The anatomy of the proximal radioulnar joint

The anatomy of the proximal radioulnar ioint Arnold-Peter C. Weiss, MD, and Hill Hastings II, MD, Providence, R.I., and

Indianapolis, Ind. This study is an anatomic investigation to define the architecture of and the stabilizing structures for the proximal radioulnar joint. Ten fresh-frozen cadaver elbows were dissected. Measurements of the radial head, annular ligament, radial fossa of the ulna, and interosseous membrane were made and were correlated to clinical observations of elbow stability. Sequential transection of the soft tissue constraints of the proximal radioulnar joint demonstrated the annular ligament and the central band of the interosseous membrane to be the main stabilizers in pronation and the central band to be the significant stabilizer in supination. Eccentric loading of the joint was noted on anterior-posterior shifting of the radial head during forearm rotation. The clinical relevance of these observations in relation to radial head dislocation, fracture pattern, and elbow stability is discussed. (J SHOULDER ELBOW SURG 7992;7:793-9.) The anatomy and biomechanical characteristics of the medial ligamentous stabilizers of the elbow have been carefully described over the past decade, and their contributions to elbow stability have been well documented. 2• 4 • 5 • 9In the last several years new attention has been directed toward the relationship of the lateral aspect of the elbow joint and the lateral collateral ligament complex to the stability of both the elbow joint and the radial hecd.v? Examination of the contribution of the radial head to elbow stability has also demonstrated that nonadjacent structures such as the interosseous membrane can play an important role in the dynamics of elbow motion and stobility.' The proximal radioulnar joint has received little attention in the orthopaedic literature." This joint does not appearto be involved in the newly described posterolateral rotatory instability of the elbow, which can be encountered after disruption of the ulnar lateral collateralligamenF Isolated dislocation of the radial head after trauma is quite uncommon. There is little un-

From the Department of Orthopaedics, Brown University School of Medicine, Rhode Island Hospital; and the Department of Orthopaedic Surgery, Indiana University School of Medicine, The Indiana Hand Center. Reprint requests: Arnold-Peter C. Weiss, MD, Department of Orthopaedics, Brown University School of Medicine, Rhode Island Hospital, 593 Eddy St., APC 10, Providence, R102903.

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derstanding of the forces around the elbow joint and the subsequent ligamentous disruptions that are required to produce this injury.lo. 11 The following study is an anatomic investigation to define the morphologic characteristics of and the stabilizing structures for the proximal radioulnar joint.

MATERIAL AND METHODS Ten fresh-frozen upper extremities were obtained and were stored at 4° C until they were used for anatomic dissection. The age and sex of the specimens were not known. No specimens had sustained previous injury to their upper extremities. Careful dissection of the superficial skin and the musculature around the elbow joint was undertaken to avoid disruption of any of the ligamentous constraints of the elbow joint. Examination of the structural elements around the proximal radioulnar joint and the radial head was undertaken in each specimen to detect any previous injury to the lateral collateral ligaments, annular ligament, or other surrounding soft tissues. In each specimen stability of the proximal radioulnar joint was assessed by sequential transection of the soft tissue constraints around the proximal radioulnar joint relative to the amount of proximal radioulnar joint divergence (measured with a micrometer caliper) on maximal manual radial distraction away from the radial

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Figure 1 Proximal radioulnar joint (PRUJj divergence is measured in millimeters after maximal manual radial distraction is performed on radial head.

Figure 2 Endpoints of radial fossa of ulna (small arrows) were used to determine inclination line (line A) relative to axis of elbow rotation (line B). Angle between line A and line B represents angle of inclination of PRUJ.

fossa ofthe ulna (Fig. 1). In every specimen each measurement was taken with the elbow in maximum pronation and in maximum supination. The proximal radioulnar joint divergence was measured as the distance between the radial head and the radial fossa of the ulna at their closest point. The line of divergence used was that in line with the radius of an arc formed by the proximal radioulnar joint itself. The proximal radioulnar joint divergence was measured under the following conditions of soft tissue constraint: (1) intact annular ligament and transected lateral collateral ligament, (2) sub luxated annular ligament and transected lateral collateral ligament, (3) transected annular ligament and lateral collateral ligament, (4) transected proximal third of interosseous membrane/ annular ligament / lateral collateral ligament, (5) transected middle third and proximal third of interosseous membrane/annular ligament/lateral collateral ligament, and (6) completely transected interosseous membrane/ annular ligament/lateral collateral ligament. After the stability of the proximal radioulnar joint was tested, anatomic positioning of the radial head was measured as it rotated within the proximal radioulnar joint. The average arc of motion of the radial head as it fell within the radial fossa was also measured in each specimen. The average angle of inclination of the

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Figure 3 Annular ligament may remain anatomic afte r late ral collateral ligament disruption (A) or may subluxate distally, impa rting PRUJ laxity

(B). radial fossa of the ulna to the elbow axis of motion was defined (Fig. 2). The radius of the curvature of the radial head was then examined by the use of a line that was perpendicular to the axis of the radial fossa of the ulna.

