Space Analysis Exercises

About these exercises

Practice problems for mixed-dentition space analysis, compiled from the OrthoInstruction module 04 - Space Analysis and Its Interpretation. Each exercise shows the casts and the question; attempt your judgment first, then expand the Answer callout to check it. See Level II Unit B Summary for the underlying theory (Tanaka-Johnston formula, leeway space, molar shift, E-line).

Case Reviews

Study the casts shown to decide whether the space analysis result a fellow student calculated is correct, or instead is an over- or under-estimate of the space needed. Unless stated otherwise, assume normal skeletal and soft tissue relationships and no molar shift.

Case Review 1

Is this patient really 4 mm short in the lower arch?

Answer

Because of the minimal irregularity of the incisors and the available leeway space, this is an overestimation of the space needed. If there is a space discrepancy, it is quite small.

It would take about 2 mm additional space to align the lower incisors, and 2–3 mm space will be available on each side during the transition from primary molars to canines — so the space analysis result of -4 mm almost surely results from an error in calculating either space available or space required.

Case Review 2

The space analysis form produced for this patient shows a -1 mm space discrepancy for both arches. Is this patient really only 1 mm short of space?

Answer

Given the availability of leeway space, this appears to be a good estimation (if the patient is skeletal Class I). Crowding of this extent in the early mixed dentition has the potential to improve during the transition to the full permanent dentition.

It would require about 3 mm space to bring the right lateral incisor into alignment, and perhaps another 2 mm to align the other incisors — but 2–3 mm leeway space on both sides will be available, so a -1 mm discrepancy is a reasonable estimate.

Case Review 3

Based on your observation of these casts, is this patient really 3 mm short of space in both arches, as the space analysis form shows?

Answer

For this patient, the space analysis result almost surely underestimates the size of the discrepancy.

The mandibular teeth are aligned, but no primary canines are present. In the maxillary arch the permanent lateral incisors have not erupted, and there is almost no space available for them. This patient is really closer to -7 mm in each arch.

In the mandibular arch there is very little space for the permanent canines — about 1 mm on one side and 2 mm on the other. The canines are about 7 mm wide (14 mm for both), so the initial discrepancy is ~11 mm; even if 4 mm is gained from leeway space, the discrepancy is about 7 mm — not the -3 mm reported. In the maxillary arch there is little or no room for the lateral incisors (~6 mm wide each), so the math is the same.

Performing a Space Analysis

Now apply the whole method. Perform a space analysis for the patient whose casts are shown here (Image 1). Note the irregularity in both arches. Using the measurement method and the prediction formula at the bottom of the form, Sections 1 through 6 have already been completed (Image 2).

Image 1: Casts for performance of space analysis.	Image 2: Partially completed space analysis form (Sections 1–6).

The patient’s face and profile tracing reveal a Class I skeletal relationship (with a mild Class II tendency?), normal vertical relationships, and slightly protrusive lip posture (Image 1). The dental casts reveal an end-to-end molar relationship on the right and left sides and incisors in both arches that look upright (Image 2). This information is recorded in Sections 7–10 (Image 3).

Now synthesize the information in all sections and write out the interpretation of the results — what you would put in the final space on the form. Do that before you reveal the answer.

Image 1: Profile photo and drawing — tells you whether there is a skeletal jaw discrepancy in the A-P or vertical planes, and whether there is incisor protrusion or retrusion.	Image 2: Dental casts in occlusion — note the end-to-end molar occlusion.
Image 3: Profile and occlusion data added to the space analysis form.

Answer — interpretation of the results

There are small maxillary (-0.9 mm) and mandibular (-1.6 mm) space deficiencies predicted when space required is compared to space available. When this is combined with the molar shift (3.4 mm), there appears to be a more significant demand for space in the mandibular arch (1.6 mm + 3.4 mm).

The upright lower incisor position suggests additional arch circumference could be created by proclining the lower incisors. But this patient’s lips appear well related or slightly protrusive, so proclining the lower incisors might make her lips protrusive or incompetent — her face is not compatible with more than very little forward movement of the incisors.

