Shoulder & Elbow Joint Biomechanics

• These are study notes for the HMS Exam 2 Review

Sternoclavicular Joint

• Classified as a saddle joint, movement is described as having 3 degrees of freedom and considered a spin

Sternoclavicular Joint Arthrokinematics

• Elevation/depression is convex on concave
  • Clavicle rolls superiorly and slides inferiorly during elevation
  • Clavicle rolls inferiorly and slides superiorly during depression
• Protraction/retraction is concave on convex
  • Clavicle rolls and slides anteriorly during protraction
  • Clavicle rolls and slides posteriorly during retraction

Effects on Periarticular Connective Tissue at the Sternoclavicular Joint

• Elevation:
  • Costoclavicular ligament is taut
  • Superior capsule and interclavicular ligament are lax
  • Lower trap lengthens
• Depression:
  • Costoclavicular ligament is lax
  • Superior capsule and interclavicular ligament are taut
  • Upper trap lengthens
• Protraction:
  • Posterior bundle of the costoclavicular ligament and posterior capsular ligament are taut
  • Elongated scapular retractor muscles also check protraction at the SCJ
• Retraction:
  • Anterior bundle of the costoclavicular ligament and anterior capsular ligament are taut
  • Posterior capsular ligament is lax

Motions Occurring at the Sternoclavicular Joint During Shoulder Abduction

• Clavicular elevation
• Clavicular retraction
• Clavicular posterior rotation

Acromioclavicular Joint

• Classified as a planar joint, degrees of freedom not typically described
• Motions described by the movement of the scapula relative to the lateral end of the clavicle

Acromioclavicular Joint Arthrokinematics

• Upward/downward rotation (frontal plane) is a natural component of shoulder flexion and abduction coupled with scapular upward and downward rotation
• Anterior/posterior tilting (sagittal plane) involves "fine-tuning," where the inferior angle of the scapula tilts toward or away from the thorax
  • ACJ anteriorly tilts during scapular elevation
  • The scapula can tilt anteriorly slightly during elevation of the scapulothoracic joint for alignment
• Internal/external rotation (transverse plane) is "fine-tuning" where the medial border of the scapula tilts toward or away from the thorax
  • ACJ IR occurs with scapular protraction

Stabilizing Acromioclavicular Joint Ligaments

• Superior and inferior acromioclavicular joint ligaments
• Coracoclavicular ligament (conoid, trapezoid)

Muscles Supporting the Superior Aspect of the Acromioclavicular Joint

• Deltoid
• Upper trap

Pathomechanics of an Acromioclavicular Joint Dislocation

• Medially and inferiorly directed force on the acromion is the mechanism of injury, such as from a fall on the tip of the shoulder
• The superior/inferior AC ligaments initially resist the force, with more severe shear forces resisted by the coracoclavicular ligament

Structures Common to Both the Acromioclavicular and Sternoclavicular Joints

• Fibrocartilaginous Articular Disc

Scapulothoracic Joint

• Considered a point of indirect contact, not a true joint
• Resting position involves 10 degrees anterior tilt, 5-10 degrees upward rotation, and 30-40 degrees internal rotation, occurring in the plane of the scapula

Functions of Scapulohumeral Upward Rotation During Shoulder Elevation

• Projects glenoid fossa upward and into external rotation, providing a structural base for the humerus
• Preserves optimal length tension relationship of GHJ abductors (middle deltoid, supraspinatus)
• Preserves volume within the subacromial space

Nerve Injury Weakening Protraction

• Injury to the long thoracic nerve, spinal nerve roots C5-C7, is likely to severely weaken the movement of protraction

Glenohumeral Joint

• Classified as a ball and socket, providing 3 degrees of freedom

Glenohumeral Joint Arthrokinematics

• Abduction (frontal plane): humeral head rolls superiorly and slides inferiorly
• Adduction (frontal plane): humeral head rolls inferiorly and slides superiorly
• Flexion (sagittal plane): humeral head spins in the glenoid fossa
• Extension (sagittal plane): humeral head spins in the glenoid
• Internal rotation in neutral abduction (transverse plane): humeral head rolls anteriorly and slides posteriorly
• External rotation (transverse plane): humeral head rolls posteriorly and slides anteriorly
• Internal/external rotation at 90 degrees abduction: humeral head spins in the glenoid fossa

