Spine Instrumentation.pptx

This is aimed at comprehensive discription of all the spine instrumentation and fixation techniques.

1. Spine Instrumentation – Cervical/ Thorasic /Lumbar Dr. Sairamakrishnan S

2. History of spine instrumentation The use of internal fixation as a tool for both stabilization and correction of deformity was a major advance in modern spine surgery. A thorough knowledge of the evolution of spinal instrumentation should yield a better understanding of both present and future developments.

3. DORSAL THORACOLUMBAR INSTRUMENTATION In 1975, the Harrington rod represented the state of the art in spinal instrumentation. Originally developed by Paul Harrington for the correction of spinal deformities. The use of a distraction system provided excellent correction of coronal plane deformities. Use of distraction as the sole correction tool resulted in the loss of normal sagittal plane alignment. ( flat back syndrome) Hook dislodgement and rod breakage also proved to be troublesome complications.

4. In addition, casting or bracing was generally required in the postoperative period, which proved to be difficult or impractical in some patients

5. Eduardo Luque advanced a major concept in the mid1970s that quietly pushed forward the future direction of spinal instrumentation: segmental spinal fixation. Luque popularized the use of a 3/16-inch steel rod secured at each spinal level with sublaminar wires. Luque reasoned that increasing the number of fixation points along a construct would reduce the force placed upon each individual point and obviate the need for a postoperative cast or brace. It increased the potential corrective power of instrumentation, reduced the potential for construct failure, and resulted in improved fusion rates

6. Some users of Harrington rod instrumentation adopted Sublaminar wires - “Tex-Mex” operation. Complications of Sublaminar wires Neurological injury Cut through Difficult to revise In response to these concerns, Drummond and colleagues developed a method for segmental fixation using a button-wire implant passed through the base of the spinous process

7. Though it does not provide as strong fixation as sublaminar wires it avoids, however, passing anything into the spinal canal and thus reduces the risk of direct neurologic injury. Hence the name “chicken- Luque ” procedure.

8. The Cotrel Dubousset (CD) system was introduced in 1986 using a 1/4-inch rough-surfaced rod. The multiple-hook design applied the principles of segmental fixation without the need for sublaminar wires. This proved a powerful force in the correction of scoliosis. Cross-linking the two parallel rods together provided further stability. Had difficulty in revision due to the inability to remove the hooks without destroying the locking mechanism. Since then multiple systems with variation in locking has been developed.

9. A major advance provided by these spinal systems was the exploitation of the pedicle as a site for segmental fixation. This innovation is generally credited to Roy-Camille of Paris Advantages of pedicle screws biomechanically superior can be placed into the sacrum they can be placed even after a laminectomy can be positioned without entering the spinal canal.

10. screw-plate vs screw-rod Most surgeons were ultimately attracted to rods because their use provides greater flexibility, reduces encroachment upon the adjacent facet joints, and leaves more surface area for fusion

11. There has been an interest in developing dynamic stabilization systems for degenerative diseases. They have been approved as an adjunct to fussion Interspinous devices that increase the intervertebral space have also been developed to treat a myriad of degenerative conditions. The primary indication is mild or moderate neurogenic claudication from spinal stenosis

12. VENTRAL THORACOLUMBAR INSTRUMENTATION Dwyer developed a ventral system for internal fixation using screws connected by a cable. The Zielke device connected transvertebral screws with a threaded rod and nuts and was more rigid than the Dwyer cables. This added both strength and the capacity for incremental correction and derotation , permitting a more powerful correction. The ventral Kostuik -Harrington instrumentation was an adaptation of short Harrington rods to achieve short-segment ventral fixation.

13. Ryan introduced a plate secured by a rostral and caudal bolt inserted through the vertebral body - offered less resistance to rotation. The Yuan I-Plate was an alternative design that consisted of a 3.5-mm stainless steel plate secured with transvertebral screws allowed for the placement of three screws at each vertebral level.

14. DORSAL CERVICAL INSTRUMENTATION Earliest methods to provide internal fixation for dorsal cervical fusions involved the use of spinous process wiring. The Brooks and Gallie techniques use sublaminar wires to compress an autologous bone graft. Halifax clamps are a pair of upgoing and downgoing sublaminar hooks tightened together with a screw that is then secured in position with a locking mechanism.

15. Magerl introduced transarticular screw placement for internal fixation of C1-2.

16. Lateral mass plate fixation with screws was introduced by Roy-Camille and associates. The first technique for screw placement was modified by Magerl and Seeman, Anderson and colleagues and An and colleagues. Lateral mass screw-rod fixation systems were designed to use 3.5-mm and 4-mm diameter lateral mass screws with polyaxial head designs attached to titanium rods for improved ability to connect fixation points.

