The Effect Of Pedicle Screw Insertion Through The Neurocentral Cartilage On The Growth Of Immature Canine Vertebra

Objective The purpose of the present study is to examine the influence of the pedicle screw insertion through the neurocentral cartilage (NCC) on the growth of the vertebra in animal experiment.Methods 20 dogs(about 2 months) were randomly assigned into 3 groups: In group A (6 dogs), posterior muscles at the surgery site(T7～L7) were dissected through a posterior midline incision, and the transverse process tip of T8, T10, and the middle point of transverse process root of the T12, L2, L4 and L6 were exposed, but the pedicles were not drilled; In group B (6 dogs), posterior muscles were dissected in the same way, and pedicles were drilled through the NCC by screws, but the screws were taken away after drilling; in group C (8 dogs), posterior muscles were dissected likewise, and the screws were placed in the pedicles through the NCC after that. Vertebras T8, T10, T12, L2, L4 and L6 were studied with the average of the adjacent two vertebras being each studied vertebra's control. The vertebras from T7 to L7 were studied by the following parameters: area of vertebra canal (AVC), transverse diameter of vertebra canal (TDVC), anterior-posterior diameter of vertebra canal (APDC), pedicle of length (PL), transverse diameter of vertebra body (TDVB), anterior-posterior diameter of vertebra body (APDB), height of vertebra body (HVB). Spiral computerized tomography (SCT) was used to measure the morphologic parameters of studied vertebras and their control vertebras by an independent radiologist on the first day after operation and 3 months after operation. Paired t-tests of studied vertebras and their control vertebras were performed to evaluate the effect of pedicle screws insertion on the canine vertebras.Results In group C, area of vertebra canal, transverse diameter of vertebra canal, anterior-posterior diameter of vertebra canal and pedicle of length of studied vertebras were smaller significantly than their control vertebras (P<0.05) 3 months after operation. In comparison to controls, the studied thoracic vertebras showed approximately 10.74% narrowing of area of vertebra canal, 7.90% shortening of transverse diameter of vertebra canal, 6.12% shortening of anterior-posterior diameter of vertebra canal, and 5.81% shortening of pedicle of length three months after the surgery. By the same time point, the studied lumbar spine showed approximately 6.38% narrowing of area of vertebra canal, 2.71% shortening of transverse diameter of vertebra canal, 5.70% shortening of anterior-posterior diameter of vertebra canal, and 5.36% shortening of pedicle of length over the control vertebras. There were no significant differences of the morphologic parameters in groups A and groups B between the studied vertebras and the control vertebras (P>0.05). And in all the groups, there were no significant variation in transverse diameter of vertebra body, anterior-posterior diameter of vertebra body and height of vertebra body (P>0.05).Conclusion 1. These findings suggest that pedicle screw placement has significant impact on the growth of the canine vertebra canal, maybe leading to iatrogenic spinal stenosis, but the placement has no significant effect on vertebra body. 2. The neurocentral cartilage can repair itself automatically. Only drilling pedicle bone through NCC by screw without leaving it in the pedicle has no obvious impact on the growth of vertebra. If a young child has been inserted with pedicle screws, taking out the screws earlier and leaving a chance for NCC to repair itself and develop vertebra pedicle, vertebra canal may minimize the influence on the growth of vertebra.