The inferior boundary condition of a continuous cantilever beam model of the human spine.

The continuous cantilever beam model of the human spine usually assumes that the beam tangent at the inferior end of the structure is exactly perpendicular to the surface in which it is built into. The model used in this paper allows for realistic imperfections in the beam so that a small non-zero rotation is allowed at its inferior end. Such a model is used to investigate the lateral deformation of the muscle-relaxed spine as it supports asymmetrical loads in the frontal plane. By comparing the model deformations with previously published results, it was easily seen how such imperfections can effect the solution quantitatively. This was found to be especially true when the model was used to estimate the gross flexural rigidity of the spine in the frontal plane. It could also explain why some spines are more prone to lateral curvature and instability than others. Considering the importance of such a parameter when used in the continuous model, an investigation into the true nature of the inferior model boundary condition could be warranted.

Recurrence of breast carcinoma following immediate reconstruction: a 13-year review.

To evaluate the effect of immediate reconstruction on the incidence, location, detection, and treatment of recurrent breast cancer, a review of 306 patients operated on according to a standard protocol during the 10-year period 1979 through 1988 was performed. Reconstruction techniques included submuscular implants (207), tissue expanders (84), and musculocutaneous flaps (15). During a minimum follow-up period of 3 years with a mean of 6.4 years, 60 patients (19.6 percent) developed recurrent disease, at a mean interval to recurrence of 31 months. The first locations of recurrences were local (16), regional (11), and systemic (33). Recurrence rates by stage included stage I, 7 patients (5.2 percent); stage II, 45 patients (32.1 percent); and stage III, 8 patients (40 percent). It was not possible to include comparisons with internal control groups of patients in our institution who were not reconstructed or who had delayed reconstructions, thereby preventing conclusions based on such comparisons. Our recurrence data are similar to literature reports of recurrence rates in patients who were not reconstructed after mastectomy. Detection and treatment of recurrences were not inhibited by the reconstructions. When radiation therapy was used in the treatment of local recurrences, the development of symptomatic capsular contracture was recorded in 58 percent of the patients.

Asymmetrical loads and lateral bending of the human spine.

The human spine is modelled as a cantilever-type beam column. Under the influence of static asymmetrical loads, muscle and low-back forces are predicted from a hypothetical but revealing model. Such forces produced by asymmetrical loads are much larger than for a corresponding symmetrical load. Asymmetrical loads can encourage, especially in young schoolchildren, lateral bending of the spine by alleviating muscle and low-back forces. This could possibly be a factor contributing to the surprisingly high percentage of schoolchildren with measurable scoliotic curves. The wearing of knapsack-type bags is advocated.

Biomechanical simulations of scoliotic spinal deformity and correction.

A new approach to surgical correction of scoliosis has been advanced by us, in the form of simulation of the surgical correction system and technique. For this purpose, we developed a finite-element model of the spinal column (SFEM), applied tractions to it and determined the model stiffness so as to watch the actual spinal geometry. Having patient-simulated this SFEM, we applied to this SFEM corrective forces and determined the optimal set of forces to gain the best correction of the spinal deformity. We then developed a special instrumentation to measure the applied corrective forces during surgery using a particular fixation system. The SFEM corrected geometry was shown to compare favourably with the post-surgical curve. We have now developed an elastic beam-column model (EBCM) to which muscle activation forces, representing asymmetrical paralysis of the vertebral column muscles, can be applied to generate a given scoliotic curve. In that process the stiffness properties of the patient-simulated EBCM are determined. Now on these patient-simulated EBCM(s), identical corrective force systems are applied as developed by the finite-element model (SFEM) and implemented surgically for these patients. It is shown that the EBCM corrected geometries compare favourably with both SFEM corrected geometries as well as with the post-surgical curves for similar corrective force systems. Thus the EBCM can be employed to presurgically simulate scoliolic correction, specify the optimal corrective system of forces so as to gain the best surgical correction.

Lifting low-lying loads in the sagittal plane.

A simple geometrical model was employed to investigate various elementary techniques during static, sagittal plane lifts. Relationships between the various joint reaction forces were deduced. In particular, it was found that it is possible to simultaneously increase (or decrease) both knee and low back forces. In terms of the total force on the low back, hip, knee, and ankle joints, vertical back lifting is generally not recommended. This is especially true when lifting low-lying objects from the ground. However, minimization of loads is not the only factor to consider when analysing the optimal technique of a certain lifting task. Several other cost functions have been previously proposed. Comparisons derived from minimizing various cost functions suggest that the minimization of the maximum necessary muscle intensity may be the most appropriate in deducing optimal load lifting configurations.

Continuous model of the human scoliotic spine.

The human scoliotic spine is mathematically modelled by employing the classical non-linear theory of curved beam-columns. A realistically representative muscle force system is included in the model. Scoliosis due to asymmetrical bi-lateral muscular contractions has been studied and arbitrary large displacements and curvatures are allowed. The two-dimensional model allowing curvature in the frontal plane can show the progression of a scoliotic curve from an initially straight configuration. For various parameter values, particularly muscle asymmetry, the model attempts to simulate the progression of actual scoliotic curves. Once these curves have been simulated, forces corresponding to corrective surgical systems are applied to the scoliotic spine. The corresponding corrected curves are then compared with those produced by a finite element model and also to the actual clinical curve. The comparisons were very favourable, considering the simplicity of the continuous model. The commonly observed phenomenon of the scoliotic curve lying to the weaker side of the back in terms of muscle strength is reproduced and explained by the model. The possible usefulness of continuous spinal models to analyse the overall deformation of the spine under various loading conditions can then be deduced.

Developmental and corrective biomechanics for scoliosis.

The developmental mechanism for scoliosis and its surgical correction are studied by modeling the spinal column as a curved nonlinear (large-deformation-sustaining) beam column to which are applied (a) muscle forces to simulate scoliosis development due to asymmetrical bilateral muscle contractions and (b) corrective forces to simulate the action of surgically implanted corrective systems. The two-dimensional model permits curvature in the frontal plane and can simulate and demonstrate the progression of a scoliotic curve from an initially straight configuration for various model parameter values. The calculation of the bonding moments is treated, and a simple algorithm for solving the model equations is presented. Results for an actual clinical case are given.