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S Afr Optom 2011 70(4) 168-174
The South African Optometrist ISSN 0378-9411
168
Induced prismatic effects due to poorly tting
spectacle frames
VR Moodley*, F Kadwa, B Nxumalo, S Penceliah, B Ramkalam and A Zama
*Discipline of Optometry, University of Kwa Zulu-Natal, Westville Campus, Private Bag X54001 Durban, 4000
South Africa
<moodleyvr@ukzn.ac.za>
Received 20 June 2011; revised version accepted 2 November 2011
Abstract
To provide patients with the best vision possi-
ble practitioners undertake comprehensive visual
examinations and optical dispensing procedures.
Ensuring proper frame alignment is an important
part of the spectacle dispensing process. Ideally,
for maximum optical benet, patients should look
through the optical centers of their lenses. Rays
entering a lens at any point other than the opti-
cal center will be deviated as long as the lens has
power, thus inducing prismatic effects. These pris-
matic effects may lead to visual complaints such as
asthenopia, blurry vision and headaches. The aim
of the study was to investigate the prevalence of
induced prismatic effects due to poorly tting spec-
tacle frames. A sample of 100 spectacle wearing
subjects was selected as participants. Question-
naires were completed and any symptoms experi-
enced recorded. The habitual spectacle position in
the primary position of gaze was noted, pupil and
optical centers marked and where relevant, induced
prism was calculated. Data was analysed using the
ANOVA Kruskal-Wallis and Mann-Whitney tests.
On initial observation of frame position, about
45% of subjects were wearing their spectacles in-
correctly. A comparison of the marked pupil cent-
ers in primary gaze and optical centers revealed
that 100% of participants were found to not be
looking through the optical centers of their lens-
es. Fifty one percent were within horizontal and
3.12% within vertical ANSI tolerances. Symp-
toms were reported by 50% of subjects experi-
encing base-out, 0% base-in and 47% vertical
induced prismatic effects. However, no correla-
tion existed between those subjects experiencing
induced prism and symptoms reported. Prism
adaptation may account for some participants be-
ing asymptomatic. Sixty three percent of subjects
were not aware of the importance of properly t-
ted spectacle frames. Proper optical dispensing
with associated patient education is necessary to
achieve optimal optical benets of spectacles and
careful attention should be given to this aspect by
practitioners. It is recommended that patients re-
turn periodically to their optometrists to have the
frame alignment assessed and the t modied if
necessary. (S Afr Optom 2011 70(4) 168-174)
Key Words: induced prism, optical center, pris-
matic effects, prism adaptation
**BOptom(UDW) MOptom (UDW) CAS(Newenco)
Introduction
A spectacle frame is the portion of the spectacles
that holds the lenses, containing the ophthalmic pre-
scription, in their proper position before the eyes.
The various parts of the frame contribute to the nal
resting position of the frame on the face1. The bridge
structure and adjustable nose pads cause the frame to
rest higher or lower on the face and the lenses to be
positioned at varying distances from the eyes. Tem-
ples help keep the spectacles from slipping down the
nose when the bridge does not help much to support
S Afr Optom 2011 70(4) 168-174 VR Moodley et al - Induced prismatic effects due to poorly tting spectacle frames
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the frame or if the prescribed spectacles tend to be
somewhat heavy1-3. Spectacles are very important,
from an aesthetic point of view, to the person wear-
ing them and the habitual wearer often needs as much
help with frame selection as the neophyte. Aside from
a frame being cosmetically appealing, the nal frame
choice should also provide the best optics with the
prescription lenses4. Various factors such as overall
facial features, frame design and colour, frame thick-
ness and the patient’s prescription should be taken
into consideration when a frame is being selected1, 3-5.
Good optical dispensing practice must include evalu-
ating the spectacles on the patient for function, t and
comfort. This will include aligning the frame before
it is adjusted on the face to simplify angling for facial
contours to ensure a good t6. The practitioner usu-
ally adjusts the spectacles for the correct position to
be attained on the face. A proper frame design and t
would be one that allows the distance optical center
of the lenses to be positioned directly in front of the
pupil.
