Arthrodesis (Fusion), Instrumentation and Bone Grafts

Arthrodesis in the lumbar spine can be approached posteriorly by placing bone graft across the transverse processes. Other lumbar fusion techniques using a posterior approach include Posterior Lumbar Interbody Fusion (PLIF) and Transforaminal Lumbar Interbody Fusion (TLIF). (73,74) Interbody fusion techniques involve removal of the disc and replacement with bone graft and/or implant to induce fusion through the disc space. Interbody fusion in the lumbar spine can also be accomplished anteriorly via transabdominal or retroperitoneal access. Combined anterior-posterior approaches are also used, depending on the goals of surgery and needs of the patient.

In the case of thoraco-lumbar deformities (e.g., scoliosis) an anterior and/or posterior approach may be employed. The surgical approach used is dependent on the magnitude, flexibility, and location of the deformity and the surgical goals. Anterior approach to scoliosis correction can usually be accomplished over a shorter fusion construct than with posterior approaches, thus sparing mobile spine segments. (75,76) Anterior arthrodesis is accomplished using thoracotomy or combined thoraco-abdominal approach, depending on the level of the curve to be corrected. (75-78) Occasionally the facet joints are fused in combination with other posterior approaches. The approach and technique depends on the particular clinical needs of the patient and the goals of surgical intervention.

Arthrodesis in the cervical spine always accompanies anterior discectomy procedures because of stability issues. Anterior plates may be used. Some posterior decompressive procedures may require arthrodesis accompanied by lateral mass plates. Instability in the upper cervical spine may require a posterior arthrodesis with plating to the occiput.

Painful disc degeneration is a controversial indication for lumbar arthrodesis. The goal of arthrodesis for painful disc degeneration is to eliminate motion across the painful disc thereby eradicating the pain generator. (79) Both anterior and posterior, and sometimes a combined anterior-posterior approach is used.

Instrumentation
Instrumentation may be indicated to aid fusion healing by internal immobilization of the fusion site. Various instrumentation systems exist for the lumbar spine, including pedicle screws and rods, plates, intervertebral cages filled with bone graft and in some applications (e.g., scoliosis and trauma) hooks and rods.

Pedicle screws are placed posteriorly through the pedicles into the vertebral bodies, and connected by rods and crosslinking devices. Intervertebral cages can be placed transforaminally (through posterior incision) into the disc space after the disc has been excised, or anteriorly via retroperitoneal approach. The cages are filled with autologous bone graft or bone growth enhancers, with the intent of creating interbody fusion. Intervertebral cages are rarely used as stand-alone devices, and are most often used in combination with pedicle screws (Figure 13).

Figure 13.
Lateral radiograph of a patient with intervertebral cages and pedicle screws.

In the cervical spine, anterior plates and screws can be used in combination with interbody fusion (Figure 14). Posteriorly, depending on the level to be fused, lateral mass plates, wiring techniques, and special occipital-cervical plates and screws may be used. In the thoracic spine, hooks and rods, anterior plates, and more recently, pedicle screw techniques can be used. (80)

Figure 14.
Lateral radiograph of a patient with C5-C7 anterior arthrodesis and plating..

Bone Graft
Arthrodesis can be achieved through the use of autograft or allograft bone. Additionally, the use of bone graft alternatives is increasing as the understanding of their use is evolving. Spinal fusion healing depends on three capabilities of bone graft material. Osteogenesis; the growth of new bone, depends on the presence of cells that synthesize new bone at the fusion site. Osteoconduction is the ability of the graft material to serve as a scaffold for new bone formation. Osteoinduction is the recruitment of stem cells to differentiate into new bone. (81)

Autograft can be harvested from the anterior superior iliac spine or the posterior iliac crest, depending on the surgical approach. Local bone can also be used, such as spinous processes from posterior lumbar surgery, or a rib that is resected for anterior approach to the thoracic spine. Autograft possesses all three properties of osteogenesis, osteoconduction, and osteoinduction. Other advantages of autograft bone include its biocompatibility and the avoidance of disease transmission risk. The disadvantages of autograft are prolonged operative times for harvest of graft and closure of the incision, increased intra-operative blood loss, the limited supply of bone, and post-operative graft site pain. (81,82)

Allograft is human bone taken from a cadaver donor. Allograft is osteoconductive but has limited osteoinduction. Because it is not living tissue allograft is not osteogenic. The advantages of allograft include its availability, avoidance of a bone graft incision, and structural utility. The disadvantages of allograft include the risk of disease transmission, slower healing of fusion, rejection from tissue incompatibility, and greater risk of fatigue failure from the processing of cadaver bone. (81-84)

The use of autograft versus allograft depends on the area to be fused, its biomechanical demands, and in certain cervical procedures, can depend on patient or surgeon preference. In some instances, both are used at the same fusion site. Autograft is still the gold standard in posterior lumbar fusions. For further discussion on bone graft enhancers and substitutes, see the section, Trends in Spine Care (link at page end).

There are conflicting reports in the literature regarding the duration of and disability related to post-operative pain from autogenous bone graft sites. The pain from a posterior iliac crest autograft harvest site tended to peak at 3-6 months and by 12 months, was considerably less in one study. (85) The pain and functional disability from anterior superior iliac crest graft sites can be significant. A retrospective study of 187 consecutive patients from one surgeon demonstrated chronic pain (greater than three months) in 26% of the patients surveyed. (86) Questionnaires were completed by mail or over the phone for 134 of these patients at an average of 48 months post-operatively (range 24-72 months).

Of the patients with chronic pain, 43% required analgesics. The mean Visual Analog Scale for pain in these patients was 3.8. Functional limitations were also reported in these patients, with 13% reporting difficulty with ambulation, 12% reporting restrictions in recreational activities, and 15% reporting either restrictions in activities of daily living or ability to complete their household chores.

In another study of 144 patients who underwent anterior cervical arthrodesis with anterior superior iliac crest autograft, bone graft site pain resolved within one week in 58.3% of patients, and within four weeks, 90.3% had resolved their bone graft site pain. (87) It is clear that patients must be informed pre-operatively of the potential long-term pain and functional limitations from autogenous bone graft sites.