Biomechanical Forces Action on Elbow - Essay Example

The elbow is a hinge-joint or “ginglymus” that connects the lower end or distal of the humerus to the radius and ulna proximal ends. The elbow joint is composed of the cup-like concavity in the head of the radius, the humeral head, and the capitellum that provides better connection with bigger sigmoid cavity of the ulna. All these articular surfaces, to prevent bone rubbing, are covered with thin layer of cartilage. The elbow joint motion is about 160° with respect to its long axis. Such motion is attained through the flexion-extension of the different muscles connected to the elbow. Meanwhile, a force is an effect on a certain body which changes its shape or motion. The elbow transforms shearing, compressional, and rotational forces into a normal motion. During flexion, the arm is bent, the pressure forces that moves along the elbow joint shift forces from the humerus to the ulna. Under extension, on the contrary, the applied forces run along the radius and the humerus, with the arm straightly open. The elbow displaces such forces largely through the muscles and ligaments that absorb and disperse stresses. Since the surface area of the ulna is bigger than that of the radial head, the ulna contributes lesser force per unit area as compared to the humerus. In addition, to avoid bone strain absorption, the elbow articular cartilages acts as a buffer between the bones. In the event that the cartilage is forced to absorb great stress for various times, the stress flattens out the cartilage and will eventually result to early wearing. Introduction Basically, the elbow is considered as a hinge joint with a single degree of freedom. However, its anatomical structure necessitates the inclusion of the articulations along with the radius and ulna. Thus, the elbow is best treated as having a mechanism of a two degree freedom that supports the supination/pronation of the forearm and the extension/flexion of the elbow. In terms of internal structure, the three synovial joints, along with subtle interactions, are radio-ulnar, humero-ulnar, and humero-radial. The humero-radial joint is of prime interest because of the combination of relative motions that occur therein: the axial rotation that involves in the forearm supination/pronation and the elbow flexion accompanying the ulna (Lockard 2006, p. 72). Flexion/extension moments are produced by the muscles that act over the elbow join, including brachioradialis, brachialis, triceps, and biceps brachii. While pronation is achieved through the muscles in the forearm, pronator quadratus, flexor carpi radialis, and pronator teres, supination is generated by the combination of biceps brachii and forearm supinator. Elbow Joint Structure and Function The elbow joint is made up of three separate joints: the radiohumeral joint, the superior radioulnar joint, and the ulnohumeral joint (Nicholson and Driscoll 1993, p. 1058). These joints are all enclosed in one joint capsule. While the movement between the humerus and the radius occurs at radiohumeral, the movement between the humerus and the ulna occurs at ulnohumeral. Likewise, the movement between the ulna and the radius occurs at superior radioulnar. The radiohumeral and ulnohumeral joints serve as hinge joints (Anderson, Hall, and Hitchings 2002, p. 154). These joints are straightened by the triceps muscles located at the back of the arms and are flexed by the brachialis, brachioradialis, and biceps (Norkin and White 2003, p. 91). The forearm rotation, turning the palm down is termed as pronation while turning the palm up by forearm rotation is called supination (Alter 2004, p. 252). On the other hand, the superior radioulnar joint acts as a pivot. It supports the pronation and supination of the wrist and forearm. An elbow normally has 135° to 155° flexion, whereas the pronator teres, flexor carpi radialis muscles, and pronator quadratus pronate and the supinator muscles and biceps supinate the elbow (De Jesus, Echevarria, Rodriguez, and Vargas 2004, p. 1). Anteroposterior line diagram of the Elbow (Norkin and White 2003, p. 91). The elbow joint is generally considered as a trochoging lymoid joint with 2° of freedom: 0°-140° and 75°-85° respectively for flexion-extension and prono-supination (Celli 2008, p. 8). Based on the study conducted by Morrey, everyday typical activities require 30°-130° and 50°-50° motion for flexion-extension and prono-supination, respectively (Celli 2008, p. 8). The rotation axis of the elbow runs through the capitellum along with the base of the trochlea solcus. This axis, during flexion-extension, has about about 4°-8° of valgus in line with the long axis of the humerus and 3°-5° internal rotation with the plane of the lateral and medial epicondyles