RESULTS Descriptive anatomy. All specimens contained a lateral collateral ligament complex that stabilized the annular ligament surrounding the radial head. In all specimens in which the lateral collateral ligament was transected, the normally taut "cup-shaped" annular ligament

complex could subluxate distally, losing its purchase on the radial head and imparting a secondary laxity to the proximal radioulnar joint (Fig. 3). The radial fossa of the ulna subtended an average arc of 66° ± 5° of the radial head at any given time, representing approximately 18% of the radial head circumference. With full pronation and supination an average arc of 215 0 ± 46° of the radial head fell at intervals within the radial fossa of the ulna. The average angle of inclination of the radial fossa of the ulna to the elbow axis of motion was 59° ± 7°.

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Figure 4 In supination radial head translates posteriorly in radial fossa of ulna, placing anterior annular ligament under tension. Table I Sequentially transeded soft tissues Intact PRUJ Subluxed ALI cut LCL Cut AL Cut distal third 10M Cut middle third 10M Cut 10M (complete)

PRUJ divergence (Average mm) Pronation

Supination

0.4

0.1

4.3 8.3

2.6

10.2 40.6 48.2

5.3 5.3 15.6 21.2

PRUJ, Proximal radioulnar joint; AL, annular ligament; LCL, lateral collateral ligament; 10M, interosseous membrane.

The radius of curvature of the radial head was on average 8% longer (relative to a line perpendicular to the axis of the radial fossa of the ulna) in neutral rotation when compared with full pronation or supination. Structural stability. Sequential transection of soft tissue constraints around the proximal radioulnar joint allowed divergence (as measured in millimeters) of the proximal radioulnar joint (Table I). The proximal radioulnar joint was less stable in pronation than in supination regardless of which soft tissue constraints had been transected. In pronation the main contributors to proximal radioulnar joint stability appeared to be the annular ligament and the central band of the interosseous mem-

brane (middle third). In pronation the contribution of the annular ligament to proximal radioulnar joint stability was lost with either transection or distal subluxation of the annular ligament. In supination the main component to joint stability was the central band of the interosseous membrane (middle third). The proximal radioulnar joint rested concentrically in neutral rotation and eccentrically with an alternating anteroposterior shifting in pronation and supination. In pronation the posterior annular ligament tightened with an anterior shift of the radial head in the radial fossa of the ulna. In supination the opposite positioning occurred, with the radial head shifting posterorly and the annular ligament tightening anteriorly (Fig. 4). At complete transection of the annular ligament the anterior-posterior shifting of the radial head within the fossa still occurred, which indicated that forces external to the proximal radioulnar joint were primarily responsible for this observation.

DISCUSSION Isolated traumatic dislocation of the radial head is uncommon.v" An overestimation ofthe frequency of this condition has occurred because the previously unrecognized posterolateral rotatory instability of the elbow resembles this condition clinicolly.' In posterolateral rotatory instability the humeroulnar joint can undergo a rotatory subluxation with or without ac-

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tual dislocation, and the radial head can subluxate or frankly dislocate from the capitellum without any dissociation of the proximal radioulnar joint. This abnormality is thought to arise from a loss of the ulnar lateral collateral ligament. True isolated radial head dislocation wh ich involves divergence of the radial head from the radial fossa of the ulna, is most commonly seen and associated with congenital or acquired neuromuscular conditions. In congenital cases embryologic evidence suggests that a dysplastic capitellum with radial head malformation and hypoplasia can develop in utero, thus precluding a normal radioulnar relationship. In traumatic dislocations of the radial head successful treatment can most frequently be obta ined by immed iate closed reduction and immobilizotion, which allow the healing of soft tissue. Immobilization of the forearm should be in supination because the annular ligament is almost invariably torn and this structure is not required for stability of the proximal radioulnar joint when the forearm is placed in this position. Most traumatic dislocations of the proximal radioulnar joint are associated with an anterior dislocation of the rad ial head or, less commonly, a lateral dislocation. The predilection of the radiol head toward anterior dislocation also favors immobilization in full forearm supination. The proximal radioulnar joint contributes to stability by shifting eccentrically with pronation and supination. In full supination the radial head shifts posteriorly on the radial fossa in the ulna, thereby moving away from the direction of the traumatic dislocation and soft tissue injury. Unfortunately this position may couse the annular ligament to heal in a relatively lengthened position because tension is present in the anterior annular ligament when the forearm is in full sup ination; the importance of this theoretic dicotomy to proximal radioulnar joint stability remains to be determined. The central band of the interosseous membrane has recently been shown to have an important role in stability with regard to longitudinal radioulnar divergence; it contributes approximately 71 % of the stiffness of the entire interosseous membrane. I This portion of the interosseous membrane is therefore extremely important in maintaining appropriate radioulnar alignment in the face of radial head fracture or excision. Our study demonstrates that the in-

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Figure 5 Transverse component of constra ints arising in cent ral band of interosseo us membrane in sup inat ion imparts stabilizing force to rad ial head at PRUJ. terosseous membrane, with its oblique nature, not only imparts longitudinal stability between the radius and the ulna but also imparts divergent stability between these two bones. In bath pronation and supination the central band of the interosseous membrane played an important role in maintaining rad ial head location in the radial fossa of the ulna. The fibers of the interosseous membrane are more taut in supinat ion than in pronation, and this was reflected in the stability of the proximal radioulnar joint as well (Fig. 5). Theoretically, disruption of the lateral collateral ligament without concomitant injury to the annular ligament could also produce incongruity in the proximal rad ioulnar jo int. Because of the nonsymmetric shape of the annular ligament (Fig. 6) and its resulting" cupping " of the radial heed," transection of the lateral collateral ligament will allow distal subluxation of the annular ligament, thus creating laxity at the prox-

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Figure 6 Annular ligament is circumferentially asymmetric around radial head. Transillumination reveals triangular shape of ligament and asymmetric orientation of collagen fiber.

