Mechanics of the Shoulder Complex

Flashcards covering joint kinematics, arthrokinematics, muscle force couples, and clinical biomechanics of the shoulder complex.

What are the four components that comprise the shoulder complex?

1) Sternoclavicular joint, 2) Acromioclavicular joint, 3) Scapulothoracic articulation, and 4) Glenohumeral joint.

Based on this anatomical diagram of the sternoclavicular joint, what are the convex-concave surface relationships for anterior/posterior and superior/inferior movements?

For anterior/posterior movements, the clavicle is concave moving on a fixed convex sternal surface. For superior/inferior movements, the clavicle is convex moving on a fixed concave sternal surface.

What arthrokinematic motion occurs at the sternoclavicular joint during osteokinematic elevation of the clavicle?

Arthrokinematic depression of the clavicle at the joint.

How many degrees of clavicular elevation and depression are available from rest at the sternoclavicular joint?

35 to 45 degrees of clavicular elevation from rest and about 15 degrees of clavicular depression from rest.

How much clavicular elevation occurs relative to glenohumeral movement during the initial 90 degrees of arm elevation?

About 4 to 5 degrees of clavicular elevation occurs for every 10 degrees of glenohumeral flexion or abduction during the initial 90 degrees.

During what range of shoulder motion does posterior rotation of the clavicle occur, and through how many degrees?

Posterior rotation occurs from 90 to 180 degrees of shoulder flexion and abduction, moving 30 to 45 degrees in each direction along the long axis of the clavicle.

Which ligament limits the anterior/posterior movement of the lateral end of the clavicle at the AC joint?

The conoid ligament.

Which ligaments limit elevation of the clavicle at the acromioclavicular (AC) joint?

The trapezoid and conoid ligaments (coracoclavicular ligaments).

How does the trapezoid ligament control clavicular rotation at the AC joint?

The trapezoid ligament acts as a hinge for rotation; its anterior fibers become taut at the limit of posterior clavicular rotation, while its posterior fibers become taut at the limit of anterior rotation (at neutral).

What are the structural components of the scapulothoracic articulation?

1) Posterior rib cage between ribs 2 through 8, 2) Serratus anterior muscle, and 3) Subscapularis and costal surface of the scapula.

What occurs during scapular upward rotation, and what functional benefits does it provide during arm elevation?

The glenoid fossa arcs upward and the inferior angle arcs laterally and upward. This increases the total range of GH flexion/abduction, maintains the subacromial space, and allows the deltoid, rotator cuff, and coracobrachialis to function near the top of their length-tension curve for longer.

What is scapulo-humeral rhythm, and what is its typical ratio?

It is the movement of the scapula relative to movement at the glenohumeral joint. The average rhythm is a 2:1 ratio (2 degrees of glenohumeral motion for every 1 degree of scapular motion).

What mechanical events characterize Phase I (0 to 90 degrees) of combined scapular movement?

Phase I involves primarily scapular elevation with slight upward rotation beginning at about 60 degrees, elevation of the lateral clavicle and AC joint, and depression of the medial clavicle and sternoclavicular joint.

What mechanical events characterize Phase II (above 90 degrees) of combined scapular movement?

Phase II involves primarily scapular upward rotation, cessation of clavicular/scapular elevation around 90 degrees, and posterior rotation of the clavicle from neutral as the acromion and glenoid fossa rotate upward.

In what position is the glenohumeral joint most vulnerable to anterior dislocation, and why?

When vertically abducted near 90 degrees and fully externally rotated, because the anterior ligaments and subscapularis tendon are rotated upward, leaving only a thin inferior-anterior capsule to prevent dislocation.

What structures make up the coracoacromial arch, and what structure lies directly beneath it to facilitate sliding?

The arch consists of the acromion and the coracoacromial ligament. Directly beneath it lies the subacromial bursa, which allows the greater tubercle of the humerus to slide under the arch.

Which muscles make up the force couple responsible for scapular upward rotation?

Upper Trapezius, Lower Trapezius, and Serratus Anterior (with Middle Trapezius assisting to neutralize protraction).

Which muscles act as downward rotators of the scapula?

Levator Scapulae, Rhomboid Major, Rhomboid Minor, and Pectoralis Minor.

Which muscles form the force couple producing an inferior glide and holding the humeral head in the glenoid fossa during elevation?

Infraspinatus, Subscapularis, and Teres Minor (along with Supraspinatus providing hold).

What is the 'accordion effect' in shoulder mechanics?

It occurs when muscles moving the humerus lack a stable proximal attachment site, causing both the proximal and distal attachments to move rather than executing isolated, effective joint motion.

How do abduction forces change following a suprascapular nerve block versus an axillary nerve block?

A suprascapular nerve block causes a 60% decrease in abduction force from 0 to 60 degrees and a 30% decrease above 60 degrees. An axillary nerve block causes a 35% decrease in abduction force from 0 to 60 degrees and a 60% to 80% decrease above 90 degrees.