33rd Annual Scientific Meeting proceedings

Clinical application of CAD-based surgical planning and 3D-printed patient-specific guide systems (CAD-PSG) is become increasingly widespread in veterinary orthopaedics and neurosurgery. This has been driven by a number of factors including a rapidly growing evidence-base showing clinically relevant benefits over traditional techniques, a greater range of available guide systems, wider availability of high-quality 3D-imaging in clinics, and improved CAD-planning and PSGs based on gained experience and technological improvements. However the technology is outside the direct experience of most surgeons, and thus embarking on a first case, and optimising the application of guided technology, and not necessarily intuitive. This presentation will review five key practical steps which will facilitate optimal application of CAD-PSG in the clinic.

Step 1 – know your indications
CAD-PSG is a tool at the disposal of the surgeon – just like CT, arthroscopy, or even a pair of Gelpis. Knowing when, and how best to deploy these tools is our responsibility as surgeons, and plays an important part in optimising clinical outcomes. Whilst some general principles will always apply, we all approach cases slightly differently – there’s rarely a single correct method. The same applies in the context of CAD-PSG. Some surgeons find this approach helpful for just one or two indications, others use a greater range of guides, some use none at all – and this will vary with experience and other practical factors such as client finances.

The key message is that it makes sense to understand what indications CAD-PSG can be used for – only then is it possible to decide whether there would be potential benefit a specific case.

Step 2 – imaging optimisation
For almost all orthopaedic and neurosurgical CAD-PSG indications this means obtaining good quality CT data. The process of extracting a virtual 3D bone model that can be exported to CAD software is called segmentation and is done in medical image viewing software from a surface-rendered reconstruction. Key factors that promote a high resolution surface-render include thin slice thickness, a scanned field-of-view collimated to the immediate area of interest, and a high-frequency reconstruction algorithm. For very small bones sub-mm slices are necessary, however in most situations 1mm slices are adequate. Thinner slices are not always better as they can adversely affect signal:noise ratio; a noisy scan significantly affects the quality of a surface-rendered reconstruction. Scan quality can also be enhanced by optimal patient positioning, for example scanning each forelimb separately (to allow minimisation of scanned FOV) or scanning thoracic spines in dorsal recumbency (to minimise breathing movement artefact).

It is always worth checking a surface-rendered reconstruction with the patient still in the scanner; note that most workstations by default display a volume-rendered reconstruction which typically appears superficially better than a surface-render.

A further consideration is to ensure all of the anatomy required for the CAD-based assessment is scanned. For example for antebrachial deformities the whole humerus, antebrachium and paw are scanned with the elbow at approximate standing angle and the antebrachium positioned without induced pronation or supination. If you are using an external CAD-PSG provider and are in doubt it’s always worth checking pre-scan.

Step 3 – surgical planning
One of the key advantages of CAD-PSG is the ability to assess deformities and plan surgery in 3D, avoiding many of the difficulties associated with traditional techniques reliant on 2D quantification of deformity components. However, the necessary CAD-based techniques require considerable training and experience, and most surgeons utilise external CAD-PSG providers. The two classic models are technician-led and surgeon-led services. In the former the surgeon works with a CAD-tech, often via screen-sharing, to determine a surgical plan; the CAD-tech then designs the necessary guides. In a surgeon-led service the assessment and planning processes are done by a surgeon in consultation with the attending surgeon; guides are then made by CAD-techs.

With either approach good communication is key – the more information the CAD-PSG provider has regarding the attending surgeon’s initial assessment of the case and vision for the correction and guide system, the better the result will be. For surgeon-led providers provision of additional information such as photos and gait videos can be extremely helpful.

Step 4 – intra-operative principles
Appropriate intra-operative use of PSGs is important. Although introducing guides for the first time can seem daunting this typically requires only minimal alteration to the normal surgical sequence, and the specific techniques required are very straightforward. Appropriately designed PSGs rarely require modification of a standard surgical approach, and are designed to avoid anatomic features that cannot be easily retracted or elevated such as neurovascular structures and collateral ligaments. Good contact between guide footprints and the cortex is key, and this does require supra-periosteal elevation of adherent soft tissues. In a well designed PSG system the guide footprints are relatively small (certainly smaller than the contact area of a plate) and are designed to fit in areas with minimal soft tissue attachments, or areas of fascial or muscular attachment where elevation and subsequent repair is minimally deleterious (e.g. analogous to hamstring muscle insertion elevation for a TPLO). However in some situations such as malunion revision more extensive adhesions may require more aggressive cortical preparation.

Once the cortex has been prepared guide fit can be assessed and if necessary compared to fit on a sterile bone model - a key element of PSG design is ensuring that a single position of fit is achievable in practice. Once correct positioning is established the guide is secured to the cortex, usually with negative-profile pins. These should be carefully aligned with the channels in the guide, and care taken during initial insertion, to ensure the guide doesn’t move or tilt as the pins are placed. Pre-drilling of the near cortex can be helpful in some cases.

The final key guide-related step is ensuring the saw blade remains parallel to, and in contact with, the saw guide plane. It can help to have an assistant viewing the blade from the side. Reduction guide placement is usually straightforward – this is usually best achieved by reducing the osteotomy and aligning the reduction pins before placing the guide. This makes application easier and reduces lever forces on the guide.

Step 5 – post-operative imaging and follow-up
Post-operative radiographs are sufficient for routine orthopaedic procedures, but post-op CT should be performed following guided vertebral stabilisations as 3D-imaging is necessary to adequately assess screw position. For orthopaedic applications post-op CT allows the accuracy of deformity correction to be assessed digitally in 6 degrees of freedom using iterative closest point segment orientation matching, however this is time-consuming and is usually only done for research purposes.

Clinical follow-up is always important but in the relatively new field of CAD-PSG this is especially the case to allow assessment of both short and long term outcomes. Additionally case-by-case feedback regarding surgical planning and guide design is possible - this is probably the most important single mechanism by which CAD-PSG techniques have been, and can continue to be refined and improved.