CHAPTER 25 Stability of the Distal Radioulnar Joint
The longitudinal axis of forearm pronation/supination passes through the center of the radial head proximally and through the foveal sulcus at the lateral base of the ulnar styloid distally. Af Ekenstam and Hagert1 defined the pole of the distal ulna as that portion adjacent to the triangular fibrocartilage (TFC) that is covered by hyaline cartilage and responsible for absorption of load transferred to the forearm from the medial carpus, through the articular disc of the TFC onto the ulna. The fovea is the recess lying between hyaline cartilage of the ulnar pole and the ulnar styloid. This fossa is richly vascularized; it serves as a point of insertion of the major distal radioulnar joint (DRUJ), stabilizing the ligamentous components of the TFC.
Advances in Understanding of Anatomy and Biomechanics of the Distal Radioulnar Joint
Research published particularly over the past 30 years now gives us a clear understanding of how the anatomical position of the fovea and the deep components of the TFC are critical to the rotational and translational stability of the DRUJ. Anatomical works by Bednar and associates, in 1991,2 and Thiru-Pathi and colleagues, in 1986,3 independently demonstrate the rich vascularity within the ulnar fovea. These authors also described the source of vascular nutrition of all peripheral tissues of the TFC, both dorsal and palmar. Their two landmark papers also describe the avascular, central articular disc of the TFC, nourished by synovial fluid from both the DRUJ and the ulnocarpal joint.
At the ginglymus ulnotrochlear joint of the elbow, the ulna participates only in forearm flexion and extension; it does not rotate. Forearm pronation/supination involves rotation of the radiocarpal unit (with the attached hand) around a rotationally fixed and stable ulna. The complex architecture of the ulnotrochlear articular surfaces and the collateral ligament stabilizers of the elbow allow the ulna to move as a hinge only, without a rotational component. The longitudinal axis around which rotation takes place can be visualized in Figure 25-1. The anatomy of the sigmoid notch of the radius, the seat of the ulna (which articulates with the notch), and the guiding, checkrein potential of the components of the TFC allow 90 degrees of forearm supination (at which point the two forearm bones are essentially parallel, and the interosseous space the widest) to 90 degrees of pronation, at which point the radius has rotated across the anterior surface of the fixed ulna (Fig. 25-2). The principal axis* of load bearing at the DRUJ tracks obliquely across the sigmoid notch as the radiocarpal unit pronates/supinates around the fixed ulna (Fig. 25-3). The tracking line is from distal/dorsal in pronation to proximal/palmar in supination.
In the human bipedal condition, with flexed elbows at the side (prepared for work), the radiocarpal unit rests “on top” of the ulnar seat, with gravity pulling the hand and its load toward the ground (Fig. 25-4). This so-called zero-rotation position of the forearm has been nicely described by Palmer.4 In this position—the most common for human forearm function—a significant joint reaction force (JRF) can develop between the sigmoid notch of the radius and the rotationally fixed ulnar seat (see Fig. 25-4). Usually, radiographs of the carpus are viewed with the fingers toward the ceiling. Students of hand surgery learn that it is easier to develop three-dimensional thinking about the anatomy of the wrist and the distal end of the ulna if (1) their patients are clinically examined with their elbows on the examining table and their fingers toward the ceiling; (2) if radiographs are viewed with the fingers toward the ceiling; and (3) if arthroscopy of the wrist is performed with the fingers toward the ceiling.
I have reoriented Figure 25-4, however, to allow the reader an appreciation of the “zero-rotation” position of the working forearm, with the radial and ulnar styloids at their widest anatomical separation and the forearm in neutral rotation (0 degrees of pronation/supination). In this position, the principal axis of load bearing at the DRUJ passes through the center of the sigmoid notch and the center of the seat of the ulna. In zero rotation, the various radioulnar ligament components of the TFC are under the least amount of tension.1,5
The structural presence and health of articular surface cartilage on the ulnar seat (as well as the sigmoid notch) are critical in providing a painless mechanical fulcrum for all radioulnar load-bearing activity. In a state of equilibrium (no forearm motion), all moments around a fixed fulcrum (i.e., the seat of the ulna) must be equal (Fig. 25-5). The loaded hand (F) times the length of the loaded hand from the fulcrum provided by the seat of the ulna (L) must be equal to the moment (F′ × L′) on the proximal side of the fulcrum:
where F′ is the stability provided at the radiocapitellar joint by the annular ligament encircling the radial head and L′ is the entire length of the forearm. Because F × L = F′ × L′, the relatively long forearm makes the requirement for proximal radius stability by the annular ligament (F′) relatively small, regardless of the load in the hand.
The sum of all moments distal and proximal to this fulcrum equals the total load on the fulcrum (the ulnar seat) itself, defined as the joint reaction force (JRF) at the DRUJ (Fig. 25-6). Based on how much load is in the hand (F), and the size of the moment distal to the DRUJ, the reader begins to appreciate the potential magnitude of the JRF at the DRUJ and the importance of surface hyaline cartilage health at both the sigmoid notch and the ulnar seat for painless DRUJ function.
