OPLL was found to occur in 1.5% to 2.4% (19, 20, 21, 22, 23, 24, 25 and 26) of adult outpatients with cervical disorders at several university hospitals in Japan (Table 80.1). In the same survey of foreign countries, the prevalence of OPLL was 0.4% to 3.0% in Asian countries (27, 28, 29, 30 and 31). In a review of plain cervical spine films by Yamauchi (27,32) and Izawa (26), the prevalence of OPLL was 2.1% among Japanese patients (143 per 6,994), 1.0% in Koreans, 0.1% in North Americans, and 0.1% in Germans. A survey in Italy in 1984 by Terayama and Ohtsuka (33), however, revealed a high prevalence of OPLL (Table 80.2). Our survey in a Utah university hospital (34) revealed OPLL in the cervical spine in 8 of 599 subjects (1.3%).

To determine the actual incidence of OPLL in various countries around the world, an epidemiologic study of the general population has been encouraged. The incidence of OPLL in the general Japanese population was reported to be 1.9% to 4.3% (35, 36, 37, 38, 39 and 40) among people older than 30 years (Table 80.3). However, a few studies have analyzed the general population. We performed a study in Taiwan on 1,004 Chinese and 529 Takasago tribe people older than 30 years (41,42). The incidence of OPLL was 0.2% in Chinese and 0.4% in the Takasago tribe. This is lower than that of Japanese people. Recently, Tomita et al. (43) carried out an epidemiologic study of OPLL in China involving 2,029 Chinese and 500 Mongolian people. According to this study, the prevalence of OPLL was 1.6% in Chinese and 1.8% in Mongolians.

Resnick et al. (17) reported DISH as a common disorder characterized by bone proliferation in axial and extra-axial sites. The most characteristic abnormalities in this condition are ligamentous calcification and ossification along the vertebral body (18). Changes in extraspinal locations are also frequent, including ligament and tendon calcification and ossification, para-articular osteophytes, and bony excrescence at sites of ligament and tendon attachment to bone. In his study of a group of 74 patients with DISH, 37 (50%) patients had concomitant OPLL on cervical radiographs (44). A similar figure of OPLL in 43% of 40 patients with DISH was reported from France (45). Whereas DISH is a fairly common disease of the general population in whites older than 50 years, its frequent association with OPLL suggests that OPLL itself is not a rare disease in whites.

In 1992, Epstein proposed a new concept regarding OPLL (12). She examined CTs of the cervical spine in whites and noted hypertrophy of the posterior longitudinal ligament with punctuate calcification. This finding was described as ossification of the posterior longitudinal ligament in evolution (OEV). Epstein noted that the prevalence of OPLL among whites with cervical myelopathy has recently been noted to be from 2% to 25% (46). Epidemiologic surveys of OPLL by Japanese researchers were done using plain radiographs of the cervical spine to detect the OPLL. Most Japanese researchers did not include OEV on their OPLL surveys. Furthermore, there is controversy regarding the definition of OPLL between Japanese researchers and North American researchers. These variables, at least in part, explain the difference.

Several papers on the clinical characteristics of OPLL have been published (4,5,47,48). The clinical characteristics of patients with OPLL in papers of Japanese and foreign researchers yielded similar findings. Terayama, a member of the Investigation Committee on Ossification of the Spinal Ligaments of Japanese Ministry of Public Health and Welfare et al. (48) performed the first Japanese national survey of OPLL in 1975. The investigators abstracted 880 hospitals, including university hospitals, for this survey, and 2,142 OPLL patients were registered. Based on the results of this survey, it was observed that OPLL typically developed in patients older than 40 years and carried a male predominance of 2:1 to 3:1. The average age of onset was 51.2 years in men and 48.9 years in women. Sixty-seven percent of patients were 45 to 65 years old. Ninety-five percent of patients had some clinical symptoms, but 5% of patients were free of symptoms. Initial complaints typically consisted of cervical discomfort in conjunction with numbness of the upper extremity. The typically recognized symptoms of OPLL were as follows: sensory and motor dysfunction of upper and lower extremities, hyperreflexia, pathologic reflexes, and bladder dysfunction. As many as 16.8% of patients required assistance in activities of daily living; 5.4% of patients showed a rapid aggravation of symptoms, and 11.4% of patients showed a chronic aggravation. Symptoms spontaneously appeared and continually progressed. Initial complaints typically consisted of cervical discomfort in conjunction with numbness or myeloradiculopathy, typically characterized by symmetric upper and lower extremity findings. In cases in which quadriparesis had rapidly evolved, sphincteric dysfunction was often also noted (5). As many as 9.7% of patients also had diabetes mellitus. As for the glucose tolerance test, 29% of patients exhibited a diabetes mellitus pattern. This prevalence was very high (5%) compared with that of an age-equivalent group without OPLL. Also, 23% of patients had a history of trauma to the cervical region. Cervical spine trauma may have precipitated the onset of symptoms, which, in some cases, included quadriparesis (49, 50 and 51). However, the prevalence of trauma that caused symptoms was only 15% in the retrospective study (48). In the prospective investigation for 368 OPLL patients without myelopathy at the time of initial consultation, only six patients (2%) subsequently develop trauma-induced myelopathy (52).