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Aims The aim of this study was to determine the risk of tibial eminence avulsion intraoperatively for bi-unicondylar knee arthroplasty (Bi-UKA), with consideration of the effect of implant positioning, overstuffing, and sex, compared to the risk for isolated medial unicondylar knee arthroplasty (UKA-M) and bicruciate-retaining total knee arthroplasty (BCR-TKA). Methods Two experimentally validated finite element models of tibia were implanted with UKA-M, Bi-UKA, and BCR-TKA. Intraoperative loads were applied through the condyles, anterior cruciate ligament (ACL), medial collateral ligament (MCL), and lateral collateral ligament (LCL), and the risk of fracture (ROF) was evaluated in the spine as the ratio of the 95th percentile maximum principal elastic strains over the tensile yield strain of proximal tibial bone. Results Peak tensile strains occurred on the anterior portion of the medial sagittal cut in all simulations. Lateral translation of the medial implant in Bi-UKA had the largest increase in ROF of any of the implant positions (43%). Overstuffing the joint by 2 mm had a much larger effect, resulting in a six-fold increase in ROF. Bi-UKA had ~10% increased ROF compared to UKA-M for both the male and female models, although the smaller, less dense female model had a 1.4 times greater ROF compared to the male model. Removal of anterior bone akin to BCR-TKA doubled ROF compared to Bi-UKA. Conclusion Tibial eminence avulsion fracture has a similar risk associated with Bi-UKA to UKA-M. The risk is higher for smaller and less dense tibiae. To minimize risk, it is most important to avoid overstuffing the joint, followed by correctly positioning the medial implant, taking care not to narrow the bone island anteriorly. Cite this article: Bone Joint Res 2022;11(8):575–584.

PurposeRadiolucencies are commonly observed in unicompartmental knee arthroplasty (UKA) patients within 1 year of arthroplasty. The objective of the study was to identify how the bone mineral density (BMD) changes up to 1 year post-arthroplasty.MethodsDual X-ray absorptiometry scans were obtained from 11 UKA patients at 10 days and 3, 6, and 12 months post-surgery. Patients were scanned in both anteroposterior and lateral knee orientations.ResultsMost subjects saw a large decline in BMD in the first 6 months following surgery, followed by some recovery in bone mass. The biggest change occurred under the tibial intercondylar eminence, which decreased significantly by an average of 18 % at 6 months and was 15 % at 1 year. The average bone loss under the tibial tray was low; however, the bone loss at the anterior portion was higher with a significant average decrease of 14 %. There was no change in BMD under the tibial keel. There was significant bone loss of 13 % under the femoral component; the regions anterior and posterior to the central femoral implant peg both had significant bone loss of 14 %. The bone response between patients was very variable, with some patients losing bone steadily, and others gaining it rapidly after an early fall.ConclusionsWhile the overall reduction in BMD under both components was low, it was significant and there was substantial individual variation superimposed on this. Improving our understanding of this response to surgery may impact on prosthesis survival.Level of evidenceTherapeutic study: case series with no comparison group, Level IV.

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