Unity and stability of dental arches
Dental arches represent an integrated system. The main factors ensuring their unity are interdental contacts, periodontium, and the alveolar process itself.
A special role in the transmission and distribution of masticatory pressure is played by the interdental ligament of the marginal periodontium. This is a thick bundle of connective tissue fibers located directly above the apex of the interdental bone septum. It extends from the cementum of one tooth root to the cementum of the adjacent one. Thanks to interdental contacts and this ligament, the load is distributed not only to a specific "working" tooth and its socket, but is also smoothly transmitted to neighboring areas, giving the arch the properties of a single organ.
Tooth inclination and jaw biomechanics
The stability of dental arches is largely determined by the orientation of the axes of crowns and roots. The nature of this arrangement varies depending on the jaw.
- Mandible: Features a high degree of stability due to the buccal convexity of the arch and specific inclination. In general, the crowns of the lower teeth are inclined lingually (inward), while the roots are inclined buccally (outward). The lower posterior teeth (molars) are additionally inclined with their crowns forward and roots backward, which biomechanically prevents undesirable posterior shifting of the entire dental arch.
- Maxilla: Has a less favorable inclination for stability. The tooth crowns here are inclined buccally (outward), while the roots point inward (palatally). This anatomical stability deficiency is successfully compensated by a natural mechanism: a greater number of roots in the upper posterior teeth compared to the lower ones.
Types and topography of arches
To describe the geometry of the dentofacial system in lateral projection (taking into account the longitudinal axes of the teeth), three main arches are distinguished:
- Dental arch — runs along the incisal edges of the anterior teeth and the cusps of the posterior teeth.
- Alveolar arch — projected along the crest of the alveolar process (approximately at the level of the tooth necks).
- Basal arch (apical base) — an imaginary line connecting the root apices.
The shape of the upper dental arch resembles a semi-ellipse, while the lower one resembles a parabola. Due to different physiological tooth inclinations, the arch dimensions on the same jaw do not match. In the maxilla (crowns outward), the dental arch is the widest, followed by the alveolar arch, and the basal arch is the narrowest. In the mandible (crowns inward), the situation is reversed: the basal arch is the widest, and the dental arch is the narrowest.
Briefly: answers to topical questions
Which periodontal structures are involved in the distribution of masticatory pressure?
The distribution of masticatory pressure involves periodontal fibers, the interdental ligament of the marginal periodontium, and tooth contact points.
- Periodontal fibers — redistribute mechanical force to the tooth and jaw bones (especially "transit" fibers penetrating the alveolar bone tissue).
- Interdental ligament of the marginal periodontium — a thick bundle of connective tissue fibers running from the cementum of one tooth to the cementum of the adjacent tooth.
- Contact points — through them, pressure is evenly transmitted to adjacent teeth.
Which muscles provide mandibular movements during mastication?
Mandibular movements are provided by the alternating action of muscles that elevate and depress it. Group of muscles elevating the mandible:
- Temporal muscle (m. temporalis) — elevates the jaw.
- Masseter muscle (m. masseter) — elevates the jaw.
- Medial pterygoid muscle [m. pterygoideus medialis (interna)] — elevates the jaw.
- Lateral pterygoid muscle [m. pterygoideus lateralis (externa)] — provides sagittal movements (forward and downward displacement).
Group of muscles depressing the mandible (suprahyoid muscles):
- Mylohyoid muscle (m. mylohyoideus) — displaces the jaw downward.
- Geniohyoid muscle (m. geniohyoideus) — displaces the jaw downward.
- Digastric muscle (m. digastricus) — displaces the jaw downward.
What is the Curve of Spee (sagittal occlusal curve) and what is its role?
The Curve of Spee (sagittal compensatory curve) is a line drawn along the incisal edges of the anterior teeth and the buccal cusps of the posterior teeth. It runs in the anteroposterior (sagittal) direction, forms a segment of a circle with its convexity facing downward, and reflects the anatomical concavity of the lower dental arch. The presence of contact in the lateral regions during anterior occlusion depends on the degree of its expression.
Which types of physiological bite provide optimal mastication biomechanics?
Types of physiological bite include orthognathic, edge-to-edge (direct), progenic, and biprognathic bites.
- Orthognathic bite — upper incisors overlap the lower ones by no more than half the crown height, with tight cusp-fissure contact.
- Edge-to-edge bite — incisal edges of upper incisors do not overlap the lower ones, contact occurs edge-to-edge.
- Physiological progeny — lower incisors overlap the upper ones, the lower dental arch is wider than the upper one.
- Biprognathia — central incisors of both jaws are inclined forward while maintaining contact and overlap depth.
Imagine the maxilla as a funnel (expanding toward the teeth) and the mandible as a mountain (expanding toward the roots). Therefore, the upper dental arch is larger than its basal arch, while the lower dental arch is smaller than its basal arch.
Class and exam questions
Why does the mandible protrude forward in complete edentulism?
This phenomenon is called senile progenia. When teeth are lost (edentulism), the alveolar processes resorb, and the jaw relationship is then determined by the basal arches. Since the lower basal arch is wider than the upper one, the mandible visually protrudes forward significantly.
How is the unfavorable inclination of the upper teeth compensated?
The unstable position (crowns outward, roots inward) is fully compensated for by the increased number of roots in the upper posterior teeth.
Which structure prevents the posterior displacement of the lower dental arch?
This is prevented by the specific inclination of the lower molars: their crowns are inclined forward, and their roots backward.
What else to study on this topic
- Structural features of the interdental ligament of the marginal periodontium
- Mechanisms of masticatory pressure transmission to adjacent teeth
- Comparison of dental arch shapes: semi-ellipse and parabola
- Stages of senile progeny development in adentia
Master the entire topic in the «Propaedeutics of Dentistry» course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Biomechanics of the masticatory apparatus" and 868 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 65 video lessons on the subject
Other topics in "Chapter 5. Biomechanics of the Masticatory Apparatus"
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.