Scaphoid variation and an anatomical basis for variable carpal mechanics

Abstract

The morphology and function of the wrist is poorly understood. Improved understanding of carpal anatomy may facilitate improved understanding of carpal mechanics and may enhance the clinical management of wrist dysfunction. Many detailed investigations of wrist structure have been reported, many of which have focussed on the scaphoid and its ligamentous supports. The results of these studies are not readily collated to provide an accurate description of the scaphoid and its supports. This study attempted to provide a detailed description of the anatomy of the scaphoid and its supporting structures. A detailed nomenclature was proposed to facilitate accurate description of the scaphoid and related structures. Gross observation enabled separation of the sample population of scaphoids into two groups. Morphometric analyses were used to determine any significant differences between the groups (type one and type two). The histological sections were then used to facilitate accurate gross identification of ligaments and computed tomographs were used to investigate the in situ variation of scaphoid orientation. The investigations suggest that two distinct populations of scaphoid existed within the sample population. The scaphoids varied in bone morphology, arrangement and degree of ligamentous support and position relative to the capitate. Articular facet shape and size differed between scaphoid types. The orientation and number of ligaments supporting the scaphoid were suggestive of variable scaphoid motion. The variation in ligamentous patterns was supported by histological investigation. Computed tomographs through the longitudinal axis of the scaphoid suggested a variable position of the scaphoid relative to the capitate. The variation of these structures was discussed in relation to the kinematic findings of others. A theoretical model of variable scaphoid function was proposed based on the anatomical findings. The data presented and the reviewed kinematic data may be extrapolated to suggest two models of scaphoid motion. The scaphoids may be divided into rotating/translating scaphoids and flexing/extending scaphoids. This must be confirmed by a combined anatomical and mechanical study. The clinical implications of different scaphoid structure and function may be profound. The ability to identify such differences in situ may facilitate varied clinical management for the various types of wrist suggested.