The use of modern technology in knee arthroplasty, like minimally invasive surgery and computer assisted systems, can help in standardization of surgical procedures and improve patient recovery. However, such procedures can affect the surgeon's ability to properly place and orient the prosthetic components on the bones. In the present study, the effects of surgical placement of components on mechanics of the joint are evaluated. A sagittal plane mathematical model of the knee with unicompartmental replacement is developed. Anatomical data and geometries of the prosthetic components were taken similar to those in literature. Ligaments were represented as bundles of elastic fibers. Net muscle forces were represented along straight lines. The prosthetic components were placed on the bones such that selected fibers in the cruciate ligaments remained nearly isometric and no ligament stretched during flexion. This defined the passive motion of the joint. The effects of external load, muscle activity and variations in component placement or size were superimposed. Component mal-placement or in-appropriate size resulted in stretched/slackened ligaments, influenced the relative positions of bones, and affected joint laxity. The model calculations show general agreement with and explain the experimental/clinical observations reported in literature. Copyright © 2010 by ASME.