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Leeway Space in Primary Dentition

August 10, 2015

Dental question written for National Dental Board Exam review course.

 

Which of the following pertains to Leeway Space in the primary dentition?

  1. The size difference in the posterior segments of both arches.
  2. The four maxillary permanent incisors are 7.6 mm wider than the primary incisors
  3. The four mandibular permanent incisors are 6.0 mm wider than the primary incisors
  4. Most arches do have somewhat wider spaces mesial to the maxillary canines and distal to the mandibular canines.
  5. The distance (in millimeters) from the facial surface of the maxillary incisors to the facial surface of the mandibular incisors when teeth are in occlusion

 

 

Answer & Explanation:                                                     

The answer is 1.

The size difference in the posterior segments of both arches is termed the Leeway Space. The combined length of the primary canine, first primary molar and second primary molar are, on average, 1.7 mm greater on each side of the mandibular arch than that of their permanent successors (canine, first premolar, and second premolar). In the maxillary arch, the primary teeth are only .9 mm greater than their permanent counterparts.

The four maxillary permanent incisors are 7.6 mm larger than the primary incisors (Choice 2) is incorrect. However; it is an important observation in evaluating space analysis during the incisor transition years (from ages 6 to 8). Taken with the observation that the four mandibular permanent incisors are 6.0 mm larger than the primary incisors, (Choice 3), these inverse size differentials have been termed the “incisor liability”.

In (Choice 4), the fact that most arches have somewhat noticeably wider spaces mesial to the maxillary canines and distal to the mandibular canines has nothing to do with the Leeway Space. Instead, these observations are termed “primate spaces”. Primate space ranges from 0 mm to 10 mm in the maxillary arch, with an average of 4 mm. The primate space range in the mandible is from 0 mm to 6 mm, with a mean of 3 mm. The primate spaces are of interest because they are utilized when the wider permanent incisors of both arches erupt.

(Choice 5) is also false. The distance (in millimeters) from the facial surface of the maxillary incisors to the facial surface of the mandibular incisors (when teeth are in occlusion) is the definition of “horizontal overjet”.

 

References:

Mathewson, R.J., Primosch, R.E. (1995). Fundamentals of Pediatric Dentistry (3rd Edition). US. Quintessence Publishing.

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