Ferrography: Its application to the study of human joint wear

Wear, 50 (1978) 115 - 125

FERROGRAPHY: JOINT WEAR

DANA

C. MEARS,

Department VERNON

ITS APPLICATION

EDWARD

of Orthopaedic

N. HANLEY,

TO THE STUDY OF HUMAN

Jr., and ROBERT

Surgery, University

of Pittsburgh,

RUTKOWSKI

Pittsburgh,

Pa. (U.S.A.)

C. WESTCOTT

Foxboro/Trans-Sonics, (Received

115

December

Burlington,

Mass. (U.S.A.)

23, 1977)

Summary Synovial fluid aspirates of 20 arthroplastic and 150 osteoarthritic joints were analyzed for evidence of wear particles. Ferrography, an industrial technique for the separation of particulate matter from samples of lubricating solutions, allows extraction of wear particles from synovial fluid. Bichromatic polarized m icroscopy and scanning electron m icroscopy permit identification and characterization of metallic, polyethylene and acrylic wear particles from arthroplastic joints as well as biological wear fragments of bone, cartilage, meniscus and synovium from osteoarthritic joints. With both techniques, the number and morphology of the wear particles within the synovial fluid specimens correlate with the rate and mechanism of wear as confirmed by examination of the joint implant or articular surfaces. Toxicity of the various types of wear particles was assessedby cytological examination of the fluid aspirate. Of significant interest is the identification of active phagocytosis of wear particles by synovial fluid white blood cells. This finding may implicate the particles as initiators of secondary inflammatory responses, as occurs in other arthritic diseases. Analysis of aspirated synovial fluid appears to be a useful method for studying the rates, mechanisms and biological responses to wear in both arthroplastic and degenerative joints. In surgical joint replacement, this technique holds much promise, not only as a test for wear and toxicity, but also as a means of assisting in the selection of materials and designs for superior articular implants. In osteoarthritic joints, this analysis provides a method for early diagnosis, serial assessment of therapy and prognostication concerning the future course of the disease. Possibly of most significance, in contrast to previous studies on intact articular surfaces, is the ability to study the principal site of degenerative changes, namely the wear particles. This ability may aid in the elucidation of the underlying cause of osteoarthritis.

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1. Introduction When first introduced, surgical joint replacement was primarily employed for the treatment of elderly patients afflicted by arthritis of the hip joint [l] . Owing to the striking clinical success of this treatment, the indications for the procedure were extended to younger individuals, and comparable implants were devised for use in other articular joints. In the past year in the United States alone, about 100 000 total hip replacements, 50 000 total knee replacements and many thousands of ankle, shoulder, elbow, wrist and finger joint replacements have been implanted [Z, 31. Most implants currently available consist of a metallic component (of either stainless steel or cast cobalt-chromium alloys) that articulates with another component of high density polyethylene. Both pieces are attached to bone with a grouting agent, polymethylmethacrylate. A few implants consist of two metallic components, with or without an interposing polymeric unit. With the phenomenal increase in arthroplastic surgery, justifiable concern has arisen about the wear characteristics of the implants and the biocompatibility of the materials and their generated wear particles. Previous analyses of wear have included retrospective clinical and radiographic studies on implants [4], histological studies of tissues obtained at surgical revision [ 5 - 81 and assessments of wear or dissolution products disseminated to local or remote areas of the body [9] . Limited analyses of synovial fluid have been performed at the time of surgical revision [ 51. No non-surgical method to detect incipient failure of joint replacements and deleterious biological responses to eroding implants has been reported. Operative intervention to provide routine assessment of implants is impractical and unduly hazardous. Degenerative arthritis is a progressive non-inflammatory deterioration of articular joints of unknown origin. Weight-bearing and other heavily stressed joints of elderly individuals are especially susceptible to this disease process and are likely to become painful, stiff and deformed, whereupon surgical intervention, often in the form of joint replacement, is undertaken. Degenerative arthritis is characterized by slowly progressive deterioration and erosion of the articular cartilage, osteophyte formation, subchondral sclerosis and cyst formation. Early microscopic changes include the loss of superficial articular cartilage and an increase in the number of chondrocytes in the residual cartilage [lo, 111. With progression of the disease, clefts appear on the articular surface and propagate into the subarticular cartilage. Erosion of the cartilage ensues, often culminating in denuded ebumated subchondral bone [12,13]. Biochemical analyses of osteoarthritic cartilage reveal a progressive decrease in the content of water and proteoglycan, variation in the quality of the matrix collagen and an increase in the concentration of free lysosomal enzymes [ 14, 201. No reliable method for the early diagnosis and assessment of degenerative arthritis has been reported. At present, therefore, clinical or radiographic findings of degenerative change, consistent with moderately

