Numerous internal fixation techniques have been

See the corresonding editorial in this issue, 24 26. J J Neurosurg Pediatrics 9:000 000, 9:27 34, 2012 A comuted tomograhy based feasibility study of translaminar screw lacement in the ediatric thoracic sine Laboratory investigation Camilo Molina, B.A., 1 Daniel M. Sciubba, M.D., 1 Christoher Chaut, M.D., 2 P. Justin Tortolani, M.D., 3 George I. Jallo, M.D., 1 and Ryan M. Kretzer, M.D. 1 1 Deartment of Neurosurgery, The Johns Hokins University School of Medicine; 3 Union Memorial Orthoedics & Sorts Medicine, Baltimore, Maryland; and 2 Deartment of Orthoedic Surgery, Scott and White Clinic Memorial Hosital, Temle, Texas Object. Translaminar screws (TLSs) were originally described as a safer alternative to edicle and transarticular screw lacement at C-2 in adult atients. More recently, TLSs have been used in both the cervical and thoracic sine of ediatric atients as a rimary fixation technique and as a bailout rocedure when dyslastic edicle morhology rohibits safe edicle screw lacement. Although authors have reorted the anatomical characteristics of the cervical and thoracic lamina in adults as well as those of the cervical lamina in ediatric atients, no such data exist to guide safe TLS lacement in the thoracic sine of the ediatric oulation. The goal of this study was to reort the anatomical feasibility of TLS lacement in the thoracic sine of ediatric atients. Methods. Fifty-two atients (26 males and 26 females), with an average age of 9.5 ± 4.8 years, were selected by retrosective review of a trauma registry database after institutional review board aroval. Study inclusion criteria were an age from 2 to 16 years, standardized axial bone-window CT images of the thoracic sine, and the absence of sinal trauma. For each thoracic lamina the following anatomical features were measured using efilm Lite software: laminar width (outer cortical and cancellous), laminar height (LH), maximal screw length, and otimal screw trajectory. Patients were stratified by age (an age < 8 versus 8 years) and sex. Results. Collected data demonstrate the following general trends as one descends the thoracic sine from T-1 to T-12: 1) increasing laminar width to T-4 followed by a steady decrease to T-12, 2) increasing LH, 3) decreasing maximal screw length, and 4) increasing ideal screw trajectory angle. When stratified by age and sex, male atients older than 8 years of age had significantly larger laminae in terms of both width and height and allowed significantly longer screw lacement at all thoracic levels comared with their female counterarts. Imortantly, it was found that 78% of individual thoracic laminae, regardless of age or sex, could accet a 4.0-mm screw with 1.0 mm of clearance. As exected, when stratifying by age and sex, it was found that older male atients had the highest accetance rates. Conclusions. Data in the resent study rovide information regarding otimal TLS length, diameter, and trajectory for each thoracic sinal level in ediatric atients. Imortantly, the data collected demonstrate no anatomical limitations within the ediatric thoracic sine to TLS instrumentation, although accetance rates are lower for younger (< 8 years old) and/or female atients. Lastly, given the anatomical variation found in this study, CT scanning can be useful in the reoerative setting when lanning TLS use in the thoracic sine of ediatric atients. (htt://thejns.org/doi/abs/10.3171/2011.10.peds11121) Key Words translaminar screw ediatrics thoracic sine scoliosis edicle screw Numerous internal fixation techniques have been develoed to manage vertebral column instability and to correct sinal deformity. Sinal fixation was originally accomlished via the use of hooks or Abbreviations used in this aer: BLW = bicortical laminar width; CLW = cancellous laminar width; LH = laminar height; PS = edicle screw; TLS = translaminar screw. J Neurosurg: Pediatrics / Volume 9 / January 2012 wires, and more recently PS-rod constructs have become the gold-standard. 2,13,14 Pedicle screw fixation offers the benefit of 3-column fixation as well as known biomechanical stability and high fusion rates. Unfortunately, This article contains some figures that are dislayed in color on line but in black and white in the rint edition. 27

