Abnormal loading and functional deficits are present in both limbs before and after unilateral knee arthroplasty.

•Joint loading and function was assessed bilaterally in unilateral knee OA.•Gait data can be summarised using a functional classification approach.•Gait abnormailities in knee OA and following arthroplasty are relatively symmetrical.•Joint loading and function frequently remains abnormal following arthroplasty.•Pre-operative function (the Cardiff Classifier) can predict post-operative function.