge of undersurface of acromion)
- Allows humeral head to be oriented more directly into glenoid fossa
What are the muscles that move the scapula known as?
- Elevators
- Depressors
- Protractors
- Retractors
- Upward & Downward Rotators
What are the muscles that move the arm known as?
- Flexors & Abductors (GH, upward rotators at ST, rotator cuff)
- Extensors & Adductors
- Internal & External rotators
What are the muscles that elevate the scapula?
- Upper trapezius
- Levator scapulae
- Rhomboids (lesser extent)
How does the upper trapezius help to maintain ideal posture of scapula?
Upper trapezius attached to lateral end of clavicle
= provides leverage around SC joint
= maintenance of ideal posture (slightly elevated & relatively retracted scapula w glenoid fossa slightly upward)
What happens when there is loss of muscular support of the shoulder girdle?
Gravity is allowed to be the dominant force in determining the resting posture of the scapulothoracic joint
= Results in depressed, protracted & excessively downwardly rotated scapula
Over time = damaging stress on other structures
What are the muscles that depress the scapula?
- Lower trapezius (acts directly on scapula)
- Serratus Anterior
- Pectoralis Minor (acts directly on scapula)
- Subclavius
- Latissimus Dorsi
How does the subclavius muscle depress the scapula?
- Acts indirectly on scapula through inferior pull on clavicle
- Compresses & stabilizes SC joint
How does latissimus dorsi depress the scapula?
Depresses shoulder girdle indirectly = primarily by pulling the humerus inferiorly
How can the depressors of the scapula help to partially unload weight of trunk & lower body?
If arm is physically blocked from being depressed, force from depressor muscles can raise thorax relative to the fixed scapula & arm
- Can only occur is scapula is stabilized to a greater extent than thorax
(impt component of transfer b/w bed & WC if paralysed waist down, etc)
What are proximal stabilizers?
Muscles that originate on spine, ribs, cranium and insert on scapula & clavicle
E.g. traps, serratus anterior
What are distal stabilizers?
Muscles that originate on scapula & clavicle and insert on humerus/forearm
E.g. deltoid, biceps brachii
What is force couple?
- Muscles working in synergism to bring about shoulder movements
What are the force couples in upward rotation?
Serratus anterior
Lower trapezius
Middle trapezius
Upper trapezius
What are the force couples in downward rotation?
Rhomboids
Levator scapulae
Pectoralis minor
What are the force couples in retraction?
Middle trapezius
Rhomboids
Lower trapezius
Which glenohumeral joint muscles are responsible for flexion of the arm?
Anterior & Middle Deltoid
Coracobrachialis
Biceps brachii
Which glenohumeral joint muscles are responsible for abduction of the GH joint?
Anterior & Middle Deltoid
Supraspinatus
Note: Middle delt & supraspinatus have nearly equal cross sectional areas & moment arm for abd.
Which muscle is the prime protractor of the scapulothoracic joint?
SERRATUS ANTERIOR
If weak = difficulty in performance of forward pushing motions
BECAUSE no other muscle can adequately provide this effective protraction force on scapula
What are the main retractors of the scapula (scapulothoracic joint)
Middle trapezius (most optimal line of force for this action)
Rhomboids
Lower trapezius
Note: rhomboids & lower traps are perform similar actions but are antagonists to e/o = during vigorous retraction effort, elevation tendency of rhomboids neutralized by depression tendency of lower trap = line of force of both combine to produce pure retraction
What rotator cuff muscles are active during arm elevation?
All four rotator cuff muscles serve as dynamic stabilizers of humeral head during arm elevation
Supra., Infra., & teres minor bind into & reinforce the superior & posterior GH joint capsule
Subscap. blends into & reinforces anterior GH joint capsule
What is the function of supraspinatus during abduction?
- Drives superior roll of humeral head
- Compresses humeral head against glenoid fossa
- Creates musculotendinous “space” that restricts excessive superior translation of humeral head
What is the function of the infraspinatus, teres minor, subscapularis (along w biceps brachii long head & passive tension in lats & teres major)?
Exert inferiorly-directed force on humeral head = neutralize deltoid’s strong superior translation force on humerus during initial phase of abduction
What is the function of the infraspinatus & terest minor?
Externally rotate humerus to clear greater tubercle under acromion
(Exert inferiorly-directed force on humeral head = neutralize deltoid’s strong superior translation force on humerus during initial phase of abduction)
How does the lower traps & serratus anterior posteriorly tile the upwardly rotating scapula?
Lower traps: pulls inferiorly on scapula
Serratus ant.: pulls anterior laterally on scapula
How does the serratus anterior & middle traps externally rotate the upwardly rotating scapula?
Serratus ant: pulls anterior laterally on scapula
Middle traps: pulls medially on scapula
