Involvement of the zygapophyseal joint in ankylosing spondylitis: Relationto the bridging syndesmophyte

Objective, To determine the prevalence of zygapophyseal (ZA) joint ankylosi s in ankylosing spondylitis (AS) and the relation of this ankylosis to the presence of bridging syndesmophytes. Methods, Roentgenograms of the cervical and lumbar spine of 50 patients wit h AS were scored by a vertebral body score according to Taylor and a newly developed score for ZA joint involvement. Results. At the cervical level 22% of the ZA joints were ankylosed and 16% of the levels presented bridging syndesmophytes. In the lumbar spine 22% of ZA joints were ankylosed and 16% of the vertebral levels showed ankylosis in both ZA joints, Bridging syndesmophytes were present in 11% of the verte bral levels. Ankylosis of the ZA joints and bridging syndesmophytes were se en simultaneously in 14% of the cervical levels and in 9% of all lumbar lev els. Ankylosed ZA joints were present in 21% of the lumbar levels without b ridging syndesmophytes and in 8% of the cervical levels without bridging sy ndesmophytes. Bridging syndesmophytes at a given level without ankylosis of the respective ZA joint were uncommon: 4 of 201 lumbar levels and 4 of 238 cervical levels. Ankylosis of the ZA joint and presence of bridging syndes mophytes is markedly associated, but in a discordant way. Conclusion. The ZA joint is affected in a major way in AS. Involvement of t he ZA joint and the presence of syndesmophytes are related. An asymmetric r elation suggests that the ZA joint is primarily involved in AS.