at Risk:
The Role of Anterior Column Release in Preventing Cage Dislodgement in Spinal Fusion Surgeries
Zaynab Khemmich
Senior Thesis | 2025
Cages at Risk: The Role of Anterior Column Release in Preventing Cage Dislodgement in Spinal Fusion Surgeries
Abstract
Spinal fusion surgery is a widely performed procedure that uses interbody fusion cages to stabilize the spine and treat various spinal conditions. With the introduction of these cages into the medical field, there has been a significant increase in improved surgical outcomes. However, complications such as cage migration, dislodgement, and subsidence remain a great concern. Anterior column release (ACR) is a common surgical technique used by a majority of spinal surgeons in order to correct spinal deformities. During this minimally invasive technique, the anterior longitudinal ligament (ALL) is cut or loosened. As a result, this may increase the chance of the cage shifting out of place after surgery. Therefore, anterior stabilization is considered the predominant goal. While surgeons commonly use anterior plating to provide extra protection of the cage from dislodgement, the necessity of additional anterior plating to prevent cage dislodgement remains debated.
This study assesses the safety of ACR, by analyzing the rate of postoperative cage dislodgement as well as evaluating whether cutting the anterior longitudinal ligament and placing an interbody cage is sufficient for spinal fusion, or if additional anterior plating is required to prevent cage migration or dislodgement. This study will also evaluate potential risk factors for increased percent migration. Some of the outcomes measured
include: Cage dislodgement, Percent cage migration, Osteophytes, Revisions, Hardware failure, Adjacent segment disease (ASD), Anterior thigh pain, Residual radiculopathy, and Vertebral body fracture.
As a result, the study found no significant need for anterior plating in order to prevent cage migration after anterior longitudinal ligament release. Additionally, osteophyte formation was found to act as a natural barrier to prevent cage dislodgement. Factors such as age, osteoporosis, and smoking were briefly examined as well, with smoking showing a positive trend toward increased migration. However, BMI was not associated with migration outcomes.
Background
Spinal Fusion is a common surgical procedure performed more than 300,000 times per year in the United States alone. During this procedure, two or more spinal vertebrae are joined together to treat various spinal conditions such as scoliosis, spinal fractures, spinal tumors, spondylolisthesis, spinal stenosis, and degenerative disc disease. During most spinal fusion surgeries, interbody fusion cages, or cylindrical metal devices, are placed to help fuse two or more vertebrae. This surgery is often performed to stabilize the spine, prevent painful movement, and correct spinal deformities. In the United States, the amount of elective lumbar fusion procedures performed increased by 62.3% from 2004 to 2015 (Martin, 2019).
Interbody cages have significantly advanced the success rate of spinal fusion procedures. Since their introduction into the
growing field of spinal surgery, a wide variety of cage designs and materials have emerged. These cages are available in different shapes, such as rectangular, trapezoidal, and banana-shaped, and come in materials including threaded titanium, carbon fiber, and polyetheretherketone (PEEK) (Jain, 2016). In addition, the cages vary based on their surgical application and patient needs. Anterior interbody cages, for example, are threaded titanium cylinders commonly used in procedures that approach the spine from the front, such as anterior lumbar interbody fusion (ALIF). During the anterior lumbar interbody fusion, a cage is inserted into the spine from the anterior (front) side of the body. This approach involves accessing the spine through an abdominal incision. Posterior interbody cages, on the other hand, are similar but are placed from the back of the spine during spinal fusion procedures, such as posterior lumbar interbody fusion (PLIF). During PLIF, a cage is inserted into the spine through a posterior approach. This involves the complete or partial removal of certain posterior elements in the spine in order to access the spinal column. Other than the difference in placement position in the spine, these two surgical techniques have very few major differences. Many studies even focus on ALIF and PLIF as comparable techniques. In a study assessing lumbar interbody fusion and comparing interbody fusion options, it was found that there was no significant difference in the complication rate between ALIF and PLIF. Possible complications include hematoma, wound infection, and non-fusion in both surgical techniques. On the other hand, it was also found that some severe complications, such as incisional hernia, retrograde ejaculation, and venous injury were specific to ALIF. Some interbody cages are designed to expand after insertion, allowing for
a more patient-customized fit that can better restore the natural curvature and alignment of the spine. Corpectomy cages are another type of interbody cages used to replace a vertebral body that has been removed due to trauma or disease. Throughout the past decades, Interbody cages have advanced and evolved to improve spinal fusion outcomes. These devices provide more precise and customizable solutions while also offering a range of designs and materials to suit various surgical and patient needs (Mobbs, 2015).
