Monovision: a review

In presbyopia, patients can no longer obtain clear vision at distance and near. Monovision is a method of correcting presbyopia where one eye is focussed for distance vision and the other for near. Monovision is a fairly common method of correcting presbyopia with contact lenses and has received renewed interest with the increase in refractive surgery. The present paper is a review of the literature on monovision. The success rate of monovision in adapted contact lens wearers is 59-67%. The main limitations are problems with suppressing the blurred image when driving at night and the need for a third focal length, for example with computer screens at intermediate distances. Stereopsis is impaired in monovision, but most patients do not seem to notice this. These limitations highlight the need to take account of occupational factors. Monovision could cause a binocular vision anomaly to decompensate, so the pre-fitting screening should include an assessment of orthoptic function. Various methods have been used to determine which eye should be given the distance vision contact lens and the literature on tests of ocular dominance is reviewed. It is concluded that tests of blur suppression are most likely to be relevant, but that ocular dominance is not fixed but is rather a fluid, adaptive, phenomenon in most patients. Suitable patients can often be given trial lenses that allow them to experiment with monovision in real world situations and this can be a useful way of revealing the preferred eye for each distance. Of course, no patient should drive or operate machinery until successfully adapted to monovision. Surgically induced monovision is less easily reversed than contact lens-induced monovision, and is only appropriate after a successful trial of monovision with contact lenses.     

长骨骨折内固定术中瞄准器校正瞄准应用的试验观察

目的探讨长骨骨折内固定术中交锁髓内钉瞄准器校正瞄准的应用效果。方法用可调式交锁髓内钉瞄准器与非可调式交锁髓内钉瞄准器在两个动物实验组（A、B组）中进行对比。结果可调式交锁髓内钉瞄准器在瞄准偏差的情况下,通过校正瞄准达到正确瞄准,非可调式交锁髓内钉瞄准器在瞄准偏差的情况下,无法校正瞄准,需改手工进行。结论可调式交锁髓内钉瞄准器有良好的适应性。     

Sensory ocular dominance based on resolution acuity,contrast sensitivity and alignment sensitivity

Background: Ocular dominance is the superiority or preference of one eye over the other in terms of sighting, sensory function (for example, visual acuity) or persistence in binocular rivalry. There is poor agreement between sighting and sensory dominance and findings are equivocal on the possible neural basis of ocular dominance and its significance. Thus, there are questions on the meaning and importance of ocular dominance. Despite the lack of clarity in this area, ocular dominance is used clinically, for example, as the basis for decisions on monovision in contact lens wear and on treatment of anomalies of binocular vision. Methods: Sighting dominance and three types of sensory dominance (based on resolution acuity, contrast sensitivity and alignment sensitivity) were compared within individuals, with the main aim of determining whether sensory dominance is consistent across spatial visual functions. Results: Our findings indicate that each type of sensory dominance is insignificant in most individuals and in agreement with previous work that sensory and sighting dominance do not generally agree. Conclusion: These results demonstrate not only that different types of ocular dominance are not in agreement but also that in the normal visual system sensory dominance as measured here is insignificant in most individuals with normal vision.     

Looking at eye dominance from a different angle: Is sighting strength related to hand preference?

Sighting dominance (the behavioural preference for one eye over the other under monocular viewing conditions) has traditionally been thought of as a robust individual trait. However, Khan and Crawford (2001) have shown that, under certain viewing conditions, eye preference reverses as a function of horizontal gaze angle. Remarkably, the reversal of sighting from one eye to the other depends on which hand is used to reach out and grasp the target. Their procedure provides an ideal way to measure the strength of monocular preference for sighting, which may be related to other indicators of hemispheric specialisation for speech, language and motor function. Therefore, we hypothesised that individuals with consistent side preferences (e.g., right hand, right eye) should have more robust sighting dominance than those with crossed lateral preferences. To test this idea, we compared strength of eye dominance in individuals who are consistently right or left sided for hand and foot preference with those who are not. We also modified their procedure in order to minimise a potential image size confound, suggested by Banks et al. (2004) as an explanation of Khan and Crawford’s results. We found that the sighting dominance switch occurred at similar eccentricities when we controlled for effects of hand occlusion and target size differences. We also found that sighting dominance thresholds change predictably with the hand used. However, we found no evidence for relationships between strength of hand preference as assessed by questionnaire or by pegboard performance and strength of sighting dominance. Similarly, participants with consistent hand and foot preferences did not show stronger eye preference as assessed using the Khan and Crawford procedure. These data are discussed in terms of indirect relationships between sighting dominance, hand preference and cerebral specialisation for language and motor control.     

The absence of lateral congruency between sighting dominance and the eye with better visual acuity

The notion that in subjects with normal binocular vision (i.e. non-strabismic cases) the eye showing sighting dominance will also most likely be that individual's better-sighted eye has proved to be persistent. A review and analysis of the literature has now demonstrated, across several population groups, the fallacy of this belief. In fact, the occurrence of lateral congruency between sighting dominance and the eye with better visual acuity is at a statistical level no greater than chance would predict. Some clinical implications of this outcome are considered.     

Sighting ocular dominance magnitude varies with test distance

Background

Ocular dominance can be defined as the preference of an individual for viewing with one eye over the other for particular visual tasks. It is relevant to monovision contact lens wear, cataract surgery and sports vision. Clinically, the measurement of ocular dominance is typically done at an arbitrary distance using a sighting test, such as the hole‐in‐card method that has a binary outcome. We investigated the effect of test distance on ocular dominance measured using a binocular sighting test that provided a continuous measurement of dominance.

Methods

Ten participants with normal binocular vision took part in this study. Their binocular sighting ocular dominance and phorias were measured at one, two, four, eight and 10 metres. During the dominance tests participants made a binocular alignment judgment and then were asked to indicate the relative alignment of each eye using a visual analogue scale as a reference.

Results

Eight participants had strong ocular dominance (five right, three left). For these participants, there was a significant increase in the magnitude of dominance with increasing test distance (p < 0.001). This could not be fully explained by changes in convergence demand. Two participants showed very weak ocular dominance across all test distances (p > 0.05), despite changes in convergence demand.

Conclusion

When ocular dominance is present, its magnitude varies significantly with test distance. This has significant implications for the accurate measurement of ocular dominance in the clinic and may reflect the neural processes that influence eye preference.

     

校正瞄准型交锁髓内钉瞄准器的研制和应用

目的：改进临床上原有交锁髓内钉瞄准器的结构,使其瞄准方式由非可变瞄准改变为可变瞄准,以提高手术中交锁钉的锁中率。方法：总体上不改变原有交锁髓内钉瞄准器的运用原理,改变瞄准方式,提高手术成功率。结果：通过改进,该器械科保证手术时锁钉准确锁中钉孔,确保手术的成功率。结论：校正瞄准型交锁髓内钉瞄准器结构合理、操作方便,能够大大提高手术效率,经实验证实,效果很好。