Reprinted from CLINICAL ORTHOPAEDICS, February, 1992Vol. 275© J. B. Lippincott Co. Printed in U.S.A.

The Distal Radioulnar Joint in Relation

to the Whole Forearm

CARL-G6tbXN HAGERT, M.D., PH.D.

The functional anatomy of the distal radioulnarjoint was studied in relation to the whole forearm,using three fresh-frozen, above-elbow amputationspecimens. The specimens demonstrate how theproximal and distal radioulnar joints togetherform a bicondylar joint of special character. Theproximal "condyle," the radial head, rotates ax-ially, whereas the distal "condyle," the ulnar head,is fixed with respect to rotation. The ordinary artic-ulation of a bicondylar joint (pure axial rotation) thereby changed into pronation-supination. Axialrotation is preserved proximally, while distally theradius swings around the ulnar head. The mobileradius is distally attached to the stable ulna.r headby the dorsal and volar radioulnar ligaments, thedorsal ligament being tight for stabilization in su-pination and the volar ligament being tight iin pro-nation. The ulnar head also serves as a keystone,carrying the load of the radius. Removal of theulnar head allows the radius to "fall in" towardsthe ulna, with narrowing of the interosseous space.

The distal radioulnarjoint(DRUJ) has pre-viously been subjected to a thorough investi-gation of its functional anatomy and patho!:ogy. ~,2,4,~ ~:~2 It should be noted that the DRUJis only the distal half of a joint, the prc,ximalhalf being the proximal radioulnar Joint(PRUJ). Together, these two enable forearmarticulation, working as a singl e :".forearmjoint" and resulting in pronatid~fl-supina-

From the Division of Hand Surgery, Departrfient ofOrthopedics, Lund University Hospital, Sweden.

Sponsored by the Swedish Medical Research Council(project 17X-09509), Stiftelsen for bist~nd ~t Vanf’Ora Sk~me.

Reprint requests to Carl-Grran Hagert, M.D., Div.Hand Surgery;. Dept. Orthopedics, Lund University Hos-pital, S-221 85 Lund, Sweden.

Received: February 2, 1991.

tion. The function of the DRUJ in relation to. ¯the rest of the forearm (i.e., the PRUJ and the:intermediary segments of the radius and’ulna) is discussed here.

MATERIALS AND METHODS

Three, fresh-frozen, above-elbow amputationspecimens from adult individuals Were used. Theamputations had been performed for malignanttumors in the shoulder region in two cases, and thethird one was a traumatic amputation. The speci-mens were resected at the level of the distal third ofthe humerus and appeared normal, each one hay-.ing a completely normal elbow joint, forearm, andwrist joint. At the time of study, the specimenslwere thawed slowly. In two of th~ specimens, allsoft tissues were removed except for .the inter-osseoiis membrane and the radioulnar ligament.Disarticulation was performed at the ~elbow andradiocarpal joints. The bones were scr~Pe~ cleanto better clarify tke PRUJ and DRUL Specimens"were ~sawed transversely at the middle of the radiusand. ulna, and ~ach half was: then further trans~sected repeatedly, ~t intervals, until the PRUJ andDRUJ were eventually reached. Photographs ofanatomic montages were taken at different levelsof transection. They are presented in reverseorder, starting with the PRUJ and DRUJ close toeach other and going towards the middle of thespecimen, to visualize progressive lengthening ofthe forearm]

The third specimen was disarticulated at the ra-diocarpal level only, sparing the elbow joint. Allsoft tissues were removed except for the bicepsbrachii and the entire pronator teres, pronatorquadratus, and supinator muscles, which were allcarefully preserved, as were the interosseous mem-brane and the elbow and radioulnar ligaments.Eventually, this specimen was also Used for sagittalsection through the distal end of the radius andulnai

56

Number 275February. 1992 DRUJ in Relation to the Forearm 57

To study DRUJ stability, the radioulnar liga-ment was carefully dissected, the triangular .carti-laginous midsection was removed, and the 6orsaland volar radioulnar fibrous strands were pre-pared and resected until just a thin strand (about1-2 mm) was left of each ligament.

