Current analysis of unicondylar knee replacements
(UKRs) by national registries is based on the pooled results of medial
and lateral implants. Consequently, little is known about the differential
performance of medial and lateral replacements and the influence
of each implant type within these pooled analyses. Using data from
the National Joint Registry for England and Wales (NJR) we aimed
to determine the proportion of UKRs implanted on the lateral side
of the knee, and their survival and reason for failure compared
with medial UKRs. By combining information on the side of operation
with component details held on the NJR, we were able to determine
implant laterality (medial versus lateral) for
32 847 of the 35 624 unicondylar replacements (92%) registered before
December 2010. Of these, 2052 (6%) were inserted on the lateral
side of the knee. The rates of survival at five years were 93.1%
(95% confidence interval (CI) 92.7 to 93.5) for medial and 93.0%
(95% CI 91.1 to 94.9) for lateral UKRs (p = 0.49). The rates of
failure remained equivalent after adjusting for patient age, gender,
American Society of Anesthesiologists (ASA) grade, indication for
surgery and implant design using Cox’s proportional hazards method
(hazard ratio for lateral relative to medial replacement = 0.88
(95% CI 0.69 to 1.13); p = 0.32). Aseptic loosening/lysis and unexplained
pain were the main reasons for revision in both groups, although
the reasons did vary depending on whether a mobile- or a fixed-bearing
design was used. At a maximum of eight years the mid-term survival
rates of medial and lateral UKRs are similar.