Anterior Lumbar Interbody Fusion.
Along the way, it was quickly determined that the key to successful bony fusion was the degree to which the affected area could be immobilized during the fusing process. It was logical, therefore, that the next step in spinal fusion was implanting metallic instruments to restrict motion. Spinous process plates and facet screws began the hardware age of spinal fusion and paved the way for pedicle screws and Harrington rods.
During the mid 1970s and early 1980s, Bagby obtained an 88% fusion rate in the cervical spines of horses by using an anterior interbody fusion "basket." This was a technological breakthrough in the field of spinal fusion. As early as 1980, anterior lumbar interbody fusions were being performed in humans. The first human implantations of the Bagby and Kuslich or "BAK" cages took place in 1992. Today, threaded fusion cages are used routinely in the anterior lumbar interbody approach.
To properly understand the radiographer's role in spinal fusion, a knowledge of the pertinent anatomy is essential. Of the structures involved in the operation, the bones and the intervertebral cartilage are most important to the radiographer.
A typical lumbar vertebra consists of a vertebral body, laterally projecting transverse processes, a posteriorly projecting spinous process, superior and inferior articular processes and a vertebral arch. The vertebral arch surrounds the spinal cord and meninges in a ring of bone. Laterally, they are protected by the pedicles and posteriorly by the lamina. (See Figs. 1 and 2.) Hyaline cartilage lines the joints of the vertebral column.
The decision to fuse depends on the condition of the structure between the vertebral bodies, the intervertebral disc. The disc is comprised of a tough, fibrous cartilage casing known as the annulus fibrosis. Inside the annulus is a gelatinous middle called the nucleus pulposus. Through trauma or disease, the intervertebral disc can become herniated and project posteriorly. This phenomenon typically impinges on the spinal cord and causes characteristic sciatic pain. A herniated disc also can cause pain, in and of itself. The outer half of the annulus is rich in sensory innervation and is believed to be the tissue of origin in most cases of low back pain. Spinal fusion can offer relief for this condition.
When the body is injured through trauma or disease, the protective housing for the spinal cord can become damaged. The results, as many spine patients know, can be debilitating. In fact, with some 70% of the American public experiencing back pain at some time, it has become one of the leading causes of morbidity in the United States. The theory behind arthrodesis, or joint fusion, is that stabilizing and fusing the diseased joint prevents the pain-inducing movement of that joint.
Indications for ALIF
The ideal candidate for an anterior lumbar interbody fusion (ALIF) has low back pain of discogenic origin. Patients should fit the dual criteria of loss of height and mobility of the diseased segment accompanied by chronic, disabling back pain. Nonsurgical methods to alleviate symptoms should have been exhausted, and candidates should not have conditions that contraindicate spinal fusion (ie, chronic pain syndrome, psychosocial disorders or economic/legal issues). ALIF can be of particular benefit to patients whose cases are complicated by osteoporotic bone. In these patients, ALIF combined with pedicle screw implantation results in more successful fusion than when pedicle screws are used alone.
Knight defines the ALIF candidate as a psychologically stable patient with 1 or 2 affected levels confirmed as pain-generating by diagnostic testing. Those with spondylolysis or a grade I spondylolisthesis also may be successfully treated, although Knight states that most patients with these conditions do not require surgery. As an isolated procedure, ALIF is not indicated in those with hypermobility of a lumbar-motion segment or in those with multilevel disease.
Studies indicate that L5-S1 ranks first among levels most often fused. Over a 3-year period, Fraser reported fusing 129 levels, with L5-S1 comprising 52.7% of those fusions. The second largest percentage was 34.9% for L4-5. The remaining 12.4% was made up of various other levels. Ray reported similar numbers (57%, 41% and 1%, respectively).
Diagnostic Imaging and Testing
According to Ray, preoperative screening studies should include routine spine radiographs (with flexion and extension) and a structural imaging technique such as CT or MR. Axial cuts help determine vertebral body morphology and can serve as a template from which the size and model of a particular cage implant can be determined. Additionally, Ray recommends functional studies, such as facet blocks and discography, because structural changes may not cause symptoms. Whitecloud and Wolfe take a somewhat different stance regarding the evaluation of facet blocks. They believe that the facet block has not been shown to indicate who will benefit from spinal fusion. Flexion/ extension radiographs are similarly controversial. Boden and Wiesel wrote that little reliable information can be obtained from them.