An alternate approach (detailed in Level III) might be space management with a lower lingual arch in the late mixed dentition to prevent loss of the leeway space and prevent the molar shift. The lower incisors can be proclined slightly to accommodate the 1.6 mm of predicted crowding, and the upper molars then distalized to obtain a Class I molar relationship.

Self-Test

Attempt each question, then expand the answer. (Reading assignment for this module: Contemporary Orthodontics 5e pp. 427–429 / 4e pp. 195–201.)

Question 1

In doing a space analysis, what is the significance of observing a skeletal Class III relationship?

1. incisor position assumption is violated
2. tooth size correlation assumption is violated
3. molar repositioning assumption is violated
4. no significance, it makes no difference to space

Answer

1 — incisor position assumption is violated. In a Class III patient, as in a Class II patient, the incisor position is likely to change, changing the space available. Usually the lower incisor moves lingually in a child with a Class III growth pattern, decreasing the available space. Tooth size correlations and molar repositioning are not a major component of the problem in these patients.

Question 2

In space analysis, how do you account for the change in first permanent molar position that may occur during the transition to the permanent dentition?

1. measure the distance from lower buccal cusp to upper groove
2. measure the distance from upper buccal cusp to lower groove
3. measure the size difference between upper and lower premolars
4. subtract the average mesial movement from space available

Answer

2 — measure the distance from upper buccal cusp to lower groove. This is the distance the lower molar would have to move forward to obtain a Class I relationship (the molar shift).

Question 3

Cephalometric analysis shows that in your patient, the lower lip is slightly behind the E line. Your interpretation is:

1. slight protrusion
2. normal lip position
3. slight retrusion
4. severe retrusion, the lip should be in front of the E line

Answer

2 — normal lip position. The lower lip normally sits slightly behind the E line, although the E line can be affected by the size of the nose or chin and should not be the sole judge of lip position.

Question 4

Why do prediction tables based on tooth size correlations work better for Caucasian than other population groups?

1. Caucasian teeth are more predictable
2. upper lateral incisor variation is less in Caucasians
3. lower premolars are extremely variable in Orientals
4. published tables are based largely on data from Caucasians

Answer

4 — published tables are based largely on data from Caucasians. There is no reason to think the teeth of other groups are less predictable, but the size relationships differ. Ideally a prediction table would exist for each population group, but these specialized tables simply don’t exist.

Question 5

In space analysis, why is the prediction formula different for the maxillary and mandibular teeth?

1. it isn’t, the same formula works
2. different formula required because the correlation coefficients are different for the two arches
3. different formula required because the correlations are different for the two arches
4. different formula required because the method is different for the upper from the lower arch

Answer

3 — because the correlations are different for the two arches. The correlations themselves differ even though the method and the correlation coefficients are the same (hence +10.5 mm mandibular vs +11.0 mm maxillary).

Question 6

During the mixed dentition, which is the preferred method of measuring space available for the permanent teeth?

1. measure the width of each tooth individually and sum the numbers
2. measure the intercanine and intermolar widths and sum the numbers
3. form a wire to the ideal catenary curve and measure its perimeter
4. measure the length of arch segments from first molar to first molar

Answer

4 — measure the length of arch segments from first molar to first molar. A catenary curve molar-to-molar is another possibility but is less accurate. Intermolar/intercanine widths cannot give space available (a perimeter measurement), and measuring teeth individually isn’t possible until they erupt — and would give space required, not available.

Question 7

In doing a space analysis, what is the significance of observing a skeletal Class II relationship?

1. incisor position assumption is violated
2. tooth size correlation assumption is violated
3. molar repositioning assumption is violated
4. no significance, it makes no difference to space

Answer

1 — incisor position assumption is violated. In a Class II patient the incisor position is likely to change, changing the space available. Usually the lower incisor moves facially in a child with a Class II growth pattern (the opposite of Class III).

Question 8

Which of the following is not a frequent cause of error in space analysis?

