Mechanical performance of standard and cannulated 4.0-mm cancellous bone screws.

The mechanical performance of bone screws is determined by their pull-out strength (holding power), compressive force, stripping torque, yield bending moment, ultimate bending moment, and fatigue strength. These parameters are related to the parameters of the screw design, including major thread diameter, minor thread diameter, thread length, pitch, shaft diameter, cannulation diameter, and material properties. The goal of the study was to theoretically predict the static performance of five 4.0-mm, 45-46-mm-long, cancellous, partially threaded standard and cannulated bone screws and compare the predictions with experimental measurements. A secondary goal was to determine if cannulation of the bone screw diminished its mechanical performance. The predicted values for pull-out force, compressive force, and stripping torque were determined by the thread length, major thread diameter, and thread shape factor. The screws with the largest major thread diameter and longest thread length had the greatest pull-out force, compressive strength, and stripping torque. However, when correcting for the thread length, a higher thread shape factor compensated for a smaller major diameter. The coefficient of determination (r2) for the correlation between the predicted and measured pull-out force improved from 0.75 to 0.90 when the theoretical model included the thread shape factor. The yield and ultimate bending moments are a function of the section modulus and material properties of the screw. The Ace solid screw had the greatest section modulus and yield and ultimate bending moments. The experimental data support the theoretical models for predicting the mechanical performance of bone screws. The design of the bone screws can be optimized on the basis of theoretical modeling. The strong correlation between the predicted and measured parameters allows comparison between bone screws without repeated experimental tests. Theoretical and experimental results show that cannulation of the bone screw did not inherently diminish its mechanical performance.

Case report: an unusual cutaneous myiasis of the foot.

A case of rare cutaneous myiasis, which has not heretofore been described in the foot, is presented. The unusual life cycle is described in order to understand the clinical presentation. The treatment options are discussed and recommendations are made to avoid treatment pitfalls.

Prolonged absence status epilepticus associated with carbamazepine therapy, increased intracranial pressure, and transient MRI abnormalities.

We describe an adolescent epileptic patient who presented in nonconvulsive status epilepticus that appeared to be related to treatment with carbamazepine. The absence status, which was resistant to multiple anticonvulsants, produced increased intracranial pressure and transient abnormalities observed on MRI.

Osseous healing after CO2 laser osteotomy.

While some limited discussion of the role of CO2 laser application in orthopaedic surgery has appeared in professional literature, a search of orthopaedic and podiatric journals has failed to uncover reports of the laser's effects on bone healing. This study examined osteotomies created with an oscillating saw and a CO2 laser in a rabbit model. The results showed that bone healing was comparable in both groups, but that healing was delayed in the laser group because inert-appearing bone fragments seemed to inhibit the healing process. Further study is indicated to determine the mechanism by which the CO2 laser delayed healing and whether lasers can enhance healing under certain conditions.

Hypoxic injury to developing glial cells: protective effect of high glucose.

Hypoxic injury to differentiating glial cells is a critical event in the development of periventricular leukomalacia, the major hypoxic-ischemic lesion of the premature infant. This study has addressed the effects of hypoxia on differentiating glial cells, primarily astrocytes. Primary cultures of dissociated newborn rat brain, which are composed predominantly of differentiating astroglia, were used. Efflux of lactate dehydrogenase, an enzyme enriched in astroglia, was used to quantitate cellular injury. Three major findings are reported. First, differentiating astrocytes were resistant to hypoxic injury for many hours, although by 24 h of hypoxia severe cellular injury (lactate dehydrogenase efflux of 86% of total and morphologic changes) was obvious. Second, increase of glucose in the culture medium from the approximately physiological concentration of 5.6 to 15 mM had a marked protective effect versus hypoxia, i.e. lactate dehydrogenase efflux was totally prevented during 24 h of hypoxia in 15 mM glucose. Third, the protective effect of high glucose appeared to be related to increased utilization by glycolysis, because there was a direct correlation between the resistance to hypoxic cellular injury and the amount of lactate generated and of glucose consumed by the cells. Thus, the cells with the lowest lactate dehydrogenase efflux (and highest glucose supplementations) had medium lactate concentrations as high as 32-36 mM. These concentrations of lactate are approximately double the reported threshold concentration of lactate considered to produce cellular necrosis in in vivo models of hypoxic injury, primarily in mature animals. The data raise the possibility that hypoxic injury to differentiating glia can be prevented or ameliorated by increase in glucose availability.

