Space Closure in Orthodontics

Orthodontic movement is the response to force applied on teeth through braces, wires, elastics, modules, elastic bands, coils. The process occurs in this manner: when a Force is applied on the tooth, it moves inside the alveolar socket, this provokes the stretching of some periodontal fibers and the compression of other fibers. At the same time the interstitial liquid of the fibers is also compressed against the osseous walls. As the liquid slowly drains out of the alveolus, it also exerts hydraulic resistance against the dental movement. Periodontal fibers and interstitial liquid act in conjunction, against the forces applied on the tooth, making it return to its original position,(is a paradox, but bone is the most malleable tissue of the human body, adapting to the forces that act upon it, it reacts by depositing osseous tissue in the areas exposed to traction forces and to resorb osseous tissue in areas where pressure is exerted. Orthodontic movement is only possible because of this malleability.

This way, the root gets even closer to the alveolar wall, compressing the periodontal ligament on the side where the force is applied and stretching the fibers on the opposite side. Osteoclasts are responsible for cortical alveolar resorption where ligament compression occurs. In the phase where ligament distension occurs, osteoblasts and fibroblasts, the cells that form bone tissue and collagen fibers, are present. Clinically, this period is characterized by moderate tooth pain submitted to pressure but without movement, Around two days after the force application, osteoclasts and osteoblasts initiate the remodeling process. Slowly the alveolus dislocates in the direction of the applied force, with the subsequent orthodontic movement liurstone defines optimal force as the one that provides a rapid dental movement, with no patienL discomfort and no tissue damage (no bone loss or root resorption), being this the most physiologic orthodontic force. Many investigators (Storey, Smith, Hrian Lee, Ricketts, among others) evaluated the optimal necessary force for dentalmovement;

Ricketts clinically showed that intrusion of the inferior incisors with utility arches is efficiently done applying 15 to 20 g per tooth or 60 to 80 g for the four lower incisors; upper incisors have a root surface transversal section that is almost double in size compared to the lower incisors, so the force required for intrusion is double compared to the force required for the tower incisors, approximately to 160 g for all four upper incisors or 40 g per tooth.

Periodontal fibers and interstitial liquid form together a shock absorbing and physiologic force dissipating system during occlusal function and orthodontic movement. When there is a rise in the orthodontic force, the periodontal ligament will present zones with excessive pressure. In these zones, more often on the compression side, blood circulation slows or shuts down, and degeneration or necrosis of the periodontal fibers sets in. This phenomenon is known as hyalinization (aseptic necrosis).

The greater the number of hyalinization areas present, the slower the orthodontic movement will be, therefore the greater the force is, the slower dental movement will be. Histologically speaking, during hyalinization we will observe periodontal tissue necrosis in the compression ssone, blood vessel obliteration, a diminished blood supply and anoxia (lack of oxygen) in the Conjunctive tissue. Clinically we can affirm that heavy forces are pathological and they cause Orthodontic movement in young patients presents less osseous resorption due to the great cellular element proliferation in the periodontal ligament and the bundles of fibers are thinner and flexible, in contrast with much older patients. Younger patients present less tissue reaction to orthodontic forces (around 2 or 3 days), in contrast with the 5 or 10 days needed for cellular proliferation in an adult, which makes adult orthodontic movement slower.

Patients with heavy complexion present reduced medullar spaces and denser cortical bone, they present a higher tendency of hyalinization and consequently a higher degree of difficulty to move teeth. Patients with hyperparathyroidism produce more osteoclasts with the subsequent bone resorption. In the same way, sexual hormones (estrogen or testosterone), when in surplus, have an effect over bone alterations. Storey, in 1954 found erratic tooth movement related to menstrual cycle phases in young adolescents.

When we determine the need to extract teeth in an orthodontic treatment we must consider some factors like dental overcrowding, anchorage, canine and incisor axial inclination, midline discrepancies, vertical dimen' sion, facial and dental esthetics, dental health, plus the main motive why the patient seeks consultation with an orthodontist. Space closure in orthodontic treatment can be done with two types of mechanics:

1. Sectional or segmented mechanics, that consist in closure loops Lhat are made on a sectioned arch. Teeth move by activation of the loop of the wire that can be designed to deliver a low load-deflection relation and a controlled moment-force relation (Burstone "T" loop).

2. Sliding mechanics, in which braces slide either on an arch wire or the wire slides on braces and tubes. One of the main factors to differentiate between the two mechanics is friction; space closure in segmented mechanics is frictionless while sliding mechanics involves friction/

Orthodontic friction is produced while braces slide upon the arch wire, In order to move a tooth we must apply a force (elastics, wires, ligatures, coils, etc.) in such magnitude as to overcome friction, this way beginning dental movement. The level of friction depends on several factors, including the type of brace and arch wire used. Stainless steel braces slide with relative ease over stainless steel arch wires and not so well on wires that contain certain percentage of titanium (beta-titanium or niclcel-titanium) thaL present a rough surface and generate more friction; furthermore, a ceramic brace has a rough surface that also increases friction, the combination of ceramic braces and stainless steel arch wires produces a great deal of friction. Adding sliding mechanics for space closure will result in a high friction coefficient and more root resorption. Recent studies have demonstrated that self- ligaL-ing braces have the lowest friction coefficient.

Some believe that we lose less posterior anchorage utilizing space closure in two phases (canine retraction first with subsequent incisor retraction) rather than with in-mass six anterior teeth retraction; but this may not be valid for all cases. In-mass space closure can reduce treatment time significantly because it is done in only one phase.

The ideal force system used for space closure must meet certain characteristics, which are:

• Provide optimal forces for tooth movement,
• Must be comfortable and hygienic to the patient.
• Must require minimal chair time.
• Must require minimal patient cooperation.
• Must be inexpensive.

According to Burstone, canine retraction mechanics can be described by three principle characteristics;

1. The monienL applied on the canine brace,
2. The main arch wire deflection.
3. The maximum force that the arch can withstand without permanent deformation.

The final result of space closure must include aligned and upright teeth with parallel roots. This implies that dental movement almost always requires certain degree ofin-mass translation and also root displacement