C. Molina et al. this technique has limitations in the sine of ediatric atients given the small edicle size, making PS lacement difficult. Furthermore, the use of edicle-based fixation is limited by structural variation in edicle conformation that is inherent to congenital sinal deformities. 13,17,19 Translaminar screws have recently been roosed as an alternative to PS fixation. First described by Wright in 2004, TLSs were originally used as a safer alternative to edicle and transarticular screw lacement at C-2. 22 In 2006 Kretzer et al. 12 reorted on the use of TLSs in the thoracic sine of adults, showing that this technique could also be utilized at T1 2 for constructs bridging the cervicothoracic junction. More recently, authors have begun reorting the use of TLSs in the ediatric oulation. For instance, in 2008 Jea et al. 9 ublished the first case reort describing the use of TLS instrumentation in the ediatric cervical sine. In 2009 Chamoun et al. 4 ublished a larger case series documenting the successful use of axial and subaxial TLS fixation in 7 ediatric atients with uer cervical instability. Most recently, Lewis et al. 13 described TLS lacement in the ediatric thoracic sine in 2 cases of severe sinal deformity correction. Given the increased use of TLSs, several authors have recently reorted the anatomical characteristics of the cervical and thoracic laminae in the adult oulation. 6,11,21 In addition, anatomical characteristics of the cervical laminae of the ediatric sine have been described. 5 However, no such data exist to guide safe TLS lacement in the thoracic sine of the ediatric oulation. Because this technique may be esecially alicable as an alternative to PS fixation and as a bailout rocedure following failed PS lacement in cases of ediatric sinal deformity, the goal of this study was to reort the anatomical feasibility of TLS lacement in the thoracic sine of ediatric atients. Comuted tomograhy scans from a randomly selected ediatric trauma oulation were used to assess the anatomical features of the ediatric thoracic laminae. The anatomical characteristics measured included lamina width and height as well as maximal screw length and ideal screw trajectory, with stratification based on atient age and sex. Methods Fifty-two CT scans of the thoracic sine in 26 male and 26 female ediatric atients were assessed in this study. Patients were randomly selected for study enrollment via a retrosective review of the trauma registry database following institutional review board aroval at the Scott and White Clinic Memorial Hosital in Temle, Texas. All atients 2 16 years of age were eligible for study enrollment rovided that they had standardized axial and sagittal bone-window CT scans of the thoracic sine without evidence of sinal trauma. All scans were obtained on a Siemens 64-slice CT scanner with a 3-mm axial image thickness (Siemens Medical Solutions). Linear and angular measurements were made using efilm Lite software (Merge Technologies, Inc.). One surgeon determined the ideal screw trajectory for TLS lacement based on the anatomical axis of the individual lamina (Fig. 1). A line bisecting the sinal canal in the anteroosterior direction Fig. 1. Left: Axial CT scan obtained at T-2, showing the otimal TLS length, outer cortical diameter, cancellous diameter, and trajectory. Right: Axial CT scan obtained at T-2, showing determination of the insertion medialization angle in relation to the midline. in the axial lane was then used to determine the angle of screw trajectory in relation to the midline. Screw trajectories were subsequently used to determine the