Implants with surfaces of various porosities and pore sizes are in clinical use. This article demonstrates how macroscopic porous metal fragments can detach from the implant surface in total hip arthroplasty (THA) and cause significant third-body damage such as deep scratches and indentations in implants bearing surfaces. Radiographs prior to revision surgery due to aseptic loosening of the acetabular component revealed the presence of numerous small metal fragments approximately 1 to 2 mm in size in the periarticular area. The size, shape, and material of the metal fragments (cobaltchromiummolybdenum [CoCrMo]) indicated that they originated from the porous metal surface.
In this case, the acetabular liner composite material consisted of two-thirds polyurethane and one-third aluminium oxide ceramic. The femoral head was made of aluminium oxide ceramic. The aluminium oxide femoral head, which had been in situ for 21 years, showed no signs of macroscopic indentations or scratches, suggesting that an aluminium oxide bearing surface, which is significantly harder than the CoCrMo debris, is not significantly affected by metal debris embedment in the counterface material. The polyurethanealuminium oxide composite material used for the acetabular liner is not comparable to a traditional ceramic bearing surface material. Debris damaged the surface of and became embedded in the liner, causing accelerated wear of the femoral head.
In porous metal surface THA, ceramic-on-ceramic bearing couples should, due to their superior hardness, be considered to prevent excessive wear, including debris embedment and scratching of the bearing surfaces, especially in revision cases.
In cementless fixation of femoral stems and acetabular components, both primary stability and secondary osseointegration are critical in achieving long-term implant survival.1-3 In an attempt to increase implant stability by surface area enlargement and secondary osseointegration, various surface structures such as sintered fiber titanium composite,4 porous metal,5 and casted beads6 have been proposed.
Today, implants with surfaces of various porosities and pore sizes are in clinical use. Bobyn et al7 determined the optimum pore size for bone ingrowth into porous-surfaced metallic implants. Gription (DePuy, Warsaw, Indiana), Trabecular Metal (Zimmer, Warsaw, Indiana), and Regenerex (Biomet, Warsaw, Indiana) are all examples of surfaces designed to promote bone ingrowth, the latter 2 also being available as stand-alone load-bearing materials.
This case study demonstrates how macroscopic porous metal surface fragments can damage the bearing surfaces of total hip arthroplasties (THAs) through third-body wear. The patient was informed that data concerning the case would be submitted for publication.
A 69-year-old man underwent revision surgery for aseptic loosening of the acetabular component after 8 years in situ. Preoperative radiographs revealed the presence of numerous small metal fragments approximately 1 to 2 mm in size in the periarticular area (Figure 1).
A cementless porous metal surface stem and acetabular component (Spongiosa Metal; ESKA Implants AG, Lübeck, Germany) had been implanted 21 years before. Thirteen years later, during the first revision, the acetabular component was revised and a cementless porous metal surface cup (Spongiosa Metal II; ESKA Implants AG) with 2 additional screws and a 32-mm composite material inlay consisting of two-thirds polyurethane and one-third aluminium oxide (ESKA-CERAM; ESKA Implants AG) were implanted. The original femoral head remained in situ.
Spongiosa Metal, which was used for the surface of the stem and the primary acetabular component, is an interconnecting porous structure.8 Spongiosa Metal II consists of individual hexapods of varying height (1-3 mm) and corresponding thickness (0.4-0.7 mm). Core and surface structure are made of cast cobaltchromiummolybdenum (CoCrMo) alloy. Each hexapod is attached to the core of the implant at 3 points.9
Visual inspection of the femoral head and acetabular component retrieved during the second revision revealed evidence of excessive wear of the acetabular liner, including macroscopic indentations and scratching (Figure 2). The indentations in the inferior region of the retrieved acetabular liner had the size of the metal fragments observed on the preoperative radiographs. The size, shape, and material of the metal fragments (CoCrMo) indicated that they originated from the porous metal surface. Examination of the head showed a small area of dark discoloration but neither scratching nor indentations. The dark spots were likely metal deposits abraded from metal fragments embedded in the acetabular liner.
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|Figure 1: Preoperative radiograph showing one of numerous metal fragments (1-2 mm) in the periarticular area (arrow). Figure 2: Photograph showing indentations and scratches in the inferior region of the 32-mm acetabular liner surface. |
Metal fragments can cause significant third-body damage, such as deep scratches and indentations, to bearing surfaces. Implants with a porous metal structure can yield metallic third bodies. The risk of debris formation and migration is increased during revision surgery, especially since the removal of porous-surfaced metal components may be complicated by otherwise desired bone ingrowth.
Metal fragments can enter the intra-articular space during instances of microseparation10,11 or subluxation due to impingement.12 The presence of metallic third bodies between the articulating surfaces has been shown to cause severe femoral head wear.13 If third bodies can leave the intra-articular space, wear acceleration may decrease over time. Roughening of the femoral head, however, causes an increase in counterface wear in cobaltchromiumultra-high molecular weight polyethylene (CoCrUHMWPE) bearing couples,14 even after all metal fragments have left the intra-articular space.
In this case, the acetabular liner composite material consisted of two-thirds polyurethane and one-third aluminium oxide ceramic. The femoral head was made of aluminium oxide ceramic. In the retrieved implant, the polyurethanealuminium oxide inlay clearly showed more evidence of wear than the aluminium oxide ceramic head. Indentations in the polyurethanealuminium oxide acetabular liner indicated that metal fragments became embedded in the liner surface, which suggests that the polyurethanealuminium oxide composite is a soft material and, although named CERAM, is not comparable to traditional hard ceramic bearing surfaces.