Glenohumeral Joint Periarticular Connective Tissues

• The GH joint capsule and supporting ligaments play critical roles in permitting or restricting arthrokinematic movements

Glenohumeral Joint Stability

• Periarticular connective tissues, along with muscles, serve as the primary source of GHJ stability
• Joint capsule has twice the size of the humeral head, which allows extensive mobility
• The joint is relatively thin, reinforced by thicker external ligaments
• Glenohumeral capsular ligaments limit extremes of rotation and translation
• Superior GH ligament
• Middle GH ligament
• Inferior GH ligament
• Axillary pouch
• Coracohumeral ligament blends with the superior capsule and supraspinatus tendon; relatively taut in anatomic position and restrains inferior translation and external rotation of the humeral head
• Glenoid Labrum is a fibrocartilaginous ring that accounts for 50% of glenoid depth

Ligament Forming the Functional “Roof” of the Glenohumeral Joint

• Coracoacromial ligament forms the functional roof

Structures Found in the Subacromial Space

• Superior GHJ capsule
• Supraspinatus muscle and tendon
• Subacromial bursa
• Long head of the biceps

Role of Rotator Cuff and Scapulothoracic Muscles as Glenohumeral Joint Stabilizers

• Rotator cuff muscles
  • Tendons blend into the joint capsule
  • Stretched tendons on the opposite side of active RC activation serve as passive restraints
  • Supraspinatus provides compression
  • Subscapularis, infraspinatus, and teres minor exert inferior-directed force to neutralize deltoid superior translatory effects
  • Latissimus dorsi and teres major have passive stretch exert inferior-directed force on the humerus in the glenoid

Contribution of the Long Head of the Biceps to Glenohumeral Joint Stability

• Restricts anterior translation of the humeral head
• Resists superior translation of the humeral head

Areas Where the Rotator Cuff Fails to Provide Dynamic Stabilization

• RC interval, the space between the supraspinatus and subscapularis tendons
• Inferiorly

Differences in Shoulder Arthrokinematics Between Abduction and Scaption

• Scaption allows greater elevation of the humerus due to larger “high point” of the coracoacromial arch
• Abduction requires more obligatory humeral external rotation

Humeroulnar Joint

• Classified as a hinge joint with one degree of freedom (flexion, extension)
• Concave on convex

Humeroulnar Joint Arthrokinematics

• During flexion, the ulna rolls and slides superiorly on the humerus (trochlea)
• During extension, the ulna rolls and slides posteriorly on the humerus (trochlea)

Humeroradial Joint

• Classified as a hinge joint with one degree of freedom (flexion, extension)
• Concave-on-convex

Humeroradial Joint Arthrokinematics

• During elbow flexion, the radius rolls and slides superiorly on the capitulum of the humerus
• During elbow extension, the radius rolls and slides posteriorly on the capitulum of the humerus

Function of the the Central Band of the Interosseous Membrane

• Firmly bind the radius to the ulna
• Attachment site for extrinsic hand muscles
• Transmit force proximally through the UE

Joints Enclosed by he Elbow Articular Capsule

• Humeroulnar joint

Structures That Resist Distraction of the Radius

• Brachioradialis
• Biceps
• Annular ligament
• Oblique cord and
• Distal oblique fibers of the interosseous membrane

Structures Lengthened in Elbow with Flexion

• Posterior capsule
• Elbow extensors
• Ulnar nerve
• Posterior fibers of the MCL
• Lateral ulnar collateral ligament

Structures Lengthened in Elbow with Extension

• Anterior capsule
• Elbow flexors
• Anterior fibers of the MCL

Proximal and Distal Radioulnar Joints

• Classified as a pivot joint with one degree of freedom
• Concave on convex

Proximal Radioulnar Joint Arthrokinematics

• During supination/pronation occurs via the radial head rotates/spins within the ring of the annular ligament
• Radial head rolls in a dorsal-lateral direction in supination
• During pronation Radial head rolls in a volar-medial direction
• Elbow extension increases compressive force on the joint during pronation, potentially creating proximal migration of the radius