maximum screw length without distal vertebral cortex violation. Outer cortical and cancellous diameters were recorded at the minimum lamina width along the ideal screw trajectory. Laminar heights were measured in the sagittal lane by using sagittally reformatted CT images. All lamina measurements were obtained bilaterally from T-1 to T-12 and were stratified by atient age (< 8 vs 8 years) and sex. Age stratification was done based on revious studies demonstrating that the human sine reaches an adult configuration at aroximately 8 years of age. 1,5,7,10 Average and standard deviation calculations were erformed using Microsoft Office Excel. The Student t-test was used for all statistical analyses (Excel, Microsoft Office). The threshold for statistical significance was set at 0.05. Results Data are resented in both tabular (Tables 1 3) and grahic (Figs. 2 5) formats. An analysis of these data follows. Patient Demograhic Data A breakdown of the number of children comosing each age grou, both overall and stratified by sex, can be found in Table 1. The overall atient age was 9.5 ± 4.8 years. When stratifying by sex, the average age for males was 10.1 ± 4.8 years and for females was 8.9 ± 4.6 years. Substratification of males and females by age (< 8 or 8 years) revealed no significant difference: < 8 years, 3.8 ± 1.9 years for males and 4.3 ± 1.5 years for females, = 0.10; 8 years, 13.0 ± 2.2 years and 12.9 ± 2.4 years, resectively, = 0.37. Bicortical Laminar Width Overall, the mean BLW for all thoracic sine levels (T1 12) was 5.4 ± 0.3 mm (range 4.7 5.9 mm, T12 4). When stratified by atient age, a significant difference in BLW was noted between the 2 grous. Those with an age < 8 years had a mean BLW of 4.8 ± 0.3 mm (range 4.2 5.1 mm, T12 4) for all levels, whereas atients with an age 8 years had a mean BLW of 5.9 ± 0.4 mm (range 5.2 6.5 28 J Neurosurg: Pediatrics / Volume 9 / January 2012

Pediatric thoracic translaminar screw lacement TABLE 1: Study grou comosition according to the number of children er age Yrs of Age Overall No. No. of Males No. of Females 2 7 4 3 3 2 2 0 4 4 1 3 5 3 0 3 6 4 1 3 7 2 2 0 8 1 0 1 9 6 3 3 10 2 1 1 11 0 0 0 12 1 1 0 13 3 2 1 14 9 5 4 15 7 3 4 16 1 1 0 all ages 52 26 26 mm, T12 4; 0.001; Table 2) for all levels. This finding shows that lamina size increases with increasing skeletal maturity and that atient age is a significant factor in determining the feasibility of TLS use in the ediatric oulation. A significant difference in BLW was also noted when stratifying by sex. Male atients had a mean BLW of 5.8 ± 2.8 mm (range 5.0 6.4 mm, T12 4), whereas female atients had a mean BLW of 5.0 ± 0.4 mm (range 4.2 5.4 mm, T12 3; 0.001; Table 3). When BLW was assessed based on sinal level, a trend of increasing BLW was noted from T1 4, followed by a reversal of this trend from T-5 to T-12 (Fig. 2). Cancellous (Isthmus) Laminar Width The mean CLW for all thoracic sine levels (T1 12) was 2.3 ± 0.3 mm (range 1.7 2.7 mm, T12 4). When stratified by either age or sex, as exected based on the aforementioned mean BLW data, a significant difference in mean CLW was noted between the 2 atient subgrous. Patients with an age < 8 years had a mean CLW of 1.9 ± 0.2 mm (range 1.4 2.1 mm, T12 4) for all levels, while those with an age 8 years had significantly larger CLWs measuring 2.6 ± 0.3 (range 1.9 3.1 mm, T12 4; 0.001; Table 2). Male atients had a mean CLW of 2.5 ± 0.3 mm (range 1.8 2.9 mm, T12 4), whereas females had significantly smaller CLWs measuring 2.0 ± 0.3 mm (range 1.3 2.4 mm, T12 4; 0.001; Table 3). When CLW was assessed based on thoracic sine level, the findings mirrored those reviously described for BLW (Fig. 2). Laminar Height The mean LH for all thoracic sine levels (T1 12) was 13.8 ± 1.7 mm (range 10.9 15.3 mm, T1 9). Following age or sex stratification, a significant difference in the mean LH was once again noted for both stratification modes. Patients with an age < 8 years had a