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The Next Generation of Acetabular Shell Design and Bearing Surfaces

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Abstract

The R3 acetabular component represents the next generation of acetabular shell with an enhanced porous ingrowth surface (StikTite, Smith & Nephew, Memphis, Tenn) to meet the needs of both primary and revision hip arthroplasty; an optimized locking mechanism; and the ability to accommodate polyethylene, metal, or ceramic liners. This prospective clinical study reports on the safety and efficacy of the new StikTite porous ingrowth surface using radiostereometric analysis (RSA). StikTite provides a superior “scratch-fit” due to its greater coefficient of friction and less micromotion using RSA measurements.

Over the past 25 years, cementless acetabular components have become increasingly popular for use in total hip arthroplasty (THA).¹ Significant improvements have been made in the bearing surface couples, socket design, locking mechanisms, and ingrowth surfaces. Today, more than 89% of primary THAs in North America utilize cementless acetabular sockets.²

Figure 1: Bearing options provided by the R3 acetabular system – XLPE cross-linked polyethylene on cobalt chrome/ceramic/Oxinium (Smith & Nephew), metal-on-metal, or ceramic-on-ceramic. FDA approval is pending for the ceramic-on-ceramic and metal-on-metal articulations.

The cementless R3 socket (StikTite, Smith & Nephew, Memphis, Tenn) is an example of a fourth-generation acetabular component design allowing the orthopedic surgeon to use either cross-linked polyethylene, ceramic, or metal liners in the same titanium alloy metal shell (Figure 1). The locking mechanism has been optimized to prevent liner micromotion, increase liner push-out strength, and minimize the risk of cross-linked polyethylene liner fracture. Although traditional porous ingrowth/ongrowth surfaces have provided good cementless acetabular fixation,^3-5 there has been a move to newer porous surfaces with greater porosity (Figure 2) and coefficient of friction (Figure 3).⁶ In keeping with this trend, the new R3 acetabular socket uses a 3-dimensional (3D) asymmetric titanium powder coating with 60% porosity compared with traditional porous coatings with 45% or less porosity (Figure 4).

The purpose of this study was to compare the safety and efficacy of the new 3D asymmetric titanium powder coating (StikTite) vs a traditional sintered bead coating (Roughcoat, Smith & Nephew) using a prospective clinical trial and radiostereometric analysis (RSA) as the endpoint. RSA is reported to be the most accurate method of determining in vivo component micromotion.^4,5,7

Methods and Materials

Study Design

A prospectively designed cohort study was initiated to examine the in vivo performance of the 3D asymmetric titanium powder coating (StikTite) applied to Reflection acetabular shells (Smith & Nephew, Memphis, TN). Permission to conduct this study was provided by the Internal Review Board of the University of Western Ontario. Patient inclusion criteria included a diagnosis of osteoarthritis, age between 50 and 80 years, sufficient bone quality to accept a cementless THA, and patient consent. Exclusion criteria included inflammatory arthritis, history of sepsis, previous pelvic osteotomy, previous pelvic fracture, previous pelvic irradiation, previous tumor, or a diagnosis of developmental dysplasia of the hip. Twenty patients were enrolled in this safety and efficacy study. Sample size was limited to 20 patients to minimize the number of patients exposed to irradiation.

All operations were performed using a direct lateral surgical approach. During acetabular preparation, the reamer was directed immediately to the quadrilateral plate and the acetabulum expanded until bleeding bone was encountered. The acetabular socket was underreamed by 1 mm in comparison with the diameter of the acetabular component. The use of supplemental screws was permitted and depended on the discretion of the surgeon (used in 8 of 20 patients).

Radiostereometric Analysis (RSA)

The use of RSA for the determination of implant micromotion has been well documented, using biplanar radiographs and radiodense tantalum beads inserted into the periacetabular pelvic bone and the acetabular component.^4,5,7,8 In this study, 12 1-mm tantalum beads were inserted into the rim of the Reflection cross-linked polyethylene liner and 10 to 12 tantalum beads within the periacetabular bone. Biplanar radiographs of the study hip were taken in a dedicated, digital RSA suite. A uniplanar RSA calibration cage was used in the analysis. Interpretation of the 3D positioning of the implanted acetabular cup and reference to the patient’s bone stock was performed using a custom RSA software system (Figure 5). Patients were assessed immediately postoperatively, at 6 weeks, 12 weeks, 6 months, and 1 year. Additional examinations will be performed at 2 and 5 years.

RSA allows the interpretation of mediolateral translation (x-axis), proximal-distal translation (y-axis), and anteroposterior translation (z-axis), as well as three planes of rotation, anteroposterior tilt (x-axis rotation), anteversion-retroversion (y-axis rotation), and increase-decrease inclination (z-axis rotation). All translations and rotations were anatomically corrected and reported in the method suggested by Valstar et al to standardize the reporting of RSA analyses.⁷ In this report, medial, proximal, and anterior translations are reported in the positive direction about their prospective axes, and lateral, distal, and posterior are negative. Similarly, positive values are used for anterior tilt, anteversion, and increased inclination, whereas negative values are used for posterior tilt, retroversion, and decreased inclination rotations. Two-dimensional and 3D cup migrations also were calculated using the Pythagorean theorem.

