CT criteria for Posterior Ligamentous Complex (PLC) injuries—insights from a multi-center study

CT scans have generally been considered to have limited accuracy and reliability when it comes to assessing PLC integrity, with reports suggesting it can fail to detect up to 17% of PLC injuries. However, recent studies have refined the approach to CT-based PLC assessment with the aim of enhancing diagnostic accuracy and improving clinical decision-making.

Multicenter External Validation of the Accuracy of Computed Tomography Criteria for Detecting Thoracolumbar Posterior Ligamentous Complex Injury. Neurosurgery, December 5, 2024. DOI: 10.1227/neu.0000000000003263

The posterior ligamentous complex (PLC) plays a crucial role in spinal stability, as recognized by both the Thoracolumbar Injury Classification and Severity Score (TLICS) and the AO Spine classification.^1,2 However, assessing PLC integrity remains a challenge. Magnetic Resonance Imaging (MRI) is considered the gold standard due to its ability to directly visualize ligamentous structures with high sensitivity. Still, it is not always available in trauma settings worldwide, which can delay the timely management of spinal fractures or associated injuries. Furthermore, MRI takes longer scanning times and is more expensive.

Computed Tomography (CT) is more accessible and widely used, but it has its own limitations. Studies indicate that CT alone may overlook up to 17% of PLC injuries, potentially leading to delayed back pain or kyphosis. 3Therefore, enhancing the accuracy of CT-based assessment is essential to minimize reliance on MRI and improve decision-making in urgent trauma care.

Challenges in using CT for PLC injury detection

CT indirectly identifies PLC injuries by detecting signs of posterior element injury, including interspinous widening, spinous process fractures, laminar fractures, and facet diastasis. However, these findings are present in various morphological types or combinations that can make interpretation complex. For example, facet diastasis can manifest as dislocation, subluxation, fracture, or simple widening. ⁴

Traditionally, PLC status on CT has been classified as either intact (when no signs are present) or disrupted (if at least one sign is present). However, this binary approach has demonstrated only 50–70% accuracy since it does not consider the varying diagnostic significance of different findings or indeed a combination of such findings. Additionally, the lack of a standardized interpretation protocol has contributed to inconsistent reliability.

In our early research, we decided to evaluate the diagnostic accuracy of combined CT findings.⁵ To do this, we compared CT findings on PLC injuries with those of MRI on the same injuries. We discovered that there were several independent predictors of injuries identified by CT. These were:

• Interspinous widening >4 mm
• Spinous process fractures
• Horizontal laminar fractures
• Facet malalignment

Conversely, we found that facet joint widening and vertical laminar fractures were not independent precursors. We then evaluated the diagnostic accuracy of a combination of the four independent predictors outlined above and discovered some interesting conclusions: 

• 0 positive findings: This indicated a low probability (9%) of PLC injury, suggesting an intact PLC (A-type injury).
• 1 positive finding: This suggested an intermediate probability (31%), indicating a possible indeterminate PLC injury (M1 modifier).
• 2 or more positive findings: This signaled a high probability of injury, suggesting PLC disruption (B-type injury), where surgical intervention is often warranted.

To improve accuracy, we also proposed a structured approach to differentiate PLC-related findings from their mimics, such as distinguishing horizontal versus vertical laminar fractures and facet subluxation versus facet joint widening.⁴

Multicenter validation of CT criteria

These findings were hugely important, and to validate them further, we recently conducted a multicenter study incorporating 614 patients across three Level 1 trauma centers—two in Saudi Arabia and one in Spain. Patients underwent both CT and MRI within 10 days of injury, and the results were published in Neurosurgery.⁶ In short, the findings reinforced our previous conclusions:

• The presence of ≥2 positive CT findings reliably indicated PLC injury (B-type injury).
• The absence of positive findings reliably ruled out PLC injury (A-type injury).
• A single CT finding suggested intermediate risk, supporting its use as an M1 modifier to determine the need for further imaging or observation.

