The patient lies supine on the X-ray couch with a support under the knee to flex it 20°. The X-ray beam is aligned 10° from the vertical (to allow for the average posterior inclination of the tibial plateau so it is parallel to the joint surfaces). The surgeon (wearing protective gloves and apron) applies a firm valgus couple of forces through the knee, while ensuring that the leg is in neutral rotation. Alternatively, a device can be used by a radiographer to apply stress. The radiographs should be examined to ensure they are of adequate quality. The radiograph should show the joint surfaces end on and the patella should be approximately central. If the quality is poor they should be repeated.
Figure 4.8 Technique of varus/valgus stress radiography.
Interpretation
1. The radiolucent joint space between the subchondral plates of the lateral compartment should measure not less than 4 mm, the sum of the thickness of two layers of normal cartilage (Fig. 4.9(b)). A gap of less than 4 mm implies thinning of the cartilage and its impending failure. It is a contraindication to UKA.
2. If the damaged medial condyles have separated to reveal a radiolucent gap of at least 4 mm (previously occupied by articular cartilage), the intra-articular deformity known as genu varum is fully correctable and the MCL is not shortened. This gap may be greater than 5 mm, depending on how much bone, as well as cartilage, has been lost (see Fig. 4.2(f)).
We do not attempt to assess the overall alignment of the leg on stress radiographs as small degrees of rotation can give a false impression. The overall alignment is not important and as long as the intra-articular deformity is correctable we will proceed with UKA.
Varus-stressed radiographs
Varus-stressed radiographs are the most reliable radiographic method for demonstrating full-thickness loss of cartilage (bone-on-bone contact) between the medial femoral and tibial condyles. As mentioned above, the commonly employed weight-bearing projections are often adequate, and if they are available and positive there is no need of further evidence. However, if there is doubt, it is best resolved by employing the technique described above, but with a varus force applied (Fig. 4.9(a)). If this does not demonstrate bone on bone, arthroscopy can be undertaken to see if there is eburnated bone on both surfaces.
Failure to demonstrate bone-on-bone contact is, we believe, a contraindication to joint replacement. Mere thinning of the cartilage and surface fibrillation with marginal osteophytes (Ahlback stage 1) (Ahlback, 1968) is a dubious explanation for disabling pain. If eburnated bone-on-bone contact cannot be demonstrated, other causes for the pain should be sought (for example, hip OA or spine pathology).
