Newsletter
The periodontal ligament (PDL) is the natural connection between the tooth root, the alveolar bone and the gingiva. It has several bio-mechanical characteristics that osseo-integrated implants do not have. For example, its flexibility provides a damping effect which protects the enamel from occlusal shocks. Furthermore, the PDL helps to avoid overloading by distributing the masticatory pressure over groups of teeth. When overloading occurs, its proprioception blocks the muscular action by neuronal reflex.
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Periodontal cells possess the best capacities for physiological tissue remodelling of all structural tissue cells. This characteristic is important to adapt the position of teeth during growth or orthodontic treatment continuously, as well as for compensation of occlusal and proximal enamel attrition over the entire lifetime. Histological studies about tooth orthodontic displacement and tooth transplantation have demonstrated the biological dynamism of the PDL. The tissue can be destroyed and rebuilt in three weeks. Tooth transplantation with double PDL stimulation is one of the best examples of its healing capacity. Fourteen days before the transplantation, the donor tooth is extracted and immediately replanted in its original alveolus. This deliberate trauma triggers a healing process within the PDL, which includes cell proliferation and differentiation. The in vivo cell culture reaches its peak of activity after 14 days, after which the transplantation of the tooth can be performed with millions of cells in full activity attached to its root by new Sharpey’s fibres.
The success rate of tooth transplantation with double PDL stimulation is 95% after ten years. With the activated cell population holding great capacity for the regeneration of bone and gingival attachment around the transplanted tooth, this surgical procedure fulfils all the criteria for good tissue engineering. Using this model in its biological and clinical aspect, we think it is now possible to obtain a similar cell culture around an artificial root using tissue engineering techniques. These cells are easy to sample from the root surface of a compromised and extracted tooth, as well as to harvest in vitro. The cells used are autologous and each implant with its own cell population is prepared in a laboratory. The cell culture needs about four weeks to grow, and enables the alveolus of the tooth to be replaced. A preliminary experiment on athymic mice with human PDL cells around porous hydroxyapatite blocks in subcutaneous localisation demonstrated that the harvested cells retained their capacity to mineralise and deposit a cement-like layer with anchored fibres.
Numerous biomaterials have been tested and found to be the most suitable, among them bioglass, alumina, zirconia, plastics and titanium. Tests have also been conducted on surface preparations. In a human trial, a regular hydroxyapatite layer was created by crystallisation in a simulated body fluid after thermal treatment of the titanium pins. After primary culturing, cells were seeded on the conical titanium implants and cultured in a bioreactor for three weeks.
The objective of this trial was to evaluate the safety of the process. Nine Ligaplants were placed in nine patients with autologous cell cultures. One patient was not able to complete the test for personal reasons. No systemic or local adverse effects were observed during the trial.
Follow-up trials could give us an idea about the efficacy of the process. The hygiene conditions and control of forces on the Ligaplants is much easier to control in humans than in animals. However, Ligaplant healing appears to be much slower than with tooth transplantations, and the first series of Ligaplants suffered occasional overloading, which compromised their preservation. The last series received a splint and could be preserved much longer. Failures were due to luxation or pocket development after one month to seven years. The hydroxyapatite layer showed numerous defects on the lost Ligaplants.
New in vitro and canine experimentations were carried out after the clinical experiments. The objective was to find superior surface treatments and culturing techniques that would allow a better differentiation of the cells. Knowledge in cell biology and tissue engineering techniques is showing rapid development, and the possibility of using periodontally integrated implants could become a clinical reality within the next ten years.
Dr. Philippe Gault