Outcomes of Allogenic Cages in ALIF and PLIF: Discussion
The purpose of interbody fusions (either anterior or posterior) are to decrease pain and increase function in relation to the existing pathology. Unfortunately, these are difficult parameters to measure objectively because people have different ways of evaluating pain and their perceptions of realistic goals from a spine surgical procedure. The criteria we used to evaluate our results were therefore based on radiographic fusions because they offered the most objective evaluation. Mature fusion appears on x-rays films at a postoperative interval of 6-9 months (Fig. 9a-c).
Figure 9. (a) Preoperative x-ray of a 59-year-old woman with degenerative disk disease, L4-L5 facet arthropathy, and spondylolisthesis. (b) 24-month lateral view x-ray of a femoral ring allograft (FRA) spacer in L4-L5 with pedicle screw fixation. (c) 24-month flexion view. (d) 36-month extension and anteroposterior views.
The combined anterior and posterior column reconstruction used in this study seemed to produce better success rates than those obtained by "stand-alone" anterior interbody arthrodesis. Previous studies have shown that, in general, the technical success rate (indicated by obtaining a fusion) is greater than the clinical success rate (indicated by decrease in pain) [7,11, 28]. Although the radiographic fusion rates in the current study were greater than 90%, the clinical results were variable and often influenced by such factors as litigation, co-morbidities, disability, and secondary gain issues.
Our results were consistent with Brodke et al.  and Enker and Steffee  that concurrent use of instrumentation has increased interbody fusion success by increasing rigidity at the fusion site. Pedicle screw fixation restores segmental stability and minimizes graft retropulsion. Lorenz and co-workers  reported 100% fusion for posterolateral fusion with segmental pedicle screw fixation, compared with 58.6% without instrumentation. Laminar-based systems have also been employed to enhance fusion rates. In comparing laminar- and pedicle-based instrumentation systems, Gurr et al.  demonstrated superior axial, torsional, and flexural rigidity with pedicle-based segmental instrumentation systems. Although pedicle screws are proven to be more effective for stabilization in patients with degenerative disc disease or instability, our results do not show any differences in fusion rates between the pedicle and translaminar screws. However, for the treatment of spondylolisthesis or Par's defect, the pedicle screws were equally successful when used with the FRA spacers or the PLIF spacers (Fig. 10).
Figure 10. Both of these implants faciliate preservation of the endplates and anatomic restoration of the sagittal alignment to provide the optimal "biological" environment to obtain arthrodesis.
In a study done by Loguidice and colleagues , non-smokers had a 14% incidence of pseudoarthrosis. Those smoking up to one pack per day had a 17% incidence of pseudoarthrosis, while those smoking more than one pack per day had a 36% incidence of pseudoarthrosis. However, the 50 smokers in our study who had either anterior or posterior interbody arthrodesis did not exhibit a greater incidence of pseudoarthrosis than the nonsmokers.
The success of every spine fusion procedure hinges on the biologic phenomenon of bone healing. Whether this biologic process occurs, however, is dependent on many factors. The extent of the instability problem being addressed, type of bone graft chosen, type of surgical construct created, even the patient's own anatomy and lifestyle can affect the success of a fusion procedure. Although a number of studies have examined the outcome of interbody fusions, the results of many studies have been contradictory. It needs to be realized that there are many varieties of surgical techniques and healing processes, and expected outcomes may vary considerably regardless of recent research.