Piezoelectric and mechanical properties in bovine cornea

The piezoelectric coefficient (d(31)) and Young's modulus (E) were investigated as a function of degree of hydration for bovine cornea. The piezoelectric and mechanical responses observed were anisotropic, and d(31) decreased, whereas E increased with decreasing the degree of hydration. The anisotropic mechanical and electromechanical properties observed seem to be caused by oriented crystalline collagen fibrils. In addition, the loss of water molecules appears to decrease crystallinity (of the collagen) in the cornea. With dehydration of the cornea, a reduction in crystallinity and changes in hydrogen bonding were observed by wide-angle X-ray diffraction and Fourier transform infrared measurements. The decrease of piezoelectricity in cornea during dehydration is most likely caused by the increase in modulus and the loss of order to a nonpiezoelectric phase in the collagen.

Symptomatic aniseikonia in unilateral and bilateral pseudophakia. A projection space eikonometer study

PURPOSE

To determine the incidence of aniseikonia and aniseikonic symptoms in pseudophakia (intraocular lens implantation after cataract extraction).

METHOD

Aniseikonia, stereoacuity, and clinical symptoms were evaluated in patients with unilateral and bilateral pseudophakia. Aniseikonia was determined with the Essilor projection space eikonometer. Eight-seven cases were studied, fifty-four of unilateral pseudophakia and thirty-three of bilateral pseudophakia. Nine of the 87 patients had insufficient stereopsis to be examined or measured for aniseikonia with the Essilor instrument, and were studied with the "Double-D", a two dimensional "direct comparison eikonometer" device programmed and viewed on a computer. Stereoacuity was determined with the Titmus Stereo Test.

RESULTS

Thirty-five (40.2%) of all pseudophakes had ophthalmic complaints referable to aniseikonia. The mean aniseikonia in the unilateral pseudophakia group was 4.1%, SD=3.41, while the mean aniseikonia in the bilateral group was 3.2%, SD=2.6.

CONCLUSION

Symptomatic aniseikonia is common in pseudophakia. Unilateral pseudophakia has the higher degrees of aniseikonia, and is at a greater risk of loss of binocularity and of asthenopia.

A study of aniseikonia and Knapp's law using a projection space eikonometer

PURPOSE

Knapp's Law, which states that anisometropias due to varying abnormal axial lengths between eyes would not result in inequality in relative retinal image size, provided the correcting spectacle lens was placed at the far point of the eye, has been shown to fall short in clinical practice in several studies using "direct comparison eikonometry". To test these findings using space eikonometry and to further elucidate this Law's clinical applicability, the following study was conducted.

METHODS

Thirteen patients with suspected axial anisometropia of at least 4 Diopters were identified, selected and examined. Cycloplegic refraction, A-scan ultrasonic ocular biometry and Essilor Projection Space Eikonometry were performed.

RESULTS

Ten of the thirteen patients had their anisometropia due primarily to ocular axial length differences. Of these ten, seven (70%) had detectable levels of aniseikonia and 3 (30%) demonstrated no aniseikonia. The other three patients whose aniso-metropia was due to combined axial and refractive components, all had aniseikonia.

CONCLUSION

As a geometric optics theory, Knapp's Law stands on its own merits. However, in clinical practice, reduction in retinal element density in high myopia limits its applicability. Such patients often do have significant aniseikonia which can produce ocular referable complaints and/or interfere with binocular vision.

Aniseikonia in relation to strabismus, anisometropia and amblyopia

PURPOSE

To study the interrelationships among these four entities which are critical to binocular vision and its precision.

SUBJECTS AND METHODS

102 selected patients (for their ability to have stereoscopic depth perception, a requisite for space eikonometry) were evaluated. Patient testing included stereoscopic testing, Essilor Projection Space Eikonometry, ultrasonic echographic axial length measurements and orthoptic evaluation. Aniseikonia was measured on the Essilor Projection Space Eikonometer.

RESULTS

1. Anisometropia alone was correlated with a marked increase in amblyopia, a moderate increase in aniseikonia and no noteworthy increase in strabismus. Statistical analysis (chi square ratio) showed that persons with elevated anisometropic values had a 4.4 fold increased risk of aniseikonia (p=.003). 2. Aniseikonia alone was not responsible for marked variations in strabismus. 3. Amblyopia was correlated with increases in anisometropia and aniseikonia. 4. Adding aniseikonia to anisometropia produced a possible increase in strabismus and a great increase in amblyopia (using Fisher's Exact Test, 2-tailed). 5. Spearman correlations of the "absolute values" (the mean of the mathematical difference between the two eyes of anisometropia and amblyopia) were as follows: anisometropia (abs) vs. aniseikonia r=.294, p=.006; anisometropia (abs) vs. amblyopia (abs) 4=.555, p=<.001; amblyopia (abs) vs. aniseikonia r=.234, p=.02.

CONCLUSIONS

Aniseikonia per se does not appear to have a major causal role in amblyopia or strabismus, but anisometropia does for amblyopia. This role is greatly augmented by aniseikonia and this combination may then produce strabismus.

Aniseikonia quantification: error rate of rule of thumb estimation

PURPOSE

To find the error rate in quantifying aniseikonia by using "Rule of Thumb" estimation in comparison with proven space eikonometry.

