MECHANISM OF ELBOW JOINT.

MECHANISM OF ELBOW JOINT.

Contents ipage No* A* ANATOMICAL CONSIDERATION... 5 * # B* BIOMECHANICS OP ELBOW - POREABM COMPLEX 68

65 A. tm/ltqmtgal gobsttitcbtttoll The elbow is an uniaxial or hinge joint consisting of two articulations t (1) humeroulnar between the trochlea of the humerus and the trochlear notch of the ulna, and (2) humeroradial between the capitulum of the humerus and the facet on the head of the radius* Thus it is a compound synovial joint* It is continuous with the superior radio- u^nar joint* Shis articulation forms a uniaxial pivot between the circume ference of the head of the radius ani the osseofibrous ring formed by the radial notch of the ulna and the annular.ligament* She -trochlea is not a part of simply pulley because its medial flange is more extensive than its lateral flange and projects downwards to a lower level so that the joint line is roughly 2 cm* below the line joining two epicon- dyles and passes downwards and medially* She trochlear.notch is not perfectly congruient with the trochlea* In full extension medial part of its upper half is not in contact on flexion*. As a result the principal to and fro movement, of the hinge is accompanied by screwing and conjunct rotation*

66 She capitulum and the head of the radius are reciprocally curyed, hut the best contact is obtained when the semiflexed radius is in the midprone position* She rim of the head which is more prominent medially, fits into the groove between th capittelum and the trochlea* She humeroulnar and humeroradial articulations together forms a largely uniaxial joint, the ligaments of which are the articular capsule, and ulnar and radial collateral ligaments* She movements of elbow consists of flexion and extension, 'the uina moving on the trochlea, and the head of *the radius on the capitulum of the liumerus.* However, the flexion - extension movements of thi ulna do not constitute a pure swing but are accompanied by a small degree of eongruhent rotation the ulna is slightly pronated during extension and supinated during flexion* She movement of extension is limited by the tension of the fibrous capsule and muscles in front of the joint, that of flexion chiefly by apposition of soft parts* When the forearm is fully extended and the hand supinated, the upper arm and forearm are not in the same line, the forearm is directed some what laterally and forms carrying angle about 163 degrees with the upper arm* As a result the ulnar border of the supinated and extended forearm cannof be brought into contact with lateral surface *

67 of the thigh when the arm is by the side* The carrying angle is partly caused by the medial edge of the trochlea of the humerus, which projects about 6 mm below the lateral edge, and partly by the obliquity of the superior articular surface of the coronoid process, which is not set at right angles to the shaft of the ulna* The angles which the articular surfaces of the humerus and ulna make wiljh the long axes of the bones are approximately equal, and as a result the carrying angle disappears on full flexion of the forearm and the two bones come to lie in the same plane* The carrying angle iq mayked in pronation of the extended fqrearm and this has the effect of bringing the upper arm, the semipronated forearm and the hand into same straight line* a The accessory ^movements of the elbow joint are limited to slight screw action, abduction and adduction of the ulna, and forward and backward movement of the head of the radius on the oapitulum of the humerus* Muscles producing flexion of elbow include brachialis, biceps and brachioradialis and extension of elbow include triceps and anconeus*

68 B«BIOMECHANICS OP THE ELBOW FOREARM COMPLEX Kinematics - Mechanics - Physiology - Perfozmance. The- elbow forearm complex represents a link in a mechanical chain of levers which begins at the shoulder and ends at the finger tips* Elbow motion serves to adjust height and length of the limb to reach any point within the sphere of shoulder motion. Kinematic aspects of elbow» forearm function She elbow is a compound joint with two degrees of freedom, the ulnohumeral part of the articulation consisting of a hinge which permits one degree of freedom i*e* angular displacement of the forearm link abqut a transverse axis with an orientation that oscillates slightly in flexion and extension. She radiohumeral articulation particpates in this action* She second degree of freedom at the elbow is added by the unusual compound arrangement afforded by the radio-humeral and radioulnar joints : these articulations permit rotation of forearm (pronation - supination) around a second longitudinal axis which is approximately perpendi- cular to the elbow axis and movable in the plane of flexion extension* In view of these combined mechanisms of motion about transverse and longitudinal axes, the elbow has been designated as a * trochoginglymus* joint (Steindler, 1955)* *

