T12

Midpoint TP

At the level of lateral pars

Fig. 2. Thoracic pedicle screw starting points.

A specially designed thoracic pedicle probe ('Lenke Probe', Medtronic Sofamor-Danek Memphis, Tenn., USA) helps probe down the small-sized pedicle channel into the vertebral body. The pedicle probe has a 2-mm tip, which will be required for navigating smaller pedicles versus a conventional 4- to 5-mm tip used as a lumbar pedicle probe. The probe tip becomes thicker as one proceeds away from the tip of the probe, which will make the probe somewhat snugger as one proceeds down into the body past the pedicle. It is also calibrated in 5-mm increments with a rectangular tip going to a round shaft at 35 mm in length. The probe is also shaped with a slight

Fig. 3. Use a 2-mm blunt-tipped, slightly curved pedicle finder (thoracic gearshift) facing laterally to enter the pedicle.

Fig. 4. Sounding device to palpate all 5 walls of the pedicle (cephalad, caudad, medial, lateral, and floor).

Fig. 5. Tap the pedicle with a tap that is 0.5 mm smaller than the proposed screw.

Fig. 3. Use a 2-mm blunt-tipped, slightly curved pedicle finder (thoracic gearshift) facing laterally to enter the pedicle.

Fig. 4. Sounding device to palpate all 5 walls of the pedicle (cephalad, caudad, medial, lateral, and floor).

Fig. 5. Tap the pedicle with a tap that is 0.5 mm smaller than the proposed screw.

Fig. 6. Use the palpating device a second time to assess the bony pedicle walls. Fig. 7. Place the screw slowly. Fig. 8. Intraoperative x-rays.

curvature, which is utilized to avoid penetration of the spinal canal at all times [17]. We almost always start our probe with the curve facing lateral for this purpose.

After a cortical window is drilled with a 4.0-mm burr, the gearshift is taken and placed perpendicular to the plane of the lamina at the level being instrumented. This 'perpendicularity' is an extremely important feature for obtaining both correct sagittal plane and transverse plane angulation to allow placement of the screw parallel to the superior end-plate of the instrumented level. The pedicle probe tip starts out with the curve facing lateral in the pedicle tract. A 'soft spot' of the cancellous pedicle is identified with an initial light push of the probe tip into the burred surface. Once this 'soft spot' is acknowledged by 'feel', then the probe is advanced deeper into the pedicle past the spinal canal. Once the pedicle probe has been engaged down to 15-20 mm (i.e. past the spinal canal and into the posterior portion of the body), the probe is removed, and turned in a medially curved direction and the probe is then advanced medially into the vertebral body. If the probe was left continually in the lateral curved position, it would deflect out of the vertebra laterally, which is less than desirable. It is important as one is advancing the probe to rotate it slightly so as to allow the tip to fall into the soft portion of the pedicle and then into the body quite readily. If resistance is being met, then the probe tip is usually positioned either too medial or too lateral. One should make multiple small permutations of probe tip positioning and orientation to seek out the best fit of the pedicle probe down the shaft of the pedicle without breaking out of the pedicle. Certainly, this 'feel' is an extremely important component to the free-hand screw placement technique. Thus, one should ideally have mastered this in the lumbar spine before proceeding up to the thoracic spine where medial penetration may render the thoracic spinal cord at risk during probe placement and/or screw placement.

After the probe has been advanced to a depth between 30 and 35 mm (proximal thoracic region) or up to 40-45 mm (lower thoracic region), the probe is removed and the pedicle tract palpated with a fine ball-tipped palpation device. It is absolutely essential to feel the floor and four bony walls (medial, lateral, superior, and inferior) confirming intraosseous placement. One must carefully rule out a ledge that will be indicative of a medial wall breech that will be present in the proximal to mid third of the pedicle tract where the spinal canal is located. A tip-off of a medial breach is fairly aggressive bony oozing of venous blood from the pedicle tract out of proportion to other levels. Although this bleeding can be indicative of intraosseous bleeding, it may also signal epidural bleeding secondary to a medial pedicle wall defect. Liberal use of bone wax will plug the hole between the various maneuvers before the screw is placed, if intraosseous borders are confirmed.

