Purpose of the study: Attempts to control growth of the spine without provoking epiphysiodesis is a promising area of investigation. The purpose of our experimentation was to achieve localized interruption of vertebral body growth without damaging the adjacent disc and ligament structures.

Material and methods: Two Yucatan micropigs weighing 7 and 9 kg were used for this study. Before initiating the experiment, a complete imaging work-up (x-ray, computed tomography, magnetic resonance imaging) of the spine of the two pigs was obtained. The animals were anesthetized for thoracotomy and a 810 nm 30W laser diode (Diomed Ltd) was implanted in the superolateral part of nine vertebral bodies (3 mm under the cartilage endplate and 3 mm in depth) in order to apply a certain quantity of heat. The nine vertebrae were divided into three groups of three vertebrae. Each group received 2W for 200, 300, or 400s (groups 1, 2 and 3 repectively). The temperature generated by the thermal delivery was recorded in the growth cartilage and in the disc using thermal probes. A complete imaging series of the spine was again obtained four months later, before sacrifice. The results presented correspond to the CT-scan findings used to analyze the effect on vertebral growth and to the MRI findings used to check for discal injury. Each vertebral level was sampled for a histological examination and (in the second part of the study) a biomechanical analysis was undertaken.

Results: Among the 18 vertebral levels studied (two micropigs), 11 levels exhibited localized interruption of growth without any alteration of the adjacent discs. We were unable to observe any significant correlation between the temperature recorded in the disc and the discal structure observed at sacrifice, although above 52°C, there appeared to be a greater risk of definitive discal damage. As disc growth can be controlled (as demonstrated in this study) without damaging the adjacent disc (which will require further study to demonstrate) is would be possible to use this technique as an alternative to treatment by corset for progressive idiopathic scoliosis in growing children.

Purpose of the study: In the growing pig, we have been able to achieve localized control of vertebral body growth by selective destruction ofhte physis using the thermal effect of a laser probe (first part of the study). The purpose of the second part of the study was to evaluate the mechanical effects in terms of 1) intersomatic disc mobility, and 2) bony resistance of the vertebral body and risk of fracture.

Material and methods: Thoracotomy was performed on two Yucatan micropigs (group A); a 510 nm 30W diode laser delivered heat applied to nine vertebral bodies. Four months later, the micropigs were sacrificed. Two normal micropigs (group B) served as controls. The specimens were dissected to the intersomatic disc-ligament complex. Three-level vertebral assemblies were thus obtained for mobility tests (flexion-extension, lateral inclination, right-left rotation). Destruction tests were pursued to fracture. Tests were performed with a Zebris 3D motion analyzer. Computed tomography images and histological findings were also assessed.

Results: Motion: In group A, when the discal space appeared normal on the specimen, no difference was noted in motion in comparison with group B. Conversely, when imaging demonstrated discal injury, joint stiffness was noted. The destruction tests showed that in group A specimens the fracture did not occur at the zone of lytic bone destruction caused by the heat delivery. Fractures observed were similar in the two groups, including epiphyseal detachements and sagittal fractures of the vertebral bodies.

Discussion: Applying laser-delivered heat to the vertebrae in micropigs enabled partial destruction of the physis without injuring the intervertebral disc. Heat delivery induced a modification in vertebral growth. When imaging showed an intact neighboring intervertebral disc, mechanical tests showed normal mobility and resistance. Bone resistance was not diminished. This result is important to consider for the treatment of vertebral osteoid osteomas with laser.

Conclusion: Laser application to vertebral bone is a reproducible method which can stop growth of the minipig vertebral bodies without injuring the intervertebral disc and without reducing bone resistance.