1. inaccurate measurements of space available
2. inaccurate measurement of incisor width
3. incorrectly trimmed dental casts
4. they’re all important causes of error

Answer

4 — they’re all important causes of error. Incorrectly trimmed casts (so molar position is shown incorrectly) is one of the most important.

Question 9

Which of the following is not a valid method for calculating the space required for the unerupted permanent teeth?

1. radiographic evaluation
2. tooth size correlation
3. jaw size–tooth size correlation
4. radiographic plus tooth size correlation

Answer

3 — jaw size–tooth size correlation. This is not a valid way to predict how much space is required; the others are valid methods.

Question 10

If your patient has protrusive lips as judged from facial form analysis, how would that affect your interpretation of space analysis results?

1. patient has more space than the analysis indicates
2. patient has less space than the analysis indicates
3. space may increase as incisor becomes more protrusive
4. space may increase as molar moves forward less than it would otherwise

Answer

2 — less space than the analysis indicates. In a child with protrusive incisors, space analysis tends to overestimate the amount of space, because some space may be needed to reduce the protrusion. The protrusion is unlikely to change spontaneously and doesn’t affect molar shift.

Question 11

Which of the following is not a factor to consider in interpreting the results of space analysis?

1. skeletal classification
2. dental classification
3. incisor position
4. dental arch growth
5. they all must be considered

Answer

4 — dental arch growth. A valid assumption of mixed dentition analysis is that there is no significant growth of the dental arches anterior to the mesial of the first permanent molars, so growth there need not be considered. Skeletal classification, dental classification and incisor position are all key variables.

Question 12

Which of the following is a way that Class I molars are achieved in a child who is skeletally Class I but has flush terminal plane primary molars?

1. distal shift of the upper molar
2. mesial shift of the lower molar
3. differential forward growth of the upper jaw
4. restriction of mandibular growth

Answer

2 — mesial shift of the lower molar (due to tooth movement and/or differential forward growth of the lower jaw). Upper molars don’t shift distally during normal development, and both forward growth of the upper jaw and restriction of mandibular growth would make things worse.

Question 13

(A) The size of the unerupted mandibular canine and premolars can be established better from the size of the maxillary than the mandibular incisors because (B) there is an excellent correlation between the size of the upper incisors and the lower canines/premolars.

1. A true, B true, A and B related
2. A true, B true, A and B not related
3. A true, B false
4. A false, B true
5. A and B false

Answer

5 — A and B both false. There isn’t a good correlation between the size of upper incisors and other teeth because upper lateral incisors are so variable in size. That’s why the size of both upper and lower canines/premolars is estimated better from the size of the lower incisors.

Question 14

(A) During the mixed dentition, space available for the permanent teeth often is less than the space required because (B) the primary incisors are significantly smaller than the permanent incisors that replace them.

1. A true, B true, A and B related
2. A true, B true, A and B not related
3. A true, B false
4. A false, B true
5. A and B false

Answer

2 — A and B both true, but not related. Crowding has multiple causes, and the size difference between primary and permanent incisors is not a major one.

Question 15

Which of the following is not a key assumption in mixed dentition space analysis?

1. There is a high correlation between the size of anterior and posterior teeth.
2. Jaw growth occurs primarily in the tooth-bearing area of the jaws.
3. The patient fits the reference population group.
4. The incisors will not move facially or lingually during further growth.
5. Repositioning of the molars during the premolar exchange is predictable.

Answer

2 — jaw growth occurs primarily in the tooth-bearing area. The key assumption is the opposite: jaw growth does not occur in the tooth-bearing areas, so this statement is incorrect. The other statements are correct as written.

Question 16

What is suggested by a unilateral Class II molar relationship in a child who is skeletal Class I?

1. loss of space in the upper arch
2. loss of space in the lower arch
3. Class III growth pattern with uprighting of the lower incisors
4. eruption along a wider perimeter

Answer

1 — loss of space in the upper arch. The lower molar is in the correct A-P position while the upper molar is further forward due to space loss, creating the Class II molar relationship on that side.