A few surgeons have been able to report clinical results and cumulative survival rates after UKA to match those of total replacement, but the general opinion, led by National Registers, is that the failure rate of UKA is not only much higher than TKA; but also is unacceptably high. If the failure rate is so high, why should surgeons bother with UKA? It may, of course, offend one’s sense of economy to replace more of a damaged joint than is necessary, but there are more practical reasons as well. The function following UKA tends to be better than following TKA; successful UKA is even more effective than successful TKA. Many surgeons who have performed both procedures have found that the range of flexion is greater and gait is more nearly normal, particularly with demanding activities like stair descent, because the biomechanics of the knee are more completely restored (Laurencin et al., 1991; Rougraff et al., 1991).
However, it is on the grounds of safety, with reduced morbidity and mortality, that unicompartmental replacement most strongly recommends itself. To examine rare events such as mortality, large data sets are necessary. As unicompartmental replacement tends to be used more in younger active patients than total knee replacement, it is essential that patients are carefully matched so as to achieve a fair comparision. Based on data from the National Joint Register of England and Wales (NJR) and other large data sets, 25,000 UKA were matched with 75,000 TKA (Liddle et al., 2014). While the revision rate of UKA was 2.4 times higher at eight years than TKA, there were many advantages of UKA. The hospital stay was shorter and readmission within one year was less. The incidence of major medical complications such as myocardial infarction, stroke, thromboembolism and deep infection was about half and the death rate was lower. During the first thirty days post-operation, the death rate was about one quarter, and even out to eight years it was 13% less. If 100 patients had a unicompartmental knee rather than a total, over an eight-year period, one life would be saved at the expense of three revisions. On the basis of these results, Cobb concluded that UKA is “unequivocally safer” than TKA (Cobb, 2014). Even taking into account the higher revision rate, UKA is still more cost effective than the TKA option (Andrews et al., 2014; Robertsson et al., 1999). A large study by Willis-Owen et al. (2009) showed nearly 50% of knees presenting with end-stage arthritis are suitable for a UKA and UKA offers a substantial cost saving over TKA (£1761 per knee).
Revision tends to be easier after UKA than TKA as it usually involves a simple conversion to a TKA. The results of revisions of UKA are better than those of revised TKAs and nearly as good as those of primary TKA (Robertsson et al., 1999). As a result, the threshold for revision of UKA is lower than that of TKA. Following a UKA, about 60% of patients with very poor results have revisions whereas only about 10% of TKA with similarly poor results have revisions (Goodfellow, O’Connor & Murray, 2010). Therefore, even though UKA tend to have fewer poor results than TKA, they have a higher revision rate. If the possibility to rectify a problem following a joint replacement can be considered to be an advantage, the higher revision rate of UKA, which is a manifestation of its ease of revision, should not be considered to be a disadvantage.