The anatomical interpupillary distance (IPD) is
dened as the distance between the centers of the en-
trance pupils of the eyes1, 7-9. According to Borish7,
the usual method of determining the IPD involves the
use of a millimeter ruler to measure the distance be-
tween the centers of the subject’s pupils. In the event
that the exact center of the pupil is difcult to esti-
mate, the IPD is measured from the nasal edge of one
pupil to the temporal edge of the other provided that
the two pupils are equal in size1, 7-9. Potential errors
when taking IPD include errors of parallelism due to
the large difference between the observer and the pa-
tient’s IPD, inappropriate distance between observer
and patient and improper placement of the ruler be-
fore the patient’s face8. The main reason for measur-
ing the IPD is to align the optical centers of ophthal-
mic lenses on the visual axis of each eye and failure
to do so results in unwanted induced prismatic effect
which may lead to eyestrain and asthenopia9-11.
Pantoscopic tilt or angle is dened as the angle be-
tween the optical axis of the lens and the visual axis
of the eye in the primary position or as the angle be-
tween the vertical plane of the face and the position-
ing of the glasses7-10. Most spectacles are tted with
downward tilts of about 7-10 degrees to the visual
axis, meaning the top of the spectacle lens is approxi-
mately 10 degrees forward when compared to the bot-
tom. Since the top of the spectacles are intentionally
tilted forward, this helps to achieve a place that al-
lows a more constant distance between the back sur-
face of the spectacle lens and the front surface of the
cornea10. A dispensing rule of thumb used is to drop
the optical centre by 0.5 mm for every 1 degree of
pantoscopic tilt11. However, anecdotal evidence sug-
gests that in practice, this position of the optical cen-
tre is often obtained with no specic instruction on
the prescription order, as modern cosmetic dispensing
dictates that the frame occupies a position such that
the centre of the pupil lies 4-8 mm above the horizon-
tal centre line.
Face-form tilt or wrap is the inclination of the tem-
poral edge of the lens towards the face, ensuring that
the frame front follows the natural frontal curvature
of the skull12. This curve serves both the cosmetic
purpose of improving the frame appearance and the
optical purpose of aligning both lens surfaces with the
wearer’s line of sight1. When prescription lenses are
ordered, the interpupillary distance is routinely meas-
ured. In addition, the eye size (measured from out-
ermost edges of lenses horizontally) and bridge size
(the distance between lenses) may also be measured
and added together, a measurement termed the frame
PD11, 13.
After the optical laboratory generates the prescrip-
tion in a spectacle lens blank, the nished lens must
be cut in such a way that 1) it ts properly into the
spectacle frame and 2) the optical center of the lens
coincides with the visual axis that passes through the
pupil center, a process termed centration10-11, 13. It
is very important that the optical centre of each lens
should occupy the desired position relative to the pu-
pil of the wearer’s eye13. Accurate pupillary (facial
PD) and centration distance (frame PD) measure-
ments are required for this process. If the centration
is not properly accomplished, the two centers will not
coincide in one or both lenses, resulting in unneces-
sary prism (induced prism)11.
Induced prism is dened as the prismatic effect
created when the patient’s visual axis does not pass
through the optical centre of an ophthalmic lens12. A
light ray will pass through a lens without deviation
only if it enters along the optic axis. Those parallel
incident rays entering a lens at points other than the
optical center will be deviated as long as the lens has
refracting power, creating prismatic effects, so termed
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S Afr Optom 2011 70(4) 168-174 VR Moodley et al - Induced prismatic effects due to poorly tting spectacle frames
due to the lens having the characteristic effect of a
prism13. Patients will experience this effect when
looking through part of a spectacle lens other than
its optical center. The amount of induced prism de-
pends upon the power of the spectacle lens and the
decentration or optical center displacement from
the pupil center11-14. Prism is frequently intention-
ally prescribed by the clinician in spectacle lenses
to eliminate diplopia or to ease asthenopia. This
prism may be incorporated by the application of
Prentice’s Rule (P=cF), which translates to each
centimeter of decentration of a lens resulting in one
prism diopter of deviation of light for each diopter
of lens power at a distance of one metre14-15. In the
equation P = cF, P is prism measured in prism diop-
ters (pd), c is optical centre displacement measured
in centimeters (cm) and F is lens power measured
in diopters (D). Jalie16 describes the application
of Prentice’s rule for horizontal and vertical merid-
ians and the methodology to be applied when the
cylinder axis is oblique and the resultant oblique
prism has to be resolved into vertical and horizon-
tal components.
The prism induced by decentration of ophthal-
mic lenses will also inuence binocular vision and
stereopsis which are important aspects of visual
functioning16-17. Jiménez et al17 showed that de-
centration changes fusional convergence and can
thereby alter certain aspects of binocular vision,
such as fusion, resulting in eye fatigue or head-
aches. Fogt and Jones18 demonstrated that the
prismatic effect of spectacle lenses results in in-
appropriate extra-retinal eye-position information
in a dark environment and Fry and Kent19 found a
deterioration of stereoacuity when they used base
in and base out prisms to induce changes in conver-
gence.
