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

Dislocation Rates

Several patient, surgeon, and implant factors affect the rate of dislocations after THA. Multiple implant options utilize the biomechanical advantage that large-diameter heads have in improving stability. Various alternatives include use of constrained tripolar heads, dual-mobility bearings, and conventional large-diameter heads with standard liners.^67-69

Large-Diameter Heads

Despite the biomechanical advantages of large-diameter metal-on-polyethylene bearings, prior studies have questioned use of these bearings because of risks for increased wear and rim failures. However, the improved wear properties of highly cross-linked polyethylene, elaborated earlier, have led to a reappraisal of this option (Table 2).^4,70 Howie and colleagues,⁷¹ in a randomized control trial of 644 patients, also found significantly lower rates of dislocation after primary THA with 36-mm heads compared with 28-mm heads (1.3% vs 5.4%; P = .012); in addition, fewer dislocations occurred with 36-mm heads than with 28-mm heads (4.9% vs 12.2%; P = .27) in a series of 44 patients in revision settings. Similarly, in a study conducted with 39,271 Medicare patients between 1998 and 2007, Malkani and colleagues⁷² found a decrease in the dislocation rate, from 4.21% to 2.14%, with use of large-diameter femoral heads. These results have been confirmed by several other authors.^34,66,73,74 Similar results were observed in 65,992 patients in the Australian National Joint Replacement Registry by Conroy and colleagues,⁷⁵ who reported a significant decrease in the risk for dislocation with large heads (≥ 30 mm) compared with 22-mm heads (relative risk, 1.0 vs 3.1; P ≤ .001).

Few studies have analyzed the role of large-diameter femoral heads in the presence of compromised soft tissues around the hip. Kung and Ries,⁷⁶ evaluating the influence of large-diameter heads in the presence and absence of a deficient abductor mechanism, demonstrated statistically significant reductions in rates of dislocation after 230 revision THAs when the abductor mechanism was intact with use of 36-mm heads compared with 28-mm heads (12.7% vs 0%; P = .015). With abductor deficiency, though, the positive effect of large heads in reducing dislocation rates was substantially reduced and was similar to that of small heads (P = .74).⁷⁶

Large heads considerably improve overall stability and lower dislocation rates in THA. With the development of newer ceramics and highly cross-linked polyethylenes, the wear rates reported in multiple studies appear to be less concerning.

Constrained Tripolar Heads

Tripolar heads have been proposed as treatment options for improving stability in patients with chronic and recurrent instability after THA. The tripolar implant consists of a metal head that snap-fits into a polyethylene liner with a polished Co-Cr backing. This bipolar head articulates with a polyethylene bearing that is press-fitted onto a metal acetabular shell and constrained by a metal ring snapped to the outer polyethylene bearing. The bipolar component behaves as a large-diameter femoral head, and the metal ring provides additional restraint, further improving stability.

Williams and colleagues⁷⁷ performed a systematic review and reported on the outcomes of constrained tripolar liners in 1199 hips at a mean follow-up of 4 years (range, 2-10 years). The mean dislocation rate was 10%, and the mean rate of revision surgery unrelated to instability was 4%. In a study of 43 hips at a mean follow-up of 4 years (range, 2-9 years), Zywiel and colleagues⁷⁸ reported on the clinical and radiographic outcomes of tripolar constrained liners. Their study group had a mean Harris Hip Score (HHS) of 82 points (range, 38-100 points) and overall survival of 91%, with no evidence of radiographic loosening during follow-up. Despite the improvements in stability with constrained tripolar liners, some authors have reported multiple mechanisms of failure with these devices.^79-81 In a study of 43 failed constrained tripolar liners with a mean time to failure of about 2 years, Guyen and colleagues⁷⁹ identified 5 different failure modes (types 1-5) involving all 4 interfaces in these components.

Encouraging outcomes have been reported at midterm follow-up with tripolar constrained liners. However, concerns about failure at the interfaces suggest that use of these components should be restricted to patients with deficient abductor mechanisms or neuromuscular compromise, low-demand elderly patients, and salvage cases of recurrent dislocations.⁷⁹

Dual-Mobility Bearings

For more than 20 years, different dual-mobility bearings have been used for difficult acetabular reconstructive scenarios and prevention of instability.^82,83 Dual-mobility cups provide constructs that snap-fit a small-diameter femoral head within a large polyethylene insert that articulates with a fixed metal shell. This effectively increases the functional head diameter.

Various authors have reported excellent survivorship rates (92%-99%) and low dislocation rates for these bearings at 5- to 10-year follow-up.^82,84-90 Philippot and colleagues,⁸⁶ in a recent study of 438 hips with dual-mobility cups, reported excellent survivorship (96%) and no early or late instability within a 15-year follow-up. Bouchet and colleagues⁶⁹ compared dual-mobility bearings (105 hips) with conventional metal-on-polythene bearings (108 hips) and found significantly (P < .05) lower dislocation rates for the dual-mobility implants at a minimum 1-year follow-up. The French Society of Orthopaedics and Traumatology performed a multicenter analysis of 3473 hips with dual-mobility cups implanted in France between January 1998 and December 2003.⁸⁷ During a mean follow-up of 7 years (range, 5-11 years), there were 15 dislocations (0.43%), 14 of which occurred early, within 3 months of implantation (0.4%). Aseptic implant survivorship was 95% at 10-year follow-up.