William B. Kleinman

25 Function of the Distal Radioulnar Joint

The distal radioulnar joint (DRUJ) is the foundation for a moving relationship between radius and ulna through their full arc of forearm pronosupination. Biomechanics of healthy, human forearm pronosupination allow the hand to be put or placed in space (▶Fig. 25.1). Perhaps this is an oversimplification of a more complicated interrelationship among the forearm, elbow, and shoulder; but the contribution of stable, painless forearm rotation to function of the entire upper limb cannot be overstated.

The purpose of this chapter is to provide the reader with a clear understanding of the details of DRUJ anatomy and biomechanics—of how the DRUJ functions. We hope that the readers will gain a clear understanding not only of the fine details of DRUJ anatomy, but of the distal forearm biomechanics that allow stable, painless pronosupination through a 180° arc, repetitively and regularly under considerable load (▶Fig. 25.2).

The DRUJ is responsible for two critical functions. The first is to provide a diarthrodial joint platform for rotation of the distal radius around a fixed and stable distal ulna. This arc of rotation is generated from full supination, where both bones of the forearm are essentially parallel and the interosseous space is at its widest breadth, to full pronation, where the radius crosses the ulna from lateral to medial, narrowing the interosseous space to its smallest dimension. The second function of the DRUJ is to provide the mechanism that allows the transfer of load from the hand, through this diarthrodial joint from radius to ulna (sigmoid fossa through the seat of the distal ulna), and up the ulna for transmission to the ginglymus ulnotrochlear joint of the elbow. In other words, the DRUJ allows the hand not only to be put or placed in space with precision but also to perform work and load transfer with stability and painlessness. Without healthy DRUJ function, forearm pronosupination and forearm load bearing would be profoundly compromised, significantly impacting the function of the entire upper limb.

The longitudinal axis of the forearm for this full, functional arc of pronosupination passes proximally through the center of the radial head at the radio capitellar joint and distally through the fovea of the distal ulna. The fovea is a well-vascularized bony recess, or sulcus, between the hyaline cartilage-covered pole of the distal ulna and the base of the ulna styloid (▶Fig. 25.3). The pole is the bony platform of the distal ulna that receives direct load-transfer from the medial carpus, through the articular disc of the triangular fibrocartilage (TFC), onto the distal ulna itself. Proximally, the ulna is attached to the trochlear of the distal humerus as a hinged, diarthrodial, ginglymus elbow joint. The ulnotrochlear joint participates only in elbow flexion and extension; it does not contribute to rotation of the forearm. The mechanics of forearm rotation only involve movement of the radius around a fixed ulna (▶Fig. 25.4). The ulna flexes and extends in a single plane at the elbow; the radius rotates around the fixed ulna in the forearm.

Affixed to the lateral border of the olecranon process is the annular ligament that holds the radial head securely at the proximal radioulnar joint (PRUJ) (Fig. 25.5). The annular ligament encircles and constrains the radial head, allowing rotation of the head around the longitudinal axis of forearm; the axis passes directly through the head’s center. The annular ligament holds the rotating radial head securely against the fixed, nonrotating ulna at the diarthrodial PRUJ. The ulna only flexes and extends on the humerus; with respect to forearm rotation, it is fixed. Therefore, movement of the radius relative to a fixed ulna generates the entire arc of prono-supination. In order to generate a full 180° arc of pronosupination (understanding that the proximal radial head is held securely by the annular ligament and only allowed to spin around the longitudinal axis of the forearm) the radius must roll from a position where both forearm bones are parallel to a position where the radius crosses the ulna (▶Fig. 25.6). In the bipedal human condition, supination puts the hand “palm up,” with the two forearm bones paralleling each other and the interosseous space at its maximum breadth. Full pronation puts the hand “palm down,” with the radius crossing the ulna, collapsing the interosseous space to its narrowest breadth. The transition from parallel to crossed forearm bones decreases the apparent length of the radius relative to the ulna, resulting in ulna-plus variance in full pronation. As a result of increasing ulna variance in pronation, load on the TFC proportionally increases. Full forearm supination draws the radius back into parallel alignment with the ulna, thus relieving pressure on the TFC as the apparent length of the radius increases, resulting in ulna-minus variance.

