Readers' forum 285

In the past decade, temporary anchorage deviceshave evolved as a mainstream orthodontic technique,and their use makes it possible to achieve tooth move-ment that is impossible with traditional orthodontic me-chanics. Your opinion makes us aware of the importanceof long-term observation, especially in patients treatedwith temporary anchorage devices.

Takeshi YanagitaOkayama, Japan

Am J Orthod Dentofacial Orthop 2011;140:284-50889-5406/$36.00Copyright � 2011 by the American Association of Orthodontists.doi:10.1016/j.ajodo.2011.08.001

REFERENCES

1. Wehrbein H, Bauer W, Diedrich P. Mandibular incisors, alveolarbone, and symphysis after orthodontic treatment. A retrospectivestudy. Am J Orthod Dentofacial Orthop 1996;110:239-46.

2. Sarikaya S, Haybar B, Ciger S, Ariyurek M. Changes in alveolar bonethickness due to retraction of anterior teeth. Am J Orthod DentofacialOrthop 2002;122:15-26.

A note about sample size

We read the article in the June 2011 issue on thecomparison of oral impacts (Wu A, McGrath C,

Wong RWK, Wiechmann D, Rabie ABM. Comparison oforal impacts experienced by patients treated with labialor customized lingual fixed orthodontic appliances.Am J Orthod Dentofacial Orthop 2011;139;784-90),and we became interested in the statistical parameters.

The authors are to be commended for their aim toconduct an age- and sex-matched prospective longi-tudinal trial on the oral impacts of 2 orthodontic ap-pliances. Even if the statistical tests used are correct,there seems to be a statistical study design issue. Ourmain concern is that it does not seem that the au-thors performed any pre-hoc sample size calculationbefore enrolling patients and undertaking this exper-imental study. In a prospective study, if statisticaltests are used, their power should be determineda priori.

For a particular experimental result to be claimed assignificant or not significant, sufficient statistical powermust be obtained, and the sample size for each groupshould be calculated a priori.

Did the authors perform a pre-hoc sample size calcu-lation? Since we don’t know whether this study was suf-ficiently powered, we don’t know whether these findingsare statistically correct and sufficiently powered. For ex-ample, the authors stated that there were significant dif-ferences in pain experiences attributed to a change intongue position and a reduction in tongue space. We

American Journal of Orthodontics and Dentofacial Orthoped

don’t know whether these “significant differences” arereally so.

Although this study is innovative, interesting, andimportant, an accurate experimental design is theonly way to draw adequate conclusions about experi-mental findings, transforming good hypotheses intostatistically supported and evidence-based scientificconclusions. We hope our suggestions will be usefulto other authors who will be involved in similar studiesin the future.

Alessandro ManganoCristina Balan

Gravedona, Italy, and Iasi, Romania

Am J Orthod Dentofacial Orthop 2011;140:2850889-5406/$36.00Copyright � 2011 by the American Association of Orthodontists.doi:10.1016/j.ajodo.2011.07.009

Author’s response

Thank you for your interest in our work. Our study wasnot a “trial,” as the letter suggested but, rather, an

observational prospective longitudinal study, and thusthe requirement for a sample power determination a pri-ori was not a necessity. Nonetheless, it is good practiceto determine the sample size if there is a standardizedassessment measure of “oral impact.”

To date, no standardized assessment measures of oralimpacts of orthodontic appliances exist to guide samplesize calculation (based on means and standard devia-tions, or the prevalence of the impact). In this study,an ad-hoc approach to oral impact assessment wasused, considering 13 factors (common oral impacts as-sociated with orthodontic treatment). Significant differ-ences were observed in 9 of the 13 oral impacts betweenthose with labial vs lingual appliances. Because signifi-cant differences were observed (across most oral im-pacts), that provided evidence that the sample size hadadequate statistical power; the issue of sample powerbecomes important when there is no significant differ-ence. Sample size calculation in itself cannot determinewhether there are significant differences, as the letterimplied.

In our study, differences in the oral impacts experi-enced were assessed via area-under-curve analyses. Ifone were to estimate sample size based on the findingsof our study, where the mean oral impact among thosewith lingual appliances was 37.4 (SD, 9.6), and hypoth-esize a 20% difference in oral impact among those withlabial appliances, then a sample size of 60 (30 per group)would have 85.4% power: ie, 85% of studies with a sam-ple of 60 (30 per group) would expect to yield

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286 Readers' forum

a significant difference, rejecting the null hypothesisthat the populations have equal oral impact. It is recom-mended that, when sample size calculations are con-ducted, a power of at least 80% should be set.

