In other cases, significant numbers of lymphocytes are present. This early phase may be associated with erosion of the underlying bone, but in due course bony outgrowth into the ligament occurs. Enthesopathy at these peripheral sites is a relatively minor problem in ankylosing spondylitis, but is prominent in other seronegative spondarthropathies. Disease in females tends to be milder and less typical. The earliest site of spinal involvement is usually the thoracolumbar junction but characteristically the condition spreads to affect all spinal segments.
Stiffness is initially due to inflammation, then to fibrous, and eventually bony ankylosis.
Spondylitis is associated with sacroiliitis in nearly all cases. Although this is a synovial joint it is likely that the initial pathology is an enthesopathy of the sacroiliac ligaments. Very rarely rigid spines may fracture in the cervical or thoracic region leading to cord transection. Ankylosing spondylitis does not only affect entheses. Specific sites in several other organs are involved.