Ultrasound assessment of injury to the posterior ligamentous complex in patients with mild thoracolumbar fractures

Abstract

Objective

To assess the value of ultrasonographic imaging of the posterior ligamentous complex (PLC) to diagnose ligamentous injuries, in patients with mild thoracolumbar fractures.

Methods

Patients with thoracolumbar fractures were included in this prospective study. Patients underwent palpation of the midline of the back, and ultrasonography was performed over the entire thoracolumbar region by an experienced sonographer. A team that included a musculoskeletal radiologist, an orthopaedic surgeon and a sonographer assessed the ultrasound results. Ultrasonographic and magnetic resonance imaging (MRI) findings were jointly evaluated in a subgroup of patients who were able to fund MRI analysis. Conflicts regarding the results were resolved by a majority vote.

Results

A total of 21 patients participated in the study, all of whom exhibited abnormal ultrasonographic echogenicity on the supraspinous or interspinous ligaments. Three patients were diagnosed with a rupture of the supraspinous ligament. In 15/17 (88.2%) patients, interspinous ligament injuries were detected caudally to the injured vertebrae.

Conclusions

Ultrasound examination is a reliable complementary diagnostic tool to identify PLC injuries in patients with mild thoracolumbar fractures.

Introduction

The posterior ligament complex (PLC) includes the supraspinous ligament (SSL), interspinous ligament (ISL), ligamentum flavum and the facet joint capsules, ¹ and is thought to contribute substantially to the stability of the thoracolumbar spine. ² The PLC plays a critical role in protecting the spine and spinal cord against excessive flexion, rotation, translation and distraction. ³ Holdsworth first emphasized the importance of the PLC in his classification system for thoracolumbar fracture, ⁴ and The Spine Study Group proposed the Thoracolumbar Injury Classification and Severity Score (TLICS) to provide a reliable classification system, in which the importance of the PLC is emphasized. ⁵

Clinical assessment of the PLC can be based on clinical examination, plain radiography images, computed tomography (CT) scans, and magnetic resonance imaging (MRI) scans. The integrity of the PLC is categorized as intact, indeterminate or disrupted. ⁶ In patients with severe thoracolumbar fracture, injury to the PLC can be easily diagnosed by a gap, which is detected on palpation or by a widening of the interspace of the spinal processes on X-radiography and CT images. However, in mild compressive/burst thoracolumbar fracture cases, the evidence for disruption has been found to be subtle. ⁴ When neurological functions are intact, the treatment procedure is inevitably based on the condition of the PLC.

Magnetic resonance imaging has an excellent ability to diagnose PLC injury with high sensitivity and specificity. ⁷ However, patient movement during imaging or the use of obsolete machines leads to MRI results of inferior quality, which may hinder PLC evaluation. ⁸ Ultrasonography has been demonstrated to be a viable imaging method for the diagnosis and assessment of the musculoskeletal system, ⁹ and the feasibility of PLC evaluation using this method has been reported.^10,11 In contrast to MRI, ultrasound examination is superior not only in its implementation and cost effectiveness, but it can also provide sequential and magnified images, allowing for a more detailed PLC evaluation. ⁸

The purpose of the current study was to evaluate the use of ultrasonography of the PLC in the detection of mild thoracolumbar fracture-related ligamentous injury.

Patients and methods

Data collection

This study prospectively evaluated the medical records of all patients who were admitted to the Department of Orthopaedic Surgery, China-Japan Union Hospital, Jilin University, Changchun, Jilin Province, China with a diagnosis of thoracolumbar fracture, between January 2012 and July 2012. Inclusion criteria were as follows: (i) single-vertebra fracture between thoracic (T)11 and lumbar (L)2; (ii) fracture of one vertebra with <30% anterior column compression, calculated using the following formula for anterior body compression:

$\frac{\frac{\text{Vs}+\text{Vi}}{\text{z}}\text{Vf}}{\frac{\text{Vs}+\text{Vi}}{\text{z}}},$

where Vs is the anterior column height of the superior vertebrae, Vi is the anterior column height of the inferior vertebrae, and Vf is the anterior column height of the fractured vertebrae; (iii) physical examination confirming the absence of any neurological deficit; (iv) no protrusion to the spinal canal of the vertebral back-wall (confirmed via a CT scan). Exclusion criteria included combined injuries to the extremities, head or visceral organs and a definite rupture of the PLC evident on CT or plain radiography examination.^12,13

Medical records and radiographs for each patient were carefully reviewed. The PLC status was examined by palpation of the midline of the back, plain radiography, and CT scanning. Ultrasonography was performed over the whole thoracolumbar region by an experienced sonographer. MRI scans were performed in those patients who were able to pay for the additional procedure, either via health insurance or other funding. A team (including a musculoskeletal radiologist, an orthopaedic surgeon and a sonographer who were not involved in the patient’s ultrasound examination) evaluated MRI and ultrasound findings in the subgroup of patients for whom both sets of data were available: In these patients, conflicts regarding the results of both MRI and ultrasonography were resolved by the specialist team, by a majority vote.