as the frame of reference. This arc of motion is possible because during the flexion-extension the elbow’s rotation center moves within the area, rather than a fixed single point. Elbow Joint Normal Forces and Motion Normal Forces in the Elbow Joint (De Jesus, Echevarria, Rodriguez, and Vargas 2004, p. 2). The forearm is slightly positioned away from the long axis of the humerus, as the elbow is supinated and extended fully, forming the “carrying angle” (Amis 2002, p. 349). The carrying angle in men is about 10° to 15° while in women it ranges from 15° to 20° (De Jesus, Echevarria, Rodriguez, and Vargas 2004, p. 1). Regarding forces, as showed in the figure, the gravity naturally exerts a downward force on the ball and on the forearm while the biceps muscle exerts an upward force. Since the lever arms at F and W are longer than at M, the force that should be exerted at M must be greater to counterbalance the moment forces exerted by F and W. During elbow flexion, the lever arm is the distance between the elbow joint axis and the tendon and the biceps muscles sustain the forearm at 90° (Ozkaya, Leger, and Nordin 1999, p. 89). Meanwhile, ligament injury and weakness of the muscles eventually result to the abnormality in the elbow biomechanics, imparting abnormal forces (Johnson and Pedowitz 2007, p. 347). Then, the abnormal forces trigger the premature damage on the articular cartilage. Nevertheless, injuries on the support structures of the elbow, including the ligaments and bones, are induced by the excessive rotational, extension, side by side, and flexion forces. During the flexion-extension as well, the olecranon runs on the trochlea, providing ground for the elbow movement, in extension to the flexion of the varus, from the valgus. Such movement creates the carrying angle or the angle, measured from the frontal plane, between the long axes of the ulna and the humerus (Amis 2002, p. 349). Men generally have smaller carrying angle than women with the average variation of 7°-12° and 2°-3°, respectively (Celli 2008, p. 9). The primary restrictions during flexion-extension include the impact of olecranon to the olecranon fossa, associated with the restrained ligaments and the anterior capsula, during extension. As well, the coronoid process and radial head impacts on the corresponding fossa cause constraints during flexion. The radial head morphology primarily influences the motion of the forearm. The rotational axis of the forearm runs proximally through the middle portion of the radial head, while the fovea of the ulna passes distally at the bottom of the ulnar styloid. In the forearm rotation, the contact between the lesser sigmoid notch and the edge of the radial head is sustained, whereas in the transverse plane, the radial shaft moves into the ulna. Furthermore, active and passive structural stabilizers support the elbow during motion. These stabilizers act differently during the elbow motion. With respect to the valgus stress, the radial head is known as a secondary stabilizer. Morrey and colleagues reported that the instability of the valgus, with intact medial ulnar collateral ligament, is not affected by the radial head resection (Celli 2008, p. 9). However, the discharge of the ulnar collateral ligament makes the radial head as a major constraint to the instability of the valgus. In the injury of the interosseous membrane, the primary longitudinal stabilized is the radial head. Based on biochemical tests, in both flexion and extension, 30% of the stability of the valgus can be attributed to the radial head (Celli 2008, p. 9). The complete discharge of the ulnar collateral ligament contributes about an-18° increase in the laxity of the valgus, which after the radial head incision, increases to 36°. The axis loading generates 40% and 60% stress distributions across the ulna and the radius, respectively. When the elbow is under supination or at 0°-35° flexion, the transmission across the radial head rises. According to An and colleagues, coronoid can resist 60%-70% varus stress while olecranon can resist 75%-85% valgus stress (Celli 2008, p. 9). The elbow gains stability against valgus-varus laxity as the olecranon takes the olecranon fossa of the humerus olecranon fossa at 20°. The elbow posterior dislocation can be avoided through the coronoid process. O’ Driscoll and colleagues reported that 30% of the coronoid resection imparts instability when the radial head is compromised during the collateral ligament injury (Celli 2008, p. 10). The coronoid fracture, associated with ligament injury, even small fragments, reduces the elbow stability. Still, with intact collateral ligaments, the coronoid is important to varus stability. Due to coronoid fracture, the radial head resection triggers more