mean LH of J Neurosurg: Pediatrics / Volume 9 / January 2012 11.2 ± 1.4 mm (range 8.8 12.8 mm, T1 10) for all levels, whereas those with an age 8 years had a lamina of significantly greater height, measuring 16.2 ± 2.5 mm (range 12.4 20.6 mm, T1 12; 0.001; Table 2). Male atients had a mean LH of 14.3 ± 1.6 mm (range 11.4 16.0 mm, T1 10) for all levels, while that in females was 13.2 ± 1.7 mm (range 10.3 15.4 mm, T1 12; 0.04; Table 3). Laminar height is esecially imortant, as TLS lacement requires one screw to be laced at a rostral entry oint within the lamina, while the second screw is laced with a more caudal entry oint. In this way, LH must be at least twice the screw diameter to allow safe TLS lacement. Based on this anatomical requirement, the data suggest that TLS lacement may be more feasible with an increased atient age and/or in males until sinal maturity is reached. When LH was analyzed based on sinal level, a general trend of increasing LH was noted as one descends the thoracic sine from T-1 to T-12 (Fig. 3). Maximum TLS Length The mean maximal TLS length for all thoracic sine levels (T1 12) was 23.5 ± 2.6 mm (range 21.6 30.5 mm, T9 1). When stratified by either age or sex, a significant difference in mean maximal TLS length was noted between the 2 atient subgrous. Patients younger than 8 years of age had a mean screw length of 21.6 ± 2.6 mm (range 19.8 28.3 mm, T7 1) for all levels, while those who were 8 years or older could accet a significantly longer screw measuring on average 24.9 ± 2.7 mm (range 22.7 32.0 mm, T9 1; = 0.003; Table 2). Female atients had a mean TLS length of 22.0 ± 2.6 mm (range 20.4 28.9 mm, T9 1) for all levels, comared with 24.6 ± 2.8 mm (range 22.6 31.9 mm, T9 1; 0.001) for their male counterarts (Table 3). In addition, a trend of decreasing maximal TLS length was noted as one descends caudally in the thoracic sine from T-1 to T-8, with stabilization of screw length from T-9 to T-12 (Fig. 4). Translaminar Screw Trajectory The mean otimal TLS trajectory for all thoracic sine levels (T1 12) was 54.8 ± 2.6 (range 50.0 58.1, T1 9). No significant difference was noted when stratifying by either age or sex. Patients with an age < 8 years had a mean TLS trajectory of 53.9 ± 2.0 (range 50.3 56.9, T1 9) for all levels, and those with an age 8 years had a mean trajectory of 55.5 ± 3.2 (range 49.8 59.0, T1 9; 0.05) for all levels. Male atients had a mean TLS trajectory of 54.4 ± 2.5 (range 50.0 57.4, T1 9) for all levels, and female atients had a mean trajectory of 55.4 ± 2.8 (range 50.0 59.0, T1 9; 0.05) for all levels. When TLS trajectory was stratified by sine level, a trend of increasing otimal trajectory was noted as one descends the thoracic sine from T-1 to T-9, with a slight reversal of this trend from T-10 to T-12 (Fig. 5). Discussion Translaminar facet screw lacement was originally develoed for use in lumbar sine short-segment fixation in the treatment of degenerative disease and sinal stenosis. 8,15 Desite its original alication as a rimary screw- 29

TABLE 2: Anatomical values for the ediatric thoracic sine and TLSs, stratified by atient age* C. Molina et al. Lamina Screw Length (mm) BLW (mm) % w/ BLW 5 mm CLW (mm) 30 J Neurosurg: Pediatrics / Volume 9 / January 2012 LH (mm) Trajectory Angle ( ) T-1 overall 30.5 5.3 70 2.2 10.9 50.0 <8 yo 28.3 <0.001 4.7 <0.001 50 1.9 0.002 8.8 <0.001 50.3 0.285 8 yo 32.0 5.8 83 2.4 12.4 49.8 T-2 overall 26.7 5.7 84 2.5 11.2 51.2 <8 yo 25.0 <0.001 5.0 <0.001 64 2.0 <0.001 9.3 <0.001 51.0 0.328 8 yo 27.8 6.2 97 2.9 12.6 51.3 T-3 overall 24.0 5.7 84 2.6 12.2 51.8 <8 yo 22.4 <0.001 5.0 <0.001 67 2.1 <0.001 9.9 <0.001 51.4 0.265 8 yo 25.2 6.2 95 2.9 13.9 52.0 T-4 overall 22.7 5.9 84 2.7 12.8 53.3 <8 yo 20.8 <0.001 5.1 <0.001 60 2.1 <0.001 10.5 <0.001 53.7 0.197 8 yo 24.1 6.5 100 3.1 14.4 53.0 T-5 overall 22.2 5.8 81 2.5 13.3 55.5 <8 yo 20.3 <0.001 5.0 <0.001 60 2.0 <0.001 11.0 <0.001 55.4 0.482 8 yo 23.2 6.1 95 2.7 15.7 58.0 T-6 