DRUJ design provides healthy articular hyaline cartilage on the surfaces of both the sigmoid fossa and the adjacent distal ulna seat. This relationship is stabilized through a healthy arc of pronosupination by intrinsic (within the DRUJ) and extrinsic (outside the DRUJ) soft-tissue stabilizers that guide the radius through its physiologic arc of forearm rotation, actively driven by the forearm muscles of supination and pronation. Since rotation of the forearm from full supination to full pronation results in a change from ulna-minus variance to ulna-plus variance as the radius rolls across the ulna, the principal axis of load bearing

aThe “principal axis of load bearing” is an engineering term used to describe the imaginary center of a cluster of surface contact points where two surfaces come together under load. As the surfaces move relative to each other, the principal axis of load bearing will track across the sigmoid fossa, along an imaginary line. In the case of the DRUJ, physiologic rotation and translation of the sigmoid fossa on the ulna seat create an oblique line from volar-proximal in supination to dorsal-distal in pronation, because of the relative radius shortening as it crosses the fixed ulna in pronation.

at the DRUJ cannot simply track transversely across the articular surface of the sigmoid fossa. Rather, the tracking line for DRUJ load bearing extends from proximal–proximal (PP) in full supination to dorsal–distal (DD) in full pronation (▶Fig. 25.7). The reader should note the transverse orientation of the sigmoid fossa in this figure. It is the larger anteroposterior dimension of the rectangular configuration of the sigmoid fossa that allows it enough surface area to rotate around the fixed ulna seat (which is covered 270° by hyaline cartilage). Rotation of the radius around the fixed ulna occurs simultaneous with oblique translation of the principal axis of load bearing at the DRUJ from dorsal–distal in supination to volar–proximal in pronation. The principal axis of radius-to-ulna load transmission at the DRUJ has plenty of room to track obliquely across the sigmoid fossa through the full arc of healthy forearm pronosupination, simply because of the adequate dimension of the fossa.

Extrinsic stabilizers of the two-bone relationship at the DRUJ include: the extensor carpi ulnaris (ECU) tendon; the VIth dorsal compartment fibro-osseous subsheath of the ECU; the two heads of the pronator quadratus (PQ); and the interosseous ligament (IOL; ▶Fig. 25.8). Recent descriptions of thickened fibers of the most distal portion of the interosseous membrane (IOM) of the forearm—adjacent to the proximal portion of the DRUJ capsule—suggest that this area of the IOM may provide some additional extrinsic DRUJ support. ^{1 , 2}

The intrinsic TFC and its components, however, are the primary and principal stabilizers of DRUJ mechanics through the full arc of forearm pronosupination (▶Fig. 25.9). In order to fully appreciate the details of anatomy of the TFC, one must first understand the nonconstrained nature of the articulating surfaces of the DRUJ. Af Ekenstam and Hagert ³ demonstrated more than 25 years ago that the radii of curvature of the sigmoid fossa of the distal radius and the seat of the ulna are not concentric. The radius of curvature of the fossa is significantly greater than that of the ulna seat (▶Fig. 25.10). In contrast to the constrained nature of the femoral head and the pelvic acetabulum, the seat of the ulna does not fit into an inherently stable, constrained socket at the medial surface of the distal radius. Rather, the DRUJ more closely resembles the relationship between the humeral head and the glenoid, requiring additional stabilizers to ensure normal biomechanics and prevent joint subluxation or dislocation. In the shoulder, stability of glenohumeral joint is provided by the musculotendinous shroud that forms the rotator cuff, and secondarily by the glenoid labrum and the glenohumeral capsule. At the DRUJ, the complex anatomy of the TFC provides primary joint stability, assisted by extracapsular soft-tissue stabilizers and the DRUJ capsule. ⁴

In their 1985 published cadaver dissections, af Ekenstam and Hagert ³ demonstrated that in full supination the radius (with its attached carpus and hand) rotates and translates across the sigmoid fossa. Hence in full supination < 10% of the volar sigmoid fossa still remains in contact with the cartilage surface of the seat of the ulna, as the radius literally rolls off the ulna seat.

Conversely, in full pronation < 10% of the dorsal sigmoid fossa still remains in contact with the edge of the ulna seat. The dissections of af Ekenstam and Hagert (in which they excised the central, articular disc of the TFC) suggest that only deep fibers of the TFC—those taking their origin from the fovea of the ulna—are responsible for exerting a “check-rein” effect on the rotating/translating radius, thereby preventing super-physiologic translation, a state in which the radio–ulna relationship would become unstable. In full supination, this checkrein effect of the TFC component (from the af Ekenstam/Hagert dissections) comes from deep, dorsal fibers originating from the fovea. Conversely, the checkrein effect precluding super-physiologic translation in pronation is the volar, deep portion of the TFC originating from the fovea. It was af Ekenstam’s and Hagert’s belief that only the deep portion of the TFC is principally responsible for rotational/translational stability of the DRUJ.

