Joint Anatomy

The lunotriquetral joint is part of the proximal carpal row, forming the articulation between the ulnar surface of the lunate and the radial surface of the triquetrum. The articulating surfaces of the lunate and triquetrum are flat and semilunar. In neutral position of the wrist, the orientation of the plane of the joint is nearly orthogonal to the frontal plane of the forearm and approximately 20 degrees oblique to the sagittal plane. In the coronal plane the joint is positioned somewhat distal to the scapholunate joint.

The lunotriquetral joint is also under the influence of the radiocarpal and the midcarpal joints. The convex proximal surfaces of the lunate and triquetrum articulate with the hybrid radiocarpal articulation: the lunate fossa of the distal radius and the triangular disc of the TFCC. Usually, no more than 50% of the lunate articulates with the triangular disc.

The relevant midcarpal articulation is between the distal articular surfaces of the lunate and triquetrum and the proximal surfaces of the capitate and hamate. A sagittal ridge can divide the lunate articular surface into a radial and ulnar fossa, and in these cases the proximal surface of the hamate articulates with the lunate. The incidence of this type II lunate varies considerably and is reported in 27% to 63% of adults.^1,2 Otherwise, the distal surface of the lunate is concave in both coronal and sagittal planes.

The hamate-triquetral articulation has a helicoid or screw-shaped configuration. The plane of the joint is not parallel to the articular surface of the distal hamate with the fourth and fifth metacarpals but is situated at a substantial angle (reported as high as 90 degrees³).

Ligament Anatomy

The lunotriquetral joint is stabilized by an intricate arrangement of ligaments (Fig. 40-1). The intrinsic (both origin and insertion are within the carpus) lunotriquetral ligament is “C” shaped and is actually composed of three discrete regions: true ligaments dorsally and palmarly with collinear fascicles of collagen and a proximal region composed of fibrocartilage (Fig. 40-2). This proximal region has a wedge-shaped cross-sectional geometry and is reminiscent of a meniscus. In normal subjects there should be no communication between the midcarpal and radiocarpal joints but the proximal region of the lunotriquetral joint can be perforated by age. Because the proximal membrane contributes very little to the overall lunotriquetral stability, a defect at this level is not a sign of instability but can be a normal physiological, albeit age-related, finding, caused by wear. Mikic⁴ found up to 55% of lunotriquetral perforations past the third decade.

FIGURE 40-1 Schematic representation of the ligaments attached to the triquetrum (Tq) as seen from the radial side, the lunate having been removed. The LTq joint is linked directly by three structures: palmar LTq ligament (1), dorsal LTq ligament (2), and proximal LTq membrane (3). The ulna is connected with the triquetrum by means of the palmar ulnotriquetral ligament (4). The radiotriquetral ligament (5) is wide and fan shaped and is key in the prevention of carpal collapse. The midcarpal joint is constrained palmarly by two fascicles: the triquetrum-hamate ligament (6) and the triquetrum-capitate ligament (7), both of which are important midcarpal stabilizers. The triquetrum is connected dorsally with the trapezium and trapezoid by means of the dorsal intercarpal ligament (8) and with the scaphoid by means of the dorsal scaphotriquetral ligament (9). TFC, triangular fibrocartilage.

(Adapted from Garcia-Elias M, Ritt MJPF. Lunotriquetral dissociation: Pathomechanics. Atlas Hand Clinics, 9(1), 2004: 7-15, with permission.)

FIGURE 40-2 Lunotriquetral joint interface. LT, lunotriquetral joint surface; D, dorsal subregion of the lunotriquetral joint; Pr, proximal subregion; Pa, palmar subregion.

There are several extrinsic (connecting the carpus with the radius or ulna) ligaments constraining the lunotriquetral joint; and from a functional point of view these may be subdivided into two major groups: radioulnocarpal ligaments and midcarpal ligaments.

Material and Constraint Properties

Results of material property testing of the lunotriquetral ligament showed that the palmar region of the ligament is not only thicker but indeed stronger than the dorsal portion of the ligament (average yield strengths: 301 N and 121 N, respectively).¹⁹ The palmar region was found to constrain primarily translation, whereas the dorsal region provides the majority of rotational constraint. The fibrocartilaginous proximal region failed at 64 N and was the least important constraint in all directions. The morphology and material and constraint properties of the lunotriquetral ligament are the exact opposite as found in the scapholunate ligament.²⁰ In a way, the lunate could be thought of as a “torque-suspended” bone between the scaphoid and the triquetrum, much in the manner of a spring. This “torque suspension” concept, for which there is now sufficient evidence, is of importance for the discussion and understanding of dorsiflexed intercalated segment instability (DISI) and volar-flexed intercalated segment instability (VISI).