Next, the pedicle tract is tapped with a tap undersized from the final pedicle screw by 0.5 mm. I use a variety of color-coded cannulated taps that range in diameter from 4.0 up to 6.5 mm with the threaded portion 35 mm in length (Medtronic Sofamor-Danek). If necessary, a short K wire can be placed into the pedicle tract as long as there is a floor for the K wire not to advance through while tapping is undertaken. The K wire can be especially helpful if more than one attempt is made to probe down the pedicle and a false passage has been created. A K wire leading down the correct pathway can guide the tap into the appropriate position. We have also found clinically, and confirmed biome-chanically, that undertapping by 0.5 mm will improve the screw ultimate pullout strength. It is not necessary to tap the entire length of the pedicle screw tract, only the portion deep into the isthmus of the pedicle usually meaning approximately 20-25 mm.

Following tapping, the pedicle is palpated again with a fine ball-tipped sounding probe. Once again the floor and four walls (medial, lateral, superior and inferior) should be confirmed intact to be completely intraosseous. There is often a very nice cortical bony ridge that is palpated following the tapping that absolutely confirms an intraosseous screw tract. It is important at this time to confirm the appropriate length of the screw by measuring the length of the sounding device while it is held deep against the floor of the vertebral body. We will place a screw of a slightly smaller length so as not to penetrate anterior or lateral to the vertebral body with an inadvertently long screw. The screw is then placed in the same orientation as the pedicle tract was probed and tapped.

If the initial pedicle probe tract is found to be either medial or lateral, it often can be redirected with the probe down the appropriate pedicle shaft. Even with a medial wall defect, a new medial wall can be created with a pedicle probe placed correctly down the pathway of the pedicle. However, this is not always the case and in very tiny pedicles, often only one pass will be allowed to successfully navigate the pedicle. Almost always if the initial pedicle probe is lateral, the true pedicle pathway can be salvaged with a more medially directed probe. Similar to the lumbar spine, if one is having difficulty finding the start of the pedicle, it is best to err slightly more lateral. If one does exit lateral initially, virtually no structures are at risk from minor lateral wall penetrations, and the correct pedicle tract can almost always be salvaged in these circumstances.

Confirmation of Intraosseous Screw Placement

Following screw placement, it is imperative that the surgeon confirm accurate screw placement. This is done intraoperatively by several methods including previously mentioned tract palpation, visual orientation and line-up of the screws, intraoperative AP and lateral radiographs and/or fluoroscopy, and pedicle screw stimulation with EMG recordings [18]. The use of CT scans postoperatively to confirm appropriate screw position is strongly recommended to those beginning their experience [19].

Probably the more important intraoperative confirmation of an intraosseous screw is the palpation method. This is utilized twice: initially after the probe is in place and then following tapping of the pedicle. Once the screw is placed, intraoperative coronal and sagittal plane radiographs or fluoroscopy should be used to check all screws. In the frontal plane, the screws should be fairly harmonious with a lack of crossing of the screw tips in the midline. Although this does not absolutely predict medially directed screws, it certainly heightens one's suspicion. Also, lateral placement needs to be checked because this is fairly common in the thoracic spine especially since the vertebral bodies thin out laterally in mid and proximal thoracic vertebral levels. In the lateral view, I prefer to have the screw parallel to the superior end-plate. And also, the screw tips should not extend past the anterior border of the vertebral body so as to make sure the screws are not excessively long. It is important to confirm that the top and bottom screws are not penetrating the superior end-plate extending into the adjacent disc space. This is especially true for the most proximal level thoracic screw, as it is often difficult to angle one's pedicle probe, tap and screw in the appropriate kyphotic angulation of the proximal thoracic region, with resultant cephalad positioning.

We perform thoracic pedicle screw stimulation with EMGs recorded from the rectus abdominis muscles to confirm intraosseous screw placement [20]. An identical technique is used for screws placed between L1 and S1 with recording in the various lower extremity myotomes. Screws are stimulated with an ascending method of simulation to obtain a compound muscle action potential from the rectus abdominis muscles bilaterally. A recent study of 677 screws placed between T6 and T12 (innervation of the rectus) predicted a medially placed screw by a combination of threshold response less than 6 mA in absolute value and also a threshold intensity averaging 65% below the mean of all other thoracic screw responses in the same patient. Although this is not absolutely indicative for a medially misplaced screw, it is another method utilized to heighten one's suspicion of a pedicle wall defect intraoperatively.

It also is advisable for surgeons beginning with the technique of thoracic pedicle screw placement to obtain postoperative CT scans of their patients [19]. It can be quite enlightening to see exactly where the screws are placed, where the tips of the screws are located, and it can only help the surgeon improve his or her technique over time.