Six years later, in 1991, Schuind et al ⁵ published conflicting data from the Mayo Clinic Biomechanics Laboratory, based on stereophotogrammetric computer studies using phosphorescent markers on the surface of cadaver TFCs, loaded through a full arc of pronosupination. The group’s data suggest increased importance of the superficial fibers of the TFC as forearm rotational stabilizers: the superficial volar fibers in supination, and the superficial dorsal fibers in pronation. These conclusions contradicted the findings of af Ekenstam and Hagert 6 years earlier.

The opinions of these two camps of thought were academically and clinically controversial until Hagert published a short, remarkable paper in 1994, ⁶ explaining how both groups of investigators were correct. (▶Fig. 25.11) is reproduced directly from Hagert’s original work; (▶Fig. 25.12 and ▶Fig. 25.13) represent my interpretation of Hagert’s thesis. Indeed, as Af Ekenstam and Hagert suggested in their 1985 work, supination of the forearm results in rotation and translation of the sigmoid fossa on the fixed seat of the ulna, the extent of which is limited by the checkrein effect of the deep dorsal fibers of the TFC, originating from the fovea of the ulna. This origin is quite lateral relative to the origin of the superficial TFC fibers, which have their origin much more medial on the ulna styloid itself. Cadaver dissections in (▶Fig. 25.14 and ▶Fig. 25.15) nicely demonstrate the origins and insertions of the superficial fibers of the TFC: (A) originating of the ulna styloid, and the deep fibers and (B) originating from the fovea. Throughout the articles I have published on this subject over the past 10 years, I have referred to the deep fibers of the vascularized, peripheral fibrous portion of the TFC (B in ▶Fig. 25.14 and ▶Fig. 25.15) as the ligamentum subcruentum.

Af Ekenstam and Hagert are correct in their interpretation of TFC biomechanics. But Schuind et al are correct as well. Superficial, vascularized fibers of the TFC (superficial radioulnar ligaments) also tighten during pronosupination. As the dorsal ligamentum subcruentum tightens in full supination, the volar superficial fibers tighten as well. Which of the two components is more crucial for DRUJ stability in full supination? (▶Fig. 25.12 and ▶Fig. 25.13) are my attempt to illustrate how, in the extremes of supination and pronation, there is so much translation of the sigmoid fossa on the seat of the ulna that the entire head of the ulna literally herniates out from under cover of the superficial component of the TFC. Even though in full forearm supination the superficial volar fibers of the TFC are taut, the position of the ulna head relative to the origin and insertion of the superficial volar fibers renders them relatively ineffective as checkreins against super-physiologic translation of the fossa relative to the seat. In contrast, the dorsal, deep ligamentum subcruentum is crucial in preventing super-physiologic rotation and translation in supination.

Conversely, in full pronation, the ulna head slips dorsally from under cover of most of the TFC, rendering the volar

superficial component ineffective as a checkrein, even though it tightens (as demonstrated by Schuind et al). The volar ligamentum subcruentum becomes the principal checkrein against super-physiologic rotation and translation of the radius on the ulna in full pronation.

Hagert’s 1994 thesis ⁶ is explained by the two artistic renderings seen in (▶Fig. 25.12 and ▶Fig. 25.13 ). These figures show the relative demands on the two connective tissue portions of the ligamentum subcruentum when subjected to excessive loads in full supination and full pronation. The differences in how the superficial and deep components of the vascularized portion of the TFC behave in stabilizing the forearm through its arc of pronosupination is based entirely on the tensile angles at which the four components are oriented from their origins to their insertions. (▶Fig. 25.12 and ▶Fig. 25.13) clarify these biomechanics.