overall 21.9 5.6 83 2.4 14.0 56.6 <8 yo 19.9 <0.001 4.7 <0.001 62 1.9 <0.001 11.7 <0.001 54.5 0.001 8 yo 23.4 6.3 98 2.8 15.1 55.5 T-7 overall 21.7 5.5 86 2.3 14.9 56.5 <8 yo 19.8 <0.001 5.0 <0.001 74 2.0 <0.001 12.1 <0.001 54.5 0.001 8 yo 23.0 5.8 95 2.4 17.0 57.8 T-8 overall 21.8 5.5 79 2.4 15.2 57.5 <8 yo 20.1 <0.001 5.0 <0.001 64 2.2 <0.001 12.8 <0.001 55.9 0.003 8 yo 22.9 5.9 90 2.6 17.0 58.6 T-9 overall 21.6 5.4 76 2.3 15.3 58.1 <8 yo 20.0 <0.001 4.9 <0.001 60 2.0 0.002 12.6 <0.001 56.9 0.026 8 yo 22.7 5.8 88 2.5 17.4 59.0 T-10 overall 22.3 5.2 68 2.2 15.2 56.0 <8 yo 20.6 <0.001 4.6 <0.001 45 1.9 0.009 12.8 <0.001 54.0 0.001 8 yo 23.6 5.6 85 2.4 17.4 57.6 T-11 overall 22.9 5.0 64 1.9 15.2 55.8 <8 yo 20.5 <0.001 4.7 0.007 47 1.6 <0.001 12.6 <0.001 53.8 0.002 8 yo 25.1 5.3 79 2.2 18.4 57.5 T-12 overall 23.7 4.7 59 1.7 15.3 55.4 <8 yo 21.1 <0.001 4.2 0.002 38 1.4 0.015 10.5 <0.001 54.8 0.166 8 yo 26.2 5.2 78 1.9 20.6 56.0 * yo = years old.

Pediatric thoracic translaminar screw lacement TABLE 3: Anatomical values for the ediatric thoracic sine and TLSs, stratified by atient sex Lamina & Patient Sex Screw Length (mm) BLW (mm) % w/ BLW 5 mm CLW (mm) LH (mm) Trajectory Angle ( ) T-1 M 31.9 <0.001 5.7 <0.001 76 2.4 0.005 11.4 0.057 50.0 0.472 F 28.9 4.8 65 2.0 10.3 50.0 T-2 M 27.9 <0.001 5.9 0.005 84 2.6 0.088 11.6 0.131 50.8 0.139 F 25.2 5.4 83 2.4 10.7 52.0 T-3 M 25.2 <0.001 6.0 0.002 83 2.8 0.002 12.7 0.095 51.3 0.115 F 22.6 5.4 85 2.3 11.6 52.4 T-4 M 23.8 <0.001 6.4 <0.001 88 2.9 0.008 13.4 0.047 52.7 0.057 F 21.4 5.3 78 2.4 12.0 54.1 T-5 M 23.2 <0.001 6.2 <0.001 83 2.7 0.005 13.9 0.113 55.4 0.395 F 20.9 5.2 78 2.2 12.7 55.6 T-6 M 22.9 <0.001 6.0 <0.001 88 2.5 0.057 14.8 0.035 56.4 0.305 F 20.6 5.0 78 2.2 13.0 56.9 T-7 M 22.7 <0.001 5.8 <0.001 95 2.4 0.011 15.4 0.158 55.5 0.018 F 20.5 5.1 76 2.0 14.3 57.6 T-8 M 22.8 <0.001 5.9 <0.001 86 2.6 0.004 15.6 0.201 56.3 0.003 F 20.5 5.1 72 2.1 14.7 59.0 T-9 M 22.6 <0.001 6.0 <0.001 91 2.6 0.001 15.7 0.191 57.4 0.049 F 20.4 4.9 59 1.9 14.7 59.0 T-10 M 23.4 <0.001 5.6 <0.001 84 2.4 0.007 16.0 0.041 55.9 0.385 F 20.8 4.6 45 1.9 13.9 56.2 T-11 M 23.9 0.004 5.3 0.002 75 2.0 0.027 15.4 0.337 55.8 0.433 F 21.3 4.5 46 1.7 14.8 55.6 T-12 M 25.3 <0.001 5.0 0.015 68 1.8 0.014 15.2 0.454 55.0 0.181 F 21.1 4.2 42 1.3 15.4 56.1 J Neurosurg: Pediatrics / Volume 9 / January 2012 based fusion technique in which the TLS trajectory was used to bridge the facet joint, this rocedure has been adated for rod-based fixation for use in both the cervical and thoracic sine. In this way, current TLSs act analogously to PSs as anchor oints for osterior rod fixation. Although TLSs do not rovide the theoretical advantage of the 3-column fixation rovided by PSs, studies on the use of C-2 TLS instrumentation have demonstrated both a diminished incidence of vertebral artery injury and the relative technical ease of screw insertion. 5,18,20,23 In addition to studies assessing the anatomy of TLS lacement at C-2, more recent reorts have described the anatomical feasibility of this technique in the uer thoracic sine. 11,12,21 Given the demonstrated feasibility of TLS instrumentation in the adult sine, 11,12,18,20,22,23 the adatation of TLSs for use in the ediatric oulation is attractive both as a rimary fixation technique when PS fixation is rohibited by dyslastic edicle morhology and as a bailout rocedure following failed PS lacement. Although clinical exerience using TLSs in the ediatric sine has been favorable, 4,5,9 no data are available in the literature describing the anatomical features of the ediatric thoracic laminae. In the resent study, CT scans were used to 31

