Modular Connections Research Articles

In vivo severe corrosion and hydrogen embrittlement of retrieved modular body titanium alloy hip‐implants

Titanium alloys are widely used in total-joint replacements due to a combination of outstanding mechanical properties, biocompatibility, passivity, and corrosion resistance. Nevertheless, retrieval studies have pointed out that these materials can be subjected to localized or general corrosion in modular interfaces when mechanical abrasion of the oxide film (fretting) occurs. Modularity adds large crevice environments, which are subject to micromotion between contacting interfaces and differential aeration of the surface. Titanium alloys are also known to be susceptible to hydrogen absorption, which can induce precipitation of hydrides and subsequent brittle failure. In this work, the surface of three designs of retrieved hip-implants with Ti-6Al-4V/Ti-6Al-4V modular taper interfaces in the stem were investigated for evidence of severe corrosion and precipitation of brittle hydrides during fretting-crevice corrosion in the modular connections. The devices were retrieved from patients and studied by means of scanning electron microscopy (SEM), X-ray diffraction (XRD), and chemical analysis. The surface qualitative investigation revealed severe corrosion attack in the mating interfaces with evidence of etching, pitting, delamination, and surface cracking. In vivo hydrogen embrittlement was shown to be a mechanism of degradation in modular connections resulting from electrochemical reactions induced in the crevice environment of the tapers during fretting-crevice corrosion.

The Optimal Metal-Metal Arthroplasty is Still a Total Hip Arthroplasty: In Opposition

The longevity of any hip arthroplasty in an individual patient cannot be predicted with certainty so revisability of the arthroplasty remains an issue. As the occurrence of loosening and osteolysis decreases, then fatigue failure of femoral stems, complications of modular connections, and infection will become relatively more common modes of failure. Metal-metal hip resurfacing conserves bone stock, which provides better options for revision surgery. Hip resurfacing favors early intervention and has demonstrated excellent outcomes in young active males, the group historically at increased risk for failure of a conventional total hip arthroplasty. The best balance of the benefit-risk ratio for hip resurfacing is in those patients at increased risk for failure of a conventional total hip arthroplasty.

Accumulation in liver and spleen of metal particles generated at nonbearing surfaces in hip arthroplasty

Systemic migration of metal particles generated at nonbearing surfaces rather than the intended primary bearing was studied in postmortem specimens from 30 patients with total hip arthroplasty. Using light and electron microscopy with x-ray microanalysis, submicrometer metal particles were identified within macrophages in the liver and/or the spleen in 11 of 15 patients with a revised arthroplasty and in 2 of 15 patients with primary hip arthroplasty. The macrophages formed focal aggregates in the organs without apparent toxicity. Fretting at ancillary fixation devices, loose components, and modular connections can generate a substantial volume of debris. These particles are in addition to those generated at the bearing surfaces, further increasing both the local and systemic particulate burdens. While all components can be associated with the distant spread of particles and metal ions, it is the environment of revision arthroplasty that provides the greatest potential for the generation and systemic dissemination of wear debris. The long-term effects of accumulated wear particles in the liver and spleen are unknown.

Invited commentary

The positive aspects of this multicenter trial are that it is prospective, involves a large number of patients, and has had data from one of its phases analyzed by an independent core laboratory. The article is an important one, since the authors have previously identified poor fixation as the cause of most ruptures following endovascular repair with the AneuRx prosthesis.1Zarins C.K. White R.A. Fogarty T.J. Aneurysm rupture after endovascular repair using the AneuRx stent graft.J Vasc Surg. 2000; 31: 960-970Abstract Full Text Full Text PDF PubMed Scopus (209) Google Scholar It is unfortunate, therefore, that migration at modular connections and between the prosthetic limbs and native iliac arteries was not addressed. Since the article reports clinically significant events almost entirely on the basis of imaging findings, it is regrettable that details of imaging and methodology of measurement are not included in the text. It is well known that parallax error in plain radiographs makes detection of endograft migration by reference to bony landmarks on images obtained at different times an unreliable method. Similarly, in the absence of any information on the thickness of CT slices, the sensitivity of this modality in detecting migration is called into question. The criteria by which the core laboratory and clinical sites assigned the dichotomous endpoint of migration or no migration are missing, together with information on the intraobserver and interobserver error for measuring migration. Migration is defined by the authors as “any postimplantation movement or displacement of the stent graft.” If this statement is taken at face value, all 1025 patients categorized as having no migration would have had to have follow-up imaging with the endograft assigned the identical distance from the reference point as in the postdeployment images, accurate to the last millimeter. Information is also lacking on imaging data coming from participating clinical sites and not reviewed by the core laboratory. If these data were not obtained under any defined protocol and were produced by observers with variable skill levels, migration may have been underreported. Although the stated objective of the article is to identify factors that may predispose to stent-graft migration, the relatively high incidence of migration itself tends to overshadow the culprit factors of low initial deployment of the device below the renal arteries and a short proximal fixation length. The incidence of migration documented in the article is a cause for some concern. Two of the clinical sites had migration rates of 30% (Fig 4), while Kaplan-Meier analysis demonstrates an increasing incidence with time (18.8% at 3 years, Fig 2). Moreover, it should be noted that the mean follow-up from implantation is limited to 23 months, which is just short of a marked increase in events, and that 70% of patients did not reach the 3-year follow-up time point. Since the cut off date for this study was August 2001, surgeons and patients will await with interest the authors' next report.

