Diffuse idiopathic skeletal hyperostosis (DISH), also known as Forestier disease, describes a phenomenon characterized by a tendency toward ossification of ligaments. It most characteristically affects the spine. [1, 2] (See the image below.) Ossification of the longitudinal ligaments (especially the anterior ligaments) of the spine produces a tortuous paravertebral mass anterior to and distinct (at least radiologically) from the vertebral bodies.  Grossly, the appearance is that of candle wax dripping down the spine.
While the thoracic anterior longitudinal ligament is ossified, the areas of ossification often meet without fusion. Motion actually is possible, in contrast to lumbar vertebral bridging, which is associated with loss of lumbar motion. The zygapophyseal and sacroiliac joints are not involved in DISH, and the intervening intervertebral disk space is preserved.
DISH is well represented in the zoologic and paleontologic record. It is found in 1-3% of baboons and monkeys, as well as in gorillas, bears, camels, horses, bison, musk oxen, canids, felids, and whales [4, 5, 6, 7, 8, 9, 10] ; DISH was also present in dinosaurs. [11, 12, 13] An age-dependent phenomenon, it occurs in 15-25% of older mammals.
Diffuse idiopathic skeletal hyperostosis (DISH) is characterized by a tendency toward ossification of ligament, tendon, and joint capsule (enthesial) insertions.  DISH is a completely asymptomatic phenomenon; no alterations are detectable based on history or through physical examination.
Diffuse idiopathic skeletal hyperostosis (DISH) is present in approximately 19% of men and 4% of women older than 50 years. Frequency information in the US was derived from the study of nonselected skeletal/cemetery populations. 
The posterior longitudinal ligament of the cervical spine is ossified in 2% of Japanese individuals but in only 0.16% of white persons.  The anterior longitudinal ligament is calcified in 24% of patients with posterior longitudinal ligament ossification.  Diffuse idiopathic skeletal hyperostosis (DISH) was reported in 17% of individuals in the Netherlands, paradoxically with male predominance. 
Diffuse idiopathic skeletal hyperostosis (DISH) appears to be a phenomenon rather than a disease. Double-blind controlled evaluation (in which controls and patients were drawn from the same population) revealed no associated pathology. Arthritis, bursitis, and tendinitis appeared no more frequently in patients with DISH than in controls. Any back pain present was no different in character or duration than that noted in control subjects. A history of back injury was actually found to be twice as frequent in control subjects as it was in patients with DISH. Back flexibility was no more limited in patients with DISH than it was in controls. In fact, patients with DISH who had decreased lumbar spinal motion had a lower frequency of back pain, implying that DISH may be protective. [15, 19] One study has showed that DISH may be protective against back pain. 
The posterior longitudinal ligament of the cervical spine is ossified in 2% of Japanese individuals but in only 0.16% of whites.
Diffuse idiopathic skeletal hyperostosis is present in approximately 19% of men older than 50 years but is found in only 4% of women in this age group.
Diffuse idiopathic skeletal hyperostosis (DISH) is uncommon in patients younger than 50 years and is extremely rare in patients younger than 40 years.
A study from Finland  revealed the age frequency in Finnish men to be as follows:
40-49 years - 0.3%
50-59 years - 2.7%
60-69 years - 8.4%
70 years or older - 11.2%
The same study revealed the age frequency in Finnish women to be as follows:
40-49 years - 0.2%
50-59 years - 1.7%
60-69 years - 4.3%
70 years or older - 6.9%