Spinal Disorders: Fundamentals of Diagnosis and Treatment Part 39

362 Section Surgical Approaches a b Figure 14. Landmarks for occipital screw insertion a Posterior view. b Axial view. Injuries to the spinal cord or vertebral artery are rare if the technique is applied The 2nd cervical nerve is at risk when exposing the C1/2 joint nar control for optimal screw placement. The medial border of the C2 pedicle (2–5 mm axial diameter) should be palpated with a dissector or a nerve hook. The screw is positioned as medially as possible to avoid injuries to the vertebral artery, which lies immediately laterally. The entry point for screw insertion is about 3 mm cranial to the lower edge of the C2 inferior facet. Usually, there is a small groove at the transition of the inferior facet to the lamina which serves as a landmark for the entry point. The drill is angled to aim at the arch of C1 in a strictly sagittal plane. The screw should pass just below the posterior border of the C1/2 joint. In some cases, the craniocaudal angulation can only be achieved if the drill is significantly inclined. Rather than dissecting all the posterior mus-cles,wepreferonlytoexposethespinefromC1toC3andchooseapercutaneous insertion of the drill usually at the level of C7–T1 with a tissueprotector. Injuries to the vertebral artery or spinal cord are rare if the technique is performed prop-erly [22, 27]. Atlantoaxial Pedicle Screw Fixation An alternative to the transarticular screw fixation is a stabilization of the spine with pedicle screws which are connected with rods [29, 64] (Fig. 15d–g). The screw entry point in C2 is more lateral (4–5 mm) than the transarticular screw trajectory. The drill is directed 20°–35° cranially and 15°–20° medially. The entry point in C1 is below the lamina and 2–3 mm lateral to the medial edge of theC1,whichcanbepalpatedwithadissector.Thescrewisaimedabout10°–15° medially and 15°–20° cranially. Care has to be taken not to injure the C2 exiting nerve root (greater occipital nerve). Anterior Atlantoaxial Transarticular Screw Fixation A second alternative is an anterior transarticular screw fixation [59]. The screw entrypointis5mmbelowtheC1/2jointlineinthegrooveformedbythebasisof Surgical Approaches Chapter 13 363 52 53 Figure 15. Landmarks for upper cervical spine screw insertions Posterior atlantoaxial transarticular screw fixation: a posterior view; b lateral view; c axial view. Atlantoaxial pedicle screw fixation: d posterior view; e lateral view; f axial view at C2. Anterior atlantoaxial transarticular screw fixation: g anterior view; h lateral view; i axial view. thedensandthelateralmass(Fig.15h–j).Thescrewtrajectoryisangled25°later-ally and cranially. However, the exposure of the entry point is not easy because it is far up in the cervical spine. During exposure great care has to be taken not to injure the: hypoglossus nerve superior laryngeal nerve Lateral Mass Screw Fixation Therearetwocommonlyusedtechniquesforscrewplacementinthelateralmass of the lower cervical spine. The screw entry point according to Roy-Camille [50] isinthecenterofthelateralmassandthetrajectoryisdirected10°outwardsrect-angular to the posterior cortex. According to the Magerl technique, the screw’s insertionpointlies2mmmedialandcranialtothefacetcenter.Thescrewtrajec-tory is parallel to the facet joints and angled 20°–25° outwards (Fig. 16a–c). Magerl’s method exhibits longer screw lengths and is therefore biomechanically 364 Section Surgical Approaches a b c d e f Figure 16. Landmarks for lower cervical spine screw insertions Lateral mass screw fixation: a posterior view; b lateral view; c axial view. Pedicle screw fixation: d posterior view; e lat-eral view; f axial view. superior to the Roy-Camille method [50]. Some studies have reported that the Magerl method is less likely to damage the neurovascular structures [51]. This screw insertion technique is reserved for the most experienced spine surgeons Lower Cervical Spine Pedicle Screw Fixation Pedicle screw fixation in the lower cervical spine is demanding and reserved for the most experienced spine surgeons [38]. The risk potential of spinal cord and vertebral artery injury is high [70]. The pedicle dimensions are not infrequently smaller than the screw [36]. Preoperative CT planning is recommended to rule out anatomical anomalies. Computer assisted surgery may reduce the rate of misplaced screws [35, 60] but does not compensate for lack of profound knowl-edge of the cervical anatomy and surgical experience [2]. The technique accord-ing to Abumi and Kaneda [1] chooses an entry point slightly lateral to the center of the lateral mass and inferior to the facet joint line (Fig. 16d–f). The cortical bone