Results and Correlations of Thoracic Pedicle Screws

Placed by the Free-Hand Technique

All thoracic pedicle screws placed by the free-hand placement technique have been reviewed by an independent spine surgeon performing a 2-year clinical research fellowship (Y.J. Kim, MD, Seoul, Korea) [21]. 2,199 thoracic pedicle screws have been placed by this technique in the last 9 years, the vast majority of which over the last 4 years. The majority of screws have been placed in the treatment of pediatric and adult spinal deformity, but also include use in nondeformity conditions such as spinal trauma, tumor and infection. Our results in spinal deformity treatment include increasing correction rates with a decreased use of preliminary anterior release and fusions in some circumstances, and the rare use of any postoperative external immobilization (fig. 9).

There were no screws removed for any type of neurologic, vascular or visceral complications [11, 21]. Specifically, there have been no screw-related neurologic deficits, unexplained thoracic chest wall pain or radicular lower extremity pain, or unexplained neurologic symptoms. There have been no cases of revision surgery needed up to this stage in the cases with thoracic pedicle

(For legend see p. 201.)

Fig. 9. a ER is a 14 + 11-year-old female who presented with a severe 113° right thoracic idiopathic scoliosis. Proximal thoracic curve measured 46° and lumbar compensatory curve 52°. b Her sagittal plane showed 43° of thoracic kyphosis between T5 and T12 and -80° of lumbar lordosis between T12 and the sacrum. c Her left side bender showed a structural proximal thoracic curve bending out to only 35°, with a flexible lumbar curve bending out to 17°. d Her right side bender showed a stiff main thoracic region as expected with correction to only 84°. e She underwent an open anterior release and fusion from T5 to T12 and a posterior segmental screw instrumentation and fusion from T3 to L3 and in a staged fashion. Her upright postoperative coronal radiograph demonstrates excellent correction of her main thoracic curve to 36°, with well balanced curves both above and below. Note the marked apical translation occurring with use of multisegmental screw instrumentation. f Her upright lateral x-ray shows normalized thoracic kyphosis of +33° and overall good sagittal balance. g Her preoperative upright photograph demonstrates her right thoracic trunk shift and rib prominence on the right side. h Her postoperative clinical photo demonstrates excellent balance of her trunk on her pelvis and level shoulders with a diminished rib prominence.

Fig. 9. a ER is a 14 + 11-year-old female who presented with a severe 113° right thoracic idiopathic scoliosis. Proximal thoracic curve measured 46° and lumbar compensatory curve 52°. b Her sagittal plane showed 43° of thoracic kyphosis between T5 and T12 and -80° of lumbar lordosis between T12 and the sacrum. c Her left side bender showed a structural proximal thoracic curve bending out to only 35°, with a flexible lumbar curve bending out to 17°. d Her right side bender showed a stiff main thoracic region as expected with correction to only 84°. e She underwent an open anterior release and fusion from T5 to T12 and a posterior segmental screw instrumentation and fusion from T3 to L3 and in a staged fashion. Her upright postoperative coronal radiograph demonstrates excellent correction of her main thoracic curve to 36°, with well balanced curves both above and below. Note the marked apical translation occurring with use of multisegmental screw instrumentation. f Her upright lateral x-ray shows normalized thoracic kyphosis of +33° and overall good sagittal balance. g Her preoperative upright photograph demonstrates her right thoracic trunk shift and rib prominence on the right side. h Her postoperative clinical photo demonstrates excellent balance of her trunk on her pelvis and level shoulders with a diminished rib prominence.

screw constructs, except for 1 patient 2 years postoperatively from an adolescent idiopathic scoliosis surgery with a chronic deep wound infection treated by implant removal. Because of these results, we have continued to utilize pedicle screws as our main anchor attachment to the thoracic and lumbar spine in all forms of pediatric and adult deformity and nondeformity conditions requiring spinal instrumentation and fusion.

Conclusions

The free-hand placement of thoracic pedicle screws in both pediatric and adult patients with or without spinal deformity can be performed in a safe and reliable manner. This technique should be mastered in the lumbar spine and thoracolumbar junction prior to extending it more proximal in the mid and proximal thoracic spine. Strict and meticulous attention to detail is required along with a precise surgical technique to safely place thoracic pedicle screws for a variety of spinal conditions.

References

1 Gaines RW: The use of pedicle-screw internal fixation for the operative treatment of spinal disorders. J Bone Joint Surg Am 2000;82:1458-1476.

2 Liljenqvist UR, Halm HF, Link TM: Pedicle screw instrumentation of the thoracic spine in idiopathic scoliosis. Spine 1997;22:2239-2245.

3 Suk SI, Kim WJ, Lee SM, Kim JH, Chung ER: Thoracic pedicle screw fixation in spinal deformities: Are they really safe? Spine 2001;26:2049-2057.

