Implant-associated sarcoma of the proximal region of the tibia in dogs with a history of TPLO1 treatment for cranial cruciate ligament injury has been proposed to be more common than naturally occurring sarcoma.2,3 Implant-associated sarcoma has been observed in large-breed dogs in which cast SS plates were implanted during TPLO,2,3 and the considerable corrosion of a cast SS TPLO plate was suggested to be associated with development of OSA at the implant site in an affected dog.2 Further investigations examined the composition of this particular TPLO plate as well as other cast SS TPLO plates and found, in addition to marked corrosion,4 inhomogeneous structure with substantial differences in chemical content (eg, varying amounts of austenite and ferrite) not only between plates, but also between regions of the same plate.5 The findings pertaining to corrosion of these plates were later challenged in an evaluation of surface chemistry and composition that detected no areas of material loss or corrosion for the bulk material.6 Nonetheless, it is generally accepted that corrosion-related metal ion release can induce negative effects in cells and tissues exposed to the ions.7–12
The objectives of the study reported here were to evaluate and compare surface and cross-sectional structure as well as localized electrochemical corrosion and ion release for cast SS TPLO plates retrieved from dogs with and without OSA and to compare these findings with similar variables for forged SS TPLO plates retrieved from dogs. On the basis of the assumption that the structure and electrochemical behavior of the cast plates were related to OSA occurrence, we sought to determine whether localized corrosion occurred at specific focal surface areas of cast plates that had a lower corrosion resistance than the major part of the implant surface; whether ion release in these areas, in the scale of several cell diameters, was significantly higher for plates retrieved from dogs with OSA than for plates retrieved from dogs without OSA; whether forged plates had a more homogeneous structure and lower ion release than cast plates; and whether surface damage resulting from intraoperative contouring of implants would be associated with higher ion release. Our null hypothesis was that there would be no differences in these variables between cast and forged plates or between cast plates removed from dogs with and without OSA.
Materials and Methods
Plate selection criteria
Cast SS TPLO platesa (manufactured in accordance with ASTM F74513) and forged (wrought) SS TPLO platesb (manufactured in accordance with ISO 5832-114 [comparable to ASTM F138-0315]) retrieved between April 2006 and October 2009 from dogs with a history of cranial cruciate ligament injury treated by TPLO and with or without a history of OSA of the proximal region of the tibia at the TPLO plate implant site were eligible for the study. Dogs with such plates and histories were identified by review of medical records and clinical research solicitation. Data collected for implants included in the study consisted of age of the dog at the time the plate was implanted, duration in situ for the plate, and age of the dog at the time of explantation. Retrieved TPLO plates were assigned to 1 of 3 groups: cast SS plates from dogs with OSA at the implant site, cast SS plates from dogs without OSA at the implant site, and forged SS plates from dogs without OSA at the implant site. The total number of plates evaluated was determined by the available retievals.
Plate retrieval
All retrievals were performed with informed owner consent. Cast plates were retrieved from dogs either after treatment for OSA (ie, limb amputation or euthanasia) or as part of a general policy to retrieve all cast plates, including from dogs without signs of clinical issues with their implants, that was instituted after implant corrosion had been identified as a possible concern with this type of plate. Forged plates were retrieved from dogs owned by individuals willing to provide the implants for additional study, even though their animals had no sign of clinical issues with the implants.
All retrievals were performed by the study investigators (RJB and RJM). Plates and tissue samples were obtained directly at the time of euthanasia or amputation; alternatively, in the case of solicited samples, the affected area (proximal tibia) was mailed en bloc to the investigators. Any overgrowth by tissue (eg, bone, fibrous tissue, or tumor) was peripherally removed to free the plate, taking care not to scratch or damage the plate in the process of removing tissue to access the screws. All plates were assessed grossly for any issues relative to screw purchase or damage, and representative tissue samples adjacent to and directly under each plate were obtained. Care was taken to handle retrieved plates appropriately so that they were not damaged, and finally, all plates were cleaned and stored prior to testing.
