Apical constriction (Physiological foramen)
is considered narrowest diameter of the root canal
located at the cement-dentinal junction.
it is the apical limit of root canal preparation preparation.
it is also known as the histological foramen, because it is located at the junction between the
pulpal connective and interstitial loose connective tissues of the periodontal ligament.
not visible on x-ray (we may find it only using WL determination methods).