120 patients undergoing total knee arthroplasty who had a BMI over 30 kg/m2 were included in this trial to determine the effectiveness of using a stemmed modular tibial implant in comparison to a standard tibial modular implant in obese patients. Randomization into the two group was stratified by the severity of obesity (moderate and severe). Patients were followed up over a two year period for pain and functional outcomes (KSS, KOOS, and VAS Pain). The results indicated that the use of a stemmed modular tibial implant was associated with significantly higher function and lower pain at 2 years postoperatively when compared to a standard modular tibial implant. No significant differences were observed for the incidence of revision or radiolucency of the tibial component within the first 2 years postoperatively. The significant differences observed in this study are not likely to be clinically important as they failed to meet minimum clinically important differences for the patient report outcomes.