To our knowledge, this study is the first to report the long-term outcomes of posterior stand-alone expandable cage fusion surgery. In this study, insertion of an expandable cage alone not only increased the segmental angle but also correlated positively with LL. However, LL was not corrected, and the SS increased. High implant failure rates, weak support of the posterior element, and compensatory mechanisms are possible factors affecting these results. On the basis of our results, we have drafted a relationship chart of these factors (Figure 5).
Are There Any Advantages to the Posterior Stand-Alone Expandable Cage Approach?
Screw placement at the pedicle has been regarded as the standard posterior stabilization procedure since 1969, and it was first introduced by Harrington and Tullos.16 The efficacy and superior support of this technique compared with other techniques, particularly the superior biomechanical strength17 and presence of three columns, which provide more support than other techniques,18,19 have been reported. However, this technique needs wide exposure for screw insertion and anatomic landmark confirmation. Furthermore, the reported rate of screw malposition ranges from 0% to 42%.20,21 Because we skipped the process of screw placement in this study, the operative time was saved, the paraspinal and posterior facet complex was preserved with a small incision, and radiation exposure was reduced. Compared with the anterior lumbar interbody fusion and lateral lumbar interbody fusion procedures, simultaneous direct decompression can be performed and abdominal organs or hypogastric nerve injury can be avoided.22,23 In our result, both the cage alone and screw fixation groups showed a decrease in the segmental angle in the long term, and expandable cages could achieve a greater angle. This shows that expandable cages have the effect of angle correction locally. In addition, we found that the rate of PJK was significantly lower in the cage-alone group than in the control group. As PJK is induced by overloading the junctional disc space,24 our facet-preserving technique might result in less overloading than the firmly fixed screw technique.
Is Interbody Fusion Without Screw Fixation Safe?
Compared with other fusion procedures, the possible complications of interbody fusion without screw fixation are totally different. In posterior fusion with a cage, owing to the wide exposure and screw placement, dural tear, rod fracture, PJK, and root damage are common complications.25 With a stand-alone anterior or oblique approach, insufficient decompression means that additional decompression is required, and psoas muscle weakness and abdominal and vessel injuries26 are common complications. In the short term, patients who received a posterior expandable cage alone reported minor complications, such as posterior leg pain, infection, and wound problems.27 However, long-term complications consisted of implant problems, especially subsidence, pseudoarthrosis, retropulsion, and cage breakage. High subsidence rate and breakage of cage can result from excessive restoration of the local angle.28 Lack of screw did not maintain stability during the initial period, as shown by the high pseudoartrosis29 and retropulsion rate. Even though our series showed that implant failure did not need replacement and revisions, it can be the cause of postoperative pain and disability during recovery.
Why Is It That an Expandable Cage Cannot Correct LL and Spinopelvic Profile?
The manufacturers have designed the expandable cages to be able to increase the lordosis by up to 9°; however, our measured mean segmental correction was only 4.66°. Subsidence28 and pseudoarthrosis29 are known factors that can reduce the lordotic angle. Cage breakage and retropulsion are possible debilitating events that can decrease LL. This may be the reason why the segmental angle did not correct LL, even though both parameters showed a significant positive relationship. Furthermore, weak posterior fixation can change the sacropelvic profile. In the normal aging process, PT and thoracic kyphosis increase. However, because PI is a consistent parameter,30 SS increases as a compensatory mechanism. However, our results in the cage-alone group were entirely different. Initially, the SS increased more in the cage-alone group than in the control group because of the lack of posterior support. Consequently, the PT was compensated for; hence, PI was preserved. Posterior screw fixation played a role in maintaining the SS in the control group, and the whole spinopelvic profile was better preserved in the control group than in the cage-alone group.
How to Solve Issues and Gain Better Outcomes
Three issues should be resolved to achieve better outcomes with this technique. First, we need to use more stable and advanced materials for interbody fusion. The use of enhanced titanium, additional bioactive glass ceramics, and other materials can reduce the rate of pseudoarthrosis.31 Second, we need to preserve posterior support. Motion-preserving total disc replacement surgery showed more stable outcomes than the currently evaluated method32 because of complete preservation of the posterior facet complex. Because this method also has a drawback (i.e., it is impossible to decompress the posterior canal), modified minimally invasive techniques, such as unilateral approaches, should be considered. Third, we need to increase bone density. The use of teriparatide in femoral fractures showed efficient prevention of bony subsidence33; thus, the use of hormones or medication may play a role in achieving better outcomes.
Limitations of the Study and Future Scope
This study has limitations and several issues that need to be resolved in future studies. First, given the lack of blinding method and retrospective study design, many patients were lost to follow-up, and many confounding factors by indication are present. Even though we narrowed the indication in multilevel degenerative pathology, misclassification and selection bias can affect operation and follow-up. Second, there may have been major advancements in medications that support bone formation and advancements in the quality of cage materials since the patients in this study were treated. Therefore, it is essential for future studies to address the effects of better bone-forming agents and the application of stronger cage materials. Future studies should also have a multicenter prospective study design.