ANATOMY
The ligamentous anatomy of the carpus is complex with multiple intrinsic ligaments (Fig. 1). Perilunate instability is common secondary intrinsic ligamentous injury and can range in severity from mild sprain to severe disruption.
Progressive instability can lead to advanced arthritis of the radiocarpal and midcarpal joints,
known as the SLAC wrist (Scaphoid Lunate Advanced Collapse).1
Initial assessment of potential wrist instability injuries commences with physical examination and orthogonal plain x-ray views. The anteroposterior (AP) view is useful to assess Gilula’s lines and the scapholunate gap (Fig. 2,
Fig. 3).
Gilula’s lines represent the arcs of the wrist,
and proximal and distal carpal rows. Breach of the lines raise suspicion for intrinsic ligamentous injury. The scapholunate gap should measure less than 3mm.
A distance greater than 5mm is highly suggestive of rupture of the scapholunate ligament (Fig. 4).2
The lateral view is useful to assess the scapholunate and capitolunate angles ( Fig. 5, Fig. 6). The scapholunate angle is constructed by the intersection of a line drawn parallel to the long axis of the scaphoid and a line drawn perpendicular to the AP axis of the lunate. This angle should normally measure between 30°- 60°. The capitolunate angle is constructed by the intersection of a line drawn parallel to the long axis of the capitate and a line drawn perpendicular to the AP axis of the lunate. This angle should normally measure less than 30°.
DISI & VISI
Disruption of the scapholunate ligament can lead to increased scapholunate and capitolunate angles with a posterior lunate tilt and clinical dorsal intercalated segmental instability (DISI).
Disruption of lunotriquetral ligament can lead to a reduced scapholunate angle but increased capitolunate angle with a volar lunate tilt and clinical volar intercalated segmental instability (VISI).
MRI can clearly identify and confirm ligamentous injury as well as reveal bone contusion,
occult fracture and provide assessment of the chondral surfaces. It can also exclude ligamentous injury even though plain XR appearances may be suggestive (Fig. 7,
Fig. 8,
Fig. 9 Fig. 10, Fig. 11, Fig. 12, Fig. 13, Fig. 14).
The pattern of perilunate injury can be divided into 4 stages described by the Mayfield Classification:
I |
Scapholunate ligament injury |
II |
Capitolunate ligament injury (midcarpal joint) |
III |
Lunotriquetral ligament injury |
IV |
Radiolunate ligament injury |
Progressive instability of the wrist leading to a SLAC wrist progresses through various degenerative stages described by the Watson Classification:
Stage I |
scaphoid and radial styloid osteoarthritis |
Stage II |
scaphoid and scaphoid facet of the radius osteoarthritis |
Stage III |
capitate and lunate osteoarthritis |
Plain XR and in particular MRI are sensitive in detecting and assessing these progressive stages . Of note,
the radiolunate joint is usually spared,
unlike other forms of wrist arthritis.3,4