The Femoral Head articulates with the Acetabular Liner (the Poly). The Femoral Head also forms a junction with the Femoral Stem (the Trunnion). Technically there is motion at both ends of the femoral head (even though motion at the trunnion would ideally not occur) and thus both are important to consider as sources of wear debris. The femoral head can be made of various materials (aka Bearing Materials) from the historical standard cobalt-chrome to ceramic to zirconium (which is a metal-ceramic hybrid). The variation in femoral head material causes different levels of friction when motion occurs between it and the acetabular liner, thus generating different amounts of wear. We discuss some of the femoral head materials below. The femoral head junction with the femoral stem - trunnion - has received a lot of attention lately as a source of corrosion (see complications section).
FEMORAL HEAD - BEARING MATERIALS.
There are a number of materials for the femoral head, but the overwhelming majority of modern day THA utilizes a Ceramic or Co-Cr femoral head on a poly liner as the bearing surface. Wear rates are low with a slight advantage to ceramic-on-poly.
Metal. A Cobalt-Chromium (Co-Cr) femoral head is a good bearing material because it possesses low wear properties, while titanium alloy scratches easily leading to rapid polyethylene abrasive wear (titanium is a great material for the acetabular shell or femoral stem due to its modulus of elasticity). The Co-Cr femoral head can articulate with a metal liner (hard on hard bearing) or with a polyethylene liner (hard on soft bearing). A Co-Cr femoral head initially has some rough spots (called "asperities") that cause accelerated wear ("run-in wear") in the first year (1 million cycles), however, these areas smooth out, and wear rates drop in subsequent years: initial wear is 0.18 mm/yr, then 0.10 mm/yr after wearing in.
When a metal head articulates with a metal liner (hard-on-hard), it forms a fluid-film layer during walking, which is a layer of lubrication between ball and socket, and it significantly decreases friction. The fluid-film layer only exists with hard bearing surfaces because the material is so smooth (Co-Cr head surface roughness is only 0.01 μm, compared to poly roughness of 7.0 μm, while ceramic roughness is even better: 0.006 μm). A larger head and high congruence between the head and liner maximize contact area and promote the fluid-film lubrication.
Complications. Metal-on-metal wear can be a major complication if significant wear triggers an immune response (more in Complications Chapter). Wear particles are tiny (0.015 - 0.5 μm, yet the overall number of particles is significantly greater than poly wear, and it creates an adverse local soft tissue reaction (ALTR).
Ceramic. An alumina-ceramic femoral head is a good bearing material because it is the smoothest material and therefore possesses the lowest wear properties (low abrasive, linear and volumetric wear). The wear rate of ceramic-on-ceramic is virtually zero (<0.07 μm/year), and even the few particles generated are completely inert; the body mounts no immune response.
Ceramic is hydrophilic, and absorbs moisture to promote fluid-film lubrication in ceramic-on-ceramic bearing.
Complications. While ceramic sounds like the wonder material, the long-term outcomes are nothing spectacular (86% survivorship at 18 years) . Ceramic liners are reserved for the “ultra-young” patient (ie < 50 years old) because of unique complications: liner fracture and squeaking.
Older designs had large pore size in the ceramic, this decreased strength, and led to ceramic fracture of 13% (a devastating complication, as the microscopic shards are near impossible to remove and contribute to significant third-body wear of the poly after revision - furthermore, despite best efforts, small shards remain and thus revision surgery requires the use of more ceramic because the density of ceramic makes it most resistant to third-body wear. 5-year survivorship is only 60% after revision for fractured ceramic. Using a ceramic head in the revision setting, placing it onto a retained stem, requires a titanium jacket to prevent the roughened neck from causing a fracture). Yet, similar to the multiple iterations to improve poly, the modern ceramics undergo hot isostatic pressing to decrease grain size by 3x. The rate of fracture is now only 4 in 100,000 at this time (0.012%), with 40% fractures occur by 1 year, 75% by 3 years. Head size decreases fracture risk. Current ceramic heads do not have cobalt-chrome liners and thus decrease trunnionosis .
note: oxidized zirconium is a new development (third generation ceramic), and is essentially a metal-ceramic hybrid (or “ceramizied metal”). A metal alloy undergoes an oxidation process to create a zirconia ceramic surface (the outer layer of metal becomes ceramic). This allows the surface to become harder with fewer bumps (ie low wear characteristics of a ceramic) without the risk of fracture and chipping that remains a concern. This material is only FDA-approved for articulation with a poly liner.
Another deterrent to ceramics is “squeaking” (incidence is low: only 14 in 2138 THA, about 0.6%). Its an audible squeak heard with every step, typically develops more than a year after implantation, and annoys patients enough that many undergo revision surgery. Its cause remains unknown, it may be associated with stripe wear due to implant malpositioning (a vertical cup > 55 degrees, causing edge loading which disrupts the lubrication of the bearing.
FEMORAL HEAD SIZE.
Size is another important technical consideration. 20 years ago, the average femoral head size was 22 mm. Today the average size is 32 mm. That’s almost 30% bigger. Does this mean that bigger is better? Most joint surgeons agree that there are both benefits and risks to a larger head size.
Benefit. A larger head increases stability for two reasons. A larger head increases the head-neck ratio (diameter of the femoral neck vs the femoral head… the neck diameter never changes). A larger head-neck ratio means a larger arc of motion before impingement (eventually the neck will impinge on the rim of the socket). Because impingement causes the head to lever out of the socket and dislocate, a larger head decreases the chances of impingement, thus increasing stability. However studies have found that 32 mm heads and larger effectively prevented impingement between components (without significant added benefit from heads larger than 32 mm). Furthermore, if the femoral neck was made trapezoidal instead of circular, impingement decreased significantly (while a skirted head dramatically increased impingement).
Additionally, a larger head stabilizes the joint because it increases the jump distance (Jump Distance = the radius of the femoral head, and represents the distance that the head must travel (sublux) before full dislocation occurs). If the head is larger, then there is a greater distance before dislocation.
Disadvantage. Increased debris formation for two reasons. A larger head increases volumetric wear of the acetabular liner and thus increases the amount of poly debris. A larger head also increases the head : neck ratio, which is great for arc of motion but it also places more stress across the head : neck junction (aka the trunnion) and is associated with corrosion (“trunnionosis”) which produces a similar reaction to the highly-undesirable Metal-on-Metal wear.
Additionally, a larger head requires a larger poly liner, which can be achieved only by making a standard poly liner thinner (a thinner poly has a higher risk of breaking or wears out to a critical level faster); or by using a larger acetabular shell (which means taking away extra bone stock).