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 ligament retaining unconstrained unicompartmental knee arthroplasty (UKA) can affect the ligament forces and, hence, the knee laxity. In the present study, a sagittal plane mathematical model of the knee with intact ligaments and unconstrained prosthetic components is used to analyze antero-posterior (AP) knee laxity during passive flexion at different force levels. Also, the effects of errors in component placement are evaluated. The model calculations show a reasonable agreement with the experimental observations reported in literature. The results show that the AP laxity during 0°120° flexion first increases from 0° to about 30°, remains nearly constant for another 10° and then decreases somewhat linearly for higher flexion angles. Some errors in the placement of femoral component of the order of 1 mm can affect the knee laxity by nearly 3 mm in some flexion positions. The analysis has clinical relevance and suggests that the UKA requires close attention to component placement. © World Scientific Publishing Company.