Outcomes in Allogenic Cages in ALIF and PLIF: Interbody Spacer
The PLIF spacer is a contoured, wedge-shaped cortical allograft that comes in five anterior heights: 9-17 mm, in 2mm increments. The surface of the spacer contains a saw-tooth pattern on the superior and inferior surfaces to minimize migration (Fig. 1). Two implants are inserted into the same disc height to ensure maximum stabilization.
There are two major techniques that may be used to distract, size, and insert the implants: distraction with the PLIF distractor or distraction with the PLIF trial spacer. The surgical technique used depends on the patient's local anatomy, the pathology, and the surgeon's preference . The majority of the posterior elements, including the facets, do not have to be sacrificed for delivery of this geometry of biological implants (Fig. 7). The PLIF trial spacer ensures accurate sizing of the PLIF spacer. Once the site has been prepared for device insertion, the PLIF distractor blades are inserted completely into the disc space so that the ridges at the end of the blade rest on the vertebral body and are lateral to the dura. The PLIF distractor distracts the vertebrae to obtain maximum implant height, while the PLIF trial spacer is inserted into the contralateral disc space to ensure accurate sizing of the PLIF spacer. Fluoroscopy and tactile judgment can assist in confirming the fit and placement of the trial spacer. If the trial spacer is either too loose or too tight, the next size is used to achieve the desired secure fit. Once the correct sizing is obtained, the trial spacer can be removed and the biological implant is introduced (using the implant holder) in the correct orientation into the contralateral disc space (Fig. 7). Autogenous cancellous bone or bone substitute (demineralized bone matrix, BMP, or allograft) is also placed in the anterior and medical aspect of the vertebral disc space prior to placement of the second implant.
Figure 7. Surgical technique using a posterior lumbar interbody fusion (PLIF) spacer.
For the surgical technique utilizing the PLIF trial spacer, it is necessary to begin with the trial spacer determined during preoperative planning. It is first inserted horizontally and then rotated vertically to size and distract the disc space. The implant corresponding to the correct trial spacer is chosen and introduced in the correct orientation into the contralateral disc space. The trial spacer is removed and the second implant is inserted. It is advisable to recess the implant 2-4 mm beyond the posterior rim of the vertebral body . Additional posterior instrumentation may also be used to enhance the fusion rate and decrease the risk of anterior column allograft migration.
Thirty implants were inserted into 13 patients between January 1999 and July 2000. Five were men and eight were women, ranging in age from 32 to 70 years (average age, 54 years) (Table 1). The follow-up on these preliminary patients averaged 18 months (range, 6-24 months). The indications for surgical implantation included recurrent disc herniation, spondylolisthesis without foraminal stenosis, and instability. The most common levels fused were L4-L5 and L5-S1. All of the patients were treated with additional posterior segmental instrumentation including pedicle screw fixation (n=12), and translaminar screw fixation (1) (Table 2). In this series of patients, there were four (33%) smokers, but results did not any differences in the fusion rates of smokers versus nonsmokers. There were no additional co-morbidities observed in this selected patient population (Table 1). One patient developed anterior migration of a single PLIF spacer that required revision and an anterior approach and reconstruction, with good clinical outcome. The clinical outcome of this posterior device is promising, with minimal incidence of migration, dislodgment, infection, pseudoarthrosis, or iatrogenic instability (Fig. 8a-c).
Figure 8. (a) Preoperative x-ray of a 69-year-old woman with spondylolisthesis, instability, and bilateral pars fracture. (b) X-ray (1 month after surgery) of a posterior interbody fusion with posterior lumbar interbody fusion (PLIF) spacer insertion and pedicle screw fixation. (c) X-ray 6 months after surgery showing a successful fusion with the PLIF spacer in L5-S1.