Wear analysis of a retrieved hip implant with titanium nitride coating

Wear analysis of a retrieved hip implant with titanium nitride coating

The Journal of Arthroplasty Vol. 12 No. 8 1997 Case Report Wear Analysis of a Retrieved Hip I m p l a n t With Titanium Nitride Coating Melinda K. ...

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The Journal of Arthroplasty Vol. 12 No. 8 1997

Case Report

Wear Analysis of a Retrieved Hip I m p l a n t With Titanium Nitride Coating Melinda

K. Harman,

M S , * S c o t t A. B a n k s , P h D , * a n d W. A n d r e w

Hodge, MD* t

Abstract: There is increasing interest in using surface modification technology to

improve the wear properties of titanium alloy and limit articular surface wear of metal and polyethylene components. This report details the in vivo wear performance of titanium nitride coating on a retrieved hip implant obtained postmortem from a low demand patient 1 year after total hip arthroplasty. Analysis of the wellfunctioning implant revealed that wear debris can originate from a titanium nitride coated femoral head, as delaminated surface asperities, and manifest as adhesive wear on the articular surface. The wear observed on this implant indicates that rigorous testing and evaluation of titanium nitride coating technology should be conducted prior to widespread use on total joint implants. Key words: titanium, titanium nitride, total hip arthroplasty, implant retrieval, wear, surface modification.

Implant retrieval analyses and in vitro wear simulations have s h o w n that limitations exist regarding the use of titanium alloy (Ti-6AI-4V) as the articular surface of total joint implants [1-14]. As a result, there is increasing interest in using surface modification technology to improve the wear properties of TI-6A1-4V. Titanium nitride (TIN) coating, 1 of several surface modifications currently being used, improves the fretting resistance of the articular surface by increasing the surface hardness [ 15-19]. This modification technique, used by a single m a n u f a c t u r e r (Endotec, South Orange, N J), employs physical vapor deposition (PVD) of pure titanium and nitrogen gas to create a thin surface layer ( 3 - 1 0 g m ) of TiN coating [16,18,20-23]. W h e n TiN is applied to polished Ti-6AI-4V femoral

heads, the TiN coating is repolished to a 0.04-btm finish and a smooth, hard articular surface is obtained [18]. The ability of TiN coating to m a i n t a i n a highly polished articular surface and reduce p o l y e t h y lene w e a r has b e e n investigated in n u m e r o u s in vitro e x p e r i m e n t s [9,15,1%19,24-28]. In hip implant and k n e e implant w e a r simulations, TiNcoated implants s h o w decreased p o l y e t h y l e n e and metal w e a r w h e n c o m p a r e d with o t h e r implant materials [9,19,24-26]. It has also b e e n s h o w n that TiN coating reduces fretting corrosion w h e n m e t a l - o n - m e t a l contact occurs b e t w e e n Ti-6A1-4V and TiN-coated surfaces [15,16]. In contrast, o t h e r investigators have s h o w n that TiNcoated surfaces increase p o l y e t h y l e n e articular surface roughness [28] and c o n f o r m i t y [26,28], indicating an increase in p o l y e t h y l e n e wear. Furt h e r m o r e , failure of TiN coating adhesion [17] and TiN fretting and coating b r e a k t h r o u g h [15,25,29] have b e e n reported to occur. To date, limited clinical data have b e e n reported regarding the survivorship of TiN-coated implants

From the *Orthopaedic Research Laboratory, Good Samaritan Medical Center, and the -i-Palm Beach Orthopaedic Institute, West Palm Beach, Florida. Reprint requests: Melinda K. Harman, MS, Orthopaedic Research Laboratory, Good Samaritan Medical Center, RO. Box 3166, West Palm Beach, FL 33402. ©1997 Churchill Livingstone Inc.

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used for total joint arthroplasty [30,31] and assessm e n t of the in vivo w e a r characteristics of TiNcoated articular surfaces has not b e e n reported. This case report details the wear observed on the articular surface of a TiN-coated hip implant retrieved p o s t m o r t e m after 1 year of in vivo function. The observed locations and extent of material wear are assessed and proposed damage mechanisms are suggested.

Case History A PVD TiN-coated hip prosthesis was retrieved p o s t m o r t e m from the right hip of a 79-year-old man. The d o n o r had a history of severe osteoarthritis and the primary total hip arthroplasty had b e e n completed w i t h o u t complications i2 m o n t h s previously. Clinical follow-up evaluation and comm u n i c a t i o n with the donor's family revealed that the d o n o r was only a h o u s e h o l d ambulator with a walker because of a sense of unsteadiness and a fear of falling. The cause of death was acute renal failure.

