Elbow and forearm biomechanics

brachialis

- 1 joint elbow flexor, pennate muscle with broad attachment to the distal humerus - capable of generating significant force; large physiological cross-sectional area with a large torque capacity - identified as the workhorse of the elbow flexors; active under all conditions (any position of the forearm, all of the elbow positions, with all types of contractions) - moment arm is greatest at slightly more than 100 degrees of flexion, greatest ability to generate force in this position - because its distal attachment is on the ulna, the length-tension curve of this muscle does not change with forearm position, it is not affected as a pronator or supinator of the forearm

biceps brachii

- 2 joint, but technically 3 joint muscle - more efficient as an elbow flexor with the shoulder in neutral or in some amount of extension (avoiding active insufficiency) - the moment arm of the biceps is largest between 80-100 degrees of elbow flexion (greatest potential to generate torque) - smallest moment arm is when the elbow is fully extended, muscle is much less effective as an elbow flexor - contraction of this muscle in full extension results in a compressive force to the humeroulnar joint - in flexion beyond 100 degrees, there is a translatory force component that is directed away from the humeroulnar joint; acts to produce a distracting force to the humeroulnar joint

triceps brachii

- 3 heads, long head only is a 2 joint muscle - greatest extensor force and greatest torque generating capacity occurs at 90 degrees of elbow flexion - the moment arm peaks early when the elbow is in the early stages of flexion and steadily decreases as the elbow flexion angle increases (moment arm changes less than the elbow flexors) - operates as a first class lever, activity is unaffected by the position of the forearm - length of the triceps increases as elbow flexion angle increases - the medial head contributes more to the extension moment than does the lateral head - neither the medial or lateral head is affected by the position of the shoulder - long head is the quietest of the three heads and tends to be recruited for maximal efforts in extension; long head contributes no more than 25% of the extensor moment at the elbow (PCSA < 1/3 of the total cross-sectional area of the triceps)

physiological cross-sectional area of the elbow flexors

- Brachialis = 5.5-7.0 cm2 - Biceps = 4.5 cm2 - Pronator teres = 4.0 cm2 - Brachioradialis = 1.3 cm2

EMG activity of the biceps brachii

- EMG activity in the biceps has been shown to be maximal when the elbow is flexing and the forearm is simultaneously supinating - low EMG activity of the biceps is noted when unresisted elbow flexion is performed with the forearm in full pronation - biceps is active in unresisted elbow flexion when the forearm is in the mid position or in supination - if we add a load, the biceps will be active in all positions of the forearm - it is recruited during high power supination activities, especially those associated with elbow flexion (using screw driver)

osteokinematics of the humeroulnar joint

- Motion: flexion/extension, abduction/adduction (coronal plane) - extension: occurs with abduction of ulna - flexion: occurs with adduction of the ulna - full passive flexion requires elongation of the posterior capsule, extensor muscles, posterior fibers of the MCL and the ulnar nerve - ulnar nerve passes through ulnar groove, if nerve is bound down by entrapment or held by scar tissue, restricts ability to reach full flexion - for complete passive extension to occur at this joint, we need the extensibility of the anterior capsule, the skin, the flexor muscles, and the anterior fibers of the MCL to make that articular cartilage between those two surfaces a more congruent fit - when the humeroulnar joint is in a position of full extension, increase in tension of many of the fibers of the MCL, anterior capsule is taut, flexor muscles play a role in extension end range stability (esp. tendon of brachialis)

coordination of elbow extensors

- a maximal effort of elbow extension is going to generate maximal levels of activity from all of the extensors - in a submaximal effort, the anconeus is the first muscle to initiate and will maintain low loads of extensor force - as extensor force is gradually increased, the medial head is the next recruited; it will remain active for most extension movements (medial head has been called the workhorse of the elbow extensors) - under moderate to high extension demands, the lateral head is the next recruited followed closely by the long head - long head has been called a reserved extensor, in that it is well suited for tasks that require high work performance

pronator teres

- active pronation by the pronator teres - activity of the pronator teres during active pronation will create compression forces at the humeroradial joint; it is associated with proximal movement of the radius - when the forearm is pronated, the interosseus membrane is slack so it cannot oppose the radial movement in the proximal direction - screw-home mechanism of the elbow: activity of pronator teres moves the radius proximally by creating compression forces at the humeroradial joint

