One of the most common predilection sites for OC in dogs is the caudal humeral head. OC is an abnormality of endochondral ossification in which the cartilage of the epiphysis fails to form subchondral bone. This results in thickened, abnormal cartilage which is susceptible to injury. OCD is the form of OC in which the articular
ML view of the shoulder, showing a small OC lesion with adjacent sclerosis involving the caudal aspect of the humeral head.
12.30 cartilage is fissured and forms a cartilage flap. In most
dogs this flap will remain within its subchondral bed, hindering ingrowth of granulation tissue. In some cases, this cartilage flap dislodges and may either be resorbed, remain a free body (‘joint mouse’) or grow in size, nour-ished by the synovial fluid, to eventually ossify. A joint mouse may cause lameness by intermittently becoming interposed between articulating surfaces or, if small enough, by entering and becoming lodged in the biceps tendon sheath (Figure 12.27). In most cases, however, the free flap will migrate towards the caudal pouch of the
Joint mice in the distal bicipital tendon sheath. (a) ML view.
(b) CdCr view. Mineralized opacities within the supraspinatus muscle are also visible (arrowed).
12.27
(a)
(b)
ML view of the shoulder of a dog showing a radiopaque joint mouse within the subscapular bursa (arrowed), which is an uncommon finding.
12.29 Congenital medial shoulder luxation in a 5-month-old Cavalier
ing Charles Spaniel. (a) ML view. (b) CdCr view. Note the flattened, abnormal form of the glenoid cavity.
12.26
(a) (b)
ML view of a radiopaque joint mouse (arrowed) within the caudal pouch of the shoulder joint. The subchondral defect and OCD at the caudal aspect of the humeral head are clearly visible.
12.28
shoulder joint (Figure 12.28) and rarely migrate to the subscapular bursa (Figure 12.29). Often these joint mice become overgrown by synovium, which can be observed arthroscopically. The final result of shoulder OCD, even when treated correctly, may be secondary osteoarthrosis. Despite apparent lameness in only one limb, both shoulders should be radiographed because often there will be bilateral lesions. It is also advisable to evaluate the hip joints for signs of hip dysplasia because this is often seen concurrently with shoulder OC.
The most common radiological finding of shoulder OC lesions is a flattening or an irregular, radiolucent sub-chondral defect of variable size involving the caudal aspect of the humeral head (Figures 12.30 and 12.31). This is
ML view of a classical shoulder OC lesion flattening and an irregular radiolucent subchondral defect involving the caudal aspect of the humeral head can be seen.
12.31
ML view of the shoulder showing an OCD calcified cartilage flap overlying a subchondral defect.
12.32
caused by thickening of the articular cartilage in that area.
If there is calcification of a cartilage flap, it is usually seen overlying the subchondral defect (Figure 12.32), although it may be seen elsewhere if detached (see Figure 12.28).
Additional findings may include subchondral sclerosis surrounding the defect (see Figures 12.30 and 12.31) and osteoarthrosis in chronic cases (see Figure 12.29 and Figure 12.33). In 20% of cases a vacuum phenomenon can be seen (Figure 12.33; see also Chapter 11).
Although a correlation has been demonstrated between the size of the subchondral defect, the presence of a vacuum phenomenon and the status of the overlying artic-ular cartilage and the associated clinical signs, plain film radiographs cannot be used to assess the status of the articular cartilage or the presence of radiolucent joint mice.
Positive-contrast arthrography helps to assess whether a non-mineralized cartilage flap is present, with an accuracy of 80%, and this finding helps to determine whether a dog should be treated conservatively or surgically. Using positive-contrast arthrography, the status of the articular cartilage covering the subchondral defect can be evaluated and classified to determine treatment options.
• Contrast medium going underneath the cartilage usually correlates with signs of pain and lameness (Figure 12.34), and the joint should be surgically or arthroscopically treated.
• The detection of thick cartilage covering the subchondral defect (Figure 12.35) or a detached
(a) ML view of an osteochondrotic shoulder joint with a vacuum phenomenon; the articular cartilage is seen as a grey line superimposed on the subchondral bone. A joint mouse within the bicipital tendon sheath is visible (arrowed), as well as arthritic changes. (b) Close-up view.
12.33
(a)
(b)
ML shoulder view of a positive-contrast arthrogram of a clinical OCD lesion. Contrast medium is seen underneath the cartilage, which correlates with signs of pain and lameness. Artefactual air bubbles, mimicking small joint mice (arrowed), are also present. They have a round and smooth appearance, in contrast to real joint mice (see Figures 12.38 and 12.39).
12.34
ML shoulder view of a positive-contrast arthrogram of a non-clinical OC lesion. Thick cartilage is covering the subchondral defect with no contrast medium visible underneath the cartilage.
12.35
ML shoulder view of a positive-contrast arthrogram showing a detached cartilage flap lodged in the caudal pouch of the joint capsule (arrowed). In most cases, such a finding is associated with no clinical signs and can be left untreated.
12.36
ML shoulder view of a positive-contrast arthrogram showing a detached cartilage flap migrating towards the caudal pouch. Notice also the joint effusion.
Such dogs are presented with an acute severe lameness.
12.37
ML view of a positive-contrast arthrogram with a joint mouse in the bicipital tendon sheath. This large joint mouse hinders the mechanical action of the biceps tendon and should be removed.
Notice also that the cartilage flap looks fragmented (arrowed), meaning that smaller cartilage fragments may break off and form joint mice.
12.38
ML shoulder view of a positive-contrast arthrogram showing a joint mouse in the bicipital tendon sheath within a larger pouch (arrowed). This joint mouse is not hindering the biceps tendon and can be left untreated.
12.39 cartilage flap lodged in the caudal pouch of the joint
capsule (Figure 12.36) is associated in most cases with lack of clinical signs, and such a lesion can usually be left untreated.
• In some dogs with acute severe lameness a detached cartilage flap migrating towards the caudal pouch can be detected (Figure 12.37).
Detached flaps in the caudal pouch may migrate cranially to interfere with joint function or enter the bici pital tendon sheath. About 10% of joint mice are located here. Not all these joint mice are calcified and they are therefore not always visible on plain film radio-graphs. Arthrography can be used to visualize joint mice in the bicipital tendon sheath and this helps in deciding whether they should be removed (Figure 12.38). The bicipital tendon sheath may have large pouches, and when a joint mouse is detected within such a pouch, it can usually be left untreated (Figure 12.39). On the other hand, if a joint mouse is hindering the mechanical action of the biceps tendon it should be removed.
OC lesions may occur in the glenoid cavity. They are usually not visible on survey radiographs. CT and arthro-scopy are necessary to obtain the diagnosis (Figure 12.40).
The results of a study (Vandevelde et al., 2006) com-paring ultrasonography with radiography, arthrography and arthroscopy suggest that all radiologically diagnosed subchondral lesions in the humeral head can be visualized by the use of ultrasonography as a concave deviation of the hyperechoic subchondral bone line with a variable length according to the extent of the lesion. The results also suggest that the presence of a second hyperechoic line at the bottom of the subchondral defect seen on ultra-sonography is a pathognomonic sign for the presence of a flap (Figure 12.41). For a clinician experienced in ultra-sonography this technique can present an alternative to positive-contrast arthrography.
Longitudinal ultrasound image of a clinical OCD lesion. Note the presence of a second hyperechoic line at the bottom of the subchondral defect, which is a pathognomonic sign for the presence of a flap. 1 = joint effusion; 2 = normal articular cartilage cranial of the lesion; 3 = subchondral bed of the OCD lesion; 4 = hyperechoic line representing the detached flap.
12.41