Prevalence of malocclusions requiring treatment according to the KIG classification

The goals of this study were to

Data acquisition took place in an orthodontic practice from the district of Viersen, North Rhine/Germany. The practice was established in the 4th quarter of 2001 as a joint practice of two orthodontists. The work phase of the practice was between 2002 and 2021, so that a period of 20 years could be scrutinized. Only two specialists for orthodontics did the KIG classifications during the 20-year practice period. All classifications were verified by the respective other orthodontist using the four-eye principle.

Apart from the founding (2001) and handover (2022) periods, the progression is divided into four 5‑year periods (Table 2) according to the operating cycles of the practice:

Possible tooth and jaw malposition are subdivided into 11 classifications of the orthodontic indication group (KIG) system. Each classification is additionally subdivided into five grades. Since only grades 3–5 are eligible for treatment, 19 possible combinations of classification and grade trigger cost reimbursement through social security. The ranking starts with A as the highest and P as the lowest possible classification, translated from the original German definition (Table 1).

The diagnoses were solely recorded through clinical inspection, as required by legislation. The extent and direction of sagittal and vertical overjet, anterior crowding and space deficits were measured intraorally using sliding calipers (Münchner Modell, Dentaurum, Ispringen, Germany) with a precision of 0.25 mm. The assessment of occlusion regarding frontal and lateral crossbites was performed visually. Only if justified by clinical reasons, x‑rays were made to diagnose possible aplasia, retention or displacement of permanent teeth. Visual assessment of the occlusion was done to detect frontal and lateral crossbites.

Primarily, children and adolescents up to the age of 18, but also adult patients requiring orthognathic surgery were examined. The classification of the patients into the respective KIG grades 3–5 with treatment need according to the valid statutory health insurance guidelines [1] always took place in the highest of the 19 possible variants. There were no multiple responses in the present study. Exclusively the two orthodontists recorded the KIG classifications and grades during the entire practice period, applying the four-eye principle.

A possible limitation of the methodology could be that the KIG classifications were set by two different examiners. According to Gesch et al. [27], there are considerable interexaminer differences in the classification of subjects into the respective indication groups and thus also different classifications into KIG grades < 3 and > 2 in borderline cases. Different data collection methods (clinic/dental cast) in the assessment of the dysgnathia by different or orthodontically inexperienced examiners may have an unfavourable influence on examiner agreement. For this reason, KIG classifications were made according to the four-eye principle without exception. Especially in borderline cases, classifications were made based on a dental cast and, if necessary, a panoramic x‑ray.

The retrospective evaluation of practice data always refers to selected patients, as most patients appear due to an individual wish for treatment and/or referral by dentists. However, when comparing the results with those of the DMS⋅6, it became obvious that the treatment indications of the most frequent findings of the selected and unselected cohorts appear approximately the same. In this respect, the selection of subjects could be considered representative.

A study on frequency and severity of dental and jaw malocclusions for a longer period is only available from one university clinic in Germany [5]. In addition, time-limited cross-sectional studies on selected patient groups in different regions of Germany have been conducted over the past 25 years to evaluate the frequency of anomalies as well as the need for orthodontic treatment [4, 6, 11‐16].

A comparison of the present results with existing cross-sectional studies is only possible to a limited extent, since either other parameters were used as a basis or the study clientele was different. In these studies [4, 6, 11‐16], the patient age was within narrow bounds, but they were not preselected by a third party. All patients in the present cross-sectional study were mainly referred by dentists, but also by paediatricians and ear, nose and throat (ENT) specialists, and are therefore only representative to a limited extent.

The most feasible comparisons are possible with the studies by Rijpstra and Lisson [5], Glasl et al. [4] and the DMS⋅6 [6], as they also used the KIG system for classification. The temporal context corresponds to periods I–III or I and IV of the present study. Unlike in the present and the university study [5], not all KIG classifications were recorded in the other two time-limited cross-sectional studies [4, 6]: “A” is missing in Glasl et al. [4], “S” and “U” are missing in the DMS⋅6 [6]. Other than investigating all possible grades, they did not record only the highest possible KIG grade [4, 6], but every possible grade 3–5 was registered. In this way, multiple responses were possible, which can lead to an overrepresentation of certain anomalies.

