Anatomical versus functional classification of ankyloglossia and their association with temporomandibular joint disorders in adults: a cross sectional study

2.1 TMD Screening and clinical examination

To screen for TMD in adults, the three question tool developed by Lövgren et al. [28] was used due to its validity and cost-effectiveness. The questions were as follows:

Q1: Do you experience pain in your temple, face, jaw or jaw joint once a week or more?

Q2: Do you have pain once a week or more when opening your mouth or chewing?

Q3: Does your jaw lock or become stuck once a week or more?

In addition to the questionnaire, clinical examinations were performed according to standardized diagnostic criteria:

E1: Palpation of the temporalis and masseter muscles, following the Diagnostic Criteria for Temporomandibular Disorders (DC/TMD) guidelines [29].

E2: Preauricular and intraauricular palpation of the TMJ, also according to DC/TMD guidelines [29].

E3: Measurement of the maximal interincisal mouth opening (MIO).

Participants who answered “yes” to at least one of the three screening questions, exhibited pain on palpation of at least one muscle or one TMJ on either side, or had a maximal interincisal opening less than 38 mm were classified as probable TMD patients.

2.2 Assessment and grading of ankyloglossia

Tongue-mobility was assessed using the Tongue Range of Motion Ratio (TRMR) protocol published in 2019 by Zaghi et al. [17]. MIO was measured with a ruler placed between the upper and lower right central incisors. This measurement was then repeated while the tip of the tongue was placed at the maxillary incisive papilla (MIO-TIP), presenting anterior tongue mobility.

To evaluate posterior tongue mobility, the MIO was measured while the patient performed a tongue-suction maneuver, in which the posterior tongue was elevated to the roof palate in a movement similar to tongue clicking (MIO-sucked). In this position at least the anterior third of the tongue was required to make contact with the palate. If a patient was unable to perform the suction maneuver, the MIO-sucked value was recorded as 0. The TRMR values were then calculated as percentages:

TRMR-TIP (%) = MIO-TIP × 100/MIO

TRMR-sucked (%) = MIO-sucked × 100/MIO

The TRMR values were used to classify the severity of ankyloglossia, as shown in Table 1 (Ref. [17]). Grades 2d, 3 and 4 were considered indicative of restricted tongue mobility, while grade 1, 2a, 2b and 2c were classified as non-restricted tongue mobility.

2.3 Anatomical classification of ankyloglossia

Ankyloglossia was evaluated using the classification proposed by Kotlow et al. [14]. The free-tongue length was measured with a Boley gauge, determining the distance between the tip of the tongue and the point of insertion of the lingual frenulum at the base of the tongue. During the measurement, a dental instrument was used to gently place the insertion point in a position approximating the tip of the tongue, thereby ensuring consistent and accurate readings. The grading criteria are summarized in Table 2 (Ref. [14]). A free tongue length of less than 16 mm was interpreted as indicative of restricted tongue mobility.

2.4 Statistical analysis

All statistical analyses were performed using IBM SPSS Statistics software, version 30.1 (IBM Corp., Armonk, NY, USA). Descriptive statistics were presented in the form of cross tables. The TRMR measurements, reflecting anterior and posterior tongue mobility according to Zaghi et al. [17] protocols, were used to assess restriction levels. To evaluate the association between TMD and ankyloglossia classifications (both Zaghi or Kotlow), as well as patient sex and age, comparative analyses were conducted using the Chi-square (χ²) test, as the variables under investigation were all categorical. A statistically significant correlation between two variables was defined as a distribution of observed frequencies that significantly deviated from the expected frequencies under the null hypothesis. A p-value of less than 0.05 was considered statistically significant. To assess the influence of age, participants were stratified into two groups: those younger than 40 years and those aged 40 years or older.

3.1 Incidence of TMD and ankyloglossia

Based on the Lövgren screening questionnaire [28], in combination with clinical examination 114 of the 129 patients were classified as probable TMD cases. The occurrence of TMD was significantly higher in females compared to males (χ²-test, p = 0.013; Table 3).

