Establishing a common instantaneous center of rotation for the metatarso-phalangeal and metatarso-sesamoid joints: a theoretical geometric model based on specific morphometrics.

Background: Previous research has identified separate sagittal plane instantaneous centers of rotation for the metatarso-phalangeal and metatarso-sesamoid joints, but surprisingly, it does not appear that any have integrated the distinctive morphological characteristics of all three joints and their respective axes into a model that collectively unifies their functional motions. Since all joint motion is defined by its centers of rotation, establishing this in a complicated multi-dimensional structure such as the metatarso-phalangeal-sesamoid joint complex is fundamental to understanding its functionality and subsequent structural failures such as hallux abducto valgus and hallux rigidus.

Methods: Based on a hypothesis that it is possible to develop an instantaneous center of rotation common to all four osseous structures, specific morphometrics were selected from a sequential series of 0.5-mm sagittal plane C-T sections in one representative cadaver specimen randomly selected from a cohort of nine, seven which were obtained from the Body Donation Program, Department of Anatomy, University of California, San Diego School of Medicine, and two which were in the possession of one author (MD). All mature skeletal specimens appeared grossly normal, shared similar morphological features, and displayed no evidence of prior trauma, deformity, or surgery. Specific C-T sections isolated the sagittal plane characteristics of the inter-sesamoidal ridge and each sesamoid groove, and criteria for establishing theoretical sesamoid contact points were established. From these data, a geometric model was developed which, to be accurate, had to closely mimic all physical and spatial characteristics specific to each bone, account for individual variations and pathological states, and be consistent with previously established metatarso-phalangeal joint functional motion.

Results: Sequential sagittal plane C-T sections dissected the metatarsal head from medial to lateral and, at approximately midway through the metatarsal head, the circular nature of the inter-sesamoidal ridge (crista) was isolated; other C-T sections defined, respectively, the elliptical characteristics of the tibial (medial) and fibular (lateral) sesamoid grooves in each specimen. A general plane model representing the most basic form of the joint was developed, and its center of rotation was established with a series of tangential and normal lines. Simplified tibial sesamoid and fibular plane models were developed next which, when combined, permitted the development of a spherical model with three separate contact points. Based on the morphometrics of each sesamoid groove and a more distally positioned tibial sesamoid, the model was modified to accurately define the center of rotation and one distinctive sagittal plane geometric and functional characteristic of each groove.

Conclusion: Consistent with our hypothesis, this theoretical geometric model illustrates how it is possible to define an instantaneous center of rotation common to all three joints while simultaneously accounting for morphometric and spatial variability. This should provide additional insight into metatarso-phalangeal-sesamoid joint complex functionality and the physical characteristics that contribute to its failure.