Elbow Mechanics PDF

Mechanics of Elbow Joint P R ES EN TED BY: A S S I STAN T P RO F. D R N E H A D A H ME D Y O U N E S S  The Objectives By the end of this day the participant could be able to: Explain the caring angle of the elbow. Illustrate the biomechanics of bone. Clarify Elbow...

Mechanics of Elbow Joint P R ES EN TED BY: A S S I STAN T P RO F. D R N E H A D A H ME D Y O U N E S S  The Objectives By the end of this day the participant could be able to: Explain the caring angle of the elbow. Illustrate the biomechanics of bone. Clarify Elbow Stability and Stabilizing Structures Elbow Anatomy Articulating Surface of Elbow Joint Distal humerus end (anterior view)  ANATOMY OF THE ELBOW Passive Stabilizers Osteology The distal humerus comprises two condyles forming the articular surfaces of the capitellum laterally and trochlea medially. The more prominent medial epicondyle is an attachment point for the ulnar collateral ligament and flexor– pronator group. The less prominent lateral epicondyle is an attachment point for the lateral collateral ligament and extensor–supinator group. Anteriorly, the coronoid and radial fossa accommodate the coronoid process of the ulna and radial head, respectively, during flexion. Posteriorly, the olecranon fossa accommodates the olecranon process of the ulna during extension. The proximal radius includes the cylindrical shaped radial head, while proximal ulna provides the elbow articulation with an inherent stability, especially in full extension.  Articulation The elbow joint is highly congruous and is made up of the articulation between the radius, ulna, and humerus bones. The ulnohumeral joint is a hinge (ginglymus) joint with motion of flexion and extension. The proximal radioulnar and radiohumeral joints are pivoting joints (trochoid) allowing rotation. The trochlea of the distal humerus is a pulley-shaped surface that is larger medially than laterally, and it articulates with the sigmoid notch of the proximal ulna. Laterally, the capitellum articulates with the radial head. The proximal radius has a cylindrical head with hyaline cartilage covering both the depression for articulation with the capitellum and also at the outside circumference of the radial head. The proximal ulna consists of the coronoid and olecranon process. These make up the saddle-shaped, ellipsoid articular surface of the sigmoid notch.  The carrying angle of the elbow The carrying angle of the elbow is formed by the longitudinal axis between the humerus and ulna when the elbow is in full extension. In females, the average valgus angle is 13° to 16°, whereas in males, it is 11° to 14°. The joint capsule normally has a thin anterior portion. The anterior capsule becomes taut in extension and lax in flexion.  Ligaments (1) The triangular medial (ulnar) collateral ligament The anterior bundle (is the significant component of the medial collateral ligament complex), posterior bundle (Bardinet ligament) is a posterior capsular thickening and is best defined at 90° flexion, and transverse segment (ligament of Cooper) contributes little to elbow stability. (2) The lateral (radial) collateral ligament complex The Radial collateral ligament the origin of the ligament is close to the axis of rotation and is therefore uniformly taut throughout flexion-extension movement. The annular ligament It originates and inserts on the anterior and posterior margins of the lesser sigmoid notch. The anterior insertion becomes taut during supination and the posterior origin becomes taut in pronation. The lateral ulnar collateral ligament functions as the primary lateral stabilizer of the ulnohumeral joint (functions in stabilizing varus) stress, and deficiency of this ligament results in posterolateral rotatory instability. The accessory lateral collateral ligament functions to stabilize the annular ligament during varus stress at the elbow. (3) The oblique ligament have limited functional importance. (4)The quadrate ligament is a stabilizer of the proximal radioulnar joint during prono-supination.  Functionof MCL Function of LCL 1. Stabilizes against 1. Stabilizes elbow against varus torque. valgus torques at the 2. Stabilizes against combined varus and medial elbow. supination torques. 2. Limits extension at 3. Reinforces humeroradial joint the end of the elbow and helps provide some resistance extension range of to longitudinal distraction of the motion. articulating surfaces. 3. Guides joint motion 4. Stabilizes radial head, thus throughout flexion range providing a stable base for rotation. of motion. 5. Maintains posterolateral rotatory 4. Provides some resistance stability. to longitudinal 6. Prevents subluxation of distraction of joint surfaces. humeroulnar joint by securing ulna to humerus.  Active Stabilizers Muscles The muscles crossing the elbow joint can be divided into four main groups. Posteriorly, the elbow extensors cross the elbow joint, and are innervated by the radial nerve. Laterally, the wrist and finger extensors and the supinator are found and innervated by the radial nerve. Medially, the flexor–pronator group and are innervated by the medial and ulnar nerves. Anteriorly, the elbow flexors cross the joint, and are innervated by the musculocutaneous nerve.  