Tissue evaluation
Histologic evaluations of bone and other tissue samples collected from the implant sites of each retrieved plate were performed by a board-certified veterinary pathologist (JHK) to confirm implant site tumor status of each dog at the time of explantation and to assess any additional histologic findings in tissues from implant sites. In addition, previous histologic diagnoses of OSA at implant sites were reviewed by the same pathologist.
Plate assessment
Surface and cross-sectional examination—After retrieval, all TPLO plates were rinsed in deionized water and then sterilized to help ensure safe handling and transport. All plates were inspected for signs of wear between screwheads and corresponding holes. In addition, plates were assessed with a stereo loupe, light microscope, and SEM for signs of surface alterations (eg, corrosion marks or impurities), and all signs of deformation (ie, as a result of intraoperative plate contouring) were recorded by a single individual (CMS). The magnetic behavior of all retrieved implants was determined by the method described previously.5 Metallographic cross sections were obtained from representative plates of each group. The cross sections were fixed in a metallographic mount, polished to a mirror finish, then examined by light microscopy and SEM by a single individual (CMS).
Evaluation of plate surface regions—Two series of measurements were obtained to compare surface heterogeneity of a representative sample of plates from each group. First, measurements of surface heterogeneity were taken from randomly selected locations on the underside (side that had been in contact with the bone surface while implanted) of plates and from regions of plate deformations (regions that had been contoured intraoperatively). Second, measurements of electrochemical behavior were performed on the same plates, with results compared between plate regions that were nondeformed (original shape as manufactured) and deformed (as a result of the intraoperative plate contouring). All plates were stored in a dry and dark environment until electrochemical investigations were performed.
Electrochemical corrosion and ion release measurements—Plate selection for electrochemical corrosion measurements and detection of local ion release for each of the 3 groups was performed on the basis of the groups having similar durations in situ. All electrochemical corrosion measurements were performed on the underside of retrieved plates by a single investigator (TS) blinded to tumor status and metallurgic evaluation results. However, differences between cast and forged plates would have been apparent when viewed microscopically as part of the electrochemical testing methods. The microcell technique16 was used to assess plate surface metal ion release and obtain electrochemical corrosion measurements at these randomly selected, 100-μm-diameter surface sites that were comparable in area with that of clusters of cells that could have been in contact with the plate in situ. Briefly, a glass microcapillary containing 1M NaCl and coated at the tip with a layer of silicone rubber to retain the solution and facilitate contact with the test surface acted as a miniature electrochemical cell that was used to obtain surface electrochemical measurements of samples (Figure 1). The microcapillary was fixed in an attachment that replaced a lens objective in the revolving nosepiece of a light microscope, and the microcapillary's 100-μm-diameter tip was pressed against the surface of the sample. The configuration with the light microscope enabled an initial search and evaluation of plate surfaces at various magnifications prior to rotation of the revolving nosepiece for selection of the microcapillary, then placement of the microcapillary tip on the exact surface site that had been observed by light microscopy. Local polarization curves (characteristic electrochemical corrosion curves) were obtained at selected sites on plate surfaces by applying currents ranging from -500 to 1,000 mV (SCE) while corresponding corrosion currents were recorded (measured and graphed) as corrosion current densities (μA/cm2). The integrated charge of the potential range between the Ecorr and Ecorr + 200 mV was equivalent to the amount of released metal ions, and it is postulated17–21 that this range is relevant for ion release by implants (in mammals).16–20 However, the range of 200 mV was chosen arbitrarily because it was assumed to cover most electrochemical conditions to which the plates could have been exposed while in situ. In addition, Epit measurements were recorded for these plates. All measurements were obtained under standardized conditions to ensure comparable data acquisition.