The discogram is of significant value in determining whether a patient is an ALIF candidate. (See Fig. 3.) Juntura acknowledges that although the discogram is controversial with respect to accuracy, specificity and complications, it appears to be the only imaging procedure that not only shows pathology of a diseased disc, but also elicits the patient's low back pain. In addition to helping determine whether a patient is a fusion candidate, the discogram can be instrumental in predicting which type of fusion will best alleviate the patient's symptoms. Following up a positive discogram with the appropriate surgical procedure has been shown to improve clinical benefit to the patient.
When performing the discogram, a radiologist injects contrast into the nucleus pulposus of a suspected diseased disc under fluoroscopic guidance. Contrast leakage beyond the borders of the nucleus is indicative of a torn annulus. Additionally, the radiologist uses a manometer to pressurize the nucleus. If the pressure builds and remains (as in an inflated car tire), the annulus is likely intact. If the pressure drops and the patient's pain is reproduced, there is likely a tear in the annulus. The patient is asked to rate the sensation on a scale of 0 to 10, with 10 being the worst pain. (See Figs. 4 and 5.) Typically the patient is sent for a postdiscogram CT so that the source of extravasation can be more precisely located.
Advantages of ALIF
The ALIF procedure has many advantages. Compared with other fusion methods, operative time and blood loss are both reduced, assuming an experienced surgeon.[1,6] Average blood loss for an ALIF procedure is 200 to 300 mL, and a one-level procedure can be completed in 90 minutes or less. The columnar support offered by the interbody cages can improve fusion rates in cases of pseudoarthrosis revision and also reduces the stress at the junction of the fused/nonfused level. Interbody cages help preserve disc height in the case of discectomy and can help to correct a preexisting deformity (eg, loss of lordosis). Other advantages cited include "lack of surgical trauma to potentially pain-generating posterior structures, complete resection of the diseased disc and avoidance of scarring in the spinal canal."
From a biomechanical standpoint, the anterior approach avoids problems commonly associated with the posterior approach. For example, muscle stripping, which is necessary with a posterior approach, may lead to weakness and fatigue. Also, pedicle screw implantation predisposes adjacent levels to fusion transition syndrome, a breakdown of the adjacent nonfused levels, usually beginning with facet degeneration. ALIF does not predispose patients to this problem.
Disadvantages of ALIF
Disadvantages of ALIF are primarily complications of the surgical procedure. In a recent study by Kuslich et al, 947 middle-aged patients underwent BAK implantation. The operations were performed by 42 surgeons at 19 different medical centers. The major overall complications were superficial infections and ileus (3.1% each). Other complications included implant migration, vessel damage, pneumonia and fracture of S1. Retrograde ejaculation occurred in 4% of the male anterior cases.
The threaded fusion cage (TFC) is a fairly uniform design, although manufacturers have made adaptations unique to their products. (See Fig. 6.) The BAK cage, as described by Kuslich et al, is a hollow, porous, square-threaded, slightly tapered, cylindrical titanium device. The cages should be porous enough to allow through-growth of cancellous bone, yet rigid enough to withstand spinal forces without deforming or breaking. Some advantages of the TFC as described by Ray are:
* The cylindrical shape of the devices permits precision and ease of cutting through the adjacent end plates and into the vertebral spongiosa.
* The recipient beds are tapped and threaded to accommodate the threads of the devices. The chance of subsequent displacement is reduced.
* Cancellous bone used for the graft is protected by the cage's metal shell during fusion formation.
* Recoil of the annulus, gravity and muscle tension stabilize the implant. Usually no additional grafting or posterolateral instrumentation is needed.
* Because of tissue compatibility and increased radiolucency in CT or MR scans, titanium is the material of choice. Nickel-titanium alloys also are well tolerated, although nickel may pose a threat of allergic reaction in some cases.
The design of the Ray TFC is such that when implanted, the perforations face the spongiosa of the adjacent vertebral bodies while a "lateral definitive blocking rib" prevents ingrowth of the remnant disc material.
The dissection process of an ALIF is rather involved and typically requires the talents of a vascular surgeon. Attention must be paid to certain anterior structures, including the peritoneum, rectus abdominus muscles and bowel loops. As the dissection progresses, the surgeon encounters such vital structures as ureters, great vessels and the sympathetic nerve chain, which runs vertically, slightly anterior and lateral to the vertebral bodies. A fusion planned at the L4-L5 level requires retraction of the great vessels to the right, while L5-S1 can be approached through the bifurcation of the great vessels.
When the dissection is complete, a needle is inserted into the disc space to check disc level and midline location. A PA spot film with the C-arm is used to check midline while a lateral projection nicely displays disc level. (See Fig. 7.) If the spine surgeon is satisfied with the needle placement, the radiographer can leave the C-arm in the lateral position and translate it cephalad to clear the operative field. At this point, the spine surgeon begins the discectomy, and the disc material is sent to the lab for analysis.
