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

Abstract

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 ...

The interobserver reliability of first metatarsal rotational component of axial sesamoid radiographs in hallux valgus

“…There were significant intraclass correlations (p=0.02) among the three observers when assessing RC of the first metatarsal in axial projections of non-weight-bearing metatarsals using the method of Mortier et al (14) We consider this measurement technique relevant for axial projections of the hallux sesamoids since it considers the first metatarsal diaphysis, as well as another line that intersects the plantar cortices of the medial and lateral sesamoid grooves. We specifically selected this assessment technique since it considers the morphology of a single bone structure (first metatarsal) and its center of rotation (16) (the head of the first metatarsal regarding the proximal diaphysis or epiphysis). Although it has been described that the morphology and relation of bony structures of the forefoot changes with the axial load (17,18) , these changes could be minimized by evaluating a single anatomical structure without considering the rest.…”

“…Regarding pathophysiology, interest has been expressed in the literature about the role of RC in first metatarsal hallux valgus deformity (7,16) , as well as dynamic factors (18) , such as muscle strength or ligament resistance, that could have a biomechanical effect on the global architecture of a weight-bearing foot (17) in addition to an isolated bone structure. Our results are relevant in morphological terms because they describe the rotation by trying to isolate the first metatarsal deformity.…”