Pacific Coast Society of Orthodontists Bulletin Spring 2020 - 30

Let's Talk About That

tion. We do not use Angle's classification system
at all (more on that later).
PT: Could you describe the WALA ridge and why
you feel it is important?
WA: The WALA ridge is a soft-tissue ridge several
millimeters below the gingival margins of the
crowns of mandibular posterior teeth. It can be
seen and palpated clinically in most individuals
with reasonably healthy periodontal tissues
(Figure 1).

Figure 1. The WALA ridge

The width and shape of the WALA ridge has been
shown to correlate with the width and shape
of the mandibular basal bone, qualifying it as a
suitable landmark for indirectly evaluating the
buccolingual positions of the mandibular posterior teeth relative to supporting bone.2-13 The FA
points of teeth (referents) are used for assessing
the positions of the crowns of the mandibular
posterior teeth relative to the WALA ridge. Studies
have shown that distance to be roughly 2 mm
when occlusion is optimal and the roots are over
basal bone (Figure 2a, b).
Posterior teeth naturally compensate for transverse skeletal discrepancies by tipping in the
buccal or lingual direction around their centers of
rotation. As a result, the positions of the crowns
change, but the centers of rotation do not. Because the WALA ridge is anatomically located at
approximately the same height as the centers of
rotation of the teeth,14 its position is left unaffected by such natural tooth compensations
(Figure 3a-c).
Posterior tooth inclinations, if errant, can and
should be corrected by buccolingual tipping using
round archwires. In this way, tooth inclinations
are improved without displacing the centers of
rotation, whose positions are inherently located over basal bone, the intention being to keep
them there. Applying torque with rectangular
archwires is contraindicated because doing so
will move the centers of rotation away from their

30

Figure 2. (a) The horizontal distance between the
facial axis (FA) points of the teeth and the WALA
ridge is used to assess the mandibular arch width
and form. (b) That distance is 2 mm for molars

PCSO Bulletin Spring 2020

Pacific Coast Society of Orthodontists Bulletin Spring 2020

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