Three adults with severe longitudinal deficiency of the tibia (LDT), in which an unossified proximal tibial anlage was present, who had been treated with proximal tibiofibular bifurcation synostosis (PTFBS) in early childhood, were evaluated between 20 and 31 years after the index procedure. All three were found to be functioning well as below-the-knee (BK) amputees. Mediolateral stability and anteroposterior instability of the knee were present in all cases. Instrumented motion analysis revealed diminished loading characteristics of the prosthetic limb, similar to that described for BK amputees in general. The most significant gait deviations at the knee unique to this study group were a quadriceps-avoidance gait pattern and an increased dynamic varus alignment. Instrumented muscle testing suggested that these deviations were a consequence of ligamentous instability. This study supports the concept that the presence of a proximal tibial anlage in severe LDT is indication for a surgical strategy that preserves the biological knee joint. The PTFBS maintains the integrity of the knee-extensor mechanism, the fibular collateral ligament, the tibiofemoral joint capsule, and the medial collateral ligament, enhancing the long-term stability and function of the knee joint.