Du Toit et al20investigated the amount of induced
horizontal and vertical prism that could comfortably
be tolerated with the use of ready-made spectacles.
Nine participants were each given plano spectacles to
wear with differing prism power for eight hours. If
visual discomfort could not be tolerated, participants
removed the spectacles, noting duration of wear and
reason for discontinuation. Distance and near visual
comfort were rated, and participants were asked if
they would be able to adapt to wearing the specta-
cles. The highest prism powers that is (1 pd up, 2 pd
out, 2 pd in) could not be worn for eight hours by the
majority of the participants. Comfort ratings for near
vision were statistically signicantly different when
the highest prism power was compared with each of
the lower powers. The results of their study20 showed
that vertically, the comfort ratings for the control and
the 0.5 pd were different from the 1 pd, whilst hori-
zontally the comfort ratings for the control, 0.5 pd
and 1 pd were all different from the 2 pd. The study
concluded that most spectacle wearers would likely
comfortably tolerate ≤0.5 pd vertical, ≤1.0 pd out, or
≤1.0 pd in induced prism. A guide to the maximum
interpupillary distance/optical center distance dispari-
ties likely to be comfortably tolerated with varying
spectacle powers was formulated. Powers −1.50 to
+1.50 D are unlikely to have sufcient lens decentra-
tion to cause discomfort20.
The allowed tolerances on prismatic power de-
pends on whether the lens is glazed or unglazed,
the lens is single vision, multifocal or progressive,
prismatic effect at the centration point or prescribed
prism is being measured and the prism is horizontal
or vertical.
Table 1: Tolerances for glazed lenses incorporating less than 2 D of prescribed prism21.
S Afr Optom 2011 70(4) 168-174 VR Moodley et al - Induced prismatic effects due to poorly tting spectacle frames
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Tolerance set by the American National Standards
Institute (ANSI):22
In 1979 the ANSI released a set of standards in-
cluding vertical prism up to 1/3 prism diopter or 1
mm decentration of the optical center was considered
acceptable and horizontal prismatic imbalance of 2/3
prism diopter or 2.5 mm decentration was considered
acceptable.
Prism adaptation is considered a form of proce-
dural learning23. When prisms are placed in front of
the eyes, the entire visual eld is displaced and the
adaptation that occurs is a phenomenon in which the
motor system adapts to new visuospatial coordinates
imposed by the displaced visual eld. The degree
and strength of the adaptation can be measured by
the spatial deviation of the motor actions in the di-
rection opposite to the visual displacement imposed
by the prisms, a phenomenon known as after-effect.
Fernández-Ruiz and Díaz23 in their study on prism
adaptation required subjects to throw balls to a tar-
get in front of them before, during, and after lateral
displacement of the visual eld with prismatic spec-
tacles. The results showed that the adaptation process
is dependent on the number of interactions between
the visual and motor system, and not on the time spent
wearing the prisms.
Osuobeni9 found that prismatic effects lead to vari-
ous symptoms, such as asthenopia, blurry vision and
diplopia. It is thus useful to take these prismatic ef-
fects into account when a patient wearing spectacles
presents with such symptoms. In these instances, it
may be incorrect to assume that the symptoms are
only due to an incorrect refraction having been per-
formed. Additionally, irrespective of how accurate
the refraction has been, if the lenses (single vision, bi-
focals or varifocals) are improperly positioned before
the eyes, the nished product will be inferior from a
quality of vision perspective. It is therefore impor-
tant that proper frame tting and regular adjustments
of spectacle frames by an optometrist or dispensing
optician occurs with the aim of alleviating symptoms
of eyestrain.
Spectacle frames also warp out of shape due to
the body heat of the wearer, particularly at the bridge
piece, causing the frames to bow slightly forwardly
and the temple pieces to spread outwardly. Exposure
to temperatures above 60 degrees Celsius, such as the
intense heat of the car dashboard or sauna bath, leads
to spectacle frame warpage as well as loosening of
the lenses24. This can result in a loose t of the frame
and temple pieces, which permits the frames to slip
down the nose, causing discomfort and distorted vi-
sion through the lens
Ophthalmic dispensing manuals and textbooks
usually highlight the importance of frame selection,
perfect alignment of the optical center of the lens with
the pupil center, and advise patients on how to cor-
rectly wear their spectacles1, 7. However, a general
observation of the multitude of frame styles and ts
being worn by patients today and the reported symp-
toms associated with induced prism in the literature
led the researchers to investigate the prevalence of
inappropriately tting frames and the effect of these
on the optical performance of their respective lenses.