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advanced deterioration, are the most sensitive indices available. By the time these tests are diagnostic, the only beneficial therapeutic modalities are surgical procedures of considerable magnitude, such as joint replacement. Previous attempts to employ hematologic, serologic and synovial fluid assessments as diagnostic tools for early disease have been unsuccessful. While Kitridou et al. [ 211 observed collagenous fibrils in synovial fluid and Hollander [22] has noted cartilaginous fragments and fibrils in synovial fluid, these studies have not proved to be suitable for clinical application. During the past decade, a sophisticated non-destructive test on machines undergoing wear has been developed. The novel improvement, Ferrography, provides a method for the orderly magnetic separation and study of wear particles from samples of the lubricant [ 231. With this technique, the wear mode of the machine can be characterized by the number, shape and size of the particulate wear matter present in a small sample of lubricant. Particles characteristic of different wear modes are identified and studied by various m icroscope techniques [ 241. The method provides a practical and accurate means for serial determinations of the mechanisms and rates of wear and facilitates prognostication concerning the future performance of such machines. Recently we have employed Ferrography for the analysis of wear particles present in the synovial fluid aspirated from surgical joint replacements and human arthritic joints. In combination with other techniques of synovial fluid analysis, the method provides a means of non-destructive repetitive assessment of the rates of wear within these joints and facilitates study of the pathological responses to the wear particles. It seems likely that this type of analysis will provide considerable new knowledge of the mechanisms and responses to implant wear and will aid in the understanding of the pathogenesis, diagnosis and treatment of degenerative arthritis and other destructive joint diseases.

2. Wear particle analysis in surgical joint replacements Synovial fluid is obtained by sterile needle aspiration from symptomatic joints after hemi- or total arthroplasty. To eliminate artifactual debris, needles and containers are washed prior to collection of specimens. The synovial fluid is then subjected to Ferrographic and histological analyses. With the Ferrograph analyzer, 2 m l of the synovial fluid are pumped across a glass m icroscope slide subjected to a magnetic field of sufficiently high gradient. Small weakly magnetic or paramagnetic particles in the fluid, often a fraction of a m icron in size, are precipitated according to their volumetric magnetic moment. Larger particles are deposited near the entry end of the substrate. Further along the flow path, progressively smaller particles are differentially deposited. After preparation of the Ferrogram, the precipitated particles are examined under a bichromatic m icroscope, simultaneously employing transmitted green light and direct red light. Metallic particles are readily identified by their red color due to their

