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Orthodontic diagnosis is complex and multifactorial, and an initial space analysis informing a decision whether or not to extract teeth, is consistently highlighted as the most significant diagnostic factor, with other clinical factors being secondary. Although most clinicians are taught a method of space analysis, few subsequently place their confidence in any formal method and furthermore may not consider the space implications of the curve of Spee. A survey of orthodontists revealed a surprising variability in the assessment of crowding, as well as a tendency to make a different diagnostic decision on the same case sometime later. This two-part series explores the current status of space analysis and suggests how we may be able to limit the potential for making poor extraction decisions. Part 1 explored the various factors that need to be considered in an orthodontic space analysis, and in particular, the space implications of the curve of Spee. Part 2 examines the various methods and tools available to the clinician in assessing orthodontic crowding. We also look at ways in which potentially poor extraction decisions may be mitigated in clinical practice.
CPD/Clinical Relevance: To help understand the clinical significance of space analysis as the key diagnostic factor informing a decision whether to extract teeth or not.
Article
The first part of this two-part series, explored the various factors that need to be considered in an orthodontic space analysis, and in particular, the space implications of the curve of Spee.1 Here we will explore the various methods and the tools available for the clinician. We will also look at the ways in which potentially poor extraction decisions may be mitigated in clinical practice.
Methods of space analysis
Although there are published average sizes of teeth, to accurately measure the all-important mesio-distal tooth widths, either a ruler, dividers and a ruler, or manual/digital calipers are required (Figure 1). However, as already highlighted, the challenge with using these instruments is being able to accurately position them at the mesial and distal contact points of the teeth. Nonetheless, some form of mesio-distal tooth width measurement is required in the following methods of space analysis.
Figure 1. Digital calipers used for measuring mesio-distal tooth widths.
The brass wire method
The brass wire method was a technique originally intended to monitor tooth movement, but it was later adapted by Nance2 as a method of quantifying crowding. This technique (still taught in some US dental schools) involves placing a brass wire around the lower arch on the occlusal surfaces, above the estimated positions of the contact points, while maintaining a smooth curve. It is assumed that by subtracting this wire length from the total mesio-distal widths of the teeth it is possible to quantify the crowding. However, a wire placed into the curve of Spee will be longer than the potential post-treatment flat arch length, therefore underestimating the degree of crowding.
Figure 2.
(a,b) The ‘Royal London’ method of measuring individual teeth and available space.Figure 3. Visualized arch form used in the Royal London space analysis.6Figure 4. Potential for measurement error of mesio-distal tooth width when contact points are obscured or difficult to access.Figure 5. Orthodontic arch length ruler with a range of 12 sizes.
Segmental measurement
The segmental measurement method described by Proffit3 divides the arch into perhaps six or eight straight segments for space analysis. This method was originally devised by Lundstrom4 and later developed by Merrifield.5 It is a quick and simple method, but prone to error in two principal ways. First, the selected points may not represent the underlying arch form, and secondly the mathematical resolution of a curve into a series of straight lines will always underestimate arch length. It is suggested that the segmental measurements can be taken from a photocopy, as well as directly from the model, but if there is a curve of Spee present, the results will differ. A photocopy of tipped teeth will tend to underestimate their mesio-distal width and will suffer the same issues with respect to the curve of Spee if only viewed perpendicular to the occlusal plane, as described in Figure 6 in Part 1.1
Figure 6. Clinical view of lower arch.
Royal London Space Analysis
Another widely taught method of space analysis in the UK is the Royal London Space Analysis, which was developed by Kirschen et al.6 In terms of diagnosis and treatment planning, this method represents a useful approach, based on a comprehensive ‘profit and loss’ analysis of dento-alveolar disproportion that also incorporates treatment objectives. Crowding or spacing is quantified by first selecting an arch form that reflects the position of the majority of teeth in the arch. With the chosen arch form in mind, the misaligned teeth can be measured, individually or in pairs, with a clear ruler (Figure 2) and compared to the space available in the arch mesial to the second molars.
Kirschen points out that it is important to quantify the crowding ‘…in relation to the archform that reflects the majority of teeth, not necessarily the imaginary arch that passes through the incisal edge of the most prominent central incisor…’ (Figure 3).6
The adjustment made to level the curve of Spee is based on a mathematical formula.7 Levelling the arch requires relative extrusion of the lower premolars, and an estimate of the depth of curve is made by measuring from the premolar cusp tips to a plane joining the lower incisor edges to the distal cusp tips of the mandibular first molars. The comprehensive space planning form requires an initial assessment of the prevailing space conditions, followed by a series of adjustments based on treatment planning. However, several aspects of this analysis may be subject to error. First, it is possible that measurements may be made with a diagnostic decision already in mind, regarding for example the need for extractions. Secondly, estimation of the space needed to correct overlaps or displaced teeth is subject to individual error (Figure 4).
