Complications of Modular Neck-Stems

Fig. 16.1

Anteroposterior (a) and lateral (b) radiographs of a patient with recalled dual-modular femoral neck-stem with trochanteric osteolysis (white arrow) juxtaposed to neck-stem modularity junction (black arrow)

Fig. 16.2

Axial T1 MARS MRI image showing adverse local tissue reaction (pseudotumor) (white arrow)

Rationale for Modular Taper

Modularity in total hip arthroplasty (THA) allows surgeons to optimize implant reconstruction to patient anatomy intraoperatively [1]. Modular femoral neck-stem THA implants possess interchangeable necks, providing additional modularity at the neck-stem interface [2, 3]. Modular taper designs have the potential to allow precise reconstruction of center of rotation of the hip by facilitating adjustments in limb length, femoral neck version, and hip offset in order to optimize hip biomechanical parameters [4]. Other purported benefits include facilitation of surgical procedures, such as revision arthroplasty [5] and minimally invasive surgical techniques [6], as well as economic benefit of enhanced implant inventory control [7]. Four modularity-related failure modes have been described, namely taper corrosion-associated adverse local tissue reaction (ALTR), modular neck fracture, dissociation of the taper junction, and mismatch of the femoral head and taper connection dissociation [8–13]. Recently, there is increasing concern regarding this stem design as a result of the growing numbers of clinical failures due to fretting and corrosion at neck-stem taper junction, in a process that has been described as mechanically assisted crevice corrosion (MACC) [14].

History of Modular Taper Corrosion

The first modular neck-stem (ANCA-Fit) was designed by Cremascoli (Milan, Italy) and possessed an elliptical taper for rotational resistance [15]. Subsequently, a myriad of dual-taper neck-stems were introduced [4]. Several of these dual-taper neck-stem designs have since resulted in taper corrosion issues at the proximal neck-stem and distal head-neck junctions [16–21]. However, it must be noted that taper corrosion is not a new phenomenon. Head-neck taper corrosion has been described in 1991 by Mathiesen et al. [22] between monoblock titanium (Ti) stems and cobalt-chromium (CoCr) heads [22]. Subsequently, MACC was recognized as a process which was dependent on multiple contributing factors, including taper geometry and metallurgical, mechanical, and solution chemistry between metal alloys [23]. In fact, several studies in the 1970s and 1980s cautioned against the use of dissimilar metals in orthopedic implants and described varying resistance to corrosion between different galvanic combinations of Ti and CoCr alloys [24, 25].

Modular femoral neck-stem THA implants have been manufactured with both titanium and cobalt chromium alloy modular necks; given the preference for titanium alloy femoral components, almost all of the “bodies” have been manufactured from titanium. While a titanium neck prevents the risk of corrosion from the use of mixed metals, titanium alloy necks would be more susceptible to fracture. Alternatively, cobalt-chromium alloy modular necks are stronger and reduce risks of facture; however, being a different metallic alloy from the stem, modular junction corrosion can occur, leading to early failure secondary to adverse local tissue reactions (ALTR) [26]. These junctions are subjected to axial and cantilever-type bending stresses, leading to metal-on-metal micromotion. This has been known to generate metal ion debris with subsequent elevation of serum Co and Cr ion levels [14, 26]. When evaluating a painful modular neck-stem, an understanding of the metals utilized to manufacture the modular neck is important. To further confuse the matter, some manufacturers have made necks of both titanium and cobalt chromium alloys at different times.

Epidemiology of Modular Taper Corrosion

The 2015 Australian Orthopaedic Association National Joint Replacement Registry Annual Report states that 9289 modular femoral neck-stems were used in primary total hip arthroplasties from September 1999 to December 2014 [27]. A significantly higher 10-year revision rate of 9.7% (95% CI: 8.8, 10.7) was reported for modular femoral neck-stems as compared to 5.1% (95% CI: 4.9, 5.2) for fixed femoral neck-stems. The main reasons for early revisions in the modular femoral neck-stems were due to loosening/osteolysis, dislocation, and modular neck fractures. In 2012, a manufacturer initiated a voluntary product recall of two modular femoral neck-stems. At that point of time, it was estimated that more than 30,000 patients received the recalled modular neck-stem implants worldwide [28].

Implant Factors Associated with Modular Neck Taper Corrosion

Implant, surgical, and patient factors have been identified as likely contributing factors responsible for taper corrosion in dual-modular neck-stem THA. Implant factors including taper cone angle, taper surface roughness, neck-stem taper metallurgy, taper geometry, and femoral head size play important roles in influencing extent of taper corrosion. Narrow cone-angled tapers have greater potential for micromotion and may explain higher fretting scores reported at the taper base of 11/13 taper designs compared to 12/14 and 14/16 tapers [29]. Rough-surface tapers were initially developed for use in ceramic heads and surface roughness has been linked to increased fretting in modular MoM head-neck taper combinations [30].

In vivo corrosion and MACC are more commonly seen in dissimilar alloy pairings (e.g., Ti alloy stem and CoCr head), and affect both head-neck and neck-stem [31]. Conversely, ceramic femoral heads have been reported to decrease taper tribocorrosion [32]. Taper geometric parameters such as length, taper contact area, and resultant lever arm contribute to taper corrosion at both the head-neck and neck-stem junctions. Femoral stems with longer modular neck lengths had significantly higher corrosion scores [33]. This corresponds with higher fretting scores noted with increased taper contact area [34]. However, there have been reports that describe increased edge loading at the base of short taper trunnions [35]. Additionally, “long varus” necks demonstrate 32.7% greater bending moments when compared to “short varus” necks [11] which potentiates cantilever bending in vivo and resultant micromotion, fretting, and corrosion [33]. In relation to this, beta titanium alloy (Ti12Mo6Zr2Fe, TMZF) has shown decreased flexural stiffness and has been associated with increased fretting and corrosion [36]. These effects may be aggravated with the use of larger head sizes (>32 mm), which increase torsional forces at the taper trunnion [37]. Larger head sizes may increase offset and varus neck shaft angle, leading to increased lever arm [35], and has been recognized as a contributing factor in the increased failure rates of modular MoM THA [38].

