PRAC 2: Arm, elbow and forearm

Joints of the elbow joint complex

Humeroulnar, humeroradial, and proximal radioulnar joints (all within a single joint capsule).

Primary elbow joints responsible for flexion and extension

Humeroulnar and humeroradial joints, acting together as a synovial hinge joint.

Articulating surfaces of the humeroulnar joint

Trochlea of the humerus and trochlear notch of the ulna.

Articulating surfaces of the humeroradial joint

Capitulum of the humerus and head of the radius.

Humeral external torsion

A slight external twist of the humeral shaft around its long axis.

Functional significance of humeral external torsion

Directs the distal humerus anteriorly, causing hand movement upward and forward toward the clavicle during elbow flexion.

Functional impact of the anterior curve of the distal humerus

Directs joint surfaces anteriorly to favor and facilitate elbow flexion.

Normal elbow flexion-extension range of motion (ROM)

0^\null extension to 140^\null flexion.

Functional elbow ROM required for activities of daily living (ADLs)

30^\null to 130^\null of flexion.

Normal elbow carrying angle

10^\null\text{--}15^\null valgus.

Definition of valgus at the elbow

Lateral deviation of the distal segment (forearm) away from the body relative to the humerus.

Definition of varus at the elbow

Medial deviation of the distal segment (forearm) toward the body relative to the humerus.

Vulnerability of normal elbow carrying angle to injury

Pre-existing valgus alignment causes axial forces through the hand to push the elbow further into valgus, predisposing it to valgus injury.

Tension states of the elbow joint capsule

Anterior capsule is taut in extension; posterior capsule is taut in flexion.

Elbow position of maximum capsular laxity

Approximately 80^\null elbow flexion.

Reason for holding the elbow at 80^\null flexion during joint effusion

Maximum capsular laxity occurs at this position, creating room for fluid accumulation and minimizing pressure and pain.

Axial loading at the elbow

Transmission of force traveling up the forearm into the arm (e.g., pushing up from a chair).

Axial load distribution across the elbow in full extension and neutral rotation

$60\%$ humeroradial and $40\%$ humeroulnar.

Functional benefit of load sharing across the elbow joints

Distributes force to reduce mechanical stress on any single articular surface.

Joint instability

Excessive or abnormal movement of a joint.

Primary joint stabiliser

A restraint whose isolated injury or release results in abnormal joint laxity.

Secondary joint stabiliser

A restraint that does not cause laxity when injured alone, but exacerbates laxity when a primary restraint is damaged.

Passive (static) stabilisers of the elbow

Bony congruency, joint capsule, ligaments, and interosseous membrane.

Active (dynamic) stabilisers of the elbow

Muscles crossing the elbow joint.

Mechanism of muscle-mediated joint stabilisation

Muscle contraction compresses joint surfaces together, enhancing stability against abnormal bone displacement.

Primary compressive dynamic stabilisers of the elbow

Brachialis, biceps brachii, and triceps brachii.

Four main directions of elbow instability

Valgus, varus, posterior, and posterolateral rotary instability.

Major bony restraint to valgus and varus force across elbow ROM

Wide, interlocking ulnohumeral (humeroulnar) articulation.

Mechanism of humeroulnar resistance to valgus and varus

Deep, broad fit between the trochlea and trochlear notch prevents sideways tilting of the ulna.

Additional bony restraint to valgus/varus in 0^\null\text{--}20^\null elbow flexion

Olecranon engaging within the olecranon fossa.

Primary stabiliser against posterior elbow dislocation

Coronoid process.

Mechanism of coronoid process in preventing posterior dislocation

Forms the anterior wall of the trochlear notch, physically blocking posterior displacement of the ulna relative to the humerus.

Primary passive restraint to valgus instability (especially in flexion)

Anterior band of the medial collateral ligament (AMCL).

Biomechanical importance of the AMCL during elbow flexion

Stiffest and strongest part of the MCL, remaining taut throughout flexion-extension to resist lateral opening of the forearm.

Three bands of the medial collateral ligament (MCL)

Anterior, posterior, and transverse bands.

Contribution of the transverse band of the MCL to elbow stability

Minimal to no significant contribution.

Tension state of the posterior band of the MCL

Taut in a flexed elbow position.

Primary structures resisting varus instability

Lateral collateral ligament complex (LCLC) and ulnohumeral articulation.

Secondary restraints to valgus instability

Humeroradial articulation, anterior capsule in extension, and common flexor muscle origin.

Role of the common flexor origin in valgus resistance

Medial forearm muscles contract to generate a varus moment, counteracting valgus stress.

Secondary restraints to varus instability

Anterior capsule in extension and common extensor muscle origin.

Role of the common extensor origin in varus resistance

Lateral forearm muscles contract to generate a valgus moment, counteracting varus stress.

Secondary restraints to posterior elbow instability

Medial collateral ligament (MCL) and brachialis.

Components of the lateral collateral ligament complex (LCLC)

Radial collateral ligament (RCL), annular ligament, and lateral ulnar collateral ligament (LUCL).

Attachments of the radial collateral ligament (RCL)

Lateral epicondyle to the annular ligament.

Attachments of the annular ligament

Anterior and posterior margins of the radial notch of the ulna.

Attachments of the lateral ulnar collateral ligament (LUCL)

Lateral epicondyle to the supinator crest of the ulna.

Posterolateral rotary instability (PLRI)

Abnormal supination and lateral rotation of the ulna and radius, causing the lateral aspect of the elbow to sublux posterolaterally.

Primary restraint to posterolateral rotary instability (PLRI)

Lateral ulnar collateral ligament (LUCL).

Dynamic muscle stabilisers against PLRI

Anconeus and brachialis.

