Elbow Complex: Anatomy and Biomechanics

Elbow Complex Objectives

• Identify the elbow complex components
• Discuss factors impacting the elbow complex's stability and mobility
• Describe the role of the musculature acting at the elbow and the effects of varying loads at the joint
• Describe open and close packed positions of the elbow and forearm complex
• Discuss structure and function at the radioulnar joints
• Describe the elbow and forearm arthrokinematics

Elbow Joint Complex Overview

• Allows for mobility of the hand in space
• Provides stability during skilled & forceful movements during manual activities
• Linked to function of shoulder and wrist via crossing muscles

Elbow Joint Complex Components

• Humeroulnar (HU) joint
• Humeroradial (HR) joint
• Proximal & distal radioulnar (RU) joints

Elbow Joint Details

• "Modified" hinge joint
• Small amount of rotation & ABD/ADD of ulna during flexion & extension
• Flexion/extension occurs in the sagittal plane
• 1 degree of freedom (dof)

Humeroulnar (HU) Joint

• Articulating surfaces include:
  • Trochlea of humerus
  • Trochlear notch of ulna

Humeroradial (HR) Joint

• Articulating surfaces include:
  • Capitulum of humerus
  • Head of radius (fovea)

Humeroulnar Joint Motion

• Primarily a sliding motion of the trochlear notch on the trochlea of humerus
• During flexion:
  • The ulna slides along the trochlea until the coronoid process enters the coronoid fossa in full flexion
• During extension:
  • The ulna slides until the olecranon process enters the olecranon fossa
• The trochlea only contacts the deepest portion of the notch when moderately loaded
• Contact b/w trochlea & deepest portion of trochlear notch from 30°-120° of flexion
• When unloaded, contact primarily occurs on the sides of the notch
• Contact area expands toward depth of the trochlear notch when loaded

Humeroradial Joint Motion

• Primary motion is the sliding of the concave radial head over the convex surface of the capitulum
• In full flexion, the rim of the radial head slides into the radial fossa
• In full extension, there is no contact b/w the articulating radial head and capitulum when the arm is unloaded/NWBing

Radioulnar Joints

• Proximal & distal RU joints are linked, function as 1 joint, and act together to produce rotation of the forearm
• Uniaxial and diarthrodial pivot joints with 1 degree of freedom (DOF)
• Pronation/supination occurs in the transverse plane
• The proximal RU joint shares a capsule with the HU & HR joints

Joint Elbow Capsule

• HU, HR, and proximal RU joints share a capsule
• Capsular Attachments:
  • Proximal: just superior to coronoid & radial fossa
  • Distal: along the margin of the coronoid process of the ulna, blending with the annular ligament
  • Medial & lateral: continuous with collateral ligaments
  • Posterior: along the upper edge of the olecranon fossa and to the back of the medial epicondyle, just inferior to the annular ligament

Joint Capsule & Associated Structures

• Capsule is relatively large and loose
  • Weaker anteriorly & posteriorly
  • Reinforced laterally & medially by collateral ligaments
• Synovial membrane
  • Folds in the membrane allow expansion for a full ROM
  • Can become inflamed & hypertrophied, causing pain

Ligamentous Function at the Elbow Joint

• Ligaments and their locations in relation to joint structures

Terminology Checkpoint: Valgus vs Varus

• Valgus: lateral projection of distal segment of a bone or joint
• Varus: medial projection of distal bone segment
• Valgus force: creates, tension on the medial aspect of the elbow & increases compression at the lateral joint
• Varus force: creates tension on the lateral aspect of the elbow & increases compression medially
• A varus or valgus force can be applied distally or at the joint itself
• Example: Valgus stress can be created with a laterally directed force to the distal forearm when the upper arm is fixed OR a medially directed force at the joint with the forearm fixed

Ligaments of the Elbow

• Medial (ulnar) collateral ligament complex:
  • Anterior bundle
    • Anterior band
    • Posterior band
• Posterior bundle
• Transverse (oblique) bundle: variable presence

Medial (Ulnar) Collateral Ligament (MCL) Complex:

• Provides overall function:
  • Stabilizes against valgus forces at the medial elbow
  • Limits hyperextension
  • Guides joint motion throughout the flexion ROM (range of motion)
  • Some resistance to longitudinal distraction of joint surfaces

Individual Medial (Ulnar) Collateral Ligament (MCL) Components

• Anterior bundle
  • It is the primary ligamentous restraint to valgus stress from 20° - 120° of elbow flexion
• Posterior bundle
  • Has a less significant role than the anterior bundle in providing valgus stability
• Transverse (oblique) bundle
  • AKA Cooper's ligament
  • Contributes little to valgus stability
  • Plays a role in the approximation of joint surfaces

Ligaments of the Elbow

• Lateral (radial) collateral ligament complex:
  • Lateral (radial) collateral ligament
  • Lateral ulnar collateral ligament
  • Annular ligament

