Original Research

Modular Versus Nonmodular Femoral Necks for Primary Total Hip Arthroplasty

Am J Orthop. 2015 September;44(9):411-414

In total hip arthroplasty (THA), proximal femoral neck stem modularity (PFNSM) has theoretical advantages over nonmodular stems, including the ability to more closely reconstruct anatomy and improve stability. However, risks of metallosis and breakage at the junction must be considered.

In this study, we compared the head centers of a modular neck system with that of its nonmodular counterpart. Of 463 primary THAs with a modular stem, 261 (56%) had a head center equivalent to that of its nonmodular counterpart, and an additional 132 (29%) had a head center within 4 mm in length and 2 mm of offset. Thus, only 70 stems (15%) had a head center that was more than 4 mm in length and more than 2 mm in offset different from the nonmodular stem. Only 12 stems had a verted neck.

These findings suggest that, in a majority of primary THAs, use of a modular stem results in head center positions also achievable with a nonmodular stem. Given the risks of modularity, PFNSM should be used with caution. We recommend PFNSM in cases that cannot be reconstructed with the nonmodular option.

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References

1. Barrack RL. Modularity of prosthetic implants. J Am Acad Orthop Surg. 1994;2(1):16-25.

2. Cameron HU. Modularity in primary total hip arthroplasty. J Arthroplasty. 1996;11(3):332-334.

3. Hozack WJ, Mesa JJ, Rothman RF. Head–neck modularity for total hip arthroplasty. Is it necessary? J Arthroplasty. 1996;11(4):397-399.

4. Holt GE, Christie MJ, Schwartz HS. Trabecular metal endoprosthetic limb salvage reconstruction of the lower limb. J Arthroplasty. 2009;24(7):1079-1089.

5. Sporer SM, Obar RJ, Bernini PM. Primary total hip arthroplasty using a modular proximally coated prosthesis in patients older than 70: two to eight year results. J Arthroplasty. 2004;19(2):197-203.

6. Spitzer AI. The S-ROM cementless femoral stem: history and literature review. Orthopedics. 2005;28(9 suppl):s1117-s1124.

7. Mumme T, Müller-Rath R, Andereya S, Wirtz DC. Uncemented femoral revision arthroplasty using the modular revision prosthesis MRP-TITAN revision stem. Oper Orthop Traumatol. 2007;19(1):56-77.

8. Wirtz DC, Heller KD, Holzwarth U, et al. A modular femoral implant for uncemented stem revision in THR. Int Orthop. 2000;24(3):134-138.

9. Lakstein D, Backstein D, Safir O, Kosashvili Y, Gross AE. Revision total hip arthroplasty with a porous-coated modular stem: 5 to 10 years follow-up. Clin Orthop Relat Res. 2010;468(5):1310-1315.

10. Bolognesi MP, Pietrobon R, Clifford PE, Vail TP. Comparison of a hydroxyapatite-coated sleeve and a porous-coated sleeve with a modular revision hip stem. A prospective, randomized study. J Bone Joint Surg Am. 2004;86(12):2720-2725.

11. Huot Carlson JC, Van Citters DW, Currier JH, Bryant AM, Mayor MB, Collier JP. Femoral stem fracture and in vivo corrosion of retrieved modular femoral hips. J Arthroplasty. 2012;27(7):1389-1396.e1.

12. Gilbert JL, Mehta M, Pinder B. Fretting crevice corrosion of stainless steel stem–CoCr femoral head connections: comparisons of materials, initial moisture, and offset length. J Biomed Mater Res B Appl Biomater. 2009;88(1):162-173.

13. Kop AM, Keogh C, Swarts E. Proximal component modularity in THA—at what cost? An implant retrieval study. Clin Orthop Relat Res. 2012;470(7):1885-1894.

14. Cooper HJ, Urban RM, Wixson RL, Meneghini RM, Jacobs JJ. Adverse local tissue reaction arising from corrosion at the femoral neck–body junction in a dual-taper stem with a cobalt-chromium modular neck. J Bone Joint Surg Am. 2013;95(10):865-872.

15. Vundelinckx BJ, Verhelst LA, De Schepper J. Taper corrosion in modular hip prostheses: analysis of serum metal ions in 19 patients. J Arthroplasty. 2013;28(7):1218-1223.

16. Kouzelis A, Georgiou CS, Megas P. Dissociation of modular total hip arthroplasty at the neck–stem interface without dislocation. J Orthop Traumatol. 2012;13(4):221-224.

17. Sotereanos NG, Sauber TJ, Tupis TT. Modular femoral neck fracture after primary total hip arthroplasty. J Arthroplasty. 2013;28(1):196.e7-e9.

18. Wodecki P, Sabbah D, Kermarrec G, Semaan I. New type of hip arthroplasty failure related to modular femoral components: breakage at the neck–stem junction. Orthop Traumatol Surg Res. 2013;99(6):741-744.