The cadaver dissection in ▶Fig. 25.15 demonstrates the acute angle of attack of the two components (volar and dorsal) of the superficial radio–ulna portion of the TFC from the ulna styloid to the medial border of the radius (A). Also demonstrated is the more obtuse angle of attack of the deeper ligamentum subcruentum (B). Using the “buckboard analogy” I first published in the Journal of Hand Surgery in 2007 ⁷ (▶Fig. 25.16, ▶Fig. 25.17, and Fig. 25.18), one begins to understand how the anatomy of the more obtuse fiber angle of the deep ligamentum subcruentum makes the volar and dorsal deep TFC components much more mechanically effective as checkreins against hyper physiologic translation of the sigmoid fossa on the ulna seat. The radius/carpus/hand unit is analogous to the team of four horses, rotating and translating as they whip the trailing buckboard around behind it. If we think of the buckboard itself as the fixed ulna and of the rotating/translating radius as the team of four horses, the seated buckboard driver (representing the origins of the radio–ulna fibers of the TFC) has the best control of his team of horses though those reins that attach to the outside harnesses of the two outside horses. The driver’s mechanical advantage for controlling the entire team is most effective when the reins are attached as widely apart as possible to the two outside horses. I have drawn these more mechanically advantageous reins in blue, since they mimic the more mechanically advantageous blue fibers of the ligamentum subcruentum (▶Fig. 25.18) in preventing super-physiologic rotation and translation of the radius sigmoid fossa off the ulna seat. The more lateral origin of the ligamentum subcruentum from the ulna fovea establishes its obtuse angle of attack on the medial radius (dorsal and palmar). The ligamentum subcruentum, therefore, is a more mechanically advantageous stabilizer of the DRUJ than the superficial TFC fibers. ▶Fig. 25.17 shows that even though the green reins that are attached to the two central horses of the team of four can be tightened by the buckboard driver, these reins (imitating the superficial fibers of the TFC) are less effective in controlling rotation and translation of the team because of their acute angle of attack on the two central horses. The analogy to the ineffectiveness of the angle of attack of the superficial fibers on the radius is clear (▶Fig. 25.19). Adding to this the anatomic/biomechanical fact that in full supination or pronation < 10% of the articular surfaces of the ulna seat and sigmoid fossa are in contact with each other, with the ulna head herniating out from under cover of the superficial fibers of the TFC, we can now understand why the ligamentum subcruentum is the more crucial intrinsic stabilizer of DRUJ pronosupination (▶Fig. 25.20 and ▶Fig. 25.21).

In addition to the vascularized superficial fibers and the ligamentum subcruentum, the TFC complex itself consists of a central fibrocartilage articular disc, responsible for load transmission from the medial side of the carpus directly onto the pole of the distal ulna (▶Fig. 25.9). The articular disc is nourished by synovial fluid washings; it contains essentially no blood supply, unlike the well-vascularized superficial and deep radioulnar ligaments that stabilize physiologic DRUJ rotation and translation. ⁸ The articular disc of the TFC does not participate in stabilizing the biomechanically sound two-bone forearm relationship through its pronosupination arc.

Additional components of the TFC that participate as stabilizers of the DRUJ are the two disccarpal ligaments. These ligaments arise along the volar margin of the radioulnar ligaments of the TFC already described in detail and insert as two distinct ligaments on the volar lunate and volar triquetrum. The origin of these secondary TFC stabilizers of the DRUJ was defined in two elegant independent embryological studies, by Garcia-Elias and Domenech-Mateu ⁹ and by Hogikyan and Louis. ¹⁰ These authors were able to demonstrate, by careful dissections and photomicrographs, that connective tissue (vascularized TFC tissue)—rather than the ulna bone itself—was the site of origin of these two important components of the TFC (▶Fig. 25.22 and ▶Fig. 25.23). The disc–carpal ligaments stabilize the volar medial carpus (lunate and triquetrum) to the volar ulna. Their orientation reduces the propensity for the radio–carpal unit to subluxate volarly off the seat of the ulna (i.e., functioning as checkreins to help prevent dorsal ulna subluxation/dislocation). The disc–carpal ligaments of the TFC should not be confused with the more superficial, thin, and vestigial ulnocarpal ligament (ulnocapitate) (▶Fig. 25.24), which has not been defined as contributing to DRUJ rotational stability.