The aluminium oxide femoral head, which had been in situ for 21 years, showed no signs of macroscopic indentations or scratches. This observation suggests that an aluminium oxide bearing surface is not significantly affected by metal debris embedment in the counterface material. This effect is due to the superior hardness of aluminium oxide compared to the CoCrMo metal alloy debris.
The polyurethanealuminium oxide composite material used for the acetabular liner is not comparable to a traditional ceramic bearing surface material. Debris can damage the surface of and become embedded in the liner causing accelerated wear of the femoral head. This can be prevented if the bearing surface is made of a material harder than the debris.
Davidson et al15 showed that the resistance to abrasive wear of metal and ceramic bearing surfaces depends on the substrate/abrasive hardness ratio and increases with increasing surface hardness. For bone debris, polymethylmethacrylate cement, and titanium debris, it has been shown that metals such as Ti-6Al-4V and CoCrMo running against UHMWPE are susceptible to abrasion because their hardness values are lower than the hardness of the third bodies, whereas ceramic bearing surfaces showed no signs of abrasion.15 Thus, in bearing materials of similar or lower hardness than the debris, abrasion of the bearing surfaces will occur.
In porous metal surface THA, ceramic-on-ceramic bearing couples should, due to their superior hardness, be considered to prevent excessive wear, including debris embedment and scratching of the bearing surfaces, especially in revision cases. In applications such as total knee arthroplasty, without the standard option of ceramic-on-ceramic bearings, the risk of macroscopic metal debris formation from porous metal surfaces must be taken into account when choosing implant components.
- Callaghan JJ, Fulghum CS, Glisson RR, Stranne SK. The effect of femoral stem geometry on interface motion in uncemented porous-coated total hip prostheses. Comparison of straight-stem and curved-stem designs. J Bone Joint Surg Am. 1992; 74(6):839-848.
- Ducheyne P, De Meester P, Aernoudt E. Influence of a functional dynamic loading on bone ingrowth into surface pores of orthopedic implants. J Biomed Mater Res. 1977; 11(6):811-838.
- Engh CA, Bobyn JD, Glassman AH. Porous-coated hip replacement. The factors governing bone ingrowth, stress shielding, and clinical results. J Bone Joint Surg Br. 1987; 69(1):45-55.
- Galante J, Rostoker W, Lueck R, Ray RD. Sintered fiber metal composites as a basis for attachment of implants to bone. J Bone Joint Surg Am. 1971; 53(1):101-114.
- Judet R, Siguier M, Brumpt B, Judet T. A noncemented total hip prosthesis. Clin Orthop Relat Res. 1978; (137):76-84.
- Lord GA, Hardy JR, Kummer FJ. An uncemented total hip replacement: experimental study and review of 300 madreporique arthroplasties. Clin Orthop Relat Res. 1979; (141):2-16.
- Bobyn JD, Pilliar RM, Cameron HU, Weatherly GC. The optimum pore size for the fixation of porous-surfaced metal implants by the ingrowth of bone. Clin Orthop Relat Res. 1980; (150):263-270.
- Götze C, Tschugunow A, Wiegelmann F, Osada N, Götze HG, Böttner F. Long-term influence of the spongiosa metal surface prosthesis on the periprosthetic bone. A radiological and osteodensitometric analysis of implantation of the S&G (ESKA) hip prosthesis [in German]. Z Orthop Ihre Grenzgeb. 2006; 144(2):192-198.
- Mittelmeier W, Grunwald I, Schäfer R, Grundei H, Gradinger R. Cementless fixation of the endoprosthesis using trabecular, 3-dimensional interconnected surface structures [in German]. Orthopade. 1997; 26(2):117-124.
- Dennis DA, Komistek RD, Northcut EJ, Ochoa JA, Ritchie A. In vivo determination of hip joint separation and the forces generated due to impact loading conditions. J Biomech. 2001; 34(5):623-629.
- Lombardi AV Jr, Mallory TH, Dennis DA, Komistek RD, Fada RA, Northcut EJ. An in vivo determination of total hip arthroplasty pistoning during activity. J Arthroplasty. 2000; 15(6):702-709.
- Shon WY, Baldini T, Peterson MG, Wright TM, Salvati EA. Impingement in total hip arthroplasty a study of retrieved acetabular components. J Arthroplasty. 2005; 20(4):427-435.
- McKellop HA, Röstlund TV. The wear behavior of ion-implanted Ti-6A1-4V against UHMW polyethylene. J Biomed Mater Res. 1990; 24(11):1413-1425.
- Shen FW, McKellop H. Surface-gradient cross-linked polyethylene acetabular cups: oxidation resistance and wear against smooth and rough femoral balls. Clin Orthop Relat Res. 2005; (430):80-88.
- Davidson JA, Poggie RA, Mishra AK. Abrasive wear of ceramic, metal, and UHMWPE bearing surfaces from third-body bone, PMMA bone cement, and titanium debris. Biomed Mater Eng. 1994; 4(3):213-229.
Drs Kleinhans, Jakubowitz, and Kretzer are from the Laboratory of Biomechanics and Implant Research, Dr Seeger is from the Department of Orthopedic Surgery, University of Heidelberg, and Dr Heisel is from Arcus Kliniken Pforzheim, Department of Orthopedics and Traumatology, Pforzheim, Germany.
Drs Kleinhans, Jakubowitz, Seeger, Heisel, and Kretzer have no relevant financial relationships to disclose.
Correspondence should be addressed to: J. Philippe Kretzer, PhD, Department of Orthopedic Surgery, University of Heidelberg, Schlierbacher Landstrasse 200a, 69118 Heidelberg, Germany.