Samik Banerjee, MD, Robert Pivec, MD, Kimona Issa, MD, Bhaveen H. Kapadia, MD, Harpal S. Khanuja, MD, and Michael A. Mont, MD
Authors’ Disclosure Statement: Dr. Mont wishes to report that he receives royalties from Stryker; consults for Janssen, Ongoing Care Solutions Inc., Sage Products, Medtronic Surgical, Stryker, and TissueGene; receives institutional support from Stryker; and is a member of Sage Products’ speakers bureau and the editorial review board of The American Journal of Orthopedics. Funding was received in support of this work. The other authors report no actual or potential conflict of interest in relation to this article.
Recent advances in the wear characteristics and material properties of highly cross-linked polyethylene and fourth-generation ceramic bearings have led to increasing use of large-diameter (≥ 36 mm) femoral heads in total hip arthroplasty (THA).
In this article, we review the current concepts and principles behind use of large-diameter ceramic or cobalt-chromium femoral heads on polyethylene bearings in THA. We specifically review the biomechanics, some of the early concerns about polyethylene wear and rim fractures, recent improvements in material properties of polyethylene and ceramic bearings, dislocation rates, and clinical and functional outcomes of large-diameter heads in THA.
Recent literature suggests that the incidence of dislocation has been substantially reduced because of improvements in jump distance and impingement-free range of motion with use of large-diameter heads. Limited evidence suggests excellent short-term and midterm clinical and functional outcomes with these heads.
A common cause for total hip arthroplasty (THA) revision is joint instability.1,2 The reported incidence of dislocation in primary THA ranges from 0.4% to 5.8%,3-5 but this rate increases after revision surgery.1,3-8 Use of large-diameter femoral heads has been proposed to decrease the risks for instability and to improve impingement-free range of motion (ROM).
The biomechanical rationale for using large-diameter femoral heads is that they must travel farther before subluxation or dislocation occurs (jump distance). Despite these benefits, there were initial concerns that catastrophic failure and high levels of volumetric wear would occur if these heads were used with conventional polyethylene liners. These concerns led to the development of alternative bearing surfaces, particularly metal-on-metal bearings, which offered theoretical benefits of large-diameter articulations that improved stability while purportedly being highly wear-resistant.9-11 However, concerns about adverse local soft-tissue reactions and high blood concentrations of metal ions tempered the initial enthusiasm for metal bearings.12-16 Fortunately, highly cross-linked polyethylene and fourth-generation ceramic bearing surfaces, with improved toughness and better wear properties, may allow use of large-diameter heads without the need for metal-on-metal bearings.17,18
In this article, we review the concepts and principles behind use of large-diameter ceramic or cobalt-chromium femoral heads on polyethylene-bearing surfaces in THA with particular attention to biomechanics, early concerns about polyethylene wear and rim fractures, recent improvements in material properties of polyethylene and ceramic bearings, dislocation rates, and clinical and functional outcomes.
Definitions
For this review, we define large-diameter femoral heads as having diameters of 36 mm or more and conventional or small-diameter femoral heads as having diameters between 22 and 32 mm.
Biomechanics
Head–Neck Ratio, Impingement-Free ROM, and Jump Distance
Several implant design principles have been proposed to reduce the risks for impingement and dislocation. Of these, large femoral head diameters have been extensively studied.19,20 It is well known that impingement of the femoral neck on the cup edge promotes edge loading and higher wear rates. In addition, impingement increases the tendency of the head to sublux from the acetabulum. One strategy for avoiding this component-to-component impingement is to increase the head–neck ratio (HNR), the ratio of the femoral head to the neck diameter. Biomechanically, increased HNRs lead to delayed contact between the femoral neck and the acetabular liner.21,22 Therefore, with large femoral heads, which have large HNRs, impingement occurs later and at larger ROMs—compared with small-diameter femoral heads, which have lower HNRs and are more prone to early impingement and subluxation (Figure 1).23-26
In a cadaveric study of 6 hips, Bartz and colleagues23 reported a significantly higher preimpingement ROM when the prosthetic head size increased from 22 mm to 28 mm (P < .05). They found a change from prosthetic to osseous impingement when the head size increased from 22 mm to 32 mm. Similar results were observed in a computer simulation model by Cinotti and colleagues,27 who demonstrated that increasing the femoral head size from 28 mm to 38 mm resulted in a 5° improvement in ROM. However, the largest gains were observed when the heads with the smallest diameters were upsized; ROM improved only marginally when femoral head size was further increased from 32 mm to 38 mm. The primary reason for the lack of expected improvement in ROM with head sizes of more than 32 mm is often bone-on-bone impingement. Burroughs and colleagues28 demonstrated that the 38-mm and 44-mm heads virtually eliminated component-to-component impingement except in extremes of external rotation. However, there were no differences in ROM between 38-mm and 44-mm heads because of osseous impingement. In addition, large heads are less likely to sublux or dislocate, as they need to travel farther before reaching the edge of the acetabular cup before dislocation. This is known as the jump distance, and it corresponds to the depth of the acetabular shell, which in turn equates with the radius of the femoral head (Figures 2A, 2B). For this reason, the larger the femoral head diameter, the farther the jump distance and, correspondingly, the lower the risk for dislocation.29
Elevated liners historically were used to increase the jump distance for dislocation.30 These liners, however, can increase impingement at the extremes of motion.31 Some of these problems can be avoided with use of larger heads, which have increased jump distances without additional risks for impingement. Moreover, large heads create a suction effect that provides passive resistance to dislocation.32 With head diameters beyond 38 mm, impingement-free ROM often plateaus. However, the jump distance required for dislocations to occur continues to increase as femoral head diameters increase in size. Thus, patients may experience fewer motion benefits but continue to benefit from overall stability with femoral head sizes increasing beyond 38 mm.
