Why do we not include full-length leg films in our preoperative radiological assessment?
Tibiofemoral varus may be due to intra-articular deformity (genu varum) (Fig. 4.16(b)), extra-articular deformity (tibia vara) (Fig. 4.16(d)), or a combination of both (Fig. 4.16(e)).
Figure 4.16 Diagrams of the knee, with femoral and tibial axes (a) showing the effect of the intra-articular deformity (genu varum) (b) and the correction achieved by OUKA (c). (d, e, and f) show the same occurring in the presence of an extra articular deformity (tibia vara).
1. The genu varum of anteromedial OA is caused by collapse of the medial compartment from loss of its cartilage and subchondral bone. The MCL is not shortened and so varus is completely correctable preoperatively, and completely corrected by OUKA. If the tibiofemoral angle was normal before the onset of arthritis (Fig. 4.16(a)), it will be returned to normal by the operation (Fig. 4.16(c)).
2. Tibia vara is a bone deformity, usually between the tibial plateau and the shaft (Fig. 4.16(d)), and is commonly developmental.
3. Not uncommonly, anteromedial OA develops in a limb with pre-existing extra-articular tibia vara, resulting in an increase in the tibiofemoral varus (Fig. 4.16(e)). This is more commonly seen in Asian patients. In these cases, OUKA corrects the intra-articular genu varum but the tibia vara persists (Fig. 4.16(f)), so that the tibiofemoral angle remains in some degree of varus. (Much less commonly, genu varum develops in a limb with extra-articular valgus, which is commonly in the femur. In this case, OUKA corrects the genu varum and restores whatever degree of tibiofemoral valgus was present before the arthritis developed. This is seen almost exclusively in female patients.)
Therefore the term ‘fully correctable’ refers only to the intra-articular component of alignment deformity. The valgus-stressed radiograph predicts full correction of the intra-articular deformity by demonstrating at least 4 mm separation of the medial femoral condyle from the tibial plateau, i.e. the MCL is not shortened (see Fig. 4.9).
Measurement of the tibiofemoral shaft angle does not distinguish between intra- articular and extra-articular deformity and therefore is not helpful.
Emerson et al. (2002) compared the effects on postoperative leg alignment of employing either a fixed-bearing UKA (Brigham) or an OUKA (Phase 2). They measured alignment on 3-foot anteroposterior radiographs taken with the patient standing. The technique of Kennedy and White (1987) was used to determine the location in the knee through which the mechanical axis passed, and the conventions of the Knee Society were used to measure the angle of alignment. The average preoperative alignment of the two groups was similar, 1.7° and 1.8° varus. After OUKA the average alignment was 5.5° valgus, and after the fixed-bearing UKA it was 2.6° valgus. The variability of alignment after OUKA was less. Figure 4.17 shows that after OUKA the mechanical axis of the limb usually passed through the inter-condylar area of the knee. The authors suggested that the greater degree of valgus achieved by the OUKA might explain the late failures from lateral compartment arthritis that they experienced. However, in a more recent study, Emerson and Higgins (2008) found that following Phase 3 OUKA, fewer patients were in valgus, suggesting perhaps with Phase 3 the incidence of OA progression might be less than with Phase 2. Furthermore the study demonstrated that, following OUKA, the leg alignment was similar to that of the other knee which suggests that predisease alignment was restored.
Hernigou and Deschamps (2004) showed that, after medial UKA, ‘severe undercorrection’ (i.e. hip–knee–ankle angle <170°) caused increased polyethylene wear in the fixed-bearing implants they used; and overcorrection into valgus (>180°) was associated with an increased risk of lateral compartment arthritis. However, their method of measurement could not distinguish between intra- and extra-articular deformity. Intra-articular overcorrection, implying as it does a damaged MCL, has been associated with lateral compartment arthritis after OUKA. However, failure of a meniscal bearing from wear-through has not been recorded, although some limbs have residual extra-articular varus.
Figure 4.17 The mechanical axis of the leg is shown with the tibial surface divided into zones. The zone location of the mechanical axis for the fixed- and mobile-bearing knee implants taken with 3-foot radiographs obtained with the patient standing are shown in graphical form. The tibial zones are according to Kennedy and White 24. (Reproduced with permission of Lippincott Williams & Wilkins from Emerson RH Jr., Hansborough T, Reitman RD, Rosenfeldt W, Higgins LL. Comparison of a mobile with a fixed-bearing unicompartmental knee implant. Clin Orthop 2002; 404: 62–70.)
We believe that no attempt should ever be made to correct an extra-articular varus malalignment intra-articularly, for example by MCL release. On the contrary, that ligament’s fibres should all be carefully preserved, and ‘overstuffing’ of the medial compartment with a bearing that is too thick should be avoided. We have found no differences in clinical outcome between those with varus and normal alignment at the end of the operation (Gulati et al., 2009).
