Modular necks are a relatively new innovation in total hip arthroplasty (THA), with several companies now offering modular neck options. The proposed advantages of reduced impingement, reduced dislocation rate, and better reconstitution of leg length and offset are compelling. However, few reports in the literature address the outcomes of these devices, and those that are published at best demonstrate equivalence to conventional THA.
There are numerous disadvantages to this new technology. Neck dissociation has been reported with a specific design of the modular taper. Numerous case reports exist of the fracture of titanium modular femoral necks, with 1 large series of 5000 cases reporting a fracture rate of 1.4%. Fractures occurred more frequently in heavy men (>100 kg), with the preponderance of fractures occurring around the 2-year mark. Retrieval analysis demonstrates failure of the titanium components at the Morse taper junction of the neck and femoral stem at the point of maximal tension, likely related to notch sensitivity. The additional interface of modular necks in the effective joint space has the potential to generate significant metal ions through a pitting corrosion process. Evidence exists of highly elevated serum cobalt and chromium ions in a modular junction used in large-head THA supporting these concerns. The use of particular neck geometries, such as long retroverted necks, may adversely affect the local biomechanical forces on the femoral component. The proposed mechanism is an increased lever arm leading to increased torque on stair climbing or rising from a chair. Finally, modular necks add significant costs to the implant and the health care system. On balance, based on the literature, the proximal modular neck in THA is a bridge too far.
The proximal modular neck in total hip arthroplasty (THA) is a relatively new concept, first reported on by Toni et al1 in 1991. Modularity at the proximal neck allows for different neck lengths, offsets, versions, and combinations of each, allowing for flexibility at the time of implantation. The proposed advantages of proximal neck modularity include reduced impingement, decreased dislocation rate, and better functional outcomes related to increased ability to balance leg length and offset. Numerous manufacturers now offer a modular neck option.
While the rationale for modular necks is somewhat compelling, there are few clinical papers on proximal neck modularity in THA. The article by Toni et al1 reports the incidence of straight vs retroverted necks in 347 primary THAs. An article by Omlor et al2 reports in more detail on the ability of modular necks to reconstitute leg length and offset in 190 primary THAs. The authors did not link improved reconstitution of offset to improved functional outcome, and as such, there is no literature to date to support such a claim. Neither article reports a reduced dislocation rate, but this is not surprising given the relatively small numbers. Based on previous reported higher dislocation rates in uncemented modular acetabular components versus nonmodular cemented components from the Australian Joint Replacement Registry, it is not necessarily intuitive that proximal neck modularity in THA will reduce dislocation rates.3
There are numerous significant disadvantages to proximal neck modularity. The most obvious is the possibility for fracture of the component at the stemneck junction. Numerous case reports exist of the fracture of modular necks (Figure). Risk factors for fracture include male sex, weight >100 kg, and long, retroverted titanium necks.4-7 A large series by Grupp et al6 on 5000 uncemented THAs with a titanium neck reported a fracture incidence of 1.4%. Neck dissociation has been reported, but this seems to be a rare event.8
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Figure: Retrieved fractured titanium modular neck demonstrating pitting corrosion and fracture below the margin of the supported Morse taper (A). Scanning electron micrograph of the fracture zone from the retrieved neck demonstrating crack propagation on the maximum tension side of the titanium implant (B).
Proximal neck modularity adds another interface in the effective joint space of the hip for the generation of particulate metal debris from pitting corrosion. In a recent article by Garbuz et al,9 the use of a modular taper adapter for large-head THAs was compared to resurfacing arthroplasty. The group with the modular taper had 10× serum cobalt and 2.6× serum chromium levels than the resurfacing group, likely directly related to the modular taper. Pitting corrosion at the taper has been identified as a potential contributor to neck fracture with these devices.7 Pitting corrosion at the proximal modular junction may prove to be problematic clinically.
