The effects of monoaxial and polyaxial screw usage on the balance in the surgical treatment of adolescent idiopathic scoliosis

Different kinds of systems were used in the surgical treatment of adolescent idiopathic scoliosis until today. Hook and hybrid systems were used in the past but today screw systems are used. There are two kinds of pedicle screws: 1) Monoaxial screw; 2) Polyaxial screw. The advantage of polyaxial screws is to have mobile heads so they facilitate the operation and shorten the operation time. Monoaxial screws have fixed heads and they lenghten the operation time. There are some studies that support the superiority of monoaxial screws about the correction rates of apical vertebra rotation and sagittal balance. On the other hand, monoaxial screws are cheaper than polyaxial screws. The lowest enstrumantation levels can be L4 or L5 in lumber and thoracolumber scoliosis. This situation decreases the amount of motion segment. In this study, we compare the sagittal and coronal balance parameters of patients who were treated with only polyaxial screws and patients who were treated with monoaxial screws in distal two segments. 56 AIS patients were included. 16 of them were treated with monoaxial screws in distal two segments and the other levels were treated with polyaxial screws. 40 of them were treated with polyaxial screws in each level. Preoperative and postoperative standing posterıoranterıor (PA) and lateral radiographs were analyzed retrospectively. Thoracal and/or lumbar 7 Cobb angles, distance between the vertical line from middle of sacrum and the vertical line from the middle of C7 were measured on preoperative and postoperative standing radiographs. Thoracal kyphosis, lumbar lordosis, sacral slope, pelvic tilt and pelvic incidence parameters were measured on preoperative and postoperative lateral radiographs. There was a significant difference in lumbar curve correction in polyaxial group when compared with monoaxial group (p=0,043). Both groups had significant corrections in lumbar lordosis and sacral slopes. But sacral slope values of monoaxial group decreased more than polyaxial group. There was no significant difference observed between two groups when compared about C7- Midsacral distance. But when the mean values of two groups were taken into account, both groups had significant decrease in postoperative period. Especially monoaxial group had more decrease than polyaxial group. Pelvic tilt and pelvic incidence parameters were not significantly different in both groups. In conclusion, monoaxial screws correct sagittal and frontal balance much better than polyaxial screws in adolescent idiopathic scoliosis except lomber cobb values. Key Words: Adolescent idiopathic scoliosis, monoaxial screw, polyaxial screw, frontal balance, sagittal balance