We are frequently sent orthopaedic and spinal radiographs for interpretation, which we offer as a free service to our colleagues in general practice. We thought it would be of interest if we offered some tips about positioning and interpretation so in this email we have distilled our thoughts on 2 of the commonest conditions that we see – cranial cruciate ligament disease and hip dysplasia. We hope you find it useful.
The Stifle
Positioning
craniocaudal view:
- Dorsal recumbency with the leg extended caudally. Use foam wedges or sandbags to elevate the contralateral limb.
The femoral condyles should be of equal size and the patella should be in the midline.
mediolateral view:
- Lateral recumbency with the affected side down. For most patients it is better to pull the contralateral limb cranially rather than caudally to avoid rotation of the stifle joint.
The femoral condyles should be superimposed as oblique views can lead to interpretation errors.
TOP TIP: Supporting the hock and weighing down the foot can help to superimpose the condyles.
Interpretation
Increased synovial mass, either as a result of synovial effusion or increased soft tissue, is an indication of joint disease. Cranial cruciate ligament (CCL) disease is the most frequent cause of synovial effusion. Cranial translation of the tibia may be seen with complete CCL rupture. Osteophytes and enthesiophytes make an appearance within 2 weeks of rupture of the cranial cruciate ligament.
The radiographs below show mediolateral views of the stifles from a dog with left CCL rupture and a normal contralateral stifle joint.
The following are signs of CCL disease:
- Infrapatellar fat pad – a radiolucent triangle caudal to the straight patellar ligament. With synovial effusion the fat pad becomes less distinct, changes shape and is smaller.
- Subgastrocnemius fascial plane – a radiolucent line at the caudal aspect of the stifle cranial to the gastrocnemius muscle. Synovial effusion and capsular thickening cause caudal displacement or obliteration of this radiolucent line.
- Osteophytes – on the trochlear ridges appear as a “tidemark” of increased radiopacity.
- Osteophytes – on the caudal aspect of the tibial plateau.
- Enthesiophytes – on the distal pole of the patella, around the fabellae and at the site of insertion of the cranial cruciate ligament on the tibial plateau.
- Cranial tibial translation – the tibial eminence which normally bisects the femoral condyles is cranially displaced and because the path of the popliteal muscle is shortened the popliteal sesamoid is displaced distally as the tendon becomes less taut.
The Hip
Positioning
extended ventrodorsal view:
- In a trough with the hind limbs fully extended and the femora parallel to each other and parallel to the table top.
- The femora are internally rotated so that the patellae are centred over the trochlear grooves.
- The X-ray beam is centred at the level of the cranial edge of the pubic bone.
abducted “frog-leg” ventrodorsal view:
- As above but with the femora abducted into a frog-legged position with the stifles equidistant from the table top.
- Use foam wedges or sandbags to support the stifles.
With both views the pelvis and hip joints must be symmetrical. The iliac wings and obturator foramina should be of equal size on both sides of the pelvis.
TOP TIP: To correct pelvic rotation elevate the side with a smaller obturator foramen and wider iliac wing.
lateral view:
- The femora should be superimposed and parallel to the table top.
- The left and right hemipelves should be superimposed.
- Use padding under the thorax to prevent rotation.
Interpretation
The radiographs show a VD view of a dog with good hip conformation and a VD view of a dog with hip dysplasia.
Normal hip joint:
- Femoral head – smooth and almost hemispherical.
- Joint margins – in the cranial one-third of the joint should be parallel.
- Dorsal acetabular rim – should cover more than 50% of the femoral head.
- Acetabular notch and acetabular fossa – seen as an indentation in the caudal medial acetabulum
- Fovea capitis – slight flattening of the femoral head at the site of attachment of the ligament of the head of the femur
The following are signs of hip dysplasia:
- Morgan’s line – a curvilinear line of enthesiophytes along the caudolateral margin of the femoral neck is an early sign of osteoarthritis (OA).
- Femoral head – in advanced cases osteophytes encircling the femoral head and are seen at the margins of the acetabulum. In more advanced cases, where there is subluxation and extensive osteophytosis, the femoral head become sclerotic and with remodelling develops an angular shape.
- Joint margins – the joint is incongruent and the joint margins along the cranial one-third of the joint are no longer parallel.
- Dorsal acetabular rim – the centre of the femoral head is not covered by the dorsal acetabular rim.
- Cats – hip dysplasia manifests as shallow acetabula and there is less remodelling of the femoral head. The predilection site for osteophytes is the cranial border of the acetabulum
