Symmetrical Face Research Articles

Rigid Fixation and Facial Asymmetry

The use of rigid fixation for the correction of facial asymmetry is in principle no different from its use in other forms of craniomaxillofacial deformity. The use of screws and plates demands more precise techniques than the use of wires. Attention to detail in passively fitting the plate to the defect and accurate insertion of the drill holes and screws enables the production of good occlusion and a symmetrical face. The precise osteotomies and bone grafts needed in the correction of all forms of skeletal asymmetry are as described for hemifacial microsomia or temporomandibular ankylosis. In a young child whose secondary dentition has not yet erupted it is not possible to insert screws, neither is the bone in a costochondral graft solid enough to permit screw fixation without the use of intermaxillary fixation. Therefore, rigid fixation can be used for osteotomies and bone grafts in patients whose main permanent dentition has erupted and in whom the cortex of the bone graft is strong enough to retain the screws against the pull of facial muscles.

Simultaneous total correction of temporomandibular ankylosis and facial asymmetry.

Ankylosis of the temporomandibular joint not only prevents mouth opening and chewing, but also affects the growth and position of the mandible. This produces progressive facial distortion with devastating psychosocial effects compounding the already difficult problem of not being able to open the mouth. Over the past 6 years, 18 patients in Canada and Taiwan were treated by excision of a large block of bone at the ankylosis and repositioning of the jaw, with the addition of osteotomies as necessary to produce a symmetrical face with good occlusion. Bilateral cases were treated at one operation in a similar way. The temporomandibular joint and absent ramus were constructed with a costochondral graft taken from the opposite chest. Some patients were treated with intermaxillary fixation for 8 weeks, while others had no fixation, but there was no difference in the effectiveness of correction of the ankylosis.

Transient Wave Propagation in a Symmetrically Layered Elastic Plate

The response of a symmetrically layered plate subjected to symmetric step normal line face loads is determined. The solution, which is based on the equations of motion from linear elasticity, is obtained in the form of an infinite series of integrals with each term being the contribution from a branch of the underlying frequency equation. Through numerical analysis several of the lower branches of the frequency equation, which depend on layered plate ratios (four material and one thickness) are evaluated and used to write the transient response in the near field. Integrations based on the first five branches show the dominance of the lowest mode in the solution. Applying arguments of stationary phase to the spectrum, yields low frequency-long wave and high frequency-short wave approximations for use in obtaining the far field-long time disturbance. The former governs a front running “head of the pulse” for the layered plate, and the latter, later arriving Rayleigh and Stoneley waves.

The Elastic Deformation of a Circular Rod of Finite Length for an Axially Symmetric End Face Loading

The problem of determining the deformation of an elastic rod of finite dimensions under a symmetrical axial loading is reduced to the solution of an infinite system of simultaneous linear equations. An approximate numerical solution is obtained for the case when the load acts uniformly over an outer annular region of the end face, and it is shown that the maximum radial displacement increases as the width of the annulus decreases. The application of the results to the problem of circumferential ridging of nuclear reactor fuel elements is discussed. It is concluded that the compression between adjacent rods probably contributes to ridge formation, but that the magnitude of the contribution cannot be determined without a knowledge of the compressive plastic stress-strain characteristics of the material.

The Hydrodynamic Lubrication of Piston Rings

Although the piston ring is a critical component in a reciprocating engine and although its successful functioning depends upon adequate lubrication, very little work has been done on the analysis of piston ring operation as a phenomenon of hydrodynamic lubrication. This paper outlines briefly the theoretical and experimental work that has been done, and produces a new analysis particularly suited to solution by computer. The analysis assumes: that the shape of the piston ring face is parabolic, but not necessarily symmetrical; that the lubricant viscosity is constant under the ring, but may vary in any fashion over the piston stroke; that oil film disruption may be represented by calculating oil pressure assuming no disruption and then discarding negative pressures; that the pressure upstream and downstream of the ring and the load on the ring may vary cyclically over the stroke; that there is circumferential symmetry; and that the motion of the ring parallel to the cylinder face is that of the piston, while the motion perpendicular to the cylinder face satisfies the force balance. Using the simple example of the symmetrical ring face, isoviscous lubricant, constant load, zero upstream and downstream pressures, and sinusoidal piston motion, it is shown that there exists a parabolic shape which maximizes the minimum oil film thickness. Comprehensive results are presented for this case. The use of the more general analysis is illustrated for a diesel engine design for which the ring face shape has been measured.

On Wave Propagation in an Elastic Plate with Nonmixed Edge Conditions

Within the framework of the “exact” linear theory, an important class of wave-propagation problems in elastic waveguides, involving nonmixed edge conditions (like stress or displacement), have remained unsolved. Basically, this is because known separation methods (classical or integral transforms) do not “ask” in a natural way for the given edge (and corner) information. A means for solving some problems in this class for the semi-infinite plate is presented here. In the method a Laplace transform is used on the propagation coordinate, say x. Exploitation of the boundedness condition of the solution, at χ→∞, generates two coupled integral equations for the edge (and corner) unknowns (displacements and strains), which depend, parametrically, on those complex wave-number roots of the governing Rayleigh-Lamb frequency equation representing unbounded waves. In some cases, these equations can be reduced to a set of algebraic ones with the aid of plate-thickness-eigenfunction expansions for the edge (and corner) unknowns. Numerical solution for the eigenfunction coefficients then determines the transformed solution of the problem, which can be inverted through known techniques. The problem of a plate with a built-in edge, subjected to symmetric face loadings, is treated as an example.