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Elbow Anatomy & Biomechanics

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Topic updated on 03/01/16 4:56am
Introduction
  • Acts as a lever arm when positioning the hand
  • Functions as a fulcrum for forearm lever
  • In patients using crutches, it functions as a weight bearing joint. 
  • During throwing, there is transfer of energy between the shoulder and elbow
  • It is crucial for activities of daily living
Osteology & Arthrology
  • Osteology
    • the shaft for humerus has a spiral groove posteriorly (contains radial nerve)
      • this lies approximately 13 cm proximal to the articular surface of trochlea
    • the distal flare of humerus includes the medial and lateral epicondyles
      • the flare accounts for half of the elbow joint
      • the trochlea is spool shaped and is located medially
      • the capitellum is located laterally
  • Arthrology
    • the joint surface is anteriorly tilted approximately 30 deg, is in 6 deg of valgus and is internally rotated by 5 deg
    • the elbow contains a hinge and pivot joint
      • the radiohumeral articulation is a pivot joint
        • radial head is covered by cartilage for approximately 240 degrees
          • the lateral 120 degrees contains no cartilage
          • this is crucial for internal fixation of radial head fractures
      • the ulnohumeral articulation is a hinge joint
        • coronoid fossa on distal humerus receives the coronoid tip in deeper flexion
        • the coronoid tip has a buttress effect in the prevention of posterior dislocations
        • the sublime tubercle on the ulna is where the anterior bundle of the medial ulnar collateral ligament attaches distally
    • the axis for elbow rotation 
      • is centered at trochlea and capitellum
      • it passes through anteroinferior medial epicondyle
    • capsuloligamentous tissues
      • capsule is maximally distended at 70-80 deg of flexion
      • distal attachment of  anterior capsule is found 6 mm distal to tip of coronoid
        • coronoid is an intra-articular structure
Muscles of the Elbow
  • Flexion
    • biceps, brachialis, brachoradialis
      • the attachment of the brachialis 11 mm distal to the tip of the coronoid
      • the distal biceps attachment is at the level of the radial tuberosity
  • Extension
    • triceps
  • Stability of elbow
    • see below
Stability of Elbow
  • Static and dynamic stabilizers confer stability to the elbow
    • static stabilizers (primary)
      • ulnohumeral joint (coronoid)
        • loss of 50% or more of coronoid height results in elbow instability
      • anterior bundle of the MCL
        • the MCL is composed of the anterior, posterior and transverse bundles
          • the MCL provides resistance to valgus and distractive stresses
          • anterior oblique fibers (of the anterior bundle) 
            • these are the most important against valgus stresses
          • the posterior bundle forms the floor of the cubital tunnel
            • if this is contracted, flexion may be limited 
        • in complex instability, repair/reconstruction of the MCL may be unnecessary because of the secondary function of the radial head as a valgus stabilizer (especially if coronoid has been repaired or is grossly intact)
      • LCL complex (includes the LUCL)
        • lateral ulnar collateral ligament
        • radial collateral ligament
        • annular ligament 
          • provides stability to the proximal radioulnar joint
        • accessory collateral ligament
          • some believe that the the accessory collateral ligament and the radial collateral ligament contribute substantially to lateral elbow stability
        • the LCL arises from isometric point on lateral aspect of capitellum
        • optimal stability is conferred with an appropriately tensioned LCL repair
    • static stabilizers (secondary)
      • radiocapitellar joint
        • this functions as an important constraint to valgus stress
          • the radial head provides approximately 30% of valgus stability
          • this is most important at 0-30 deg of flexion/pronation
      • capsule
        • greatest contribution the capsule on stability occurs with the elbow extended
      • origins of the flexor and extensor tendons
    • dynamic stabilizers
      • includes muscles crossing elbow joint
        • anconeus
        • brachialis
        • triceps
      • they provide compressive stability
    • secondary varus stability 
      • LCL, anconeus, and lateral capsule 
Nerve of the Elbow
  • 4 major nerves
  • musculocutaneous nerve (lateral cord of the brachial plexus)
    • innervates elbow joint
    • it supplies the biceps and brachialis
      • the nerve runs between these muscles 
      • it exits laterally, distal to the biceps tendon 
      • it will terminate as the LABC (forearm), which is found deep to the cephalic vein
  • radial nerve (posterior cord of the brachial plexus )
    • it leaves the triangular interval (teres major, long head of triceps and humeral shaft)
    • found in spiral groove 13 cm above the trochlea
    • pierces lateral intermuscular septum 7.5 cm above the trochlea
      • this is usually at the junction of the middle and distal third of the humerus
      • lies between the brachialis and the brachioradialis
    • distally it is located superficial to the joint capsule, at the level of the radiocapitellar joint
  • median nerve (medial/lateral cords of the brachial plexus)
    • it courses with brachial artery, running from lateral to medial
      • lies superficial to brachialis muscle at level of elbow joint
    • it gives branches to elbow joint
    • it has no branches in upper arm
  • ulnar nerve (medial cord)
    • runs medial to brachial artery, pierces medial intermuscular septum (at the level of the arcade of Struthers) and enters posterior compartment
    • it traverses posterior to the medial epicondyle through the cubital tunnel
    • it gives branches to elbow joint
    • it has no branches in upper arm
      • first motor branch to FCU is found distal to the elbow joint
Blood Supply of Elbow
  • Brachial artery
    • is located medially in the upper arm
    • it enters cubital fossa laterally
      • contents-- biceps tendon (lateral), brachial artery, median nerve (medial) 
      • lateral border--brachoradialis
      • medial border--pronator teres
      • proximal border --distal humerus
    • at the level of elbow it splits into the radial and ulnar arteries
  • Principle branches
    • superior/inferior ulnar collateral
    • nutrient/muscular
    • supratrochlear
Kinematics
  • Functional ROM
    • 30° to 130 flexion
      • total ROM is 0-150 degrees
    • 50° supination
    • 50° pronation
  • Motion vectors
    • flexion & extension
      • the axis of rotation is found at the center of trochlea
    • pronation (pronator teres and quadratus) & supination (biceps and supinator)
      • the axis of motion is found at the capitellum through to the radial/ulnar heads
        • this effectively forms a cone
  • Joint reaction force
    • there are large joint reaction forces due to short and inefficient lever arms around elbow (biceps inserts not far from center of rotation)
    • this contributes to degenerative changes of the elbow
  • Center of rotation
    • is a line through isometric points on the capitellum about trochlea
    • the axis of pronation / supination is a line drawn from capitellum, through radial head, to distal ulna
  • Valgus alignment
    • normal valgus carrying angle
      • 7° for males
      • 13° for females
      • this diminishes with flexion 
  • Axial loading
    • in extended elbow 
      • 40% of weight is through ulnohumeral joint
      • 60% of weight is through radiohumeral joint
Free Body Diagram
  • Free body diagram demonstrate inefficiencies of elbow
    • sum M = 0
    • 5B = 15W
    • B = 3W
  • Static loads are close to body weight
  • Dynamic loads are greater than body weight
Arthrodesis
  • Optimal position
    • in a unilateral arthrodesis
      • 90° of flexion
      • 0-7° of valgus
    • in a bilateral arthrodesis
      • one elbow in 110 ° of flexion for feeding
      • one elbow in 65 ° of flexion for perineal hygiene

 

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