6: Prediction planning

6

Prediction planning


Chapter overview
Intended learning outcomes
6.1 Introduction
6.2 Prediction planning for orthognathic surgery
6.3 Methods of prediction planning

6.3.1 Model planning
6.3.2 Soft tissue profile prediction planning
6.3.3 Computerised prediction
6.3.4 Factors affecting the accuracy of prediction planning
6.3.5 Hard tissue and soft tissue ratios
6.3.6 Summary
6.4 The two dimensional planning process

6.4.1 Acquisition of lateral cephalogram
6.4.2 Acquisition of the lateral profile photograph
6.4.3 Superimposition of lateral cephalogram and profile photograph (Matching)
6.4.4 Planning
6.4.5 Simulation
6.4.6 Surgical planning sheet
6.5 Three dimensional prediction software
6.6 The three dimensional planning process – acquisition of different imaging modalities

6.6.1 CBCT
6.6.2 Three-dimensional stereophotogrammetry
6.6.3 Planning
6.7 Navigation surgery

6.7.1 Summary
6.8 References


Intended learning outcomes
By the end of this chapter the reader should:
  • Be aware of the current methods of prediction planning.
  • Understand and be able to apply the two-dimensional planning process in routine clinical practice.
  • Be aware of the advantages and disadvantages of two-dimensional planning.
  • Understand the steps in producing a three-dimensional ‘virtual patient’.
  • Have knowledge of navigation surgery and its potential future role in orthognathic surgery.

6.1 Introduction

The aim of this chapter is to introduce the different methods of orthognathic prediction planning, highlighting current practice to state of the art three-dimensional planning. The strength and short comings of each method of planning will be discussed. This chapter will describe in detail the methods of planning that are currently used by the Team; both two dimensional prediction software (CASSOS – Computer Assisted Surgical Simulation for Orthognathic Planning) and three dimensional prediction software (Maxilim)*. The majority of computer planning programs apply a similar methodology which will help the reader transfer knowledge and techniques learned in this chapter into their own clinical practice.

6.2 Prediction planning for orthognathic surgery

The successful outcome of orthognathic surgery requires precise surgical technique and comprehensive orthodontic treatment. More importantly the final outcome must provide an aesthetic improvement that meets the patient’s expectations and an occlusion that is functional. Involving the patient in the decision making process is an important part of pre-surgical planning and as such providing patients with a ‘realistic’ simulation of the post-surgical result is becoming a necessity. The current method of planning uses profile views of the patient in the form of lateral cephalograms and profile photographs. The challenge of planning is to position the teeth in an ideal class I incisor relationship whilst at the same time producing the correct final soft tissue appearance bearing in mind that the skeletal hard tissue lays in between (Figure 6.1).

Figure 6.1 Shows the interaction between soft tissue, hard tissue and teeth during the planning and execution phases of orthognathic surgery. Source: Ayoub, A.F. and Khambay, B. (2012) A paradigm shift in the diagnosis and management of dentofacial deformities. The Saudi Dental Journal, 24, 121–125. Reproduced with permission of Elsevier.

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Orthognathic surgical planning should achieve the following objectives:

1. Determine the final post-surgical dental occlusion – termed ‘model planning’.
2. Illustrate to the patient and Team the potential post-surgical soft tissue profile facial appearance – termed ‘soft tissue profile prediction planning’.
3. Determine the magnitude of skeletal hard tissue movement necessary at the time of surgery.

6.3 Methods of prediction planning

6.3.1 Model planning

Model surgery prediction is based on surgical procedures performed on study models, which are sectioned to achieve normal occlusal relationships. This gives an indication of the surgical skeletal movements required to correct the malocclusion but provides no information about the soft tissue change. It was soon realised that this approach may be detrimental to the facial profile and that an approximately ideal occlusion may not be accompanied with the best facial aesthetic results. However, model planning is still essential to predict the final dental occlusion and to construct the intermediate and final orthognathic wafers to guide the surgical procedure. This is dealt with in greater detail in Chapter 5 Orthognathic technical procedures.

6.3.2 Soft tissue profile prediction planning

Combined cephalometric tracings and photographs

Henderson (1974) introduced a technique that combined cephalometric radiographic tracings with photographs. A lateral transparent positive 1:1 photograph was superimposed on the patient’s lateral cephalometric radiograph. A tracing was made of the relevant hard tissue structures from the cephalometric radiograph onto the photograph. The photograph was then sectioned along the planned osteotomy sites and moved according to the desired surgical movements. The soft tissues were then moved according to the known ratios of hard to soft tissue movements, as in Figure 6.2.

Figure 6.2 1:1 prediction planning. Pre-operative profile (a). Superimposed lateral cephalogram radiograph showing predicted hard and soft tissue movements following a maxillary advancement (b).

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These photocephalometric planning techniques have proved useful in orthognathic planning and have been used in many centres, and are still being used in many units, to plan the outcomes of orthognathic surgery. These techniques, however, have a number of limitations:

1. The method of planning is complex and time-consuming.
2. The ratios of hard tissue to soft tissue movement have become increasingly complex and the surgeon or orthodontist must be very experienced to be able to predict the response of the soft tissues to bony movements. A degree of artistic skill is required when using such planning techniques.
3. The methods are not consistent and reproducible and are open to human error.
4. Differential movement of the soft tissues can be complex. It requires multiple cuts and small numerous segments to reproduce the soft tissue profile. This results in a number of step defects in the profile outline which does not provide a realistic prediction of facial appearance to patients.
5. The final plans are easily distorted and with time the adhesive degrades with parts of the plan moving or even being lost. This makes audit and research from these records difficult.
6. An experienced surgeon or orthodontist is able to visualise the results of prediction planning, but for the patient the sectioned photographs are difficult to interpret.

