Passive knee laxity is an important clinical measure to assess function after joint replacement. Clinical observations suggest that the use of minimally invasive surgical techniques in knee arthroplasty may affect the surgeon's ability to orient and position the prosthetic components accurately. Further, recent studies suggest that malplaced prosthetic components in unicompartmental knee replacement (UKR) can affect the ligament forces and, hence, the knee laxity. In the present study, a sagittal plane mathematical model of the knee with unicompartmental replacement is used to analyze the passive antero-posterior (A-P) laxity during flexion using different force levels. Also, the effects of errors in component position are evaluated. The results show that for all force levels, the A-P laxity first increases from 0 to about 30 flexion, remains nearly constant for another 10 and then decreases somewhat linearly for higher flexion angles. Changes in the position of the femoral component by 1mm can affect the knee laxity by 2mm or more in some flexion positions. The analysis has clinical relevance and suggests that the UKR requires close attention to component placement. © 2011 IEEE.