Elbow and Forearm Biomechanics Flashcards
What are the 2 joints of the elbow?
Humeroulnar and humeroradial joints
Which 2 joints allow the palm of the hand to be supinated and pronated without requiring motion of the shoulder?
proximal and distal radio-ulnar joints
What provides most of the elbow’s structural stability?
The tight fit between the trochlea and trochlear notch at humero-ulnar joint
What type of joint is the elbow? Describe why.
Modified hinge joint because the ulna experiences a slight amount of axial rotation and side-to-side motion as it flexes and extends
Elbow flexion and extension occur around what AOR? Where does this AOR pass through?
Medial lateral
Through the vicinity of the lateral epicondyle
Is the trochlea symmetrical along the ML axis? What does this cause?
No, there is a distal prolongation of the medial lip of the trochlea.
This asymmetry causes the ulna to deviate laterally relative to the humerus producing a valgus angle
What is normal cubitus valgus?
about 13°, with standard deviation close to 6°
Valgus angle is typically greater in which extremity?
on the dominant arm
It is considered excessive valgus when the forearm is deviated laterally more than __°.
30
Excessive cubitus valgus may overstretch and damage which nerve?
The ulnar nerve as it crosses medial to the elbow
It is considered cubitus varus when the forearm is deviated medially _°.
5
Medial collateral ligament consists of _____, ______, and ______ fiber bundles.
anterior
posterior
transverse
Which fibers of the medial collateral ligament are the strongest? Therefore they provide the most resistance against which force to the elbow?
Anterior
Valgus (abduction)
Where do the anterior fibers of the medial collateral ligament originate and insert?
They arise from anterior part of medial epicondyle and insert on medial part of coronoid process of ulna
Where do the posterior fibers of the medial collateral ligament originate and insert?
They originate on the posterior part of medial epicondyle and insert on medial margin of olecranon process
Where do the transverse fibers of the medial collateral ligament originate and insert?
From olecranon to coronoid process of ulna
**They do not provide significant articular stability because they originate and insert on the same bone
In addition to MCL, the proximal fibers of what groups of muscles resist excessive valgus-producing strain at elbow?
The proximal fibers of wrist flexor and pronator group of muscles (especially the flexor carpi ulnaris)
When is the MCL susceptible to injury?
When fully extended elbow is violently forced into excessive valgus, often from fall onto an outstretched arm and hand
Also from repetitive, valgus-producing forces to elbow in non–weight-bearing activities, such as pitching baseball and spiking volleyball
Where does the lateral collateral ligament originate?
on the lateral epicondyle
Ligamentous complex originates on lateral epicondyle and immediately splits into two fiber bundles known as what?
radial collateral ligament and the lateral (ulnar) collateral ligament
Once the radial collateral ligament splits off of the lateral epicondyle where does it go and attach?
It fans out to blend with the annular ligament
Once the ulnar collateral ligament splits off of the lateral epicondyle where does it go and attach?
It attaches distally to the supinator crest of the ulna
When are LCL ligament fibers taught?
during flexion
The ulnar collateral ligament and anterior fibers of the medial collateral ligament function as collateral “___ wires” to the elbow providing medial-lateral stability to ulna during sagittal plane motion
guy
What elbow structures are the primary stabilizers against varus-producing forces?
LCL complex and posterior-lateral aspect of capsule
A compete rupture of the LCL complex and posterior-lateral aspect of capsule causes increased varus of the elbow but also what?
posterior-lateral rotary instability
Posterior-lateral rotary instability of the elbow is expressed as what?
excessive external rotation of forearm with subsequent subluxation of both humero-ulnar and humeroradial joints
What 3 motions increase tension in the anterior fibers of the MCL?
- valgus
- flexion
- extension
What 2 motions increase tension in the posterior fibers of the MCL?
- valgus
- flexion
What 2 motions increase tension in the radial collateral ligament?
- varus
- external rotation
What 3 motions increase tension in the lateral (ulnar) collateral ligament?
- varus
- external rotation
- flexion
What 2 motions increase tension in the annular ligament?
- distraction of the radius
- external rotation
Which ligament is the primary valgus stabilizer?
anterior fibers of the MCL
Which ligament is the primary varus stabilizer?
Lateral (ulnar) collateral ligament
What 2 pathologies might someone who cannot fully extend their elbow suffer from?
- Flexion contractures
- stiffness in anterior capsule and some anterior fibers of MCL
What is maximal ROM at the elbow?
- 5° through 145°
What is the functional ROM at the elbow?
30° - 130°
Humero-ulnar joint is articulation b/w _____ trochlear notch of ulna and _____ trochlea of humerus
concave
convex
How much of the articular surface on the trochlea is covered in hyaline cartilage?
300°
How much of the articular surface on the trochlear notch is covered in hyaline cartilage?
180°
The natural congruency and shape of the humero-ulnar joint limits motion primarily within what plane?
sagittal
In order for full extension to occur at the humero-ulnar joint what things must be extensible?
The dermis anterior to the elbow, the flexor muscles, the anterior capsule, and the anterior fibers of the medial collateral ligament
During full elbow flexion which structures are required to elongate?
The posterior capsule, extensor muscles, ulnar nerve and certain portions of collateral ligaments, especially posterior fibers of MCL
Repetitive elbow flexion activities can lead to damage of what nerve?
ulnar nerve
The humeroradial joint is an articulation between what two structures?
The radial head and the capitulum of the humerus
During passive flexion which ligament is taught at the humeroradial joint?
lateral (ulnar) collateral ligament
What 3 things bind the radius and ulna together?
