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Biomedical subjects

M W Johnson

Publications and source records attributed to M W Johnson.

153 records · Page 9Linked to original sources
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Surface dose perturbation due to air gap between patient and bolus for electron beams.

The effect of air gaps under bolus material on the surface dose for electron beams is investigated. Dose measurements were performed for air gaps from 0.0 to 3.0 cm and bolus thicknesses of 0.5 and 1.0 cm using the various electron energies and cone sizes available on an electron linear accelerator. Our results show that the surface dose decreases for lower electron energies, smaller field sizes, thicker boluses, and larger air gaps.

Electrons↗
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Peripheral choroidal neovascularization.

Peripheral choroidal neovascularization can result in an elevated subretinal lesion which can simulate a choroidal tumor. We reviewed 8 eyes with 11 peripheral areas of subretinal fluid and exudate which were subsequently determined to be secondary to peripheral choroidal neovascularization. Previous reports of peripheral choroidal neovascularization have emphasized the hemorrhagic nature of these lesions which can simulate a choroidal melanoma. In contrast, turbid subretinal fluid and exudate characterized the majority of peripheral lesions in this series and the majority of these patients were referred with a diagnosis of choroidal metastasis. Clinical examination with fluorescein angiography and echography can effectively distinguish these areas of peripheral choroidal neovascularization from choroidal metastasis.

Aged↗
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Visual field results after vitreous surgery in a case series of patients with open-angle glaucoma.

BACKGROUND AND OBJECTIVE: Visual field loss is a complication of vitrectomy and a concern for patients with glaucoma. Our objective was to determine whether vitrectomy is associated with new field defects in patients with glaucoma. PATIENTS AND METHODS: This retrospective case series involved 7 eyes of 7 patients, who had open-angle glaucoma and underwent vitrectomy for macular hole (2) or epiretinal membrane (5). RESULTS: Acuity improved by at least two lines in 5 eyes. Visual fields showed a change in mean deviation following vitrectomy (mean difference -1.50 dB, P=0.0006, paired Student's t-test). One eye had confirmed progression of a pre-existing visual field defect, but no new defects were apparent in the other eyes. Transient intraocular pressure rises requiring therapy occurred in 3 eyes. One eye developed macular edema, but no other operative complications occurred. CONCLUSIONS: The results of our small case series suggest that vitrectomy may be an acceptable intervention in eyes with glaucoma and co-existing macular problems.

Aged↗
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Electrical properties of compact bone.

Dielectric properties of compact bone tissue have been measured in the wet, i.e., fluid-saturated, state. Comparison of these with other measurements at high relative humidity (RH) shows that the dc conductivity of wet bone is about 100 times larger than that of the high RH sample. Thus, the extrapolation of the high RH results to in vivo situations is not valid. In addition, the results of electrical measurements on dry bone samples cannot be extrapolated to the in vivo state because of the dominance of the fluid-filled pores. The difference in the results for longitudinal, tangential, and radial samples, both in dc resistivity and relaxation time, reflects the difference in connectivity of the pores on bone in these three orientations. Quantitative estimates of the cross-sectional area of connected pores are obtained from measurements on photomicrographs and correlated with dc conductivity of the samples. Further evidence for the dominance of the fluid-filled pores in determining the properties of the tissue comes from the results for bone conductivity g measured as a function of saline conductivity g0. The ratio g/g0 is approximately constant with respect to changes in g0 over a range corresponding to the conductivities of various body fluids. The influence of the dielectric properties in all but destroying the piezoelectrically generated voltage in going from the dry to the wet state is discussed. It is suggested that some mechanism other than the piezoelectric effect (e.g., streaming potentials) must be considered to account for the magnitude and decay time of the electromechanical voltage measured in wet bone. Our studies suggest that fluid transport plays a significant role not only in various aspects of bone metabolism such as mineralization, but also in the electrical, mechanical, and electromechanical properties of bone.

Animals↗
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Regeneration of peri-implant infrabony defects using PerioGlas: a pilot study in rabbits.

PerioGlas is a silicate-based synthetic bone augmentation material that has been used to fill periodontal defects with bonding and integration to both soft tissue and bone. The purpose of this research was to determine the PerioGlas interface with titanium dental implants and bone. Seven live rabbits were used; however, one rabbit was euthanized at 3 days as a result of a tibial fracture through the implant placement site. Each rabbit received four 3.3 x 8 mm Imtec titanium plasma-sprayed dental implants, two in each proximal tibia. One implant in each rabbit was placed in the standard fashion. Two implants in each rabbit had a surgically created defect adjacent to one side of the coronal aspect of the implant. The defect was subsequently filled with PerioGlas. One implant in each rabbit had a surgically created defect that was not filled with PerioGlas. The rabbits were sacrificed at 1, 2, 3, 6, 12, and 24 weeks. Each specimen was prepared for histologic viewing, yielding a nondecalcified specimen demonstrating the interface of bone, implant, and PerioGlas. The results demonstrate peripheral formation of osteoid, followed by bone deposition within the defect from host (surgical margin) bone, toward the implant. The new osteoid and bone form around the PerioGlas particles. Newly formed trabeculae connect these areas of osteoid and new bone around the PerioGlas, interconnecting the PerioGlas particles. The new bone eventually reaches the implant, and osseointegration occurs with incorporation of the PerioGlas particles.

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