17. VENTRAL CERVICAL INSTRUMENTATION First system was developed by Bohler in the mid1960s Potential for screw backout was recognized as a possible cause of serious complications Earlier systems consisted of simple plates with slots or holes but without any locking devices. Constraint of the screws depended on obtaining bicortical purchase and “blocking” backout by screw angulation. This led to the development of the Cervical Spine Locking Plates.

18. The CSLP used a titanium expansion screw that secured the screw head to the plate and, thus, allowed for unicortical purchase without the risk of screw backout. Although this plate was widely used and had good reported surgical results, some surgeons felt that the system was too rigid and shielded the graft from stress, thereby promoting a significant rate of pseudarthrosis. Ventral fixation of odontoid fractures can be achieved with the placement of one or multiple screws.

19. Lateral Mass Fixation (C3-6)

20. Entry point The screw entry point is found slightly superomedial to the intersection of these lines.

21. Opening of the cortex Penetrate the cortex with a thin burr or an awl.

22. Medio-lateral angulation The drill trajectory should be aimed 25 degrees laterally to avoid the vertebral artery which is located directly anterior to the entry point.

23. Cranio-caudal angulation To identify the cranio-caudal angulation, a Penfield elevator is inserted in the facet joint which will be included in the fusion. The drill trajectory is then then parallel to this elevator, avoiding compromise of the facet joints.

24. Monocortical vs Bicortical Utilizing 14 mm screws will be safer but only monocortical purchase can be achieved. A longer screw providing bicortical purchase will result in a more stable construct. However, the screw tip should not extend too far beyond the second cortex as it may compromise the nerve root.

25. Drilling If a monocortical screw is planned, the drill is set for a 14mm screw hole. For bicortical screw t he drill bit is advanced only for a short distance, then pulled back before advancing again. This maneuver is repeated until the second cortex can be felt and crossed.

26. Screw insertion A screw of appropriate diameter (3.5 mm) and length is carefully inserted into the same created trajectory.

27. Cervical pedicle screws Pedicle screws offer three-column fixation and have greater pullout strength than lateral mass screws The small mid-cervical pedicles and the proximity of the cord, vertebral arteries, and nerve roots limit enthusiasm for routine use of pedicle screw fixation. Most frequently, C3-6 pedicle screw placement is recommended for posterior-only corrections of markedly unstable three-column injuries or for maintenance of correction after cervical osteotomy or postlaminectomy kyphosis

28. Standard entry point is 3 mm below the superior facet joint. The drill is angled 45 degrees medially and advanced in a vertical line parallel to the endplate. Alternatively, Abumi recommended removal of the lateral mass with a high-speed bur to provide a direct view of the pedicle introitus.

29. Due to the low margin of error superior laminotomy is recomended

30. Entry points The starting point is just below the facet joint at the half way point between the medial and lateral margins of the lateral mass.

31. Opening of the cortex Open the superficial cortex of the entry point with a burr.

32. Medio-lateral angulation Depending on the exact location of the starting point, the angle is around 45°. Angulation decreases somewhat as you progress cranial to caudally, approaching 50° at C3 and 40° at C6.

33. A trajectory roughly perpendicular to the axis of the posterior elements is required. This trajectory can be fine tuned by palpating the inferior and superior margins of the pedicle through the laminotomy.

34. Screw incertion

35. Anterior Subaxial Cervical Fixation FIRST-GENERATION PLATES Allowed motion at the screw-plate interface and as such were considered nonrigid implants graft exposed to greater compressive forces thereby promoting fusion

36. SECOND-GENERATION PLATES The second-generation plates were rigid implants that were best utilized in trauma They also reduce the need for postoperative immobilization However, they may stress shield the bone graft and result in either implant failure or a pseudoarthrosis

37. THIRD-GENERATION PLATES The third-generation plates improved on the original Caspar plate design by preventing screw backout while allowing for some motion at the screw-plate interface, thereby enabling load sharing between the bone graft and the implant Two subtypes: (1) rotational and (2) translational. The rotational dynamic plates allow screws to rotate or toggle at the screw-plate interface Translational dynamic plates allow for axial translation and rotation of the plate

38. HARRINGTON DISTRACTION FIXATION

39. Pedicle screw

40. Screw charecteristics Diameter- 4.5mm to 7mm Length – 30mm to 55mm Self-tapping and non tapping screws Monoaxial and polyaxial screws

