BIOMECHANICAL PRINCIPLES IN DIARTHROSES A ND SYNARTHROSES .4. MECHANICS OF THE LUMBAR SPINE - A PILOT-STUDY

Theoretical considerations help define the requirements for an apparat us that is to localize the instantaneous helical axes (IHA) of axial r otations of lumbar segments. Result: Since the range of axial rotation of an L3/4 segment is in only approximate to +/- 1.5 degrees the rota tional angle intervals have to be smaller than 0.3 degrees with an res olution less than 0.03 degrees in order to be able to determine the lo ci of the IHAs. For the first time in vitro measurements are presented that satisfy this requirement. The data prove that the guidance by th e artt. zygapophysiales critically influence the possible positions of the IHA. Comparatively, ligaments and intervertebral disk play a marg inally role. During axial torques T-z the IHAs lie dorsal to the inter vertebral disk and migrate from one joint to the other depending on ax ial rotation (length of migration: approximate to 3-4 cm). The IHAs li e almost parallel to the axial torque vector. When the joints are remo ved the IHA is stationary and almost perpendicular to the intervertebr al disk and intersects the disk's central region. The screw inclinatio n (pitch) of the instantaneous screw movement is proportional to the r otational angle. Therefore, depending on the direction of rotation? on e obtains left or right handed screw movements. This means: axial tors ional load leads to an increase in thickness of the intervertebral dis k. During preloads that produces extensions the fixed centrodes (paths of axis migration) of intact segments are dorsally beaten out, wherea s during flexional loads they are ventrally beaten out. Then, the IHAs migrate through the canalis vertebralis. By the concept ''dimeric lin k chain'' the different shapes of the fixed centrode are traced back t o the morphology of curvature of the articulating surfaces. The measur ements suggest the hypothesis that the distinct nonlinearity of the lo ad displacement curves (s-shape of alpha = alpha(T-z) funktion is an a ffection of IHA migration. Comparatively, the influence of ligaments c an be neglected. The measurements suggest the hypothesis that the hyst eresis of the load displacement curves (neutral zone) is an artefact t hat does not appear in vivo. Altogether, the experiments prove that th e loci of the IHAs are determined by the interplay of preload, structu re of the applied force system and morphology of curvature of the arti culating surfaces. By that the possibility is clinically given to calc ulate the multitude of possible movements as function of muscle activi ty when in the individual the shape and the position of the articulati ng surfaces are measured in vivo (by NMR-methods e.g.). A physically b ased classification of pathological cases seems to be possible.