Distal Radioulnar Joint

• Concave ulnar notch of the radius slides and rolls over fixed ulna in supination/pronation
• Radius rolls and slides in dorsal direction on fixed ulnar head

Soft Tissue Structure That Is An Integral Part of the Distal Radioulnar Joint

• Triangular Fibrocartilage disc
• Transmits 20% of compressive force into the ulna, while 80% goes through the scaphoid and lunate to the radius

Structures Limiting Supination At the Elbow

• Pronator teres and quadratus
• FCR
• Extrinsic finger flexors
• TFCC - palmar capsular ligament
• Quadrate ligament

Structures Limiting Pronation At the Elbow

• Biceps
• Supinator
• Radial wrist extensors
• EPL
• TFCC - dorsal capsular ligament

Workhorse of the Elbow

• Brachialis
• Largest cross-sectional area, attachment on ulna allow involvement in any elbow position

Greatest Elbow Flexor Torques During Flexed Position

• Supination - biceps and brachioradialis increased flexor moment near supination positions

Radiocarpal Joint

• Condyloid, with 2 degrees of freedom
• Motion occurs primarily at the capitate

Radiocarpal Athrokinematics

• Flexion/Extension is convex on concave
  • Lunate rolls palmarly and slides dorsally during flexion
  • Lunate rolls dorsally and slides palmarly during extension
• Ulnar/radial deviation is convex on concave
  • Scaphoid, lunate, and triquetrum roll radially and slide ulnarly during radial deviation
  • Scaphoid, lunate, and triquetrum roll ulnarly and slide radially during ulnar deviation

Joint Contributing Most Motion to Wrist Radial and Ulnar Deviation

• Midcarpal joint

Natural Wrist Kinematics

• Dart throwers motion during extension with radial deviation and flexion with ulnar deviation

Most Vulnernable Carpal

• Scaphoid transmits direct force through the wrist and is most often fractured

Organization and function of the distal and proximal row

• Proximal row of carpals is loosely joined, which allows the formation of the palmar concavity
• Distal row is firmly joined, which provides a rigid and stable base for articulations with metacarpals

Most Unstable Carpal

• The lunate - lacks muscular and ligamentous attachments

Function of the Transverse Carpal Ligament

• Attachment for many intrinsic hand muscles
• Passageway for the median nerve and extrinsic finger tendons, prevent “bowstringing”

Dorsal Radiocarpal Ligament

• Is thin and blends with wrist joint capsule
• Courses in an ulnar direction
• Reinforces the posterior side of the radiocarpal joint
• Fibers attach to lunate to resist dislocation
• Is richly innervated providing a dominant role in wrist proprioception

Palmar Radiocarpal Ligament

• Thick, strong ligament
• Courses obliquely toward ulna
• Maximally taut in wrist extension

Structures Composing the Triangular Fibrocartilage Complex (TFCC)

• Triangular fibrocartilage disc
• Palmar ulnocarpal iigament (2 distinct ligaments)
• Distal radioulnar joint capsular ligaments
• Ulnar collateral ligament
• Fascial sheath of the ECU tendon

TFCC Functions

• Primary stabilizer of the distal radioulnar joint
• Reinforces ulnar side of wrist
• Forms part of the concavity of the radiocarpal joint
• Transfers forces from the hand to the ulna

Structure Securing Wrist Extensor Tendons

• Extensor retinaculum, it overlies 6 fibro-osseous compartments of tendon with their synovial sheaths

Function of Wrist Extensors

• Stabilize the wrist during active flexion of the digits and gripping

Optimal Grip Wrist Force

• Occurs at 30 degrees wrist extension

Wrist in Full Flexion

• Finger flexors are shortened on the length-tension curve
• Finger extensors are stretched and provide passive extensor torque

Greatest Wrist Extensor Moment Arm

• Wrist Extension - ED
• Wrist Flexion - FCR
• Radial deviation = EPB
• Ulnar Deviation - ECU

Questions on the biomechanics of the shoulder and elbow joint. Includes ligamentous configurations, ACJ separation, scapular rotation and mobilization to improve elbow extension. Focuses on musculoskeletal rehabilitation.