Although this procedure is becoming increasingly performed, as with all medical procedures, there are certain risks associated with it as well. One specific risk in spinal fusion procedures is cage migration. This occurs when an interbody fusion cage moves more than 3mm beyond the vertebral body wall, potentially shifting into a different area of the spine after being implanted. Cage migration can occur either anteriorly, posteriorly, or laterally. Anterior migration is defined as the shifting of the interbody cage towards the front of the spine, posterior migration is defined as the shifting of the interbody cage towards the back of the spine, and lateral migration is defined as the shifting of the interbody cage towards the side of the spine. These different instances of cage migration can cause increased pain, tissue damage, and spine instability. When the migrated cage is completely out of the spine, it is referred to as cage dislodgement. This movement can occur either immediately after surgery or at any point during the healing process. This type of complication can cause significant problems such as nerve compression, instability in the spine, and further tissue damage (Jin, 2020). Another
potential type of cages at risk complication is cage subsidence. This complication occurs when a cage sinks or settles into the vertebrae, causing it to lose height. This can cause decreased disc space in the spine, spinal deformity, and nerve compression. On a side note, this specific cage-at-risk complication is the most common after spinal fusion surgeries (Rickert, 2023).
This surgical risk is a rare but serious complication that affects several aspects of a patient’s recovery after spinal fusion surgery including low back pain and neurological deficits such as difficulty walking or controlling bladder function in extreme cases. Cage migration was also found to be associated with age. As the age of the patient increases, cage migration becomes more common. Specifically, a study found that cage migration occurred in 44.1% of those under 60 years of age and in 72.7% of those over 60 years of age (Rickert, 2023).
To prevent these complications, Anterior Column Release (ACR) is a commonly used and minimally invasive procedure used to correct spinal deformities. With the rise of minimally invasive spine surgery techniques, ACR is becoming increasingly used for deformity correction and spinal balance. The anterior longitudinal ligament (ALL) is a thick band of connective tissue that runs vertically down the front of the spine. During this procedure, there is a partial or complete cut of the anterior longitudinal ligament to place the interbody cage. After cutting this ligament, almost all surgeons use anterior plating to support or stabilize the spine and prevent dislodgement. Anterior plating refers to a surgical procedure in which a metal plate is placed on the front (anterior) side of a bone (Godzik, 2020).
To further increase the effectiveness of spinal deformity correction and reduce the rate of complications, anterior and posterior approaches to lumbar interbody fusion provide efficient and direct access to the disc space in the spine. This provides a precise and smooth placement of the interbody fusion cage within the spinal column. One of these techniques is the minimally invasive surgery antepsoas approach (MIS-ATP). During this technique, the lumbar spine is accessed through an abdominal approach to perform a fusion between the vertebrae. A key step of this surgery is a full discectomy, or removal of the spinal disc, without significant violation of the spinal endplate. As a result, this reduces the stresses that are thought to initiate cage dislodgement and ultimately achieve spinal stability. Oblique Lateral Interbody Fusion (OLIF) is another minimally invasive surgical technique that accesses the lower portion of the spine from the front and side of the body and limits cutting into muscles to access the spine disc. These advanced and minimally invasive approaches are gaining increasing attention for their ability to balance precision with minimal patient trauma. Similarly, posterior percutaneous fusion (PPF) is another minimally invasive technique where a surgeon surgically accesses the spine from the back through small incisions. During this procedure, bone graft material is implanted to fuse the vertebrae together. In addition to effectively stabilizing the spine, this procedure also minimizes the impact on surrounding tissues (Mobbs, 2015).
Osteophytes, also referred to as bone spurs, are bony lumps that grow from bony tissue excess on spinal bones or around joints. They typically appear as smooth lumps on the outside of spinal
bones. Osteophytes typically form when the body tries to repair damaged bone tissue. Because spinal fusions typically increase stress on nearby spinal areas, the body produces this bone growth as a way to stabilize the area of the stress. Bones that osteophytes typically grow on include the spine, or on vertebrae where joints meet (Rickert, 2023).
If not fixed, spinal cage migration can cause serious complications. These complications can include neurological deficits such as peripheral neuropathy, weakness, ataxia, muscular atrophy, and low back pain that occur when migrated cages compress parts of the spinal canal. In addition, cage migration can lower the fusion rate in spinal surgeries. Repeated migration of a fusion cage can further damage spinal tissues and require more demanding revision surgery (Lee, 2013). Depending on the severity, if a patient develops neurological symptoms such as loss of sensation in the extremities, they will likely need revision surgery, which has complications on its own. To prevent mortality and morbidity, and achieve the best quality of life for all patients involved, it is crucial to further research targeted toward finding novel treatments to alleviate post-operative pain and discomfort for cage migration.
Objective
Almost all surgeons place anterior plating to protect the cage from dislodgement after cutting the anterior longitudinal ligament. The objective of this study is to determine whether cutting the anterior longitudinal ligament and placing an interbody cage is sufficient for spinal fusions.