This study stands out due to its large sample size and robust methodology, both of which enhance the precision and reliability of its conclusions, and it has important clinical implications for CT-based PLC assessment.

Firstly, it will strongly facilitate decision-making in trauma centers worldwide, particularly in resource-limited settings where MRI is either unavailable, contraindicated (e.g., pacemakers), or can’t be done in a timely fashion due to emergency surgery. These new CT-based criteria provide a reliable alternative for identifying B-type injuries requiring surgery. But even in well-equipped centers, such criteria can reduce unnecessary MRI scans, thereby lowering costs and minimizing delays in treatment.

Previously, the TLICS and AO Spine used the "indeterminate PLC" category, also known as the M1 modifier, to address uncertainty in diagnosing PLC injuries. For patients with neurologically intact burst fractures, the M1 modifier would place treatment recommendations in a so-called “grey zone," making decisions more dependent on a surgeon’s judgment. The M1 modifier is commonly overused as a middle ground option in light of uncertainty about PLC status.

Using a single CT scan finding as a criterion for the M1 modifier offers a more objective approach, reducing bias in treatment decisions. More importantly, the M1 modifier can help determine when further MRI testing is needed. If an MRI is unavailable, alternatives such as standing X-rays or close monitoring for kyphosis in non-surgical cases can be considered.

The cost-effectiveness of this approach is shown by the fact that only 15% of patients had a single CT finding; performing MRI for such a group will avoid 75% of missed PLC injuries on CT.

The usefulness of CT criteria can be further improved by considering their accuracy in different spinal regions, AO fracture classifications, neurological impairments, and unstable burst fractures. A machine learning algorithm is currently being developed to analyze all these factors and improve decision-making.

CT vs. MRI: Complementary tools in PLC evaluation

Rather than debate the merits of CT versus MRI, we believe both imaging methods should complement each other in guiding treatment decisions. Using CT and MRI together can improve the detection of PLC injuries while keeping costs and delays in treatment to a minimum.

Notably, interpreting MRI results for PLC injuries can be more challenging for spine surgeons than for neuroradiologists, but when two or more CT findings strongly suggest PLC injury, a detailed MRI review or a radiologist’s opinion may improve MRI accuracy.

Additionally, the significance of subtle MRI findings, such as an isolated and unclear supraspinous ligament injury, is still uncertain. In such cases, CT findings may help confirm or rule out instability. CT indicators of bone displacement, like interspinous widening, may provide more useful insights into instability than MRI findings alone in such case.

On the treatment side, there are a lot of controversies about when to consider surgery versus conservative management, short- versus long-segment instrumentation, minimally invasive versus open procedures, and posterior versus combined approaches. Most spine surgeons aim to establish clear treatment guidelines for managing burst fractures.

However, the first and most crucial step in addressing this issue is creating a reliable diagnostic process. Our validated CT criteria now provide an objective framework for addressing this diagnostic dilemma and represent the first step toward establishing clearer treatment guidelines.

Understanding PLC injuries 

The Posterior Ligamentous Complex (PLC) is a crucial stabilizing structure of the spine. It consists of the supraspinous ligament, interspinous ligament, ligamentum flavum, and facet joint capsules. These structures work together to maintain spinal alignment and prevent excessive motion that could lead to instability or neurological injury. Injuries to the PLC are serious and often indicate significant spinal trauma, requiring careful assessment and management.

PLC injuries are commonly caused by high-energy trauma, such as vehicular accidents, falls from heights, and sports injuries.

Identifying PLC injuries is crucial but proper diagnosis can be challenging. MRI findings are sometimes subtle or open to misinterpretation. CT scans and X- rays may not adequately visualize the true extent of the damage, which can lead to delayed or inappropriate treatment. For example, in stable fractures, conservative management such as bracing may suffice, but with severe ligament disruption, surgical stabilization is usually required.

If PLC injuries are not properly managed, several complications can arise, including chronic pain, progressive spinal deformity. Therefore, PLC injuries are serious and often indicate significant spinal trauma, requiring prompt and accurate diagnosis.