Strength and fatigue resistance of thoracolumbar spine implants: an experimental study of selected clinical devices.

Results of strength and endurance tests of several screw/rod thoracolumbar spinal implants are presented. The devices tested were: AcroMed ISOLA, CD systems, Depuy systems, and Synergy systems. The AcroMed ISOLA devices had the greatest stiffness and strength. The Synergy and AcroMed systems had the greatest fatigue resistance. The predominant failure mode was screw bending, although several instances of rod fracture and screw fracture were observed. A number of design improvements are identified, including reducing the number of components, thereby reducing the number of failure mechanisms; avoiding notches and knurled surfaces to provide greater resistance to crack initiation; and strengthening attachments between components, particularly between pedicle screws and horizontal rods.

Effects of cross-linkage on fatigue life and failure modes of stainless steel posterior spinal constructs.

This study tested the effects of cross-linkage on the fatigue performance of posterior spinal constructs (i.e., AcroMed stainless steel Isola systems). The failure modes encountered during fatigue were also examined. The results of this study confirmed earlier findings that the use of cross-linkage does not significantly affect the stability of posterior constructs during axial loading. Their influence in torsion loading is much more pronounced. During the fatigue tests, posterior stainless steel spinal implants instrumented without cross-linkage reached 1 million cycles at 500- and 750-N loads. When the load was increased to 1,000 N, the number of cycles to failure dropped by two-thirds. These findings demonstrate that the endurance limit was between 750 N and 1,000 N for spinal constructs without cross-linkage, with the limit being closer to 750 N. Devices equipped with one or two cross-linkages reached 1 million cycles at 500 N. The number of cycles to failure dropped dramatically as the load was increased to 750 and 1,000 N. It appears that the endurance limits for spinal devices using cross-linkage should be 500 and 750 N, with the limit closer to the 500-N load. All rod fractures occurred near the junction between the longitudinal and transverse rods. Stress concentration was greatly in the vicinity of that contact point. These results should provide a basis for future improvement in endurance limits of spinal implants equipped with cross-linkage. Higher endurance limits will reduce the toxic effects encountered during fracture modes. The implants will also be better able to withstand the high physiologic loads experienced by obese individuals.

Resident documentation of diagnostic impression in sexual abuse evaluations.

In sexual abuse evaluations, the documentation of the examiner's diagnostic impression is essential. If the diagnostic impression is not documented, the examiner will have to rely on memory rather than the medical record when called to testify. The purpose of this study was to determine whether pediatric residents adequately document their diagnostic impression in child sexual abuse evaluations. We performed a three-year retrospective chart review from patients 0-17 years of age who were evaluated at our emergency room for suspected sexual abuse. We reviewed 1,487 charts for historical information, physical findings, and diagnostic impression. Physical findings were categorized as normal, nonspecific, suggestive, or indicative of penetration. In 77% of cases (N = 256) with hymenal findings indicative of penetration and 84% of cases (N = 31) with vaginal findings indicative of penetration, residents recorded no impression or a nonspecific impression. Results were similar for vulvar and rectal findings indicative of penetration. Residents fail to document an adequate interpretation of their physical examinations in sexual abuse evaluations.

Biomechanical assessment of titanium and stainless steel posterior spinal constructs: effects of absolute/relative loading and frequency on fatigue life and determination of failure modes.

The goal of this study was to examine the effects of absolute/relative loads and frequency on the fatigue life of titanium and stainless steel posterior spinal constructs, and to determine the failure fracture modes. The stainless steel constructs had higher stiffness and yield strength than the titanium constructs, but the ultimate static strength was almost equal for both types of constructs. Titanium constructs, however, exhibited higher variability than the stainless steel constructs. In fatigue tests, the stainless steel constructs were significantly affected by the external load and were frequency independent. It appears from fatigue curves that 500 N can be approximated as the endurance limit for the stainless steel constructs. Titanium constructs were load-frequency dependent, and their endurance limit was somewhere between the 500 and 750 N load levels. There were no differences in performance between the stainless steel and titanium constructs at 16 Hz. At 4 Hz, titanium constructs performed as well or better than stainless steel constructs. Also, the titanium constructs resulted in better performance than the stainless steel constructs in the elastic region, and with smaller differences in the plastic region. Most of the failure modes for stainless steel constructs were in screw bending at 16 Hz with a smaller percentage of rod fractures at high loads, with a higher percentage of rod fractures observed for the stainless steel constructs at 4 Hz. Most of the failure modes for titanium constructs occurred in screw bending or fracture.

Unwanted sexual experiences in adolescents. Patterns of disclosure.

More than 300 patients who presented consecutively to a sexual abuse clinic, a family planning clinic, and a family practice clinic filled out an anonymous survey regarding unwanted sexual experiences. In the family planning and family practice clinics, 40% of females and 16% of males reported having at least one unwanted sexual experience prior to their 18th birthday. Although 85% had disclosed their unwanted sexual experience, time to disclosure was prolonged (mean 2.3 years, median 5 to 6 months). Fear and embarrassment were the most common reasons for delay or lack of disclosure. The most common reasons for disclosure reflected internal rather than external or environmental influences. Increased awareness of unwanted sexual experiences and pattern of disclosure may enhance detection and treatment in children and adolescents.