C. Molina et al. Fig. 2. Grah showing the mean BLW and mean CLW against the thoracic sine level, with stratification based on age, that is, < 8 versus 8 years. determine the maximal TLS length, laminar width, LH, and otimal screw trajectory to determine the anatomical feasibility of this technique. In terms of the clinical utility of TLS lacement, laminar width is the most critical factor. Some authors have used a minimum lamina thickness of 5 mm to safely accet a TLS, allowing for the lacement of a 4.0-mm screw with 1.0 mm of clearance. 5,16 Previous studies that characterized laminar width using these criteria have demonstrated significant variability among atients, with reorted values ranging from 53% to 72% at C-2. 6,21 Given the variability of laminar width reorted in rior studies in adults, it was imortant in the resent study to characterize the BLW of the ediatric thoracic sine. The mean BLW for all ediatric thoracic sine levels was 5.4 mm. A trend of increasing BLW from T-1 to T-4 was noted, with a reversal of this trend below T-4 (Fig. 2). Interestingly, this trend is oosite to that tyically found in thoracic PS studies in which minimum edicle diameters are generally noted at the T4 5 levels. This finding of a maximum thoracic laminar width at T-4 may make TLS use esecially attractive in the midthoracic sine where edicle diameter is at a minimum. In terms of clinical feasibility, 78% of individual thoracic laminae were at least 5 mm in diameter in the resent study. Notably, the maximal rate of accetance occurred at T-7 (86%) and the minimum at T-12 (59%). Imortantly, Chern et al. 5 rovided data that only 41% of ediatric C-2 laminae could accet a 3.5-mm screw with 1.0 mm of clearance, suggesting that the ediatric thoracic sine may be better suited to TLS lacement than the cervical sine. When BLW was stratified by atient age in the resent study, 59% of the laminae in atients with an age < 8 Fig. 4. Grah demonstrating the mean maximal TLS length against the thoracic sine level, with stratification based on age, that is, < 8 versus 8 years. years were 5 mm in diameter as comared with 92% in atients with an age 8 years. When combined with the observation that BLW was significantly smaller at every thoracic level for atients younger than 8 years old, these results indicate that atient age significantly affects the feasibility of TLS instrumentation in the ediatric oulation (Table 1). When stratifying by atient sex, only 67% of females had laminae 5 mm, whereas 83% of males had laminae 5 mm. Based on these data, one can infer that TLS instrumentation is most feasible in male atients who are 8 years or older, and thus ractitioners should be articularly vigilant regarding laminar anatomy when deciding whether to use TLS instrumentation in female atients younger than 8 years. Furthermore, it is evident that TLS instrumentation is least feasible in the lowermost thoracic levels (T11 12) given that these levels consistently have the lowest accetance rates regardless of a atient s age or sex (Table 3). Although bicortical width is considered to be the most imortant factor in the selection of TLS diameter, the isthmus diameter, or cancellous bone content, is also an imortant arameter given its biomechanical imlications. Recent studies comaring TLSs and PSs have shown that TLSs have both greater inline ullout strength and insertional torque. 3 Cardoso et al. 3 roosed that this biomechanical advantage is attributable to the smaller magnitude of cancellous bone in the lamina in comarison with the edicle, allowing for increased cortical bone-thread urchase during laminar screw insertion. In the resent study, the mean CLW for all levels was 2.3 mm (Table 2). As this value is less than the clinically relevant screw diameter of 3.5 or 4.0 mm, it is likely that screw urchase within Fig. 3. Grah showing the mean LH against the thoracic sine level, with stratification based on age, that is, < 8 versus 8 years. Fig. 5. Grah demonstrating the mean ideal screw trajectory angle against the thoracic sine level, with stratification based on age, that is, < 8 versus 8 years. 32 J Neurosurg: Pediatrics / Volume 9 / January 2012

