The articular process-synovial intervertebral articulation (AP-SIVA) complex is located dorsally to the vertebral canal. It is composed of the caudal articular process of one vertebra, the synovial joint (with articular cartilage, synovial fluid and membrane, and articular capsule) located at the base of the interspinal space, and the cranial articular process of the following vertebra. Eight types of abnormal radiographic findings have been identified in the AP-SIVA complexes of the equine thoracolumbar spine (see Lameness in Horses: Abnormal Radiographic Findings in Articular Process-Synovial Intervertebral Articulation (AP-SIVA) Complexes). These findings were mainly observed at the thoracolumbar junction and in the lumbar area. These abnormal findings were reported in adult horses but also in 3- to 6-yr-old race and sport horses. AP-SIVA lesions are much more likely to be associated with back pain than kissing spines or any other vertebral lesions.

Table 1
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Abnormal Radiographic Findings in Articular Process-Synovial Intervertebral Articulation (AP-SIVA) Complexes Type General Features Radiographic Signs 1 Asymmetry No clear joint space; double joint space 2 Modification of density of the articular processes Sclerosis of the subchondral bone; increased density of the AP-SIVA complex 3 Modification of density of the articular processes Radiolucent images in the subchondral bone; increased density of the articular processes 4 Periarticular proliferation Dorsal periarticular proliferation; increased size of the AP-SIVA complex; often associated with modification of density 5 Periarticular proliferation Ventral periarticular proliferation 6 Ankylosis Dorsal bridge between 2 following vertebrae 7 Ankylosis Osteolysis of the AP-SIVA complex; no joint space 8 Fracture Radiolucent line on the caudal or cranial articular processes

With ultrasonography, loss of the joint space because of periarticular osteophytes and dorsal periarticular proliferation (types 4 and 6) can be detected. This procedure helps determine whether the proliferation is symmetric or, if not, which side is most affected.

Treatment and management include periarticular ultrasonographic-guided injections of steroids, mesotherapy over the painful area, and rehabilitation using tolerated exercises after progressive warm-up at a slow canter.

Last full review/revision March 2012 by Stephen B. Adams, DVM, MS, DACVS; Andrew L. Crawford, BVetMed, CertES (Orth), MRCVS; James K. Belknap, DVM, PhD, DACVS; Jane C. Boswell, MA, VetMB, CertVA, CertES (Orth), DECVS, MRCVS; Peter Clegg, MA, Vet MB, PhD, CertEO, DECVS, MRCVS; Marcus J. Head, BVetMed, MRCVS; C. Wayne McIlwraith, BVSc, PhD, DSc, FRCVS, DACVS; James Schumacher, DVM, MS, DACVS, MRCVS; John Schumacher, DVM, MS, DACVS, MRCVS; Roger K. W. Smith, MA, VetMB, PhD, DEO, DECVS, MRCVS; Chris Whitton, BVSc, FACVSc, PhD; Jean-Marie Denoix, DVM, PhD, Agregé; Joerg A. Auer, DrMedVet, Dr h c, MS, DACVS, DECVS