Materials and Methods The implants were carefully dissected during retrieval to preserve the pelvic b o n e surrounding the acetabular c o m p o n e n t and 30 m m of bone distal to the femoral stem. The u n c e m e n t e d acetabular cup and u n c e m e n t e d femoral stem were m a n u ally assessed to be stable, and radiographs did not reveal any radiolucencies surrounding the implants. The c o m p o n e n t s consisted of a polyethylene liner in a 6I m m porous-coated Ti-6A1-4V acetabular cup with TiN coating, a TiN-coated Ti-6AI-4V femoral stem with a 16-mm-diameter modular distal tip, and a 3 2 - m m - d i a m e t e r modular TiNcoated Ti-6A1-4V h e a d (Buechel-Pappas System, Endotec). The acetabular cup had 5 holes for screw fixation, although screws had not b e e n used in this case. The articular surface of each c o m p o n e n t , as well as any exposed implant surfaces, was e x a m i n e d using a stereomicroscope. The orientation of the TiN head was marked, and the head was r e m o v e d from the femoral stem to facilitate analysis. Similarly, the orientation of the polyethylene cup was m a r k e d and r e m o v e d from the metal backing, These c o m p o n e n t s were submersed in 5% sodium hypochlorite to r e m o v e a n y tissue debris, ultrasonically cleaned, and t h e n air-dried. The articular surface of the acetabular cup was carbon coated in preparation for scanning microscopy. The acetabu-



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lar cup, the femoral stem, and the surrounding bone tissues were t h e n cut into 10-ram sections, fixed in 10% neutral-buffered formalin, dehydrated in ascending grades of ethyl alcohol, and e m b e d d e d in polymer (Technovit 7200, Exakt Medical Technologies, Oklahoma City, OK). These sections were polished and coated with carbon. A scanning electron microscope (JSM 6100, JEOL USA, Peabody, MA) was used to assess the condition of the implant materials and to quantify the surface wear. Titanium nitride coating integrity was assessed on the femoral head using secondary electron and backscattered electron imaging to generate n u m e r o u s photographs at magnifications up to 4,000x. Similarly, the porous coating on the acetabular cup and proximal femoral stem sections was examined. Energy-dispersive x-ray spectrometry (Kevex DehaPro Plus, Fisons Instruments, San Carlos, CA) was used to determine the elemental composition of the implant materials [29,32] and any third-body particulate debris f o u n d adhered to the implant, in the tissue specimens, or in the polyethylene cup [7, 33,34].

Results Femoral Head Surface Wear Analysis Gross examination showed a dull gray discoloration on the gold-colored TiN coating localized to the posterosuperior region of the femoral head. Scanning electron microscopic images of intact TiN coating appeared smooth with uniformly dispersed voids 1-2 btm in diameter. Energy-dispersive x-ray spectrometric analysis in this region produced titanium and nitrogen energy peaks, consistent with intact TiN coating (Fig. 1). Numerous larger voids, up to l0 btm in diameter, were visualized in the discolored regions, with circular droplets of pure titanium evident in some of the voids (Fig. 2). On the posterosuperior head, there was debris 5-75 btm in width affixed to the TiN surface. Tilting the specim e n in the scanning electron microscope revealed that the debris was raised 1-10 btm above the articular surface with a structural appearance consistent with adhesive wear (Figs. 3, 4). Energy-dispersive xray spectrometric analysis of the debris produced energy spectra consistent with pure titanium (Fig. 5) and also spectra consistent with Ti-6A1-4V (Fig. 6).

Polyethylene Acetabular Liner Surface Wear Analysis Gross examination of the polyethylene liner and the rim of the metal acetabular cup did not reveal

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Fig. l. Energy-dispersive xray spectra of intact titanium nitride coating, with energy peaks for titanium and nitrogen.