characteristics of the superior and inferior radioulnar joint

- allow for movements of forearm pronation and supination; occur around axis of rotation that extends from radial head down through the ulnar head distally; axis of rotation crosses both bones - radius is the moving component of both articulations - in the anatomical position (full supination of the forearms), radius and ulna are parallel - in pronation, radius crosses over the ulna - minimal amounts of ulnar movement, occurs in the frontal plane; during pronation, tiny bit of ulnar abduction occurs; with supination of the forearm, tiny amount of adduction occurs

carrying angle

- angle made by the long axis of the humerus and the long axis of the ulna in the anatomical position (fully extended and supinated) - in the frontal plane, medial flare of the trochlea extends more distally than the lateral flare => lateral deviation of the ulna - carrying angle that is evident in anatomical position will disappear in a position of full elbow flexion with the forearm pronated, or when a supinated forearm is fully flexed at the elbow - average carrying angle is 13-15 degrees - females tend to have a larger carrying angle than males (2 degree increase) - males: 5-10 degree carrying angle - females: 10-15 degree carrying angle

arthrokinematics of the humeroulnar joint

- articular surface does not sit at the end of the bone; articular surface is at 45 degree angle to the shaft of the ulna, this must be taken into consideration with mobilization techniques - hyaline cartilage covers about 300 degrees of articular surface on trochlea; only covers 180 degrees of trochlear notch - on the posterior aspect, contact between olecranon process and olecranon fossa - as we move into flexion, the concave trochlear notch is going to role and glide anteriorly (joint is a snug fit, small amounts of rolling and gliding)

inferior radioulnar joint

- articulation between convex ulnar head and the ulnar notch on the distal radius - articulation also includes the proximal surface of an articular disk (more commonly referred to as the triangular fibrocartilage) - primary role: stabilizes distal forearm during pronation and supination - network of connective tissue helps to provide stability to the joint - TFC (triangular fibrocartilage) - anterior and posterior edges are continuous with the palmar and dorsal radioulnar capsular ligaments (holds ulnar head snuggly against the ulnar notch of the radius during pronation and supination)

humeroradial joint articular surfaces

- articulation between the superior surface of the radial head (concave articular component) with the convexity of the capitulum of the humerus - head of the radius articulates with only a portion of the capitulum as we get close to positions of full extension - in full extension, there is no contact between the articular surfaces - amount of contact between the articular surfaces is going to increase with increasing amounts of elbow flexion - given a load is being lifted, there is less pressure per unit of surface area in positions of flexion, because there is more contact area

superior radioulnar joint

- articulation formed by the convexity of the radial head and the concave radial facet of the ulna - pivot joint; bony architecture does not provide much stability - stabilizing structures: capsule, lateral collateral ligament, annular ligament, interosseus membrane (stabilizes both radioulnar joints), oblique cord, quadrate ligament

posterior medial collateral ligament (PMCL)

- attaches to olecranon process - plays a role in resisting valgus forces - becomes taut after 60-90 degrees of elbow flexion - most taut in positions of end range flexion - resists distraction forces (common in athletes with lots of acceleration of elbow)

characteristics of the elbow flexors

- biceps and the brachialis function as third class levers: the range and speed are favored at the expense of power - these muscles are best suited to generate large flexor moments at the elbow - the flexor force of the elbow flexors is maximal between 65 and 90 degrees of elbow flexion - with the forearm in the mid position, there is an increased internal moment arm of the biceps and brachioradialis

biceps brachii: active and passive insufficiency

- biceps brachii is a 3 joint muscle: shoulder, elbow, forearm - positioning of patient is critical to test ROM or strength - active insufficiency - flex shoulder, flex ebow, supinate forearm - passive insufficiency - extend shoulder, extend elbow, pronate forearm

bony stability at humeroulnar and humeroradial joints

- bony architecture: equivalent of a tongue in groove fit that occurs between the humerus, the ulna, and the radius - because of tongue in groove fit, the ability to medially and laterally glide between the joint surfaces of both joints is nearly impossible - radial head as part of tongue in groove fit helps to limit valgus excursion at the elbow - studies show that if radial head is removed, valgus instability is increased by 30%

elbow capsule

- capsule is loose anteriorly and even more loose posteriorly - capsule is so loose, has folds within it - anterior capsule - folds with flexion, unfolds with extension - posterior capsule - folds with extension, unfolds with flexion

lateral collateral ligament (LCL)