Humans are bipedal animals. Our hands are usually working in front of us, with elbow flexion neutralizing the force of gravity on our hand–forearm unit. Most of our activities of daily living take place with our forearms in neutral rotation. ▶Fig. 25.25 demonstrates this usual work attitude in X-ray form, showing how the ulna is hinged at the ginglymus ulnotrochlear joint only for flexion/extension, with the radius/carpus/hand unit sitting balanced on top of the ulna. Gravity pulls the unit (and whatever is in the hand) toward the floor. This anatomic arrangement establishes the ulna seat of the DRUJ as the fulcrum for forearm mechanics. In neutral forearm rotation, with the radius perfectly balanced atop the fixed, nonrotating ulna, all peripheral, vascularized radioulnar ligaments of the TFC are lax. Neutral rotation is the only position where both the superficial fibers and ligamentum subcruentum fibers of the TFC are not under tension; nor are they functioning as checkreins against super-physiologic rotation/translation of the forearm. As the forearm rotates either into supination or pronation, specific components of the TFC described above will tighten. But in neutral rotation, muscle tone of the forearm is enough to maintain balance without any static checkrein effect by the components of the TFC.

Applying elementary principles of physics, in equilibrium (static forearm in neutral, no rotation), the length from the hand to a DRUJ fulcrum, defined by the seat of the ulna (L), times the weight of the hand and anything held in it (F), establishes a moment arm distal to the DRUJ. In equilibrium, this moment arm must equal a proximal moment arm, defined by the length of the entire forearm from DRUJ to the elbow (L′) times the restraining force of the annular ligament on the radial head (F′) (▶Fig. 25.25). The load borne by the ulna seat is the joint reaction force (JRF) of the DRUJ, equaling the sum of the

moments on both the distal and proximal sides of the ulna seat fulcrum. It is easy to appreciate from this model the enormity of load bearing potential at the DRUJ. The JRF can easily exceed multiple times body weight without any component of forearm rotation.

Remember that in neutral or zero rotation, the dorsal and volar radioulnar ligaments are lax. TFC laxity is only seen in this position. Once forearm rotation is initiated from the neutral position, some parts of the vascularized peripheral fibers of the TFC begin to tighten. Consider the tensile strength of the fibrous connective tissue required to maintain DRUJ stability in the extremes of supination and pronation. Elegant histological sections performed by Chidgey et al in 1991 ¹¹ demonstrate fibrocyte alignment of the connective tissues of the superficial and deep components of the vascularized periphery of the TFC. At the extremes of supination and pronation, where < 10% of the sigmoid fossa is in contact with the ulna seat, the radioulnar ligaments can be under tremendous load in stabilizing the DRUJ—particularly when the DRUJ JRF is high (▶Fig. 25.21 and ▶Fig. 25.22). Chidgey et al, in their cadaver TFC sections, showed at a cellular level the significance of orientation of connective tissue fibers in both the superficial and deep components of the TFC (i.e., the superficial dorsal and volar fibers, and the dorsal and volar ligamentum subcruentum). Cellular alignment is a requirement for maximizing load-bearing capacity of these checkrein ligaments. The cellular orientation physiologically favors the extraordinary demands for tensile strength in this tissue. Only with longitudinal fiber orientation in the radioulnar ligaments can they serve as effective checkreins against DRUJ instability. In supination under load (▶Fig. 25.20), and under conditions of static equilibrium (no rotational movement), the moment arm distal to the DRUJ must still equal the moment arm proximal to the DRUJ. This fact is defined by the physical state of equilibrium. In this position, the principal checkrein keeping the radius/carpus/hand unit from being pulled by gravity off the fixed, nonrotating ulna is the dorsal ligamentum subcruentum. In full pronation (▶Fig. 25.22), only the volar ligamentum subcruentum keeps the radius/carpus/hand unit from being pulled off the ulna seat by the gravitational force on the loaded hand. At the extremes of pronosupination, the superficial fibers of the vascularized periphery of the TFC are less critical than the deep components in preventing DRUJ instability and subluxation (▶Fig. 25.19). The dorsal ligamentum subcruentum blocks super-physiologic translation of the DRUJ in supination; the volar ligamentum subcruentum prevents super-physiologic translation in pronation. The superficial radioulnar ligaments (cadaver dissection (▶Fig. 25.14, ▶Fig. 25.15, and ▶Fig. 25.19) participate in stabilizing the DRUJ by holding the two bones together at the articular surface, but the ligamentum subcruentum prevents radius translation of the notch beyond the final 10% contact area between articular surfaces at the extremes of supination and pronation.

DRUJ function anatomy is complex, but the intrinsic and extrinsic factors that provide stability of this diarthrodial joint through a physiologic arc of DRUJ pronosupination are even more complex. Health of all the components is critical in providing the human forearm with a full arc of painless, stable motion under load.

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