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attenuation of the green transmitted light and their reflection of the red direct light. Non-metallic compounds appear to be green, yellow or pink depending upon their density and thickness. Surface conditions are distinguished by the intensity of the light reflected from the particles. Polarized light m icroscopy permits characterization of anisotropic compounds such as the high density polyethylene and the polymethylmethacrylate commonly used in orthopaedic implants. To assess the inflammatory changes within the arthroplastic joints, we examine the cytological characteristics of the aspirated synovial fluid, A clot is induced in a portion of the liquid specimen and the resultant coagulum is subjected to routine histological preparation and staining. The method permits excellent resolution of the intra-articular inflammatory response. In order to assessthe accuracy of these techniques, specimens of synovial fluid, synovium and implants were obtained at operation from 20 patients in whom surgical revision was necessary after implantation periods of 2 - 10 years. Synovial tissue was prepared for histological analysis in routine fashion. Alternatively, the synovial surfaces were washed in methanol to release superficial cells and associated particles, and the washings were then used to prepare a Ferrogram. After removal, implants were examined macroscopically and m icroscopically for evidence of wear. The composition of the implants and of the metallic wear particles was confirmed by scanning electron m icroscopy (SEM) energy dispersion X-ray analysis. In order to assessthe role of surgical instruments in particle contribution, hemostats, osteotomes, scissors, disposable needles and scalpel blades were examined using a m icroscope. Washings from the tools were analyzed Ferrographically. Subsequently, the chemical elements of the metallic particles were identified with the SEM X-ray apparatus. Our analysis reveals that metallic, polyethylene and polymethylmethacrylate particles present in synovial fluid are readily retrieved and identified employing the techniques described. After surgical joint replacement with metallic articular implants, metallic particles are always identifiable in the synovial fluid; larger numbers of particles are evident when both articular surfaces are composed of metal. With metal on polyethylene joint replacements, most of the wear particles are polyethylene, although analysis with polarized light usually reveals particulate polymethylmethacrylate cement. The number and morphology of the particles in the specimen appear to correlate with the rates and mechanisms of wear in the implants. The metallic particles identified range in size from less than 0.25 pm in specimens from joint replacements experiencing a relatively small amount of wear to 1 m m in length in instances where evidence of abrasive or fatigue wear is found (Fig. l(a)). Where implants of cobalt or titanium base alloys have been studied, synovial fluid analysis reveals wear particles from both the implant and stainless steel surgical tools. Surgical instrument analyses show that most of the wear particles from these tools originate in those which undergo repetitive use, such as hemostats, scissors and osteotomes. A

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small quantity of metallic debris arises from disposable needles and scalpel blades. The needles are relatively free of particles if flushed prior to use. Polyethylene, most often in the form of shredded fibers, is seen in a variety of sizes, ranging in diameter from 1 to 10 pm and up to several hundred microns in length (Fig. 2(a)). Polymethylmethacrylate particles generally have the appearance of irregular granular chunks with a larger variation in size (Fig. 2(b)). The diameter is less than 1 pm in the case of identifiable granules and up to 1 mm or more in the larger pieces. This acrylic material is found in larger quantities in the knee joint replacements which have been studied and is sometimes noted as being adherent to metallic fragments (Fig. l(b)). Cytological examination of synovial fluid reveals the presence of inflammatory cells, macrophages, sheets of synovial cells and, in the presence of hemi-arthroplasties, free chondrocytes. In addition, cartilaginous and bony fragments have been identified in the latter cases. The wide variety of cells seen in synovial fluid after surgical joint replacement approximates the spectrum of cells found in many different human joint diseases; it follows no pattern distinctive of any heretofore described disease process [ 251. The quantity of cells, however, appears to correspond to the degree of inflammation noted in the synovial tissue obtained from these same joints. This description refers solely to cases where no evidence of infection, either by clinical findings or culture, has been present. Specimens of synovium reveal the presence of particles of comparable composition and morphology to those found in samples of joint synovial fluid (Fig. 3). Observations on removed implants show that conditions of abrasive wear engender significant acceleration in the rates of wear of the components with wear particles of characteristic morphology. Abrasive wear is found with metal on metal implants and in instances where particulate polymethylmethacrylate or bone erodes polyethylene.

