Orthopedics

Clinical Presentation

Traditionally, ossification of the posterior longitudinal ligament was thought to mostly affect an East Asian patient population. Literature specific to East Asian populations frequently reported that patients with ossification of the posterior longitudinal ligament presented in the fifth decade of life and were more commonly men.^1,2 More recent investigations have reported non-Asian patients displaying demographic, diagnostic, and therapeutic similarities, indicating that this disease is not limited to East Asia.

A recent study by Wang and Thambuswamy³ found that, among non-Asian patients, a male predominance exists and symptoms peak in the sixth decade of life. In addition, a systematic review by Saetia et al⁴ reported an incidence of ossification of the posterior longitudinal ligament ranging from 0.16% to 2.4%, the former representing a non-Asian patient population.

Chronic insidious neck pain is a frequent presenting symptom of ossification of the posterior longitudinal ligament. Given the nature of these symptoms, the initial differential diagnosis includes cervical disk herniation, intervertebral disk degeneration, and facet or uncovertebral osteoarthritis. Other axial pain syndromes, such as overuse and trauma, should have an appropriate clinical history to suggest a diagnosis. Systemic diseases, such as rheumatoid arthritis, ankylosing spondylitis, and calcium pyrophosphate disease, may also be preexisting diagnoses. More indolent processes, such as diffuse idiopathic skeletal hyperostosis, may be difficult to differentiate from ossification of the posterior longitudinal ligament clinically.

Symptoms of radiculopathy or myelopathy are often present. Radiculopathy results from neural foraminal narrowing, whereas myelopathy is due to spinal cord compression. Therefore, in addition to radiating pain, patients with ossification of the posterior longitudinal ligament may present with paresthesias, weakness, clumsiness, or gait instabilities. The differential diagnosis for myelopathy includes spondylotic myelopathy or congenital/developmental stenosis. Less frequently, trauma may unmask ossification of the posterior longitudinal ligament in asymptomatic patients who have borderline compression transitioning to frank canal compromise associated with hemorrhage, edema, or fracture. Noncompressive etiologies, such as multiple sclerosis or other primary degenerative or demyelinating diseases, should be ruled out in the evaluation of patients with cervical myelopathy.

Imaging

Radiographs

Radiographs of the cervical or thoracic spine are often the initial imaging study for patients with neck or dorsal back pain. In ossification of the posterior longitudinal ligament, a longitudinal, coalescent, or segmented band of calcification or ossification exists at the posterior margin of the vertebral bodies (Figure 1A). This may be a solitary abnormality, but additional signs of degenerative disease may exist given the older presenting age. Although oblique images may be of benefit to evaluate foraminal narrowing in the presence of facet and uncovertebral proliferation, these images do not provide additional information regarding the ligament ossification.

Figure 1: Lateral cervical spine radiograph of a 60-year-old man showing dense, layering ossification of the posterior longitudinal ligament (arrows) resulting in severe narrowing of the bony spinal canal (A). Sagittal T2-weighted magnetic resonance image showing signal void of the ossified posterior longitudinal ligament and loss of cerebrospinal fluid high T2 signal adjacent to the spinal cord secondary to thecal sac impingement. Note deformation of the spinal cord with internal high-T2-signal myelomalacia (arrow) (B). Axial computed tomography scan showing dense ossification of the posterior longitudinal ligament (arrow) with marked impingement on the spinal canal and thecal sac (C). Axial computed tomography scan from myelogram showing slightly eccentric, dense ossification of posterior longitudinal ligament (arrow) with severe spinal cord impingement and deformity (filling defect within white cerebrospinal fluid after contrast injection) (D).

Advanced Imaging

Advanced imaging with CT or MRI is recommended to help further characterize the extent of disease, especially the degree of spinal canal narrowing. Even when radiographs are normal, further imaging is recommended in the setting of patients with clinical myelopathy or a frank neurological deficit. Magnetic resonance imaging is often considered the gold standard and is used as the first-line diagnostic modality. Magnetic resonance imaging has the advantage of providing exquisite soft tissue detail without the need for myelography. Both the ossified and unossified portions of the posterior longitudinal ligament show signal void on all sequences (Figure 1B). The volume of the narrowed spinal canal is seen on axial and sagittal images. On T2-weighted spin echo sequences, the myelographic effect of high T2 signal of cerebrospinal fluid contrasted with the signal void of the ligament provides easy visualization of the severity of thecal sac impingement. In addition, MRI provides the added benefit of direct visualization of the spinal signal abnormalities, including intermediate or high T2 signal indicative of myelomalacia secondary to impingement (Figure 1B). This finding may further correlate to clinical findings.

Computed tomography is the preferred modality to further delineate the extent of calcification and for surgical planning given its superior bone resolution. Although some surgeons may obtain routine CT scans prior to cervical spine surgery, no evidence exists to support the effectiveness of this protocol. Nonetheless, CT scans should be obtained if MRI or plain radiographs suggest the presence of ossification of the posterior longitudinal ligament. Axial images show well-formed ossification within the spinal canal intimately applied to the posterior cortex of the vertebral bodies (Figures 1C, D). Because the ossification develops in the ligament, the layering mineralization affects the central aspect of the body, as opposed to osteophytosis at the perimeter. Sagittal reconstruction images are also of benefit for depiction of the longitudinal extent of the ossification (Figure 2A). If ossification of the ligament contributes to an anteroposterior diameter <10 mm, ossification of the posterior longitudinal ligament likely accounts for clinical symptomatology.³ Precise delineation of secondary effacement or deformity of the thecal sac can be problematic, especially in the cervical spine, which has a relative paucity of epidural fat. Therefore, injection of contrast into the intrathecal sac can be more specific for determining the presence or severity of thecal sac impingement (Figure 2B). Intravenous infusion of iodinated contrast agents is generally not indicated, nor does it provide additional information.

