a titanium shell and polyethylene liner for the  acetabular implant

The current acetabular implant is a cementless 2-component cup.  A metal shell for bony ingrowth + a polyethylene liner (sometimes metal or ceramic) that snaps into the shell and  articulates with the femoral head. 

The modern acetabular implant is a "press-fit" design (it is not cemented).  The actual implant is about 1-4 mm smaller than the reamer of the same size (i.e. a size 50 implant is slightly smaller than a size 50 reamer).  This creates hoop stresses that hold the shell rigidly in place, allowing bone to grow into the shell.  Lets look into this process more.

The acetabulum is prepared via sequential reaming - removing the remaining cartilage and subchondral bone to create a bed of vascular cancellous bone that encourages bone ingrowth.  

If the target is cup size is 50, a surgeon will start smaller, ie 46 reamer, and work up to a size 49 or size 50 reamer.  If a size 50 reamer is the final one used for a 50 cup - this is called "line-to-line" fitting.  If a size 49 reamer is the final one used for a 50 cup - this is called “oversizing” the implant to increase hoop-stress on the implant.  Why oversize?  The hemisphere created by sequential reaming may not be a perfect circle if the surgeon moves the center-point of each ream, creating more of an oval, which decreases hoop stress.  Oversizing can compensate for imperfect reaming.  Alternatively, poor quality bone, as seen with avascular necrosis or osteoporosis, may reduce the viscoelasticity of bone, which in turn decreases its ability to generate hoop stresses, and therefore, oversizing may generate better hoop stresses (however, note that too much hoop stress will create a fracture, and poor bone is most susceptible).  

Hoop-stress create the press fit.  If the acetabular implant is undersized (ie a size 52 reamer for a size 50 cup), then stress is overly concentrated centrally (the implant doesn't fully touch the outer walls because reaming diameter is bigger), and there may be gaps between the component and bone at the periphery, preventing stability and proper ingrowth.  In contrast, if the component is too large for the prepared cavity (ie size 47 reamer for a size 50 cup), excessive stress is transferred peripherally with risk of acetabular rim fracture.  The center of the component also may not seat completely, also risking instability via ingrowth failure. 

note: hemispherical cups are often “oversized” as described above (ie 50 cup prepared with a 49 reamer), while, elliptical design cups are usually prepared “line-to-line” (ie 50 cup prepared with a 50 reamer), or even over-reamed by 1 mm.  This is due to the difference in geometry: you are using a hemispherical reamer but implanting an elliptical shell, this mismatch creates the requisite hoop stresses.

The metal shell implant is highly polished on the inside and the outside is completely coated to allow for bony ingrowth (in reality only about 30% ingrowth occurs).   The biologic fixation occurs by either bony ingrowth or bony ongrowth. The optimal pore size is between 100 and 400 μm.  Bone ongrowth occurs with a “roughened” (but not porous) surface.  This roughened surface occurs by grit blasting (a pressurized spray of aluminum oxide particles to produce an irregular surface at 3-8 μm depth, and a thickness of 50-150 μm), plasma spraying (apply molten metal in a argon gas environment), or hydroxyapatite coating (which is an osteoconductive calcium phosphate coating applied by plasma spray).  

note: newly developed highly porus metals (ie Trabecular metal) increases the friction against cancellous bone and improves initial stability (it also encourages rapid and extensive bone ingrowth). 

Many surgeons supplement press-fit fixation with screws to improve stability (although in theory, the hoop stress from press-fit should provide sufficient stability).  Retrieval studies show that most bone ingrowth occurs around these screws, highlighting their efficacy. 

Most metal shells are sized 40 to 70 mm (referring to their outer diameter), increasing in size by 2 mm.  Metal shells are ~ 5 mm thick to prevent fatigue fracture.  A shell can accommodate varying thickness of poly liner.  For example, a size 50 shell can be fit to match both a size 28 and size 32 femoral head.  The difference is the thickness of the liner. 

matching acetabular shell with femoral head

The most common size in women is 48, the most common size in men is 52.  

Historically cups were monoblock polyethylene (no shell) and were cemented into the acetabulum.  The poly was designed with backside grooves to improve the cement mantle.  However, studies showed cementing a poly cup leads to higher rates of aseptic loosening,  because cement works well against compressive forces (as seen with the stem in the femoral canal), but works poorly against shear forces (which you see in the acetabulum).   A titanium shell that allows bone ingrowth was the solution for aspetic loosening of cemented cups, and while it appears to improve implant longevity, a poly liner with metal backed shell has unique problems.  

Disadvantages to metal backed cup. 

1. Need for a “locking mechanism” (fix the poly to the metal shell).  Its just one more thing that can fail, and there are reports of dislocated poly liners due to improper insertion or failure of the locking mechanism. 

2. “Backside wear”: Micromotion can occur between any two segments, no matter how well fixed they appear.  “Backside wear” refers to micromotion between the back of the poly liner and the metal-cut.  Many of the current designs have a polished inner poly shell to minimize the wear particle production, which can lead to osteolysis.

3. Increased bone loss. The metal-backed cup must be >5 mm thick to prevent fatigue fracture.  The poly must also be >5 mm thick to prevent fatigue fracture and early wear.  Thats a total of at least 10 mm thickness.  A single-component (ie metal only, or poly only) requires only 5 mm overall and thus takes up 50% less volume.  Adding the metal backing to a poly liner thus requires extra bone resection, unless you ream the same amount of bone and then make the poly smaller and use a smaller femoral head.  Yet many surgeons associate smaller heads with larger risk of dislocation.  Therefore, to preserve the larger head size, they will ream more native bone from the acetabulum.  Its follows the saying “robbing paul to pay peter”  (femoral head size vs. acetabular bone stock). One advantage to metal-on-metal design is that it can accommodate a larger femoral head because it’s a single component.

4) There is less concern about stress shielding in the acetabular component as compared to the femoral component, however it does occur with about 20% decreased density anteriorly (yet the clinical significance is less apparent).  This is particularly seen in monoblock cups made of cobalt chromium for metal-on-metal hips (as opposed to standard titanium shells which have modulus of elasticity that better matches native bone).