Analysis Of Safety And Effective Of Thoracic Pedicle Screws In The Surgical Treatment Of Scoliosis

Objective1. To evaluate the safety of technique of free-hand thoracic pedicle screw (including stainless steel versus titanium screws).2. To discuss the influence of volumes of spinal canal when the instrumentations were placed (including thoracic pedicle screws, thoracic laminar hooks and pedicle hooks).3. To evaluate the spontaneous correction of the un-fused proximal thoracic (PT) curve after isolated correction of the main thoracic (MT) curves by either anterior or posterior correction.Methods1. 4 adult cadavers were instrumented with thoracic pedicle screws. 3 cadavers were instrumented with titanium screws and 1 with Stainless steel screws. The spines were imaged with computed tomography. Three observers evaluate pedicle violation independently scored the placement of each pedicle screw on three separate occasions. Interobserver and intraobserver agreements were determined by using the kappa statistic.2. Volumetric analysis of canal intrusion of pedicle screws and hooks was performed by computer aided design CAM. All implants were product by a single manufacturer (Moss Miami DePuy spine Johnson & JohnsonCompany). Intrusion of pedicle screws with medial positioning was analyzed in 0.5-mm increments, including a calculation of the "screw shadow," representing additional space not available for the spinal cord between screw threads and lateral to a medially positioned screw with intrusion greater than the screw radius. The length of screw intrusion was determined from postoperative CT scans in patients with thoracic pedicle screw instrumentation. All hook styles were analyzed. The volume of the footplate in line with the dorsal surface of the footplate was considered the intruding volume for laminar hooks, half of the volume of the footplate was considered to be the intruding volume for pedicle hooks since a properly positioned pedicle hook straddles the pedicle. 3. 30 patients with adolescent idiopathic scoliosis underwent operative instrumentation and fusion of their MT curve from 2001-2003year. All patients had a PT curve ^20° (average 27°, range 20-47°;average residual side-bending 18°, range 7-38°) and were evaluated for preoperative PT curve flexibility and postoperative curve correction after PSF and ASF . Minimum average follow-up was 2.8 years (2.4 -4.1years). Preoperative, lweek postoperative, and latest follow-up full-length radiographs were evaluated for the PT, MT , and thoracolumbar-lumbar coronal, side-bending, and sagittal Cobb measurements, as well as Tl tilt, clavicle angle, radiographic shoulder height, and the PT, MT, and thoracolumbar-lumbar apical vertical translation. A patient outcomequestionnaire was also completed to correlate patient satisfaction with respect to their shoulder balance and overall appearance.Results1. The mean kappa score for interobserver agreement for all 96 screws (including titanium and stainless steel screws) was 0.51, which correlates with a moderate degree of agreement. Although the interobserver kappa statistics for titanium ([kappa] = 0.53) and stainless screws ([kappa] = 0.44) showed a moderate degree of agreement, the intraobserver reliability was substantial ([kappa] = 0.63). The mean intraobserver kappa for titanium screws was 0.63 and for stainless steel screws was 0.62.2. Volumetric intrusion for a 4.75-mm screw ranged from 2.4 5mm3 (0.5 mm medial perforation) to 93.45 mm3 (3.0 mm perforation). For a 5.5-mm screw, intrusion volume range was from 1.50mm3 to 93.1mm3. Accounting for the "screw shadow," the volumetric intrusion was 10.88mm3 to 130.25 mm3 and 11.35 mm3 to 145.26 mm3, respectively. Hook volumetric intrusion ranged from 54.75 mm3 for a pediatric narrow-blade pedicle hook to 114.5 mm3 for a wide-blade laminar hook.3. The two groups were found to be statistically equivalent (P = 0.66) in terms of preoperative PT curve, MT curve, and MT side-bending curves, The spontaneous improvement in the PT curve was significant (P < 0.0001) in both groups. However, the spontaneous PT curve correction was significantly greater after an ASF versus PSF correction of the MTcurveConclusions1. Our data show that interobserver agreement is moderate and intraobserver agreement is substantial when computed tomography is used to assess placement of thoracic pedicle screws. We conclude that computed tomography is reliable for evaluating thoracic pedicle screw placement throughout the thoracic spine.2. 4.75-mm or 5.5-mm thoracic pedicle screw must have a medial perforation of 1.5 mm to have the same volumetric spinal canal intrusion as a pediatric narrow-blade pedicle hook, the smallest hook footplate. Further, the medial violation must be3 mm to approach the same volumetric intrusion as the largest hook. Accounting for the "screw shadow," a thoracic pedicle screw must have a medial perforation of 2 mm to approach the same intrusion volume as a standard pedicle hook.3. Spontaneous proximal thoracic curve correction consistently occurs after instrumented correction of the main thoracic curve. Furthermore, this spontaneous correction is as good as or slightly better after an ASF versus PSF of the MT curve. The preoperative side bender radiographs (PT curve flexibility) positively correlate with the postoperative spontaneous PT curve correction.