The ulna, fixed with respect to rotation and es-sentially moving in only one single plane (exten-sion and flexion), represented the reference axisfor the following defined points: flexion side ofulna--the surface of the ulna facing flexion of theelbow; extension side of ulna--the surface facingextension of the elbow; anteroposterior (AP) viewof ulna--ulna seen in the extension-flexion planeof the elbow; lateral view of ulna--ulna seen per-pendicular to the extension-flexion plane of theelbow; forearm neutral position--radius posi-tioned relative to the ulna so that the radial styloidis the leading point during flexion of the elbow.

RESULTS

FLEXION SIDE OF THE ULNA

In an oblique axial view of the specimenfrom the flexion side of the ulna from proxi-

mal to distal and with radius maximallypronated, the PRUJ and DRUJ are obvi-ously similar, since the joint concavities areseen together on one side, and the convexitieson the opposite side (Fig. 1A). After osteoto-mies close to the two radioulnar joints, theradial and ulnar shafts were removed. Theremaining fragments are the proximal ulnaand distal radius with concave articulatingsurfaces on one side, and the radial and ulnarheads with convex articulating surfaces onthe other side (Fig. 1B). As the proximal anddistal articulations are approximated, a truebicondylar joint results (Fig. I C). The axis rotation of this bicondylar "forearm joint" isseen to go through the center of the radial andulnar heads (Fig. 1C).

EXTENSION SIDE OF THE ULNA

The specimen shown in Figure 1C has, inFigure 2A, been rotated to be seen from theextension side of the vlna, and the bones are

FiGs. 1A-IC. Drawings illustrating the montages of the PRUJ and DRUJ. (A) The forearm bones areseen from the flexion side of the ulna and radius in full pronation. The two lines indicate the sites ofosteotomy. (B) The radial and ulnar shafts are being removed and the two distal and the two proximalsegments approximated. (C) The distal radial segment is brought close to the proximal ulnar segment andthe ulnar head close to the radial head. The montage demonstrates the PRUJ and the DRUJ to form abicondylar joint. Dotted line indicates the axis of rotation.

Clinical Orthopaedics58 Hagert and Related Research

FIGS. 2A-2C. The montage in Fig IC has been rotated to see the extension side of the ~alna. (A) The twojoint compartments recede as the intermediary segments "grow" out. The dotted line indicates axis ofrotation. (B) As the segments continue to "grow," the full length of the forearm is eventually reached. Themontage still demonstrates a bicondylar joint where the proximal radius-distal ulna may just rotatearound the axis of rotation centrally positioned in the two segments (dotted lin,e). (C) The proximal part the radius (Fig. 2B) has now been rotated 180° in supination and "fused" with the distal radial segment toform radius, and, correspondingly, the distal part of the ulna (Fig. 2B) has been rotated ° insupinationand "fused" with the proximal ulnar segment to form ulna. Because of this chiiasma-shaped "fusion," theradius can now move around the ulna in pronation-supination: proximally, a rotation around an axiscentrally positioned in the radial head and, distally, a swing around an axis; positioned about 10 mmoutside the radius. The dotted line indicates the axis of pronation-supination.

Number 275February, 1992 DRUJ in Relation to the Forearm 59

now allowed to "grow" out. The two con-dyles of the bicondylar forearm joint will re-cede gradually until the full length of the, fore-arm is reached (Fig. 2B). At this stage, theyare separated from one another by abe,ut 25cm. Still, the function is that of a bicortdylarjoint, meaning only axial rotation is po~,;sible,not pronation-supination.

The montages of the forearm (Figs. 2A and2B) closely resemble ventral views of the leg,which is also its extension side. The proximalulnar and distal radial segments create a unitsimilar to the tibia, where the olecranon re-sembles the patella fused with the tuberosityof the tibia, and the radial styloid resemblesthe medial malleolus at the ankle. The proxi-mal radial and distal ulnar segments create aunit reminiscent of the fibula with the ulnarstyloid resembling the lateral malleolu~,;.

