C-shaped mandibular molars are so named because of the cross-sectional morphology of their fused roots and their root canals. Instead of having several discrete orifices, the pulp chamber of a molar with a C-shaped root canal system is a single, ribbon-shaped orifice with an arc of 180 degrees or more. It starts at the mesiolingual line angle and sweeps around either to the buccal or the lingual to end at the distal aspect of the pulp chamber (see Fig. 5-176, A). Below the orifice, the root structure can show a wide range of anatomic variations. These can be classified into two basic types: those with a single, ribbon-like, C-shaped canal from orifice to apex, and those with three or more distinct canals below the usual C-shaped orifice. Fortunately, molars with a single swath of canal are the exception rather than the rule. More common is the second type, with discrete canals that take unusual forms. Other investigators determined that C-shaped canals in mandibular second molars can vary in shape and number along the root length.
之所以命名为c型根管， 是因为他们融合的牙根和融合的根管，横截面形态呈c型。其他的磨牙髓室通常有几个独立的根管开口，但C型根管的开口通常呈单一的带状，这条带为一个180度甚至更大的圆弧。根管开口下方，牙根结构的解剖变异很大。这些可以分为两类：一类是单一、带状，从根管开口到根尖都是C型；另一类是在根管口下方有3个或以上的独立根管。所幸的是，单个条带状的根管并不多见。更常见的是第二类，形态多不常见的独立根管。其他研究者认为，下颌第二磨牙的C型根管沿途的形态和数目都不同。
Definition of the exception rather than the rule
: not common or usual : not often done, seen, or happening : rare
Friendly customer service seems to be the exception rather than the rule nowadays.
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