This article focuses on two systems, one anterior and one posterior that have recently been developed to facilitate the correction of spinal deformity.
The Expedium Spine System is a posterior pedicle screw system which features two internal closing mechanisms, the so called Single Innie (SI) and Dual Innie (DI) screws. DI screws employ independent locking technology to combine the flexibility of polyaxial screw placement with the controlled correction capability of fixed angle screws.
With the rod in situ, an outer ring compresses a collar onto the polyaxial screw shaft, locking it in the desired angle. The fixed angle screw can still move freely around the rod during reduction manoeuvres. A centre-set screw finally secures the screw head to the rod.
The development of the Expedium Max screw (Figure 1) provides yet further flexibility to allow the surgeon to approximate the spine to the desired sagittal or axial profile. The combination of the long tab screw and the favoured angle characteristics of the screw head increase the excursion of the screw head, thus facilitating rod capture. The favoured angle of 45 degrees together with a further 15 degrees from the long tab compare with the standard 30 degrees of other screws. The centre set screws then reduce the spine onto the rod.
The advantages of this screw design can be seen throughout the various reduction manoeuvres utilised by surgeons during deformity correction:
For 'en bloc' derotation, the rod is contoured into the desired sagittal plane and then placed into all of the concave screws. This is facilitated by the forgiving nature of the long tab, favoured angle screws which allows rod capture even when there is considerable discrepancy between the coronal plane deformity and the desired sagittal profile. The set screws should be placed within the screw head but not tightened. The rod is then rotated into the correction position before being secured.
For thoracic curves, direct vertebral body translation can be very effective. The rod is again precontoured into the correct sagittal contour. It is first placed in the proximal implants which are then closed loosely. The rod is then sequentially reduced into each adjacent implant. The long tabs/favoured angle characteristics, particularly around the apex, again facilitate rod capture. The rod is they loosely secured to the most distal implants before being rotated into the desired sagittal and coronal orientation. The inner screws on the apical implants are then slowly and sequentially tightened to achieve reduction of the deformity by pulling the spine to the rod. This technique allows the forces of reduction to be distributed over each segment and the extended tabs allow rotation of the rod into its final position without large stresses being applied to the rod.
|The AARO Anterior Spinal System screw|
When using the cantilever technique, most frequently used for kyphosis but also to treat hyperlordotic thoracic curves, the rod is first secured proximally. The combination of long tab/favoured angle helps to sequentially capture each screw and reducing the spine to the rod.
For further fine tuning of the correction, the DI screws lock the desired screw angle, preventing toggling of the screw heads during distraction and compression manoeuvres.
Early results of the use of this system has been recently reported to show that the immediate post operative correction using polyaxial screws is similar compared to monoaxial screws (G Wynne-Jones et al at British Scoliosis Society Meeting 2006.)
The AARO System
Anterior correction for deformities of the thoraco-lumbar spine has several well documented advantages compared to posterior surgery including improved coronal plane correction, reduction in the number of levels fused and avoidance of damage to the posterior spinal musculature.
|The AARO Anterior Spinal System|
The AARO anterior spinal system capitalises on these benefits of anterior surgery but also offers further advantages due in part to the unique design of the rectangular rod. This design innovation makes the construct flexible in the frontal plane, but rigid in the sagittal plane.
The AARO anterior spinal system uses compressive forces only applied on the convexity of the curve and places minimal stress on the spinal cord as distraction forces are not applied. It is designed to reduce the pull-out forces and permits genuine segmental correction, applied gently on a “step-by-step” basis until optimal correction is achieved.
The system is low profile, with a maximum height of only 9mm. Thus in the thoracic spine, unlike other anterior systems, the AARO implants because of their very low profile can be covered by the pleura and important blood vessels are not compromised.
|Post-operative Anterior-Posterior X-Ray|
The AARO Anterior Spinal System
The AARO anterior spinal system is manufactured in both titanium and stainless steel. The rods have a rectangular profile. Both straight and curved vertebral body plates are available in three sizes to suit most clinical situations. Bi-cortical cancellous screws are used to attach the plates to the vertebral body.
T-shaped segmental implants link the vertebral body plates with short segmental rectangular rods. Set screws to connect the rods to the plate provide security and maintains the principle of low profile.
Compared to a conventional double-rodded anterior system, AARO has the benefits of quicker surgery, less blood loss and lower profile. The AARO system was first used in April 1995. Over three hundred case have been performed, the vast majority of them since 2002.
The AARO system has been successfully used on tho-racic, thoraco-lumbar and lum-bar curves and indications include idiopathic and neu-romuscular scoliosis.