Page 362 - Adams and Stashak's Lameness in Horses, 7th Edition

328 Chapter 3

Articular Cartilage and Subchondral Bone Surface endochondral ossification. Ossification radiates out
toward the articular surface so that articular cartilage of
With ultrasonography, reflections of the sound beam
VetBooks.ir occur at interfaces between tissues of different material with the subchondral bone irregular. A more distinct
immature joints can appear thickened and the interface
densities. Articular cartilage is predominantly water
cartilage–subchondral bone interface develops with
producing a sonolucent (anechoic to hypoechoic) image
between the echogenic joint capsule/synovium and sub­ maturity. Thus, measurements of articular/epiphyseal
cartilage thickness in foals may vary greatly with age and
chondral bone; however, the distinction between joint possibly with the joint involved. In neonates, articular/epi­
fluid and cartilage cannot always be appreciated. The physeal cartilage thickness on the medial trochlear ridge
ultrasound beam cannot pass through the bone, so the of the femur can be as much as 25 mm; the articular car­
subchondral bone interface is seen as a dense, hypere­ tilage is about 1 mm thick, and the remaining thickness is
choic line that follows the normal contour of the joint unossified epiphyseal cartilage, although the differentia­
surface. Normally, there are distinct soft tissue–cartilage tion between the two types of cartilage is not appreciable
and cartilage–bone interfaces. When synovial fluid is ultrasonographically. The thickness of articular cartilage
31
seen between the synovial membrane and the cartilage can be quite variable depending on the stage of devel­
surface, the interface between the fluid and the cartilage opment and the amount of calcification that exists. As
may create a thin, echogenic line. 17 epiphyseal ossification continues, the thickness of carti­
To accurately define the thickness and to detect lage covering the epiphyseal bone decreases. 38,39
changes in surface and internal characteristics of carti­
lage, the sound beam must be perpendicular to the carti­
lage surface. Normal articular cartilage thickness in Osteochondral Lesions
horses remains to be well defined in all but a few joints. A Ultrasonography complements radiography in the
recent study in healthy adult horses showed that the mean early diagnosis of osteochondrosis in growing animals.
cartilage thickness on the lateral and medial trochlear In young horses, articular cartilage lesions associated
ridges of the talus and on the distal intermediate ridge of with osteochondrosis can cause significant joint effusion,
97
the tibia was 0.57, 0.58, and 0.70 mm, respectively. The and in many cases, there are thickened cartilage or
trochlear ridges of the talus were chosen in that study osteochondral fragments either in situ or free‐floating
because of the high incidence of osteochondrosis and the within the joint. Areas of thickened cartilage or osteo­
potential for cartilage thickening at these sites. Fetlock chondral fragment(s) are often apparent over an irregu­
cartilage thickness was found to be thicker proximally lar subchondral bone surface (Figure 3.115). This
(0.8 mm) more than distally (0.4 mm). In humans, carti­ irregular surface is thought to be associated with a con­
23
lage thickness varies somewhat between joints and even tinuum of changes that occur in the development of
within a particular joint. For example, in the human knee osteochondrosis lesions starting with a linear fissure or
the cartilage thickness on the femoral condyles normally fold formation in the thickened articular cartilage that
ranges from 1.2 to 1.9 mm, with cartilage thickness on can progress to an undermined cartilage flap with lysis
the medial condyle being less than that on the intercondy­ of the underlying bone. 68,69 Developmental abnormali­
lar notch and the lateral condyle. From the limited num­ ties in the articular cartilage and subchondral bone can
2
ber of equine studies and based on clinical impression, occur bilaterally, so use of the contralateral limb for
there is also variation in cartilage thickness both between comparison may be misleading.
and within joints in horses. Compare, for example, the Clinical signs often precede radiographic evidence of
difference in articular cartilage thickness between the lat­ osteochondrosis in young horses. Radiographic assess­
eral and medial trochlear ridges of the femur in the nor­ ment of osteochondrosis lesions commonly underesti­
mal femoropatellar joint. The cartilage is thinner on the mates the extent of the articular lesion when compared
medial trochlear ridge; this is normal and should not be with what is seen arthroscopically. Ultrasonography has
mistaken for a pathologic process. proven to be a very useful imaging technique to evaluate
the cartilage/subchondral surface and is indicated in any
Normal Variations young horse with joint effusion suspected of having
Articular cartilage has a fairly uniform thickness and a osteochondrosis. Serial examination of affected joints
smooth contour. There are however a couple of areas where can be useful when deciding whether surgery is neces­
cartilage thickness and contour vary normally. Synovial sary. Progression of an osteochondrosis lesion is a good
fossae occur in some joints being bilaterally symmetrical indicator for surgical debridement. When serial exami­
and most often located in a non‐weight‐bearing part of the nation demonstrates a static osteochondrosis lesion or
articular surface (e.g. humeroradial, tarsocrural, and cox­ even progression to a normal cartilage–subchondral
ofemoral joints). If an area is found that is of concern, the bone surface, surgery may not be indicated.
same site in the opposite limb should be evaluated for
comparison (bearing in mind that some developmental Osteoarthritis
lesions often occur bilaterally). The articular cartilage at
the perimeter of a concave epiphysis and the cartilage at the Osteoarthritis manifests as thinning and erosion of the
center of a convex epiphysis are thickest. 55 articular cartilage and progresses to osteophyte forma­
tion and periarticular soft tissue changes such as enthesis
Immature Cartilage and Bone new bone at the joint capsule attachments. In the horse
early cartilage thinning has been difficult to identify ultra­
Epiphyseal cartilage ossifies from a secondary ossifi­ sonographically. More advanced stages of the disease
cation center, and the carpal and tarsal bones ossify by may be easier to demonstrate in the high‐motion joints