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a n y evidence of h e a d dislocation. A thin fibrous m e m b r a n e was f o u n d b e t w e e n the m e t a l cup a n d p o l y e t h y l e n e liner o n disassembly. The articular surface of the p o l y e t h y l e n e cup was e x a m i n e d at 30x magnification a n d the d a m a g e m o d e s w e r e d o c u m e n t e d {35]. There w e r e scratches on the posterior inferior edge of the cup, covering m o r e t h a n 50% of the available area (grade 3), a n d o n the anterior section, covering less t h a n 10% of the surface (grade 1). Polishing of grade 3 was m o s t extensive on the anterior inferior portion of the cup. The p o l y e t h y l e n e articular surface m a i n t a i n e d its h e m i s p h e r i c shape a n d creep d e f o r m a t i o n was not seen. Scanning electron microscopy revealed u n i f o r m linear scratches m e a s u r i n g a p p r o x i m a t e l y i 0 ~ m in width, w h i c h is the s a m e order of m a g n i tude as the size of the a d h e r e n t debris on the femoral head. Third-body particulate debris was not observed using secondary electron a n d backscattered electron imaging.

Porous Coating on the Acetabular Cup and Proximal Femoral Stem The porous coating on b o t h c o m p o n e n t s was grossly intact. The p o r o u s coating on the acetabular cup was not fully covered w i t h tissues, allowing e x a m i n a t i o n of the TiN-coated b e a d surface. The TiN coating on the beads was r o u g h a n d n o n u n i form, consistent with a surface that was not polished after the PVD coating process. There was n o evidence of b e a d separation or migration on a n y of the cup or stem sections, a n d backscattered electron imaging did not reveal a n y metallic debris in

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the s u r r o u n d i n g b o n e tissue. Energy-dispersive xray spectrometric analysis of the b e a d substrate p r o d u c e d e n e r g y spectra consistent with p u r e titan i u m , and the b e a d surface p r o d u c e d e n e r g y spectra consistent w i t h TiN.

Discussion This case report d o c u m e n t s the in vivo w e a r perf o r m a n c e of a PVD TiN surface modification on the articular surface of a total hip i m p l a n t retrieved after 1 year of in vivo function. Circular voids without TiN coating a n d voids filled with circular droplets of p u r e t i t a n i u m w e r e evident on the femoral h e a d (Fig. 2), a n d p u r e t i t a n i u m and Ti-6AI-4V debris was f o u n d a d h e r e d to the TiN coating (Figs. 3, 4). During the PVD TiN coating process, n o n u n i f o r m t i t a n i u m droplet size and adhesion variations can occur due to fluctuating processing controls. These droplets do not b e c o m e a n integral part of the TiN coating at the substrate interface or on the coating surface, w h i c h creates a n o n u n i f o r m TiN coating (personal c o m m u n i c a t i o n , Endotec). Aggressive final polishing of the TiN coating on the femoral heads is m e a n t to r e m o v e these asperities during i m p l a n t m a n u f a c t u r i n g ; however, r e m o v a l of the larger droplets can leave a void in the TiN-coated surface. Detection of a n y voids greater t h a n 3/1,000 inch (76 ~m) in diameter during visual assessment of the polished surface causes the i m p l a n t to be w i t h h e l d f r o m distribution (personal c o m m u n i c a t i o n , Endotec). During in vivo articulation, surface asperities that are smaller t h a n

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Fig. 2. (A,B) Scanning electron photomicrographs of surface asperities (a) and voids (b) in discolored region of titanium nitride (TIN) coating. (C) Magnified view of Figure 2B showing a circular droplet of pure titanium (a) within a void (b).

the size tolerance used by the m a n u f a c t u r e r could d e l a m i n a t e f r o m the TiN surface, creating the voids that we observed in the coating a n d acting as thirdb o d y particulate. M a n u f a c t u r i n g procedures used for the femoral heads m a y n e e d to be e x a m i n e d to avoid the introduction of a n y surface c o n t a m i n a n t s that could interfere w i t h TiN coating adhesion a n d to ensure that coating variability is m i n i m i z e d during the PVD process [17,36]. Quality control m e t h ods that can detect TiN coating discontinuities that are smaller t h a n the 76-btm visual tolerance currently in use m a y help to eliminate the t i t a n i u m droplets and surface asperities w e observed. The limited time in situ for this i m p l a n t a n d the activity history of the patient indicate that low

d e m a n d was placed on the articular surface, and yet the TiN coating did not p e r f o r m to the level predicted by in vitro joint w e a r simulations [9,19, 24-26]. Pappas et al. reported less t h a n 2 g m of w e a r on 4 7 - r a m - d i a m e t e r TiN f e m o r a l heads after articulation against p o l y e t h y l e n e for 48 million cycles in a hip w e a r simulator [19]; however, previous lower-cycle tests (10 million cycles) using 32m m - d i a m e t e r TiN heads resulted in regions of exposed substrate. These data led to the use of thicker TiN coatings on implants m a n u f a c t u r e d for clinical use, but additional w e a r data t r o m 3 2 - m m heads with the thicker coatings h a v e not b e e n reported, m a k i n g direct c o m p a r i s o n w i t h our in vivo observations difficult. F u r t h e r m o r e , filtration