Bilateral hip and femur fractures.

Bilateral hip and femoral fractures are rare. A 26-year-old male with multiple injuries including such fractures was successfully treated with a variety of internal fixation techniques, allowing early mobilization. It is important to consider the possibility of this fracture pattern in high-energy trauma.

Three experimental designs testing orthopedic casting material strength.

Three methods for testing the comparative strength of orthopedic casts have been considered. Two of the tests, three-point-beam-bending and diametral compression, were expected to yield information about "material" strength, and a third simulated an actual cast yielding information about "structural" strength. In one study using these three methods, differences in strength were detected in the material tests, but not in the structural test. Thus, the authors believe that tests of material strength alone are insufficient to establish clinically relevant differences between the strength of orthopedic casts. Adequate testing must include structural tests like the cast-cylinder test described in this article if meaningful and clinically relevant conclusions are to be drawn regarding the comparative strength of the actual cast. Since both the three-point-beam-bending and diametral compression tests produced essentially the same results, both tests are probably not necessary. However, each of these two tests does serve to substantiate the results of the other.

A comparative study of synthetic cast material strength.

The strength of five commercially available synthetic orthopedic casting products are discussed. Using three testing designs, significant differences could be consistently determined. These testing methods provide a means of comparing the various casting products on the market. The authors point out that this study was a comparison of ultimate strength only, and did not compare the other characteristics of the products. It also was noted that all these synthetic casting materials were stronger than plaster-of-Paris casting material.

The effect of hydration water temperature on orthopedic plaster cast strength.

The effect of hydration water temperature (dip water) on the strength of orthopedic casts is discussed. Three methods of testing are used in an attempt to detect any differences and also to assess their clinical relevance. It was found that warmer water produced stronger casts, however, the differences were not found to be clinically relevant. It is suggested that water temperature be chosen on the basis of considerations other than strength.

Intervertebral disc impingement syndrome in a child. Report of a case and suggested pathology.

Nerve root impingement syndrome is rare in children and adolescents. The symptomatology of the pediatric group sometimes differs from that of the adult. An accurate diagnosis is often delayed because of concerns for neoplasm, infection, and spondylolisthesis. A case report of an 11-year-old boy with symptoms and myelographic findings consistent with a herniated disc is presented. A review of the literature disclosed only four cases of herniated disc in children under the age of 12 years. Our case represents one of the youngest patients with documented intervertebral disc impingement syndrome. Rather than a herniation due to degenerative changes in the annulus, a slipping of the entire disc, and vertebral end-plate (a "slipped vertebral apophysis") may precipitate a central disc impingement in children and adolescents. This has been reported in cases of trauma previously. However, the authors think that this slipped vertebral apophysis may, in fact, represent a large number of the so-called herniated discs in adolescents. It is also suggested that this may be an identifiable syndrome with a susceptible age range, activity level and body habitus.

Congenital dislocation of the knees associated with a partial chromosome I deletion.

Congenital dislocation of the knee is a rare disorder that has been reported previously in association with neuromuscular disorders and ligamentous laxity syndromes and is related to mechanical factors. Concurrent chromosomal abnormalities have not been reported previously with congenitally dislocated knees. The authors present a case of congenitally dislocated knees in a patient with an extremely rare and only recently described partial deletion of chromosome I. There have been three previously reported cases of this particular chromosomal deletion, none of which included dislocated joints.

Cement dam. A simple device to prevent cement extrusion.

Cement extrusion through cortical defects during total hip arthroplasty can be prevented by a simple technique utilizing one or more Penrose drains.

Intensity perception. X. Effect of preceding stimulus on identification performance.

This article describes some results on the effect of the preceding stimulus on performance in an intensity-identification experiment with feedback. Based on previous research and our own subjective experiences as listeners, we had expected that sensitivity would increase when the intensities of the preceding and current stimuli were close together. The results of our experiments show, however, that this is not the case: sensitivity is indepenent of the preceding stimulus. The fact that the response variance tends to decrease when the intensities of the preceding and current stimuli are close together is found to be caused solely by sequential effects in response bias. These findings create an interesting puzzle concerning the mechanism by which feedback improves sensitivity in identification. Apparently, the most obvious hypothesis--namely, that the feedback enables the previous stimulus to be used as a standard for judging the current stimulus--is false.