Muscles of the anterior compartment of the arm

Coracobrachialis, biceps brachii, and brachialis.

Primary actions of coracobrachialis

Flexion and adduction of the glenohumeral (shoulder) joint (not a primary elbow flexor).

Primary flexors of the elbow joint

Brachialis, biceps brachii, and brachioradialis.

Biomechanical formula for muscle torque at a joint

$$\text{Torque} = \text{Muscle Force} \times \text{Moment Arm}$$

Physiological cross-sectional area (PCSA) significance

Indicates potential force output; larger PCSA means more muscle fibers in parallel capable of generating greater force.

Elbow flexor with the largest physiological cross-sectional area (PCSA)

Brachialis.

Elbow flexor with the longest moment arm

Brachioradialis.

Key factors making brachialis a potent elbow flexor

Largest PCSA (high force capacity) and single-joint action dedicated entirely to the elbow.

Reason brachialis force is unaffected by forearm rotation

Inserts on the ulna, which does not rotate during pronation or supination.

Reason biceps brachii function is affected by forearm position

Inserts on the radial tuberosity, which rotates with the radius during pronation and supination.

Reason biceps brachii function is affected by shoulder position

Crosses both the shoulder and elbow joints, altering its length with shoulder movement.

Elbow angle of maximum flexor torque

Approximately 70^\null\text{--}90^\null elbow flexion.

Biomechanical reason for optimal flexor torque at 70^\null\text{--}90^\null

Favorable moment arms combined with optimal muscle length allow maximum pull efficiency.

Sliding filament explanation for reduced force in shortened muscles

Excessive overlap of actin and myosin filaments limits effective cross-bridge formation.

Three heads of the triceps brachii

Long, lateral, and medial heads.

Reason triceps long head function varies with shoulder position

Originates at the infraglenoid tubercle of the scapula, making it a biarticular muscle crossing both shoulder and elbow.

Distal insertion site of all three triceps brachii heads

Olecranon of the ulna.

Biomechanical role of the olecranon in triceps extension torque

Projects posterior to the elbow axis of rotation, increasing the triceps moment arm and leverage.

Elbow extensor torque relative to flexor torque strength

Approximately $80\%$ as strong as flexor torque.

Strongest extensor of the elbow

Triceps brachii.

Triceps head with higher activity at 0^\null shoulder flexion

Long head.

Triceps head with higher activity above 90^\null shoulder flexion

Medial head.

Advantage of recruiting monoarticular triceps heads in isometric extension

Medial and lateral heads cross only the elbow, allowing force production without influence from shoulder position.

Functional roles of anconeus

Assists elbow extension near end-range, dynamically stabilizes against PLRI, and abducts the ulna during fixed-hand pronation.

Mechanism of anconeus in PLRI protection

Anterior fibers overlie and run parallel to the LCL/LUCL, resisting posterolateral subluxation when contracted.

Reason primary elbow flexors/extensors generate minimal varus/valgus force

Lines of action are centrally aligned across the joint, pulling straight rather than from lateral or medial sides.

Three components of the forearm structural complex

Proximal radioulnar joint (PRUJ), interosseous membrane (IOM), and distal radioulnar joint (DRUJ).

Functional rationale for PRUJ, IOM, and DRUJ coupling

Operate as a unified kinetic chain for pronation-supination and axial load distribution between radius and ulna.

Joint classification of the proximal radioulnar joint (PRUJ)

Synovial pivot joint.

Articulating surfaces of the PRUJ

Circumference of the radial head and radial notch of the ulna.

Primary stabiliser of the PRUJ

Annular ligament.

Primary role of the annular ligament

Secures the radial head in the radial notch while permitting rotation during pronation and supination.

Interosseous membrane (IOM) structure

Fibrous sheet linking radius and ulna, featuring a prominent central interosseous ligament.

Fibrillar orientation of the central interosseous ligament

Distal and medial orientation from radius to ulna.

Functions of the interosseous ligament

Muscle attachment site, rotational axis hinge, prevents bone separation/rotation, stabilizes PRUJ/DRUJ, and transfers load.

Role of the IOM in axial load transmission

Transfers proximal force from the radius to the ulna, preventing load concentration on a single joint.

Joint classification of the distal radioulnar joint (DRUJ)

Synovial plane joint.

Articulating structures of the DRUJ

Distal ulna with ulnar notch of radius, and distal ulna with triangular fibrocartilage disc.

Primary passive stabilisers of the DRUJ

Palmar and dorsal radioulnar ligaments of the TFCC.

Anatomical components of the TFCC

Triangular fibrocartilage, palmar/dorsal radioulnar ligaments, ECU tendon sheath, and meniscus homologue.

Active dynamic stabilisers of the DRUJ

Extensor carpi ulnaris (ECU) and pronator quadratus.

Normal pronation range of motion

75^\null\text{--}85^\null.

Normal supination range of motion

80^\null\text{--}90^\null.

Functional significance of load sharing across forearm joints

Decreases mechanical stress on individual structures, reducing injury risk.

Primary supinator muscles of the forearm

Biceps brachii and supinator.

Primary pronator muscles of the forearm

Pronator teres and pronator quadratus.

Primary action of brachioradialis

Elbow flexion regardless of forearm position.

Secondary rotational action of brachioradialis

Assists pronation from full supination back to neutral position.

Biomechanical reason brachioradialis only pronates to neutral

Line of pull is optimal for returning an overly supinated forearm to mid-position, not for full pronation.

Forearm muscles originating from the medial epicondyle

Pronator teres, flexor carpi radialis (FCR), palmaris longus, flexor carpi ulnaris (FCU), and flexor digitorum superficialis (FDS).