Pediatric thoracic translaminar screw lacement the ediatric lamina will largely be cortical, thereby theoretically increasing insertional torque and screw ullout strength. This theoretical benefit is offset by the fact that TLS does not rovide 3-column fixation. Similar to BLW, LH can be an anatomical factor limiting TLS lacement. Thresholds for the minimum height requirement for TLS lacement vary in the literature. Mandel et al. 16 indicated that TLS lacement is overwhelmingly difficult for C-2 LHs smaller than 5 mm. However, given that TLS lacement most often occurs bilaterally, Chern et al. 5 stated that the LH threshold for safe screw lacement should be 9.0 mm, assuming bilateral lacement of a 3.5-mm screw with 1.0 mm of clearance on each side. Based on these calculations, we determined that a minimal LH of 10 mm was required to accet bilateral lacement of 4.0-mm thoracic TLSs with 1.0 mm of clearance. Using this threshold, we found that thoracic LH was not a significant factor limiting TLS instrumentation in the ediatric thoracic sine regardless of stratification by either atient age or sex (Tables 1 2). In the ediatric cervical sine, Chern et al. 5 found a range of 33.3% (C-4) to 85.5% (C-7) of laminae meeting the resecified minimum safe LH of at least 9.0 mm. Based on results in the current study, thoracic LH data rovide further evidence that the ediatric thoracic sine may be more accommodating than the cervical sine in terms of TLS instrumentation lacement. Note, however, that measured LH based on CT scanning is not equivalent to an instrumentable functional height given that vertical laminar morhology is redominantly oval, and thus not all of the measured LHs can safely allow screw urchase. Hence, the surgeon should be aware of this discreancy when choosing to lace TLS instrumentation in a atient with an LH close to the resecified safe threshold. Laminar length and insertion trajectory angle are not necessarily anatomical factors limiting TLS lacement; however, their characterization rovides ractical objective anatomical data to guide safe screw lacement. For examle, determining laminar length is essential in choosing ideal screw length. The mean otimal TLS length for all levels in the resent study was 23.5 mm, with the longest screw length found at T-1, which was consistent with rior studies. 11 In addition, age and sex once again significantly affected the choice of ideal TLS length. Insertion trajectory angle describes the ideal medial inclination for roer screw lacement in relation to the midline sinous rocess. Ideal medial inclination is essential to minimize the risk of ventral cortical wall violation, which could result in CSF leakage or sinal cord injury. In the oerating room, the roer medialization angle is tyically determined by aroximating the sloe of the outer cortical laminar surface during ilot hole drilling and subsequent screw lacement. Based on CT scanning, the ideal trajectory angle ranged from 50.0 at T-1 to 58.1 at T-9, reresenting a slight trend of increased medial inclination as one rogresses caudally in the thoracic sine, which was in line with rior studies 11 (Fig. 5). As exected, no differences were noted when the data were stratified by atient age or sex (Tables 1 3). Desite the aarent anatomical feasibility of TLS instrumentation in the ediatric thoracic sine, TLS J Neurosurg: Pediatrics / Volume 9 / January 2012 lacement, in comarison with PS lacement, is associated with a variety of technical and ractical limitations that must be noted when using the technique. First, there is a learning curve for safe screw insertion, articularly in the setting of comlex sinal deformity due to changes in sinal morhology. Second, coronal deformity correction rocedures tyically require derotational maneuvers that lace significant forces on the screws, which in the case of TLSs could result in breaching the ventral laminar wall and entering the sinal canal. Third, rimary fixation within the lamina limits the surface area for fusion, as the as the laminar surface can no longer