- comes off of lateral epicondyle of humerus and inserts onto the annular ligament - as a whole, the ligament remains taut throughout the entire arc of elbow motion, stabilizes against excessive varus stress on lateral elbow as well as compressive forces between the surfaces of the humeroradial joint - lateral collateral ligament complex is divided into parts: lateral ulnar collateral ligament, radial collateral ligament, accessory lateral collateral ligament, annular ligament

lateral ulnar collateral ligament (LUCL)

- considered to be the primary lateral stabilizer to the elbow joint - helps the radial collateral ligament to protect against excessive varus stress - taut in both flexion and extension of the elbow - helps to stabilize throughout the ROM - in full flexion of the elbow it is considered to be taut (recent studies indicate that this ligament may be more protective dynamically than as a static stabilizer), - plays a role in stabilizing posterolateral aspect of the elbow

posterior bundle of AMCL

- does not have a large role in valgus stabilization - becomes taut in later stages of elbow flexion

anconeus

- does provide an assist to the triceps as an elbow extensor - tends to be active in early extension - has a very small PCSA, very small moment arm, and therefore, its role as an elbow extensor is very limited - it accounts for only 10-15% of the total extensor force

pronator quadratus

- fiber orientation & line of force is almost perpendicular to the axis of rotation within the forearem, so force potential is maximized - functions to compress the ulnar notch of the radius against the ulnar head; helps to stabilize distal radioulnar joint during pronation movements - the most active and consistent pronator - active under all conditions of forearm pronation, regardless of the angle of the elbow or the power demands

oblique cord

- flat band of fascial tissue - fibers run distally from the ulnar tuberosity to just distal to the radial tuberosity (oriented in opposite direction of the orientation of the fibers of the interosseus membrane) - contribution to function is unclear

arthrokinematics of supination

- forearm pronation and supination are a function of movement at both radioulnar joints; can't have movement of one without the other - superior radioulnar joint: convex radial head articulates with concave radial notch of ulna (does not follow the convex concave rules: radial head spins within enclosure of annular ligament, no roll-gliding occurs during pronation or supination) - distal radioulnar joint: convexity of ulnar head articulates with concavity of ulnar notch of the radius, ulna does not move - supination: posterior rolling and gliding of ulnar notch of radius on ulnar head - at the end of supination at the distal joint; palmar radioulnar ligament is maximally taut, creating a stiffness that will stabilize the distal joint

biceps as a supinator

- from a position of full pronation, the biceps can spin the radius sharply into supination, due to the biceps tendon wrapping around the proximal radius when the forearm is fully pronated - biceps is a powerful supinator, corss-sectional area is 3 times that of the supinator - the biceps is most effective as a supinator when the elbow is flexed 90 degrees - biceps as an elbow flexor augments the torque potential of the biceps as a supinator (using screwdriver: elbow is flexed to increase efficiency of supination, angle of the elbow has to remain stationary, activity of triceps is necessary to nullify elbow flexion of biceps and allow only supination)

arthrokinematics of the humeroradial joint

- in a position of full extension at the elbow, no contact between articular surfaces - position of flexion, esp. with active muscle contraction, creates pull on radial head so it comes into contact with capitulum (further into flexion => greater surface area contact) - flexion: concave surface moves on convex surface; anterior rolling and gliding of superior surface of radial head relative to the capitulum - extension from a flexed position: posterior rolling and gliding

characteristics of pronators and supinators

- in order for the forearm muscles to pronate or supinate, there has to be one attachment on the humerus or ulna and another attachment on the radius or hand - must have a line of force that intersects the axis of rotation for pronation and supination - Example: pronator quadratus is very close to the perpendicular axis of rotation

brachioradialis

- inserts a great distance from the elbow; because of this, its largest component of muscle force causes joint compression of the humeroradial joint - widely accepted as an elbow flexor - peak moment arm is between 90-120 degrees of elbow flexion, this is the range where it is most efficient as an elbow flexor - more active when the forearm is in the mid position, also active when the forearm is fully pronated - acts eccentrically during activity such as hammering to control rate of extension to hit nail

