Both the PRUL and DRUL are non-elastic. In neutral rotation both are tensioned equally. In a dry bone specimen, as the forearm is pronated the distance from the fovea of the ulna and the palmar margin of the sigmoid notch diminishes, thus de-tensioning the PRUL (Fig. 11.3). The distance from the fovea of the ulna to the dorsal margin of the sigmoid notch increases with this movement. As a consequence, to maintain a constant distance from the sigmoid notch to the dorsal lip of the sigmoid notch (since the DRUL cannot elongate) the radius must translate in a palmar direction (bringing the ulna head up to the dorsal lip of the sigmoid notch) [3, 4]. In supination, the reverse happens [5] (Fig. 11.4).

However, the above supposition is not universally accepted, in that the reverse of this tensioning has been proposed [6]. However, simple observation of a dry bone articulation will confirm the distance from the fovea of the ulna and the dorsal and palmar edges of the sigmoid notch. In addition, research has also shown that the superficial and deep parts of the PRUL and DRUL behave differently as discreet entities [7]. Other histological research, however, has failed to demonstrate these discrete areas and transducer studies, which measure the tension in these ligaments, confirm the expected translation and ligament tension on rotation. Specifically, that the dorsal capsule of the DRUJ is tight in pronation and the palmar capsule is tight in supination.

Much of the literature has concentrated on the key role of the TFCC for stability of the DRUJ. Clearly it must have an important role and, for instability to occur, it must be disrupted at least in part. It is self-evident, by observation of the anatomy, that when there is a dorsal dislocation of the ulna relative to the radius, then the distance between the palmar margin of the sigmoid notch and the fovea of the ulna is greatly increased. In this circumstance, the PRUL must be disrupted. The reverse is true in palmar dislocations. Thus, in either dorsal or ulna dislocations, the TFCC must be either at least in part or completely disrupted. This also includes avulsion of the TFCC from the sigmoid notch.

The ulno-carpal ligaments and capsule are also tensioned in pronation and supination. Biomechanical transducer studies have demonstrated the importance of the ulna-carpal ligaments throughout these movements.

Experimental analysis of the stability of the DRUJ, following sectioning of the soft tissue stabilisers, has been undertaken. Gofton et al. (2004) studied the effect of sequential sectioning of the dorsal and palmar radioulnar ligaments, the triangular cartilage, the ulnocarpal ligaments, the extensor carpi ulnaris and its sub-sheath, the pronator quadratus and finally the interosseous membrane [8]. They demonstrated that whilst the radioulnar ligaments and triangular cartilage could maintain stability in the absence of other soft tissue restraint, if the radioulnar ligaments and triangular cartilage were divided, the intact other soft tissue restraints were able to stabilise the DRUJ with normal kinematics. Thus, a tear of the TFCC alone will not create instability. For instability to occur, other major ligamentous disruption must take place.

Whilst Palmar (1989) has classified the injuries of the TFCC, this does not necessarily relate to instability, since other structures must be injured for this to occur [9]. Lichtman and Collins, however, have proposed a combined classification of DRUJ injuries. This includes all injuries of the DRUJ, rather than focusing on instability alone (Table 11.1) [19].

Instability can be considered as congenital or acquired. Causes of congenital instability include Madelungs deformity and other dysplasias (Fig. 11.5).

Acquired instability may be caused by a combination of bony and soft tissue injury, or soft tissue injury alone. When considering the bony injuries, one should consider fractures that give rise to angular or axial deformity, or injuries to the sigmoid notch. Distal radial fractures with angulation, or angular malunion are the commonest cause of DRUJ dislocation and instability (Fig. 11.6). The more proximal the radial fracture, the smaller the angular deformity required to cause symptoms [10]. Axial instability occurs when there is radial shortening. The Essex-Lopresti injury is a complex lesion, including a radial head fracture, disruption of the interosseous membrane and axial instability of the DRUJ. More distal radial fractures, which have healed in a shortened position with disruption of the interosseous membrane distal to the injury, will lead to axial subluxation. However, in this injury the proximal interosseous membrane and radial head are still intact, ensuring that axial instability does not occur.

Sigmoid notch fractures may include dorsal or palmar marginal injuries or more complex patterns. In these, the ligamentous injury can be much less severe for dislocation and subluxation to occur, since the bony restraint for subluxation is diminished.