BIOMECHANICS OF THE ELBOW Elbow Stability and Stabilizing Structures Passive Stabilizers: The passive and active stabilizers provide biomechanical stability in the elbow joint. The passive stability results from both the highly congruent articulation between the humerus and ulna and the soft tissue constraints. The active stability is caused by joint compressive forces provided by the muscles.  Passive Bony Stabilizers The ulnohumeral joint is a highly congruous joint and is a dominant factor as a passive bony stabilizer. The contact areas in the elbow joint vary with the type of applied stress. In a laboratory study, contact areas of the elbow have been shown to occur at four facets in the sigmoid fossa, two at the coronoid and two at the olecranon. With varus and valgus loads, the contact changes medially and laterally, respectively. The carrying angle orientation changes from a valgus orientation in extension to varus orientation in flexion. [For simplicity, one may assume that the ulnohumeral joint is a pure hinge joint, and that the axis of rotation coincides with the trochlea so that the change in carrying angle with flexion is caused by anatomic variations of the articulation].  Passive Soft Tissue Stabilizers It includes the medial and lateral collateral ligament complexes and the anterior capsule. The lateral collateral ligament originates from the lateral condyle at the point where the axis of rotation of the elbow passes through. This ligament has a fairly uniform tension throughout range of motion, because it originates at the axis of rotation. The medial collateral ligament consists of two main components. As the point of origin of the different components of the medial complex do not occur at the axis of rotation, the different components are not uniformly taut during elbow flexion and extension. Interplay Between Passive Stabilizers The contributions of the articular geometry and ligaments to varus and valgus loads were studied. In 90° elbow flexion, the medial collateral ligament is the primary stabilizer to valgus stress. In extension, the medial collateral ligament, anterior capsule, and bony fit (articulation) are fairly equally resistant to valgus stress. With the elbow in both flexion and extension, the bony articulation provides much of the stability to varus stress. Eighty-five percent (85%) of the resistance of the joint to distraction is caused by the anterior capsule in extension, whereas only 8% of resistance is caused by the anterior capsule in 90° elbow flexion. With elbow flexion, 78% resistance to traction is provided by the medial collateral ligament complex.  Active Stabilizers Muscles crossing the elbow joint and their function have been previously described. The line of pull and contraction of muscles across the elbow joint create forces within the joint at the humerus, radius, and ulna. These balanced forces likely function as dynamic stabilizers of the joint. From the muscles crossing the elbow joint, the brachialis and triceps muscles have the largest work capacity and contractile strength. Maximal isometric elbow flexion, when the elbow is fully extended can produce joint compression forces of as much as two times body weight.  Force Transmission Through Elbow Compressive load during activities such as dressing and eating=300N,1700 N when the body is supported by arm when raising from chair ,and 1900 N when pulling table across the floor. During moderate activities ,Humeroulnar forces up to 1600N,humeroradial forces up to 800 N. - During push-up exercise peak force in each elbow reach 45% of body weight. - During hand medially rotated, greater posterior and varus force are generated more than during hand in natural or laterally rotated position. Since the attachment of triceps tendon of ulana is closer to elbow joint center than the attachment of the brachialis on the ulna and biceps on the radius, the extensor moment arm is shorter than the flexor moment arm. This mean that the elbow extensor must generate more force than elbow flexor to produce the same amount of joint torque. Function of Elbow Supination and Pronation The primary motion of the forearm is supination and pronation, with the axis of rotation passing from the proximal radial head to the convex articular surface of the ulna at the distal radioulnar Joint ( In ADL 50° pronation and 50° supination). Flexion and Extension In elbow flexion and extension (in ADL 30° to 130°), the deviation of joint rotation is minimal, and elbow motion can be thought of as a uniaxial joint except at the extremes of flexion and extension. With this simplification, the axis of rotation can be thought of a line passing from the inferior medial epicondyle through the center of the lateral epicondyle. The elbow is often mistakenly thought of as a simple hinge joint because of the congruous and stable ulnohumeral articulation. Studies have shown that in addition to flexion and extension, the ulnohumeral joint also has 6° axial rotation secondary to the obliquity of the trochlear groove.  