Large-Diameter Femoral Heads in Total Hip Arthroplasty: An Evidence-Based Review

Current evidence suggests there may be substantial benefits toward improved stability from increasing head diameters from 22 mm to 38 mm because of the increase in jump distances and improvements in prosthetic impingement-free ROM. However, there may be little gain in ROM from increasing the head diameters beyond these dimensions because of the potential risks of bony impingement. Nevertheless, there may be some additional benefits toward stability from improvement in jump distances with incremental head sizes
beyond 38 mm.^29,33,34

Finite Element Analysis Studies

Finite element analysis of large-diameter heads in THA has shown that, at optimal cup inclination (45°), most stresses occur on the articular surface of the liner. However, these stresses remain well below the yield strength of the polyethylene liners.²⁹ With increasing abduction angles, the stress concentration increases substantially because of the decreased contact surface area. At these angles, the point of maximum contact moves toward the rim of the polyethylene liner, which can lead to rim fractures or failure of locking mechanisms.^29,35,36

Early Concerns With Large-Diameter Femoral Heads: Wear, Liner Failure, and Fracture of Ceramic Components

Use of small-diameter femoral heads started with the first report by Charnley³⁷ of “low frictional torque arthroplasty.” Charnley initially considered a 41.5-mm femoral head, but he thought it would increase risks for acetabular loosening from high frictional torque generated by the large head, and he switched to a small-diameter (22.5 mm) design. One of the tradeoffs with smaller diameter heads was decreased jump height in addition to increased linear wear.

Large femoral heads used with cemented polyethylene acetabular components historically have been associated with increased rates of volumetric wear but low rates of linear wear, which potentially may increase the risk for osteolysis.^38-40 However, newer highly cross-linked polyethylene liners have shown improved in vitro and in vivo volumetric wear characteristics and potentially lower linear wear rates compared with earlier designs (Table 1).^28,41-43

Another concern about earlier generations of large femoral heads was the risk for catastrophic liner failure on conventional polyethylene. This was originally reported by Berry and colleagues,⁴⁷ who described wear-through and failure in patients with thin (< 5 mm) acetabular cups. However, these concerns have been largely addressed by the development of highly cross-linked polyethylene, which has improved wear characteristics and fatigue resistance.⁴⁸

Recent Improvements in Material Properties of Polyethylene and Ceramic Bearings

The development of highly cross-linked polyethylene and fourth-generation ceramics has renewed interest in large-diameter bearings in THA. These bearing surfaces improve wear, enhance material properties, and have superior oxidation resistance.^42,48-53

We now briefly describe the methods used to improve the material properties of polyethylene and ceramics. Studies have shown that increasing the radiation dose (up to 200 kGy) increases cross-linking and causes an inverse exponential decrease in polyethylene wear.^28,41,48-51 However, increasing radiation doses also increases production of free radicals, which diminish the material strength of these polyethylenes. The current generation of highly cross-linked polyethylene liners is produced through a variety of manufacturing strategies to improve cross-linking and reduce wear. These strategies include differential radiation doses (50-100 kGy), techniques (electron beam, radiation), and thermal treatments (melting, annealing). Moreover, to enhance the material properties and reduce the incidence of rim cracking and delamination, authors have proposed using vitamin E supplementation to minimize the amount of subsurface oxidation that occurs as an inevitable consequence of free radical formation during fabrication.^54,55 A terminal sterilization process (eg, gas plasma, ethylene oxide, or gamma sterilization in nitrogen) is needed to make commercial, highly cross-linked polyethylene.^52,53

Fourth-generation ceramics manufactured with nano-sized yttria-stabilized tetragonal zirconia particles in a stable alumina matrix have more fracture toughness and improved wear characteristics.^54,55 In addition, oxide additives (eg, chromium oxide, strontium oxide) improve hardness and dissipate energy by deflecting cracks to prevent their propagation.⁵⁶ Moreover, the smaller grain sizes of fourth-generation ceramic bearings compared with third-generation designs (0.8 µm vs 1-5 µm) cause less disruption of the fluid film layer, which ultimately results in improved wear performance.⁵⁷

Multiple studies have found reduced wear rates with metal and ceramic large heads coupled with highly cross-linked polyethylene-bearings (Table 2).^17,41,50,58 Bragdon and colleagues,⁵⁸ using radiostereometric analysis in 25 patients, found no significant differences in mean head penetration rates between 36-mm and 28-mm cobalt-chromium (Co-Cr) heads articulating with highly cross-linked polyethylene cups at a mean follow-up of 3 years (0.035 mm/y vs 0.046 mm/y; P = .11). Geller and colleagues,⁶⁴ in their study of 42 patients with large-diameter (> 32 mm) Co-Cr femoral heads, found low mean (SD) linear wear rates of 0.06 (0.41) mm/y at a mean follow-up of 3 years. D’Antonio and colleagues,⁶⁵ in a multicenter study, reported low average linear wear (0.015 mm/y) and volumetric wear (12.1 mm³/y) over 5 years using sequentially annealed cross-linked polyethylene. In vitro reports suggest that large-diameter ceramic heads may have lower wear properties than Co-Cr heads do. Galvin and colleagues,⁶⁶ in an in vitro hip simulator study, found that large-diameter ceramic heads on highly cross-linked ultrahigh-molecular-weight polyethylene had 40% reductions in steady-state wear rates compared with Co-Cr heads on highly cross-linked bearings (4.7 vs 8.1 mm³/million cycles; P < 0.01).