41. The transverse width of the pedicle is the limiting factor in terms of screw size The strength or resistance to bending and breaking of a screw is proportional to the third power of its minor diameter. The pullout resistance of a screw is related to the amount of bone that can be incorporated between the threads of the screw. The distance between the threads (pitch), major diameter, and thread shape all influence the pullout resistance of a screw

42. Pedicle screw entry techniques Intersection technique Pars intraarticularis technique Mamillary process technique

43. Although the midline of the transverse process corresponds to the location of the pedicle at L4, this relationship does vary at different lumbar levels. Above L4, the midline of the transverse process is rostral to the pedicle, and at L5, it is an average of 1.5 mm caudal to the pedicle

44. Pars intaarticularis technique relies on the easy identification of pars and the lateral border of the lamina. Since pars intraarticularis is the junction between the pedicle and the lamina the entry point is directy over the posterior aspect of the pedicle.

45. The mamillary process technique uses the mamillary process which is a small prominence at the base of the transverse process. This entry point is the most lateral entry point of all the techniques. It provides the most medio-lateral angulation.

46. Thoracic entry points The transverse process is rostral to the pedicle in the upper thoracic spine and caudal to the pedicle in the lower thoracic spine. The crossover occurs at T6-7. Hence entry point with relation to the transverse process varies based on the level.

47. The entry point of the pedicle screw for the lower thoracic segments is defined after determining the intersection of the mid portion of the facet joint and the superior edge of the transverse process. The specific entry point will be just lateral and caudal to this intersection. The entry point tends to be more cephalad as you move to more proximal thoracic levels.

48. Sacral pedicle screw Entry point is superior and lateral to the S1 foramen just inferior to the inferior articular process of the L5 vertebra. The trajectory should aim for the sacral promontory. Bicortical purchase increases the strength of the fixation Unicortical - 1 Bicortical /S1 endplate purchase - 1.5 Tricortical purchase – 2 Advantage of Pedicle Screw Placement Into the Sacral Promontory (Tricortical Purchase) on Lumbosacral Fixation - Kato, Minor et al, Journal of Spinal Disorders and Techniques, 10.1097/BSD.0b013e31828ffc70

49. Insertion techniques - freehand, fluoroscopy-based, and frameless stereotaxy systems. The screw entrance site is decorticated with a drill or rongeur The pedicle is probed with a blunt-tipped pin or small curet. Intraoperative radiographs are used to check pin placement. Holes are tapped with successively larger taps until a desired diameter is reached The walls of the pedicle should be palpated from within after each tap to verify the integrity of the cortical bone.

50. Screws should be placed with as much lateral-to-medial angulation as possible so as to maximize the beneficial effects of triangulation on screw pullout. No significant advantage is gained by penetration of the ventral cortex

51. Entry point Open the superficial cortex of the entry point with a burr or a rongeur.

52. Cranial-caudal angulation A pedicle probe is used to navigate down the isthmus of the pedicle into the vertebral body. The appropriate trajectory of the pedicle probe in the cranial caudal direction occurs by aiming to be parallel to the superior endplate

53. Medio-lateral inclination The medio-lateral inclination will depend on the location up to 45° in L5 or 0° in T5. The main goal is to avoid medial penetration of the spinal canal superficially and lateral or anterior penetration of the vertebral body cortex at the depth of insertion. Ideally, the two screws should converge but stay entirely within the cortex of the pedicles and body.

54. Probing Once the pedicle track has been created, it is important to confirm a complete intraosseous trajectory by pedicle and body palpation using a pedicle sounding device. At any point in the process, radiographic confirmation can be obtained.

55. Screw insertion A screw of appropriate diameter and length is carefully inserted into the same created trajectory.

56. Entry point T1 to T3 The entry point lies just below the rim of the upper facet joint, 3 mm lateral to the center of the joint near the superior border of the transverse process.

57. Opening the cortex Open the superficial cortex of the entry point with a burr or an awl.

58. Medio lateral angulation Their transverse angulation ranges from 30° at the level of T1, to 15° at the level of T3.

59. Cranial-caudal angulation Anatomic trajectory Subchondral/straightforward trajectory 

60. Screw placement

61. The cortical bone trajectory Described by Santoni as an alternative for osteoporotic patients due to higher amount of interface between screw and cortical bone This technique requires a more medial entry point and cephalad-lateral trajectory The starting point is in the inferior pars. The CBT will require shorter screws

62. Grading System Used for the Assessment of Screw Placement Proposed by Abul-Kasim

63. Laminar Hook Insertion

64. Preoperative imaging studies are useful for determining the adequacy of the spinal canal for sublaminar hook placement. laminotomies are performed, removing the caudal portion of the lamina above and the rostral portion of the lamina below the level of hook application. Once a hook is placed, it should be compressed against its lamina to prevent migration into the spinal canal

65. Pedicle Hooks

66. Thoracic pedicle hooks are placed between the superior and inferior articulating surfaces of the facet. The caudal portion of the inferior articular process is removed by using a drill or osteotome.

