We found that in prosthodontic patients one OHIP-49 point corresponded to 15.2 impacts over the period of one month. This translates to approximately one impact every other day.
This number may be used to better interpret previously observed OHIP results. For example, prosthodontic patients receiving autologous bone grafts prior to insertion of dental implants would experience about 38 impacts a day when seeking treatment
. The clinically relevant difference between patients with FPDs and patients with RPDs would correspond to about 6 impacts a day and the minimal important difference for the OHIP translates to 3 impacts per day
. An increase of 1 missing occlusal unit in patients with shortened dental arches would be associated with an increase of 1 impact a day
. If this number was generalized to another patient population such as TMD patients (a population with known psychosocial impact) they are likely to experience between 15 and 31 daily impacts, depending on the diagnoses. This is at least three times more often than the mean of 5 daily impacts among general population subjects without TMD
. Reference values could be compared across countries, e.g., general population subjects wearing complete dentures in Germany reported daily impacts 4 times as often compared to those subjects in Hungary (median: 12 vs. 3)
[12, 25]. These numbers give an impression of how frequently OHRQoL problems occur, how patients suffer from oral disease and how they benefit from interventions. Patients, clinicians and researchers have an easy and practical guide to interpret OHRQoL in more detail.
The study population comprised of prosthodontic patients who either had an appointment for an annual examination or for regular treatment. We considered our study population to be typical as they suffered from common dental diseases - caries and periodontitis. The oral status of the patients ranged from only natural teeth, fixed partial dentures or removable partial dentures to complete dentures. Differences in OHRQoL impairment with respect to gender and age group were not statistically significant and, therefore, might be due to chance. However, despite the fact that our patients experienced a wide spectrum of oral health problems, it is not clear how far our results can be generalized to other populations. It might be that the oral impacts corresponding to one point of the OHIP are different in populations with lower impaired OHRQoL. In our study, we included both patients with highly impaired OHRQoL indicated by treatment needs and patients with less impaired OHRQoL at their annual dental check-up.
We did not encounter any difficulties when applying the OHIP in the personal interview. However, since patients had to think about the exact frequency of each item (and not only making a estimation on a ordinal 5-point scale), the interviews with the numerical impact frequencies as the response scale took longer compared to the self-administered completion of the questionnaire. Counting oral health impacts seems easy, but in practice it is actually quite challenging
. This is a reason why the counting method using the OHIP numerical response format is not very practical for most settings. Based on our clinical experience, we believe that it may be interesting for settings where the specific number of oral health problems is small but the impacts are severe.
Our methodological approach has strengths and limitations. To calculate the mean number of impacts for each OHIP response category is sound and does not rest on many assumptions. We considered the OHIP items to be interchangeable indicators of one construct and there is some evidence for this assumption from factor analytic studies showing that OHIP has a dominating general factor even if the construct is considered multidimensional
. In contrast, fitting a straight line through OHIP’s five ordinal response categories and deriving one problem count per OHIP point rests on several assumptions. In addition to the one mentioned above, it is assumed that the relationship between ordinal and numerical OHIP values is linear, that the difference between the ordinal categories is equal and that the straight line fits reasonably well through the individual data points to name major points. Although we found some evidence that supported these assumptions, our results should nevertheless be interpreted with caution. The distributions of the numerical frequencies corresponding to the ordinal responses were of a substantial magnitude. However, this has already been observed previously in other studies investigating the relationship between ordinal and numerical responses, e.g., pain ratings on an ordinal scale and a visual analogue scale
. We condensed a complex phenomenon into one number and our results are intended as first step into an innovative interpretation of OHIP scores. Nevertheless, we believe using this single number is worthwhile as it is a simple and practical guide to interpret OHIP scores. As OHIP scores are potentially influenced by recall periods
[20, 28], memory effects
, order effects
[15, 16], and administration method
; these methodological factors may also potentially influence the problem count per OHIP point. However, it has been shown that OHIP scores are rather robust against the influence of methodological factors. Therefore, there is no compelling evidence why the problem count per OHIP point should be substantially influence by methodological factors.
A more challenging question is whether our results can be informative for the several available OHIP short forms. We assume that the number of oral impacts for one OHIP point of a single item (problem) should be identical in long and short forms. For summary scores, extrapolation of our results rests on the assumption that OHIP has a dominating underlying general factor and that all items are basically interchangeable. Under this assumption and using OHIP-5 as an example, this would result in a summary score of approximately one-tenth of the OHIP-49 if administered simultaneously, and the number of oral impacts for one point of the OHIP-5 summary score should be tenfold the number for the OHIP-49 to yield comparable results. For the OHIP-14 summary score, the number of oral impacts for one OHIP point should be multiplied with 3.5, respectively. Such results should be carefully checked whether they fit with expectations and other findings.