4 Suk SI, Lee CK, Kim WJ, Chung YJ, Park YB: Segmental pedicle screw fixation in the treatment of thoracic idiopathic scoliosis. Spine 1995;20:1399-1405.

5 Hamill CL, Lenke LG, Bridwell KH, Chapman MP: The use of pedicle screw fixation to improve correction in the lumbar spine of patients with idiopathic scoliosis. Is it warranted? Spine 1996; 21:1241-1249.

6 Hirano T, Hasegawa K, Takahashi HE, Uchiyama S, Hara T, Washio T, Surgiura T, Yokaichiya M, Ikeda M: Structural characteristics of the pedicle and its role in screw stability. Spine 1997;22: 2504-2510.

7 Krag MH, Weaver DL, Beynnon BD, Haugh LD: Morphometry of the thoracic and lumbar spine related to transpedicular screw placement for surgical spinal fixation. Spine 1998;13/1:27-32.

8 Liljenqvist UR, Link TM, Halm HF: Morphometric analysis of thoracic and lumbar vertebrae in idiopathic scoliosis. Spine 2000;25:1247-1253.

9 Lonstein JE, Denis F, Perra JH, Pinto MR, Smith MD, Winter RB: Complications associated with pedicle screws. J Bone Joint Surg Am 1999;81:1519-1528.

10 Vaccaro AR, Rizzolo SJ, Balderston RA, Allardyce TJ, Garfin SR, Dolinskas C, An HS: Placement of pedicle screws in the thoracic spine. II. An anatomical and radiographic assessment. J Bone Joint Surg Am 1995;77:1200-1206.

11 Kim Y, Lenke LG, Bridwell KH, Riew KD, Rhee JM, Hanson DS: Free hand pedicle screw placement in the thoracic spine (poster). Scoliosis Research Society 36th Annual Meeting, Cleveland, 2001.

12 Cinotti G, Gumina S, Ripani M, Postacchini F: Pedicle instrumentation in the thoracic spine. A morphometric and cadaveric study for placement of screws. Spine 1999;24/2:114-119.

13 Ebraheim NA, Xu R, Ahmad M, Yeasting RA: Projection of the thoracic pedicle and its morphometric analysis. Spine 1997;22/3:233-238.

14 Kothe R, O'Holleran JD, Liu W, Panjabi MM: Internal architecture of the thoracic pedicle. An anatomic study. Spine 1996;21/3:264-270.

15 Vaccaro AR, Rizzolo SJ, Allardyce TJ, Ramsey M, Salvo J, Balderston RA, Cotler JM: Placement of pedicle screws in the thoracic spine. I. Morphometric analysis of the thoracic vertebrae. J Bone Joint Surg Am 1995;77:1193-1199.

16 O'Brien MF, Lenke LG, Mardjetko S, Lowe TG, Kong Y, Eck K, Smith D: Pedicle morphology in thoracic adolescent idiopathic scoliosis: Is pedicle fixation an anatomically viable technique? Spine 2000;25:2285-2293.

17 Ebraheim NA, Jabaly G, Xu R, Yeasting RA: Anatomic relations of the thoracic pedicle to the adjacent neural structures. Spine 1997;22:1553-1557.

18 Brown CA, Lenke LG, Bridwell KH, Geideman WM, Hasan SA, Blanke K: Complications of pediatric thoracolumbar and lumbar pedicle screws. Spine 1998;23:1566-1571.

19 Kim YJ, Lenke LG, Bridwell KH, Riew KD, O'Brien M, Rhee JM, Hanson DS: CT scan accuracy of 'free hand' thoracic pedicle screw placement in pediatric spinal deformity (poster). Scoliosis Research Society Annual Meeting, Cleveland, 2001.

20 Raynor BL, Lenke LG, Kim Y, Bridwell KH, Hanson DS, Padberg AM: Can triggered EMG thresholds accurately predict thoracic pedicle screw placement? Spine, in press.

21 Kim Y, Lenke LG, Bridwell KH, Riew KD, Rhee JM, Hanson DS: Thoracic pedicle screw placement in deformity: Is it safe? (poster). Scoliosis Research Society 36th Annual Meeting, Cleveland, 2001.

Lawrence G. Lenke, MD

One Barnes-Jewish Hospital Plaza, Suite 11300, St. Louis, MO 63110 (USA) Tel. +1 314 747 2509, Fax +1 314 747 2599, E-Mail [email protected]

Haid RW Jr, Subach BR, Rodts GE Jr (eds): Advances in Spinal Stabilization. Prog Neurol Surg. Basel, Karger, 2003, vol 16, pp 204-212

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