On completion of the discectomy, the intervertebral disc space is tapped to accept the fusion cage. Lateral spot films are obtained with the C-arm to confirm depth of the tapping. When this portion of the operation is complete, the cages are packed with graft bone (usually cancellous bone taken from the patient's iliac crest) and screwed into place with clockwise turns of 180 [degrees] increments. The position of each cage is progressively checked with lateral C-arm spot film exposures.
Considerations for the spine surgeon are depth of the cage (without impingement on the spinal canal) and sufficient end-plate bite. Typically, 2 cages are implanted for each affected level. When the cages are in place, they should be completely in the disc space and parallel to each other. A lateral C-arm projection should show the cages to be superimposed, while a PA projection reveals the cages to be equidistant from the midline of the vertebral bodies. (See Fig. 8.) When it has been determined that the cages are correctly placed, the closing process begins. Before the patient is awakened, a portable KUB should be obtained to rule out foreign bodies and instrumentation in the peritoneum.
Radiation Dose Considerations
As with every procedure, the assumption is that the risks involved in x-irradiation of a patient are outweighed by the benefits of a correct diagnosis and treatment plan. Nevertheless, cumulative radiation dose should be considered. During the course of work-up and treatment for low back pain, a patient can expect to receive a routine lumbar spine series (AP/lateral with flexion/extension), CT and discogram. Intraoperative films will be obtained, as will follow-up routine radiographs, AP Ferguson projections (see Fig. 9) and CT scans.
A quality Ferguson projection is essential for determining whether there is "halo" formation around the cages and whether fusion is occurring. To obtain the Ferguson, Long and Rafert recommend placing the patient in the supine position (legs extended) and using a cephalic tube angle of 30 [degrees]. However, positioning for the AP Ferguson can be difficult due to differences in anatomy from patient to patient.
An accurate Ferguson can be obtained by shooting the lateral L-spine film first and using it to determine the cephalic angulation needed to "look down the barrels" of the interbody cages. If a straight line (Line A) is drawn on the lateral film connecting the bodies of the upper lumbar vertebrae, a goniometer can be used to measure the angle that a second line bisecting the disc space (Line B) forms with Line A. A right triangle can be formed by dropping a third line (Line C) from the x-ray tube to the table. By subtracting angle [Sigma] from 90 [degrees], the radiographer can calculate the cephalic tube angle [Theta]. (See Fig. 10.)
Intraoperative fluoroscopic exposure time can be drastically reduced if the radiographer is experienced and educated about the surgical procedure. Because the dose that a patient receives during an ALIF is directly proportional to the amount of fluoroscopy time, the issue of radiation protection is largely in the hands of the radiologic technologist (provided that the technologist is, in fact, controlling the fluoroscopy). A recent survey at the author's institution revealed an average fluoroscopy time of 37 seconds per fused level and an average skin dose of 4.42 Gy/[cm.sup.2] per level, based on a 9-inch field size.
The skill of the operating room radiographer and his or her ability to provide the spine surgeon with prompt, quality images should not be underestimated. Understanding the surgical procedure and anticipating the surgeon's requests can substantially reduce patient dose and operative time.
Follow-up and Success Rates
The Ray follow-up protocol requires that the ALIF patient be seen at 6 weeks and then at 3, 6, 12, 18 and 24 months (yearly thereafter, if indicated). An analysis by Turner et al of 47 articles found that an average of 68% of patients having various types of lumbar fusion surgery achieved a satisfactory outcome. However, reports ranged from 16% to 95%. In a 10-year follow-up study by Penta and Fraser, 78% of ALIF patients rated themselves as having "complete relief" or "a good deal of relief." In fact, 83% of those questioned indicated that they would "recommend the procedure to a friend with a similar problem."
For those who do not have success with fusion, studies are in progress to investigate correlation with the radiologic evidence. According to the BAK protocol, a difference at 4 years postprocedure in vertebral angle greater than 3 [degrees] on flexion/extension radiographs or the presence of radiolucency around the margin of the device (a halo) indicates nonunion. The Ray study is a bit more stringent, citing any motion on flexion/ extension as fusion failure, within a postprocedure time of 2 years. Neither the Ray nor the BAK study found a significant difference between fusion rates of smokers and nonsmokers.
