The notch that the prosthesis fits into needs to be close to the thickest part of the ilium. The dashed line in the image (above) are proposed levels of ilium resection depending on lesion location.1 Illustration below courtesy of Malawer et al.15
Benevenia et al.1 found that improper notch prep can lead to instability.
The Mark II saddle prosthesis articulates with the ilium and is allowed to have some movement to help with the overall functionality of the prosthesis. Although, this is a controversial issue. Some authors claim that this saddle-notch articulation can cause pressure erosions, superior saddle migration and premature failure.
Menendez et al., claim that their custom saddle prosthesis (PAR) is less likely to move as it is fixed with three screws.
For stability some surgeons attach Gore-Tex artificial ligament around saddle to maintain stability for the Mark II design (see below).3