Soft tissue injuries that give rise to instability in the absence of a fracture are rare and represent a severe disruption. As stated previously, it is not enough for the TFCC and dorsal and palmar radioulnar ligaments to be disrupted in isolation for instability to occur. There must be enough ligamentous disruption for the radius and ulna to drift apart, such that the ulna can escape over the edge of the sigmoid notch. This will require the distal interosseous membrane to be disrupted, as well as various active restraints. Finally, fractures of the ulna styloid, particularly through the base, is in effect a complete tear of the TFCC. Both the dorsal and palmar radio-ulna ligament will be detached. This does not occur with fractures of the tip of the ulna styloid, as this is distal to the ligament attachment [11].

Instability can also present in inflammatory arthropathies, typically rheumatoid arthritis. This is due to persistent synovitis causing capsular and ligamentous laxity. Arthrosis of the DRUJ in the absence of an inflammatory arthropathy is usually post traumatic. Primary osteoarthritis is rare.

Patients with instability present with ulna sided wrist symptoms, typically pain and clicking. They will also describe a feeling of ‘giving way’, or sudden pain on small movements. These symptoms are intermittent. The congenital group will have a self-evident history of wrist abnormality, but may develop new symptoms due to DRUJ instability in their late teens or early twenties, usually related to new sports or activities taken up at that age. Whilst the DRUJ is often affected early in rheumatoid arthritis, it would be unusual for a patient to present with DRUJ instability in the absence of an obvious diagnosis or other clinical signs of a polyarthropathy. In all other cases there will be a history of injury, most commonly with an associated fracture, but if not, a history of significant trauma with persisting symptoms thereafter.

The differential diagnosis will include soft tissue problems, such as TFCC injury, injury to the luno-triquetral ligament or less commonly capito-hamate ligament and instability of the Extensor Carpi Ulnaris (ECU) tendon. Persistent symptoms from fractures which were not recognised, or remain un-united, may mimic DRUJ instability, such as the less common carpal fractures of the lunate, triquetrium or hamate, including the hook of the hamate. Capitate neck fractures can also give rise to ulna sided wrist pain. Intra-articular fractures in the radio-carpal joint that have healed with a step may give pain and clicking as can any fracture within the sigmoid notch. Keinböck’s disease and pathology of the piso-triquetral joint must also be considered in the differential diagnosis.

In inflammatory arthropathies the difficulty is in the differentiation of the instability from the symptoms of arthrosis or synovitis.

Examination may reveal asymmetry of the DRUJ, if there is a subluxation or dislocation. Tenderness round the DRUJ may be elicited, as well as pain and clicking on forearm rotation. A piano key sign may be present (Fig. 11.7). This is a dorsally displaced ulna (more correctly the palmar displacement of the radius), which is reducible, but re-subluxes when the pressure on the distal ulna is removed. The translation test for the distal ulna should be diagnostic. The radius is held firmly between the thumb and fingers of the examiners hand. The ulna is then held in the other hand and moved in the antero-posterior plane. This should be done in neutral, full supination and full pronation. A difference in the range of translation, clicking during the movement or the reproduction of the patient’s symptoms during this manoeuvre should confirm the diagnosis. However, this test is more difficult to evaluate correctly and identify instability than the description of the sign would suggest. This is due to the small difference between normal translation, which is in the order of 4–5 mm and subluxation, which may add only 1–2 mm of further movement. Different positions of forearm rotation will give different degrees of translation and thus more accurate assessment, comparing the translation to the normal side may be compromised. The key to the test is to demonstrate the escape of the ulna head over the margin of the sigmoid notch. This may be easier to feel as a re-location test. This is performed by compressing the ulna head into the radius, after maximum translation and then attempting to translate the ulna head back. In the normal wrist, this will still be a smooth movement, but in the subluxed ulna head, where the ulna head is either perched on or passed beyond the margin of the sigmoid notch, the ulna will relocate with a definite jump or click. A variation of this is known as the “press test” [12]. In this, the patient rises from a chair using his or her hands for assistance, pushing against a table top located to the front. Instability is shown by greater depression of the ulna head on the affected side and is often painful.