Page 234 - Adams and Stashak's Lameness in Horses, 7th Edition

200 Chapter 3

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Figure 3.13. Lateromedial projection of the front foot with
irregular osseous proliferation along the dorsal distal aspect of the
middle phalanx (arrow), consistent with enthesopathy of the distal
interphalangeal joint capsule. On magnetic resonance imaging of
this region, marked diffuse articular cartilage damage was
Figure 3.12. Lateromedial projection of the tarsus. Multiple,
small gas opacities are visible within the proximal aspect of the visualized within the joint.
tarsocrural joint (arrows). This horse was subsequently diagnosed
with septic arthritis of the joint, and the presence of intra‐articular cortical bone), sclerosis, fragmentation, and adjacent
gas was likely a result of anaerobic bacterial infection. Degenerative soft tissue mineralization. Enthesopathy can provide
changes within the distal tarsal joints were an incidental finding. indirect evidence of other pathologic processes such as
joint disease or ligamentous or tendinous injury.
wounds, needle centesis, or gas‐producing bacterial
organisms. Radiographic signs of soft tissue emphysema Radiology of Bone
include radiolucent regions within soft tissue structures
(the radiolucencies should be differentiated from fat) Fundamental Patterns of Bone Response
and a focal accumulation of gas with an air–fluid level The response of bone to different pathologic pro­
that occurs with abscesses. cesses is limited and consists in one or a combination of
Localization of intra‐articular gas is important diag­ the following processes: new bone production/forma­
nostically and prognostically. When gas is identified in a tion, bone resorption/destruction, or shape alteration.
joint (Figure 3.12), there are three primary causes: (1) Bone remodeling and bone modeling are two terms that
iatrogenic, incidental gas due to synovial centesis, (2) are often used interchangeably when referring to muscu­
septic arthritis, and (3) “vacuum phenomenon” (non­ loskeletal radiology. Physiologically, osteoclastic and
clinically significant) gas that occurs when a joint is osteoblastic activities occur in both processes.
flexed. 20,54 In rare cases, idiopathic gas is found in mark­ During the bone remodeling process, the osteoblasts
edly osteoarthritic joints; in the authors’ experience, this and osteoclasts are coupled and act together; therefore,
primarily occurs in the carpus and is of uncertain origin. bone resorption and formation occur at the same time
Because of the high clinical importance of diagnosing and at the same site on a bone surface. On the other
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septic arthritis, it is preferable to obtain radiographs of hand, during the bone modeling process, the bone alter­
a joint suspected of being septic prior to synovial cente­ ations result from independent actions from osteoclasts
sis to reduce confounding factors for interpreting sources and osteoblasts. This means that the bone resorption
of intra‐articular gas. and formation may occur on different sites. In addition,
modeling may cause large changes in bone structure,
Entheses whereas remodeling will replace bone, maintaining the
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current amount of bone structure. Therefore, the cor­
An enthesis is a point in the bone at which a soft tis­ rect term for describing visible bone structural changes
sue structure attaches. The soft tissues involved can is usually modeling rather than remodeling.
include tendons, ligaments, or joint capsule. A patho­ Radiographically, new bone production is frequently
logic change at these sites is known as enthesopathy and manifested as a periosteal or endosteal reaction (adja­
can be secondary to many disorders, but most com­ cent to the cortex), new bone at periarticular margins or
monly includes trauma or degenerative or inflammatory entheses, or increased bone opacity (sclerosis). Areas of
conditions and may be intra‐ or extra‐articular bone destruction are seen as bone lysis (aggressive bone
(Figure 3.13). In cases of an acute traumatic event, an lesions or osteoarthritis [OA]) or decreased bone opac­
avulsion fracture at the enthesis may occur. Radiographic ity (osteopenia). Pathologic shape alteration is com­
changes associated with enthesopathy include bone ero­ monly seen radiographically in, but not limited to,
sion, bone proliferation or hyperostosis (thickening of skeletally immature horses, usually secondary to physeal