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Fig. 3. (A) Scanning electron photomicrograph of adhesive titanium alloy (Ti-6A1-4V) debris (a) on the surface of the titanium nitride (TiN)-coated femoral head. The labeled region (a) indicates the location used for energy-dispersive x-ray spectrometric analysis. (B) Magnified view of Figure 3A imaged at a 30 ° tilt to show the adhered debris (a) profile.

of the lubrication m e d i a during cyclic loading r e m o v e d a n y w e a r debris g e n e r a t e d at the articular surface. This p r o c e d u r e does not replicate the high v o l u m e of debris particulate that can be present in vivo [6], a n d a n y d e l a m i n a t i o n of the surface asperities we observed in vivo w o u l d be filtered out in the simulation. W h e n articulated against polyethylene, TiN coatings h a v e b e e n s h o w n to significantly decrease polyethylene w e a r c o m p a r e d with cobalt-chromium, t h o u g h conflicting results h a v e b e e n r e p o r t e d because of variations in the m e t a l surface finish and

the w e a r m e a s u r e m e n t techniques used [24-26, 281. Based on visual observation of m e t a l wear, Peterson et al. concluded that c o b a l t - c h r o m i u m and TiN coating each p e r f o r m superior to Ti-6A1-4V a n d are equally scratch resistant w h e n articulated against carbon fiber-reinforced p o l y e t h y l e n e tibial inserts [9]. Other in vitro investigations h a v e s h o w n that TiN coating decreases fretting corrosion w h e n m e t a l - o n - m e t a l contact occurs [15,16], but it does not significantly improve the wear performance of Ti-6AI-4V w h e n articulated against b o n e or b o n e - c e m e n t [16[. Failure of TiN coating adhesion a n d coating b r e a k t h r o u g h h a v e also b e e n reported to occur [15,17, 25,29], reducing the w e a r properties of the i m p l a n t to those of the exposed Ti-6A1-4V substrate in the presence of e x t r e m e l y h a r d t h i r d - b o d y particulate debris [15,29]. In applications in w h i c h devices are not required to p e r f o r m for e x t e n d e d p e r i o d s - - a s with b o n e cutting instruments, for e x a m p l e - - i m p r o v e d perform a n c e a n d increased durability h a v e b e e n realized w h e n i n s t r u m e n t s are coated w i t h TiN [22, 23].

Conclusion

Fig. 4. Scanning electron photomicrograph of adhesive pure titanium debris (a) on the surface of the titanium nitride (TiN)-coated femoral head.

Particulate debris is a leading concern after total joint arthroplasty {2-9,12,37-46], and surface modifications are being explored in an effort to limit articular surface wear of metal and polyethylene components. Our retrieval observations show that wear debris can originate from a TiN-coated femoral h e a d as delaminated surface asperities and manifest as adhesive w e a r on the articular surface. In the event

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Fig. 5. Energy-dispersive x-ray spectra obtained from pure titanium debris adhered to the titanium nitride (TiN)-coated articular surface. The absence of a nitrogen peak distinguishes these spectra from those obtained from the intact TiN coating (Fig. 1).

of coating breakthrough, the exposed Ti-6A1-4V substrate would be susceptible to third-body w e a r [9-11, 29,47-49]. Considering that this TiN surface modification involves manufacturing processes that are not otherwise used during the fabrication of uncoated implants, the surface variations we observed are confined only to implants that have undergone similar

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TiN coating processes. Given the p r o v e n clinical performance of c o b a l t - c h r o m i u m total hip arthroplasty femoral heads [47], the use of TiN-coated implants should continue to be monitored dosely until their in vivo performance and cost-effectiveness are proven equivalent or superior to those of this widely accepted material.

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Fig. 6. Energy-dispersive x-ray spectra obtained from titanium alloy (Ti-6A1-4V) debris adhered to the titanium nitride (TiN)-coated articular surface. The distinct aluminum peak and the absence of a nitrogen peak distinguish these spectra from those obtained from the intact TiN coating {Fig. 1).

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