Figure 7 Bicipital tuberosity is slightly offset from articular region of radial head. Region of nonarticulation (between black dots) is opposite bicipital tuberosity and encompasses from 25% to 50% of radial head, depending on individual.

imal radioulnar joint. Our study demonstrates that laxity at the proximal radioulnar joint with subluxation of the annular ligament is worse in pronation than in supination. The amount of instability that occurs with motion at the proximal radioulnar joint containing an intact annular ligament is significant. Because of this instability degenerative joint disease might appear secondary to the previously described joint mechanics during long-term cyclic loading.

The steep angle of inclination of the radial fossa of the ulna in relation to the elbow axis of motion indicates that radial head dislocation is likely to occur in the anterolateral direction. This is seen clinically. Although a significant portion of the radial head falls within the radial fossa of the ulna during a full arc of forearm motion, a significant portion of the radial head never comes in contact with the proximal radioulnar joint. This area can be located radiographically with the bicipital tuberosity of the radius as a reference, and it may play an important role when one is considering the indications for operative repair of displaced intraarticular radial head fractures (Fig. 7)8. With appropriate radiographic technique it should be possible to define whether an intraarticular radial head fragment will impinge on the radial fossa of the ulna during full forearm motion. If the fracture fragment should fall outside the normal arc of the radius encompassed by the proximal radioulnar joint, operative reduction and repair would seem to be less frequently warranted. The anterior-posterior shifting of the radial head in the radial fossa of the ulna in pronation and in supination is partially due to the slightly elliptic shape of the radial head. In fact, the radial head does not appear to rotate precisely around a specific center in full pronation and supination. It has a small component of ante-

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Anatomy of the proximal radioulnar joint

rior-posterior translation along the radial fossa of the ulna. With pronation the radial head translates anteriorly, placing the posterior annular ligament under tension. With supination the radial head translates posteriorly in the radial fossa of the ulna, placing the anterior portion of the annular ligament under tension. It might therefore be argued that if an operative procedure is undertaken on a congenital dislocation of the radial head, which most often occurs posteriorly, then postoperative immobilization should be undertaken with the forearm in full pronation. This would allow anterior translation of the radial head and would compensate for the deficiency of the posterior radial fossa of the ulna. Several aspects of the proximal radioulnar joint were not addressed by this study. The articulation between the radial head and the capitellum could at varying degrees of flexion play an important role in the overall stability of the radial head in the radial fossa of the ulna. In addition, varus and valgus stresses on the elbow at varying degrees of flexion might also impart changes in the overall stability pattern of the proximal radioulnar joint, depending on the soft tissue constraints. This relatively small joint will require further biomechanical studies in conjunction with the adjacent elbow joint ar-

ticulations to define its role stability.

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overall elbow

REFERENCES 1. Hotchkiss RN, An KN, Sowa DT, Basta S, Weiland AJ. An anatomic and mechanical study of the interosseous membrane of the forearm: pathomechanics of proximal migration of the radius. J Hand Surg [Am] 1989;14:25661. 2. Hotchkiss RN, Weiland AJ. Valgus stability of the elbow. J Orthop Res 1987;5:372-7. 3. Martin BF. The annular ligament of the superior radioulnar joint. J Anat 1958;92:473-81. 4. Morrey BF, An KN. Articular and ligamentous contributions to the stability of the elbow joint. Am J Sports Med 1983;11 :315-9. 5. Morrey BF, An KN. Functional anatomy of the ligaments of the elbow. C1in Orthop 1985;201:84-90. 6. Morrey BF, An K, Stormont TJ. Force transmission through the radial head. J Bone Joint Surg [AmI 1988;70A:250-6. 7. O'Driscoll SW, Bell DF, Morrey BF. Posterolateral rotatory instability of the elbow. J Bone Joint Surg [AmI 1991 ;73A:440-6. 8. Radin EL, Riseborough EJ. Fractures of the radial head. J Bone Joint Surg [Am] 1966;48A: 1055-64. 9. Schwab GH, Bennett JB, Woods GW, Tullos HS. Biomechanics of elbow instability: the role of the medial collateral ligament. C1in Orthop 1980;146:42-52. 10. Wiley JJ, Loehr J, Mcintyre W. Isolated dislocation of the radial head. Orthop Rev 1991;20:973-6. 11. Wiley JJ, Pegington J, Horwich P. Traumatic dislocation of the radius at the elbow. J Bone Joint Surg [Br] 1974;56B:501-7.