Range of Motion Normal elbow range of motion is from 0° to 150°, and forearm rotation averages 75° pronation and 85° supination. Factors limiting extension: - Include the impact of the olecranon process on the olecranon fossa. - Tension of the anterior bundle of the medial collateral ligament, and the flexor muscles. Factors limiting flexion: - Include the impact of the coronoid process against the coronoid fossa. - The impact of the radial head against the radial fossa, and the tissue tension from the capsule and triceps muscle. Pronation and supination motions are restricted more by the passive stretch of antagonistic muscles than by ligaments, although the quadrate ligament has been shown to provide static constraint to pronosupination motion.  Range of motion - In supination the radius and ulna lie parallel to one another, so normal range is 81 degree. - In Pronation the radius’s crossing over the ulna at the superior radioulnar joint, so normal range is 71 degree. - Most activities are accomplished within the functional range of 50- degree pronation to 50-degree supination.  Elbow injuries Lateral epicondylitis, commonly known as tennis elbow. The amount of force to which the lateral aspect of the elbow is subjected during tennis play increase with poor technique and improper equipment. Medial epicondylitis is commonly known as golfer's elbow Elbow injuries in pitchers can be related to the large rotational force - called "torque" - needed to slow down the cocking of the arm and accelerate the forearm, hand, and ball forward. From the cocked position, the ulnar collateral ligament (UCL) pulls the forearm forward with the rotating upper arm. Injury or stretching of UCL can result in valgus instability , repeated valgus overload during repetitive throwing provoke bony changes. This seen in individual who throw repetitively. Elbow sprain and dislocation: forced hyper extension of elbow----posterior displacement of ulna resulting in dislocation. continued hyperextension-----cause distal humerus to slide over the ulna. The mechanism in adult is typically falling on an outstretched hand or a forceful ,twisting elbow. In children radial head subluxation is generally results from a sudden pull on the upper limb, such as that exerted by an adult to prevent the child from falling; in children aged 1 to 3 years it is referred as nursemaid's elbow. Radial tunnel syndrome (happens when the radial nerve is squeezed where it passes through a tunnel near the elbow). Olecranon bursitis is the inflammation of a small sac of fluid located on the tip of the elbow. Elbow arthritis may be surgically treated with a procedure called interposition arthroplasty. The term "interposition" means that new tissue is placed between the damaged surfaces of the elbow joint.  Arthrokinematics at the Humero-Ulnar Joint The humero-ulnar joint is the articulation between the concave trochlear notch of the ulna and the convex trochlea of the humerus  Arthrokinematics at the Humeroradial Joint The humeroradial joint is an articulation between the cuplike fovea of the radial head and the reciprocally shaped rounded capitulum. The arthrokinematics of flexion and extension consist of the fovea of the radius rolling and sliding across the convexity of the capitulum Arthrokinematics at the Proximal and Distal Radio-Ulnar Joints  Aging Effects on Forearm Muscles Muscle force and shortening velocity may be affected more by muscle function than by physiological cross-sectional area and that the effects of aging on muscle shortening velocity becomes apparent after age 50.  Compression Injuries Resistance to longitudinal compression forces at the elbow is provided mainly by the contact of bony components; Falling on the hand when the elbow is in a close-packed (extended) position may result in the transmission of forces through the bones of the forearm to the elbow. If the forces are transmitted through the radius, as may happen with a concomitant valgus stress, a fracture of the radial head may result from impact of the radial head on the capitulum. If the force from the fall is transmitted to the ulna, a fracture of either the coronoid process or olecranon process may occur from the impact of the ulna on the humerus.  Distraction Injuries Ligaments and muscles provide for resistance of the joints of the elbow complex to longitudinal traction. A tensile force of sufficient magnitude exerted on a pronated and extended forearm may cause the radius to be pulled inferiorly out of the annular ligament. This injury is common in children younger than 5 years and rare in adults. Lifting a small child up into the air by one or both hands nursemaid’s elbow or pulled elbow Nerve compression, bony fracture, or dislocation may also result from muscle contractions. Repetitive forceful contractions of the flexor carpi ulnaris muscle may compress the ulnar nerve as it passes through the cubital tunnel between the medial epicondyle of the humerus and olecranon process of the ulna These stresses can cause an injury called cubital tunnel syndrome, in which motion of the fourth and fifth fingers is impaired. Varus/Valgus Injuries