In the treatment of scoliosis (incidence approximately 20,000 worldwide), surgery is commonly performed if bracing has not resulted in sufficient arrest or correction of the deformity. Generally, the deformity is corrected per-operatively, and the correction is preserved by attaching a rigid structure (rod-shaped implant) to the corrected spine. Intentional fusion of the instrumented vertebrae is facilitated by the rigid implant, and in fact is often encouraged by measures such as removal of facet joints and insertion of bone debris between vertebrae.
Implications of a fused spine are life-long reduced spinal mobility, risk of overloading of adjacent vertebrae, and arrested spinal growth. If spinal instrumentation cannot be postponed until after the growth period (puberty) the implant needs to be lengthened periodically to avoid hypokyphosis/ hyperlordosis or even “crankshafting”. Currently, the option to allow some growth after implantation or even avoid fusion, is being explored (eg. orthobiom™, MAGEC ™). What these systems have in common with conventional implants is their rigidity, by which a corrected shape is sought to be imposed on the deformed spine.
Researchers of the University of Twente, in co-operation with the University Medical Centres of Groningen and Utrecht, are developing a system which acts not by imposing a corrected shape but by applying corrective forces.
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This same principle is already applied in orthodontics where bony geometry is gradually changed by continuous application of relatively low forces, and the correction is obtained over a period of time. To attain this objective, the implant acts as a torsional spring, attached at three points to the spine (ie. the apex and the most tilted vertebrae). The torsional spring places a corrective torsional force on the apical vertebra, thus “rotating it back in line” with the rest of the spine. The implant then corrects both torsion and lateral deviation. The specially shaped U-shaped ends of the implant can slide in their anchor points. Therefore, both normal physiological motion capability and growth potential of the spine are preserved while the deformity is being corrected. After spinal maturity has been reached, the implant can be removed.
A prototype of this system was successfully tested in a reversed manner in a porcine model. That is, the researchers were able to induce a combined rotation and lateral deviation of the middle instrumented vertebrae, under preservation of integrity of the facet joints. Currently clinical trials are being prepared.