Dorsal Implants
From Head-neck-joints instability conditions
Dorsal cervical fusion differs from ventral fusion in that it is performed with onlay grafts and facet fusion placed under tension, posterior to the instantaneous axis of rotation (IAR). In ventral fusion, conversely, compressive (bone healing) forces are used. Studies examining the use of allograft bone for dorsal cervical fusion have had poor results -- possibly an indication of the biology of the bone healing response.[48,49] For these reasons dorsal cervical fusion might benefit from the use of more rigid implants and constructs. Posterior internal fixation can be divided into 3 categories based on anatomic levels: occipitocervical, C1-2 f, and subaxial C-spine fixation.
Occipitocervical Methods of occipitocervical fusion include combinations of onlay graft and wire, rod and wire, or plates and screws. Indications for occipitocervical fusion include instability resulting from inflammatory arthropathy, trauma, and congenital deformity, as well as tumor and infection. Occipitocervical fusion may also be indicated after anterior resection of the odontoid, and after occipitocervical decompression or partial resection of the occipital condyle. In order to prevent cranial settling, occipitocervical constructs must be rigid and able to resist shear, rotation, and axial loads.[117]
Wertheim and Bohlman described a technique of wire passage that avoids intracranial placement of the wire in stabilizing the onlay bone to the skull. One wire is passed through the occiput, one around the arch of C1, and one through and around the base of the spinous process of C2. Two slabs of corticocancellous graft containing 3 holes are placed on either side of midline onto the decorticated bone surface and anchored in place by wires (Figure 8).[118,119] This method may enhance the security of the bone graft, thus improving the fusion rate.
Figure 8. Bohlman technique of occipital cervical wire/fusion.
Anderson PA, Steinman JC. Internal fixation of the cervical spine. In: Frymoyer JW (ed).The Adult Spine. Principles and Practice. 2nd ed. Philadelphia: Lippincott-Raven; 1997:1119-1147. Figure 1.
Ransford and colleagues described the technique of stabilizing the occipitocervical junction using a contoured Luque rod, wired to the skull and cervical spine with sublaminar wires.[120] When the dorsal elements of the upper cervical spine cannot be used as points of fixation, plate fixation to the occiput with screws is a particularly useful strategy. Because the skull tends to thicken toward the midline, longer screws providing better fixation can be placed here. Hence, inverted Y-plate constructs and plates with a medial curve have been designed.[121,122] Another useful technique for fixation to the occiput is the use of an inside-outside flat-head screw or stud (occipital button) placed through a burr hole into the epidural space and secured to the plate by way of nut (Figure 9).[123]
Figure 9. The inside-outside technique of Pait et al. A keyhole is made in the occiput, a button is placed in the keyhole. The plate is affixed to the button and the subaxial spine.
Benzel EC. Biomechanics of Spine Stabilization: Principles and Clinical Practice. 2nd Edition (in press). AANS Publications.
Figure 28.5.
Atlantoaxial Atlantoaxial arthrodesis is performed for translational instability due to traumatic ligamentous disruption or fracture, inflammatory arthropathy, congenital abnormalities, skeletal dysplasias, or genetic disorders, as well as for rotational instability. Dorsal atlantoaxial fusion can be accomplished by 1 of 3 methods and their variations: (1) C1-2 wiring; (2) transarticular screw fixation; and (3) interlaminar Halifax clamp technique.
The Gallie and Brooks techniques, as well as a combination of the 2, have been used for C1-2 wiring and fusion (Figure 10).[124-126] Both techniques minimally resist sagittal plane deformation, which occurs in a parallelogram-like manner. This type of motion is minimally resisted given the axial orientation of the C1 and C2 facets. The Gallie technique uses a single sublaminar wire at C1 passed around the base of C2 and around a sculpted interposed corticocancellous iliac crest bone graft. The Brooks technique involves passage of a double wire loop beneath the lamina of C1 and C2. Two autologous iliac corticocancellous grafts are fashioned on either side of the midline and secured using the 4 sublaminar wires. Hanley and colleagues reported that the Brooks technique achieved twice the stiffness in flexion and extension and 5 times the stiffness in rotation compared with the Gallie technique.[127] Sonntag and coworkers described a combination technique that provides a biomechanical advantage similar to that of the Brooks technique, without the disadvantage of bilaminar sublaminar wire passage.