humeroulnar joint articular surfaces

- joint is formed between the convexity of the trochlea of the humerus that articulates with a concave trochlear notch on the ulna - very congruent fit, but from a functional position, it doesn't exhibit its greatest congruency until the joint is well-loaded (load is going to deform the articular cartilage, allowing for a better fit between the articular surfaces) - congruent joint surfaces provide most of the elbows structural stability - primary motion at humeroulnar joint: elbow flexion/extension (sagittal plane motion)

medial collateral ligament (MCL)

- larger than the LCL - because it is on the medial aspect of the elbow, it helps to reinforce the capsule on the medial side - as a whole, MCL resists excessive valgus forces at the elbow (important because at carrying angle, already in position of valgus, need to keep these forces form becoming excessive) - each part of ML helps to resist excessive valgus forces during movements of flexion and extension - 3 parts: anterior band, posterior band, transverse band

cubitus varus (varum)

- less than normal carrying angle - example: arm is measured to be in 2 degrees of valgus - since carrying angle is only 2 degrees, considered to be cubitus varus

combined MCL and LCL function

- limit medial and lateral rotations at the humeroulnar joint - about 10 degrees of lateral rotation occurs at humeroulnar joint - less then 5 degrees of medial rotation at humeroulnar joint (greater rotation than this is indicative of ligamentous insufficiency)

pronator teres

- median nerve runs between two heads - primary pronator, secondary elbow flexor - position of the elbow has no effect on muscle recruitment - considered to be a reserve pronator in that the pronator teres is only recruited with resistance to pronation or during rapid active pronation movements - greatest EMG activity occurs during high power pronation activities (triceps acts as a synergist to neutralize the tendency towards elbow flexion)

anterior medial collateral ligament (AMCL)

- most anterior of the components of the MCL - distal attachment on coronoid process - fibers are relatively stiff - function is the primary ligament restraint to valgus forces between 20-120 degrees of flexion, some fibers of the anterior band are taut throughout the ROM (some of anterior fibers cover both sides of the axis of rotation) - 3 subdivisions of fibers: anterior bundle, middle bundle, posterior bundle

anterior bundle of AMCL

- most anterior part - taut in extension of the elbow - shown to be primary valgus restraint at 30, 60, 90 degrees of elbow flexion - co-primary valgus restraint up to 120 degrees of elbow flexion

function of the interosseus membrane

- muscle force (generated by active muscle contraction) transfer from radius to the ulna - plays a role in the arthrokinematics of the elbow - the membrane decreases the amount of force that crosses a limited surface area at the humeroradial joint - FOOSH injury: radius is more distal, takes the force first, would cause compression of the humeroradial joint if unchecked (radial head fractures, subluxation, dislocation) - interosseus membrane dissipates some force away from the radius to the ulna (due to orientation of fibers of interosseus membrane) - most of the elbow flexors and major pronators and supinators of the forearm have their distal attachments on the radius; when they contract, they cause compression at the humeroradial joint

EMG activity of the brachiradialis

- no activity during eccentric elbow flexion when performed slowly with the forearm supinated - no activity with slow unresisted concentric elbow flexion - when the speed of motion increases there is moderate activity of the brachioradialis if the load is applied with the forearm in the mid position or in full pronation - high activity during rapid alteration between forearm pronation and supination

alignment of elbow bony landmarks

- one of the things we can evaluate by observation is the relationship between the medial and lateral epicondyles and the olecranon process - in full elbow extension: line connecting medial and lateral epicondyles should be a straight line with the olecranon along that line - 90 degrees of flexion: connecting bony landmarks will form an isosceles triangle, olecranon inferior to epicondyles of humerus

middle bundle of AMCL

- plays small role in providing valgus stability - middle bundle is taut in midrange of motion

radial collateral ligament (RCL)

- primary supporter to lateral aspect of elbow - reinforces the humeroradial joint - protects against excessive varus stress in some positions of elbow flexion/extension - provides resistance against longitudinal joint distraction forces > anterior fibers of RCL: taut in extension of the elbow > middle fibers: taut throughout ROM > posterior fibers: taut in flexion

contractile stabilizers at distal radioulnar joint

- pronator quadratus - tendon of the extensor carpi ulnaris - joint capsule - distal fibers of interosseus membrane

supinator m.