Using elegant cadaver dissections, in 1985 af Ekenstam and Hagert1,6 demonstrated that the concave radius of curvature of the sigmoid notch is greater than that of the ulnar seat (Fig. 25-7). This incongruity of articular surfaces creates a geometrically nonconstrained articulation at the DRUJ, subject to translational dorsal and palmar instability of the sigmoid notch on the ulnar seat. Not only does the radiocarpal unit rotate around the fixed ulnar seat in pronation/supination, but the flatter surface of the sigmoid notch also has enough inherent instability through incongruous cartilage surfaces to allow translation of the notch palmarly or dorsally on the fixed ulna as the forearm rotates into pronation or supination, respectively. The DRUJ is not a ball-and-socket joint. Figure 25-3 reveals the exposed cadaver sigmoid notch (the ulna has been dissected free and hinged palmarly out of the notch to the reader’s left), showing the oblique tracking line of the principal axis of forearm load bearing through a full pronation/supination arc at the DRUJ (see earlier). Because rotation of the radius across the ulna from supination to pronation results in relative ulnar-positive variance4 at the end arc of forearm pronation, the tracking line of the principal axis of load bearing across the sigmoid notch must—by definition—be oblique. It tracks from slightly more proximal at the palmar edge of the notch in full supination to slightly more distal at the dorsal edge of the notch in full pronation.
FIGURE 25-7 Transverse section through the DRUJ. The radius of curvature of the sigmoid notch is greater than the radius of curvature of the seat of the ulna, leading to an inherent instability of the DRUJ through an arc of pronation/supination. Rotation of the forearm around a longitudinal axis of rotation (see Fig. 25-1) is manifest at the DRUJ by rotation and translation of the sigmoid notch against the ulnar seat.
(From Af Ekenstam F, Hagert CG: Anatomical studies on the geometry and stability of the distal radioulnar joint. Scand J Plast Reconstr Surg. 1985; 19:17-25.)
Stability of the DRUJ
With inherently unstable, nonconstrained articular surfaces, anatomical stability of the DRUJ is achieved through extrinsic extracapsular as well as intrinsic intracapsular structures. Extrinsic stability is provided principally by (1) dynamic tensioning of the extensor carpi ulnaris (ECU) as its tendon crosses the distal head of the ulna7,8; (2) the semi-rigid sixth dorsal compartment itself, constraining the ECU tendon7,8; (3) dynamic support provided by the superficial and deep heads of the pronator quadratus9; and (4) the interosseous ligament of the mid forearm10 (Fig. 25-8). The extrinsic DRUJ stabilizers are of secondary importance compared with the more biomechanically effective intrinsic radioulnar components of the TFC.1,2,6 Dorsal and palmar radioulnar TFC fibers arise from the medial border of the distal radius and insert on the ulna at two separate and distinct sites: (1) the fovea at the base of the ulnar styloid and (2) the ulnar styloid itself. Figure 25-9 emphasizes the well-vascularized nature of the dorsal and palmar radioulnar ligaments. Figure 25-10 illustrates the critical and clinically significant difference between the angle of attack of the dorsal and palmar superficial radioulnar fibers inserting on the ulnar styloid and the deep fibers inserting onto the fovea. Well-vascularized, longitudinally oriented connective tissue fibers of the TFC anchor the radius to the ulna along both its dorsal and palmar margins. The blood supply to both these areas of the periphery of the TFC is through branches of the posterior interosseous artery (see Fig. 25-9).2,3 These vessels course along the dorsal and palmar radioulnar ligaments penetrating and nourishing the dorsal 20% and palmar 20% of the TFC.2,3 Between these two sets of DRUJ checkreins, the articular disc is nourished by synovial fluid washing from the ulnocarpal joint distally and the DRUJ proximally. The articular disc is primarily responsible for load transmission from the medial carpus to the forearm, particularly with the hand-forearm unit in ulnar deviation. In neutral deviation of the hand-forearm unit, the principal axis of load transmission passes from the hand, through the head of the capitate, then the scapholunate ligament, and finally onto the articular surface of the distal radius at the interfossal ridge, which separates the elliptical (scaphoid-bearing) and spherical (lunate-bearing) fossae of the distal radius (see Figs. 25-10 and 25-11). Af Ekenstam and colleagues6 have demonstrated in the laboratory that, in neutral position of the wrist, 84% of hand load is transferred to the radius and only 16% is transferred through the central articular disc of the TFC. With ulnar deviation of the hand-forearm unit (Figs. 25-12 and 25-13), the principal axis of load bearing shifts medially, placing more load on the articular disc and the pole of the distal ulna, rather than on the interfossal ridge of the distal radius when the hand-forearm unit is in neutral deviation.
How the TFC Components Guide the Radiocarpal Unit around a Fixed Ulna: The Significance of Rotation and Translation
The dorsal and palmar radioulnar ligaments consist of superficial components inserting directly onto the ulnar styloid and deep components inserting significantly more laterally, into the fovea adjacent to the articular surface of the pole of the distal ulna (Fig. 25-14). These two components of the TFC are distinct in both their anatomy and function. Scrutiny of Figure 25-14 reveals that fibers of the superficial component form an acute angle as they converge on the ulnar styloid from the medial radius. This narrow angle of attack gives the superficial TFC a poor mechanical advantage for guiding the radiocarpal unit through an arc of pronation/supination. The deep components of the TFC (arising along the medial border of the distal radius, but inserting into the fovea), however, form an obtuse angle of attack, which is much more mechanically advantageous in stabilizing rotation of the radius around the fixed ulna (see Fig. 25-14