Methods
After obtaining Institutional Review Board approval, this research was conducted as a retrospective chart review to examine patients who underwent minimally invasive surgery antepsoas (MIS-ATP) with posterior percutaneous fusion (PPF) at Boston Medical Center between 2007-2023. Demographic and clinical data were collected in the process.
During this experiment, there were several inclusion and exclusion criteria. The inclusion criteria include adult patients diagnosed with spinal disease and who underwent a combination of MIS-ATP and PPF for the thoracolumbar and/or lumbosacral spine. In addition, patients had to have postoperative lateral standing X-rays taken within six months of surgery to evaluate the progress of anterior-posterior migrations as well as A/P standing X-rays within six months to evaluate lateral cage migrations. Finally, all patients had to have been treated at Boston Medical Center. The exclusion criteria include patients under 18 years of age or with incomplete medical records as well as those with significant comorbidities, such as severe cardiovascular or metabolic conditions, that could complicate surgery or recovery. Additionally, individuals with active infections were at risk for the procedure.
In this study, postoperative cage migration is defined as any portion of the cage migrating past the endplate in an anterior, posterior, or lateral direction.
To measure and detect cage migration, different radiographs were examined including standard standing
anteroposterior (AP) radiographs, lateral radiographs, and dynamic radiographs. All of these measurements were calculated in millimeters and performed using the SECTRA measuring system, which is a medical imaging platform that helps healthcare professionals accurately measure specific features on medical images and aids in diagnosing, surgical planning, and following up. Using this platform, measurements were taken in segments, measuring both the distal (further) and proximal (closer) segments of the cage migration. These segments were then divided by the total length (in mm), making sure to control for radiograph magnification and potential errors. Percent migration was calculated as the portion of the cage not contained within the disc space. This was measured as a percentage of the full cage length and the direction of cage migration was taken into account.
In the research, some of the variables that were examined include osteophyte presence, subsidence measurement (in mm), hardware failure, and alignment. Instances of hardware failure include: rod breakage, screw breakage, screw pull-out, and proximal/distal junctional failure. Statistical analysis was then conducted using univariate and multivariate logistic regression to determine the percentage of cage migration as well as the overall correlation with common postoperative outcomes.
Results
From the patient sample, a total of 302 patients were found to have 478 at-risk cages. The average age of the patients was 58.98 years with a standard deviation of 11.40 years, and their average BMI was 31.54 years with a standard deviation of 616. Among the patients, 204 (67.55%) were female, 156 (51.66%) were white, 120 (39.74%) were smokers, 83 (27.48%) consumed alcohol, and 81 (26.48%) had diabetes. Additionally, the most common reason for surgery was spondylolisthesis, which is defined as a condition where a bone in the spine slips out of place (92 patients, or 30.46%) (See Table 1).
Of the 478 total cages, the most frequently used cage location was in the lower back location or at the L4-L5 location (128 cages, or 26.78%). The average migration of the cages was 11%, with a standard deviation of 13%, and only 3 cages were completely dislodged (see Table 2).
Factors associated with higher cage migration include sex (p=0.041), BMI (p=0.031), smoking status (p=<0.001), alcohol status (p=0.035), and osteopenia (p=0.032). On the other hand, the presence of osteophytes was not significantly associated with cage migration (p=0.052) (See Table 3).
Table 1: Patient Characteristics of Patients with at-Risk Cages.
Table
2: Cage Characteristics of Cages at Risk.
Cage Characteristics
L1-L2
L3-L4
Table 3: Univariate and Multivariate Logistic Regression with associations with Increasing Percent Cage Migration.
Limitations
A potential limitation of this study is the limited diversity within the sample population, given that it primarily consists of patients from Boston Medical Center who underwent a combination of minimally invasive surgeries between 2014 and 2023. While the sample size is relatively large, it may not fully reflect the diverse demographics of the broader population. This could limit the generalizability of the findings to a wider population. In addition, the retrospective study design is commonly prone to selection bias because it relies on pre-existing data, the inclusion of patients is not random, and there could be unaccounted factors that influenced the selection process. Furthermore, while the radiographic analysis was an important aspect of the study, there is potential for measurement error in the radiographic analysis. Even subtle differences in radiographic technique could have inaccuracies that interfere with the results. These potential limitations influence the need to exercise caution in interpreting the findings and overall conduct of the study.
Discussion
This study aimed to investigate the role of anterior column release (ACR) and the potential protective effect of osteophytes in preventing cage migration and cage dislodgement in spinal fusion surgery. Analysis of the results showed that there is no need to use anterior plating to protect the cage from dislodgement after cutting the anterior longitudinal ligament (ALL). Instead, cutting the anterior longitudinal ligament and placing an interbody cage was sufficient for spinal fusions.