- proximal attachment is on the lateral epicondyle; passes too close to the medial-lateral axis of the elbow to generate any significant force as an elbow flexor - supinator is active during forearm supination, regardless of the elbow angle and the speed and power of the contraction - usually recruited for low power, slow tasks that require only supination (biceps is inactive under these conditions) - during moderate to high demand supination activities there is significant EMG activity of the biceps; biceps acts as a reserve supinator until forearm supination is resisted - with elbow extended, supinator is especially active, biceps is inhibited

position to test/measure pronation and supination

- put elbow at 90 degrees flexion; adducted by side => pen in fist with thumb up toward ceiling (midway point between pronation and supination - elbow at 90 degrees flexion allows us to negate any shoulder rotation; if elbow is out, shoulder will also IR and ER

secondary supinators

- radial wrist extensors - extensor pollicis longus - extensor indicis - brachioradialis (can help in supination to neutral from fully pronated position)

annular ligament

- relatively circular, but not completely (4/5 of a ring), firbo-osseus ring of connective tissue that surrounds the radial neck and attaches to the anterior and posterior margins of the radial notch on the ulna - described as a sling (wraps around radial head and supports it) - provides little to no effect on limiting normal motion at the joint - primary restraint to inferior subluxation/dislocation of the radial head - helps to check lateral excessive movement or subluxation of the radial head - during supination of the forearm, anterior aspect of annular ligament tightens - during pronation, the posterior aspect of the annular ligament will tighten

accessory lateral collateral ligament (ALCL)

- runs from annular ligament to supinator crest

interosseus membrane

- runs from medial surface of radial shaft and passes medially and distally to the interosseus border of the ulna - fibers are taut in supination and lax in pronation - function: bind radius to ulna along the length of the bones (keeps them connected, helps to maintain the space between them, esp. during movement of the forearm), helps to distribute compressive loads applied to distal radius over to ulna

quadrate ligament

- runs from radial notch on ulna to the neck of the radius - lends some structural support to the inferior capsule - maintains the contact between the radius and the ulna at this joint

coordination of elbow flexors

- seem to be finely coordinated in performing elbow flexion, seems to be a functional pattern that exists for each one of the elbow flexors - biceps activated as an elbow flexor with the forearm supinated regardless of the speed of flexion or the resistance - when the forearm is partially pronated, the biceps is only recruited with resistance - in full pronation, the biceps will only be partially recruited with resistance - brachialis is active under all conditions of elbow flexion due to its distal attachment on the ulna; able to move the elbow through a full range of motion due to its short moment arm - brachioradialis is active with resisted elbow flexion, especially when the forearm is partially and fully pronated; functions against resistance even when the forearm is supinated - brachioradialis compensates for inefficiencies of biceps in pronation - pronator teres is recruited as an elbow flexor when resistance is applied

transverse medial collateral ligament (TMCL)

- spans the medial aspect of the trochlear notch - runs from olecranon to the coronoid process (same bone) - does not provide any significant articular stability

triangular fibrocartilage complex (TFCC)

- structural area at the joint composed of the triangular fibrocartilage disk (TFC), the dorsal and palmar (radioulnar) capsular ligaments, and ulnar collateral ligament - problem for vocation requiring lifting and force generation through wrist and hand; lost integrity of TFCC => significantly unstable wrist - TFCC occupies most of the space between the distal end of the ulna and the proximal carpal bones on the ulnar side (ulnocarpal space) - end of radius extends more distally than ulna - function: primary stabilizer of distal radioulnar joint during movement of the forearm and movement of the wrist - if disrupted, it will result in instability of the distal radioulnar joint and create wrist dysfunction - after the age of 20-30, TFCC starts to degenerate

arthrokinematics of pronation

- superior radioulnar joint: convex radial head articulates with concave radial notch of ulna (does not follow the convex concave rules: radial head spins within enclosure of annular ligament, no roll-gliding occurs during pronation or supination) - distal radioulnar joint: convexity of ulnar head articulates with concavity of ulnar notch of the radius, ulna does not move - pronation: anterior rolling and gliding of ulnar notch on ulnar head - in pronation, the dorsal radioulnar ligament is maximally taut, providing end range stabilization

