Transmission of Pressures Across the Elbow Joint ALBERT A. HALLS AND ANTHONY TRAVILL’ Department of Anatomy, Creighton University School of Medicine, Omaha, Nebraska ABSTRACT To ascertain the pathway (s) along which forces are conducted from the hand to the humerus in the human cadaver specially designed pressure sensitive transducers were introduced into the radio-capitular and ulnar-trochlear components of the elbow joint. A constant force was applied to the hand, and changes in electrical resistance and hence pressure across the elbow joint where recorded. It was observed that 57% of the force applied crossed the radio-capitular joint while 43% crossed the ulnar-trochlear joint. The pathway of this force transmission is discussed with particular reference to the involvement of the interosseous membrane. Early in his anatomical training nearly every medical student is taught “ex cathedra” that forces applied to the palm are transmitted along a devious route from the hand to the humerus by way of the radius, interosseous membrane and ulnar-trochlear joint. This explanation of force transmission first appears in the anatomical literature in a paper by Lopes (1860). He suggested that fractures of the distal end of the radius are particularly prevalent because of the structural relationship of the interosseous membrane to the two bones it connects. Hennequin (1894) and others so supported and embellished this concept that by the beginning of the present century it had become firmly entrenched in the French anatomical teaching (Testut and Jacob, ’14). However, Destot and Gallois (1898) had offered experimental evidence that this was not so and that possibly the interosseous membrane acted onIy in a secondary role in the transmission of forces parallel to the axis of the forearm. The view that the major function of the interosseous membrane is one of force transmission arises naturally from close inspection of the ligament and an appreciation of the direction of its constituent fibers. The constancy of the downward and medially directed fibers led to a consideration of their potential as force transmitting instruments. The same observations have led others to ascribe different functions to the membrane. Strasser (’17) considered that the interosseous membrane acts as a check ligament to restrain excessive supination. Hennequin (1894) had ANAT. REC., 150: 243-248. pointed out that Weitbrecht also held this view, while Cruveilhier considered the membrane acted as an aponeurosis for the insertion of deep flexor and extensor muscles, and Sappey considered that the interosseous membrane’s sole function was to prevent the separation of the radius and the ulna. There has been to date no direct experimental evidence to permit one to confirm or deny whether forces applied to the hand pass directly across the radio-capitular joint or whether they are conducted to the ulna and thence across the ulnartrochlear joint. Consequently we carried out the present investigation in an attempt to elucidate this problem. MATERIAL AND METHODS Seven human upper limbs were used for this investigation after they had been dissected by students in the dissecting room. They were separated from the body at the gleno-humeral joint. Care was taken not to disarrange the ligaments and capsule about the elbow joint. Thus a fibro-osseous specimen was prepared with metacarpal, carpal, antebrachial, and brachial bony and ligamentous elements left intact. A scaffolding and platform were constructed so that the upper limb could be supported and maintained in an upright position with the hand uppermost (fig. 1). To record forces transmitted across the components of the elbow joint special micro-transducers were designed. Both 1 Present address: Department of Anatomy, Queen’s University, Kingston, Ontario, Canada. 243 244 ALBERT A. HALLS AND ANTHONY TRAVILL of pressure. The effect of applying pressure is analogous to increasing the number of resistances in parallel in an electric circuit. The thickness of the completely insulated transducer was 1.0 mm. The micro-transducers were introduced between the articular surfaces of the radius and capitulum and then the ulna and trochlea through small incisions made in the anterior aspect of the capsule of the elbow joint. An electric circuit for recording changes of resistance in the transducers was completed by connecting in series an ammeter and the galvanometer of a pen writer. The power supply was provided by a 45 volt battery. Each transducer was connected to such a circuit. The scaffolding with the arm attached and the transducers in situ was placed on a Toledo beam scale (fig. 2 ) . Pressure was then applied by one of us to the palm of the extended hand so that the beam scale read 33 pounds (15 Kg). With each application of pressure to the hand recordings were made by the pen writers and at the same time readings of the ammeters were noted. For each limb a series of five applications of pressure, each repeated five times, was recorded. Between each application the system was allowed to come to mechanical and electrical rest. The results of 25 readings for each limb were averaged and tabulated. After the major investigation had been completed three of the limbs were set in turn on the scaffolding; and while pressure was being applied to the hand the interosseous membrane of each limb was divided by a single swift downward stroke of the scalpel. Changes in pressure across the radio-capitular and ulnar-trochlear joints were recorded. OBSERVATIONS Fig. 1 Scaffolding and platform with supported extremity. sides of brass shim stock, 3 mils (0.125 mm) thick, were painted with a thin layer of pressure sensitive paint (CELAB Clark Electronic Laboratories). After drying for 24 hours this paint undergoes changes in electrical resistance with the application The changes of pressure across the radio-capitular and ulnar-trochlear joints following application of pressure to the extended hand were recorded by the ammeters in circuit with the transducers. The averages of these recordings are summarized in table 1. In all limbs the changes of resistance, hence pressure, across the radio-capitular joint were greater than those across the ulnar-trochlear joint. The average of the 245 PRESSURES