Fracture of the distal radius is the wrist injury most often encountered by orthopedic and hand surgeons.1 The number of fractures of the distal radius in the United States was estimated to be 640,000 in 2001, and the incidence is increasing.2,3 Recent evidence has shown a substantial increase in treating these fractures with internal rather than closed fixation, even in the elderly.4
Treatment of complex intra-articular fractures of the distal radius requires an accurate diagnosis of the fracture pattern and a thoughtful approach to fixation. Although a majority of the fractures that meet the operative criteria are now treated with various anterior locked-plating techniques with good results, a subset requires more technically demanding fixation approaches, including fragment-specific approaches, dorsal and palmar plating, and combined internal and external fixation.
The sustentaculum lunatum, as we have named the palmar lunate facet, deserves specific attention because of its importance in load transmission across the radiocarpal joint and its key role in restoring the anatomy of the palmar distal radial metaphysis during internal fixation. This fragment in comminuted fractures was first ascribed special importance by Melone5 in his description of common fracture patterns. In the present article, we describe the anatomical characteristics of the sustentaculum lunatum and the clinical relevance of this fragment to management of fractures of the distal radius.
Classification
A variety of classification systems have been proposed to characterize and guide treatment of fractures of the distal radius. The earliest descriptions of fracture patterns were presented by Castaing6 and Frykman7 in the 1960s. The Frykman classification historically has been popular but is limited in accuracy in its characterization of fragments and their displacement and is limited in its ability to guide treatment. The classification system proposed by Melone and colleagues5,8-10 was the first to truly describe fracture of the distal radius fragments in a relevant manner, including their characteristic “4 parts” (Figure 1). The authors emphasized the importance of the “medial complex” as the cornerstone of the radiocarpal and radioulnar joints.
The classification system developed by Müller and colleagues,11 which was adopted by the AO (Arbeitsgemeinschaft für Osteosynthesefragen), might be the most descriptive and informative system, and it is widely used to conduct research and direct treatment. This system classifies fractures into A (extra-articular), B (partial articular), and C (complete articular) types and subclassifies them according to fracture location and comminution. These classifications, along with a conceptualization of the distal forearm as a 3-column structure involving the radial, ulnar, and intermediate columns (including the lunate facet), as proposed by Peine and colleagues,12 gave us a framework for approaching fixation of fractures of the distal radius.
Etymology and Definition
Sustentaculum, from the Latin sustinere, “to support, check, or put off,” and taculum, “receptacle or holding space,” is a fitting description of the most distal portion of the palmar lunate facet, as it supports and holds the carpus, and specifically the lunate, on the radial articular surface. This portion is analogous to the sustentaculum tali, the named portion of the calcaneus that supports and articulates with the middle calcaneal articular surface of the talus13 and provides a reliable fragment for internal fixation of the calcaneus.
Anatomical and Biomechanical Considerations
The distal radial articular surface is composed of distinct scaphoid and lunate facets that articulate with their respective carpal bones. Several studies have characterized the anatomy of the distal radius.14-17 Linscheid14 found that the lunate and scaphoid facets account for 46% and 43% of the contact area across the radiocarpal joint, respectively; this has been corroborated by others.15 A biomechanical study by Genda and Horii18 showed that the majority of stress across the wrist joint was concentrated at the palmar side of the distal radius in the neutral position. Although it is recognized that the scaphoid facet bears most of the load across the wrist in the neutral wrist position, most activities of daily living place the wrist in a slightly extended and ulnarly deviated position. This position results in a shift of the majority of load to the radiolunar articulation, constituting 53% of total force transmission.18 Subchondral bone density analyses have supported this lunate-predominant loading pattern across the radiocarpal articulation in most people.19 This loading pattern is also supported by the observation that failure of fixation and carpal subluxation generally occurs at the radiolunate articulation.
The palmar lip of the distal radius traditionally has been depicted and conceptualized as a flat extension of the metaphysis, leading to the development of implants that are not ideally designed for capturing this area in the fracture setting. A 3-dimensional (3-D) computed tomography (CT) study of the distal radii of healthy volunteers, conducted by Andermahr and colleagues,20 showed that the contour of the palmar lunate facet projects from the palmar cortex of the radius by 3 mm on average and is about 19 mm in width (radial to ulnar dimension) (Figures 2A-2C). In the axial plane, the anterior cortex of the distal radius slopes in a palmar direction, from radial to ulnar. This presents a challenge in attempts to support the entire surface (scaphoid and lunate facets) with a single palmar implant.20-25