The ability to substitute a straight conventional neck in a THA for a modular, retroverted neck, the most common variety used other than straight, may not necessarily be benign with respect to the local biomechanical environment and potentially long-term survivorship.1 In a 2002 article by Gill et al,10 the RSA migration pattern of some types of cemented femoral components was affected by the version of the femoral component. Stems with less anteversion had higher maximum total point motion and overall migration. The proposed hypothesis is that anteversion shortens the rotatory lever arm (ie, torque) placed on the hip when rising from a chair or stair climbing. Subsequently, it can be postulated that substitution of a straight neck for a modular neck with varying geometries may have unintended effects on the long-term survivorship of the component. More research on the effect of neck modularity on the biomechanics of the hip is required.
The use of proximal neck modularity in THA adds significantly to the cost of the prosthesis. This is problematic in todays health care economy, particularly when there is a lack of evidence for proven benefit, let alone equivalency.
Given the lack of literature to support the proposed benefits of proximal neck modularity, and the published theoretical and known complications with significantly increased costs, the available evidence would suggest that the proximal modular neck in THA is a bridge too far.
- Toni A, Sudanese A, Paderni S, et al. Cementless hip arthroplasty with a modular neck. Chir Organi Mov. 2001; 86(2):73-85.
- Omlor GW, Ullrich H, Krahmer K, Jung A, Aldinger G, Aldinger P. A stature-specific concept for uncemented, primary total hip arthroplasty. Acta Orthop. 2010; 81(1):126-133.
- Conroy JL, Whitehouse SL, Graves SE, Pratt NL, Ryan P, Crawford RW. Risk factors for revision for early dislocation in total hip arthroplasty. J Arthroplasty. 2008; 23(6):867-872.
- Dangles CJ, Altstetter CJ. Failure of the modular neck in a total hip arthroplasty. J Arthroplasty. In press.
- Skendzel JG, Blaha JD, Urquhart AG. Total hip arthroplasty modular neck failure. J Arthroplasty. In press.
- Grupp TM, Weik T, Bloemer W, Knaebel HP. Modular titanium alloy neck adapter failures in hip replacementfailure mode analysis and influence of implant material. BMC Musculoskelet Disord. 2010; (11):3.
- Wilson DA, Dunbar MJ, Amirault JD, Farhat Z. Early failure of a modular femoral neck total hip arthroplasty component: a case report. J Bone Joint Surg Am. 2010; 92(6):1514-1517.
- Sporer SM, DellaValle C, Jacobs J, Wimmer M. A case of disassociation of a modular femoral neck trunion after total hip arthroplasty. J Arthroplasty. 2006; 21(6):918-921.
- Garbuz DS, Tanzer M, Greidanus NV, Masri BA, Duncan CP. The John Charnley Award: Metal-on-metal hip resurfacing versus large-diameter head metal-on-metal total hip arthroplasty: a randomized clinical trial. Clin Orthop Relat Res. 2010; 468(2):318-325.
- Gill HS, Alfaro-Adrián J, Alfaro-Adrián C, McLardy-Smith P, Murray DW. The effect of anteversion on femoral component stability assessed by radiostereometric analysis. J Arthroplasty. 2002; 17(8):997-1005.
Dr Dunbar is from Dalhousie University, Halifax, Nova Scotia, Canada.
Dr Dunbar is a consultant for Stryker Orthopaedics and has received institutional research support from DePuy, Inc, Wright Medical Technology, Inc, Smith & Nephew, and Zimmer, Inc.
Presented at Current Concepts in Joint Replacement 2009 Winter Meeting; December 9-12, 2009; Orlando, Florida.
Orthopaedic Crossfire is a registered trademark of A. Seth Greenwald, DPhil(Oxon).
Correspondence should be addressed to: Michael J. Dunbar, MD, FRCSC, PhD, Dalhousie University, Ste 4822, Halifax Infirmary Hospital, 1796 Summer St, Halifax, Nova Scotia, Canada B3H 3A7 (firstname.lastname@example.org).