Representative images of equipment and results associated with the microcell technique used to measure local surface corrosion of cast and forged SS TPLO plates retrieved from dogs. A—Configuration of a microcapillary held in a device mounted in place of a lens objective in the rotary nose piece of a light microscope. B—A 100-μm-diameter tip of a microcapillary that has been sealed with a layer of silicone rubber. C—Scanning electron microscope image of a site on a cast SS plate at which local corrosion had occurred. Notice the uneven material dissolution in the circular area tested. Bar = 20 μm. D—Illustration of a polarization curve with Ecorr, metastable events (asterisks), and Epit identified. The shaded area between the Ecorr and Ecorr + 200 mV represents conditions under which metal ions were generated. E—Representative polarization curves of SS TPLO plates retrieved from dogs and measured at deformed regions (cast plate [white triangle] and forged plate [white circle]) as well as nondeformed regions (cast plate [black triangle] and forged plate [black circle]). The amount of metal ions generated by cast plates was generally much higher than that generated by forged plates.
Citation: American Journal of Veterinary Research 79, 9; 10.2460/ajvr.79.9.970
To assess variability in local ion release for individual plates, 9 locations were measured on 1 plate selected from each of the 3 groups. Tested locations on the underside of plates were situated randomly in regions between the narrow (distal) and wider (proximal) portions of the plate. Selected regions were those typically deformed during intraoperative plate contouring (Figure 2).
Mediolateral radiographic views of the left stifle joint in a 3-year-old castrated male Bouvier des Flandres obtained immediately after TPLO with a cast SS plate for treatment of a ruptured cranial cruciate ligament (A) and 62 months later (B). Notice the radiographic evidence of a large OSA of the proximal region of the tibia at the implant site. C—Explanted plate retrieved from the same dog after euthanasia. The soft tissue side of the plate is shown on the left; the underside of the plate that had been adjacent to the tibia is shown on the right. Deformed regions from intraoperative contouring are evident (white arrows). D— Photomicrograph of a section of the OSA obtained alongside decalcified bone (asterisks). H&E stain; bar = 200 μm. The region inside the rectangle is shown at higher magnification (E), and accumulated corrosion products (black arrows) are evident. H&E stain; bar = 20 μm.
Citation: American Journal of Veterinary Research 79, 9; 10.2460/ajvr.79.9.970
In a second series of measurements for ion release, paired measurements, 1 from a randomly selected nondeformed region (ie, with no visible contour tool marks) and 1 from a randomly selected deformed region (ie, with contour tool marks), were obtained for each plate. These measurements were performed on 9 cast SS plates from dogs with OSA at the implant site, 9 cast SS plates without OSA, and all 3 forged plates.
Statistical analysis
Statistical analyses were performed with available software.c Data from each group of plates were assessed for normal distribution with the Shapiro-Wilk test, and homogeneity of variance among groups was checked with the Levene test. Normally distributed data were analyzed with ANOVA. Bonferroni and Games-Howell post hoc tests were used to account for multiple comparisons with and without homogeneity of variance among groups, respectively. Wilcoxon signed rank tests followed by Mann-Whitney U tests with Bonferroni correction to account for multiple comparisons were performed when a normal data distribution was not detected. Values of P < 0.05 were considered significant for all statistical tests.
Results
Plate retrievals
Between April 2006 and October 2009, 47 TPLO plates were retrieved from 45 client-owned dogs (22 cast platesa from 22 dogs reported to have had OSA at the implant site, 22 cast platesa from 22 dogs without OSA at the implant site, and 3 forged platesb from 3 dogs without OSA at the implant site; Figure 3; Supplementary Table S1, available at http://avmajournals.avma.org/doi/suppl/10.2460/ajvr.79.9.970). Four cast plates were retrieved from 2 dogs, each previously treated bilaterally with TPLO and each with reported OSA at the implant site in 1 limb but not at the implant site in the contralateral limb. All dogs reported to have had OSA at the implant site had characteristic radiographic evidence of tumor development with substantial osteolysis directly under the implanted plates (Figure 2), whereas there was no radiographic evidence of osteolysis observed under the plates of dogs without OSA at implant sites.