To properly rate the success of ALIF, the medical community needs to employ uniform standards of evaluation. If these standards include lateral flexion and extension positions of the lumbar spine, a protocol should be developed so they are executed in a uniform manner. Some hospitals require that the aforementioned projections be obtained with the patient standing, while recent versions of the 2 major radiographic positioning textbooks, Bontrager and Ballinger, describe them being performed in the lateral recumbent position. (Bontrager states in an aside that they also can be performed upright, seated on a stool.) Long and Rafert describe the standing positions, but the method first described by Knutsson (ie, having the patient sit while flexing forward and stand, bracing the buttocks against the table while extending backward) or a variation thereof may assess flexion/extension best while minimizing patient motion during exposure.
ALIF is still a relatively new procedure and long-term follow-up data are not yet abundant. Developments in cage structure and grafting material are continually improving the fusion rate of these surgeries. Bone morphogenic protein has shown promise as a grafting material in cases of subtalar arthrodeses. Duplication of this technique has been attempted with respect to spinal fusion, in an operation that could potentially eliminate the need to harvest iliac crest graft. Because the bone graft donor site is sometimes a source of continuing pain for the patient, eliminating this part of the procedure would have much benefit.
The anterior approach is, generally speaking, a faster, less invasive tool in the spine surgeon's repertoire. Although not a cure-all for the 70% of Americans who will experience back pain at some time in their lives, ALIF does serve a special segment of the low back pain population. For those with unrelenting discogenic pain and especially for patients who already have been helped, anterior lumbar interbody fusion is a welcome advancement.
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[2.] Kuslich SD, Ulstrom CL, Griffith SL, Ahern JW, Dowdle JD. The Bagby and Kuslich method of lumbar interbody fusion. History, techniques, and 2-year follow-up results of a United States prospective, multicenter trial. Spine. 1998;23:1267-1278.
[3.] Penta M, Fraser RD. Anterior lumbar interbody fusion. A minimum 10-year follow-up. Spine. 1997;22:2429-2434.
[4.] Nachemson A, Zdeblick TA, O'Brien JP. Controversy. Lumbar disc disease with discogenic pain. What surgical treatment is most effective? Spine. 1996;21:1835-1838.
[5.] Zdeblick TA. The treatment of degenerative lumbar disorders. A critical review of the literature. Spine. 1995;20:126S-137S.
[6.] Whitecloud TS, Wolfe MW. Indications for internal fixation and fusion in the degenerative lumbar spine. In: Bridwell KH, DeWald RL, eds. The Textbook of Spinal Surgery. 2nd ed. Philadelphia, Pa: Lippincott-Raven Publishers; 1997:1581-1600.
[7.] Knight RQ, Thomassen PF. An introduction to open anterior lumbar interbody fusion (ALIF). Indications, approaches, techniques and nursing care. Available at: http://www .spineuniverse.com/surgery/procedures /ag_antlumb_fus.html. Accessed July 26, 2000.
[8.] Boden SD, Wiesel SW. Lumbosacral segmental motion in normal individuals. Have we been measuring instability properly? Spine. 1990;15:571-575.
[9.] Juntura TL. The diagnostic value of discography. Radiol Technol. 1998;702:127-128.
[10.] Long BW, Rafert JA. Orthopaedic Radiography. Philadelphia, Pa: WB Saunders; 1995;260-267.
[11.] Turner JA, Ersek M, Herron L, et al. Patient outcomes after lumbar spinal fusions. JAMA. 1992;268:907-911.
[12.] Bontrager KL. Textbook of Radiographic Positioning and Related Anatomy. 3rd ed. St. Louis, Mo: Mosby-Year Book Inc; 1993:270.
[13.] Ballinger PW. Merrill's Atlas of Radiographic Positions and Radiologic Procedures. 9th ed. St. Louis, Mo: Mosby-Year Book Inc; 1999:362.
[14.] Knutsson F. The instability associated with disk degeneration in the lumbar spine. Acta Radiol. 1944;25:593-609.
[15.] Scranton PE. Comparison of open isolated subtalar arthrodesis with autogenous bone graft versus outpatient arthroscopic subtalar arthrodesis using injectable bone morphogenic protein-enhanced graft. Foot Ankle Int. 1999;20:162-165.
Patrick J. Burke, B.A., Pt. T. (R), is a staff technologist at Nay England Baptist Hospital in Boston, Mass. His professional interests include sports medicine, general orthopedics and spinal surgery.
Reprint requests may be sent to the American Society of Radiologic Technologists, Communications Department, 15000 Central Ave. SE, Albuquerque, NM 87123-3917.
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|Author:||BURKE, PATRICK J.|
|Date:||May 1, 2001|
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