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The combined mesio-distal width of the permanent canine and premolars is usually less than that of the deciduous canines and molars.
This surplus space is called Leeway Space of Nance.
Described by Nance in 1947
According to Nance, this difference is 1.7 mm in the mandible and 0.9 mm in the maxilla, in each quadrant.
Maxillary arch=1.8mm (0.9mm on each side)
Mandibular arch=3.4mm (1.7mm on each side)
Utilized for the mesial drift of the mandibular molars to establish Class I molar relation.
On average 0.7 - 1.3 mm excess space exist in each upper quadrant and 1.6 - 2.7 mm in the lower quadrants with significant individual variation
During normal occlusal development the change of the buccal segment starts approximately at the age of 9 ¾ in girls and 10 ½ in boys and finishes 1 ½ years later when the permanent canine of the maxilla erupts, varying individually.
The Leeway Space is available particularly because the second primary molars are on average 2 mm larger than the second permanent premolars.
Significance of Leeway space of Nance
- Presence of excessive Leeway space provides for mesial movement of permanent molars.
- Leeway space in mandibular arch is more than of maxillary arch
- because primary mandibular molars
- Wider than primary maxillary molars.
- Leeway space differential between two arches cause mandibular 1st molar to move mesially more than maxillary 1st molar.
- Such arrangement causes change in molar relationship from end on in early mixed dentition period (late mesial shift) to class 1.
- Leeway space deficiency seen in some individuals when size of un erupted premolars and permanent canine are larger than space available.
- In some cases the Leeway Space is reduced either when the deciduous teeth concerned develop cavities on approximal surfaces or have to be extracted too early due to decay . In such cases an orthodontic treatment can become necessary to maintain the space for the permanent teeth.
- The leeway space provides adequate space to resolve crowding that is present in the mixed dentition in the majority of individuals.
- This space can be maintained by preserving arch length with a lingual arch as the primary teeth begin to exfoliate, unless conditions such as the premature loss of a primary canine require earlier intervention.
- A lip bumper can also be inserted after the eruption of the first premolars to preserve arch length.
How to measure?
- For the measurements two sets of plaster casts were used from each subject, one of the deciduous dentition, the other of the permanent dentition.
- The mesio-distal crown size of the following groups of teeth at the points of mutual contact in each quadrant of each cast was measured:
- Group 1: deciduous canine, first and second molar
- Group 2 : permanent canine, first and second premolar
- In order to calculate the Leeway Space the sums of the sizes of the teeth in these two groups was formed. The Leeway Space is then the difference between the sums of the deciduous and the permanent teeth, which is group 1 minus group 2.
- Before starting the actual measurements, digital and analogue callipers were compared by repeating several measurements with both types of callipers.
- The actual measurements were then carried out using a digital sliding calliper. The branches of the calliper were sharpened to assure a precise positioning of the spikes.