Figure 10. The Gallie (A) and Brooks (B) C1-2 wiring techniques.
Anderson PA, Steinman JC. Internal fixation of the cervical spine. In: Frymoyer JW (ed).The Adult Spine. Principles and Practice. 2nd ed. Philadelphia: Lippincott-Raven; 1997:1119-1147. Figures 6A and 7B.
Magerl described the use of C1-2 transarticular screw fixation as an adjunct to C1-2 wiring in order to counteract the rotatory and translational forces of C1-2 instability.[128] Transarticular screws are nearly 10 times stiffer in rotation than C1-2 wiring or Halifax clamp techniques and improve resistance to shear forces.[129] This allows for minimal postoperative immobilization. This technique is contraindicated in cases in which the lateral masses of C2 are destroyed or subluxed about C1 as occurs with inflammatory arthropathy. Complications associated with screw malposition include potential damage to neural structures or vertebral arteries. Damaged vertebral arteries have been reported to occur in 4% of cases.[130] The Halifax interlaminar clamp is associated with fewer risks than are wires or screws. The major disadvantage of the Halifax interlaminar clamp is the associated technical challenges of application and the inherent potential for rotational dislocation.[118]
Subaxial Cervical Spine Dorsal cervical wiring can be used in the subaxial cervical spine to simulate the function of the dorsal ligaments by providing a tension band effect and by fixating bone graft or rods. The most common indication for these procedures is the treatment of distractive flexion injuries resulting in unilateral or bilateral facet disruption and dislocation. Biomechanical studies suggest that in this clinical setting, dorsal fixation and fusion techniques are superior to ventral cervical plating.[131,132] In clinical practice, however, ventral fusion and plating may restore the stability of the acutely injured spine with posterior ligamentous injury. This is particularly important when considering the treatment of traumatic facet dislocation associated with an extruded disc.[133,134]
Most wiring techniques have evolved from the technique reported by Rogers.[135] Rogers' technique is a simple interspinous process wiring technique wherein the wire is passed around and through the bases of adjacent spinous processes with corticocancellous bone grafts laid under the wires bridging the interspace (Figure 11A).[135] The Bohlman triple-wire technique is an adaptation of Rogers' technique in which separate wires are used to compress bridging corticocancellous grafts to the lamina and spinous processes. Some studies show that the Bohlman triple results in flexural or torsional stiffness that is superior to Rogers' wiring alone (Figure 11B).[136] In the Dewar technique, wires are fixed to the spinous process by means of Kirschner wires.[62] If there is multilevel insufficiency of the dorsal elements, a technique involving facet wiring described by Robinson and Southwick may be used.[137] The technique involves passing wires through drill holes in the inferior articular processes and binding 2 longitudinal struts of corticocancellous bone (Figure 11C). An oblique facet wiring can also be used in cases of facet fracture dislocation with rotational instability (Figure 11D). This technique involves bilateral wiring of the facets to the spinous process of a caudal level while preventing the need for fusing an additional level to reach an intact spinous process (Figure 11E).[138,139]
Figure 11. Lower cervical spine wiring techniques: (A) Rogers technique; (B) Bohlman triple wire technique; (C) Dewar technique; (D) Robinson and Southwick facet wiring; (E) oblique facet wiring.
Weis JC, McAfee PC. Lower cervical spine arthrodesis: wiring techniques. In: The Cervical Spine Research Society Editorial Committee (ed). The Cervical Spine. 3rd ed. Philadelphia: Lippincott-Raven;1998. Figure 35-4 (A), Figure 35-8 (B), Figure 35-12 (D), and Figure 35-13 (E).
Liew SM, Simmons, ED. Cervical deformity: rationale for selecting the appropriate fusion technique (anterior, posterior and 360 degree). Orthop Clin North Am. 1998;29(4). Figure 1(C).
Roy-Camille popularized dorsal cervical plating as an alternative to wiring techniques.[140] This technique is particularly useful in situations where the laminae are incompetent. Screws can be placed into the lateral masses of C3-C6. Pedicle screws are more typically used in the cervicothoracic junction (C7-T2). Two methods of screw placement into the lateral masses are commonly used. The original technique, as described by Roy-Camille, uses an entrance point in the middle of the lateral mass and directs the screw in a straight sagittal direction and 10° laterally.[140] The second method, the Magerl technique, involves placing a screw parallel to the facet joint and angling the screw laterally 25° to 30°.[141]