instability. On the other hand, lateral and medial collateral ligament complexes are soft tissue stabilizers. Based on the analysis of Morrey and An, in extension, the bony fit, medial collateral ligament, and anterior capsula are uniformly resistant to valgus stress, while the primary stabilizer against valgus stress, at 90° of flexion, is the anterior band of the medial collateral ligament (Celli 2008, p. 10). Further, the anterior band and the mid-portion of the medial collateral ligament, during elbow motion, sustain tension where the posterior bundle at 65° is stretched to full flexion. In order to recuperate the full flexion, the portion of this posterior area does not affect the valgus instability. Moreover, Morrey and colleagues found that the medial collateral ligament is the major restraint against the valgus stress while the radial head is the secondary stabilizer (Celli 2008, p. 10). As mentioned earlier, in the insufficiency of the medial ligament complex, the radial head becomes the major restraint. Since the origin of the lateral collateral ligament reclines near the elbow’s axis of rotation, the lateral collateral ligament is stretched in the elbow range of motion. Likewise, O’ Driscoll and colleagues reported that lateral collateral ligament functions mainly as the ligamentous stabilizer to postero-lateral and varus rotatory instability (Celli 2008, p. 10). During supination, the anterior of the annular ligament is stretched whereas the posterior is stretched during pronation. Meanwhile, muscles crossing the elbow joint are active stabilizers. The compressive forces around the radius, ulna, and humerus are induced by the contraction and line of pull of those muscles. Large forces are observed at the axial of the distal humerus during full extension, which decrease during elbow flexion. These forces serve as dynamic stabilizers of the elbow joint (Lockard 2006, p. 75). Conclusion The rotational axis of the forearm runs proximally through the middle portion of the radial head, while the fovea of the ulna passes distally at the bottom of the ulnar styloid. During rotation, the contact between the lesser sigmoid notch and the edge of the radial head is sustained, whereas in the transverse plane, the radial shaft moves into the ulna. In addition, active and passive structural stabilizers support the elbow during motion. These stabilizers act differently during the elbow motion. Thus, the radial head morphology primarily influences the motion of the forearm. With respect to the valgus stress, the radial head is known as a secondary stabilizer. Nonetheless, the discharge of the ulnar collateral ligament makes the radial head as a major constraint to the instability of the valgus. In this connection, most of the axial loads that will be placed on the elbow will be transmitted by means of the humeradial joint than by the humero-ulnar articulation. Stresses applied on the elbow, depending on the lever arm length, resultant force, and load applied, vary. As such, the load may reach 2-3 times and 8-10 times, respectively, of the body weight and the lifted weight. In fact, by means of modeling approaches, Chadwick and Nicol (2000) computed the task predicting loads 800 N and 1600N, respectively, for the humero-radial and humero-ulnar joints. List of References Alter, M. J. (2004). Science of flexibility. Champaign, Ill: Human Kinetics Amis, A.A. (2002). ‘Mini-symposium: the elbow, biomechanics of the elbow.’ Current Orthopaedics 16, pp. 349-354 Anderson, M. K., Hall, S. J., and Hitchings, C. (2002). Fundamentals of sports injury management. Baltimore: Williams and Wilkins Celli, A. (2008). ‘Anatomy and biomechanics of the elbow.’ In A. Celli, L. Celli, and B.F. Morrey Treatment of elbow lesions: new aspects in diagnosis and surgical techniques. Milan: Springer Chadwick, E. K. and Nicol, A. C. (2000). ‘Elbow and wrist joint contact forces during occupational pick and place activities.’ Journal of Biomechanics 33, (5) pp. 591-600 De Jesus, W., Echevarria, L., Rodriguez, J., and Vargas, A. (2004). ‘Biomechanics of elbow prostheses.’ Applications of Engineering in Medicine pp. 1-25. University of Puerto Rico Johnson, D. and Pedowitz, R. A. (2007). Practical orthopaedic sports medicine and arthroscopy. Philadelphia, Lippincott: Williams and Wilkins Lockard, Margary (2006). ‘Clinical Biomechanics of the Elbow.’ Journal of Hand Therapy 19, pp. 72-81 Nicholson, D.A. and Driscoll, P.A. (1993). “The elbow.’ British Medical Journal 37, 6911 pp. 1058-1062 Norkin, C.C. and White, J.D. (2003). Measurement of joint motion: a guide to goniometry, 3rd ed. Philadelphia: F.A. Davis Ozkaya, N., Leger, D. L., and Nordin, M. (1999). Fundamentals of biomechanics: equilibrium, motion, and deformation. New York, NY: Springer Read More