Scatterplots of the ages of 45 dogs at the time of SS TPLO plate implantation (A) and explantation (B) as well as box-and-whisker plots of the duration in situ for plates selected for electrochemical testing (C) from each of 3 groups of plates: 22 cast SS TPLO plates from dogs with OSA at the implant site (circles), 22 cast SS TPLO plates from dogs without OSA at the implant site (squares), and 3 forged SS TPLO plates from dogs without OSA at the implant site (triangles). For the box-and-whisker plots, the box represents the interquartile (25th to 75th percentile) range, the horizontal bar within the box represents the median, and the whiskers represent the range. Four cast plates were retrieved from 2 dogs, each previously treated bilaterally with TPLO and each with reported OSA at the implant site in 1 limb but not at the implant site in the contralateral limb.
Citation: American Journal of Veterinary Research 79, 9; 10.2460/ajvr.79.9.970
Eleven different veterinary centers contributed by sending 1 to 4 specimens of intact tissues with TPLO plates from 18 dogs, reported to have had OSA at implant sites, to the study investigators (RJB and RJM) who performed implant removal and tissue collection. Additional retrievals from 4 dogs reported to have had OSA at implant sites were performed at the Tufts Cummings School of Veterinary Medicine by the same investigators. None of the plates were damaged during removal. Access to the screws was straightforward, despite some osseous and soft tissue proliferation around the plates. No screws were found to be damaged or to have protruding screwhead. All screws were found fully seated; however, a small number of the screws in some dogs were found to have had loose purchase, which was attributed to the OSA and osteolysis under the plates.
Retrievals of all 25 TPLO plates (cast, n = 22; forged, 3) from dogs without OSA at implant sites were performed by the study investigators (RJB and RJM). Twenty-three specimens had been collected at the Tufts Cummings School of Veterinary Medicine (cast plates, n = 20; forged plates, 3), and 2 had been collected at different veterinary centers. The latter 2 were collected from dogs that had been euthanized for OSA in their contralateral limbs and also previously treated with TPLO. Intact tissues and plates from the contralateral limbs of these 2 dogs were also included in the present study. None of the plates were damaged during retrieval. Access to the screws was straightforward, despite some minimal osseous and fibrous soft tissue proliferation around the plates. No screws were found to be damaged, and all had secure purchase in the bone.
Mean duration in situ for cast plates retrieved from dogs with OSA at the implant site was significantly longer (P < 0.003; Supplementary Table S2, available at http://avmajournals.avma.org/doi/suppl/10.2460/ajvr.79.9.970), compared with that for cast plates retrieved from dogs without OSA at the implant site.
Tissue evaluation
Histologic examination was repeated on tissue sections from all 22 dogs with a previous histologic diagnosis of OSA at the implant site. For this, H&E-stained sections of tissue samples collected from areas adjacent to and directly under the explanted plates were examined. Microscopic characteristics of inflammation (eg, scattered intralesional macrophages, lymphocytes, and neutrophils) as well as intra- and extracellular particulate debris were present in all specimens from this group. In addition, results confirmed OSA in 21 dogs, but indicated histiocytic sarcoma in the remaining dog (Supplementary Table S1). The plate from the dog with histiocytic sarcoma was not included in electrochemical analyses.
All 25 dogs reported not to have had OSA at the implant site were confirmed by repeated histologic evaluations to have had no evidence of neoplasia in tissue sections obtained from areas adjacent to and directly under their plates (cast plates, n = 22; forged plates, 3) at the time of explantation. However, histologic evidence of inflammation (eg, mononuclear inflammatory cells, including macrophages, lymphocytes and neutrophils), locally extensive fibrosis, and heterogeneous particulate debris were observed in tissue samples corresponding to each of the 22 cast plates. Histologic evaluation of tissue samples from implant sites of the 3 forged SS plates indicated fibrous tissue membranes surrounding the plates, but no evidence of inflammation or intra- or extracellular particulate debris.