Ultrasonographic assessment

An ultrasound scanner with a 7–10 MHz linear transducer was used (Vivid 7; GE Healthcare, Piscataway, NJ, USA). Patients were transferred to the examining table and placed into the prone position. Thoracolumbar localization was performed by palpation of the spinous process with reference to the sagittal X-radiography findings. An ultrasound probe was applied between the tips of the spinous processes in the sagittal plane, with the appropriate cephalad orientation to avoid the spinous process. The evaluation standards were as follows: (i) any disruption in the echo of the SSL was defined as a complete rupture of the PLC; (ii) any hyperecho on the ISL was considered as an indeterminate injury of the PLC in the TLICS scoring. Integrity of the PLC was categorized as intact, indeterminate or disrupted. ⁵

The study protocol was reviewed and approved by the Ethics Committee of the China–Japan Union Hospital (no. 20120046). All patients provided verbal informed consent prior to participation.

Statistical analyses

As both MRI and ultrasound results were only available in a small subgroup of patients, the sample was too limited to perform reliable statistical analyses.

Results

The study included 21 patients (16 males; five females) with mild thoracolumbar spinal fractures, diagnosed by plain radiography and CT scans. The mean ± SD age of the patients was 42.2 ± 13.7 years (range 20–79 years). The causes of trauma were falling injury and road traffic accidents. The fractures were classified as follows: compression fractures (n = 18); burst fractures (n = 3). Fracture levels were as follows: T11 (n = 3); T12 (n = 7); L1 (n = 8); L2 (n = 3).

All 21 patients presented an abnormal echo in the SSL or ISL, according to the ultrasound findings. Three cases were diagnosed with a rupture of the SSL and underwent surgical treatment (Figure 1). Surgical findings corresponded with those from ultrasonographic examinations: one patient had a compression fracture and two had burst fractures (Table 1).

Figure 1.

Representative ultrasonographic image showing a patient with a lumbar (L)2 fracture. Ultrasonography identified a rupture of the supraspinous ligament and a haematoma in the interspinous ligament (ISL). This patient underwent surgical treatment. The arrow identifies an abnormal curve that indicated the rupture of the ISL, which was confirmed during surgery.

Table 1.

The locations of ligamentous injuries, as identified by ultrasonographic scanning of the posterior ligament complex in patients with mild compression or burst thoracolumbar fractures (n = 21).

Ligament involved	Thoracolumbar fracture type
	Compression fracture	Burst fracture
ISL	16	1
ISL + SSL	1	–
SSL rupture	1	2

Data presented as n patients.

ISL, interspinous ligament; SSL, supraspinous ligament.

Ultrasonography imaging in the remaining 18 patients demonstrated that the SSL was intact, but an abnormal echo in the ISL (or under the SSL) was detected above or below the spinal process of the fractured vertebrae (Figure 2). Of these 18 patients, one had a burst fracture and 17 had compression fractures (Table 1). All abnormal ultrasound findings were located on (or adjacent to) the injured vertebrae. In one of the 18 patients, ultrasonography indicated a haematoma signal, located on the ISL and SSL (Figure 3). Similar to the patients with SSL ruptures, most ISL injuries were detected caudally to the injured vertebrae (15/17), and two were located in the cranial direction.

Figure 2.

Representative ultrasonographic image showing an abnormal echo (arrow) on the interspinous ligament, below the spinal process of the fractured vertebrae, in a patient with a mild thoracolumbar fracture.

Figure 3.

Representative ultrasonographic image showing a haematoma signal, located on the supraspinous and interspinous ligaments, in a patient with a mild thoracolumbar fracture. Arrow shows an abnormal echo that indicated a haematoma signal. L, lumbar.

Six patients also underwent MRI scans, and the results of these procedures were available for comparison with the findings of ultrasonography examination. Four of the MRI results showed high T2-weighted images on the ISL. One MRI examination showed a multilevel (two-segment) abnormality within the ISL, but an ultrasound examination detected only a single-segment lesion. The MRI results for the two other cases were negative.

Discussion

The stability of the spinal column has been found to be strongly influenced by the integrity of the ligamentous structures.^4,13 Biomechanical loading studies have shown that the PLC, as a whole, plays a primary role in dictating the flexion resistance of the thoracolumbar spine. ¹⁴ The SSL and ISL represent the main load-bearing elements and are often the first ligamentous structures to be torn in a flexion injury. ¹⁵ The most recent thoracolumbar fracture classification system, TLICS, considers the integrity of the PLC to be of high importance. ⁵

The use of ultrasonography for the evaluation of the musculoskeletal system has been increasing since its early use in the evaluation of the rotator cuff. ¹⁶ With the use of advanced sonography equipment and through the combined efforts of radiologists and orthopaedic specialists, musculoskeletal sonography is considered to be an important complementary tool to MRI. In some situations, ultrasonography may be preferable to MRI. ⁸ Moon et al. ¹¹ described the use of ultrasonography as an alternative to MRI for PLC detection. However, reliable reports regarding its sensitivity and specificity are lacking. The structures of the PLC system and spinal processes are relatively simple and subcutaneous: they can be easily detected by ultrasonography, and the images that are produced can be easily interpreted.

In the present study, the ISL was found to be more vulnerable than the SSL in cases of mild thoracolumbar fracture, but more research is needed to determine the impact on the tissue structure and the mechanism of tissue injury. An inferential hypothesis can be made that, like other fractures, a soft-tissue injury centred around the fracture point may be alleviated as the space increases.

In conclusion, the present study showed that ultrasonography can be a reliable complementary diagnostic tool for evaluating the PLC in patients with mild thoracolumbar fracture, when MRI is not available. With its high sensitivity and feasibility, ultrasound examination of the PLC should be widely used for patients with spinal disorders other than spinal fracture. A database of ultrasonographic images of PLC structures should be collected, to establish a new diagnostic standard with a reliable classification system.