The significant differences between the legand the forearm are evident when comparingthe montage in Figure 2B with the naturalpicture of the forearm in Figure 2C where theforearm is seen from the extension side of theulna and in full pronation. The proximal partof the radius has been rotated 180° in supina-tion and then "fused" with the distal radialsegment. Correspondingly, the distal part ofthe ulna has been rotated 90° in supinationand then "fused" with the proximal ulnarsegment. By means of this chiasma-shaped"fusion," the ulna and radius are positionedso that the radius can move around the ulnain what is defined as pronation-supination.There is no longer pure axial rotation of onebone relative to the other. This is characteris-tic of the movement of a bicondylar joint.Pronation-supination is a combined motion,with axial rotation confined only to the veryproximal portion the radius (the radial head),whereas its distal counterpart swings aroundan axis positioned about 10 mm outside itsmedial edge, distally, at the center of the ul-nar head. The authors’ anatomic montagespoint out this uniqueness of the DRU.I.

The extension side AP view of the ulnaclearly reveals marked bowing just proximalto the middle of the ulna (Fig. 2C), as well

the radius at its midshaft. The two bones lie;across one another, forming an elongated "X."

THE AXIS OF PRONATION-SUPINATION AND

THE CURVATURE OF THE RADIUS

The axis of pronation-supination is drawnthrough the centers of the radial and ulnarheads (Fig. 2C). From about 2 cm distal the radial tuberosity to about 2 cm proximalto the ulnar head (Figs. 3A and 3B), the axisof pronation-supination is positioned be-tween the two bones, in the interosseousmembrane (Fig. 3A). The fibers of the inter-osseous membrane go from proximal radialto distal ulnar, approaching the axis ofprona-tion-supination at an angle of 10°-15° (Fig.3A). During supination of the forearm, themembrane folds along a line correspondingto the axis of rotation (Fig. 3B).

In a straight lateral view of the ulna, i.e., inthe view perpendicular to the extension-flex-ion plane of the elbow, the radius is picturedin a straight AP orientation if the forearm iskept in the neutral position (Fig. 3A). In thisposition, the following three points should beobserved: (1) The radial and ulnar styloidsare positioned opposite each other, and a lineconnecting these two parts will more or lesscoincide with the extension-flexion plane ofthe elbow. Comparing the wrist with the an-kle, where a line connecting the tips of thelateral and medial malleolis is positioned per-pendicular to the extension-flexion plane ofthe knee, this is a change of about 90° intosupination, indicating an axial torque of theradius and ulna (Figs. 2B and 2C). (2) Proxi-mally, the radius has a bend directed towardthe ulna, the tuberosity being the point fur-thest from the axis of rotation, whereas al-most exactly at midshaft, the outer contourofthe radius shows a bend facing radially(Fig. 3A). The S-shaped configuration of theradius around the axis of pronation-supina-tion determines, anatomically, the sizes ofthe lever arms of the pronating-supinatingmuscles (Fig. 3A). (3) Relative to the axis pronation-supination, the articulating sur-

Clinical Orthopaedics60 Hagert and Related Research

FIGS. 3A-3C. (A) Right forearm specimen in neutral: lateral view of the ulna, AP view of the radius. Theaxis ofpronation-supination is marked by a Kirschner wire. The fibers of the ir~terosseous membrane runfrom radial-proximal to ulnar-distal at an angle of approximately 10°- 15 ° relative to the axis of prona-tion-supination. Due to the curvatures of the radius, the sizes of the lever arms of the rotating muscles willbe defined: dotted lines--LB (lever arm biceps brachii), LPT (lever arm pronator teres), and LPQ (leverarm pronator quadratus); unbroken line--LS (lever arm supinator). (B) The specimen in full supination:AP view of both radius and ulna. The interosseous membrane folded along a line corresponding to theposition of the axis ofpronation-supination. (C) Sagittal section centrally through the distal ends of radiusand ulna in neutral. The articulation surfaces of the DRUJ slant distalward 15°-20° relative to the longaxis of ulna.

faces of the DRUJ, being in maximum con-tact in the neutral position, show a distallydirected slope of about 15°-20° toward theaxis (Fig. 3C).

THE MOTORS OF THE DRUJ

There are four major muscles that provideactive motion of the DRUJ: the pronatorteres and pronator quadratus, innervated bythe median nerve, for pronation; and the bi-ceps brachi, innervated by the musculocuta-

neous nerve, and the supinator, innervatedby the radial nerve, fi~r supination.

Biceps Brachii and Pronator Teres. Thesetwo muscles have their origins outside theforearm and can therefore be designated as"extrinsics." They both approach the radiusat its volar aspect, arid they insert on eitherside of the axis of rol:ation, thus counteract-ing each other.

