From Head-neck-joints instability conditions

[edit] Dorsal Implants

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.

[edit] 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.

[edit] 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]