A study conducted by Harness and colleagues24 showed that the majority of palmar shear fractures are composed of multiple fragments of the lunar articular facet. Anatomical studies of the distal radiocarpal articulation have also described the ligamentous attachments to the sustentaculum lunatum.26 The short radiolunate ligament, which originates from this fragment and inserts onto the lunate, provides stability to the carpus and, if not adequately fixed, leads to an incompetent restraint to palmar carpal translation. Isolated injuries of the short radiolunate ligament or fractures of the palmar lunate facet have been shown to result in palmar carpal translation.27,28 In addition, attachments of the palmar radioulnar ligament and other more ulnar radiocarpal ligaments act as deforming forces on the palmar lunate facet.24,26
Fracture Pattern Recognition
Although the AO type B palmar shear fracture pattern, also known as the Barton fracture, has classically been recognized as the fracture involving the palmar lunate facet and requiring special attention, many complete articular fractures feature involvement and fragmentation of this portion of the distal radius (Figures 3A-3F).29 In highly comminuted complete articular and palmar shear fracture patterns, the morphology of the sustentaculum lunatum should be appreciated, and its adequate fixation to the radial metaphysis ensured, to prevent loss of reduction.
Visualization of the palmar lunate facet as a distinct fragment might be difficult in cases of highly comminuted fracture patterns. Standard CT or more recently described 3-D CT techniques with subtraction of the carpus might facilitate appreciation of this fragment for preoperative planning of approach and fixation.29,30 Our institutional protocol involves obtaining preoperative traction radiographs of every fracture of the distal radius. These radiographs have reduced the need for CT in understanding the fracture pattern and aid in decision making.31
Besides appreciating the existence of the sustentaculum lunatum fragment, we should recognize that some injury patterns that split the lunate facet into unstable dorsal and palmar fragments might necessitate a separate dorsal approach to reduce and fix the dorsal lunate fragment. Traction radiographs can be especially useful in recognizing these patterns (a V sign is present) (Figures 4A, 4B).
Open Fractures
Highly comminuted fractures of the distal radius presenting with displaced lunate facet fragments can have high-energy mechanisms of injury. Although open fractures of the distal radius are associated with lower risk for infection (compared with open fractures of other long bones), they deserve special attention because of associated tendon and neurovascular injuries. Few studies have specifically assessed open fractures of the distal radius.32-35 Only the study by Rozental and Blazar34 listed associated injuries at the wrist level. The authors identified 4 patients (out of 18) with concomitant flexor tendon or neurovascular injuries that included radial or ulnar artery injury. In our experience, many open fractures of the distal radius are caused by an inside-out mechanism and present with an open wound either over the ulnar styloid or in the area of the ulnar side of the palmar radial metaphysis corresponding to the metaphyseal spike that mates with the sustentaculum lunatum (Figures 5A, 5B). Given these findings, we approach this intermediate column with particular care in cases of open fracture, paying attention to important structures (flexors, neurovascular) and looking for contamination from the environment into the fracture.
Fixation Techniques
The approach to fixation of partial articular palmar shear fractures is fairly straightforward. Buttress plate fixation has been well described and has had reliably good results.36 However, in very distal fracture patterns and in cases in which the palmar lunate facet is fragmented as part of a complete articular fracture, a fragment-specific approach to fixation with or without spanning external fixation often is necessary.37 The unrecognized sustentaculum lunatum fragment in comminuted complete articular fractures can lead to inadequate fixation constructs, resulting in loss of reduction and carpal subluxation in a palmar direction.24,34,38
Our surgical approach uses the standard anterior interval between the radial artery and the flexor carpi radialis, as described by Henry.39 The flexor pollicis longus is retracted ulnarly, revealing the pronator quadratus. We then reflect the pronator quadratus from the distal radial metaphysis until the most proximal and ulnar extent of the fracture is easily visualized. The palmar ulnar metaphyseal cortex that mates with the displaced sustentaculum lunatum is, in our experience, often the least comminuted portion of the metaphysis, thus providing a cortical key for restoration of height and alignment (Figures 5A, 5B). At our institution, fixation typically is achieved by contouring miniplates (1.3 or 1.5 mm) to capture and buttress the sustentaculum lunatum (Figures 6A, 6B). In our experience, the screw lengths in the most distal fixed-angle constructs at the palmar lip are limited to 6 mm or less to avoid penetration of the articular surface, though this has not been previously reported in the literature. After restoring the length and tilt of this intermediate column of the distal radius, we proceed with “rebuilding” the remainder of the fragments to our stabilized initial construct.