19. Dorn U, Neumann D, Frank M. Corrosion behavior of tantalum-coated cobalt-chromium modular necks compared to titanium modular necks in a simulator test. J Arthroplasty. 2014;29(4):831-835.

20. Carothers JT, White RE, Tripuraneni KR, Hattab MW, Archibeck MJ. Lessons learned from managing a prospective, private practice joint replacement registry: a 25-year experience. Clin Orthop Relat Res. 2013;471(2):537-543.

21. Archibeck MJ, Cummins T, Carothers J, Junick DW, White RE Jr. A comparison of two implant systems in restoration of hip geometry in arthroplasty. Clin Orthop Rel Res. 2011;469(2):443-446.

22. Duwelius PJ, Hartzband MA, Burkhart R, et al. Clinical results of a modular neck hip system: hitting the “bull’s-eye” more accurately. Am J Orthop. 2010;39(10 suppl):2-6.

23. Duwelius PJ, Burkhart B, Carnahan C, et al. Modular versus nonmodular neck femoral implants in primary total hip arthroplasty: which is better? Clin Orthop Relat Res. 2014;472(4):1240-1245.

Femoral stem modularity in total hip arthroplasty (THA) has a checkered past. Developments such as the modular head–trunnion interface, which allows for placement of femoral heads of different sizes and offsets, and the modular midstem, which allows for version adjustments independent of patient anatomy (S-ROM, Depuy) and for bypassing proximal bone defects in the revision setting (Restoration Modular, Stryker; ZMR-XL, Zimmer), have proved very successful.^1-10However, even these successful advances have been associated with failures at the modular junction.^11-13 Proximal femoral neck–stem modularity (PFNSM) has had mixed results, with notable failures and recalls associated with the neck–stem junction.^14,15 Failures at this junction have occurred secondary to corrosion and breakage of the modular neck.^16-18 Nevertheless, proximal modular stems remain available for implantation. One such system, the M/L Taper stem with Kinectiv technology (Zimmer), is an all-titanium construct that allows for adjustment of several variables (length, offset, version), providing numerous combinations beyond those of the original M/L Taper offerings. Advantages of these offerings include closer reconstruction of patient anatomy, stability improvements, and easing of the process of revision in polyethylene/femoral head exchanges or in infections in which single-staged irrigation and débridement and polyethylene/head exchange are chosen.

These theoretic advantages must be judged in the context of the possible disadvantages of the modular neck junction. The mechanical environment of the junction places it at risk for failure as well as for metallosis from fretting, crevice corrosion, and recurrent repassivation.¹⁹ Although the titanium necks are at less risk for degradation than their cobalt-chromium counterparts, they are at higher risk for breakage.^13,19 For one of the surgeons in our practice, the M/L Taper stem with Kinectiv technology is the stem of choice for primary THA.

We conducted a study to determine, in the setting of primary THA, how often a neck–stem combination choice resulted in a reconstructive geometry that would not have been possible had the surgeon opted for the traditional M/L Taper stem. Every Kinectiv stem has numerous neck options with a head center position that would not be possible with the nonmodular M/L Taper. However, in a high-volume community practice, how often is a modular neck that results in an otherwise unavailable head center being used for the reconstruction?

Materials and Methods

This study was approved by our local institutional review board. The Kinectiv stem is used by 1 of the 4 high-volume joint replacement surgeons in our practice (not one of the authors). From our community practice joint registry, we identified every stem–neck combination used since the Kinectiv stem became available in 2006.²⁰ Each case was performed using a posterior approach. A trabecular metal acetabular component (Zimmer) secured with 2 screws was used, and an M/L Taper stem with Kinectiv technology was implanted in each case.

Once the neck–stem combination was determined, its position on the head centers map was compared with that of the standard M/L Taper head centers (Figures 1, 2) for each stem size as the relationship of the Kinectiv head center varies with each stem size compared with the head center of the M/L Taper stems. If the head centers were in contact on the map, the geometry was considered identical. If the head centers were not in contact, we noted where the nearest standard M/L Taper head center lay in terms of length and offset. As the head centers are laid out in regular, 4-mm increments, this estimation was relatively easy. Any anteverted or retroverted neck was considered to have no adequate substitution in the standard M/L Taper stem offerings. This initial evaluation was performed by Dr. Carothers.

We then reviewed the head center comparisons independently. For every Kinectiv head center that did not contact an M/L Taper counterpart, the difference between those head centers was reviewed. Each of us noted whether the difference between the head centers was clinically relevant, as many of the head center positions are extremely close. The head centers that were so close as to be deemed clinically irrelevant were recorded.

Results

Between January 2008 and October 2013, 463 primary THAs were performed using the M/L Taper femoral stem with Kinectiv technology. Of the neck options used, 205 (44%) had a head center identical to that of a nonmodular M/L Taper stem. In another 56 cases (12%), all 3 reviewing surgeons agreed that the M/L Taper head center was so close to the Kinectiv head center as to be clinically indistinguishable. Of these 56 cases, 54 had a head center difference of less than 1 mm in length or offset; the other 2 had a 2-mm difference in offset.