Differences in the fretting corrosion of metal–metal and ceramic–metal modular junctions of total hip replacements

The use of modular interlocking components is a central design feature of total joint replacements. In this investigation we hypothesized that clinically available ceramic–metal modular connections used in total hip arthroplasty release more metal through fretting corrosion than traditional metal–metal modular connections. This was investigated using an in vitro comparison of ceramic (zirconia, ZrO 2) and metal (Co-alloy) femoral-head fretting upon Co-alloy stem components. In vitro fretting corrosion testing consisted of potentiodynamic monitoring and analysis of metal release from zirconia and Co-alloy 28 mm femoral heads with similar surface roughnesses ( Ra=0.46 μm) on identical Co-alloy stems at 2.2 kN for 1 × 10 6 cycles at 2 Hz. In contrast to our original hypothesis, we found greater metal release (approximately 11-fold increase in Co and 3-fold increase in Cr) and potentiodynamic fretting of metal–metal modular junctions when compared to ceramic–metal. Potentiodynamic testing demonstrated that lower initial voltages (−266 < 153 mV), greater maximum voltage changes (116 > 56 mV, p<0.05, t-test) and voltage variability (3 > 0.5 mV, p<0.05, t-test) were associated with the open circuit potentials of Co-alloy on Co-alloy junctions when compared to zirconia on Co-alloy junctions. In this study of a single total hip replacement stem and head design, zirconia heads mated with Co-alloy stems produced less fretting than Co-alloy heads mated with Co-alloy stems. Although further studies are necessary with a variety of implant designs and under different experimental conditions, the evidence presented here should, in part, alleviate concerns of increases in fretting corrosion at modular junctions of ceramic–metal coupled components.

Orthopedic Implant Fretting Corrosion

Fretting corrosion occurs to some extent on all load bearing metallic orthopedic implants. Fretting of total hip and knee replacements occurs at such places as the bone-stem interface, the stem-cement interface, and on the interfaces of modular connections between implant components. Fretting also occurs at areas of cyclic load bearing contact on metallic bone-plates and screws, such as screw-plate connections, bone-plate interfaces and screw-bone interfaces. Of primary clinical concern is the generation of ionic and particulate debris through fracture and abrasion of the metal oxide protective layers covering implant alloys /1/. This debris produced locally within crevices of component connections can migrate away from junctions and cause injury both systemically and in the local tissue. Clinical concern about the release and distribution of metallic degradation products is justified by the known potential toxicities of the elements used in modern orthopedic implant alloys (titanium, aluminum, vanadium, cobalt, chromium, molybdenum and nickel) and known pathologies associated with metal implant degradation (i.e. hypersensitivity, particle induced inflammation, etc.) /2/. However, little is known of the short and long-term biologic impact of soluble metal-protein complexes formed from the degradation products of implants.

Efferent connections of the anterior hypothalamic nucleus: a biocytin study in the cat

The efferent connections of the anterior hypothalamic nucleus (AH) were examined using biocytin as anterograde tracer in the cat. The results provide several new findings in addition to confirming earlier observations. In the hypothalamus, the AH projections terminated mainly in the medial regions which are related to the defensive, reproductive and feeding behaviors, and autonomic functions. Moreover, we found dense patches of the AH terminals in the medial preoptic area and ventromedial hypothalamic nucleus, which suggests the existence of modular connections between sub-regions of each nucleus. In addition, the AH projected to regions which may be related to the emotional and autonomic responses, i.e., such regions in the amygdala, midline thalamus, septum, subthalamus, and midbrain. The data suggest that the AH may play an important role in the autonomic functions and behaviors between animals, and thus may play a key role in the defensive behavior elicited in the medial preoptic area and ventromedial hypothalamic nucleus.

