Discussion

Background:
The periodontium is constituted of 4 components: the alveolar bone, the dental cementum, the periodontal ligament (PDL), and the gingiva. The PDL is a fibrous connective tissue interposed between the dental cementum and the alveolar bone. It provides fixation of the tooth and withstands masticatory forces at the same time. These functional requirements are met by a unique architecture of a collagen fibre bundle system in combination with an ample blood vascular system. Due to the lifelong eruption of the equine tooth the PDL has to provide mechanisms for continuous tissue remodelling and repair.
Morphological studies were thus performed to identify specific periodontal components that are involved in tooth support, acceptance of forces and tissue remodelling. Biomechanical studies obtained data about periodontal loading during the equine chewing cycle.

Specific structures and properties of the equine periodontium:
Tooth support is achieved by collagen fibre bundles aligned in multidirectional arrangements. Thus, intrusive movements of the tooth and the sideward motion during the equine laterolateral chewing cycle cause tensile load only in distinct groups of fibre bundles. Eruption is assured by a well-coordinated remodelling of the periodontal fibre apparatus. Collagen degradation is initiated by matrixmetalloproteinase-1 and occurs only within certain single fibre bundles. In this way the function of tooth support is always maintained while the collagen apparatus is submitted to remodelling in order to become adapted to the dynamic morphological changes.
The blood vascular system is a well adapted arrangement consisting of an inner capillary layer (near the tooth) and an outer venous layer (near the alveolar bone). This system contains specific vascular structures, i.e. blind vessels and large ampullae, but no venous valves. These structural features provide the requirements for the distinct functions of the blood vascular system, like nutrition and shock absorbency. Furthermore, it is assumed that the vascular system facilitates an intra-extravasal shift of fluids in order to generate hydrostatic pressures necessary for tooth eruption.
Characteristic fibro-vascular arrangements apparently meet the biomechanical requirements of the PDL during the chewing cycle. In a so-called type I arrangement, a sheath of fibres and cells protects the blood vessels from the deformations of the surrounding tissue. In a type II arrangement, tractive forces in collagen fibres are transferred to compress adjacent blood vessels (by what is called a lateral compression mechanism); in a type III arrangement wide venules are assumed to act as a haemodynamic cushion, and collagen fibres prevent the vessels from shifting. The distinct combination of the physical properties of the collagen fibres (i.e. elasticity) and the intravasal blood content (i.e. viscosity) forms a visco-elastic system.
Biomechanical examinations using finite element simulations revealed the direction and range of tooth movements during the chewing cycle. Periodontal areas subjected to remarkable high loads are found in the periapical region and at the alveolar crest. Loads increase with dental age.
Initial lymphatic vessels are present in the PDL and in the adjacent bony spongiosa of the jaws. Their distribution leads to the assumption of 2 alternative ways of lymphatic drainage: ventrally into the mandibular lymph nodes, and caudally into the retropharyngeal lymph nodes.
The periodontal cells play a pivotal role in regulation and orchestration of periodontal remodelling. This is reflected by an exceptionally high rate of cell proliferation in the equine PDL compared to other species. Further, the equine PDL houses a population of multipotent mesenchymal stromal cells which are capable of differentiation into certain cell lineages.

Conclusion:
The most notable aspect of the equine PDL is the constant, essential process of tissue remodeling, which is necessary for the continuous eruption of teeth. It is also an indicator for remarkable regenerative properties of the periodontal tissues. By utilising the PDL specific multipotent mesenchymal stromal cells new therapeutic concepts in equine periodontology might be available in the future. Biomechanical findings might help to develop new concepts for therapies to prevent fatal periodontal diseases.