Computer assisted navigation is known to improve tunnel placement in ACL reconstruction even compared to use of direct arthroscopic view due to image distorsion by the wide angle optics in the arthroscope. However the earlier software and instrumentation has been relatively cumbersome. The use of new materials and further software elaboration has allowed to increase the navigational precision and to accommodate more different ACL repair techniques. The relevant developments of such an upgrade which in addition allows stability testing before and after the repair are presented.

In contrast to the acetabular cup where the close to spherical shape of the implant allows a precise alignment and positioning, the femoral stem implant positioning has always been a compromise between anteversion, angulation and length of the prosthetic femoral neck and the congruence of the implant shaft with the inner anatomical shape of the proximal femur. Balancing these reduces the risks of dislocation and eccentric wear of the acetabular implant and of unfeasible loading of the femoral implant with loosening. Nevertheless neither the anchorage of the stem nor the alignment of the neck can ever be ideal as it would too much jeopardize the other aspect even if cement is used for stem fixation. Customary stem navigation only guides this compromise more precisely than eye balling.

With the introduction of modular necks it has become possible to infringe this restrictive fix relation and after fully fitting stem fixation the neck alignment and length is optimized separately. With regard to computer assisted navigation the guidance takes into consideration the definitive stem position after best anatomical fixation. A sound navigation of cemented stems becomes also possible and brings up the opportunity to choose cementing or uncemented fixation based on whether the bone quality needs reinforcement by cement to get closer to the implant e-module or the bone promises to strengthen on its own on the same purpose.