3. Wear particle analysis in degenerative arthritis Synovial fluid is aspirated from degenerative joints of patients by routine sterile technique. Identifiable biological wear particles of cartilage, bone, synovial tissue and meniscus are diamagnetic and, under the influence of the magnetic field of the Ferrograph analyzer, these particles are repelled to the periphery of the glass microscope slide. To obviate this problem, a proprietary solution of magnetic ions is added to the synovial fluid. The magnetic ions bind to the biological wear particles and induce them to behave in a paramagnetic fashion. Subsequently, Ferrograms can be prepared in the routine way. Bichromatic microscopy and SEM of the Ferrograms reveal the presence of cartilaginous, osseous, meniscal and synovial particles, each with unique optical properties and morphological characteristics (Fig. 4(a)). The

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volumes of the cartilaginous and bony particles appear to be directly related to the severity of the degenerative process. Cartilage fragments exhibit shapes characteristic of flaking or peeling from the parent articular surface. Numerous sizes and shapes of osseous particles are seen, although an irregularly surfaced chunk predominates (Fig. 4(b)). Meniscal and synovial tissue fragments vary greatly in quantity in different synovial specimens (Fig. 4(c)). Cytological analyses of the synovial fluid from osteoarthritic joints reveal findings similar to those previously described by other investigators [ 251. Inflammatory cells in the form of polymorphonuclear leukocytes and lymphocytes are present in small numbers, although the numbers of these cells and free sheets of synovial cells are increased where there is evidence of reactive synovitis. With transmission electron m icroscopy, we have observed active phagocytosis of particulate matter, presumably of eroded bone and cartilage, within synovial polymorphonuclear leukocytes aspirated from degenerative arthritic joints (Fig. 5). Confirmatory studies to identify the elemental composition of the ingested particles through X-ray diffraction analysis are underway.

4. Significance of wear particles in human joints Ferrography and allied techniques of synovial fluid analysis permit serial assessment of the wear of surgical joint replacements and prognostication concerning their future function. The methods permit a correlation between the mechanisms and rates of wear of the implants and the biological responses of the adjacent tissues. It is conceivable that different wear modes may stimulate unique deleterious pathological reactions, either locally within the joint or systemically. In addition, our ability to monitor wear in presently available surgical joint replacements may provide information that will assist in the selection of future implantable materials and in the design of superior articular replacements. Ferrographic analysis of human degenerative joints provides a means of studying the site of articular cartilage or denuded bone which has undergone the primary deterioration. In addition to its value as a diagnostic and prognostic tool, it enables future studies to shift their attention from the bulk cartilage, which may not as yet have undergone the critical biochemical or biomechanical alteration, to the site of maximal degeneration. Currently, we have initiated biochemical characterization of the wear particles. Further analysis of the wide variety of shapes and sizes of cartilaginous and osseous wear particles may ultimately permit the recognition of several degenerative processes of peculiar biochemical and mechanical origin, initiation and progression. It is conceivable that biological wear particles may contribute to the pathogenesis of degenerative arthritis. Willoughby has identified hydroxyapatite crystals within inflammatory cells from aspirated degenerative joints [ 261. He postulates that the crystals arise from an inborn error of metabo-

(4 (b Fig. 1. (a) A bichromatic photomicrograp lh ofn of a patient with a failed joint replacemel 1t (lO( of a large cutting-wear metallic particle in the sy knee replacement (1000 x ).

the synovial flui ..l.-C..-:,.r.T”..“r

Fig. 2. (a) Polyethylene particles as seen under polarized light. Note the characteristic birefringence. These olarizin properties are due to the highly ordered polyethylene structure (400x). (b)!’ o ly met f ylmethacrylate particles as seen under polarized light. Note the characteristic amsotropic optical properties of this granular substance (400 x ). (These photographs have been reduced in reproduction by %)

Fig. 3. A synovial tissue biopsy from an arthroplastic joint. Note polyethylene particles embedded in the synovial tissue. The polyethylene is highly birefringent under polarized light (100 X ).

