Computer-assisted Spinal Surgery for Deformity – A Review


Haigron et al. are the lack of effective demonstrations of patient benefits in terms of cost-effectiveness or cost savings, the reluctance of surgeons to use it and the continual emergence of newer techniques. There is clearly a matter of cost savings, but there are no studies looking at the benefit in terms of reduced hospital stay, lower infection rate and improved functional outcome when an expensive technology is implemented.55


With the lower incidence of pedicular


cortex violation using CASS, and the majority of cases affecting the lateral cortex, one would assume that a reduction in neurological complications would result in early rehabilitation of the patient back into the community. A surgeon’s future practice is commonly dependent on experiences during training, so, as fewer centres are implementing CASS, fewer future consultants (current trainees) would be experienced and hence comfortable in practising CASS. Also, to improve accuracy, newer techniques are inevitably developed, making it imperative for the surgeon to keep up to date with current technology. This is time-consuming and difficult in a busy surgical practice.


Computer-assisted Spinal Surgery Other than Navigation


Software has also been developed using fuzzy logic to assist surgeons with pre-operative selection of curve instrumentations and fusion levels in surgery for adolescent idiopathic scoliosis.56,57


Finite element modelling is a useful biomechanical simulation for obtaining clinical evidence by allowing integrated measurement of mechanical behaviour in a model. Hato et al. had conducted a finite element analysis on closing-opening correction osteotomy for angular kyphosis and concluded that such a model allows testing for biomechanical stability and hence planning for surgery.58


Intra-operative realtime assessment of external trunk geometry is usually by visual means and can be inaccurate. Duong et al. used an optoelectronic camera system that performed continuous tracking of trunk geometry. However, further clinical studies have been suggested to validate the procedure.59


Discussion of Computer-assisted Spinal Surgery in Scoliosis and Specific Anatomical Regions


Computer-assisted Spinal Surgery and Scoliosis Scoliosis is a deformity of the spine characterised by lateral curvature, changes in the sagittal profile and rotation towards the convexity. As well as the overall changes, the individual vertebrae and discs undergo remodelling with wedging and changes in the posterior elements. Most studies have concentrated on the thoracic vertebrae as the consequences of a misplaced pedicle screw in this region are most severe.


Endosteal pedicle width is smaller at the concavity of the curve, being thinnest at the apex of the curve. The medial wall of the pedicle was also found to be thicker than the lateral wall. The overall length of the pedicle is greater on the concave side of the curve. Combined with vertebral torsion, this results in the overall length from the base of the pedicle to the anterior cortex of the vertebral body being shorter on the convexity of the curve.60,61 The spinal cord is also shifted towards the concavity of the curve resulting in the cord often being compressed against the medial wall of the smallest pedicles.


EUROPEAN MUSCULOSKELETAL REVIEW


There are also changes in the relationship of the great vessels to the spine. The aorta generally lies posterior and lateral in relation to the normal spine;62–64


there is also movement of the aorta in relation to the spine during correction of deformity.65–68


Release, correction and stabilisation of the scoliotic curve can be achieved by anterior-posterior surgery or a combination of approaches. Open anterior approaches allow good visualisation of anatomy and direct placement of cages and plates with screws. Posterior approaches use a combination of fixation devices such as hooks, wires and pedicle screws.69–77


Pedicle screws have


However, for the morphological changes noted earlier instrumentation of the pedicle is difficult and more hazardous in scoliosis. Thoracoscopic anterior techniques are also hazardous and benefit from a detailed knowledge of the patient’s cross-sectional anatomy and the use of navigation.9,10


A significant non-navigated computer-aided technique has been the ability to analyse and represent CT data graphically with multiplanar reconstruction and 3D reconstructions that can be rotated and viewed from multiple angles on a monitor. Especially if the relevant images are available in the operating theatre, this can be a valuable tool in accurate pedicle screw placement.82


technique is the use of rapid prototyping using the technique of selective laser sintering to produce polystyrene models.83


These can


be sterilised and held by surgeons in a sterile environment. Additionally, pre-operative planning is made more realistic and even ‘virtual’ operations can be performed to practise metalwork placement and plan osteotomy bone removal.


Surgery can also be planned and performed in the ‘virtual world’. Pettigrew et al. have developed a system to predict sagittal alignment, in which a computerised model of the spine, to which simulated surgical manipulations can be applied, is constructed from pictures and imaging data.69


become more popular due to their ability to act both on the posterior pedicles and the centrum and hence, in addition to posterior stability, apply derotation forces and re-create lordosis.69,78–81


Another interesting


Additionally, the force–feedback systems that are extensively used in industrial and design fields can be used to get a true feel of the morphology and required interventions such as osteotomy can be tried ‘virtually’ before surgery. This gives a feeling of déjà vu during surgery and seems to increase surgical accuracy and reduce error (personal communication).


CNSS has mainly been focused on the safe and optimal placement of pedicle screws. The results of this have been discussed previously; however, one study should be emphasised. Rajasekaran et al. performed a randomised control trial that compared navigated (iso-C 3D-based) and non-navigated placement of pedicle screws in scoliosis. They looked at 27 patients with scoliosis and six with kyphosis, with a total of 478 screw insertions. Pedicle screw accuracy was much better in the navigated group for all parameters with only 2% breaches compared with 23%. Of concern, in terms of the potential for vascular injury, 38 screws in the non-navigated group (16%) penetrated the anterior or lateral cortex of the vertebral body, 16 by >4mm. Additionally, the screw insertion time was much less in the navigated group, 2.37±0.72 minutes compared with 4.61±1.05 minutes as was required for the C-arm to be moved into the operative field.42


There is


a trend towards minimal access surgery in scoliosis, which could potentially be augmented by navigation-assisted techniques.


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