The aim of this study was to determine the prevalence
and the associated prismatic effects of poorly tting
spectacle frames. The specic objectives of the study
were to determine the percentage of spectacle wear-
ers whose frames do not t properly, the prevalence
of induced horizontal and vertical prismatic effects in
participants not looking through the optical centers of
their spectacle lenses, the participants’ knowledge on
the importance of properly tted spectacles and lastly
the relationship between induced prism and the par-
ticipants’ symptoms.
Methodology
Ethical clearance to conduct the study was obtained
from the University of KwaZulu - Natal (UKZN) Fac-
ulty of Health Science Research Committee. A quan-
titative, descriptive study design was utilized. From
a study population of students at the UKZN Westville
campus, a study sample of 100 students wearing spec-
tacles for refractive correction were selected. The
study sample comprised of a conveniently selected
group of students from across the campus. This sam-
ple included participants of different race groups and
males and females were included. Participants with
a recently compensated refractive error and wearing
single vision spectacles were included in the study
and individuals with a refractive error wearing bifo-
cal or multifocal spectacles and participants with a
strabismus were excluded from the study.
A standardization workshop was conducted to
S Afr Optom 2011 70(4) 168-174 VR Moodley et al - Induced prismatic effects due to poorly tting spectacle frames
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standardize techniques used and ensure that each
of the researchers obtained accurate readings with
all techniques. This was followed by a pilot study,
conducted on a convenient sample of 10 students,
to assess the validity of the questionnaire and rene
techniques used. A consent form in English or Isi-
Zulu containing information relating to the purpose,
signicance and intended procedures of the research
study was completed and signed by each participant.
Participants completed a questionnaire providing in-
formation on their visual and ocular complaints, date
of the last visual examination and knowledge on the
importance of properly tting spectacle frames. A
unilateral cover test was conducted to rule out any
strabismus. An initial observation of the position of
the participant’s spectacles was made whilst being
worn in the habitual position in primary gaze. This
subjective observation was noted on the participant’s
result sheet and photographs of poorly tting frames
were randomly taken.
With the frames in the habitual wearing position,
the pupil centers were marked with a blue ink marker
and thereafter the distance IPD was measured. The
frame PD was measured and compared to the distance
IPD to ascertain the face form tilt which was record-
ed on the result sheets. The optical centers of each
of the lenses were then marked with a black colour
marker using a vertometer. The distances between
these markings were then measured to determine the
centration distance. The power of each spectacle lens
was then determined using the Iso-Lys vertometer.
The oblique cylinders were initially resolved into hor-
izontal and vertical components. Any prismatic ef-
fects present were thereafter calculated and recorded
using Prentice’s Rule.
Data was captured using SPSS Version 15 and Mi-
crosoft Excel 2007. The prevalence of poorly tted
frames was derived from the data. A descriptive anal-
ysis was done using the ANOVA Kruskal-Wallis and
Mann-Whitney Test, with a probability of p<0.05 to
correlate the participant’s symptoms and the induced
prism as well as to correlate the refractive errors and
the induced prism. To maintain condentiality the
use of a code rather than the participant’s name was
employed.
Results
The majority (63%) of the participants claimed
that they were not informed by their eye-care practi-
tioners and hence were unaware of the importance of
properly tting spectacles (Figure 1).
Figure 1: showing the subjects awareness of the importance of
properly tted spectacles.
Forty ve percent of the study population was
symptomatic with the most common visual complaint
being blurry vision (28%) and other symptoms ex-
perienced included asthenopia (12%) and headaches
(5%).
Clinical Findings
Figure 2 shows that on observation 45% of subjects
appeared to be wearing improperly tted spectacles,
however, a comparison of the marked pupil and opti-
cal centers revealed that 100% of subjects were not
looking through the optical centers of their spectacle
lenses. Participants with base out prismatic effects
experienced the most symptoms (50%), although
there was no correlation between the amount of in-
duced prism and the symptoms experienced. Fifty
one percent of the induced prism fell within the hori-
zontal and 3.12% within vertical ANSI tolerances.
