Periodontal ligament: embryology and anatomy
In the chapter on joints, the periodontal ligament one is considered a distinct entity in the context of synarthroses (properly referred to as swelling), not only as it connects a bone with a non-bone structure – the tooth – but also by virtue of its particular structural features.
The periodontal ligament consists of dense and highly specialized fibrous tissue, which connects the root of the tooth, covered with cementum, with the alveolar bone, also interfacing with the soft periodontal tissues.
It mainly consists of collagen grouped with variously oriented fibers, inserted in a matrix in the form of a gel.
From an embryological point of view, fibroblastic activity mainly occupies the period preceding eruption, while the root is forming in the dental follicle. The follicle contains ectomesenchymal tissue which differs from cementoblasts: initially, the fibers are found to be included inside the cementum, in an orientation almost parallel to that of the root, with the future Sharpey’s fibers in a coronal position. Insertion on the opposite side, or the alveolar bone, will take place later, little by little during the formation of the root.
The main fibers constitute an important component of the ligament: they are bundles of collagen fibers, oriented at functional inclinations along the root’s surface. These are principally responsible both for keeping the tooth in place and for the physiologically guaranteed mobility. Sharpey’s fibers are the extensions of the main fibers in the cementum (where they are more numerous) and in the bone (where they are thicker). They progressively calcify and associate with non-collagen proteins.
With regard to elastic fibers, the human periodontal ligament contains the so-called oxytalan fibers, simple bundles of microfibrils that run vertically from the cementum surface, forming a sort of mesh that covers the root of the tooth. They respond to changes in tension and can be functionally connected to vascular structures.
The interstices are occupied by a rich vascular network and nerve plexuses. The arterial vessels, branches of the alveolar arteries, reach the ligament mainly in the form of perforating arteries, while the venules take the waste blood from the coronal area to the apical one, where the largest caliber vessels are located.
Innervation is provided by branches of the second and third branches of the trigeminal nerve, the fifth pair of cranial nerves. Some of the fibers are accessed from the apical foramen, while others perforate the lateral wall of the alveolus.
Knowledge of the anatomy of the ligament, and particularly of the nerve fibers that run therein, seems fundamental to understanding the anesthesiological techniques involving this structure.
Injection at the level of the periodontal ligament, usually referred to as intraligamentary local anesthesia, is a local anesthesia technique that has always been used in dental clinics and has undergone further development in recent years. It has classically been used as an additional anesthesia technique, indicated in complex cases for pain management: the clearest example is the state of irreversible pulpitis, the clinical picture most frequently requiring some additional form of anesthesia.
The technique’s popularity is first recorded in the 1970s, with the introduction of the Peripress syringe, capable of administering the ligament with a limited volume of anesthetic solution at high hydrostatic pressure. Instruments of this type, although popular among clinicians, have not always found favor among patients, due to the possible intraprocedural pain stimulation.
The most recent evidence shows the intraligamentary technique to be versatile, and potentially usable as an isolated anesthetic procedure, as an alternative to conventional techniques such as the inferior alveolar nerve block.
The basic rationale for intraligamentary anesthesia involves inserting a 30-27 gauge needle into the periodontal space between the tooth root and the alveolar bone. The insertion direction follows an angle of 30-40° from the element’s major axis. The depth is approximately 2-3 mm from the sulcus in the periodontally healthy tooth.
The approach for a single-rooted tooth is indicated for at least two sites, while the multi-rooted tooth should receive one injection per root. The first injection should last at least 20 seconds, and all the subsequent ones a few seconds longer. Histological studies have shown that the anesthetic solution spreads within the alveolar bone: time management is essential to overcome the tissues’ resistance, avoiding a risk of excess pressure in the periodontal compartment. The onset of the anesthetic effect, which involves the tooth and surrounding tissues, is expected after about 40 seconds, for an indicative duration of 30 minutes.
In recent years, some authors have raised certain critical issues regarding the intraligamentary technique. It seems to induce a state of transient inflammation, with an indicative duration of 24 hours, in the periodontium, without however causing actual damage. The excess pressure can lead to postoperative pain and, in extreme cases, the non-compressibility of the liquid induces the extrusion of the tooth (we speak of “hydraulic lever”).
These possible complications, related not to the technique but to the practitioner, have supported the study of new methods, based on the same rationale but independent of the clinician’s sensitivity.
From intraligamentary anesthesia to single tooth anesthesia
The development of innovative techniques based on the rationale of intraligamentary anesthesia not only responds to the need to innovate this method, but also to the desire to provide an alternative to more conventional techniques. Think, for example, of the inferior alveolar block, a standard in the approach to the lower molars, which however presents the discomfort – relevant when working on an isolated dental site – of involving the entire half-jaw, and moreover is burdened by a failure rate of 20-25%.
The main development in this regard has been the introduction of computer-controlled local anesthesia delivery (CCLAD) methods.
These instruments are, in the first place, capable of directing the practitioner to the target site with a higher tactile accuracy than conventional syringes. The handpiece has the shape and weight of a pen, which helps with handling; it also has a “friendly” appearance which is useful for managing anxiety, an aspect which will be dealt with later. This depends on the fact that the handpiece – entirely disposable – is basically the only needle holder; the injection vial to draw from is instead placed in the central unit, which has the role of the “brain” regulating the location of the site and the release of the anesthetic. The system’s third component is a pedal, which lets the operator activate the release system.
Once the needle is inserted into the sulcus, the system, through dynamic pressure sensing (DPS) technology, monitors the outlet pressure in real time and keeps the needle in an optimal position, also providing audiovisual feedback. These messages warn the operator in the event of needle blockage or leaks in the system.
The procedure that essentially derives directly from CCLAD technology is single tooth anesthesia (STA). It shares the biological assumptions of intraligamentary anesthesia, the effectiveness of which is related to pressure. This does not mean this must necessarily be high: it just has to be ensured that the tissue resistance is overcome and, for this reason, the time will need to be extended.
As the name suggests, being able to act on a limited substrate is a major advantage. This does not mean, however, that these types of instruments lack versatility: in fact, they lend themselves to multiple techniques, also useful when anesthesia is required on larger portions of the arch.
In the final analysis, it should be emphasized that local anesthesia represents one of the fundamental triggers of dental anxiety, in both pediatric and adult subjects. CCLAD methods must also be considered in the context of so-called painless anesthesia, as they promise to reduce pain levels, not only compared with conventional intraligamentary anesthesia, but also compared with classic infiltration techniques. Studies have established that blood pressure control can actually reduce pain during the injection and consequently the anxiety that derives from it, thus promoting a positive perception of dental care or, at least, of this specific method.