METHODS

Study 1: 24 adult pseudophakic individuals were measured for anisometropia, and astigmatic interocular difference. Rule of Thumb quantification for prescription was calculated and compared with aniseikonia measurement by the classical Essilor Projection Space Eikonometer. Study 2: parallel analysis was performed on 62 consecutive phakic patients from our strabismus clinic group.

RESULTS

Frequency of error: For Group 1 (24 cases): 5 ( or 21 %) were equal (i.e., 1% or less difference); 16 (or 67% ) were greater (more than 1% different); and 3 (13%) were less by Rule of Thumb calculation in comparison to aniseikonia determined on the Essilor eikonometer. For Group 2 (62 cases): 45 (or 73%) were equal (1% or less); 10 (or 16%) were greater; and 7 (or 11%) were lower in the Rule of Thumb calculations in comparison to Essilor eikonometry. Magnitude of error: In Group 1, in 10/24 (29%) aniseikonia by Rule of Thumb estimation was 100% or more greater than by space eikonometry, and in 6 of those ten by 200% or more. In Group 2, in 4/62 (6%) aniseikonia by Rule of Thumb estimation was 200% or more greater than by space eikonometry.

CONCLUSION

The frequency and magnitude of apparent clinical errors of Rule of Thumb estimation is disturbingly large. This problem is greatly magnified by the time and effort and cost of prescribing and executing an aniseikonic correction for a patient. The higher the refractive error, the greater the anisometropia, and the worse the errors in Rule of Thumb estimation of aniseikonia. Accurate eikonometric methods and devices should be employed in all cases where such measurements can be made. Rule of thumb estimations should be limited to cases where such subjective testing and measurement cannot be performed, as in infants after unilateral cataract surgery.

Piezoelectric response of scleral collagen

The piezoelectric coefficients (d31) for a number of bovine and human scleral collagen samples were determined as a function of drying time at room temperature. The measured values of d31 decreased with drying time. There were significant differences in the values of the d-coefficient between the human and bovine eyes as well as in the values obtained from different regions of the eye.

Prediction of late enophthalmos by volumetric analysis of orbital fractures

The purpose of this study was to determine whether orbital volume assessment by computerized tomography (CT) could provide additional information for the initial evaluation of orbital blowout fractures and guide optimal treatment. The medical records of 30 patients with orbital blowout fractures, either surgically or conservatively managed, were retrospectively reviewed. Orbital volumetric analysis was then determined from digitized CT scans. Fracture-related volume expansion relative to the unaffected fellow orbit was correlated with motility deficits and location and degree of enophthalmos. Early Hertel's measurements (< 4 weeks) were available in 21 patients and did not correlate with the computer volumetric values or with subsequent late enophthalmos. Late Hertel's measurements (> 4 weeks) were obtained in 13 of 15 nonrepaired fractures and in 5 of 15 surgically repaired patients (late presentation; 18 patients). When seen at more than 4 weeks, 11 (92%) of 12 patients with > or = 13% orbital volume expansion manifested significant enophthalmos (> 2 mm) compared with 1 (17%) of 6 patients with < 13% orbital expansion (p = 0.004). Fractures presenting with enophthalmos on initial examination had extensive medial wall involvement in addition to the floor fracture (p = 0.003). CT measurements of orbital volume can predict the final degree of late enophthalmos and may facilitate the planning of surgical intervention.

Cyclic changes in conjunctival smears from menstruating females

Cytohormonal evaluation has become an accurate procedure for the assessment of estrogenic effect on the vaginal and buccal mucosa. Both show a cyclical variation in the maturation index (an "estrogenic value" based on epithelial maturation). Conjunctival smears were taken from the temporal conjunctiva of 11 premenopausal and 7 postmenopausal females on a near-daily basis. No females had evidence of ocular surface disease, including keratoconjunctivitis sicca. After fixation and Papanicolaou staining, maturation indices for each specimen based on the differential morphology were red in a single blind fashion by a Papanicolaou cytologist. Cyclical variations paralleling the menstrual cycle in eight of nine premenopausal women were noted with maturation index values ranging from 30 (low estrogen effect) to 82 (high estrogen effect), peaking on day 11 or 21 of the menstrual cycle. No cyclical change was found in postmenopausal smears and smears of subjects with endocrine abnormalities. Maturation index values ranged from 30 to 38. Conjunctiva appears to be an estrogen-sensitive epithelium.

A fiberoptic fixation target for tonography

Substituting a fiberoptic core for the standard Müller tonography plunger provides a fixation target for tonography. The fiberoptic is particularly useful for testing patients who are difficult to examine with the conventional plunger (those with monocular vision, phoriatropia, and the like).

The inverse Marcus Gunn phenomenon. An electromyographie contribution

A patient with the inverse Marcus Gunn anomaly is presented. Electromyographic studies showed the salient fact that the closing of the involved eye on opposite motion of the lower jaw was not mediated by seventh nerve stimulation of the orbicularis oculi but by inhibition of the homolateral levator palpebrae superioris. As a consequence of this observation, a specific and effective operation could be planned.

The ophthalmologist and the reading problem

Images