A special feature of the DMS⋅6 [6] due to its methodology is that it conducted a nationwide random data collection and thus presented superregional results. All other studies were always regionally limited to the immediate surroundings of the respective study locations.

In 2004, Glasl et al. [4] determined a treatment indication according to SGB V (KIG grade ≥ 3) in 41.4% of the subjects. Of these, 10.6% were assigned to KIG grade 3, 29.4% to KIG grade 4 and 1.4% to KIG grade 5. The most frequent indication groups were “K” with 32.2%, “E” with 21.0% and “D” with 20.9%. The most frequently diagnosed treatment needs grades were “D4” with 17.4%, “K4” with 15.3%, “M4” with 14.9% and “T3” with 13.1% (Fig. 6). Subdivided according to spatial plane or tooth position anomaly, the sagittal deviations “D” and “M” with 37.5% and “E” and “P” with 36.4% were almost equally represented. Deviations in the transverse “B” and “K” occurred in 25.9% and those in the vertical “O” and “T” in 14.6% of those examined. However, the summation of the proportions of the indication groups results in 126.8% due to multiple responses, which is why not all individual results of the degrees of treatment need can be reconstructed from the study. Patients with craniofacial anomalies were not present.

In contrast, all data can be reconstructed from the study by Rijpstra and Lisson [5]. This cross-sectional study over several years described the frequency and percentage distribution of KIG grades 3–5 in a majority of nonselected 1766 patients over a period of 12 years. However, university hospitals are characterized by a high occurrence of complex disease patterns that require interdisciplinary treatment. This explains the high proportions of the indication groups “M” with 20.3%, “D” with 20.2%, “K” with 15.5% and especially “A” with 13.9%. The rarest indication group here was “T” with 1.5%. In the individual degrees of treatment need, “A5” was at the top with 13.9%, followed by “K4” with 13.8%, “M4” with 13.6% and “D4” with 12.4%. The sagittal deviations “D” and “M” together had a frequency of 40.5%, whereas the vertical deviations “O” and “T” had a frequency of only 3.3%, the transverse deviations “B” and “K” had a frequency of 18.3% and the tooth position anomalies “E” and “P” had a frequency of 10.1%. Of the 1766 patients in this study, 10.6% had malocclusions (KIG grade 3), 55.4% had pronounced malocclusions (KIG grade 4) and 33.9% had extremely pronounced malocclusions (KIG grade 5) (Table 8).

In the DMS⋅6 [6], 268 of 704 8‑ to 9‑year old and previously untreated subjects showed a KIG grade 3–5 (Table 9). In all, 45.9% had a KIG rating of “D”. More than 10% each had KIG grades “T” (12.4%), “P” (11.6%), and more than 5% each had KIG grades “K” (9.7%), “M” (8.5%) and “E” (6.6%). Of the 16 possible therapy-relevant KIG grades 3–5, “D4” occurred most frequently with 37.8% (182 patients). In addition, only KIG grade “T3” (12.4%) was represented with more than 10%. Of 286 patients requiring treatment, 25.9% had malocclusions (KIG grade 3), 61.0% had pronounced malocclusions (KIG grade 4) and 13.1% had extremely pronounced malocclusions (KIG grade 5).

The sagittal deviations “D” and “M” together had a frequency of 54.4%. The vertical deviations “O” and “T” came to 16.3%, the transverse deviations “B” and “K” to 10.1% and “E” and “P” to 18.2%. The indication groups “S” and “U” were not recorded due to the methodology and were therefore also missing from the billing data of the National Association of Statutory Health Insurance Dentists (KZBV) from 2020 published for comparison within the framework of the DMS⋅6 (Table 10; [6]).

Thus, the distribution of findings requiring treatment in the KZBV data is reduced to 9 classifications and 16 KIG grades, analogous to the DMS⋅6, although all age groups were recorded in the KZBV billing data. 33.7% of the nationwide patients had the indication group “D”. More than 10% each had the indication groups “M” (18.6%), “K” (16.6%), “P” (12.4%), and “E” (10.1%). The vertical anomalies “T” with 1.5% and “O” with 1.3% as well as the craniofacial anomalies with 0.3% were registered extraordinarily rarely.