The interosseous membrane and proximal and distal radio-ulnar joints
The proximal and distal radio-ulnar joints allow what movements?
pronation and supination
What are the postions of the radius and ulna during full supination and full pronation?
In full supination the radius and ulna are parallel. In full pronation the radius crosses over the ulna.
During pronation and supination what 2 arm bones do not move?
The ulna and humerus
What is the axis of rotation for pronation and supination?
It extends obliquely across the forearm from the radial head to the ulnar head
What is the neutral or zero reference position of the forearm?
The “thumb-up” position, midway b/w complete pronation and supination
the forearm rotates through about __° of pronation and __° of supination
75
85
What is the functional ROM of forearm rotation?
50° of pronation through 50° of supination
Can someone who lack last 30° of complete forearm rotation still perform many routine ADLs?
YES
Decreased pronation and supination can be compensated by what motion?
Internal and external rotation of the shoulder
Pronation and supination requires movements at what 3 joints?
proximal and distal radio-ulnar joints humeroradial joint
At the proximal radio-ulnar joint the radius is con__ and the ulna is con__.
convex
concave
At the distal radio-ulnar joint the radius is con__ and the ulna is con__.
concave
convex
What are the arthrokinematics at the proximal radio-ulnar joint during supination/pronation?
The radius rotates in the same direction of the movement around the fixed ulna, but there is no slide because the tight annular liagament holds the radial head in place
What are the arthrokinematics at the distal radio-ulnar joint during supination/pronation?
The radius rolls and slides in the same direction of the movement around the fixed ulna
Describe the arthrokinematics at the humeroradial joint
The fovea of the radial head spins against the rounded capitulum of the humerus
What muscle is the primary pronator of the forearm and also causes a compression force which stabilizes the joint?
Pronator teres
The natural proximal migration of the radius and increased joint compression at the humeroradial joint during pronation is called what?
The “screw home” mechanism of elbow
When the hand is fixed what bones move and which are fixed?
The humerus and ulna rotate around the fixed radius and carpals
Describe the at the kinematics at the proximal radio-ulnar joint during pronation when the hand is FIXED.
The humerus and ulna externally rotate (same direction as pronation)
Describe the at the kinematics at the proximal radio-ulnar joint during supination when the hand is FIXED.
The humerus and ulna internally rotate (same direction as supination)
What mechanism is used to pronate the forearm from a weight-bearing position?
“Force-couple”
What muscle rotates the humerus relative to fixed scapula?
infraspinatus
What muscle rotates ulna relative to fixed radius?
pronator quadratus
Describe the at the arthrokinematics at the proximal radio-ulnar joint when the hand is FIXED.
The annular ligament and radial notch of the ulna rotate around the fixed radial head and the capitulum of the humerus spins relative to the fovea of the fixed radius
Describe the at the arthrokinematics at the distal radio-ulnar joint when the hand is FIXED.
The head of the ulna rotates around the fixed ulnar notch of radius
Muscles that attach distally on the ulna participate in what motions?
Flexion and extension
Muscles that attach distally on the radius participate in what motions?
Flexion and extension
Pronation and supination
4 primary elbow flexors
- Biceps brachii
- brachialis
- brachioradialis
- pronator teres
Where does the biceps brachii originate and insert?
Originates on the scapula and inserts on the radial tuberosity
Is the biceps brachii more active when flexion is performed with supination or pronation?
supination
Where does the brachialis originate and insert?
Originates on the anterior humerus and inserts on the extreme proximal ulna
What is the brachialis’ sole function?
to flex the elbow
The brachialis generates the greatest force of any muscle crossing the elbow, why?
It has the largest cross-section of any muscle crossing the elbow
Where does the brachioradialis originate and insert?
Originates on the lateral supracondylar ridge of the humerus and inserts near the styloid process of the radius
During what movement is the brachioradialis most active?
During rapid flexion against high resistance
What are the 2 elbow extensors?
triceps brachii and anconeus
Where does the long head of the triceps originate?
on infraglenoid tubercle of scapula
Where does the medial head of the triceps originate?
posterior side of humerus below the radial groove (opposite to the brachialis on the anterior side)
Where does the lateral head of the triceps originate?
Posterior surface of humerus, above radial groove
Where do all three heads of the triceps insert?
on the olecranon process of the ulna via a common tendon
Where is the anconeus located?
Between the lateral epicondyle of humerus and strip along posterior aspect of proximal ulna
What is the function of the anconeus?
It provides important longitudinal and ML stability across humero-ulnar joint that is beneficial during extension activities
Shoulder internal rotation often occurs with _____, whereas shoulder external rotation often occurs with ______.
pronation
supination
Why is it important that internal and external rotation occur with supination and pronation?
Combining these shoulder and forearm rotations allows hand to rotate nearly 360° in space, rather than only 170° to 180° by pronation and supination alone
What are the 2 primary supinator muscles?
supinator and biceps brachii
What 4 muscles/muscle groups are secondary supinators?
- radial wrist extensors because they attach near lateral epicondyle of humerus
- extensor pollicis longus
- extensor indicis
- Brachioradialis (from a pronated position)
What 4 msucles are active during the clockwise rotation of tightening screw?
- biceps
- supinator
- extensor pollicis longus
- triceps (isometrically)
What are the 2 primary pronator muscles?
pronator teres and pronator quadratus
What 3 muscles are secondary pronators?
- Flexor carpi radialis
- palmaris longus
- Brachioradialis (from a supinated position)