Figure 2: showing the percentages of subjects not looking
through the optical centers of their lenses as observed and meas-
ured.
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Figure 3: showing the ranges of the induced BI and BO horizon-
tal prismatic effects experienced by participants (ANSI = 0.75
Δ).
Whilst viewing in the primary position 49% of the
participants experienced horizontal induced prisms
with the majority of subjects (75%) experiencing base
out prismatic effects in the range < 1 pd. The induced
prism in these cases could have resulted from myopic
patients looking through a point temporal to the dis-
tance optical centers or hyperopic patients looking
through a point nasal to the distance optical centers.
Twenty three percent of subjects experienced base
in prismatic effects (Figure 3). Figure 4 shows that
96.8% of participants experienced vertical prismatic
effects with the majority falling in the < 1 pd range.
The vertical induced prism was not further classied
in this study.
Figure 4: showing the ranges of the induced vertical prismatic
effects experienced by participants (ANSI = 0.25 Δ).
Discussion
Despite the majority of the subjects’ frames ap-
pearing to t properly on naked eye observation,
when measured, a clinically signicant number of
the sample was found to be wearing their spectacles
incorrectly, with all subjects not looking through the
optical center of their lenses. This could be avoided
by practitioners ensuring proper alignment during the
dispensing of the spectacles and providing good pa-
tient education which, as shown in this study, is not
always done by practitioners. If the laboratory made
an error with the spectacles then practitioners should
reject them as indicated by Fowler et al21 who state
that if a pair of spectacles does not conform to the
tolerances then the work should be rejected. One can-
not rely solely on patients to ensure that frames t
well to avoid induced prism if the proper alignment
of the optical centres are not initially conrmed by the
practitioner prior to dispensing the lenses. The poor-
ly tting frames in this study resulted in all subjects
being exposed to a certain amount of induced prism
in primary gaze and according to Osubeni9 they may
experience symptoms such as asthenopia and blurry
vision. This is also supported by Topliss15 who states
that centration is important with modern large lens
sizes or the patient will have to overcome excessive
prismatic imbalances in order to see the object clearly.
In some cases this may cause uncomfortable vision
and headaches. This was further conrmed in this
study where 45% of the subjects reported symptoms
such as blurry vision, asthenopia and headaches. As
shown in Figures 3 and 4, 49% of the subjects expe-
rienced induced horizontal and 96.8 % vertical pris-
matic effects respectively, both of which were greater
than the accepted ANSI tolerance levels. There was
however no correlation between the amount of in-
duced prism and the symptoms reported by the par-
ticipants. Literature25-27 indicates that the possible
reason for those who were asymptomatic, despite
experiencing induced prism that exceeded acceptable
tolerances, could be prism adaptation. According to
Grifn and Grisham25 heterophoric patients having
normal binocular vision with no ocular symptoms
typically show strong prism adaptation. The symp-
toms that were reported, largely by participants with
induced base out prism, may be as a result of the in-
duced prism compounding other possible near vision
anomalies such as convergence insufciency.
The expected clinical protocol to be performed
S Afr Optom 2011 70(4) 168-174 VR Moodley et al - Induced prismatic effects due to poorly tting spectacle frames
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by optometrists during the dispensing of spectacles is
to ensure that the patient is looking through the opti-
cal center and assuring proper frame alignment. As
noted, and of concern in the study, was that the major-
ity of participants’ frames were not aligned for proper
optical benet and patients were not adequately edu-
cated about the importance of the spectacle frame be-
ing well aligned on the face. To ensure that patient
care is not compromised practitioners must adhere to
the minimum standards of optical dispensing practice.
Conclusion and Recommendations
This study showed that all the subjects were not
looking through the optical centers of their spectacles
and most claimed that they were not made aware by
the practitioners of the need for proper frame tting.
The pantoscopic tilt, which could have also contrib-
uted to the induced prism, was not taken into consid-
eration as the study focused only on the centration of
the lenses. Practitioners should however check the
pantoscopic tilt as well as centration when dispens-
ing the spectacle frame. Proper optical dispensing
practices, accompanied by appropriate patient educa-
tion is a critical part of the visual consultation and
careful attention should be given to this aspect by
practitioners. It is thus recommended that patients
return periodically to their optometrists to have the
frame alignment assessed and the t modied if nec-
essary. Consideration should be given to include this
as part of the minimum standards for optical dispens-
ing practice.
Recommendations for further studies will be to
use a larger sample size, include the effects of panto-
scopic and face form tilts, determine the effects of the
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