Biomechanics of elbow joint .

BIOMECHANI
CS OF ELBOW
JOINT
BIOMECHANI
CS OF ELBOW
Presenter: Mr. Aragya Khadka

 a complex joint that functions as a fulcrum for the forearm lever system that is
responsible for positioning the hand in space.
 Anatomy:
 Movements:
I. Humeroulnar and humeroradial: flexion and extension
II. Radioulnar articulation: supination and pronation

The trochlea and capitellum of the distal
humerus are internally rotated 3° to 8° and
94° to 98° of valgus with respect to the
longitudinal axis of the humerus

The distal humerus is anteriorly
angulated 30° along the long axis of the
humerus.

The articular surface of ulna is oriented approximately 4 to 7
degree of valgus angulation with respect to longitudinal axis of
the shaft.

 The distal humerus is divided into medial and lateral columns that terminate distally with
the trochlea connecting the two columns.
 The medial column diverges from the humeral shaft at a 45° angle and ends approximately
1 cm proximal to the distal end of the trochlea.
 The distal one-third of the medial column is composed of cancellous bone, is ovoid in shape,
and represents the medial epicondyle.
 The lateral column of the distal humerus diverges at a 20° angle from the humerus and
ends with the capitellum.

 The articular surface of the ulna is rotated 30° posteriorly with respect to its long axis.
 This matches the 30° anterior angulation of the distal humerus, which helps provide
stability to the elbow joint in full extension

 The radial neck is angulated 15° from the long axis in the anterior-posterior plane
away from the bicipital tuberosity
 Four-fifths of the radial head is covered by hyaline cartilage.
 The anterolateral one-fifth lacks articular cartilage and strong subchondral bone,
explaining the increased propensity for fractures to occur in this region.

 The normal range of flexion-extension is from 0° to 146° with a functional range of 30° to
130°.
 The normal range of forearm pronation-supination averages from 71° of pronation to 81°
of supination
 As the elbow is flexed, the maximum angle of supination increases, while the maximum
angle of pronation decreases.
 Most activities are accomplished within the functional range of 50° pronation to 50°
supination.

 patients can tolerate flexion contractures of up to 30°, which is consistent with the
functional range .
 Previously, the axis of rotation for flexion-extension has been shown by several
investigators to be at the center of the trochlea.
 Later, discovered a changing axis of rotation with elbow flexion .
 the axis of rotation passes through the center of concentric arcs outlined by the bottom of
the trochlear sulcus and the periphery of the capitellum.
 the surface joint motion during flexion-extension was primarily of the gliding type and
that with the extremes of flexion-extension (the final 5°–10° of both flexion and
extension), the axis of rotation changed and the gliding/sliding joint motion changed to a
rolling type motion

 The rolling occurs at the extremes of flexion and extension as the coronoid process comes
into contact with the floor of the humeral coronoid fossa and the olecranon contacts the
floor of the olecranon fossa.
 In addition, internal axial rotation of the ulna has been shown to occur during early
flexion and external axial rotation during terminal flexion.
 Thus, the elbow cannot be truly represented as a simple hinge joint.

 Pronation and supination take place primarily at the humeroradial and proximal
radioulnar joints with the forearm rotating about a longitudinal axis passing through
the center of the capitellum and radial head .
 During pronation-supination, the radial head rotates within the annular ligament and
the distal radius rotates around the distal ulna in an arc outlining the shape of a cone.
 internal axial rotation of the ulna occurs with pronation while external axial rotation
occurs with supination.

 Carrying angle:
The valgus position of the elbow in full extension is commonly referred to as the carrying
angle.
defined as the angle between the anatomic axis of the ulna and the humerus measured in
the anteroposterior (AP) plane in extension or simply the orientation of the ulna with
respect to the humerus, or vice versa, in full extension.
The angle is less in children as compared to adults and greater in females as compared to
males, averaging 10° and l3° of valgus.

 Valgus forces at the elbow are resisted primarily by the anterior band of the medial collateral
ligament (MCL).
 The MCL complex consists of an anterior bundle, posterior bundle, and the transverse ligament.
 The anterior bundle of the MCL tightens in extension whereas the posterior bundle tightens in
flexion.
 The throwing motion illustrates the role of the MCL in a common functional activity. Baseball
pitchers are frequently at risk for MCL injury due to the repetitive valgus stress placed on their
elbows by the nature of the throwing motion.

 The LCL complex consists of the
radial collateral ligament that originates from the lateral
epicondyle and inserts on the annular ligament;
the lateral ulnar collateral ligament, which originates from the
lateral epicondyle and passes superficial to the annular ligament, inserting on the supinator crest of
the ulna;
and the accessory lateral collateral ligament
The origin of the LCL complex lies at the center of the axis of elbow rotation, explaining its
consistent length throughout the flexion-extension arc

 O’Driscoll et al. described the entity of posterolateral rotatory instability of the elbow in
which the ulna supinates on the humerus and the radial head dislocates in a
posterolateral direction
It has been shown that the elbow can dislocate posterolaterally or posteriorly with an
intact MCL.
 This can occur with combined valgus and external rotation loads across the elbow joint
 The lateral ulnar collateral is the primary restraint to posterolateral rotatory instability
of the elbow followed by the radial collateral ligament and capsule

 Structures limiting passive flexion include the capsule, triceps, coronoid process, and the
radial head.
 Structures limiting elbow extension include the olecranon process and the anterior band of
the MCL.
 Passive resistance to pronation-supination is provided in large part by the antagonist
muscle group on stretch rather than ligamentous structures.
 Longitudinal stability of the forearm is provided by both the interosseous membrane and
the triangular fibrocartilage.
 Lee et al. (1992) demonstrated marked proximal migration of the radius only after 85% of
the interosseous membrane was sectioned.

