## Introduction
In cementless Total Hip Replacement, **Primary Stability**鈥攖he immediate mechanical lock鈥攊s the prerequisite for long-term **Secondary Stability** (bone ingrowth). This is achieved through the “Press-fit” principle of the acetabular cup.
## The 1mm “interference” Rule
Surgeons typically ream the acetabulum to a specific size and then insert a cup that is **1mm to 2mm larger** than the hole.
* **The Mechanic:** This “interference fit” creates massive friction and compression against the bony walls of the socket.
* **The Indicator:** A solid “thunk” sound during impaction and zero movement when tested with a lever indicate successful primary stability.
## Ortholyx Cup Design Features
Ortholyx acetabular cups utilize several engineering features to enhance this lock:
1. **Hemispherical Geometry:** Ensures uniform contact across the entire socket surface.
2. **Porous Titanium Coating:** A rough, sandpaper-like surface (high coefficient of friction) that prevents the cup from spinning or tilting.
3. **Equatorial Grip:** Many Ortholyx designs are slightly wider at the “equator” to wedge firmly into the densest part of the pelvis.
## The Transition to Ingrowth
Once the cup is mechanically stable, the body’s bone cells begin to migrate into the porous coating. Within 6-12 weeks, the mechanical lock is replaced by a permanent biological bond.
## Conclusion
A stable cup is the cornerstone of a successful hip. By combining precise reaming techniques with high-friction Ortholyx press-fit designs, surgeons can achieve the rock-solid stability required for early patient mobilization.