69 Surface Motion (a) Motion in sagittal plane : (i) Range t Flexion-extension capability of the elbow ranges from the extended position of zero degree or slight hyper extension upto 160 degrees of flexion* fhis range of motion is limited by the geometry of the joint surfaces and surrounding bone, by passive supporting structures represented by collateral, capsular and other ligaments and by the active motivating structures represented by muscles and tendons* * (iix Nat use of motion s She mechanism of elbow flexion-extension is one.of gliding or.sliding^as in the knee, but the congruity of joint surfaces is somewhat more accurate, at least between the trochlea.and olecranon* (iii) Loading *and Pressure effect s A high percentage of the joint surfaces are in contact at any given time so that pressure or stress in the cartilage is comparatively well distributed during loading and it has been shown that nearly all part of the two articular surfaces are in contact at some point during the full range of motion, consequently degenerative change in this joint is relatively infrequent in the absence of a specific existing cause* However, this is not the case in the radio humeral joint where contact and loading are lees evenly distributed and erosion is seen often on the posteromedial part of the rim of the ' radial head (Gpod fellow and Bullough»(l967)*

(iv) Axis of motion % She elbow axis for flexion extension motion is determined by the transverse axis of the distal humerus which is slightly oblique to the longitudinal axis of the bone, in particular because of the increased depth of the medial lip of the trochlea compared with the lateral lip, this accounts for the carrying (cubital) i angle which refers to the anatomical deviation between the long axis of the humerus and the extended and fully supi- nated forearm..(v) Carrying angle's She normal carrying angle lies within the-range* f 0 degree to SO degrees of valgus with a* tendency towards the higher side of the range ;Ln women* Despite this deviation in extension,* which is more apparent in supination, it is essential to recognise that the angle disappears in flexiod as the forearm and humerus become closely aligned with the hand lying in front of the shoulder (Steindler, 1955)i in fact Morrey and Chao (1976) indicated that the long axis of forearm change linearly from valgus to slight varus during flexion, independent of forearm position in pronation-supination* Geometrically, these relationships depend on the fact that the axis of the elbow joint normally bisects the carrying angle, so that the angle of the longitudinal axis of the humerus to the elbow axis and the angle of the forearm axis to the elbow axis are the same* If this relationship is disturbed sufficiently, not oix^y. does an abnormal carrying angle appear in extension, but the arm deviates in flexion as well because the elbow no longer bisects the carrying angle* *

(b) Motion in transverse -plane In the most interesting analysis of passive motion of the elbow, Morrey and Chao (1976) noted that the ulna and hence the entire forearm rotates about its long, anatomical axis during flexion extension, irrespective of the degree of pronation-supination* Internal rotation of 5 degrees oeeurs during early flexion and external.rotation of 5 degrees during terminal flexion, these changes probably due to the configuration of the articular surface and ligaments* A point approximating the instant centres of rotation for flexion.extension lies on a line passing through tlje centre of the trochlea and in the plane of the anterior surface of the distal humerus* These observations may have clinical significance in e^laining loosening of hinged elbow prosthesis s nqt only does a hinged prosthesis prevent normal rotation, but misplacement of the axis can compound the problem* Also external rotation of the ulna during extension could explain the reason of findings of degenerative changes on the medial side of the tip of the olecranon in baseball pitchers* (c) Motion in Coronal plane : Hotation of forearm, in terms of pronation supination is permitted by the radio capitular articulation and the superior, and inferior radioulnar joints which move in paired synchronous motion* Classically pronation is sskd to be accompanied by slight abduction of the ulna and supination by slight adduction.

Forearm Rotation. Morrey and Chao (1976) pointed out that these ideas were the result of observing composite mdtion and that no lateral displacement of the ulna occurs during pronation supination when the hand is.free 5 during pronation, however, the long axis of the radius does deviate medially 6-10 degrees with respect to the humerus. The axis of pronation - supination passes through the capitttlum and the centre of the head of the radius and extends distally through the head of the ulna and on down'to the extended little finger V (Burman, 19^2; Steadier, 1955? Yasely, 1967). This axis. of.mechanical rotation of the forearm 4s thus oblique to the. anatomioal long axle of the forearm Bones. Biomechanical concent of prosthesis 8 London (1931) Reported Kinematic analysis of eight normal elbows (four in cadavers and four in living subjects) using true lateral roentgenograms made by a special technique showed that flexion occurs about a single axis. This axis passes through the centres of the arcs outlined by the bottom of the trochlear sulcus and the periphery of the capit&ltum. Motion of the Joint surfaces is of a sliding type except at the extremes of flexion and extension where rolling motion occurs. Based on these findings, total elbow prosthesis ideally should be uniaxial, with the axis of flexion of the prosthesis the same as the axis of the normal. Joint. These^Kinematic findings, of course do not explain