Fig. 4(a) (These photographs have been reduced in reproduction by l/z)

Fig. 4(b) Fig. 4(c) Fig. 4. Scanning electron micrograph of “biological” wear particles from synovial fluid sample: (a) cartilaginous particle (100 X ); (b) osseous particle (100 X ); (c) synovial tissue fragment (100x ). Note the characteristic morphologies.

(a)

(b)

Fig. 5. Transmission electron micrograph of a polymorphonuclear leukocyte containing phagocytosed particulate matter: (a) low power (12 300~ ); (b) high power (38 300~ ). Note the extended cellular processes and the recently engulfed particles.

lism. In contrast, our studies suggest that the intracellular inclusions represent phagocytosed osseous and cartilaginous wear particles. Possibly the ingested particles .may initiate the production, release and activation of lysosomal enzymes, as has been shown for calcium pyrophosphate crystals in pseudo-gout [ 271. Alternatively, the wear particles may induce other cellular or humoral inflammatory responses. Regardless of the etiology of degenerative arthritis, subtle changes in the biochemical constitution of articular cartilage or denuded subchondral bone may initiate the erosion of particles of peculiar morphology and number. The fates of the various particles may range from innocuous degradation to the provocation of several deleterious inflammatory responses. This may account for the great variation in the clinical courses observed in patients with degenerative joint disease.

5. Conclusions Analysis of aspirated synovial fluid appears to be a useful method for the study of rates, mechanisms and biological responses to wear in surgical joint replacements and in osteoarthritic joints. Ferrography, an industrial technique of magnetic separation of particulate matter from samples of

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lubricating solutions, allows the separation of wear debris from synovial fluid. Bichromatic m icroscopy and SEM X-ray analysis permit identification of metallic particles from arthroplastic joints as well as biological fragments from degenerative joints. Polarized light m icroscopy characterizes and differentiates polyethylene and polymethylmethacrylate debris. The number and morphology of the wear particles in synovial specimens from arthroplastic joints correlate with the rate and the mechanism of wear, as confirmed by examination of the implant and the adjacent synovial tissue. The toxicity of the implant and the wear debris may be assessedby cytological examination of the fluid aspirate. Ferrography and synovial fluid analysis permit characterization of degenerative mechanisms. In synovial fluid samples from these joints, wear particles of articular cartilage, bone, meniscus and synovium have been identified. Unlike previous studies on intact articular cartilage, biochemical characterization of the principal site of degenerative change, i.e. the wear particles, may permit elucidation of the underlying cause of degenerative arthritis. The identification of active phagocytosis of wear particles by synovial fluid polymorphonuclear leukocytes implicates the particles as initiators of secondary inflammatory responses, as occurs in other inflammatory arthritides. These secondary reactions may be responsible for the wide variations seen in the pathogenesis and clinical course of the disease.

Acknowledgments The underlying knowledge about the characteristics and morphology of wear particles and their relation to wear modes resulted from research sponsored by the U.S. Department of Defense, Advanced Research Projects Agency, Dr. E. C. van Reuth, under Contract N00014-74-C-0135, with the Office of Naval Research, Dr. Richard M iller, scientific officer. We wish to thank Dr. William Ruff of the National Bureau of Standards, Washington, D.C., for his invaluable aid in providing SEM X-ray analysis of particles on selected Ferrograms connected with this work. We also acknowledge the research efforts of Messrs. John Bowen and E. Roderic Bowen of Foxboro/Trans-Sonics, Inc., who developed the necessary magnetic fluids and prepared the Ferrograms used in this investigation. We also thank Helga Georgescu, Marcia Williams and Warren Thompson for skilled technical assistance in the preparation of the histological specimens. Scanning and transmission electron m icroscopy studies were performed with the assistance of Richard Matta. References 1 J. Charnley, Clin. Orthop. Relat. Res., 72 (1970) 7 - 21. 2 S. Farner, personal communication, 1976.

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