Plate assessment
Surface and cross-sectional examination—All retrieved cast plates (n = 44) were magnetic; however, none of the retrieved forged plates (3) were magnetic. Visual inspection of all cast plates by use of gross examination and also with a stereo loupe, light microscope, and SEM revealed similar macro- and microscopic findings: rough surfaces, sharp-edged material, and signs of local corrosion (Figure 4). Local notches and severe tool marks were found next to deformed regions of plates, especially near where the upper end of the plate widened (Figure 2). In these deformed regions, local corrosion events were observed 1.2 times as frequently as on surface areas without signs of deformation. Major signs of wear and local corrosion at screw holes were not observed. On forged plates, tool marks from surgery were also present, but no signs of local corrosion were found.
Representative SEM images of surfaces (A through C) and metallographic cross sections (D through F) of explanted TPLO plates represented in Figure 3. A— Circular areas of corrosion can be seen between scratches on the surface of a cast plate. Bar = 20 μm. B—Smaller circular area of corrosion on the surface of a cast plate. Bar = 2 p.m. C—Heterogeneous inclusion on the surface of a cast plate visually enhanced with SEM in back-scattered electron mode. Bar = 500 nm. D—Cross section of a cast plate illustrating a biphasic structure and inclusions of various sizes. Bar = 50 pm. E—Cross section of a cast plate with inclusions (arrows); the embedding material used for preparation of the cross section can be seen (asterisk). Bar = 10 pm. F—Cross section of a forged plate illustrating a monophasic, homogeneous structure and few inclusions. Bar = 50 pm.
Citation: American Journal of Veterinary Research 79, 9; 10.2460/ajvr.79.9.970
Metallographic examination was performed on 8 cast plates (4 from dogs with and 4 from dogs without OSA at the implant site) and 2 forged plates. Cast plates had a heterogeneous biphasic structure with many variably sized inclusions on the surface and in cross sections (Figure 4). In contrast, forged plates appeared monophasic and homogeneous with rare, small inclusions. No differences were observed between the surfaces and centers (in cross section) of forged plates.
Electrochemical corrosion and ion release measurements—Twenty-one plates (9 cast plates from dogs with OSA at the implant site, 9 cast plates from dogs without OSA at the implant site, and 3 forged SS plates from dogs without OSA at the implant site) were selected for electrochemical corrosion measurements and detection of local ion release. There was no significant difference among the 3 groups of plates selected for analysis with regard to duration in situ (Figure 3). Results indicated circular corrosion events on cast plates but not on forged plates (Figure 4). In addition, the polarization curves illustrated heterogeneity of plate surfaces among the 3 groups, with higher susceptibility for the onset of localized corrosion (pitting corrosion) and metal ion release at lower potentials for both groups of cast plates, compared with the group of forged plates (Figure 5). All cast plates demonstrated an inhomogeneous superficial composition, with high spatial variability of the electrochemical surface properties in vitro. No significant difference in the Epit was observed between the 2 groups of cast plates; however, the Epit of both groups of cast plates (from dogs with and without OSA at the implant site) varied by factors up to about 30 and differed significantly (P < 0.003 and P = 0.003, respectively) from that of the forged plate group (Figures 1 and 6; Supplementary Table S3, available at http://avmajournals.avma.org/doi/suppl/10.2460/ajvr.79.9.970). Although the Epit for all cast plates varied and an Epit as low as 42 mV for cast plates was noted on a deformed region of a cast plate, no significant (P = 0.110 and P = 0.063, respectively) difference in corrosion was found between regions of individual plates in the either of the groups of cast plates from dogs with or without OSA at the implant site (Supplementary Table S4, available at http://avmajournals.avma.org/doi/suppl/10.2460/ajvr.79.9.970).