 DeFrate et al. (2001) showed that interosseous membrane transfers more force from the
radius to the ulna in supination than in pronation.
 The coronoid process also plays a role in longitudinal stability and has been shown to
prevent posterior displacement of the ulna.

 Kinetics:
The primary flexor of the elbow is the brachialis, which arises from the anterior aspect of the
humerus and inserts on the anterior aspect of the proximal ulna.
the biceps arises via a long head tendon from the supraglenoid tubercle and a short head tendon
from the coracoid process of the scapula and inserts in the bicipital tuberosity of the radius. It is
active in flexion when the forearm is supinated or in the neutral position.
The brachioradialis, which originates from the lateral two thirds of the distal humerus and inserts
on the distal aspect of the radius near the radial styloid, is active during rapid flexion movements
of the elbow and when weight is lifted during a slow flexion movement

 The primary extensor of the elbow, the triceps, is composed of three separate heads. The long
head originates from the infraglenoid tubercle, and the medial and lateral heads originate from
the posterior aspect of the humerus.
 The three heads coalesce to form one tendon that inserts onto the olecranon process of the ulna.
The medial head is the primary extensor, and the lateral and long heads act in reserve
(Basmajian, 1969).
The anconeus muscle, which arises from the posterolateral aspect of the distal humerus and
inserts onto the posterolateral aspect of the proximal ulna, is also active in extension.
This muscle is active in initiating and maintaining extension

 Muscles involved in supination of the forearm include the supinator, biceps, and the
lateral epicondylar extensors of the wrist and fingers.
 The primary muscle involved in supination is the biceps brachii.
 The biceps generates four times more torque with the forearm in the pronated position
than in the supinated position (Haugstvedt et al., 2001).
 The supinator arises from the lateral epicondyle of the humerus and the proximal lateral
aspect of the ulna and inserts into the anterior aspect of the supinated proximal radius

 Muscles involved in pronation include the pronator quadratus (PQ) and pronator teres (PT).
 PQ and PT are active throughout the whole rotation, being most efficient around the neutral
position of the forearm (Haugstvedt et al., 2001).
 The pronator quadratus originates from the volar aspect of the distal ulna and inserts onto the
distal and lateral aspect of the supinated radius.
 The pronator teres is more proximally located, arising from the medial epicondyle of the humerus
and inserting onto the lateral aspect of the midshaft of the supinated radius.
 The pronator quadratus is the primary pronator of the forearm regardless of its position
 . The pronator teres is a secondary pronator when rapid pronation is required or during resisted
pronation

 Elbow joint forces:
43% of longitudinal forces are transmitted through the ulnotrochlear joint and 57% are
transmitted through the radiocapitellar joint.
Ewald et al. (1977) determined that the elbow joint compressive force was eight times the
weight held by an outstretched hand.
An and Morrey (1991) determined that during strenuous weightlifting, the resultant force at
the ulnohumeral joint ranges from one to three times body weight.
The coronoid process bears 60% of the total compressive stress when the elbow joint is
extended.
Force transmission through the radial head is greatest between 0 and 30° of flexion and is
greater in pronation than supination.

 In extension, the force on the radial head decreases from 23% (of total load) in neutral rotation to
6% in full supination (Chantelot et al., 2008).
 This is secondary to the “screw-home” mechanism of the radius with respect to the ulna, with
proximal migration occurring during pronation and distal translation occurring during
supination.
 Disruption of the triangular fibrocartilage complex (TFCC) and the interosseous membrane in
the presence of an intact radial head does not result in proximal radioulnar migration.
 Absence of the radial head due to fracture or resection and a concomitant disruption of the TFCC
and interosseous membrane will result in proximal migration of the radius

The 'screw-home' mechanism is the rotation between the tibia
and femur and is considered to be a key element to knee stability
for standing upright. This mechanism serves as a critical function
of the knee and it only occurs at the end of knee extension,
between full extension (0°) and 20° degrees of knee flexion.

 During elbow flexion, the ulna is posteriorly translated as contact occurs at the coronoid.
 During the forced extension that occurs during the follow-through phase of the throwing motion,
impaction of the olecranon against the olecranon fossa has been demonstrated in the overhead
athlete.
 This impaction may result in the formation of osteophytes at the olecranon tip

 The force generated in the elbow has been shown to be up to three times body weight
with certain activities (An et al., 1981). Nicol et al. (1977), using three-dimensional
biomechanical analysis, found that during dressing and eating activities, the joint
reaction forces were 300 N.
 Rising from a chair resulted in a joint reaction force of 1,700 N and pulling a table 1,900
N, which is almost three times body weight

 Articular surface forces:
Contact areas of the elbow occur at four locations: Two are located at the olecranon and
two on the coronoid
The humeroulnar contact area increases from elbow extension to flexion.
In addition, the radial head also increases its contact area with the capitellum from
extension to flexion.
During valgus/varus loads to the elbow, Morrey et al. (1988) demonstrated the
varus/valgus pivot point to be located at the midpoint of the lateral aspect of the trochlea.

Biomechanics of elbow joint .

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