Towards dark current suppression in metallic photocathodes by selected-area oxidation.

Oxide-free surfaces of polycrystalline Cu are prepared using acetic acid etching after chemical-mechanical polishing. UV ozone treatment is shown to increase the work function of the cleaned Cu by up to 0.5 eV. There is also a large reduction in quantum efficiency at 265 nm. Cu sheet can be easily masked from ozone exposure by Si or glass, meaning that selected-area oxi-dation is possible. Oxygen plasma treatment has a similar effect to the UV ozone but is more difficult to mask. There is no increase in surface roughness after oxidation, meaning that the larger work function could significantly re-duce dark current in accelerator photocathodes without affecting the desired photoemission region.

Spatial regularity of InAs-GaAs quantum dots: quantifying the dependence of lateral ordering on growth rate.

The lateral ordering of arrays of self-assembled InAs-GaAs quantum dots (QDs) has been quantified as a function of growth rate, using the Hopkins-Skellam index (HSI). Coherent QD arrays have a spatial distribution which is neither random nor ordered, but intermediate. The lateral ordering improves as the growth rate is increased and can be explained by more spatially regular nucleation as the QD density increases. By contrast, large and irregular 3D islands are distributed randomly on the surface. This is consistent with a random selection of the mature QDs relaxing by dislocation nucleation at a later stage in the growth, independently of each QD's surroundings. In addition we explore the statistical variability of the HSI as a function of the number N of spatial points analysed, and we recommend N > 103 to reliably distinguish random from ordered arrays.

Interaction of Mn with GaAs and InSb: incorporation, surface reconstruction and nano-cluster formation.

The deposition of Mn on to reconstructed InSb and GaAs surfaces, without coincident As or Sb flux, has been studied by reflection high energy electron diffraction, atomic force microscopy and scanning tunnelling microscopy. On both Ga- and As-terminated GaAs(0 0 1), (2 × n) Mn-induced reconstruction domains arise with n = 2 for the most well ordered reconstructions. On the Ga-terminated (4 × 6), the Mn-induced (2 × 2) persists up to around 0.5 ML Mn followed by Mn nano-cluster formation. For deposition on initially ß2(2 × 4)-reconstructed GaAs(0 0 1), the characteristic trench structure of the reconstruction is partially preserved even beyond 1 monolayer Mn coverage. On both the ß2(2 × 4) and c(4 × 4) surfaces, MnAs-like nano-clusters form alongside the reconstruction changes. In contrast, there are no new Mn-induced surface reconstructions on InSb. Instead, the Sb-terminated surfaces of InSb (0 0 1), (1 1 1)A and (1 1 1)B revert to reconstructions characteristic of clean In-rich surfaces after well defined coverages of Mn proportional to the Sb content of the starting reconstruction. These surfaces are decorated with self-assembled MnSb nanoclusters. These results are discussed in terms of basic thermodynamic quantities and the generalized electron counting rule.

Depth-dependent magnetism in epitaxial MnSb thin films: effects of surface passivation and cleaning.

Depth-dependent magnetism in MnSb(0001) epitaxial films has been studied by combining experimental methods with different surface specificities: polarized neutron reflectivity, x-ray magnetic circular dichroism (XMCD), x-ray resonant magnetic scattering and spin-polarized low energy electron microscopy (SPLEEM). A native oxide ∼4.5 nm thick covers air-exposed samples which increases the film's coercivity. HCl etching efficiently removes this oxide and in situ surface treatment of etched samples enables surface magnetic contrast to be observed in SPLEEM. A thin Sb capping layer prevents oxidation and preserves ferromagnetism throughout the MnSb film. The interpretation of Mn L(3,2) edge XMCD data is discussed.

Verifying and evaluating progressive addition lenses in clinical practice.