Supinator and Pronator Quadratus. Thesetwo muscles have theiir origin within the fore-arm (except for a min~or part of the supinator

Number 275February, 1992 DRUJ in Relation to the Forearm 61

that comes from the lateral epicondyle of thehumerus) and can therefore be designated as"intrinsics." They both insert on the sameside of the axis of rotation but on either sideof the radius, thus counteracting each other.It should be noted that the supinator muscleinserts into the radius at an angle of about45 °, whereas the pronator quadratus insertsinto the radius at an angle of about 80°. Thismeans that these muscles will have a vector,which gives rise to a dynamic compressionforce over the DRUJ, the end of the radiusbeing pressed against the ulnar head.

STABILIZATION OF THE DRUJ

Removal of the whole carpus makes it pos-sible to have a full "end-on" view of the ulnarhead and the DRUJ. Resection of the triangu-lar cartilage (preserving only a thin strand ofthe dorsal and the volar radioulnar liga-ments) allows study of the DRUJ articulationand its stabilization in pronation and supina-tion.

PRONATION

During pronation, there is gradually de-creasing contact between the articulating sur-faces, so at the end of pronation, only anarrow, dorsal part of the semilunar notch isin contact with a correspondingly narrow,medial part of the ulnar head. In this posi-tion, the volar radioulnar ligamentous strandis tight and prevents joint subluxation. Thejoint stability is not destroyed by the dorsalradioutnar ligament’s being removed com-pletely. However, if the volar radioulnarstrand is, instead, detached from the fovea ofthe ulnar head and the dorsal strand is pre-served, the DRUJ will dislocate (Fig. 4).

SUPINATION

As in pronation, the contact area betweenthe articulating surfaces will gradually de-crease during supination. At the end of su-pination, there is only a narrow volar portionof the semilunar notch that is in contact witha narrow lateral portion of the ulnar head. In

FIG. 4. End-on view of the distal radius andulna, the radius in pronation. The dorsal radioul-nar ligament strand is preserved whereas the volarone has been detached from the ulna, after whichthe stabilization of the radius has been lost and theDRUJ luxated.

this position, the dorsal radioulnar ligamen-tous strand is tight to prevent joint disloca-tion. Joint stabilization is not affected by thevolar strand being detached from the fovea ofthe ulnar head. However, if the volar strand ispreserved but the dorsal strand is detachedfrom the ulnar head, the joint becomes sub-luxated (Fig. 5).

DISCUSSION

The movement of the DRUJ is defined aspronation-supination. It is generally agreedthat this movement plays an essential role inintegrated hand function. Generally, theDRUJ is considered a separate joint per seincluded in the wrist joint complex. Thisstudy reinforces the well-known fact that pro-nation-supination is a movement that in-volves not only the DRUJ but also the PRUJand, in particular, the interposed two seg-ments. Any impairment of pronation-supi-nation in terms of impaired range of move-ment, strength, or stability, therefore, shouldbe analyzed in its relation to the wholeforearm.

In his work on forearm rotation, Kapandji9

emphasized the coupling between the proxi-

Clinical Orthopaedics62 Hagert and Related Research

FIG. 5. End-on view of the distal radius andulna, the radius in supination. The volar radioul-nar ligament strand is preserved whereas the dor-sal one has been detached from the ulna, afterwhich the stabilization of the radius has been lostand the DRUJ luxated.

mal and distal radioulnar articulations. Hestated that the two radioulnar joints are coax-ial, and he compares them with two hinges ofa door. The door can be op,ened freely as longas the two hinges are aligned, whereas theopening of the door will be hindered if thehinges are malaligned. According to Ka-pandji,9 this is exactly what will happen toforearm rotation when a fracture of the ra-dius or ulna is allowed to unite in a poorlyreduced position. Unlike Kapandji, who im-plicates malalignment of both "hinges," thispresent study implicates only the distal"hinge," the DRUJ. The DRUJ is the onlyone of the two that could possibly cause limi-tation in pronation-supination when mal-aligned. The PRUJ is, in fact, not a "hinge"but rather a ball-and-socket-type of articula-tion. The radial head has a 3150° convex artic-ulating surface, which mainly bears againstthe flexible anular ligament. Beside the pureaxial rotation during pronation-supination,this allows a simultaneous lateral deviation tooccur.9 Thus, the PRUJ has more than onedegree of freedom. At the same time, it is wellstabilized with complete contact between the

articulating surfaces in any position of fore-arm rotation.