Characteristics of childhood sexual abuse in a predominantly Mexican-American population.

Little has been written about the characteristics of childhood sexual abuse among Mexican Americans. In addition, certain aspects of the epidemiology of reported sexual abuse in the United States have changed significantly over the past 12 years. To better understand how characteristics of sexual abuse vary with ethnicity, race, gender, and age, we reviewed the records of 2,130 children under age 18 who were evaluated at our university-based facilities for possible sexual abuse from 1987 to 1989. The demographic, historical, and physical findings of the cases from our predominantly Mexican-American population were analyzed. T-test, analysis of variance, and chi-square analyses were used to test for differences related to ethnicity, race, gender, and age. Significant differences in these factors were found with respect to the relationship of the perpetrator to the child, the number of perpetrators, the interval from the abuse to the evaluation, the types of abuse, and the findings of the physical examination. This study demonstrates the important influences of ethnicity, race, gender, and age on characteristics of childhood sexual abuse. Awareness of these influences can be helpful in understanding factors that contribute to childhood sexual abuse, preparing for legal proceedings, and designing appropriate prevention programs.

Chlamydia trachomatis infections in children evaluated for sexual abuse.

The purpose of this study was to elucidate the conditions under which children evaluated for sexual abuse should be tested for Chlamydia trachomatis. Children were seen in an outpatient sexual abuse clinic over a nine-month period. Eight patients, ranging from 1.5 to 14.5 years of age, had positive C. trachomatis cultures. The records of these patients were examined for historical information and physical findings suggestive of rectal or vaginal penetration. Neither historical information of abuse nor physical findings were helpful in predicting the C. trachomatis infections. These results suggest that all children being evaluated for sexual abuse should be cultured for C. trachomatis.

Effect of a program to facilitate parent-child communication about sex.

U.S. teens have high rates of premarital sexual activity resulting in alarming rates of teenage pregnancy and sexually transmitted disease. One possible way to combat the problem of teenage sexual activity is to promote sexuality education within the family. The purpose of this study was to increase parent-child communication about sex through an educational program for parents. The effect of the educational program was evaluated by a nonrandomized, controlled trial. Volunteer parents were recruited from three middle schools (grade six through eight) located in middle to upper-middle socioeconomic class neighborhoods. The parents participated in four 2-hour sessions which included factual information about sexuality and exercises to improve communication skills. The experimental group (N = 47) were requested to fill out questionnaires immediately before and one month after the program. The control group (N = 17) were requested to fill out questionnaires one month before and again immediately before the program. Parents were asked to report the number of times they talked with their adolescents about 11 sex-related topics. The difference in reported frequency of communication before and after the program was compared using a two-tailed, matched pairs t-test. Twenty-four (51%) experimental group parents and eight (47%) control group parents completed both questionnaires. There was a significant increase in communication reported by the experimental group. The mean difference of the number of topics discussed was 10.9 (SD 7.3) for participants versus -2.5 (SD 5.9) for controls (p = .00053). This study shows that parent-child communication about sex can be facilitated by an educational program for parents.

Analysis of opening shock of a chest mounted reserve parachute.

The effect of opening shock of a chest-mounted reserve parachute on the head/neck dynamics is studied. Comparisons are made with similar effects of a back-mounted reserve parachute. The analysis employs a biodynamic computer model developed in earlier research. The results show that there are significant advantages of a chest-mounted over a back-mounted reserve parachute.

Numerical advances in gross-motion simulations of head/neck dynamics.

An efficient method for gross-motion simulation of head/neck dynamics in accidents and high acceleration environments is presented. The method uses finite-segment modelling to develop a 3-body model of the head/neck system. The model is shown to compare favorably with an analogous 9-body model and with experimental data. The model is expected to be useful for: efficient analysis of gross-motion head/neck dynamics during accidents; for developing increased intuitive understanding of head/neck behavior; and for use with gross-motion, whole-body, crash-victim simulators.

Effect of protective helmet mass on head/neck dynamics.

The crash helmet which provides protection against injury due to direct head impact may actually contribute to injury in indirect head impact (e.g., hyperflexion or "whiplash") situations because of the added mass of the helmet on the head/neck system. It has been suggested that it may be possible to reduce this hyperflexion/hyperextension injury hazard while retaining the beneficial protection against direct impact through use of the helmet restraining collars, such as styrofoam or inflatable airbags. These claims are quantitatively and qualitatively examined and discussed in this paper. The UCIN HEAD/NECK computer stimulation model is used for the quantitative analysis. It is shown that the helmet can indeed contribute to the hyperflexion/hyperextension injury hazard and that the proposed restraining devices can potentially provide protection against this hazard.

Supplemental dietary carnitine and lipid metabolism in exercising rats.

