Ossification of the Posterior Longitudinal Ligament

Summary of Key Points

The morbidity rate of cervical ossification of the longitudinal ligament (OPLL) is higher in East Asia than in Europe or North America.
The ratio between men and women with cervical OPLL is 2:1, but the incidence of thoracic OPLL is 1.5 to 3 times higher in women than in men.
Around half of the patients with cervical OPLL present with progression of ossification. This phenomenon may lead to cervical myelopathy and an increased risk of spinal cord injury after minor trauma.
The average age of onset of symptoms is around 50 years. The subjective symptoms at the time of diagnosis are radiculopathy, myelopathy, stiffness, and neck pain, and only a few patients have no subjective symptoms. Spinal cord symptoms are more common in patients, with a spinal canal stenosis rate exceeding 60%.
The use of both x-ray and reconstructed computed tomography (CT) imaging can increase the accuracy of diagnosis and help determine the appropriate type of OPLL treatment.
If cervical or thoracic OPLL is detected by imaging, whole-spine evaluation using CT images can be useful to understand the mechanism underlying the development of concomitant spinal ligamentous ossification.
A few studies have evaluated the utility of conservative treatment for OPLL. If patients who have myelopathy at initial examination do not respond to conservative treatment, surgical therapy should be recommended because of the high rate of symptomatic progression during the follow-up period.
Several techniques have been used in the treatment of cervical OPLL, including anterior and posterior decompression fusion and laminoplasty. As each procedure has advantages and shortcomings, their indications should be cautiously considered according to the pathological OPLL condition.
Posterior fusion surgery, anterior decompression via a posterolateral approach, and circumferential decompression and fusion via a combined anterior and posterior approach are the major procedures used for the treatment of thoracic OPLL. However, the complication rate is 50%, and among the adverse events, motor palsy in the lower extremities has the highest incidence (32%).

Ossification of the posterior longitudinal ligament (OPLL) is an important spinal disorder that may be associated with significant disability. OPLL involves heterotopic bone formation in the spinal canal, including the posterior longitudinal ligament. OPLL involving the cervical spine is associated with the worst morbidity, but the condition may be present in all spinal segments, often concurrently. The purpose of this chapter is to describe the pathophysiology of OPLL and discuss treatment options for symptomatic patients.

Demographics

OPLL is mainly prevalent in Asian countries, but must be suspected in patients of all ethnicities. The morbidity rates for cervical OPLL are 3% to 6% in East Asia and 0.1% in North America and Europe. Compared with cervical OPLL, thoracic OPLL has less morbidity (0.5%–1.5%). ^, The ratio between men and women with cervical OPLL is 2:1, but thoracic OPLL is 1.5 to 3 times more common in women than in men. ^, ^, ^, Cervical OPLL occurs in the third and fourth decades of life, and the frequency increases at the age of 50. ^, Even if ossification is discovered incidentally at a younger age, most of the symptoms only appear after middle age, particularly at the age of 50 years.

Natural History

Progression of Ossification

Around half of patients with cervical OPLL show progression of ossification. In a recent evaluation, enlargement of ossification was determined using three-dimensional computed tomography (CT) imaging. During the 2-year follow-up period, the OPLL ossification volume increased significantly. The annual mean increase was 4.1%, and the risk factors were as follows: younger age, higher weight, and greater body mass index. Among these factors, younger age has the strongest association with enlargement of ossification volume. Another previous study also revealed that progression of cervical OPLL is associated with younger age, involvement of multiple levels, and mixed-type morphology. In patients with nonprogression, the segmental type was significantly predominant.

Progression of Myelopathy

Morio and colleagues identified the risk factors for deterioration of myelopathy as part of the natural disease course: developmentally narrowing spinal canal and wider range of motion at the disc level, which promotes myelopathy. In addition to compression because of ossification, cervical disc herniation contributes to the development of compressive myelopathy and was found to be present at the maximum compression level in 60% of OPLL patients. The incidence of disc protrusion was higher among patients with segmental OPLL (81%).

The presence of OPLL increases the patient’s risk of developing traumatic spinal cord injury (SCI). A large multicenter study from Japan demonstrated that, among OPLL patients with SCI, around 90% had no bone injury and presented with incomplete paralysis. Jung et al. identified the risk factors for acute progression of myelopathic symptoms associated with OPLL after minor trauma, which revealed a higher rate of stenosis of the spinal canal as an independent predictor. Patients with acute progression exhibited unfavorable neurological outcomes. Kato et al. reported that older patients with cervical SCI without bony injury have spinal canal stenosis, cervical spondylosis, or OPLL.

Genetic Expression and Biomarkers

The association between genetic predisposition and OPLL has been reported in previous family surveys, twin studies, studies of HLA haplotypes, and genetic studies. Multiple reviews have been conducted to evaluate the genes associated with OPLL. A recent genome-wide association study conducted in Japan identified six susceptibility loci for OPLL: 20p12.3 (rs2423294), 8q23.1 (rs374810), 12p11.22 (rs1979679), 12p12.2 (rs11045000), 8q23.3 (rs13279799), and 6p21.1 (rs927485). Among these loci, rs374810, which is located in the chondrocyte promoter region of RSPO2 , plays an important role in the differentiation of chondrocytes. A further study identified RSPO2 as a susceptibility gene for OPLL.

OPLL biomarkers have been evaluated and identified as calcium phosphate metabolism markers, bone turnover markers, sclerostin, DKK1, SFRP1, fibroblast growth factor-23, fibronectin, menatetrenone, leptin, pentosidine, and hypersensitive C-reactive protein. During the evaluation, we observed that there were only a limited number of targeted markers, a small number of cases, and a lack of reproducibility. Therefore, definitive biomarkers still remain elusive.

Symptoms and Diagnosis

The subjective symptoms at the time of diagnosis are radiculopathy, myelopathy, stiffness, and neck pain, while a few patients had no subjective symptoms. In some cases, severe spinal cord palsy occurs because of the compression exerted by ligamentous ossification. In fact, spinal cord symptoms are common in patients, with a spinal canal stenosis rate exceeding 60%.

The average age of onset of symptoms is around 50 years. Patients with cervical OPLL commonly experience head/neck pain, limb pain/numbness, muscle weakness, and bladder and rectum dysfunction. In contrast, patients with thoracic OPLL frequently experience numbness of the lower limbs and trunk, gait disturbance, and bladder and rectal dysfunction. Most patients with thoracic OPLL have more severe gait disturbance than those with cervical vertebrae OPLL. Bladder and rectal dysfunction also tend to be more intense in thoracic OPLL patients than in cervical OPLL patients. Lumbar OPLL is less frequent, but may present with numbness in the lower limbs and intermittent claudication.

More than half of the patients have increased tendon reflex. Babinski reflex and Hoffmann reflex (only for cervical OPLL) are occasionally positive. However, if the OPLL exists at the level of the thoracolumbar spine, tendon reflexes can be reduced or disappear owing to the compression of conus medullaris.

Imaging Evaluation and Classification

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