Although we also support the notion of prestudysample size calculation, without a standardized assess-ment measure and evidence from previous studies, allthat can be determined is a “guestimate” rather thana sample power estimate. We hope that our clarificationswill be useful to those involved in similar studies in thefuture.

Abby WuHong Kong

Am J Orthod Dentofacial Orthop 2011;140:285-60889-5406/$36.00Copyright � 2011 by the American Association of Orthodontists.doi:10.1016/j.ajodo.2011.08.002

Incidence and effects of genetic factorson canine impaction

We read with interest the article, “Incidence and ef-fects of genetic factors on canine impaction in an

isolated Jewish population” (Chung DD, Weisberg M,Pagala M. Am J Orthod Dentofacial Orthop 2011;139:e331-5).

First, we congratulate the authors for their contri-bution of identifying a genetic factor contributing tothe etiology of palatally displaced maxillary canines.The exact etiology of palatally displaced maxillary ca-nines was unknown until recent years. The authors de-termined the incidence and effects of genetic factorson palatally impacted canines in a genetically isolatedcommunity of ultraorthodox Hassidic Jews of Ashke-nazi descent. But, there are some aspects of this studyto which we want to draw attention and give additionalinformation.

The authors stated that “A canine was considered im-pacted if it was not erupted within the normal chrono-logic age range and had to be surgically exposed tobring it into the oral cavity.” However, the normal chro-nologic age range and the systemic condition of patients(endocrine deficiencies, febrile diseases, and so on) werenot clarified and discussed by the authors.

As stated in other studies, a tooth would be consid-ered impacted if its complete eruption to the oral cavitywas prevented by abnormal contact with an adjacenttooth in the same arch and root development of the im-pacted tooth was at least 75% complete or the patientwas over 16 years of age.1-3 In our recent study, a toothwas recorded as impacted when it was not exposed inthe oral cavity and the patient was over 16 years.

September 2011 � Vol 140 � Issue 3 American

Therefore, the mean age of the patients was 19.17 years(range, 16-25 years) in our study.1

Several etiologic situations including localized,systemic, and genetic factors could contribute to causecanine impactions. The authors provided some informa-tion about localized and genetic factors, but nothingabout systemic factors was included. If some patientshad systemic disorders, we would like to know whetherthe findings of this study are reliable.

In this study, the incidence of impacted canines—4.9%—was higher than reported in previous studies.Even if these anomalies do not occur frequently, it is im-portant to make an early diagnosis to begin treatment atthe optimum time.

Hasan KamakG€ulen Kamak

Kirikkale, Turkey

Am J Orthod Dentofacial Orthop 2011;140:2860889-5406/$36.00Copyright � 2011 by the American Association of Orthodontists.doi:10.1016/j.ajodo.2011.07.007

REFERENCES

1. Celikoglu M, Kamak H, Oktay H. Investigation of transmigrated andimpacted maxillary and mandibular canine teeth in an orthodonticpatient population. J Oral Maxillofac Surg 2010;68:1001-6.

2. Aydin U, Yilmaz HH, Yildirim D. Incidence of canine impaction andtransmigration in a patient population. Dentomaxillofac Radiol2004;33:164-9.

3. Cho SY, Ki Y, Chu V, Chan J. Impaction of permanent mandibularsecond molars in ethnic Chinese schoolchildren. J Can Dent Assoc2008;74:521.

Authors’ response

First, we thank Drs Kamak and Kamak for their com-ments about and interest in our article. To prevent

the selection of an ambiguous impaction, our samplecomprised obvious maxillary canine impactions asjudged by clinical and radiologic examinations. Theseimpactions were severe, each warranting surgical inter-vention to allow their proper alignment into the dentalarch.

In stating that “a tooth was considered impacted if itwas not erupted within the normal chronologic agerange,”we assumed the reader’s knowledge of a typicallynormal range; maxillary canines emerge into the oralcavity at mean ages of 10 to 12 years in girls and 11to 13 years in boys.1 However, because of immense indi-vidual variations in the chronologic timing of toothemergence, a deviation in the usual tooth eruption se-quence should be more alarming for clinicians. For ex-ample, the complete eruption of a second molar before

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