By contrast, the.’ distal end of the radius atthe DRUJ performs a swing of about 160° ormore, around an axis positioned outside thebone. During this large arc of movement, theshallow, narrow, concave articulating surfaceof the radius is congruent with the narrowconvex articulating surface of the ulnar headas a result of the w~ry complex architecture ofthe radial and ulnar shafts. Any interferencewith anatomic integrity of the radius or ulnacould easily disturb the DRUJ swing of theradius. Malalignment of the PRUJ in itselfwill hardly give rL,;e to any impaired forearmrotation--provided the radial head is not dis-placed as in the Monteggia fracture or de-formed or split into fragments. It is theDRUJ that is easily malaligned as a result ofmalunion of a radial or ulnar fracture, at anylevel, from just pr,aximal to the DRUJ to theproximal part of the forearm. Therefore, itseems most reasonable that such a malalign-ment of the DRUJ is what actually causes theimpairment of forearm rotation in any, or all,of the following clinical parameters: range ofmotion, strength, and stability.

The ligamentc,us stabilization of theDRUJ seems to be a subject of controversy.According to Kapandji,9 the triangular fibro-cartilage complex (TFCC) is tensed when theforearm is in neutral, whereas it becomesslack in pronation and supination. In eachposition, the interosseous membrane be-comes the main stabilizing structure that pre-vents dislocation of the DRUJ. Recently,Olerud et al.11 presented an in vivo study ofthe DRUJ using magnetic resonance imag-ing. They found that in pronation, when thedorsal rim of the semilunar notch restsagainst the medial edge of the ulnar head, thevolar rim of the ~;emilunar notch was at alarger distance from the center of the headthan in neutral position or supination, andvice versa in supination. That finding coin-cided with the one previously presented by afEkenstam and Hagert. 4 However, the volarand dorsal radioul:aar ligamentous strands of

Number 275February~ 1992 DRUJ in Relation to the Forearm 63

the TFCC have their origins at some distancefrom the center of the ulnar head. In light ofthis, Olerud et al. 11 have suggested that in pro-nation, the dorsal radioulnar strand pressesthe articulating surfaces together to providestability, whereas in supination the volarstrand is responsible. This is in agreementwith the old concept presented by Lipp-man.w The present study was able to confirmthe previous findings presented by af Eken-stam and Hagert,4 namely that in pronaEon,the volar radioulnar ligament becomes tightand prevents the DRUJ from dislocating,whereas in supination, it is the dorsalradioal-nar ligament that becomes tight and preventsdislocation. In general, joint stabilization isaccomplished by the combined action of ten-sion forces in the ligament and compressionforces over the articulating surfaces. Accord-ing to Olerud et al.] ~ the tension forces andthe compression forces will appear on the

same side of the joint. According to the find-ings in this study, as well as in the previousone,4 the tension forces and the compressionforces will appear on opposite sides of thejoint. This is typical of many joints. The con-cept of stabilization of the DRUJ presentedhere, therefore, seems to be the most reason-able one, and this ~ is also consistent withBowers. ~

The results of the present study have alsoindicated the forces acting over the DRUJ, asinduced by the supinator and pronatorquadratus (Fig. 6A). Adding the gravity anything held in the hand, it is very possiblethat many forces act over the ulnar head. Itshould be pointed out that the ulnar head isthe most distal part of the ulna, which is verystably attached to the distal humerus in analmost pure hinge joint. Also, the strongflexor of the elbow, the brachialis muscle, isattached to the proximal ulna. Furthermore,

FIGS. 6A AND 6B. (A) Right forearm specimen in neutral. The supinator and pronator quadratusgenerate a vector that gives rise to a dynamic compression force over the DRUJ (arrow). This is restrainedby the ulnar head, maintaining normal interosseous space. (B) Following removal of the ulnar head, thiscompression force (arrow) will cause the radius "fall in" towards the ulna, resulting in a reduced inter-osseous space.

