A congruent contact, unconstrained, multiaxial ankle replacement has been developed for use without cement. A talar onlay component with a trochlear surface and central fixation fin uses a cylindrical articulating axis that reproduces the lateral talar curvature. A tibial inlay component with a 7 degree anteriorly inclined short fixation stem uses a flat loading plate, recessed anatomically into the distal tibia to distribute tibial loads to the ankle joint. For both components, made of cast cobalt-chromium-molybdenum, a 275-micron pore-size, sintered-bead, porous coating is used to allow tissue ingrowth stabilization. A congruent ultra-high molecular weight polyethylene bearing is inserted between the metallic implants. Its upper surface is flat, whereas its lower surface conforms to the trochlear surface, thereby providing unconstrained, sliding cylindrical motion with low contact stress on the bearing surfaces. Contact pressure and collateral ligaments maintain ankle stability during both static and dynamic loading conditions. Clinically, 23 total ankle arthroplasties were performed in 21 patients. The follow-up period ranged from 24 months to 64 months with a mean of 35.3 months. Diagnoses included rheumatoid arthritis, 6 patients (26.1%); osteoarthritis, 4 patients (17.4%); post-traumatic arthritis, 10 patients (43.5%); avascular necrosis of the talus, 2 patients (8.7%), and painful ankle fusion, 1 patient (4.3%). Pain was the primary reason for surgery in all cases. Postoperatively, 87% of ankles had no pain or, at most, mild pain. Postoperative complications included poor wound healing in four ankles, reflex sympathetic dystrophy in two ankles, deep infection in one ankle, and one bearing subluxation. No ankle replacements were removed and no fusions were performed for failed implants, although one bearing was exchanged without disrupting the metallic elements. In this report, the suggestion is made that total ankle arthroplasty may have an improved application in various arthritis disorders when used with biologic fixation and unconstrained mobile bearings.

New Jersey Low Contact Stress Total Ankle Replacement: Biomechanical Rationale and Review of 23 Cementless Cases

A physiologic constitutive expression is presented in algorithmic format for the nonlinear elastic response of wavy collagen fibrils found in soft connective tissues. The model is based on the observation that crimped fibrils in a fascicle have a three-dimensional structure at the micron scale that we approximate as a helical spring. The symmetry of this wave form allows the force/displacement relationship derived from Castigliano's theorem to be solved in closed form: all integrals become analytic. Model predictions are in good agreement with experimental observations for mitral-valve chordae tendinece.

Elastic model for crimped collagen fibrils

The various factors affecting the strength of bolted or riveted joints in advanced composites are discussed qualitatively. The mechanisms associated with each failure mode — tension, bearing, shearout, cleavage, and pull-through — are explained. The work is based mainly on experimental observations, from which conclusions have been drawn to integrate the study. A simple theory to account for the elastic stress concentrations at loaded bolt holes, in terms of the various geometric parameters, is deduced from experimental and analytical evidence for isotropic materials. Further tests on composite materials are used to deduce correlation factors which relate this theory to the limited, but significant, stress concentration relief observed prior to failure. The combination of these theories permits a considerable generalization from limited test data to geometries for which no test data are available, but does need some testing for that specific composite material and fiber pattern. The interaction of stress concentration factors caused by bearing and bypass loads is explained. The theories above are shown to cover such combined load cases, as at multi-row bolted joints, and along spar caps and consistency with test data is shown. Not all load cases are usually covered by test programs, so the use of this theory to extend results to such cases as bearing loads applied orthogonally to the bypass load is explained, along with cautions about known limits on such techniques. No attempt is made to compile a comprehensive data bank of test data, but the examples presented do cover a sufficiently diverse range of fiber patterns to elucidate the known failure phenomena. The need to consider advanced composites not as homogeneous orthotropic materials but as two-phase mixtures of fibers and resin is highlighted by the observations on the benefits and problems associated with interference-fit fasteners in composites. The sequence of first failure of the resin (by delamination or disbonding of the fibers) and second of the fibers (by tension) is shown to explain the strength increases resulting from nonlinearity.

Mechanically-Fastened Joints for Advanced Composites — Phenomenological Considerations and Simple Analyses

Stability of superdislocations and shear faults in L12 ordered alloys

Load distribution model along the line of contact for involute external gears

Introduction. Theories of Stress and Strain. Linear Stress-Strain-Temperature Relations. Inelastic Material Behavior. Applications of Energy Methods. Torsion. Bending of Straight Beams. Shear Center for Thin-Wall Beam Cross Sections. Curved Beams. Beams of Elastic Foundations. The Thick-Wall Cylinder. Elastic and Inelastic Stability of Columns. Flat Plates. Stress Concentrations. Fracture Mechanics. Fatigue: Progressive Fracture. Contact Stresses. Creep: Time-Dependent Deformation. Appendix A: Average Mechanical Properties of Selected Materials. Appendix B: Second Moment (Moment of Inertia) of a Plane Area. Appendix C: Properties of Steel Cross Sections. Author Index. Subject Index.

https://physicstoday.scitation.org/doi/pdf/10.1063/1.3061599?download=true

James O. Smith

Papers

Advanced Mechanics of Materials