Patient selection for cementless knee arthroplasty (TKA) remains challenging. 106 consecutive TKAs performed with a cementless porous tantalum tibial component of PS-design were reviewed. Nine failures (8 male/1 female) occurred at a mean 18 months and demonstrated a characteristic failure mode of medial tibial collapse. The mean height of 72.5 inches in the failure group was greater than the 65.8 inches in the well-functioning group (P < 0.001). The mean weight of 260.4 lb in the failure group was greater than the 204.1 lb in the well-functioning group (P = 0.006). The high early failure rate occurred predominantly in tall, heavy, male patients and may be related to medial tibial overload resulting from increased flexibility of the implant in this patient type.