Figure 2: Sagittal computed tomography scan showing multilevel, layering ossification of the posterior longitudinal ligament (arrows) (A). Sagittal computed tomography scan showing intermittent ligament ossification (arrows) at the cervical and thoracic levels (B).

Differential Diagnosis on Imaging

The radiologic findings of ossification of the posterior longitudinal ligament, in the absence of additional abnormalities such as degeneration, erosion, or bone destruction, are specific for the diagnosis. Imaging should be inspected carefully for additional findings that may direct the clinical investigation to alternative diagnoses.⁵ Cervical involvement in rheumatoid arthritis can be indicated by erosive arthropathy of the C1–C2 articulation, abnormal pre-dental interval due to ligamentous disruption, and synovial inflammation and pannus with enhancing soft tissue. Although ankylosing spondylitis is also a proliferative spondyloarthropathy that can be associated with ossification of the posterior longitudinal ligament, ossification of the annulus fibrosis/anterior longitudinal ligament and smooth flowing marginal syndesmophytes also exist, in addition to erosive/ankylosing arthropathy of facet articulations (Figure 3).

Figure 3: Lateral cervical spine radiograph showing discontinuous ossification of the posterior longitudinal ligament and marginal annulus (black arrows) at C2–C3 and C3–C4. Note margin-to-margin flowing syndesmophytes anteriorly (white arrows) in this patient with ankylosing spondylitis.

Similarly, diffuse idiopathic skeletal hyperostosis is a proliferative enthesopathy that may manifest with ossification of the posterior longitudinal ligament and preferentially involves the cervical and thoracic spine, a distribution similar to ossification of the posterior longitudinal ligament. However, the hallmark of diffuse idiopathic skeletal hyperostosis is proliferation and ossification of the anterior longitudinal ligament, often resulting in multilevel ankylosis (Figure 4).

Figure 4: Lateral cervical spine radiograph showing dense contiguous ossification of the posterior longitudinal ligament (black arrows) at C3–C5. Note bulky anterior longitudinal ligament ossification (white arrows) bridging C4–C6 in this patient with diffuse idiopathic skeletal hyperostosis.

Calcium pyrophosphate dihydrate deposition affects the spine as with other extra-axial joints due to crystal deposition in ligaments. However, the more typical manifestations result in small mineralized masses caudal to the anterior arch of C1 and calcification of the stabilizing ligaments at the C1–odontoid articulation, rather than frank ossification of the posterior longitudinal ligament. A mineralized hard disk, due to protrusion or frank disk herniation, may occasionally be difficult to differentiate from focal nodular ossification of the ligament, but ossification of the posterior longitudinal ligament usually has more extensive, multilevel longitudinal involvement.

Treatment

Treatment decisions for ossification of the posterior longitudinal ligament are based on a correlation of imaging findings to the patient’s clinical presentation. Both conservative and surgical measures have been reported. In a systematic review of 480 patients, Pham et al⁶ reported that patients without myelopathic symptoms who were managed conservatively and observed over time were unlikely to develop symptoms. These non-myelopathic patients were more likely to present after trauma, with imaging showing incidental ossification of the posterior longitudinal ligament.⁶ This supports observing asymptomatic, specifically nonmyelopathic, patients with ossification of the posterior longitudinal ligament and intervening with further evaluation or treatment when symptoms develop. Pham et al⁶ also determined that patients presenting initially with myelopathic symptoms were more likely to progress. Therefore, these patients typically require a more direct approach for management.

Although not uniformly agreed on, 2 surgical approaches are used for symptomatic stenosis: direct removal of the ossified ligament through an anterior approach and posterior spinal decompression. The anterior approach can remove the offending ventral compression. However, surgical complications associated with anterior resection are significant, specifically the risk for persistent cerebrospinal fluid leak and dural defects after decompression. In addition, given the multilevel nature of the disease process, the multilevel cervical corpectomy often needed for decompression has been associated with higher nonunion and instrumentation complication rates.^7,8

The posterior approach avoids these risks by decompressing the spinal canal, allowing spinal cord expansion and posterior migration away from the offending ventral compression. Posterior approaches, including posterior laminectomy, posterior laminectomy and fusion, and posterior laminoplasty, have been reported.^9,10 Motion-preserving techniques may lead to symptom recurrence because the underlying pathologic process of calcification and ossification will continue.¹¹ In addition, patients with significant cervical kyphosis or large ventral compression may not improve with posterior decompression due to insufficient cord migration.

Conclusion

Ossification of the posterior longitudinal ligament, an uncommon cause of neck pain and cervical myelopathy, affects patients of various ethnicities, although an Asian predominance exists. Although many patients are asymptomatic and diagnosed incidentally, some patients experience significant and progressive neurologic sequelae.

Imaging and clinical suspicion are keys for the diagnosis of ossification of the posterior longitudinal ligament in the absence of additional abnormalities. Radiographs show confluent ossification of the posterior longitudinal ligament. Cross-sectional imaging techniques can precisely delineate the extent of canal and thecal sac impingement, and MRI can show myelomalacic findings of the spinal cord. Although asymptomatic patients can safely be observed, symptomatic patients with myelopathic findings more commonly require surgical therapy for restoration of spinal cord function and symptomatic relief.