64 HagenClinical Orthopaedics

and Related Research

it should be observed that the ulnar head, po-

sitioned about 25 cm distal to the elbow, hasthe convex articulating surface of the DRUJ

directed toward the flexion side of the elbowand that the ulnar head does not rotate but isfully stable. One can thus conclude that theradius and the hand, as well as what is held inthe hand, are resting on the ulnar head, whichis a keystone of the DRUJ, the wrist, and theforearm as a whole. The authors usually find

that following the Darrach procedure, theulna approaches the radius.l’13 However, the

radius, which "falls in" towards the ulna, cer-tainly gives a more correct and precise under-standing of what is the real effect of removingthe ulnar head and its stabilizing function.The effect is removal of the keystone, the ful-crum of pronation-supination, and conse-quently, there will be a narrower interosseousspace (Fig. 6B). As a result, the lever arms the pronator teres, pronator quadratus, andsupinator will decrease significantly, result-ing in impaired power of pronation-supina-

tion, which has been demonstrated clini-cally.1,3

The results of the present study clearly sup-port the concept of restoring DRUJ functionwhenever possibIe, if necessary by doing a cor-rective osteotomy at the site of a malunitedfracture. This holds not only for a maluniteddistal radial fracture, as is generallyagreed,5’6’s’~4 but also for a malunited fracture

at any site of the radius or ulna, to correct anymalalignment of the DRUJ. Osteotomy mayhave to be combined with restoration ofliga-mentous function by reinserting the TFCC tothe center of the ulnar head or by reattaching

a loose, nonunited ulnar styloid or both.

REFERENCES

1. Bowers, W. H.: The distal radioulnar joint. InGreen, D. P. (ed.): Operative Hand Surgery,, ed. New York, Churchill Livingstone, 1988, pp. 939-989.

2. afEkenstam, F. W.: The distal radioulnarjoint--Ananatomical, experimental and clinical study withspecial reference to malunited fractures of the distalradius, Doctoral thesis at the University of Uppsala.Acta Universitatis Upsaliensis 505, 1984, pp. 1-55.

3. af Ekenstam, F. W., Engkvist, O., and Wadin, K.:Results from resection of the distal end of the ulnaafter fractures of the lower end of the radius. Scand.J. Plast. Reconstr. Surg. 16:177, 1982.

4. af Ekenstam, F. W., and Hagert, C.-G.: Anatomicalstudies on the geome.try and stability of the distalradioulnar joint. Scand. J. Plast. Reconstr. Surg.19:17, 1985.

5, af Ekenstam, F. W., Hagert, C.-G., Engkvist,T/3rnvall, A. H., and Wilbrand H,: Corrective osteot-omy of malunited fractures of the distal end of theradius. Scan& J. Plast. Reconstr. Surg. 19:175, 1985.

6. Fernandez, D. L.: Correction of post-traumatic wristdeformity in adults by osteotomy. J. Bone JointSurg. 64A:1164, 1982.

7. Hagen. C.-G.: The distal radioulnar joint. HandClin. 3:41, 1987.

8. Jupiter, J. B., and Masem, M.: Reconstruction ofposttraumatic deformity of the distal radius andulna. Hand Clin. 4:3’77, 1988.

9. Kapandji, I. A.: The Physiology of the Joints. Vol-ume One, Upper Limb. Edinburgh, Churchill Liv-ingstone, 1982. pp. 101-127.

10. Lippman, R. K.: La;dty of the radioulnar joint fol-lowing Colles’ fracture. Arch. Surg. 35:772. 1937.

i1. Olerud.,, C., Kongsholm, J., and Thuomas, K.-,~.:The congruence of the distal radioulnarjoint. A mag-netic resonance imaging study. Acta Orthop. Scand.59:183, 1988.

12. Palmer, A. K.: The distal radioulnar joint. Anat-- omy, biomechanics, and triangular fibrocartilage

complex abnormalities, Hand Clin. 3:3I, 1987.13. Watson. H. K., and Brown, R. E.: Ulnar impinge-

ment syndrome after Darrach procedure. Treatmentby advancement lengthening osteolomy of the ulna.J. Hand Surg. 14A:302, 1989.

14. Watson, H. K., and Castle, T. H.: Trapezoidal osteot-omy of the distal radius for unacceptable articularangulation after Colles’ fracture. J. Hand Surg.13A:837, 1988.