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Definition

Enthesopathy characterized by ossification which originates within or spans the space between two discs to involve the PLL

Epidemiology

Japanese have high incidence; prevalence of about 2% in asymptomatic adults. Incidence of 0.2 to 0.7% in Caucasians.

Increases with age. In Japanese men >60 years of age, more than 10% have OPLL.

Male predominance.

Pathogenesis

Etiology unknown

High incidence of DISH among OPLL patients suggests a hereditary diathesis of spinal ligament ossification

OPLL in up to 50% patients with DISH

DISH in over 20% patients with OPLL

Resnick D. “Calcification and Ossification of the posterior Spinal Ligaments and Tissue.” Bone and Joint Imaging, 1989; p 455.

Possibility of an autosomal dominant inheritance.

Incidence of OPLL among family members of second order kinship 15 times the general incidence

High incidence of diabetes among patients with OPLL

28% of patients with OPLL have diabetes and 18% are borderline diabetics. The incidence of OPLL in diabetic patients was 16%.

Process similar to heterotopic bone formation

Cartilage cells proliferate first in the periosteum of the vertebral body and then in the annulus fibrosus, longitudinal ligament and dura.

The ligament becomes calcified by endochondral ossification.

Mature lamellar bone is eventually formed.

Rate of growth varies. May be cuboidal, triangular, or polypoid in shape.

Only a minority of patients with OPLL are symptomatic.

Natural history of OPLL is that of progressive enlargement laterally, posteriorly, and longitudinally.

Mean annual increase in OPLL size was 1.0 mm in S-I dimension and 0.67 mm in the AP direction.

Relatively good spinal cord function perserved

due to a high degree of tolerance to slowly increasing mechanical pressure

decompensation usually occurs after 50-60% of the AP diameter of the spinal canal is occupied.

Minor trauma may cause quadraplegia.

OPLL can occur at any level: cervical, 75%; thoracic, 15%; and lumbar, 10%

Types:

1. Segmental. Discrete sites of ossification adjacent to a single vertebral body.

2. Localized. Span a single disc but involve two adjacent vertebral bodies.

3. Continuous. Span two or more discs.

4. Mixed

Cervical spine: 32% segmental, 35% continuous, 32% mixed, 3% localized.

Most commonly seen at C4, C5, and C6.

Average number of levels involved is three.

The narrowing ratio (ratio of maximal thickness of OPLL to the AP diameter of the spinal canal on lateral C-spine films) is higher in the mixed and continuous types than in the segmental and localized types.

May be difficult to appreciate on plain films

CT best imaging modality for the detection of OPLL.

MR

On T1 may be indistinguishable from the ventral CSF

>3mm usually readily seen on T2WI

evaluates cord signal abnormality

35% with continuous

16% with segmental.