triceps brachii: active and passive insufficiency

- two heads are 1 joint muscles, one head is a 2 joint muscle - passive insufficiency - flex elbow, flex shoulder - active insufficiency - extension of the shoulder, extension of the elbow

elbow and forearm complex

composed of 4 joints: 1. humerounlar joint 2. radiohumeral joint (functionally, these two joints act as one in that they allow flexion and extension of the elbow) 3. superior roadioulnar joint 4. inferior radioulnar joint - there is a single joint capsule that encompasses the humeroulnar joint and the humeroradial joint, and the superior radioulnar joint

cubitus valgum (valgus)

excessive carrying angle above normative values (excessive valgus appearance of forearm related to arm)

muscular coupling

muscles that work together to negate a muscle's action

muscular coupling at the elbow and forearm

• Biceps + posterior deltoid - shoulder sagittal plane movement is negated • Triceps + anterior deltoid - shoulder sagittal plane movement is negated • Biceps +Triceps + supinator - supination is allowed while preventing elbow flexion/extension • Pronator teres + triceps - pronation is allowed while preventing elbow flexion/extension

Peak moment arms of elbow flexors

• Biceps - 80 ° and 110 ° • Brachialis - Peaks at 90 ° • Pronator teres - Peaks around 75 ° • Brachioradialis - peaks between 90-120 °

osteokinematics of the elbow

• Flexion/extension - Active ROM (AROM) =5-0-145 (5 degrees of hyperextension) - Passive ROM (PROM) flexion = 150-160º Osteokinematics • Functional ROM needed for ADL => 30-130 º - limit of flexion is the depth of the soft tissue, developed biceps = less flexion - limit of extension is the bony contact between the olecranon and the olecranon fossa - axis for flexion and extension is variable - axis does not lie completely in transverse or frontal plane

osteokinematics of pronation and supination

• Neutral or zero position = Thumbs up (Midway between pronation & supination) • Supination - 0-85 degrees • Pronation - 0-75 degrees (clinicians often assume 90 degrees of pronation and supination) • Most ADL require only 100 degrees of motion: 50 of pronation, 50 of supination

limiting structures to end range supination

• Opposing muscle tightness - Pronator teres and quadratus • Interosseus membrane • Tightness in: - Joint capsule - Palmar capsular ligament at inf. RU joint - Oblique cord - Quadrate ligament - TFCC complex

overall function of MCL

• Stabilizes against valgus torques at medial elbow • Limits extension at end of extension ROM • Guides joint motion throughout flexion • Provided some resistance to longitudinal distraction joint surfaces • Most taut when elbow is extended and near full extension (0-30 degrees) • Provide articular stability throughout ROM in sagittal plane flexion/extension

overal function of the lateral collateral ligament complex

• Stabilizes elbow against varus torque • Stabilizes against varus and supination torques • Reinforces humeroradial joint - Some resistance to longitudinal distraction of joint • Stabilizes radial head - Provides stable base for rotation • Prevents subluxation of HU joint - Secures ulna to radius • Prevents forearm from rotating off humerus in valgus and supination, from full extension through flexion

limiting structures to pronation

• Supinator muscle tightness • Joint capsule tightness • Passive tension in biceps brachii • Approximation of radius on ulna • Dorsal capsular ligament at distal RU joint • Tension in posterior fibers of medial collateral ligament • TFCC complex

restraints in and around the elbow in position of full extension

• Valgus Restraints - 31% MCL - 31% bony articulation - 38% joint capsule • Varus restraint - 55% articulation • Extension Restraints - Flexor Tightness - Anterior capsule - Anterior bundle MCL • Distraction - 70% capsule

restraints in and around the elbow in position of full flexion

• Valgus restraint - 54% MCL - 33% bony articulation - 10% joint capsule • Varus restraint - 75% articulation • Flexion Restraint - Soft tissue approximation (in really thin people with no real end point from soft tissue approximation; end point comes from coronoid process moving into the coronoid fossa) • Distraction - 78% MCL - UCL tensile strength