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. Based on 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.14 The efficacy and superior support of this technique compared with other techniques, particularly the superior biomechanical strength15 and presence of three columns, which provide more support than other techniques,16,17 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%.18,19 Because we skipped the process of screw placement in this study, 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.20,21 Our results show that both the cage-alone and screw fixation groups had decreased segmental angles in the long-term, with expandable cages achieving a greater angle change. This shows that expandable cages have the effect of local angle correction. In addition, we found that the rate of PJK was significantly lower in the cage-alone group when compared to the control group. As PJK is induced by overloading the junctional disc space,22 our facet-preserving technique might result in less junctional disc space overload 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 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.23 With a stand-alone anterior or oblique approach, insufficient decompression means that additional decompression is required, and psoas muscle weakness and abdominal and vessel injuries24 are common complications. During the short-term period, patients who received a posterior expandable cage-alone reported minor complications, such as posterior leg pain, infection, and wound problems.25 However, long-term complications included implant issues, especially subsidence, pseudoarthrosis, retropulsion, and cage breakage. High subsidence rate and cage breakage can be caused by an excessive restoration of the local angle.26 The lack of screw during the initial period decreases support, which subsequently leads to higher rates of pseudoartrosis27 and retropulsion. Even though our series showed implant failure did not need revision and replacement, it could be the reason for postoperative pain and disability during recovery.
Why Is It That an Expandable Cage Cannot Restore Sagittal Balance?
The manufacturers have designed the expandable cages to be able to increase the lordosis by up to 9 degrees; however, our measured mean segmental correction was only 4.66 degrees. Subsidence26 and pseudoarthrosis27 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,28 SS increases as a compensatory mechanism. However, our results in the cage-alone group were 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 or bioactive glass ceramics can reduce the rate of pseudoarthrosis.29 Second, we need to preserve posterior support. Motion-preserving total disc replacement surgery showed more stable outcomes than the currently evaluated method30 because of complete preservation of the posterior facet complex. Due to the fact that 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 subsidence31; 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 that need to be overcome in further studies. First, because of the retrospective study design, many patients were lost during follow-up, and many confounding factors were present. Despite our strict patient inclusion criteria, the possibility of selection bias could not be eliminated. 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.