When teeth are displaced, it may also be difficult to maintain, in the mind's eye, an ideal arch form against which the measurements are made. Thirdly, a standard ruler introduces the potential for further inaccuracy when fractions of millimetres are being visualized. Fourthly, assessment of the space needed to level the curve of Spee is based not only on a subjective estimation of the depth of the curve, but also on a potentially inaccurate mathematical model. Once the space deficit of an arch has been quantified in millimetres, this comprehensive analysis is developed beyond that point to include a range of diagnostic factors, such as the potential to procline lower incisors, that may modify the figure. A further problem therefore, is that the modifications may have been based on an inaccurate assessment of any space deficit in the first place. Finally, it is necessary to consider the contact points in detail. In this analysis it is suggested that a curve of Spee is essentially the outcome of a series of vertically slipped contacts. However, while some teeth may have an acceptable contact area, which allows very slight tipping of adjacent teeth, others may not when it comes to contributing to the curve of Spee. It is very difficult on a study cast to determine the exact nature and acceptability of each contact. A series of very slightly tipped teeth with acceptable but high contact areas will still lead to a curve of Spee that requires space for levelling.
Computerized methods of space analysis
There are an increasing number of scanning methods providing a computerized three-dimensional image. Using specific software, a comprehensive range of diagnostic information may be obtained ‘on screen’, including an assessment of crowding. Within the software itself, ‘digital calipers’ can be used to measure individual tooth widths, and arch length is determined by regulating a series of control points to produce a ‘soft wire’. However, digital scanning can lead to three basic sources of error. The first is that some of the displaced contact points may be obliterated and so it is necessary to guess where they are; the second is the subjective aspect of deciding exactly where to place the arch form over the teeth, the third being the reduced mesio-distal image of tipped teeth when viewed from above.
Space analysis ruler
This orthodontic arch length ruler (Figure 5) was developed because there are currently no specific tools available to accurately quantify arch length, incorporating the space needed to align the teeth and level the curve of Spee.8 In developing the ruler, the mathematical arch form that offered the best fit to an average of 200 randomly selected arch forms drawn from pre-treatment cases was determined. The best-fit mathematical shape was the fourth polynomial curve, which agreed with the findings of other researchers who identified this as the common European arch form.9,10 If arch levelling is a treatment goal, then it is necessary to measure the arch using a flat rigid ruler, ie two-dimensionally, and within this, to estimate the proposed final labio-lingual position of the incisor teeth using the inbuilt anterior millimetre rule. In addition, it also requires the mesio-distal tooth widths of the teeth to be measured on a study model.
Simple instructions are printed on the bottom of the ruler, namely:
Visualize the most appropriate arch form above contact points;
Record the arch length to most distal contact point and deduct from total mesio-distal tooth widths;
The amount of crowding includes space needed to level curve of Spee.
The following clinical case (Figures 6–8) indicates that approximately 6.7 mm of additional space is required to align and level the arch. The total mesio-distal tooth widths equal 83.7 mm, the ruler indicates an arch length of approximately 77 mm.
Figure 7. Lateral view showing curve of Spee.Figure 8. Direct measurement of study model with flat ruler showing approximate arch length to the distal of the first molars of 77 mm.
Although it is suggested that the ruler could be a useful tool to identify outliers, plan borderline extraction cases and have a place in ‘self-calibration’, it is yet to be fully validated with further research and will be subject to some of the limitations of each of the methods, namely accurate measurement of tooth widths when the contact point are not visible.
How can we justify and therefore defend a decision to extract?
Although clinicians may be called upon to justify their decisions regarding extraction choices at a later date, it would seem the majority still use ‘eyeballing’ to assess crowding, despite the evidence suggesting this method can be unreliable and surprisingly variable. However, the space analyses available to the orthodontist have their own limitations, and this often starts with the difficulty in identifying the true mesio-distal widths of the teeth within the arch, and estimating the pre- and post-treatment arch length and shape.
Decisions can be supported by referring to a range of clinical factors, although the focus may be more specifically on quantifying the crowding, particularly in borderline situations. Generally accepted criteria do exist, such as the recommendation by Proffit3 that lower arch crowding in excess of 4–5 mm may warrant extractions. Occasional ‘self-calibration’ may further support the notion of ‘experienced eyeballing’, which would appear to be a reasonable and defendable stance, as would peer review where estimating crowding and treatment planning decisions are regularly discussed.
To further overcome issues with possible litigation, it is recommended that clinical diagnostic records could include notes to highlight any key factors regarding the extract/non-extract decision, especially in perceived ‘borderline’ cases. At a later date, it may be difficult to recapture the reason for the original diagnostic decision. By way of an example a highlighted note might include:
In this case a non-extraction approach was made because:
There is only mild crowding of approximately 4 mm;
Some space can be made by rounding out the arch in the premolar area;
The patient has a retrusive profile, and a small degree of lower incisor proclination followed by the provision of a bonded retainer would be acceptable.
The complexity and potential for errors in quantifying crowding is further complicated by natural underlying skeletal changes, as well as an unpredictable tissue response to the treatment itself. It is therefore suggested that the borderline has a significant bandwidth that may justify an initial non-extraction approach. This is because, even in some essentially moderately crowded cases, there may be an advantage to delaying extractions in the early stage of treatment in order to gauge compliance with treatment.
Thirty years and many thousands of volumes of orthodontic research have passed since the warning by Graber in 1991 that orthodontists may struggle to defend a decision regarding the need for extractions.1 As outlined in this two-part series, it is interesting to see how over 60 orthodontists can have such wide opinions regarding degrees of crowding and the need for extractions, as well as the tendency to make a different decision on the same case several months later. In such a complex and dynamic environment as a set of teeth within an active appliance, with unpredictable underlying changes, it is perhaps no surprise that most orthodontists will occasionally need to reconsider an initial diagnosis. In such cases, a review of the initially deemed space requirements will continue to inform and improve our diagnostic ability.