Rats were physically trained by treadmill running and fed normal, high fat, or identical diets supplemented with 1.0% D,L-carnitine. Fatty acid oxidation, stimulated by exercise and/or by a high fat diet was studied to determine if it might be further augmented by supplemental dietary carnitine. Fatty acid oxidation in vitro or in vivo was unaffected by carnitine feeding. Carnitine feeding was associated with an increase in heart and muscle palmitylcarnitine acyltransferase activity and reduced serum cholesterol in trained rats fed the high fat diet. Under the conditions of this study, supplemental dietary carnitine did not greatly alter glyceride synthesis, lipolysis, or fatty acid oxidation, which indirectly indicates that the increased demands for fatty acid oxidation resulting from exercise or from fat feeding are adequately supported by endogenous levels of carnitine.

Biodynamic model of a parachutist.

A three-dimensional, gross-motion, finite-segment model of a parachutist is presented. The model is a modification of the UCIN Crash Victim Simulation Code. It is designed for study and analysis of the effects of opening shock and wind loading on the dynamics of a parachutist. The model consists of 11 rigid bodies linked together to simulate the human figure of a parachutist. Springs and dampers are used to model the joint forces and moments. The parachute forces are modelled by riser forces applied to the torso of the model. The wind loading is modelled as a profile drag force on each body of the model. The governing dynamical equations of motion for the system are coded into a computer program and they are then integrated numerically. Comparison with experimental data on parachutist head acceleration shows good agreement between model and experiment.

Comprehensive, three-dimensional head-neck model for impact and high-acceleration studies.

A three-dimensional, 54-degree-of-freedom computer model of the head/neck system is presented and discussed. The model consists of nine rigid bodies representing the head and vertebrae together with a series of nonlinear springs and dampers modelling the soft tissue. The soft tissue modelling involves the discs, muscles, and ligaments. The discs are modelled as two-parameter viscoelastic solids; the muscles are also modelled as two-parameter viscoelastic solids, but only able to exert force in tension; and the ligaments are modelled as nonlinear elastic bands exerting force only in tension. Equations of motion are written for this model by using Lagrange's form of d'Alembert's principle, a virtual work type principle. Computer algorithms are written to efficiently compute the numerical coefficients of these equations. The equations are integrated numerically for a number of specific cases where experimental data are available. Results show excellent agreement between the model and the experiments.

Fatty acid and ketone body metabolism in the rat: response to diet and exercise.

This study was designed to measure the response of key enzymes of ketone body metabolism in heart, skeletal muscle, and liver to diet and exercise, two conditions known to influence ketone body utilization. A 3 (diet: control, high fat, or high carbohydrate) X 2 (kill condition: rested or exhausted) X 2 (training: trained or untrained) factorial design was used to estimate main experimental effects as well as identify significant interactions of the variables. Physical training (treadmill running) was associated with a doubling of the activity of skeletal muscle 3-oxoacid CoA transferase, a key enzyme in extrahepatic ketone body utilization. The activity of the rate-limiting enzyme of liver ketone body production, hydroxymethylglutaryl CoA synthetase (HMG CoA synthetase), was not greatly influenced by training or exhuastive exercise indicating that the metabolic control of the ketosis of exercise may more likely be a function of the supply of fatty acids to the liver rather than the activity of HMG CoA synthetase. Feeding a high fat diet, on the other hand, significantly increased the activity of liver HMG CoA synthetase, indicating that the ketosis of fat feeding may be of a different nature than that of exercise. The results of this study indicate that physical training is associated with biochemical adaptations in ketone body metabolism as well as fatty acid oxidation, and that trained individuals are metabolically better endowed to benefit from the ketosis of exercise than untrained individuals.

Adipose tissue cellularity and lipolysis. Response to exercise and cortisol treatment.

Male rats a 5 wk of age were subjected to 13 wk of intensive treadmill running to study the effect of exercise on adipose tissue cellularity and lipolysis. Untrained controls of the same age remained sedentary in their cages for the duration of the experiment. Adipocyte numbers were similar in eqidiymal fat pads from trained and untrained rats (12.7 plus or minus 1.3 X 10(6) vs. 15.3 plus or minus 1.3 X 10(6) cells/pad), however trained rats had smaller fat pads containing smaller cells (0.09 plus of minus 0.01 vs. 0.20 plus or minus 0.04 mug triglyceride/cell). Adipocytes from trained rats possessed greater epinephrine-sensitive lipase activity than sedentary rats on a per cell, per milligram protein, per gram adipose tissue, or per fat pad basis. Although the smaller cells of the trained rats had greater epinephrine-sensitive lipase activity than the larger cells of the untrained rats, lipolysis was positively correlated with cell size within both treatment groups. Cortisol treatment of intact animals did not significantly affect in vitro adipose tissue lipolysis. The results of this study indicate that exercise training increased the potential of adipose tissue cells to release free fatty acids in response to epinephrine stimulation. Exercise training initiated at 5 wk of age had only a small effect on adipose tissue cell numbers but significantly decreased cell size.