Lunate dislocation: a serious injury not to be missed

A 28-year-old patient with a painful and deformed wrist after a fall from a ladder presented for a wrist CT examination.

Fig. 1 Normal anatomy of the wrist.

Fig. 1A PA x-ray view shows:

Carpal bones are: Scaphoid (SCA), Lunate (LUN), Triquetrum (TRI),
Pisiform (PIS), Trapezium (TRM), Trapezoid (TPD), Capitate (CAP) and
Hamate (HAM).

The three carpal (Gilula’s) arcs are : The first (blue) one outlines the
proximal convexities of the scaphoid, the lunate and the triquetrum;
the second (red) one follows the distal concave surface of the same bones
while the third arc (black) traces the proximal curvature of the
capitate and hamate.

Fig. 1B and Fig. 1C show normal alignment of the distal radius, the
lunate, the capitate and 3rd metacarpal on lateral x-rays and
sagittal CT views.

Fig. 2A (sagittal), 2B (sagittal), 2C (axial), 2D (coronal) and 2E (VR
image) CT show fracture dislocation of the lunate, triquetrum and
scaphoid bones in a 28-year-old male. There is disruption of
the first and second carpal arcs and anterosuperior displacement of
both the lunate and triquetrum bones.

The wrist is composed of the distal radius and ulna, eight carpal bones aligned in two rows and several ligaments.
Carpal dislocation most often occurs after a severe trauma on an outstretched hand.
Lunate and perilunate dislocations constitute a potentially serious injury involving ligaments that may cause nerve or vascular disorder and long-term wrist disfunction. Lunate dislocation must be differentiated from the perilunate dislocation in which the lunate remains in normal alignment with the distal radius [6, 7].

Clinically the wrist is greatly enlarged and deformed. Oedema and haematoma are often present and palpation is very painful.

Imaging plays an important role in the diagnosis, by demonstrating the alignment of the carpal bones.
Conventional x-ray can show disruption of carpal arcs (Gilula's arcs), abnormal overlapping of lunate with other carpal bones, widening of radiolunate space and allows measurement of scapholunate angle (normal if 30-60°) and capitolunate angle (normal if < 30°) to look for a carpal instability [1, 8, 11]. The lateral wrist film will look for misalignment of the radius, lunate, and capitate. However, plain radiograph may miss carpal occult fractures and underestimate displacements which can alter treatment planning [2, 6].
CT acquisition and reformatted images (MPR, 3D, VR) makes it easier to detect bone misalignment and allows in addition to detect occult fractures [2, 3].

Mayfield et al. classified carpal instability in four stages [5, 6]. Radiographic findings are:

Stage I: Scaphoid rotation and scapholunate dissociation.
Stage II: Capitate dislocation.
Stage III: Perilunate dislocation: Malrotated triquetrum and scaphoid, triquetrolunate diastasis, dislocated triquetrum.
stage IV: Lunate dislocation.

Lunate dislocation is the last stage of perilunate injuries. These are rare and serious, involving ligaments with risk of nerve and vascular damage, requiring early diagnosis and treatment. The standard treatment in the acute phase, which consists of surgical reduction of dislocation, stabilisation of the fracture, and repairing any ligament damage [4, 9]. Long-term complications are not uncommon despite the correct treatment, including carpal instability, osteoarthritis, decrease of motion and grip strength [1, 9].

Imaging plays a major role in identifying lunate and other carpal dislocations.
Dislocations of the wrist should be diagnosed on conventional x-ray.
Displacements and misalignment are however best evaluated with CT which in addition allows to detect occult fractures giving a better assessment of lesions prior to definitive treatment [2, 7, 10].