Comparative evaluation and co-relation in variation of curve of Spee and curve of Wilson in Class II div. 1, Class II div. 2, and Class III as against Class I malocclusion in central India population- an in vitro study

Background and rationale

Planning for orthodontic treatment begins with achieving acceptable cosmetic outcomes in the soft tissues of the face. In order to achieve healthy static and dynamic occlusal interactions, it is also crucial to place the teeth in the basal bone with the correct angulation, inclination, and location. In order to prevent periodontal disease, provide stability, and achieve a functional occlusion, it is critical to identify the patient’s dental arch form before commencing treatment so that mechanics can be used to conform arch form throughout the therapy. Periodontal problems in teeth are caused by teeth moved orthodontically that go beyond the basal border and compromise the stability after treatment. The occlusal plane should be flattened and the Wilson Curve should be levelled as the end results of orthodontic therapy.¹

The human dentition exhibits the natural phenomenon known as Curve of Spee (COS). For a successful masticatory system, the anteroposterior curve of occlusion is necessary.² In 1890, F. Graf von Spee (1855-1937), German anatomist, described the Curve of Spee. He discovered that the occlusion line in skulls with worn away teeth was a line on a cylinder perpendicular to the mandibular incisors’ incisal borders, surface of occlusal of 2^nd molar, and the anterior border of the condyle.³ The COS, which is essential to attaining a stable occlusion, is present in every single patient undergoing orthodontic treatment. In orthodontics, the arc that is perpendicular to the incisal margins is currently referred to as the “Curve of Spee”.⁴

Recent studies have shown that the mediolateral curve plays a biomechanical role in mastication by improving the efficacy of forces of occlusion during mastication and the crush/shear ratio between the posteriors. Dental malocclusions with significant overbites typically have an inflated Curve of Spee.³ As a result, there will be an inappropriate functional occlusion since the muscle imbalance will be altered.

Wilson was the very first to distinguish the lower grinding teeth’s lingual inclination and the higher grinding teeth’s buccal inclination. The Curve of Wilson is the name given to this occlusal curve in the coronal plane. The Wilson Curve allows for lateral mandibular excursions without posterior obstructions.²

For adequate function, the amount of buccolingual tooth inclination must be determined and quantified in order to support treatment objectives.² The palatal and buccal cusps of the posteriors are in functional contact as a result of this curvature, which appears to necessitate a concave mandibular arch as well as a concave and convex maxillary arch.⁵ It would be logical to expect that the incline of bone would be oriented in this direction for the best masticatory stress given the alignment of the anatomical parts outlined by Dawson. Okeson outlined the purpose of the Curve of Wilson, which prevents balancing interferences and ensures the most efficient utilisation of cuspal contacts.²

Andrews came up with 6 keys of normal occlusion out of which 3^rd key is tooth inclination, which is the labiolingual as well as buccolingual inclination of the crown. He saw a lingual tilt in the posterior crowns of the mandible and maxilla.² One of the elements included in the six keys to occlusion is the Curve of Spee. According to Andrews, COS in people with proper occlusion varies from flat to moderate.¹

The COS and COW in the lower arches of people living in central India will be determined in this study. In this study, reverse engineering is used to determine the Wilson curve and the Spee curve, which is useful for understanding its comprehensiveness as well as the impact it has on stable occlusion. The measurement and significance of compensatory curves have also been discussed in various studies.

In literatures prior to our study, some of the compensatory curves were shown to be correlated with dentoskeletal malocclusions and normal occlusion. The link between the Curves of Spee and Wilson and their effect on morphology of dentoskeletal, their potential to maintain stability of occlusion have all been the subject of a few studies. This investigation investigates the connection between compensatory curves and how it might aid in the development of treatment plans for specific malocclusions.

Objectives

Trial design: In-vitro study

Study setting

Following study will be done at the Department of Orthodontics and Dentofacial Orthopedics, Sharad Pawar Dental College, Sawangi (M), Wardha, in cooperation with the Department of Prosthodontics.

Eligibility criteria

Inclusion criteria:

Exclusion criteria:

Intervention

Figure 1. Curve of Spee will be taken from that tangent line to the deepest point on the premolar region.

Figure 2. With WALA point used as a reference line Curve of Wilson will be measured.

Outcomes

To obtain a functional occlusion, it is essential to identify the patient’s dental arch form before the procedure and to apply mechanics that conform with the arch form throughout the therapy.

Sample size

Formula for sample size when comparing two means

Where;

Zα represents the degree of significance at 5% i.e., 95%

Confidence interval $=$ 1.96

Zβ is the test’s power $=$ 80% $=$ 0.84

δ₁ $=$ SD of COS depth in Class I $=$ 0.717

δ₂ $=$ SD of COS depth in Class II $=$ 0.732

$∆=\text{Distiction between}\mathrm{two}\text{means}=2.472–1.954=0.518$

n $=$ 30 patients needed in each group

Total sample size is 120

Study Reference: IIknur Veli et al.⁷

Statistical method

All the results will be calculated using R version 4.2.3 (Shortstop Beagle). Data for outcome variables will be tested for normality using Kalmogorov-Smirlov. Comparative analysis over the outcome of functional occlusion in different malocclusion will be evaluated and measurement of depth of curve of Spee and Wilson in millimeters respectively. ANOVA will be used to find significant difference in mean in comparison of 4 groups. Tukey test will be used for comparative evaluation of measurement in between 2 groups pairwise. P-value ≤ .05 will be considered significant 5% level of significance and 95% confidence of interval.

The results and measurements will be evaluated by experienced technical engineers for Inter-rater reliability test to check the consistency of result obtained.

Dissemination

This protocol will assist us in evaluating and comparing the Spee and Wilson curves in Class II Div-1, Class II Div-2, & Class III malocclusions as against Class-I malocclusions, & this will help us design our treatments in order to avoid periodontal issues, ensure stability, and achieve effective occlusion.

Study status

Not started yet.

Ethical considerations

Ethical approval received from Datta Meghe Institute of Higher Education and Research, Sawangi, Wardha

IEC reference number - DMIHER (DU)/IEC/2023/569