Results

Our study population of 20 patients included 14 women, an average age of 75 years, and an average body mass index of 28.7 (Table 1). StikTite-coated Reflection sockets were press fit in 12 patients, and supplemental screw fixation was used in the remaining 8 patients (6 patients with 2 screws and 2 patients with 3 screws). Figure 6A illustrates a representative patient with a StikTite-coated Reflection cementless socket and a cementless Synergy stem. Figure 6B depicts the total 3D translations (mediolateral, distal/proximal, and anteroposterior) for this patient.

RSA revealed minimal cup migration with mean individual direction translations well below 0.1 mm and mean rotations below 0.35º at all follow-up visits (6 weeks, 3 months, 6 months, and 1 year) (Table 2). The majority of any migration occurred within the first 6 weeks of implantation, after which little change was noted (Figure 7,A-D).

Discussion

There is considerable interest in new porous coatings with greater porosity because they provide a greater coefficient friction for initial implant fixation and the potential for a greater bony ingrowth.⁶ The introduction of any new technology such as a new porous coating requires rigorous testing before it is introduced into widespread clinical use. Malchau has suggested a stepwise process for the introduction of new technologies including preclinical testing, randomized clinical trials with the use of RSA, multicenter studies, and postmarket surveillance.⁹

The purpose of this study was to assess a new porous coating, asymmetric titanium powder (StikTite) applied to a cementless acetabular shell with a known track record. The average RSA translations and rotations measured at each follow-up are presented in Table 2.

Zhou et al have reported on the same Reflection acetabular shell coated with a sintered porous bead coating.⁸ Our study has revealed that the 3D asymmetric powder coating (StikTite) resulted in a significant decrease in migration in all translations and rotations compared with a sintered bead porous coating (Roughcoat) (Table 3). The reduction in translations and rotations was striking in the first 6 weeks of follow-up, perhaps indicative of the increased coefficient of friction of centered asymmetric porous titanium coating (StikTite) compared with a centered bead coating (Roughcoat).⁶ The StikTite-coated cups showed a 64% decrease in medial translation, an 81.1% decrease in proximal translation, and no difference in anterior to posterior translation. Rotational movements were equally affected with markedly increased stability from 43.6% to 79.6.7%. At 1 year, StikTite revealed a slight (8.2%) decrease in medial migration, an 83.6% decrease in proximal migration, and a slight (18.2%) increase in anterior translation. Rotational components at 1 year were decreased by 56.2% for anterior tilt, over 91.7% for anteversion, and by 48% for increased inclination.

Conclusion

Three-dimensional asymmetric titanium powder (StikTite) coating seems to be an ideal fourth-generation porous coating for use in orthopedic implants. In this study, we have demonstrated a greater initial stability as indicated by less micromotion at 6 weeks and translations and rotations lower than commonly used sintered bead porous coatings. Therefore, it seems reasonable to apply the new asymmetric titanium powder coating to newer implants to take advantage of the greater coefficient of friction for initial stability and the potential for enhanced porous ingrowth.⁶

References

1. Barrack RL. Primary total hip arthroplasty: Uncemented acetabulum “In” Advanced Reconstruction – Hip, editors JR Lieberman and DJ Berry, American Academy of Orthopaedic Surgeons, 2005, 55-61.
2. Canadian Joint Replacement Registry – 2005 Report Total Hip and Total Knee Replacements in Canada. Ottawa, Canada: Canadian Institute for Health Information; 2005.
3. Clohisy JC, Harris WH. The Harris-Galante porous-coated acetabular component with screw fixation. An average ten-year follow-up study. J Bone Joint Surg, 1999:81A:66-73.
4. Rohrl SM, Nivbrant B, Strom H, Nilsson KG. Effect of augmented cup fixation on stability, wear and osteolysis – a 5-year follow-up of total hip arthroplasty with RSA. J Arthroplasty. 2004:19:962-971.
5. Rohrl SM, Nivbrant B, Snorrason F, Karholm J, Nilsson KG. Porous-coated cups fixed with screws: a 12-year clinical and radiostereometric follow-up study of 50 hips. Acta Orthop, 2006; 77(3):393-401.
6. Heiner AD, Brown TD. Frictional coefficients of a new bone ingrowth structure. Trans Orthopaedic Research Society, p 1623, 2007.
7. Valstar ER, Gill R, Ryd L, Flivik G, Borlin N, Karrholm J. Guidelines for standardization of radiostereometry (RSA) of implants. Acta Orthop. 2005;76(4):563-572.
8. Zhou Z, Li MG, Borlin N, Wood DJ, Nivbrant B. No increased migration in cups with ceramic-on- ceramic bearing: an RSA study. Clin Orthop Relat Res. 2006;448: 39-45.
9. Malchau H. On the Importance of Stepwise Introduction of New Hip Implant Technology [thesis]. Goteborg, Sweden; 2005.

Disclaimer: StikTite has not been approved by the U.S. Food & Drug Administration for ceramic insertion.

Authors

Drs Bourne, McCalden, Naudie, Charron, and Yuan are from the London Health Sciences Centre, University of Western Ontario, London, Ontario, Canada.

Dr Bourne is designer of the R3 acetabular component and consultant for Smith & Nephew, Inc. Drs McCalden and Naudie are consultants for Smith & Nephew, Inc. Dr Naudie receives institutional support from Smith & Nephew. Dr Charron, Yuan, and Holdsworth have no relevant financial relationships to disclose.

This work was supported by a research grant from Smith & Nephew, Inc, Memphis, Tennessee.

Correspondence should be addressed to Robert B. Bourne, MD, FRCSC, University Hospital, 339 Windermere Road, London, Ontario N6A 5A5 Canada.