The size and grade of the bone particles are important for the initial mechanical stability of the impacted morselized graft. Particles should be as large as practical to ensure stability, i.e. they should be the largest size that can be fitted between the host cortical bone and the phantom prosthesis. That size is thought to be between 3 and 5 mm in diameter for femoral revisions.37, 38 On the acetabular side, the ideal size is larger; research suggests 8 to 10 mm diameter chips provide the best initial stability.39-41 More proximal in the femur, these larger grafts are now also used to improve the initial stability of the stem. Another advantage of larger particles is that they result in a more porous and more permeable impacted graft. This is important because a higher degree of impaction may make it more difficult for new bone to grow into the impacted mass. A study comparing bone ingrowth into non-impacted and impacted bone showed that a higher degree of impaction reduces bone ingrowth. Also larger voids facilitate cement penetration, which may contribute to an improved initial stability. Implants supported by impacted morselized bone graft can migrate as a result of shear within the layers of the impacted graft. Shear strength of the graft layer is improved by using morselized grafts with a range of particle sizes.42 However, using a range of particle sizes reduces graft permeability, since the pores between the larger particles will be filled with smaller particles. A reduction in permeability may reduce bone ingrowth, but the improved mechanical resistance to shear can offset this. More work is needed to clarify the interaction between these mechanical and biological factors.