ACROSS THE ELBOW JOINT Fig. 2 Supported extremity with transducers i n situ mounted on beam scale. TABLE 1 Ammeter readings and ratios following applications of pressure across the elbow joint Av. ammeter readings (milliamps) Specimen 1 2 3 4 5 6 7 ulnar- Radioc apitular joint Ratio trochlear joint 7.2 7.6 7.5 5.8 3.4 9.0 6.8 9.2 9.1 9.8 8.2 8.8 9.2 9.1 45:55 46:54 44:56 41 :59 28 : 72 50:50 42 : 58 Each average is that of 25 readings. ammeter readings indicate that 57% of the pressure applied to the hand was transmitted the length of the radius and across the radio-capitular joint. While only 43% was transmitted across the ulnar-trochlear joint. Comparable results were obtained for the ulnar-trochlear joint when its transducer was in position alone or accompanied by the radio-capitular transducer in its joint. Tracings taken while pressure was being applied to the hand are shown in figure 3. The coincident onset and decline of pressure in both joints is also discernible. When the interosseous membrane was cut while force was being applied to the hand there was recorded a minimal amount of instability across the ulnartrochlear joint which rapidly settled down ULNAR-TROC HLEAR 1 R A D I O - C APlTU L A R Fig. 3 Traces showing changes of pressure across components of elbow joint. Upright and inverted arrows indicate application and release of pressure respectively. 246 ALBERT A. HALLS AND ANTHONY TRAVILL to its previous state. At this time there was no evidence of any radio-capitular pressure change (fig. 4 ) . our recordings when the transducers are within the joint. The observations indicate that though the articular surfaces are in point contact UL N A R - T R O C H L E A R at rest there is no detection of transmission of forces until pressure is applied. When pressure is applied to the hand we note that 57% of the forces are transmitted BEFORE DURING AFTER through the radius to the humerus and 43% through the ulna. However, with the l _ 1 7 - J - l - n p T L interosseous membrane cut, 43% of the t forces applied to the hand are still transmitted across the ulnar-trochlear joint. RADIO- C A P I T U L A R Why 100% of the forces are not transFig. 4 Traces showing changes of pressure across components of elbow joint before, during mitted directly across the radio-capitular and after cutting the interosseous membrane. joint is still not clear from our investigaArrows same as in figure 3. tion. However, the ulna is still bound to the radius at the proximal and distal radioDISCUSSION ulnar joints. At either one or both of these The proponents of the concept that joints forces initially applied to the hand forces are transmitted in a retrograde man- may be transmitted to the ulna. Further ner across the interosseous membrane base studies of force transmission at these joints their views, presumably, on the necessity will be necessary to elucidate the interreof a significant space being present be- lationships. To explain the frequent occurrence of tween the articular surfaces of the head of the radius and capitulum of the humerus. fractures of the distal end of the radius, in Mason ('54) stated that in the cadaver as contradistinction to the normal occurrence well as in the living there is a one-quarter of midshaft fractures of most other long inch space between the cartilaginous sur- bones, previous investigators have based faces of the elbow joint. Grant ('58) in their explanation on: (1) the presence of his schematic diagram of the radius and a significant joint space between the head ulna indicates the presence of a wider of the radius and the capitulum, and ( 2 ) space between the capitulum and the the force transmitting capability of the interosseous membrane due to its obliquely radius than between the trochlea and the fibers. However, we feel confiulna. With such a space the radius would directed dent, from these initial observations on the be impelled, following the application of of cadavera, that the interosseous force to the hand, toward the humerus limbs membrane does not play a major role in unless restrained by the interosseous membrane. Such restraint would be followed the transmission of forces along the forearm. naturally by the transmission of force across the membrane and thus substantiACKNOWLEDGMENT ate the concept. However, should the The authors wish to thank Mr. Fred space between the articular facets be non- Bantin B.S., who designed and built the existent or filled with an extremely thin scaffolding and platform. layer of synovial fluid, as is now generally This investigation was supported in part accepted (Barnett, Davies and MacConaill, by Public Health Service Research Grant '6 1; Gardner, '64), then any possible move- NBO5154-01 from the National Institutes ment provoked in the radius would be re- of Neurological Diseases and Blindness. strained by the capitulum acting as a buffer, and any forces would of necessity LITERATURE CITED be directly transmitted across the radio- Barnett, C. H., D. V. Davies and M. A. MacConaill Such a capitular joint to the humerus. 1961 Synovial Joints, Their Structure and condition is confirmed by our visual obMechanics. Springfield, Ill., Charles C Thomas, g . 177. servations on the limbs of cadavera and PRESSURES ACROSS THE ELBOW JOINT Destot, and Gallois 1898 Recherches physiologiques et expkrimentales sur les fractures de rextrkmitk hfkrieure du radius (radiographie). Revue de Chir. Pans, 18: 886-915. Gardner, E. 1964 Personal Communication. Grant, J. c. B. 1958 A Method of Anatomy, Descriptive and Deductive. and Wilkins Company, p. 124. Hennequin, J. 1894 Considkrations sur le mkcanisme, les symptomes et le traitement des fractures de l'extrkmitk inf6rieure du radius conskcutives aux chutes sur le poignet. Revue de Chir. Paris, 14: 557-583. 247 Lopes, H. 1860 Des fractures du radius et du role physiologique du ligament interosseux de l'avant bras. Thesis, University of Paris, p. 30. Mason, M. L. 1954 Observations on fractures of the head of the radius with a review of one hundred cases. Brit. J. Surg., 42: 123-132. Strasser, H. 1917 Lehrbuch Muskel-und Gelenkmechanik. Band nT, Berlin, Julius Springer, Pa 314-317Testut, L., and 0. Jacob 1914 Trait6 d'Anatomie Topographique. 3rd ed., Tome 11, Paris, Octave Doin et Fils, p. 789.