In addition, there is also evidence to suggest that osteophytes may be protective in terms of cage migration by acting as a natural barrier to protect the cage from being dislodged. Since osteophytes have not been proven to increase cage migration, osteophyte development may act as a physiological protective mechanism that prevents the cage from dislodgement or excessive migration. Contrary to other studies conducted, in a study assessing the risk factors and scoring system of cage retropulsion after posterior lumbar interbody fusion (PLIF), it was found that age and osteoporosis may not be factors affecting cage dislodgement or severe cage migration. A possible reason for this is that osteoporosis is only a risk for backward migration, but the specific time of occurrence of backward migration cannot actually be calculated. The specific time may be caused by many different factors, but osteoporosis is not a risk factor on its own (Peng, 2021).
While some studies found no association between smoking and the success rate of spinal fusions, others observed that smoking has shown detrimental effects on the bone healing of spinal fusions, ultimately increasing the percent migration. For instance, in a study assessing the effect of smoking on spinal fusions, Berman et al. concluded that smoking does indeed increase the rate of perioperative complications for patients undergoing spinal fusion surgery in addition to increasing the risk of other perioperative complications such as infection (Berman, 2017). These results agree with the finding that previous or current smoking trended towards increased percent migration. In addition,
BMI was not associated with an increased incidence of percent migration or other intraoperative or health-related complications.
While many different studies reported different influencing factors that can cause cage migration, dislodgement, or subsidence, they still seem controversial. Some factors that may contribute to increased incidence of cage migration include disc size and shape, cage material, positioning of the cage within the disc space, surgical technique, endplate injury, bone mineral density, age, and BMI. In a meta-analysis conducted to investigate the risk factors influencing the incidence of cage migration, it was found that bony endplate injury, pear-shaped disc, and screw loosening are significantly correlated with cage migration (Hou, 2023).
Some of the different surgical options that have been used in the past include but are not limited to transforaminal lumbar interbody fusion, posterior lumbar interbody fusion, and anterior lumbar interbody fusion. During the transforaminal lumbar interbody fusion procedure, a cage is inserted into the spine from a posterior or lateral approach through the opening where the nerve roots exit the spine. This procedure involves removing the intervertebral disc and inserting a cage before fixing it with screws and rods. This procedure is gaining popularity as an alternative to ALIF and PLIF combined with pedicle screw instrumentation. In a study assessing TLIF, it was found that the complication rate, blood loss, and operation time of this technique were similar to those of the PLIF and the ALIF techniques (Hackenberg, 2005).
Conclusion
This study suggests that anterior column release (ACR), especially when paired with the minimally invasive surgery antepsoas (MIS-ATP) approach, provides stability for spinal fusion procedures without the need for additional anterior plating to prevent cage dislodgement. Out of the 4,447 cages examined, 478 (10.67%) were considered at-risk, and only 2 (0.007%) were fully dislodged. Additionally, cage migration rates were proven to be extremely low. This indicates that ACR alone when performed using the MIS-ATP approach, may be sufficient for maintaining cage stability during the spinal fusion process.
Osteophytes didn’t seem to be a risk factor for cage dislodgement, they instead were found to be somewhat beneficial. As Osteophytes formed within the spine, it increased the surface area of the vertebral endplate, preventing cage migration. Osteophytes did not appear to contribute to cage dislodgement, though it may serve as a natural protective barrier against migration.
Moreover, previous or current smoking trended towards increased percent migration. This highlights the negative impact of smoking on bone healing and spinal fusion outcomes. In contrast, body mass index (BMI) was not associated with increased cage migration, suggesting that weight alone may not be a significant factor in the risk of cage migration, dislodgement, or subsidence. The study also found a significant association between increased percent cage migration and postoperative hardware failure. This
highlights the importance of monitoring cage position during the recovery period.
Given the low incidence of cage migration, subsidence, and dislodgement observed in the study’s cohort as well as taking into account the associated risks of revision surgery, a strategy focused on the observation of cages risk rather than repositioning the cages may be a better approach in some cases.
Future Directions
While the findings of this study provide significant insights into the different factors influencing spinal fusion stability, there remain many future research opportunities. One key direction is to expand the sample size and incorporate more diverse patient populations from multiple medical centers. This would not only increase the generalizability of the findings but also ensure that the results are applicable across different demographics and clinical settings. In addition, the use of more advanced medical imaging would aid in improving the accuracy of measuring cage migration and the effects of osteophytes. Improving the imaging technique would also help to better visualize the subtle changes over time. Another valuable area of future research would be extending the follow-up periods of studies to examine the long-term effects of cage migration and osteophyte development. Furthermore, investigating other possible risk factors for cage migration like bone density, different cage materials and shapes, and different surgical techniques could improve the success rate of spinal fusion procedures and generally provide important input into this growing field of research.
Advisor Information
Tony Tannoury, Email: tannoury@bu.edu
Rehan Khan, Email: rrkhan@bu.edu
References
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