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.

First metatarsophalangeal joint motion in Homo sapiens: theoretical association of two-axis kinematics and specific morphometrics.

BACKGROUND: The metatarsal head and proximal phalanx exhibit considerable asymmetry in their shape and geometry, but there is little documentation in the literature regarding the prevalence of structural characteristics that occur in a given population. Although there is a considerable volume of in vivo and in vitro experiments demonstrating first metatarsal inversion around its longitudinal axis with dorsiflexion, little is known regarding the applicability of specific morphometrics to these motions. METHODS: Nine distinctive osseous characteristics in the metatarsal head and phalanx were selected based on their location, geometry, and perceived functional relationship to previous studies describing metatarsal motion as inversion with dorsiflexion. The prevalences of the chosen characteristics were determined in a cohort of 21 randomly selected skeletal specimens, 19 of which were provided by the anatomical preparation office at the University of California, San Diego, and two of which were in the possession of one of us (M.D.). RESULTS: The frequency of occurrence of each selected morphological characteristic in this sample and the relevant summary statistics confirm a strong association between the selected features and a conceptual two-axis kinematic model of the metatarsophalangeal joint. CONCLUSIONS: The selected morphometrics are consistent with inversion of the metatarsal around its longitudinal axis as it dorsiflexes.

Radiographic image distortion between the distal edge of the first metatarsal and the tibial sesamoid: establishing a reliable radiographic relationship.

BACKGROUND: After failing to statistically confirm a perceived pattern noted on radiographs that the sesamoids were proximally positioned in patients with hallux limitus compared with a control population without evidence of the deformity, the probable causes of this failure were examined. Measurement error was briefly considered but rejected owing to the careful manner in which the measurements were taken. The most plausible explanations were that the observations were incorrect and that the radiographs, which were retrospectively analyzed, were taken in a manner that distorted the spatial relationships between the metatarsal and the tibial sesamoid to a point that the results did not reflect reality. METHODS: This study examines potential difficulties in obtaining consistently reliable radiographic data regarding the spatial relationships of the metatarsal and the tibial sesamoid and establishes guidelines to minimize experimental error. Criteria for measuring metatarsal sesamoidal distances to the radiographic plate are established, along with application of the criteria to a control population. The principle of radiographic image distortion as it relates to these objects is presented, and, based on a predetermined range of radiographic angles, radiographic image distortion is calculated for the metatarsal head and the tibial sesamoid separately. RESULTS: By using accepted trigonometric principles, a mathematical model is developed that makes it possible to collectively quantify image shift between the two objects. CONCLUSIONS: Criteria are established that, if followed, should minimize image distortion when it is important to measure metatarsal sesamoidal distances.