Recently, immunohistochemical analysis of wrist ligament innervation showed that sensory important ligaments were primarily related to the triquetrum while mechanically important ligaments were primarily located in the radial, force-bearing column of the wrist. The triquetrum and its ligamentous attachments are regarded as key elements in the generation of the proprioceptive information necessary for adequate neuromuscular wrist stabilization.²¹

Kinematics

The bones of the proximal carpal row form a highly functional adaptable unit, permitting a large range of motion of the hand: flexion/extension, radioulnar deviation, and even some rotation. During hand motions, the proximal carpal row undergoes adaptive geometric changes, dictated by the geometry of the articular surfaces of the carpal bones as well as ligament functions.

The scaphoid is forced to flex palmarly, largely based on its bony geometry, and the lunate is passively forced to follow this motion. There is a strong tendency by the lunate to rotate dorsally owing to its dorsopalmar wedge shape. The shape of the scaphoid, aided by the peripheral location of the interosseous scapholunate ligament, results in a large moment arm, which forces the lunate to follow against its tendency and which therefore demonstrates less flexion than the scaphoid. In its turn, the lunate moves the triquetrum via the interosseous lunotriquetral ligament during scapholunate interaction. These motions and tendencies to move are arrested by the scaphoid/trapezium/trapezoid geometry distally. However, the position of the scaphoid, between trapezium and trapezoid, aided by the specific geometry of the scapholunate ligament proximally, which follows the lunate, prevents exaggerated dorsal displacement of the proximal pole of the scaphoid. By the difference in curvature at the radiocarpal level, the lunate demonstrates less flexion than the scaphoid. Extension is facilitated by the wedge-shaped proximal part of the scaphoid, the tendency of the lunate to move into the same direction according to its wedge shape, and the helicoid articular facet of the triquetrum with the hamate. Geometrically, a helicoid facet represents a surface generated by the rotation of a plane or twisted curve about a fixed line, so that each point of the curve traces out a circular helix with the fixed line as axis. Besides facilitating extension of the wrist, this geometry of the triquetrohamate joint plays a role in ulnar deviation of the wrist as the triquetrum is allowed to extend as it glides distally on the hamate. Aided by the oblique shape of the lunotriquetral interface, the lunate is forced to follow the triquetral motion and extends, taking the scaphoid along into erect position.

In radial deviation of the wrist, the scaphoid is forced to flex over the palmar radioscaphocapitate ligament largely because of its bony geometry. This motion is transmitted by the scapholunate ligament, and the lunate is forced to flex. Transmission of this inclination by the lunotriquetral ligament, aided by the helicoid triquetrohamate surface, causes the triquetrum to flex.

All the above mechanisms make it necessary for the proximal carpal bones to take very specific positions, with small pronation/supination motions and mainly ulnar deviation as conjunct motion to flexion/extension of the wrist. In 60 degrees and 30 degrees of wrist extension, the lunate extended 29.7 degrees and 15.4 degrees and the triquetrum 39.3 degrees and 20.1 degrees, respectively. With 60 degrees and 30 degrees of flexion, the lunate flexed 23.0 degrees and 11.5 degrees and the triquetrum 30.6 degrees and 15.5 degrees, respectively, with simultaneous ulnar deviation of both lunate and triquetrum. In extension/flexion motion of the wrist there is some progressive pronation of the triquetrum. Thus, the total arc of lunotriquetral intercarpal motion in the sagittal plane was 16.8 degrees, with the wrist moving from 60 degrees of extension to 60 degrees of flexion. With radial and ulnar deviation of the wrist, the magnitude of the conjunct motion of both the lunate and triquetrum is greater than the primary motion: that is, flexion/extension motion of these carpals is greater than the prime radioulnar deviation motion.²²

These results are all in broad agreement with previous in vitro studies^23–25 concerning normal carpal kinematics as well as with more recent in vivo studies.^26,27