BACKGROUND: Despite the fact that more than 50% of multifocal lenses dispensed in the United States are progressive addition lenses, adequate methods for clinical verification of these lenses have been lacking. Using automated lens meter techniques, the author describes a simplified method for verification of these complex lenses. METHODS: Thirty pairs of progressive lenses were measured in a modified method using a Humphrey 330 Lens Analyzer. Fifteen pairs were "premium-quality" progressive lenses: fifteen pairs were "non-premium-quality" progressives. Five criteria were assessed on each lens: Distance Zone Width (DZW). Intermediate Zone Width (IZW), Near Zone Width (NZW), Drop Distance (DD), and Maximum Astigmatic Distortion (MAD). RESULTS: "Premium-quality" progressive lenses failed to demonstrate clear-cut superiority over "non-premium-quality" progressive lenses in the five specified criteria. Individual measurements indicate considerable product inconsistency affected every brand tested. CONCLUSIONS: Premium- and non-premium-quality progressive lenses demonstrated similar performance characteristics in this study. Zone size variation in these lenses was found to be considerable, a characteristic that seemed to cut across brand lines. The AO Compact lens seemed to demonstrate a shorter drop distance than other lenses, which does enhance its suitability for use with small frames. A comparison of the Essilor Natural PAL to the Younger Image lens showed little difference in the categories measured, although peripheral distortions seemed closer to the reading zone in the image. A comparison of the MAD of lenses in this study to lenses tested in 1986 indicates a considerable improvement has been made in that important characteristic.

Cranial nerve palsy as a complication of operative traction.

STUDY DESIGN: Case report. OBJECTIVE: This report documents one case of diplopia from abducens (sixth cranial) nerve palsy after spinal surgery using a Jackson table and cranial traction. SUMMARY OF BACKGROUND DATA: Cranial nerve deficits have frequently been described in the orthopedic literature after trauma, halo pelvic traction, and halo skeletal fixation. The theorized mechanism of injury to the abducens nerve involves stretch or traction force, which causes localized ischemia or a change in nerve position. An extensive literature search failed to show this type of injury using Gardner-Wells tongs in conjunction with the Jackson table. METHODS: This is a case report that included a chart review, examination of the patient, and a literature search. RESULTS: The patient had complete spontaneous resolution of abducens nerve dysfunction within 6 months. CONCLUSIONS: It is important for the surgeon to be aware of this potential complication and to inform patients who have diplopia that develops from abducens nerve palsy that most of these cranial nerve deficits spontaneously improve.

Newer management options in patients with spinal metastasis.

For some patients with spinal metastasis and spinal cord compression, newer surgical techniques are better than laminectomy or radiotherapy alone in relieving pain and restoring function. While radiotherapy remains the standard for spinal metastases due to myeloma, lymphoma, and many types of adenocarcinoma, proper surgical treatment can significantly improve function and outcome in selected patients.

Ischaemic optic neuropathy after spinal fusion.

PURPOSE: We report a case of ischaemic optic neuropathy which occurred after prolonged spine surgery in the prone position in an obese, diabetic patient. CLINICAL FEATURES: The patient was a 44-yr-old, 123 kg, 183 cm man for decompressive laminectomy and instrumented fusion of the lumbar spine. Anaesthesia was induced with thiopentone, fentanyl and succinylcholine and maintained with nitrous oxide, oxygen, isoflurane and a fentanyl infusion. He was positioned prone on the Relton-Hall frame and had an uneventful intraoperative course. Estimated blood loss was 3,000 ml. He was taken to the surgical intensive care unit (SICU) and the trachea was extubated 3.5 hr later. He had no pulmonary or haemodynamic problems and went to a regular nursing floor in the morning. He was discharged home on postoperative day #5. He telephoned his surgeon on postoperative day #7 to say that his vision had been blurry since surgery. His visual acuity was decreased, and on examination, he had a bilateral papillary defect, optic swelling and a splinter haemorrhage in the right eye. Magnetic resonance imaging (MRI) scan of the head and orbits detected no other abnormality. Based on this examination, he was felt to have bilateral ischaemic optic neuropathy and treated conservatively. By postoperative day #47, his visual acuity was greatly improved and near normal. Careful review of possible contributing factors suggests that the cause of the ischaemic optic neuropathy was venous engorgement. CONCLUSION: This patient developed ischaemic optic neuropathy from a prolonged interval in the prone position of the Relton-Hall frame, which may be related to venous engorgement.

Spinal anesthesia for elective lumbar spine surgery.

STUDY OBJECTIVE: To evaluate a large series of elective lumbar spine surgical procedures by a single surgeon whose patients were all offered spinal anesthesia. DESIGN: Retrospective chart review. SETTING: Tertiary-care teaching hospital. MEASUREMENTS AND MAIN RESULTS: The records of all elective lumbar spine procedures between 1984 and 1995 performed by one surgeon (GRB) were obtained, and 803 were identified. Of those 803 patients, 611 accepted spinal anesthesia. Data collected included patient demographics, details of the spinal and general anesthesia, perioperative complications, and impact of the spinal anesthetic options on the outcome of spinal anesthesia. General and spinal anesthesia patients were comparable for age, gender, height, and ASA physical status. Patients who received spinal anesthesia were significantly heavier than the general anesthesia patients. Among perioperative complications, nausea and deep venous thrombosis occurred significantly more often in the general than spinal anesthesia patients. Mild hypotension and decreased heart rate (HR) were the most common hemodynamic changes with spinal anesthesia, whereas hypertension and increased HR were the result of general anesthesia. Among spinal anesthetic drugs, plain bupivacaine was associated with the lowest incidence of supplemental local anesthetic use intraoperatively compared to hyperbaric bupivacaine or hyperbaric tetracaine. CONCLUSION: Spinal anesthesia is an effective alternative to general anesthesia for lumbar spine surgery and has a reduced rate of minor complications.

The surgical management of patients with rheumatoid cervical spine disease.

Rheumatoid arthritis is an inflammatory disease that commonly affects the cervical spine. This article reviews the incidence of spinal involvement, the clinical manifestations of spinal disease, and the types of instability patterns that may occur and their radiographic diagnosis. Appropriate guidelines for obtaining imaging studies and for surgical referral is also described. Finally, the role of surgical stabilization is discussed.

Surgical treatment of spinal tumors.

The surgical treatment of spinal tumors depends on a host of factors that include: the type of tumor and its location within the spine, the presence or absence of neural compression, the portion of the vertebrae involved, the anticipated mode of spinal failure, the biology of the tumor, the anticipated life expectancy of the patient, and the type of prior or subsequent adjuvant treatment. Two thirds of all spinal tumors arise from the vertebral body and only 1/3 originate from the posterior elements. Malignant tumors more commonly involve the vertebral body and benign lesions usually are located posteriorly. Malignant tumors, because of their aggressive nature and their propensity for anterior vertebral body involvement, are associated with a higher incidence of neurologic deficit than are benign lesions. The surgical treatment of spinal tumors is dictated largely by the location of the tumor within the spine; anterior vertebral body tumors generally should be approached anteriorly, whereas posterior lesions should be approached posteriorly. Because most malignant tumors, whether primary or secondary (metastatic), are located anteriorly within the vertebral body, most surgery for malignant tumors should be approached anteriorly. Anterior decompression should be accompanied by reconstruction with biologic materials such as autogenous bone graft unless life expectancy is certain to be very limited (<6 months).

Calcium pyrophosphate dihydrate crystal deposition disease as a cause of lumbar canal stenosis.

STUDY DESIGN: This study measured the incidence of calcium pyrophosphate dihydrate crystal deposition in specimens of ligamenta flava in consecutive patients undergoing decompressive laminectomy between 1984 and 1991. The results were compared to determine the difference between calcium pyrophosphate dihydrate-negative patients with lumbar canal spinal stenosis. OBJECTIVES: The results were compared with cadaver specimens and literature values to determine if calcium pyrophosphate dihydrate crystal deposition disease contributes to the thickening of the ligamentum flavum and thereby contributes to spinal stenosis. SUMMARY OF BACKGROUND DATA: Calcium pyrophosphate dihydrate crystal deposition disease has been described in the axial skeleton. Hypertrophy of the ligamentum flavum has been suggested to contribute to stenosis. The association of calcium pyrophosphate dihydrate disease and hypertrophied ligamenta flava has not been fully defined nor linked to neurologic symptoms and signs. METHODS: The incidence of calcium pyrophosphate dihydrate crystal deposition in specimens of ligamenta flava obtained from four groups was measured: specimens obtained during surgery from 102 consecutive patients undergoing decompression laminectomy between 1984 and 1991, 47 additional pathologic specimens of ligamentum flavum tested between 1984 and 1991, 222 calcium pyrophosphate dihydrate-positive Pathology Department specimens collected between 1980 and 1991, and, as control specimens from 20 cadavers. The associated patient histories were reviewed for the first two groups; no histories were available for the cadaver group. RESULTS: The incidence of calcium pyrophosphate dihydrate crystal deposition was 24.5% in the ligamentum flavum among the surgical patients, 31% among the Pathology Department specimens, 33.8% among the calcium pyrophosphate dihydrate-positive Pathology Department specimens, and 5% among the cadavers. No associated medical conditions with calcium pyrophosphate dihydrate crystal deposition were found among the medical histories. Patients with the symptoms of spinal stenosis who were also calcium pyrophosphate dihydrate-negative patients with symptoms of less than 6 months' and less than 24 months' duration (P < 0.001). Except for time to presentation, calcium pyrophosphate dihydrate-negative patients had similar signs and symptoms of lumbar canal spinal stenosis. Having previous spine surgery did not produce a statistically significant risk of having calcium pyrophosphate dihydrate crystal deposition. No specific laboratory tests were found to be of predictive value. CONCLUSIONS: These findings suggest that calcium pyrophosphate dihydrate crystal deposition may indeed be associated with the thickening of the ligamentum flavum, if so, patients may benefit from medical treatment before undergoing surgical treatment of lumbar canal spinal stenosis.

Application of frameless stereotaxy to pedicle screw fixation of the spine.

Interactive frameless stereotaxy has been successfully applied to intracranial surgery. It has contributed to the improved localization of deep-seated brain lesions and has demonstrated a potential for reducing both operative time and morbidity. However, it has not been as effectively applied to spinal surgery. The authors describe the application of frameless stereotactic techniques to spinal surgery, specifically pedicle screw fixation of the lumbosacral spine. Preoperative axial computerized tomography (CT) images of the appropriate spinal segments are obtained and loaded onto a high-speed graphics supercomputer workstation. Intraoperatively, these images can be linked to the appropriate spinal anatomy by a sonic localization digitizer device that is interfaced with the computer workstation. This permits the surgeon to place a pointing device (sonic wand) on any exposed spinal bone landmark in the operative field and obtain multiplanar reconstructed CT images projected in near-real time on the workstation screen. The images can be manipulated to assist the surgeon in determining the proper entry point for a pedicle screw as well as defining the appropriate trajectory in the axial and sagittal planes. It can also define the correct screw length and diameter for each pedicle to be instrumented. The authors applied this device to the insertion of 150 screws into the lumbosacral spines of 30 patients. One hundred forty-nine screws were assessed to be satisfactorily placed by postoperative CT and plain film radiography. In this report the authors discuss their use of this device in the clinical setting and review their preliminary results of frameless stereotaxy applied to spinal surgery. On the basis of their findings, the authors conclude that frameless stereotactic technology can be successfully applied to spinal surgery.

The anterior approach to the cervical spine.

This article focuses on the indications for anterior cervical surgery, relevant surgical anatomy, a brief description of the technique of anterior surgery followed by a discussion of results and complications. It is hoped that the reader will achieve a better understanding for the clinical syndrome of cervical disc degeneration and be better able to put into proper perspective the role of diagnostic imaging in the overall evaluation of the patient.

Biomechanical considerations of high myopia: Part II--Biomechanical forces affecting high myopia.

Assuming the possibility of a scleral pathogenesis hypothesis for the development of high myopia, an examination of biomechanical forces affecting myopic eyes is presented. The biomechanical considerations include Laplace's law applied to the eye, balloon expansion dynamics, and the "porthole" effect derived from mechanical engineering principles. Other biomechanical considerations include Coleman's pressure head theory of accommodation, extraocular muscle stresses, and the role of uveoscleral outflow. Balloon expansion dynamics cast doubt on the common usage of Laplace's law governing fluid filled spheres for estimating coat stresses in myopia. Coleman's theory of accommodation, featuring an accommodation induced pressure head between the anterior and vitreous chambers is reanalyzed. Experimental testing of this hypothesis is still lacking. Uveoscleral outflow computations show that the vitreous chamber distension of high myopia may dramatically increase the importance of uveoscleral outflow in the pressure dynamics of high myopia. These biomechanical force phenomena clearly provide fertile ground for several areas of research that could illuminate the great puzzles of myopia.

Biomechanical considerations of high myopia: Part I--Physiological characteristics.

Prior to a discussion of biomechanical considerations in high myopia, a review of anatomical and physiological patterns in high myopia is presented. A scleral pathogenesis or mesodermal hypothesis of the anomaly is presented with the inclusion of supporting evidence. Retinal, choroidal, and vitreous malformations in high myopia also are highlighted. Notable is a pattern consistent with the concept of gradual stretching of the vitreous chamber. The association of high myopia with other diseases is shown, with special attention being given to the concomitant features of high myopia and glaucoma. A discussion of animal deprivation studies examines their relevance to cases of clinical myopia. The dopamine deficit found in the retinas of visually deprived animals appears to be an area that will attract considerable future research. However, there may be no connection of these experimental findings in deprived animals to most cases of high myopia.

Biomechanical considerations in high myopia: Part III--Therapy for high myopia.

Clinical procedures for the treatment of high myopia are updated in this third part of a report on high myopia. After a discussion of testing techniques and refractive approaches for high myopia, an examination of therapeutic drug experiences shows their relative merits. The author believes that more investigation of beta blocker and epinephrine topical solutions is needed pertaining to the treatment of high myopia. Theoretically, such agents could improve deficient arterial perfusion of the highly myopic eye, and could be helpful in retarding scleral creep. Lifestyle recommendations include patient advice on accommodative reduction and proper exercise techniques. Low impact aerobics may have merit for highly myopic patients since the improved cardiovascular efficiency they can provide may improve the deficient arterial perfusion of their retinas. A summary of the three-part report on high myopia concludes the article. The physiological patterns of high myopia dovetail into the biomechanical considerations showing that a scleral pathogenesis hypothesis of myopic development is a viable working theory. The author's clinical procedures reflect the influence of that theory. If research were redirected from deprivation studies to investigating biomechanical considerations and pharmacological approaches to high myopia, the author contends that dramatically improved treatment regimens could result.

Diagnosis of nerve root compression. Myelography, computed tomography, and MRI.

Current technology and published studies support the preferential use of MRI in the diagnosis of cervical myelopathy. In cervical radiculopathy, data support the use of either contrast-enhanced CT or MRI. We prefer MRI as the initial diagnostic test because of the obvious advantage of its noninvasive nature. Similar to myelography, MRI permits visualization of the entire cervical canal and spinal cord. Unlike routine CT, MRI visualizes the spinal cord and nerve roots in two planes and is better in detecting unsuspected pathology at other cervical segmental levels. Because the quality of MRI scans in the cervical spine is subject to more variation than in the lumbar spine, anticipated pathology is sometimes not clearly visualized by cervical MRI. Under such circumstances, we obtain a water-soluble myelogram followed by contrast-enhanced CT. Because inaccurate radiographic diagnosis is one of the identifiable causes of poor outcome in surgical therapy for cervical spondylosis, accurate visualization of pathology in terms of its location, nature, and extent is imperative before performing decompressive cervical spine surgery.