Of the 16 KIG grades relevant to treatment, “D4” occurred most frequently with 26.2%. More than 10% each had KIG grades “M4” (17.9%) and “K4” (14.8%). 19.9% of the patients had malocclusions (KIG grade 3), 71.4% had pronounced malocclusions (KIG grade 4) and 8.7% had extremely pronounced malocclusions (KIG grade 5). The sagittal deviations “D” and “M” together accounted for 52.3%, whereas the vertical deviations “O” and “T” accounted for only 2.8%. The transverse deviations “B” and “K” (22%) as well as “E” and “P” together (22.5%) each represented just under a quarter of the KIG grades requiring treatment.

All previous studies agree with the present study in that most patients exhibit KIG grade 4 and thus pronounced malocclusions. In Rijpstra and Lisson [5], the extremely pronounced malocclusions with KIG grade 5 was overrepresented due to the frequent classification “A5”. The most frequent malformation in the KIG group “A5” was cleft lip, jaw and palate. They occurred independently or as part of a syndrome [17, 18]. Craniofacial anomalies also included patients with trisomy 21, when serious functional disorders, mostly of the tongue, needed to be corrected by therapy in infancy and early childhood [19]. All diseases in this group of findings have extensive pathological findings in the dental and jaw region, which must be treated in an interdisciplinary manner. In many cases, this can only be done at university hospitals with their specialized treatment centres.

In all studies, the sagittal deviations “D” and “M” occurred most frequently. The vertical deviations “O” and “T” were found least frequently in four out of five studies. One discrepancy is found in the DMS⋅6 [6], because there, as in the study by Glasl et al. [4], the treatment need grade “T3” was overrepresented. A reason for this may be that those studies collected multiple treatment indications. Not only the highest-ranking grade was recorded, but each existing KIG classification and grades > 2 were registered separately for the individual subjects.

It was discussed by Rijpstra and Lisson [5] that the grade “T3” rating is objectively very difficult, and “T3” is therefore a rarely used grade. The Index of Orthodontic Treatment Need (IOTN), for example, already considers any contact of teeth with the opposite mucosa as T3, clearly visible impressions in the gingiva are considered as T4 [20]. In Germany, 4.5 years after the introduction of the KIG system, certain formulations were clarified [21]. Since then, impressions in the opposite mucosa are not considered a treatment indication at the expense of the statutory health insurance funds. Only if a deep bite has led to recessions or other permanent damage to the mucosa is the finding considered as “T3”. Since gingival problems with recession and inflammation usually only occur with advancing age [22], it is understandable that in both multiyear observations (current study and [5]) as well as in the comparative data of the KZBV [6], “T3” was hardly ever recorded. This also explains the frequency decrease in the present study from 3.4 to 1.1% between periods I and IV.

It remains a problem that not all 11 KIG classifications were recorded in the DMS⋅6 and the KZBV data [6] due to study limitations. Thus, there can be no statement made regarding the prevalence of certain findings. Studies show that the prevalence of aplasia of permanent teeth (“U”) is between 3.5 and 6.5% [23, 24] and that of retention and/or displacement (“S”) is 6% [25]. However, this can only be detected deductively or not at all without x‑raying the patient. In the present study, the proportion of patients with KIG classification “S” was even higher (13.1%) and comparable to KIG classification “U” (5.2%).

It is critical to note that the KIG classification was not primarily developed as an epidemiological index. Rather, it is an instrument to determine whether patients from the late mixed dentition (approx. 10th year) can be treated at the expense of the National Statutory Health Insurance system. Applying the KIG classification to 8‑ and 9‑year-olds as in the DMS⋅6 [6] and to 9‑ to 11-year-olds in Glasl et al. [4] is not without problems, since orthodontic anomalies become more pronounced during growth and ageing [15, 26]. Thus, studies with an age-restricted study population have a risk of underestimating actual prevalence and thus the need for orthodontic care.

For factors that are justified in the study design—i.e., representative population average, age distribution as well as no university specifics—a comparison of the present results with the KZBV data is easiest, as there is the greatest possible agreement for all measured parameters. Both the frequency of classifications and grades and the age distribution correspond to the national average represented by the KZBV data [6].