Maintains Space in Medullary Canal

For long-term prosthesis success, the surrounding bone must remain healthy and nourished. The femur's medullary space is vital for bone nutrition. Some stem designs fill the entire medullary canal, destroying the endosteal blood supply.

The SL-PLUS®, with its flat sides, does not completely fill the medullary canal. By maintaining this area, the endosteal blood supply is still maintained, increasing long-term prosthesis success by promoting healthy bone.

Trochanter Wing

Primary stability is critical for a cementless stem system. In addition to the double taper rectangular geometry, the trochanter wing acts as stabilizing element. It also increases the overall proximal surface area for cementless fixation.

Trunion
 
The interface between the ball head and the trunion is critical especially with ceramic ball heads. The ball must fit securely and have optimal contact for a correct and even load transfer. High spots at the interface can lead to stress concentrations and failures. Our trunion combines an industry standard 12/14 taper, with a micro-ridge surface. The micro-ridge surface deforms slightly to absorb local incongruencies upon ball impaction, which produces a conforming interface between the stem and ball head. Load distribution is then optimized for optimal strength.

Insertion/Extraction Device

Long-term success depends on precise surgical technique. The insertion device is purposefully aligned with the long axis of the stem to ensure accurate surgical placement and prevent tilting.

Rounded Tip

A complaint from patients with some stems is post-operative thigh pain. This is often caused by a sub optimal transition of the stem tip to the bone coupled with a material that is too stiff. To avoid this problem, the SL-PLUS® has a rounded, tapered tip and utilizes titanium alloy for its low stiffness. This significantly reduces localized stresses, avoiding thigh pain.

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 1000x magnification.