Lateral Collateral Ligament Complex Overview

• Overall function:
  • Stabilizes the elbow against varus forces
  • Stabilizes against combined varus & supination
  • Reinforces HR (humeroradial) joint
    • Provides resistance to longitudinal distraction of the joint
  • Maintains posterolateral rotatory stability
  • Stabilizes the radial head for rotation (annular ligament)

Lateral Collateral Ligament Components

• Lateral (radial) collateral ligament
  • Reinforces the HR (humeroradial) joint
  • Resists varus stress at the elbow
    • Assists in resisting longitudinal distraction of joint
• Lateral ulnar collateral ligament
  • Is a 2° restraint to combined forced varus & supination stresses
  • Assists in resisting varus stress
• Annular ligament
  • Resists Distraction of the Radius
  • Stabilizes the radial head against the ulna at PRUJ (Proximal Radioulnar Joint)

Mechanics & Musculature of the Elbow Complex

• Overview of elbow complex anatomy

Carrying Angle

• Forearm typically lies slightly lateral to humerus when the elbow is fully extended in anatomic position
• "Carrying angle" normal range: 8 - 15°
• Female carrying angle tends to be greater than males

Cubital Positions

• Cubital (cubitus) valgus
  • Excessive carrying angle (>15°)
• Cubital (cubitus) varus
  • Reduced carrying angle (< 5°) or reversed

Cubitus Varus

• Also known as "gunstock deformity"
• Term used when the deformity results from a malunion following a supracondylar fracture of the humerus

Axis of Motion

• Axis of motion for flexion/extension
  • Oblique axis of rotation
  • Axis runs through the middle of the Trochlea

Axis of Rotation: Flexion of Elbow

• The ulna is guided medially when moving from extension to full flexion (2° to oblique axis)
  • Typical carrying angle: Forearm comes to rest in line with humerus
  • Cubital valgus: Forearm comes to rest medial to humerus in full flexion
  • Cubital varus: Forearm may come to rest lateral to humerus

Musculature of Elbow Joint

• Muscles crossing anterior to the elbow joint
  • Primary elbow flexors:
    • Brachialis, biceps brachii, & brachioradialis
  • The following muscles have primary functions at the wrist/hand
    • FCR, FCU, FDS, & palmaris longus
      • Can serve as weak elbow flexors
  • Supinator & pronator teres have primary functions at the RU joints

Joint Musculature

• Elbow joint muscles and points of action
• Biceps brachii & brachialis mobility muscles; brachioradialis stabilizer muscle

Brachialis

• Mobility muscle that:
  • Has an insertion close to the axis of rotation
    • Favors a large arc of motion
  • Large CSA contributes to a greater potential for force generation
  • MA (muscle activity) is greatest at ~100° of elbow flexion

Biceps Brachii

• Mobility muscle that:
  • Inserts close to the axis
  • Shoulder position impacts force production
  • Muscle activity is greatest b/w 80-100° flexion
  • Elbow flexion > 100°
    • Changes the line of action and creates a distraction force at the joint
  • Most active if the forearm is supinated or in neutral
  • Less active if the forearm is pronated
  • Active in all positions of the forearm when resistance is greater than the weight of the forearm
  • Muscle activity decreases as the elbow moves closer to full extension, making it less effective as a flexor

Brachioradialis

• Stability Muscle that has an insertion at a greater distance from the joint axis
  • The largest component of muscle force goes to joint compression
  • Relatively small muscle, but has a large average peak of muscle activity as compared to other elbow flexors
    • Peak muscle activity occurs b/w 100° - 120° of elbow flexion
      • Muscle activity increases when the speed of flexion increases - Moderate activity when the forearm is loaded in a neutral or fully pronated position

Musculature of Elbow Joint

• Muscles crossing posterior to the elbow joint
  • Extensors:
    • Triceps Brachii: Long Head, long head crosses shoulder joint
    • Anconeus
  • The following Originate at the lateral epicondyle & primarily act at the wrist/hand:
    • ECRL, ECRB, ED, ECU and EDM
      • The Effect at the elbow is predominantly compressive for stabilization

Elbow Joint Triceps

• Three heads, long head crosses shoulder joint
• Primary extensor with Long head force production ability impacted by shoulder position
• Medial & lateral heads are not impacted by shoulder position
• Muscle torque production at ~90 deg elbow flex
• Muscle activity will vary depending upon shoulder position
• Muscle acts as a stabilizer & synergist
  • Acts as a stabilizer when biceps is acting as a supinator

Elbow Joint Anconeus

• Assists in Elbow extension
• Contributes ~15% of the total extension torque at the elbow
• Act as a stabilizer during muscle activity

Elbow Mobility

• Factors impacting flexion/extension motion at the elbow
  • Type of motion (active or passive)
    • Passive flexion is typically > active flexion
  • Position of the forearm (pronation/supination)
    • Flexion is greater when forearm is supinated
  • Body mass index (BMI)
    • Flexion ROM tends to be less with higher BMI due to tissue approximation
  • Position of the shoulder
    • Active or passive insufficiency of2 joint muscles can impact elbow motion

