Background: Early post-operative medial tibial bone loss in both unicompartmental knee replacement and total knee replacement has been reported in our previous studies and many other studies. Significant bone loss can contribute to a tibial stress fracture, bone pain and early implant failure. The bone loss appeared to be greater in total knee replacement. Therefore, the aim of the study is to look for any significant difference in medial tibial bone loss in both unicompartmental knee replacement and total knee replacement in the first 3 years and to investigate the underlying pathophysiology. Methods: Cases of fixed-bearing unicompartmental knee replacement and posterior stabilising total knee replacement performed in 2015–2016 were recruited. The change in medial tibial bone loss (expressed in grayscale Gy) over a three-year post-operative period was measured using the method of digital radiological densitometry. Potential predictors and correlations were analysed. Results: Forty-four cases of unicompartmental knee replacement and 52 cases of total knee replacement were recruited. The cumulative drop in 3 years was 23.3% in unicompartmental knee replacement and 33.7% in total knee replacement, respectively, a difference of up to 10%. The cumulative drop between the two groups at 12 months (p < 0.05) and 36 months (p < 0.05), respectively, were significantly different. Angle correction has not been shown to affect medial tibial bone loss in this study. No surgical complication was documented during the follow-up period. Conclusion: Total knee replacement results in 10% greater medial tibial bone loss than unicompartmental knee replacement at the three-year time. The effect is greatest in the first year. In addition to possible stress shielding, early physiological bone remodelling in response to surgical trauma can contribute to the difference in medial tibial bone loss of unicompartmental knee replacement and total knee replacement. This is supported by the insignificant correlation between angle correction and medial tibial bone loss in the result.

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Radiographic retrospective cohort on medial tibial bone loss for fixed bearing unicompartmental knee arthroplasty and total knee arthroplasty at a three-year period