INTRODUCTIONUnicondylar knee arthroplasty (UKA) is a surgical procedure whose popularity has grown these last years after good-to-excellent long-term results have been constantly reported. [1][2][3][4] It is favored to total knee arthroplasty because of its lesser invasive nature, quick recovery, and more anatomic post-operative kinematic results. [5][6][7] Despite lateral compartment applications having gained popularity recently, UKA is still primarily performed on the medial side. [3] Furthermore, many studies have previously analyzed the radiological changes taking place on the adjacent bone structures after the procedure. [8][9][10] Increased bone strain is observed on the medial tibial metaphysis after a unicondylar arthroplasty. [8][9][10][11][12] The medial tibial metaphysis has been shown to get overloaded due to the loss of subchondral bone and remodeling is expected to follow to accommodate the new metallic structures. [9,10] However many of these studies, were either finite-element analyses or cadaveric studies, and radiological studies in real patients are still lacking.The purpose of this study is to radiologically investigate the effects of the remodeling process in the medial tibial metaphysis after a unicondylar replacement, by measuring the cortical thickness of the proximal tibial metaphysis and establishing a new ratio parameter.Although not yet radiographically proven, it has frequently been stipulated that overloading of the new subchondral bone should lead to some degree of remodeling which itself may present as change in the cortical thickness or the increase in sclerosis Objective: Unicondylar knee arthroplasty (UKA) is a surgical procedure primarily used for the resurfacing of the medial compartment and many studies have previously analyzed the changes taking place on the surrounding osseous tissues after the procedure. The purpose of this study is to investigate the effects of bone strain in the medial tibial metaphysis after a cementless unicondylar replacement.Methods: Patients treated with a cementless UKA between March 2015 and March 2019 was selected for this study. Inclusion criteria were a minimum of 1 and a maximum of 2 years follow-up and presence of standard radiographs of the operated knee at yearly intervals. A total of 109 patients were included in the study. Two lines at a distance of 5 and 7 cm from the lateral tibial eminence were horizontally drawn and the medial cortical thickness and the total cortical distance were measured. A cortex-to-metaphysis (CTM) ratio was established. The increase of density in the metaphysis was analyzed though the "reactive triangle" phenomenon. We initially hypothesized that some degree of increase in sclerosis would be detected in the medial tibial metaphysis and that the increase would be greater in patients with an implant underhanging, since more cancellous bone would come under strain. Results:The CTM ratio showed a decreasing pattern during the 1 st post-operative year for all measured levels. An increase in density at the metaphysea...