67. Cross-Fixation Cross-fixation increases the stability of a construct by preventing rotation or translation. Screw pullout resistance is also markedly improved with the use of rigid cross-links combined with toeing in of the screws

68. Interbody Cages A cage should ideally have a hollow region of sufficient size to allow packing of bone graft or bone graft substitute. It should be structurally sound so that it can withstand the great forces applied to it in the immediate postoperative period and allow immediate patient mobilization. It should have a modulus of elasticity that is close to that of vertebral bone to optimize fusion and avoid subsidence. It should have ridges or teeth to resist migration or retropulsion into the retroperitoneal space or the spinal canal.

69. Serrations on the top and bottom surfaces of the cage may improve fixation strength and diminish motion at the cagebone interface It should be radiolucent to allow visualization of fusion on radiographs and may have radiopaque markers to localize the precise location of the implants on intraoperative and postoperative radiographs. If inserted from a dorsal approach (TLIF or PLIF), it should be tapered, with a bullet-shaped tip to allow easier initial insertion into the disc space with minimal trauma to the adjacent thecal sac and nerve roots. This is especially beneficial when introducing the graft into narrowed disc spaces for distractive purposes.

70. The stiffness of a cage has been found to influence fusion rates. Ideally, a cage would have a modulus of elasticity that is similar to that of vertebral bone, which would optimize the load transfer between the cage and the adjacent vertebral bodies and reduce the effects of stress shielding on the graft material. Carbon fiber cages have a modulus of elasticity closer to that of cortical bone,34 while metal and titanium cages exceed the stiffness of the vertebral bone.

71. The modulus of elasticity of stainless steel and titanium implants is 200 and 110 GPa , respectively, compared with that of vertebral trabecular and cortical bone, which is 2.1 and 2.4 GPa , respectively. Titanium cages also have the disadvantage of incomplete radiographic assessment of the fusion mass. Furthermore, owing to the mismatch of modulus of elasticity of titanium and vertebral bone, the stiffness of titanium cages may cause subsidence into the vertebral end plates

72. To create a more suitable modulus bone-cage-bone transition, PEEK cages have been developed and are routinely used as interbody devices. PEEK is a semicrystalline aromatic polymer that is radiolucent and can be formed into any shape. Radiopaque markers are routinely incorporated into the borders of the cage so that the surgeon can precisely localize the implant on radiographs.

73. Despite a more biologic modulus of elasticity of PEEK as compared to metal, there is some biomechanical evidence showing lower primary fixation and initial stability of PEEK cages compared to titanium cages of equal dimensions. But they provide same clinical outcomes when augmented with posterior instrumentation.

74. Some studies have evaluated the fusion rate and its relationship to cage stiffness, evaluating a poly-L-lactide (PLLA) cage versus a titanium cage. An in vitro study showed that PLLA cages were mechanically sufficient directly after implantation After 6 months, increased interbody fusion was seen with the PLLA cages. Smit TH, Müller R, Dijkvan M, et al. Changes in bone architecture during spinal fusion: three years follow-up and the role of cage stiffness. Spine (Phila Pa 1976). 2003;28:1802-1809.

75. One of the goals of lumbar interbody fusion is to increase/ restore disc space height and maintain segmental lordosis. Increasing disc space height is relatively easy to accomplish in the prone or supine patient intraoperatively and is usually maintained on short-term follow-up. However, over time, settling of the cage into the vertebral end plates can occur. If significant subsidence occurs, it can lead to segmental loss of lordosis and loss of anterior column support.

76. These changes may result in an unfavorable biomechanical environment contributing to pseudarthrosis and possibly compression of the neural elements. The causes of subsidence are multifactorial and may be due to any combination of improper graft selection, poor bone quality, insufficient bony healing, lack of supplemental open or percutaneous fixation, and overexuberant end-plate preparation.

77. Cervical cages Reduces the need for bone graft harvesting and reduces donor site morbidity. Promotes osteointegration and provides adequate resistance to compressive forces. They are zero profile devices and do not warrant removal during revision surgeries. Latest generation cages can be used as standalone systems in cervical fussion .