6.3.3 Computerised prediction

In an attempt to overcome the limitations of the manual techniques, a number of computer-based packages have been developed. The early computer packages were able to generate simple line drawings of profile prediction. Following the evolution and development of computer hardware and software, prediction imaging packages are able to incorporate lateral profile photographs of the patient, which could then be morphed to produce photo realistic predictions.

All of the modern commercially available packages are in essence computerisations of the manual planning technique. Firstly, specific landmarks are digitised on both the lateral cephalometric radiograph and the profile photograph; the number of landmarks vary for each program. The prediction software then automatically superimposes the radiograph and photograph. Obviously the soft tissue on both the cephalogram and the profile photograph needs to be identical for the best superimposition. The method of superimposition is often not known to the operator and the factors which may effect the accuracy are therefore unknown. The orthodontist or surgeon is then able to analyse dental, skeletal and soft tissue variables against standardised sets of linear and angular measurements and plan the surgery on-screen. The software is programmed using mathematical algorithms to allow the simulation of hard and soft tissue movements. The final profile prediction is a ‘morphed and smoothed’ photo realistic prediction of the proposed surgical outcome. Some packages allow the clinician to “tidy-up” the lips if they appear incorrect; this however introduces a subjective element to the prediction which may introduce even larger inaccuracies.

Advantages of computerised prediction

1. Computerised planning has greatly simplified the prediction process and reduced operator time. For most packages digitisation of the landmarks on the radiograph and profile photograph is time consuming but the real advantage comes from being able to rapidly and accurately analyse and predict surgical results. The operator can easily explore a number of orthodontic and surgical options.
2. The software may be programmed with a number of complex non-linear mathematically derived algorithms to predict the soft tissue response to the surgical movements. The software can easily be updated and the software algorithms adjusted, based on any new data that becomes available.
3. The computer software package is able to morph and smooth the image meaning less artistic skill is required. Some packages do have the option of using touch-up tools to enhance the final prediction.
4. The use of computer packages has improved the reproducibility of orthognathic surgical planning. The process of planning is standardised from digitising landmarks and superimposition through to morphing and smoothing of the final prediction. Orthodontists and surgeons are able to produce similar predictions and in the same format.
5. Computerised predictions are easily stored and retrievable at a later date. This method lends itself to audit and research more so than the manual prediction technique. It is relatively easy to back up patient data and plans and electronically transfer information between various hospital sites.
6. The greatest advantage of software prediction packages is the ability to produce photorealistic post-surgical predictions. This greatly improves patient participation in the planning process and communication between orthodontist, surgeon and patient. This has a clear positive impact on patient satisfaction following surgery.
7. As a result of the ability of computer packages to calculate calibrated distances and angles it is more accurate to calculate the desired surgical movements.

Disadvantages of computerised prediction

1. Initial set up cost of the hardware and software.
2. As with any new technique there is a learning process.
3. The patient may perceive the photo realistic images to imply a guarantee of treatment result. In reality the actual surgical outcome is usually more aesthetically pleasing than the prediction. To prevent patients believing that the prediction will be a guarantee of the actual surgical outcome some authors have advocated adding the phrase ‘Treatment Simulation Only’ to all predictions. It has also been suggested that the patient should sign a waiver stating that the prediction is only an approximation of the actual surgical outcome. The Team has however determined the validity of CASSOS scientifically (Jones et al., 2007).
4. The increased accuracy of prediction planning must be matched by surgical accuracy.
5. The majority of computer programs cannot deal with ‘incompetent lips’ and as a result the final prediction around the lips is often poor. Some manufacturers have added ‘tools’ which allow for correction but this can undermine the process and once again introduces subjectivity.

6.3.4 Factors affecting the accuracy of prediction planning

The accuracy and reliability of prediction planning is dependant on a number of factors. These include:

1. Cephalometry. Lateral cephalograms are subject to errors of projection during capture as well as landmark identification and measurement errors.
2. Photography. Photographs are two-dimensional representations of three-dimensional objects and are therefore subject to the same errors as lateral cephalometric radiographs.
3. Superimposition of radiographic and photographic images. Manual methods of superimposition have been shown to be inaccurate. The errors are the result of three factors: differential magnification and distortion of the cephalometric radiograph and photograph, errors of landmark identification on both the photograph and radiograph are combined during superimposition increasing potential errors.
4. Soft tissue response to the underlying hard tissue movements. The ratios of hard to soft tissue movement currently used in prediction planning are average responses. Ratios will only describe a relationship between two specific points and it is unlikely that a series of ratios will be able to predict the response of the entire soft tissue drape. Unfortunately there is considerable individual variation and this can lead to inaccurate predictions of the outcome of orthognathic surgery.

6.3.5 Hard tissue and soft tissue ratios

Regardless of the method of prediction planning the hard and soft tissues are broadly the same and based on historical data, Figure 6.3. Orthognathic surgical simulation programs are based on algorithms that relate soft tissue response to the underlying skeletal change. The soft tissues are stretched, or become thin­ner as the maxilla is advanced in surgery and interpreting this interaction is important in predicting the outcome. The relative changes are expressed as mean ratios of soft tissue movements versus those of the hard tissues, and the software packages are based on these values. Therefore, though clinically useful, computerised predictions of soft tissue profile changes after orthognat/>

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Dec 31, 2014 | Posted by in Orthodontics | Comments Off on 6: Prediction planning
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