Materials

Bone ongrowth on an SL-PLUSstem at 25x magnification.


Stiffness of the SL-PLUS stem in comparison to a canal filling stem.

Bone has the ability to remodel by altering its size, shape, and structure to meet the mechanical demands placed on it. This phenomenon, in which bone gains or loses cancellous and/or cortical bone in response to the level of stress and strains sustained, is summarized as Wolff's law, which states that bone is laid down where needed and resorbed where not needed. The biological mechanisms that produce these effects are still not completely understood but involve electrical, chemical, and hormonal mechanisms as well as mechanical processes at both the cellular and organ level.

Typically, all load is handled exclusively by bone. Following hip arthroplasty, load is now shared by the prosthesis, reducing load on the bone. If this reduction is high, it can induce atrophic remodeling (according to Wolff's law) and lead to decreased bone density and fractures. The phenomenon is called "stress shielding."

To reduce stress shielding, the SL-PLUS is made from a highly biocompatible forged titanium-aluminum-niobium alloy (Ti-6Al-7Nb). Ti alloy features an elasticity that more closely matches bone than other materials used traditionally, thus load is distributed more equally. This ensures bone is loaded and kept healthy to support the prosthesis.

Bone more readily attaches to a roughed than a smooth surface. The SL-PLUS is precision grit blasted to create a uniform 4 - 6 microns surface roughness throughout. As opposed to a secondary coating, the surface of the SL-PLUS will remain constant throughout its functional life. Because the entire stem is roughened, bone will integrate over the entire length, providing a physiologic and functional load transmission to surrounding bone.

SL-PLUS stem surface morphology at 200x magnification.
SL-PLUS stem surface morphology at 1000x magnification.