Periprosthetic femoral osteolysis around an uncemented nonmodular moore prosthesis

Periprosthetic osteolysis is a major problem in total joint arthroplasty surgery today. The particulate debris from ultrahigh-molecular-weight polyethylene, polymethyl methacrylate, and corrosion products from modular connections have been implicated in this process. A case of femoral osteolysis at the tip of a nonmodular Moore prosthesis, in the absence of polyethylene, polymethyl methacrylate, and modular connections, is reported. In the area of osteolysis, histochemical and in situ hybridization techniques established the expression of messenger ribonucleic acid encoding for certain cytokines implicated in bone resorption, preferentially in the area of osteolysis. This case illustrates that the etiology of periprosthetic osteolysis is multifactorial and can occur in the absence of polyethylene, methacrylate, or modular components. All joint implants should be monitored for the development of this complication.

Wear Debris in Total Joint Replacements.

In vivo degradation of prosthetic implant materials is increasingly recognized as a major factor limiting the durability of total joint arthroplasty. In vivo degradation occurs primarily by means of wear processes that can generate large quantities of particulate debris. This debris can stimulate an adverse local host response leading to periprosthetic bone loss, which can compromise implant fixation and bone stock. The authors review the basic mechanisms of implant degradation and the host response to particulate degradation products, particularly in the context of the pathogenesis of osteolysis. Submicron polyethylene particles (mean size, 0.5 um) are the dominant type of wear particle present in periprosthetic tissues associated with uncemented hip replacements. Polyethylene wear can be minimized by improving the quality of the polyethylene, avoiding use of large-diameter (greater than 28 mm) femoral heads in total hip arthroplasty, and improving the design and fabrication of modular connections, which can be important sources of three-body wear particles. Advances in the understanding of the basic mechanisms of osteolysis are critical to the development of preventive measures that will minimize the clinical impact of this phenomenon.

Wear Debris

There is a growing concern that osteolytic lesions, often adjacent to otherwise stable implants, are a recent phenomenon caused by some recent change in polyethylene, metal, or other aspect of the total hip construction. This study investigates the possibility that bearings and modular connections used in modern hip replacements are an unappreciated source of particulate debris. Measurements taken from contemporary femoral bearings show a significant mismatch in both surface finish and sphericity of mating metal and polyethylene components, with sphericity of inserts being much worse then sphericity of femoral heads. The tolerances for sphericity of polyethylene inserts were further changed by the placement of an insert into its metal shell. Hip simulator tests of assembled inserts and shells showed greater polyethylene weight loss for metal-backed shells than for inserts alone. Bending and torsional tests of metal/metal modular connections showed that dynamic loads can release large numbers of debris particles from taper junctions. Because osteolytic lesions clearly are associated with overload of tissue by debris particles, the design, manufacture, and tolerances of modular connections in total hip replacement all seem to require reevaluation.

Mechanisms of Failure of Modular Prostheses

The expectations of wear and longevity of total hip components are based in large part on Charnley's early work. The evolution of the total hip from the one-piece, all-polyethylene acetabular component and fixed-head femoral component to the myriad of parts that comprise many of today's total hip designs has brought with it an array of potential mechanisms for failure that were not present in the earlier design. The risk/benefit ratio of these new designs may need to be reevaluated based on the additional mechanisms for failure that they provide. One hundred eleven acetabular hip prostheses and 139 femoral prostheses, all of modular configuration, retrieved by surgeons in the field, and sent for histologic examination, were analyzed for this study. A number of component characteristics were found to be correlated to early failure. These included acetabular designs with thin polyethylene bearings, poor fixation of the polyethylene to the metal shell, and geometries that permitted a moment to be applied to the bearing insert, tending to cause it to rotate in the metal shell. Modular femoral components were observed to be susceptible to corrosion, with titanium-alloy stems mated to cast cobalt-alloy heads at greatest risk attributable to a galvanic effect. All modular connections of femoral and acetabular components are at risk for disassociation and fretting; therefore, clever design and precision machining are necessary to produce prostheses in which the benefits of modularity exceed the risks.

Modular Connections between Areas V2 and V4 of Macaque Monkey Visual Cortex.

We have studied the connections between two visual areas of macaque monkey cortex, V2 and V4, by injecting wheat-germ agglutinin horseradish peroxidase (HRP-WGA) into V4 and examining the distribution of labelled cells and terminals in V2, in relation to its characteristically striped cytochrome oxidase architecture. The cells projecting from V2 to V4 are arranged in bands and the number of bands per cycle of cytochrome oxidase stripes varies (one cycle consists of a thin stripe, a thick stripe and two interstripes). In the Type 1 connectivity pattern, there is just one band per cycle, centred over the thin stripes but normally spreading into the neighbouring interstripes. In the Type 2 connectivity pattern there are two bands per cycle, generally rather narrower and centred over the interstripes. Thick stripes are mostly free of labelled cells. The return projection from V4 to V2, whilst being concentrated in the vicinity of the labelled cells, is more diffusely distributed and invades the territory of all the stripes.