the inordinately high incidence of loosening reported after hinged prosthetic arthroplasty of the elbow* However, it would seem that ideally an uniaxial joint should be replaced with a uniaxial hinge.d prosthesis* She observed changes from sliding to rolling motion at the extremes of flexion and extension should be of no consequence since few patients, if any, achieve a full range of flexion or extension after replacement arthroplasty* Of the many possible causes of loosening of hinged elbow prostheses, one could be improper positioning of the axis of the hinge* Many surgeons have recommended aligning the axis of the hinge with the epicondylee As* demonstrated in this stucly, the normal axis of rotation is internally rotated relative to the epicondylar plane., Also, the axis of the hinge of many prostheses may be posterior to the normal axis of* rotation of the elbow* Another likely cause of loosening may be the relatively large forces across the elbow* especially the torques* With hinge prostheses, these forces are probably not dissipated in the surrounding soft tissues but are transmitted directly to the bone cement interface* In designing a prosthetic elbow replacement not only the Kinematics of the joint but also the loading properties of the elbow, material' characteristics and so forth should be considered* An ideal replacement, prosthesis would be uniaxial and unconstrained, with its fixation i* surfaces placed as far from the centres of the intramedullary canals as possible* _

Mechanical aspects (Kinetics)of elbow - forearm function* Motion of a limb does not occur spontaneously but is caused by mechanical forces) hence, study of elbow forearm function must progress from the geometric realm of Mnematice to that of mechanics the science describing actions of the foreces on physical objects* In the most simple configuration of the elbow, the forearm hangs relaxed at the side and in this position there are no active muscle forces rather than muscle tone; this is a linear force system in static equilibrium, the forties acting at the elbow balancing each other** If additional forces are applied to the N hanging arm, stress in the tissues would increase proportiona- «lly to maintain equilibrium* If the.applied force continues to increase, critical stress or strength of the tissues eventually would be exceeded and damage occur as a sprain or fracture* Ihree major forces are active on the flexed forearm* First, gravity or forearm weight with the action line through the centre of gravity, generates a moment tending to cause rotation into extension* Second in order to maintain the arm at 90 degrees another force must be applied by the flexor musculature to generate a counteracting moment, tending to cause rotation into flexion* As the distance from the elbow joint axis to the flexor insertion is much shorter than from the joint to the centre of gravity e^the forearm, a relatively high muscle force must be applied *"* if the muscle foment is to equal the gravity moment in terms * of the product of force and distance*

The third force is represented by the joint reaction forces* Moment equilibrium requires a large muscles force acting upwards to oppose the torque of the small forearm weight acting downwards s the joint reaction force is developed in order to equalise the imbalance in linear forces because the oppositely directed forces must summate to ZBEO if position in space is to be maintained* Physiological Aspects of Elbow - forearm function. Difinitive work on electromyography 1ms been compiled by Basmajian (1974). He showed that both the biceps and brachialis ate active.in slow flexion and that the brachio-. ra^ialis is recruited as load increased on the partially flexed forearm (Basmajian, 1957). Brachialis develops considerable power in all position during both slow and quick motion*. Regarding other muscles, Basmajian (1974) indicated that the pronator quadratus and pronator forces are both active during pronation with the prime mover being the quadratus* Supination is initiated by elastic recoil from pronation and is continued by the supinator augmented by the biceps as fesistance develops, there is little supination action by the biceps with the forearm is pronation and extension as the muscle is inhibited in this position. Muscle activity during elbow extension has also been

is the work horse* and prime extensor, similar in action to the hrachialis in flexion* The lateral and long heads of the triceps are recruited during heavier loads and the anconeus in-active during extension as well as pronation and supination this small muscle serves a stabilising function rather than simply causing ulnar deviation during pronation* Hoekzema (1971) showed that the long head of "the triceps inserting centrally in the triceps tendon, was 30-40 per cent more efficient in extension effort, provided the olecranon was intact to provide a lever arm* With the olecranon removed»all*three heads showed equal mechanical " efficiency.. ##