Using the TRMR classification by Zaghi et al. [17], 51 patients (39.6%) had anterior tongue restriction, indicated by a below-average TRMR-TIP value, and 45 patients (34.9%) had posterior tongue restriction, indicated by a below-average TRMR-sucked value (Table 4, Ref. [17]). In total, 63 patients (48.8%) exhibited either anterior and/or posterior tongue restriction. No significant correlation was found between the presence of ankyloglossia classified by TRMR and age (χ²-test, p = 0.278) or sex (χ²-test, p = 0.060).

The free tongue measurement according to Kotlow et al. [14] was performed in 128 patients, with values ranging from 4 mm to 28 mm. According to this classification, 76% of patients were diagnosed with mild, moderate, or severe ankyloglossia; none had complete ankyloglossia (Table 5, Ref. [14]). In addition, we observed no significant correlation between Kotlow-defined ankyloglossia and age (χ²-test, p = 0.116) or sex (χ²-test, p = 0.523).

3.2 Association between TMD and ankyloglossia severity

On evaluating the incidence of TMD in relation to TRMR-TIP and TRMR-sucked measurements based on the classification by Zaghi et al. [17], the χ² test revealed no significant correlation, neither between TMD and anterior restriction, nor between TMD and posterior restriction, nor between TMD and the presence of anterior and/or posterior restricted frenulum (χ²-test, p = 0.918; Table 6 (Ref. [17])).

In contrast, when using Kotlow’s classification, the presence of mild, moderate, or severe ankyloglossia was significantly associated with TMD (χ²-test, p = 0.026; Table 7 (Ref. [14])).

4.1 Study strengths and limitations

The consecutively recruited participants represent a clinical population from a practice with a specific focus on TMD and ankyloglossia, which may likely explain the high incidence of both conditions observed in the present cohort but limits the generalizability of the findings to broader populations.

Several limitations must be considered. First the sample size was relatively small. A formal power analysis was not performed as the study was designed as a pilot investigation.

Second, TMD screening was conducted using the three Lövgren questions, which are validated and appropriate for use in primary dental healthcare. The tool has a specificity of 82% and a sensitivity of 79%. For at least one affirmative answer, it shows a low positive predictive value of 30% but a high negative predictive value of 97% [28]. These predictive values are highly dependent on TMD prevalence. In a specialized setting, the positive predictive value increases to 79%, while the negative predictive value decreases to 81% [28]. Given the specialized context of the present study, it can be assumed that probable TMD patients were identified with high probability.

Third, in addition to the anamnestic data, clinical examinations were performed to identify symptom-free functional disorders of the temporalis and/or masseter muscles, myogenous or arthrogenous limitations of the mandibular mobility, as well as capsulitis and/or synovitis. However, malocclusion was not included in the clinical assessment, despite the potential for ankyloglossia to influence jaw development and therefore occlusion. This is a limitation that should be recognized and addressed in future studies.

Fourth, all patients included in this study were older than 18 years. Information regarding potential corrections of developmental disorders such as the previous orthodontic, orthopedic, or prosthetic treatments was not collected. These factors may have influenced both tongue mobility and TMD symptoms.

Fifth, although the association between TMD and occlusion remains under ongoing discussion, with several studies reporting a weak or no connection [22], the exclusion of occlusal assessment in this study limits the ability to explore this relationship further.

Lastly, since there is currently no internationally accepted, validated diagnostic tool for ankyloglossia, we depended on the anatomical classification by Kotlow et al. [14] and the functional TRMR classification by Zaghi et al. [17] to evaluate the presence and severity of tongue restriction. However, it should be noted that all the measurements were performed by the same well-trained examiner to ensure consistency and enhance reliability.

4.2 Interpretation of associations between ankyloglossia and TMD

To the best of our knowledge, no previous studies have specifically examined the correlation between restricted tongue mobility and TMD, and existing studies have primarily focused on frenoplasty techniques and their effects on sleep-related breathing disorders or breastfeeding outcomes [3, 6, 7, 8, 16, 25, 30, 31]. In the present study, while the visible classification of ankyloglossia according to Kotlow showed a significant correlation with TMD, the functional classification based on the TRMR protocol by Zaghi did not support this association. We believe that these differing results could be explained by the fundamental differences between the classification approaches.

Consistent with findings from other studies, a higher prevalence of probable TMD was observed in female patients [18, 28]. The association between TMD and age narrowly missed statistical significance (p = 0.060), likely due to the limited sample size, although patients were stratified into only two cohorts, those younger than 40 years and those aged 40 years or older.