atthe entry point is openedwithaburr and theholeis enlargedto bury the pedicle screw (3–4 mm). The screw trajectory is angled 25°–45° medially. A thin pedicle finderis used to dilate thepedicle under lateral image intensifier control. Perforationscanbedetectedwithafinepedicleprobe(feeler)(Fig.17).Inexperi-enced hands, the complication rate is low [2, 38]. Thoracic Spine Pedicle Screw Fixation Screw placement in the thoracic spine requires a detailed knowledge of the anat-omy of the thoracic spine. However, it can be done with a high safety margin Surgical Approaches Chapter 13 365 Figure 17. Surgical instruments for screw hole preparations a Fine awl. b Thin pedicle finder. c Thick pedicle finder. d Pedicle feeler. when the proper technique is applied [20]. The pedicle morphology of the thoracic and lumbar spine has been thoroughly investigated in several studies [49, 65–67, 73]. The landmarks for screw insertion T2–T11 are below the rim of the inferior facet. Sometimes it is necessary to osteotomize the lateral inferior part of the facet toclearlyidentifythebaseofthesuperiorfacet.Theentrypointisatthelateralbor-der.Thescrewtrajectoryisangled20°mediallyand10°caudally.Whentheextrape-dicular technique [14] is used, the entry point is slightly more lateral and the angle to the midline is higher (Fig. 18a–c) (see Chapter 3 ). This inside-out-inside tech-nique involves a reduced risk of injuring the medial border of the pedicle [14]. The entry point at T1 is slightly more medial and the screw trajectory is less angled to themidline.TheentrypointforthepedicleofT12isatthelevelofthemammillary process,whichisopened/removedwitharongeur(Fig.18d–f).Thescrewtrajectory isangledmoremediallysimilarlytothelumbarspine.Thescrewsforadultpatients usuallyhaveadiameterof5(lowerthoracicspine)and6mm(lowerthoracicspine) and have a length of 30–35 mm at T1 and 45–55 mm at T12, respectively. Our preferred technique (Fig. 17) is to use a sharp fine awl to open the cortical boneattheentrypoint.Thispositionischeckedinthelateralplaneusinganimage intensifier. A thin pedicle finder is used to probe the pedicle again under fluoro-scopic guidance. A fine pedicle feeler is entered into the pedicle hole to verify that thecorticalshellofthepedicleisintactparticularlymedially,inferiorlyandanteri-orly. In the lower thoracic spine, a thicker pedicle finder is used to further widen the pedicle. In questionable cases, the screw is inserted somewhat deeper than the baseofthepedicle,whichcanbecheckedinthelateralviewwithanimageintensi-fier. The screw is then removed and the medial pedicle wall is palpated with the pedicle feeler. When the medial wall is intact the screw can be reinserted. Check for potential perforations with a fine pedicle feeler Lumbar Spine Pedicle Screw Fixation The pedicle morphology of the lumbar spine has been accurately described in several studies [41, 49, 56, 62, 67, 74]. 366 Section Surgical Approaches a b c d e f Figure 18. Landmarks for thoracic pedicle screw insertions ThoracicpediclefixationatthelevelofT6:aposteriorview;blateralview;caxialview.Notethealternativeextrapedicu-lar screw position on the right side. Thoracic pedicle fixation at the level of T12: d posterior view; e lateral view; f axial view. A double sacral screw fixation provides a strong sacral anchorage Several techniques have been described. We prefer a more lateral insertion point with a larger angulation to the midline, which is also biomechanically more sta-ble than a straight anterior screw insertion. The pedicle entrance point is at the lateralborderofthebaseofthesuperiorarticularprocess.Thesametechniqueis used as described for the insertion of thoracic screws. The screw trajectory is angled 20°–25° to the midline. In the sagittal plan the screws take a course paral-lel to the upper vertebral endplates (Fig. 19a–c). Knowledgeofthesizeandanatomyofthepedicleisrequired,butalsoanunder-standingofthetopographyofnerveandvascularstructuresinrelationtothepedi-cleisindispensableforsafepedicleplacement.Thenerverootsarelocateddirectly at the medial-inferior border of the pedicle. Screws should not penetrate the ante-riorcortexexceptincasesinwhichthisisabsolutelynecessarytoenhancethepull-out resistance. The screws should not be in contact with an artery because pulsa-tion can cause vessel wall erosion and the formation of an aneurysm. Sacral and Iliac Screw Fixation Themostfrequenttechniqueisscrewplacementinthefirstsacralpediclelocated just below the L5/S1 facet angled medially 20° cranially toward the anterior cor-ner ofthepromontorium.Anotheralternativeistoinsertthescrewsata30°–45° lateral and cranial direction into the sacral alae (Fig. 19d–g). Both screw posi- ... - tailieumienphi.vn