Most recently it has been demonstrated that the kinematics of a type I lunate are different from those of a type II lunate during radioulnar deviation of the wrist: wrists with a type I lunate show statistically greater scaphoid translation with radial deviation, whereas wrists with a type II lunate show statistically greater scaphoid flexion with radial deviation.^28,29

It is obvious that the intercalated lunate is in need of constraints on both sides to counteract the just-mentioned torques. Although the connections between triquetrum and lunate are strong, they are not so taut as not to allow mutual mobility. In fact, during maximal radioulnar deviation there is an average 2.5-mm proximal shift of the triquetrum relative to the lunate, ranging from 0 to 5.2 mm.³⁰ Thus, from a kinematic viewpoint, the lunotriquetral joint is not a functional block without intrinsic mobility.

Trauma

There is general agreement that injuries to the lunotriquetral ligaments are part of a spectrum of progressive ligament disruption. This is associated with perilunate dislocation either in the classic direction or more often in the reverse direction.

In the latter situation, isolated injuries to the lunotriquetral ligaments appear as the consequence of a fall backward on the outstretched hand, with the arm being externally rotated and the wrist positioned in radial deviation and extension. In such circumstances, the impact concentrates on the pisiform. The triquetrum, already in extended position, is translated dorsally but the lunate does not follow: it is effectively constrained by the radius and the long radiolunate ligament. Furthermore, it is likely that this mechanism, with concomitant midcarpal pronation, is able to tighten the ulnocarpal ligamentous complex to a point where isolated failure of the lunotriquetral fibers may occur, whereas the radiocarpal ligaments are spared. Substantial shear stress appears at the lunotriquetral joint, causing progressive stretching and, ultimately, tear of the different lunotriquetral stabilizing ligaments. Complete rupture of the strong palmar portion of the lunotriquetral ligament rarely occurs unless there is additional violent pronation of the distal row, by which the palmar triquetrum/hamate/capitate ligament adds the extra destabilizing force that is required for this strong part of the ligament to fail. Supporting this theory is the fact that the palmar lunotriquetral and triquetrum/hamate/capitate ligament are seldom both disrupted.

Frequently encountered associated regional injuries are a combination of a partial or complete rupture of the lunotriquetral ligaments, a peripheral tear of the TFCC, and a distal avulsion of the ulnotriquetral ligament. The mechanism of injury may be similar to the just discussed mechanism for the isolated lunotriquetral injury except for the presence of radial deviation and pronation as the predominant torque-inducing vectors. It is reported that this combination of injuries is not unusual, and it is easy to miss any one of its components.³¹ Avulsion fracture of the palmar rim of the triquetrum as documented by Smith and Murray³² should be interpreted as a subtle sign of a more extended lunotriquetral injury and would correlate well with the previously described concept of a “reversed Mayfield perilunate dislocation model” discussed by Viegas and coworkers.³³

In other circumstances the progression of injury seems to follow a more direct perilunate destabilization process, as described in the well-known paper by Mayfield.³⁴ This classic pattern of progressive perilunate instability begins with palmar to dorsal disruption of the scapholunate interosseous ligament. In these instances, injury to the lunotriquetral ligaments occurs in stage III, after rupture of the scapholunate ligaments (stage I) and the lunocapitate dislocation (stage II). Lunotriquetral dissociation is an integral part of a progressive perilunate dislocation, but it is difficult to imagine as an isolated finding according to this classic progression pattern. Perhaps isolated lunotriquetral instability represents a residual problem from a previous perilunate injury in which the scapholunate problem heals spontaneously or with intervention.

Acute lunotriquetral ligament injury is less frequently seen with distal radius fracture than one of the TFCC or scapholunate ligament injuries. Still, partial or complete lunotriquetral ligament disruption is reported to be associated with intra-articular fractures in a considerable number of cases, ranging from 7% to 24%.^35–37 Because additional disruption of the palmar arcuate ligament and/or the dorsal radiocarpal ligaments is necessary to produce a VISI stance, this pattern is seldom seen acutely in distal radius fractures.

Degeneration

A lunotriquetral instability pattern may also appear as the consequence of an ulnocarpal abutment resulting in degeneration of (mainly) the proximal portion of the lunotriquetral ligament by a wear mechanism. Patients without history of trauma, inflammatory arthritis, or ulnocarpal abutment also may have degenerative lunotriquetral lesions because this region is prone to age-related degeneration.⁴ In both situations the lunotriquetral joint is not dissociated and no progressive derotational changes will develop.