be decorticated to avoid screw failure. Fourth, safe TLS lacement may be limited by the racticality of checking for aroriate screw lacement. While PS lacement can be checked via intraoerative anteroosterior radiograhy, correct TLS insertion is oorly visualized on standard radiograhs and may require intraoerative CT scanning. Beyond the technical considerations of TLS lacement, there are several limitations within this study that should be noted. First, axial and sagittal CT scans rovide a 2D reresentation of the sine and must be translated into the 3D oerating room setting, meaning that it may not always be ossible to achieve otimal screw diameter, length, and trajectory in every case. Second, because of the 3-mm axial CT image thickness, some anatomical information ertaining to the thoracic laminae may be omitted. Third, laminar anatomy in the current study was analyzed in children devoid of sinal deformity. It is ossible that laminar dimensions in the setting of sinal deformity may not be comletely reresented by data derived from atients with normal sinal alignment. However, it is resumed that, unlike edicle conformation, laminar dimensions may not be as affected by congenital sinal deformities, and thus will minimize this limitation. Conclusions The data collected rovide reliminary information regarding otimal TLS length, diameter, and trajectory for each sinal level in the ediatric thoracic sine. With the excetion of TLS trajectory, all other features measured (width, height, and screw length) were significantly greater in older atients (age 8 years) comared with the younger cohort (age < 8 years) for all thoracic levels. In addition, when stratified by sex, male atients had larger values for all anatomical features measured with the excetion of TLS trajectory. Imortantly, the data collected demonstrate no anatomical limitations within the ediatric thoracic sine as regards TLS instrumentation, although accetance rates are lower for younger (age < 8 years) and/or female atients. Bicortical laminar width and height data were greater than those reviously reorted in the ediatric cervical sine, and thus show that TLS use may be more feasible in the thoracic region. Measured CLW arameters may also rovide theoretical biomechanical benefits because of increased cortical bone urchase. Given the anatomical variation found in this study, CT scanning can be useful in the reoerative setting when lanning TLS use in the ediatric thoracic sine. 33

C. Molina et al. Disclosure Dr. Sciubba has received honoraria for study grou articiation. Dr. Tortolani is a consultant for and has received clinical or research suort for the study described, including equiment or material, from Globus Medical. Dr. Kretzer is a consultant for Chestnut Medical, receives suort from DePuy Sine for non study related clinical or research effort, and has ownershi in Neuraxis Technologies, LLC. Author contributions to the study and manuscrit rearation include the following. Concetion and design: Chaut, Kretzer. Acquisition of data: Molina. Analysis and interretation of data: Molina. Drafting the article: Molina, Kretzer. Critically revising the article: all authors. Reviewed submitted version of manuscrit: all authors. Aroved the final version of the manuscrit on behalf of all authors: Molina. Statistical analysis: Molina. References 1. Baker C, Kadish H, Schunk JE: Evaluation of ediatric cervical sine injuries. Am J Emerg Med 17:230 234, 1999 2. 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Wright NM: Translaminar rigid screw fixation of the axis. Technical note. J Neurosurg Sine 3:409 414, 2005 Manuscrit submitted March 27, 2011. Acceted October 10, 2011. Please include this information when citing this aer: DOI: 10.3171/2011.10.PEDS11121. Address corresondence to: Camilo Molina, B.A., CRB-II 264, 1550 Orleans Street, Baltimore, Maryland 21205. email: cmolina2 @jhmi.edu. 34 J Neurosurg: Pediatrics / Volume 9 / January 2012