Factors Contributing to Elbow Joint Stability

• Configuration of joint surfaces
  • Inherent articular stability at end range of flexion & extension
• Ligaments
• Joint capsule
• Active & passive tension in muscles crossing the joint
  • Contributes to stabilization via compression & co-contraction
  • Co-contraction of muscles at the elbow assists in creating a stable base for forceful motions of wrist & fingers

Elbow Stability with Extension

• In full extension:
  • Bony components provide resistance to varus & valgus stress
• The Olecranon process contacts the olecranon fossa limiting extension
  • The HU (humeroulnar) joint is in close-packed position
  • Additional resistance to the varus/valgus forces comes from the capsule & ligaments
  • Resistance to joint distraction is provided entirely by soft tissue
  • Anterior capsule provides major resistance to anterior displacement of the humerus

Elbow Stability with Flexion

• At 90° of flexion
  • The Anterior portion of the MCL provides 1° resistance to distraction & valgus stresses
  • The Majority of resistance to varus stress is provided by osseous joint structures, with slight contribution from the LCL & capsule
• In full flexion
  • Approximation of the coronoid process with the coronoid fossa and the radial head in the radial fossa limit extremes of flexion

Radioulnar Joints again

• Linked proximal and distal, function as 1 unit
• Rotation occurs throughout the arm
• 1 degree of freedom
• Pivotal and diarthrodial

Proximal RuJ

• Radial notch on the ulna articulates with the radius
• Concave surfaced is lined with hyaline cartilage
• Annular ligament attaches anteriorly and posteriorly to stabilize the joint

More Details on RU Joints

• Fovea spins against the capitulum
• Spinning matches axis of rotation through PRUJ and DRUJ

Support for Proximal

• Annular ligaments form ring around radial head
• Quadrate ligaments stabilize radial head
• There is also an Oblique Cord that runs ventrally

Distal Radioulnar (DRUJ Details

• Transmits loads from hand to forearm.
• This is a uniaxial pivot joint
• Joins Distal Radius with the Fibrocartilage triangle called the TFCC,
• This unit is the main stabilizer

The unit for DRUJ (Distal Radioulnar Joint)

• Composed of Concave ulnar notch
• Convex head of ulna
• Connected by lax capsule
• Reinforced anteriorly with palmar ligaments
• Reinforced posteriorly with looser capsules that allow expansion

The Complex of the Triangle Fibro-Cartilage

• Stability for Radioulnar Unit (DRUJ)
• Enhances Joint Stability
• Reinforces distal support for the wrist
• Is part of concave aspect
• Supports compression up the chain

More Details on Triangle Fibro-Cartilage

• There is an Articular Disc
• There are Distal-Radio-Ulnar Ligaments
• Palmar Support
• Ulnar Collateral Support in the Ligaments
• It also enclosed extensor carpi ulnaris

Muscles That Control

• Pronators include teres and quadrants
• Supraspinatus include biceps and other rotating stabilizing units

Muscle Actions • Pronator teres • Major action at RU jts, but long head contributes to stab of prox RU jt & HR jt • Pronator quadratus • Acts as dynamic stabilizer maintaining compression of distal RU jt • Supinator • May act alone during unresisted slow supination in all positions of elbow • Also can act alone during unresisted fast supination with elbow extended • Biceps • Active when supinating against resistance and/or during fast supination with elbow flexed to 90° • Brachioradialis may contribute to rapid pron/sup when elbow is at 90°

How RU Joint Functions

• Proximal and Distal operate with synchronized movement
• Radius spins within proximally and Distally on either TFCC or stabilized ulna

RU Motion Explained

• Its an Open Chain of Motion
• Movement linked from radius from ulna

Measuring Ru Joint Functions

• Total motion is 150 degrees
• Prone or supination are linked to Elbow Joint Position
• With Elbow flex, supraspinatus is 90
• Then as it Prone it moves 60
• Extended with Supraspinatus 100

Joint Details and Stability

• Proximal

1. Quadrate and Annular Ligaments
2. Interosseus membrane
3. Biceps
4. With assistance from Teres muscle to head.

• Distal

1. Interrosseus Support Structure
2. Radioulnar Ligaments
3. TFC

Umeroulnar Arthrokinematics

• Flex the Ulna on humerus
• Then it slides on Ventral surface
• With Extension it glides Dorsally

Humeroradial joint Arthrokinematics

• Radius flex on Humerus with slide on ventral units
• Or moves dorsally with extending

Wrist Arthrokinematics

• Concave joint is stabilized with interosseus ligaments

Closed Chain RU Arthrokinematics

• Stable pronation- resulting from internal and external rotation
• HU is in closed pact.

Open and Close Packed

• The positions of the joint relative to each other is linked with function