Modular heads were first introduced in the 1980s and became the standard of care by the early 1990s [1]. One advantage of modularity at the head–neck junction is the ability to make small adjustments to a head size, offset, and length to optimize soft-tissue tension, joint stability, leg lengths, and overall hip biomechanics. Additionally, modularity allows surgeons to choose from a variety of bearing materials, ranging from cobalt–chromium (CoCr) to ceramic, individualized for each patient. Furthermore, revision surgeries may be simplified if the revision only requires a head and liner exchange allowing a well-fixed, properly positioned stem to be retained (e.g., in managing an acute periprosthetic joint infection [PJI]). However, while advantages remain, this chapter discusses the potential disadvantages of modularity at the head–neck junction, and offers potential solutions to the recently seen problems of corrosion at the head–neck junction.

Corrosion refers to the environmental degradation of a material that occurs on its surface due to a series of electrochemical reactions (reduction–oxidation reactions); for metallic materials the degradation products are in the form of metal ions, metal salts, and/or adducts of metal ions such as metalloproteins. Passivation is a process that limits corrosion in which a thin layer, typically tens of nanometers in thickness, forms on the metal surface. In the case of surgical implant alloys, this layer consists of an oxide of one or more of the alloying elements. For titanium alloy implants, the passivating layer is composed of titanium oxides, whereas for CoCr alloys and stainless steel, the passivating layer is composed primarily of chromium oxide. Normally, with no external forces acting on the system to disrupt this passive layer, the transport of ions and/or electrons is impeded limiting the redox reactions involved in corrosion.

Eight different modes of corrosion have been described: uniform, galvanic, fretting, crevice, pitting, intergranular, selective, and stress [2].

These eight different modes of corrosion can occur simultaneously and can be synergistic such that the actual corrosion process occurs synergistically (e.g., at modular junctions of a total hip arthroplasty). The manufacturing method, implant design, material selection, metallurgical history, and surface finish, in addition to the chemical and mechanical environment (i.e., pH, oxygen tension, presence of micromotion, and stresses on the implant), all affect the magnitude of corrosion.

In addition to defining and understanding the definition of corrosion, another important category of material degradation is wear . Wear is defined as material loss from the surface of a material due to contact and relative motion with a second body. Fretting is a specific type of wear relevant to total hip implants as it describes material loss due to micromotion (typically less than 100 μm) between two materials under a sustained load [2]. All modular head–neck junctions are subject to fretting [3, 4]. As mentioned above, fretting disrupts the outer passive oxide layer and allows for unimpeded redox reactions (corrosion) in the contact zone between the head and the neck. This is exacerbated by the crevice geometry of the contact zone. Since fretting initiates and potentiates this corrosion process, it has been described as “mechanically assisted crevice corrosion ” (MACC) [5]. Many aspects of the geometry, assembly, and material properties of the trunnion and head can influence MACC, highlighting the complexity of this process.

Tribocorrosion is a term that is increasingly being used to describe the combined and synergistic mechanical and electrochemical (redox) processes that occur at metal contact zones. Tribocorrosion includes both wear and corrosion and is relevant when describing material degradation at metal bearing surfaces and metal contact zones in modular junctions.

Although the use of metal-on-metal (MoM) THAs has been almost entirely abandoned, the catastrophic consequences seen as a result of metal bearing wear in MoM THAs have led to increased scrutiny and caution at other metal–metal junctions, most notably the head–neck junction. History has shown that with an increase in modularity, where metal contacts metal, complications due to fretting , corrosion, and wear debris may become clinically relevant over time and thus must be studied and followed clinically. While protocols and clinical algorithms exist for the workup and management of MoM THAs, the ideal management of a “tribocorroded” THA remains unclear. Part of what makes taper corrosion so difficult as a clinical problem is that no threshold has been established to predict what amount of tribocorrosion products at the trunnion produce clinically significant complications, specifically an adverse local tissue reaction (ALTR) .

While the incidence of clinically significant corrosion at the modular head–neck taper has not been quantified, McGrory et al. reported an incidence of 1.1% in a population of patients having undergone a primary total hip replacement of one particular design [6]. In a consecutive series of 569 revision THAs, 1.8% of the revision surgeries were performed due to ALTR associated with MACC at the head–neck junction in patients with a metal-on-polyethylene bearing THAs [7]. Another center reported that of 519 revision THA cases between November 2011 and December 2013, 3.3% were to treat ALTR (i.e., pseudotumors) due to trunnion corrosion in metal-on-polyethylene THAs [8]. However, one of the challenges to determining the incidence of ALTR in association with MACC is that the presence of MACC at the head–neck junction does not always lead to a clinically significant ALTR that will require revision surgery. In a study of a recalled dual-modular stems, even some patients with abnormal imaging or lab values, such as elevated metal ion levels, were asymptomatic. In all asymptomatic cases in which the implants were removed because of the recall, there was macroscopic evidence of corrosion [9]. What is concerning about patients with “silent” corrosion is that in a retrieval study, when presenting clinical symptoms were correlated to assessed corrosion and wear damage found in retrieved implants, more severe abductor destruction and bone loss were seen when there was a preponderance of corrosion, whereas those cases with a preponderance of wear and less corrosive damage had more moderate soft-tissue involvement [10].

The presence and amount of tribocorrosion debris at the head–neck junction are determined by many factors. Material properties, including the inherent elastic modulus, flexural rigidity, and reactivity of the comprising elements , are believed to play a large role. Furthermore, the taper geometry, size, design, and lateral offset also play a role. Patient-specific factors have also been shown to affect fretting at the modular head–neck junction—male gender, younger age, and longer implantation time have all been shown to lead to increased fretting; however, body mass index (BMI) did not show a correlation [11]. Surgeon-controlled factors such as maintenance of clean and dry conditions during intraoperative engagement of the head–taper junction, and the application of impaction forces sufficient to engage the head–neck taper , have also been shown to affect tribocorrosion at this location [12, 13].

The femoral head material is likely the most significant factor leading to ALTRs associated with MACC. In simple terms, femoral heads are either made out of metal or ceramic. With improvements in metallurgy and manufacturing, implant materials have had enhanced longevity and resistance to tribocorrosion. However, they still harbor the potential for tribocorrosion corrosive damage. The metal used for the femoral stem in contemporary implants is usually a titanium alloy (Ti–6Al–4V or Ti–Mo–Zr–Fe), and less commonly a cobalt–chromium–molybdenum alloy. Titanium (Ti) is inherently more inert than CoCr, but has inferior wear properties, a lower elastic modulus (i.e., is less stiff), and poor abrasion resistance (is more susceptible to surface deformation and wear as it slides along another metal).