Representative polarization curves for 3 cast plates from dogs with OSA at the implant site (A), 3 cast plates from dogs without OSA at the implant site (B), and 3 forged plates from dogs without OSA at the implant site (C) illustrating heterogeneity of surface electrochemical properties. The Epit occurred at lower potentials for cast plates from dogs with OSA than for cast and forged plates from dogs without OSA. The highest Epit values (ie, best corrosion resistance) were measured on forged plates.
Citation: American Journal of Veterinary Research 79, 9; 10.2460/ajvr.79.9.970
Box-and-whisker plots of local Epit measurements for the groups of SS TPLO plates represented in Figure 3 (A) and local Epit values for original (orig) and deformed (deform) areas on the plates (B).
Citation: American Journal of Veterinary Research 79, 9; 10.2460/ajvr.79.9.970
In contrast to the corrosion test results, cast plates from dogs with OSA at the implant site had significantly (P = 0.024) greater metal ion release values, compared with values for cast plates from dogs without OSA at the implant site. Further, cast plates from dogs with and without OSA had significantly (P < 0.003 and P = 0.033, respectively) greater metal ion release values, compared with values for forged plates (Figure 7; Supplementary Table S3).
Regarding regions of individual plates, metal ion release of cast plates at nondeformed and deformed regions of the same plate varied, but were not meaningfully different. Forged plates had more consistent ion release values with less variability between deformed and nondeformed regions in individual plates (P = 0.285), compared with that of individual cast plates from dogs with OSA (P = 0.214) or without OSA (P = 0.086; Supplementary Table S4). All cast plates (from dogs with and without OSA at the implant site) had significantly higher metal ion release values than did forged plates for both nondeformed and deformed surface regions (P = 0.002 and P = 0.002, respectively; Supplementary Table S5, available at http://avmajournals.avma.org/doi/suppl/10.2460/ajvr.79.9.970).
Box-and-whisker plots of metal ion release measurements for the groups of SS TPLO plates represented in Figure 3 (A) and local metal ion release measurements for original (orig) and deformed (deform) areas on the plates (B).
Citation: American Journal of Veterinary Research 79, 9; 10.2460/ajvr.79.9.970
Discussion
Results of the present study indicated that retrieved cast SS TPLO plates had localized corrosion and metal ion release in situ and that electrochemical characteristics of these plates varied across plate surfaces. The calculated metal ion release was substantially higher for the group of cast plates from dogs with OSA at the implant site, compared with the 2 groups of plates from dogs without OSA at the implant site. These results were consistent with the idea that tumor development could have been a random event that was more likely to occur in locations with higher ion release. The absence of substantial differences between the 2 groups of cast plates with regard to corrosion suggested that limited areas of corrosion, although perhaps not relevant to the overall behavior of the bulk material, could have had important biologic effects. Classical electrochemical corrosion studies typically focus on implant or construction element failure as a consequence of corrosion. In the present study, in contrast, we focused on localized sustained metal ion release, which could have occurred at a level that did not affect bulk material stability measurements but could still have potential biological effects (eg, OSA development at the implant site). Another important finding was the notably lower metal ion release across the collective surface sites (including deformed regions) of forged plates, compared with cast plates, which was likely a result of the greater homogeneity observed during surface and cross-sectional examination of the forged plates. Although we had suspected prior to the present study that deformation would reduce the local electrochemical surface properties of plates, in the present study, we did not identify substantial differences between deformed and nondeformed regions for any of the 3 groups of plates.
