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Inflammation is a response to injury, and the presence of polymorphonuclear leucocytes and chronic inflammatory cells is indicative of failure of VPT. Swelling is also a feature of the inflammatory response, but the unique anatomy of the dentine–pulp complex and the rigidity of the surrounding dentine prevent expansion of the pulp. Additionally, after pulpal exposure, the buffering effect of the dentine is lost and the pulp tissue is rendered sensitive to potential adverse interactions from materials or microbes [70]. Notably, inflammation is also important in driving the soft tissue response during healing following placement of a pulp‐capping material [39]. Calcium hydroxide produces a mild irritation of the pulp and stimulates repair. If pulp capping is successful, then after a few days there will be a reduction in the number of inflammatory cells present under the necrotic zone, whilst under the capping material, the pulpal cells will proliferate, migrate, and form new collagen in contact with the necrotic zone [71]. Although the process is similar with HCSC, the pulpal irritation is less than that with calcium hydroxide (Figure 2.2) [9]. Tertiary reparative dentinogenesis is then initiated, odontoblast cells are formed, and mineralized matrix is secreted [72]. This matrix forms the so called ‘hard tissue’ bridge, which walls off the pulp and offers further protection to the soft tissue adjacent to the wound site.

Figure 2.2 Histological response to pulp capping. (a) Macrophotographic view of the mesial half of a human maxillary third molar demonstrating the remnants of the restorative material (A) and ProRoot MTA capping material (B) at one month. Note the distinct hard tissue bridge (arrow). Original magnification ×8. (b) Photomicrograph of histological section of the specimen in (a) of an MTA pulp cap at one month. Note that the mineralized barrier (arrow) stretches across the entire width of the exposed pulp (C). Original magnification ×16. (c) Higher‐magnification photomicrograph from (a) and (b). Cuboidal cells (arrows) line the hard tissue barrier (D). Note the absence of inflammatory cells in the pulp (E). Original magnification ×85. (d) Photomicrograph of a selected serial section of hard‐setting calcium hydroxide cement (Dycal) at one month. Engorged blood vessels are prominent and inflammatory cells are present. Note the presence of Dycal particles (arrows) in the pulp (F). Original magnification ×16.

Source: Images adapted from Nair, P.N., Duncan, H.F., Pitt Ford, T.R., Luder, H.U. Histological, ultrastructural and quantitative investigations on the response of healthy human pulps to experimental capping with mineral trioxide aggregate: a randomized controlled trial. Int. Endod. J. 2008; 41(2):128–50.