MORPHOLOGICAL CHANGES OF THE SPINAL CANAL AFTER PLACEMENT OF PEDICLE SCREWS IN NEWBORN PIGS

Dezso Jeszenszky, MD
Department of Orthopaedic Surgery, Cantonal Hospital, St. Gallen, Switzerland

PURPOSE:
The application of pedicle screws in spinal deformities offers strong corrective forces and allows stable fixation of the corrected deformity. The use of pedicle screws in children is rare because of fear of enlargement of the growing pedicle with subsequent spinal canal stenosis. This study examines the effect of pedicle screws in the growing skeleton in a pig model.

METHODS:
13 pigs (age 40 days, weight 6 to 11.5 kg) underwent surgery under general anesthesia. A 4–mm wide titanium pedicle screw was inserted into the second lumbar vertebra. Only one pedicle either left or right was instrumented in a randomized fashion. The animals were sacrificed after 6 months, 3 animals died earlier due to causes unrelated to surgery. The spines of the pigs underwent computer tomographic examination with 2 mm axial and coronal cuts through the 2nd and 3rd lumbar vertebra. 4 parameters were chosen to measure the vertebral body. The spinal canal was evaluated by measuring the transverse and sagittal diameter. All measurements were obtained digitally from the computer tomogram. The operated 2nd lumbar vertebra was compared to the intact 3rd lumbar vertebra. All dates were normalized with respect to the 3rd lumbar vertebra.

RESULTS:
10 animals could be used for the analysis. The weight of the animals after 6 months had increased ten fold (94.5 – 103 kg). The transverse diameter of the spinal canal at L2 measured 11.6 mm (10.9 to 12.8) on the day of the operation. After 6 months the diameter increased to 16.5 mm (15.0 to 17.5). There was a minimal 5.4% (–12 to +4) loss in diameter compared to the 3rd lumbar vertebra. The sagittal diameter of the spinal canal at L2 measured 7.0 mm (6.5 to 7.7) immediately postoperative and 12.0 mm (11.1 to 13.4) after 6 months. There was a loss of 9.5% of the sagittal diameter after 6 months (–32 to +19) compared to the third lumbar vertebra.

SUMMARY:
This is the first study which examines the effect of placement of pedicle screws in the immature skeleton. The results of this animal model demonstrate only minimal pathologic narrowing of the spinal canal by implantation of pedicle screws. The results are encouraging. Further studies will be necessary to evaluate the long term effect of pedicle screws in the immature spine