Results of histologic evaluation of tissue samples from implant sites of cast plates retrieved from dogs with OSA complemented the electrochemical findings. In addition to neoplasia, characteristics of chronic inflammation21 as well as the presence of corrosion products were observed in the tissue samples. Corrosion debris is thought to be responsible for macrophage activation and osteolysis,22 and it was interesting to note that osteolysis was noted in all dogs of the present study with neoplasia at the implant site, but not in any of the other dogs in the study. Metal ion release, which was found in the present study to be higher in cast plates from dogs with OSA, likely contributed to the chronic inflammation at the implant sites and may have had oncogenic effects.8–10 The inert nature of the particulates and their resistance to digestion by cells may cause chronic inflammation; furthermore, direct toxic effects of these ions may contribute to a foreign-body reaction and subsequent carcinogenesis.8–10,23
The cast plates investigated in the present study had structural heterogeneities and surface alterations (eg, scratches, chipped borders, and deformed regions caused by implant contouring at the time of surgery). Regions of structural heterogeneity and surface alterations were preferential sites for localized corrosion and ion release. To investigate processes that could have occurred at the cellular level during the time plates were in situ, the microcell technique was the electrochemical tool for local measurements at an area the size of cell clusters on a 100-μm scale. The most sensitive method to detect a surface reaction of an electrochemical nature is to immerse the material in a solution and record the current passing through the metal-solution interface at various potential gradients.19 However, it is impossible to obtain local information at a scale relevant to clusters of individual cells when a large surface is immersed; therefore, classical electrochemical corrosion tests (eg, with samples of a few square centimeters) provide information only on the average behavior of a large surface of material. Such studies, which address overall surface chemistry and composition for the bulk material,6 are not very useful for testing local corrosion processes of passive implant materials because they simply cannot assess surface heterogeneity.
The Epit is an accepted material parameter used to describe the susceptibility for localized corrosion.19,20 However, evaluating only Epit would likely overlook a considerable amount of biological impact on living tissues. During most of an implant's duration in situ, the surface of the plate would likely have been in the potential range from Ecorr to Ecorr + 200 mV. Metal ions will be released in this potential range, resulting in high local ion concentrations, which are likely to lead to adverse local cell responses, such as an immune response, osteolysis, and sarcoma development at the implant site.8–10,23
Forged plates are the current standard of care for orthopedic use,17,24 and forged plates had less corrosion in the present study, compared with cast plates, and notably higher Epit values (most > 1,000 mV). The absence of localized corrosion in the forged plates of the present study was consistent with findings of a previous study.25 In addition, results of histologic evaluation of tissues from implant sites adjacent to forged plates indicated only a fibrous tissue reaction around the plates, without the inflammation associated with the cast plates.
A link between chronic inflammation and neoplasia is well established, with a number of pathways postulated for how this may occur, including direct cell transformation, uncontrolled cell growth, and immunosuppression that results in a lack of response to abnormal or modified growing cells.26 Despite this, implant-associated neoplasia is considered rare, given the millions of implants used for fracture fixation and the low number of subsequent reports of tumor development, and no significant association between metal implants and tumor development was shown in a case-control study.25
Findings of the present study indicated that inhomogeneous implant surface areas were related to variable susceptibility to localized corrosion, which in turn resulted in ion release. The magnitude of this ion release appeared to support the assumption that implant surface behavior was related to OSA occurrence, in that ion release from plates retrieved from dogs with OSA was significantly higher than that from plates retrieved from dogs without OSA. This was also consistent with the concept of metal-induced sarcoma and tumorigenesis risk, in which an average latency period of > 5.5 years has passed with no other additional predisposing factors (eg, infection, implant failure, and delayed or nonunion27), that were similarly not observed in dogs of the present study.
Corrosion-related neoplastic transformation related to metal ion release has been discussed previously,8–10 and corrosion-related tumor development around SS and titanium implants has been reported.11,28,29 Recently, it has been shown that even low concentrations of hexavalent chromium ions trigger a selective mutagenesis effect that damages DNA in various mesenchymal cell types.30 On the basis of our and previous findings, we believe that localized corrosion is more likely to occur in areas of deformation (eg, tool marks from intraoperative plate contouring) on heterogeneous plate surfaces and that this could lead to neoplastic transformation in adjacent connective tissue. Further, we believe that such transformation would be less likely to occur in association with forged plates, compared with cast plates, because of the absence of corrosion with forged plates. Importantly, however, results of the present study did not establish a definitive cause and effect with regard to tumorigenesis, although results did provide considerable circumstantial evidence that plate surface structure and electrochemical behavior were related to tumorigenesis in affected dogs.