Injury to the Lunotriquetral Supporting Ligaments: Effects on Carpal Kinematics*

In the laboratory, several attempts to ascertain the consequences of lunotriquetral ligament disruptions have been made.^14,15,25,38 The first published investigation of the role of the lunotriquetral ligaments in the carpal kinematics was reported by Reagan and associates,³⁹ who also were the first to analyze a series of such ligament injuries. They showed that in patients with severe lunotriquetral sprains there was often a static VISI deformity and abnormal motion of the carpus. That clinical study also showed that on lateral radiographs the longitudinal axis of the triquetrum, which in normal subjects exhibits an average 16 degrees of flexion relative to the longitudinal axis of the lunate, shows an extension posture of more than 14 degrees in severe lunotriquetral disruption: this is a more than 30-degree change in the lunotriquetral angle compared with normal. Reagan and associates³⁹ also described an increased proximal migration of the triquetrum on ulnar-deviated posteroanterior radiographs.

In 1990, Viegas and coworkers³³ showed that even in partial disruptions of the lunotriquetral ligament there is increased motion between the elements of the lunotriquetral joint, a finding later confirmed by Horii and colleagues²⁵ using biplanar radiography. All intercarpal joints have altered kinematics after complete lunotriquetral ligament sectioning. The changes were especially marked at the lunotriquetral joint, with the lunate adopting a palmar-flexed position and the triquetrum rotating into supination. Li and associates⁴⁰ concluded that the palmar lunotriquetral ligament is the major stabilizer of the lunotriquetral joint during wrist extension and that the rest of the lunotriquetral ligament provides stability during ulnar deviation.

Horii and colleagues²⁵ and Viegas and coworkers³³ emphasized the importance of the dorsal radiotriquetral and scaphotriquetral ligaments in the prevention of a global carpal collapse when the lunotriquetral ligaments are disrupted. In most studies, a carpal collapse, typically in the form of a VISI pattern, occurred only when the ligaments were sectioned in association with complete lunotriquetral ligament division. Li and associates⁴⁰ did not observe a static or dynamic VISI deformity after sectioning only the lunotriquetral ligaments.

More recently, Ritt and colleagues,³⁸ from the Mayo Clinic, reported on a series of cadaver specimen using stereoradiographic techniques to study carpal kinematics after different sequences of lunotriquetral ligament sectioning. According to that investigation, neither the isolated section of the proximal lunotriquetral membrane nor the disruption of both proximal and dorsal regions of the lunotriquetral capsule creates any significant change in carpal alignment (stage II in Table 40-1) or carpal motion. From a kinematic viewpoint, partial tears of the proximal lunotriquetral membrane need not be treated because they have minimal mechanical consequences. In contrast, when the palmar lunotriquetral ligament is sectioned selectively, significant changes in carpal kinematics occur. Although the intrinsic tendency of the scapholunate complex to rotate into flexion is still balanced (no carpal malalignment yet at this stage), increased lunotriquetral instability becomes evident, especially in ulnar deviation. Only after dividing the dorsal radiotriquetral and scaphotriquetral ligaments does carpal malalignment occur in a consistent fashion: lunate and triquetrum supinate, flex, and radially deviate (stage IV in Table 40-1). The strong dorsal radiotriquetral and scaphotriquetral ligaments can act as rotational constraints for the lunotriquetral joint in the absence of the dorsal lunotriquetral ligament.

One factor that should be taken into account is the effect of cyclic loading on the generation of abnormal patterns of motion. Horii and colleagues²⁵ applied “some degree of manipulation” after sectioning of the ligaments. It may require secondary attenuation from prolonged cyclic loading to produce the clinical findings. Ritt and colleagues³⁸ simulated gradual attenuation of the remaining ligaments by repetitively loading the joint before analyzing carpal motion. The values of static carpal malalignment changed dramatically: the static malalignment increased in magnitude (stage V in Table 40-1).

Most laboratory attempts to explain the pathomechanics of one particular injury have an important drawback: the experiments cannot simulate what happens in living systems in which healing and remodeling have a great effect on the end results. Nevertheless, from most of these cadaver studies, results of which seem to be in accordance with the observations made in clinical practice, one can conclude the following: