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Biomedical subjects

Charles W McMonnies

Publications and source records attributed to Charles W McMonnies.

10 recordsLinked to original sources
PubMed2006

Biomechanically coupled curvature transfer in normal and keratoconus corneal collagen.

PURPOSE: To examine the hypotheses that a "pushed" biomechanically coupled intrafibrillar transfer of curvature may be involved in corneal warpage (pseudo cone) development, which is observed with superior cornea-bearing rigid contact lens fittings, and that a "pulled" biomechanically coupled intrafibrillar transfer of curvature may be involved in keratoconus cone development. METHODS: The mechanical properties of corneal stromal fibrils have been reviewed to examine the histologic basis for fibril inextensibility and the associated potential for them to demonstrate biomechanical coupling. DISCUSSION: The hypothesis that apical clearance rigid contact lenses may promote pushed biomechanical responses that aid cone development in some normal or keratoconic corneas is examined. CONCLUSIONS: Stromal fibrils that run limbus to limbus and have circumferential stability at the limbus, appear to be relatively inextensible: a finding that is consistent with the need for stable refraction. The property of inextensibility could also support intrafibrillar biomechanically coupled curvature transfer. For example, in some normal corneas, definite apical clearance fittings may be associated with biomechanically coupled push forces generated by midperipheral bearing that squeeze the central cornea to greater curvature. A disease-related biomechanically coupled pull mechanism for cone formation may be accelerated by an apical clearance fitting push mechanism, with additional synergy generated by subatmospheric pressure in the postlens tear pool. These fitting-related mechanisms appear likely to increase with the degree of apical clearance. Cone formation in keratoconus and models of external forces acting on the cornea (tonometry and contact lens-related) are mathematically analyzed for an idealized one-dimensional elastic model of a corneal stromal fibril. The classic theory of "elastic rods" is adopted, and despite the nonlinear nature of the differential equations involved, a method of dealing with biomechanically coupled curvature transfer in an "exact" manner is described and used to model the related clinical phenomena.

Animals↗
PubMed2005

The biomechanics of keratoconus and rigid contact lenses.

PURPOSE: To examine aspects of the genesis and progression of keratoconus through an analysis of the biomechanical forces associated with this condition, including those generated by rigid contact lenses. METHODS: The biomechanics of applanation tonometry and rigid contact lenses serve as a basis for examining contact lens adherence and the potential for contact lens induced mid peripheral corneal applanation and apical molding with apical clearance fittings. DISCUSSION: Some physiologic and pathologic mechanisms for increases in intraocular pressure are reviewed. The possibility is raised that hard squeeze blinks, for example, could significantly increase the distending forces that bear on the corneal apex in keratoconus and that some apical clearance contact lens fittings could increase the risk of progressive ectasia. Support for these hypotheses is found among healthy eyes and eyes with keratoconus that show increased curvature when apical clearance fittings are worn. CONCLUSIONS: The known risk of scarring responses to excessively flat fitting rigid contact lenses must be balanced against the possible risk of molding and ectasia advancement responses to tight definite apical clearance lenses. Fittings within the range of minimal apical clearance and minimal apical contact (divided support) may be the most appropriate. The appearance and performance of these fittings may be difficult to distinguish clinically. Because some patients may develop pathologic levels of intraocular pressure with vigorous eye rubbing, strong squeeze blinks, inverted body positions, and strenuous muscular effort, patients with, or at risk for, keratoconus, glaucoma, or progressive myopia should be advised of the possible adverse significance of these activities.

Biomechanical Phenomena↗
PubMed2004

Keratoconus fittings: apical clearance or apical support?

PURPOSE: To examine the relative merits of apical support and apical clearance fitting of rigid gas-permeable contact lenses for keratoconus. METHODS: After an historic review of fitting approaches for keratoconus, a case report is described in which an adventitious apical clearance fitting for early keratoconus might have been associated with accelerated progress of the ectasia. DISCUSSION: The hypothesis that apical clearance fittings increase the risk of accelerating ectasia progression in early keratoconus is examined in counterpoint to the hypothesis that apical support fittings increase the risk of apical scarring. Reference is made to the responses of normal corneas to apical clearance fitting and to apical contact fittings used in orthokeratology fittings. The tendency for corneas to mold to contact lens curvature is reviewed. The possibility that reduced corneal thickness or tissue softening and associated changes to the biomechanical properties of the cornea in keratoconus may facilitate molding with apical clearance fitting is examined. CONCLUSIONS: Known and putative risk factors for fitting complications that are associated with apical clearance and apical touch contact lens fitting are given as a basis for the reader to draw conclusions about the management of contact lens fitting for keratoconus. The possibility of symptomless adverse responses is a strong indication for frequent routine aftercare reviews.

Child↗
PubMed2003

Keratoconus, allergy, itch, eye-rubbing and hand-dominance.

BACKGROUND: One hypothesis for the pathogenesis of keratoconus includes teenage allergy, ocular itch and associated eye-rubbing. METHODS: This study examined the prevalence of these factors for teenage and adult patients. The results for a sample of 53 subjects with bilateral keratoconus were compared with those for a control sample of non-keratoconus subjects, who also routinely wore RGP contact lenses. The strongest dominant hand and the eye with more advanced keratoconus were also determined, to examine for a relationship between them. RESULTS: The keratoconic sample reported significantly higher levels of allergy, itch and rubbing as teenagers and as adults. However, all distributions were bimodal, consistent with the hypothesis that allergy, itch and rubbing are relevant in the pathogenesis of keratoconus only when the highest levels of these factors are present. For example, a significant relationship between the stronger dominant hand and the more advanced eye was evident only in subjects who reported the most severe rubbing. CONCLUSIONS: This finding adds weight to the circumstantial evidence that rubbing contributes to the pathogenesis of keratoconus. Low levels of teenage rubbing by some keratoconic subjects suggest a non-rubbing pathogenesis and that emphasis on rubbing management is not warranted in these cases. However, high levels of adult rubbing reported by many keratoconic subjects indicate that the standard advice to avoid vigorous and prolonged rubbing is often not effective, even when repeated. There appears to be an indication for the need to improve the management of eye-rubbing for some patients with keratoconus or at risk of developing this disease.

Adolescent↗
PubMed2001

Control of chart memory for retesting acuity.

BACKGROUND: Enhancement of visual acuity has been demonstrated when retesting takes place within 24 hours and appears to be due to chart memory. Significant chart letter subset memory has been found after a single acuity measurement and can still be present 10 days after an examination involving several acuity measurements. Control of chart memory may increase the precision of acuity measurement. METHOD: A single acuity measurement was used to establish chart letter subset memory. Retroactive interference methods for the inhibition of chart letter memory have been compared using a letter recognition memory test. The experimental methods used to promote forgetting included a spelling backward exercise and near acuity assessment using a contra-chart. The contra-chart is constructed from the 16 letters not involved in the distance chart that was used to establish chart memory. RESULTS: Assessment of near vision using a contra-chart was found to inhibit chart letter memory. CONCLUSIONS: The contra-chart appears to have research and clinical applications that require control of measurement imprecision due to chart memory. Contra-charts obviate the need to use equivalent charts, which have the disadvantages of being difficult to use and control only for memory of letter sequences. The contra-chart can be included easily in an examination protocol for research or in a clinical examination sequence for which it will have the additional purpose of providing a guide to near vision performance.

Journal Article↗
PubMedDate not supplied

Chart memory and visual acuity measurement.

BACKGROUND: A small, but statistically significant, improvement has previously been demonstrated when retesting visual acuity within 24 hours. Because letter chart testing procedures are familiar to most adult patients, this improvement may be due to memory of the chart letter subset, rather than increased familiarity with the test procedure. Chart letter subset memory does not appear to have previously been measured directly. METHODS: A letter recognition task was used to measure chart letter subset memory for groups of routine clinical patients. Comparisons were made between assessments after a single acuity measurement and assessments at increasing intervals after a routine examination involving several references to the same letter chart for both eyes. RESULTS: The recognition score for the 'one-minute-after a single acuity measurement group' was an average of 2.5 letters (range 0.7 to 4.7). Low levels of chart letter subset memory were found to persist for 10 days. CONCLUSION: The chart letter subset memory found is consistent with enhanced acuity scores that occur on retesting. Scores appear likely to have a greater chance of enhancement for a second eye measurement when it follows immediately after the first. Similarly, scores at the end of a full examination may be enhanced by memory that is accumulated during multiple measurements for both eyes using the same chart. Chart memory may be acquired when acuity is measured frequently in monitoring the progress of disease. Chart memory, or its loss over time, may contribute to measurement imprecision and associated expansion of the confidence limits for significant change in acuity.

Journal Article↗