Over the years we have gained a good insight into why loosening of cemented components occurs and how it can be prevented. It would seem factors causing loosening of cementless components are different and, as cementless loosening is rare, we have little insight into this. Loosening is one of the commonest causes of failure in the national registers. In the NJR, the loosening rate is 4.01 (CI 3.73 – 4.32) per 1000 patient years (The NJR Board, 2014). It is much less common in the published series. For example, our series of 1000 cemented OUKA with up to 15 years follow-up, we have encountered one case each of femoral and tibial loosening (Pandit et al, 2015). A possible reason for the high rate seen in registers relates to misdiagnosis of physiological radiolucencies (see Chapter 9). The incidence of loosening of cementless components is less than cemented in the registries.
Diagnosis
In OUKA, the only reliable radiographic evidence for loosening of a metal component is its displacement. For example, a loose tibial component may tilt (Fig. 11.7) or a femoral component rotate (Fig. 11.8). As has been discussed elsewhere, stable radiolucencies are very common at the bone–cement interfaces and are not evidence of loosening. Displacement is diagnosed by comparing two radiographs taken with a time interval between them; however, small changes in position can only be detected if the X-ray beam was aligned, on both occasions, in the same relation to one of the components. The required accuracy can only be achieved if both radiographs were taken with the beam aligned by fluoroscopy or by carefully adjusting it while taking multiple low dose images with a digital system (Fig. 11.7).
Figure 11.7 (a) Aligned radiograph showing a thick poorly defined pathological radiolucency around the tibial component. Comparison with a similarly aligned post operative radiograph (b) shows that the tibial component has tipped and is therefore loose.
Figure 11.8 (a) This (fluoroscopically aligned) radiograph was misinterpreted as evidence of loosening of the tibial component. However, the radiolucencies beneath that component are thin and defined by a radiodense line, suggesting they are stable and benign. (b) Careful examination of the lateral radiograph shows a possible pathological radiolucency around the peg and posterior part of the femoral component. When the AP radiograph is compared with the (similarly fluoroscopically aligned) post-operative radiograph (c) it can be seen that there is displacement of the femoral component but the tibial component has not moved. At the revision operation, the tibial component was found to be firmly fixed and the femoral component was loose.
Misinterpretation of the ‘physiological’ lucencies, so commonly seen on aligned radiographs, has resulted in unnecessary revision by surgeons unfamiliar with their benign nature. Femoral component loosening is difficult to diagnose because the pathological radiolucencies can be difficult to see (Fig. 11.8). If it is suspected, it is recommended that lateral X-rays are taken in flexion and extension (Monk et al, 2009) (Fig. 11.9). These may show evidence of movement of the femoral component which is diagnostic of loosening.
Bone scans can be very misleading so it is recommended that they are not used. Typically, whether there is loosening or not, there is increased uptake under the tibial component, presumably due to remodelling, which persists for many years. On the femoral side, there is usually no increased uptake. The bone scan will be reported as indicative of tibial loosening. We are aware of a surgeon who revised an OUKA on the basis of a typical scan appearance and a report suggesting a loose tibial component and a secure femoral component, and found at operation, a secure tibia and loose femur.
Occasionally, dislocation of the bearing draws attention to a loose component.
Figure 11.9 Flexion and extension radiographs with magnified views showing the femoral component moving which is diagnostic of loosening.
Causes
Early failures are probably the result of poor initial fixation. Immediate postoperative radiographs and retrieved specimens have often revealed unsatisfactory cementing of the tibial component. In addition, the tibial component is often too small and does not reach the posterior cortex. We have had several reports of sudden displacement of the femoral component in the immediate postoperative period, but usually it occurs a few years later and is associated with clunking, attributable (in retrospect) to poor initial fixation of its posterior facet and lack of cement in the 6 mm hole. It is hoped that use of the two-peg femoral component will decrease the incidence of femoral loosening. Some evidence from the retrieval studies suggests that late tibial loosening may be due to the accumulated effects of impact loading from impingement of the front of the bearing on the femoral condyle in full extension (Psychoyios et al, 1998).
Treatment
In early loosening, if the bone has not been seriously eroded, cementing a new component is a possible option and has been successful on several occasions.
However, in late loosening, the bone will already be more extensively damaged and revision to TKA is better undertaken immediately.