There is published evidence indicating that plates manufactured for surgical fracture stabilization are not always produced under the highest quality standards in certain parts of the world and have been shown to have less resistance to corrosion, compared with forged plates.d Cast plates in the present study were magnetic and had a biphasic structure, characteristics identical to plates previously evaluated,2,5 and they did not comply with the ASTM F745 standard, including a maximum of 1% ferrite.13 We believe that cast implants manufactured in full compliance with the ASTM or ISO standards, controlling for chemical composition, impurities, and heterogeneity, may be safe. Although the type of cast plates evaluated in the present study are no longer manufactured and the ASTM F745 standard has been withdrawn, our findings may give some insight as to the risks posed if plates with similar corrosion properties would be produced for orthopedic use as a result of poor adherence to appropriate standards, most often in an effort to control implant cost.
An important limitation of the present study was the inclusion of only 3 forged SS TPLO plates. The number of forged plates was limited by a lack of available implants with similar duration in situ as the cast plates. The forged plates included in the present study were all manufactured in compliance with ISO and ASTM standards, in which stringent metallurgical requirements ensure a single-phase austenitic microstructure and high corrosion resistance.31 Thus, the limitation of the low number of forged implants was mitigated because of the minimal structural variability and demonstrated biocompatibility of these plates.32
Another limitation of the present study was that the proposed biological link could not be tested; therefore, our results did not prove that corrosion and metal ion release are directly related to OSA development. Our results indicated that more corrosion and greater ion release was observed for cast plates retrieved from dogs with OSA at the implant site, compared with plates retrieved from dogs without OSA at the implant site, and that forged plates had minimal corrosion and ion release. It could be suggested that the occurrence of OSA at the implant sites for 1 group of plates was a coincidence, especially because cases were retrospectively selected from a large cohort of clinical cases. Nevertheless, in our opinion, findings from the present study add considerable circumstantial evidence suggesting a relationship between the cast SS TPLO plates that were evaluated and the development of OSA at the implant sites. Alternative explanations for development of OSA at implant sites include continuing inflammation resulting from the simple presence of a plate, complicated healing, or infection.33
In conclusion, cast SS TPLO plates (from a single manufacturer and no longer produced) evaluated in the present study had an inhomogeneous surface and cross-sectional microstructure, with isolated areas of corrosion and high metal ion release, whereas these effects were not observed with forged SS TPLO plates. The in vitro corrosion behavior observed in the present study suggested a possible relationship with neoplastic transformation.
Acknowledgments
Funded by the AO Foundation, AOVET Network grant No. ARI_2008-01.
Dr. Boudrieau was a member of the AO Technical Commission, Veterinary Expert Group of the AO Foundation from 2004 through 2014. The authors declare that there were no other conflicts of interest.
Presented at the European College of Veterinary Surgeons 25th Annual Scientific Meeting, Lisbon, Portugal, July 2016.
ABBREVIATIONS
| ASTM | American Society for Testing and Materials International |
| Ecorr | Corrosion potential (or open circuit potential) |
| Epit | Pitting potential |
| ISO | International Organization for Standardization |
| OSA | Osteosarcoma |
| SCE | Saturated calomel electrode |
| SEM | Scanning electron microscopy |
| SS | Stainless steel |
| TPLO | Tibia plateau leveling osteotomy |
Footnotes
316L cast SS TPLO plate, Slocum Enterprises Inc, Eugene, Ore.
Forged SS TPLO plate, DePuy Synthes Co, 4528 Zuchwil, Switzerland.
SPSS Statistics, version 22.0, IBM Corp, Armonk, NY.
Dewo P. Evaluation and redesign of an osteosynthesis plate, produced in Indonesia. Doctoral thesis, Faculty of Medical Sciences, University of Groningen, Groningen, Netherlands, 2011.
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