Surgical Assembly Associated with Modular Neck Taper Corrosion

Intraoperative surgical assembly may play an important role. In vitro tests have demonstrated greatly reduced (>50%) load to failure with a contaminated assembly compared to a well-cleaned assembly [39] and has been shown to affect cantilever micromotion of dual-modular taper neck-stems [40]. Although impaction technique and force of impaction have been studied in head-neck taper corrosion, the effect of impaction at the modular neck-stem junction has not been examined to date. It is recognized that cleaning of interfaces before impaction, avoidance of angular mismatch between the neck and the stem, and use of similar alloys for the neck and the stem are essential in preventing modular neck taper corrosion.

Patient Factors Associated with Modular Neck Taper Corrosion

Potential patient factors associated with modular neck taper corrosion-related ALTR include metal hypersensitivity, body mass index, and activity level. Implant-related metal hypersensitivity has been reported since 1990s [22]. Although ALTR has been associated with taper corrosion secondary to MACC, ALTR has also been observed in the absence of high wear or metallosis [41–43]. Histological examination of periprosthetic tissues in dual-taper THA patients undergoing revision surgery has demonstrated features suggestive of metal hypersensitivity [44, 45]. Although increased body mass index (BMI) and increased activity levels would potentially increase the stresses borne by the modular trunnions, to date, no correlations have been reported between ALTR in dual-modular stem neck THA and BMI or increased activity levels [42, 46].

Systematic Evaluation

A painful dual-modular neck THA can present with a myriad of symptoms and may be attributable to various intrinsic and extrinsic causes . In general, the clinician should look to rule out common cause of pain or failure, including infection and implant loosening, prior to initiating an evaluation for modes of failure specific to a modular neck-stem. A systematic evaluation should include focused clinical history, detailed physical examination, laboratory tests, and cross-sectional imaging to identify potential differential diagnoses in a painful dual-modular neck THA [47, 48]. A consensus systematic risk stratification algorithm is available to guide physicians based on currently available evidence [49] to initiate prompt and appropriate treatment.

Clinical Evaluation

The physician should obtain a complete history when evaluating patients with a dual-modular femoral neck-stem total hip arthroplasty. The characteristics, site, severity, duration, and onset of the pain provide important information [48, 49]. Joint sepsis must be suspected in patients with a history of delayed wound healing, or hip pain after recent gastrointestinal or dental procedures. The physician should also ask about any discomfort caused by fullness or swelling around the hip as this may suggest a fluid collection secondary to an ALTR. Physical examination should begin with inspection of the skin for signs of infection and previous scars. Palpation around the hip may reveal the presence of soft-tissue masses which again might suggest ALTR. Range of motion of the affected hip should be tested as reproduction of pain on passive extension and active flexion may indicate iliopsoas tendinopathy. Strength of hip abduction should also be examined. A comprehensive spine and neurovascular examination is essential to exclude potential confounding neurogenic and vascular causes of pain [48].

Implant Modularity

It is important to recognize that different types of material options exist at the neck-stem junction of a dual-modular femoral neck THA [49]. Different combinations of neck and stem materials may be used at the neck-stem modular junction of dual-taper stem THAs, and include:

(a)

Titanium modular neck on titanium stem (Ti/Ti)
(b)

Cobalt-chromium modular neck on titanium stem (CoCr/Ti)
(c)

Cobalt-chromium modular neck on cobalt-chromium stem (CoCr/CoCr)

To date, the majority of taper corrosion-related adverse tissue reactions have been reported in dual-modular neck femoral THAs with cobalt-chromium modular necks on titanium stem modular junctions (CoCr/Ti).

Inflammatory Markers and Hip Joint Aspiration

Serum inflammatory markers (ESR and CRP) are frequently elevated in a setting of taper corrosion and this can occur in isolation, or may suggest presence of concurrent or isolated periprosthetic infection (PJI) [50, 51]. Taper corrosion may also mimic the diagnosis of PJI and this warrants hip aspiration for culture and differential counts [52]. However, synovial fluid counts are also affected by the presence of metallic debris which requires manual counting to reduce errors from metallic debris contamination or the erroneous measurement of dead white blood cells [53]. Although ESR and CRP have limited value in the diagnosis of PJI in dual-modular neck implants with corrosion, these inflammatory markers may be useful in excluding PJI. In addition, there should be a low threshold to perform synovial fluid hip aspiration in the setting of elevated inflammatory markers as the intraoperative appearance can be deceiving with purulent material frequently seen. Hence, a preoperative aspiration with a cell count and culture can be very useful to perform preoperatively to avoid confusion at the time of surgery. Newer methods to determine the presence of infection such as leukocyte esterase strip tests, alpha defensin, and PCR methods may be potentially useful to detect PJI in the presence of taper corrosion in dual-modular neck-stem THA patients although some reports have suggested false-positive alpha defensin testing in patients with corrosion reactions [54].

Serum Metal Ion Levels

Cobalt and chromium ion levels are influenced by the implant type, metallurgy, design of neck and stem taper interface, head size, and positioning of the implant [49]. Elevated metal ion levels have been documented in dual-modular neck-stem THA patients with taper corrosion-related adverse tissue reactions [4, 14, 55]. In general, the magnitude of elevation, however, has been lower in dual-modular neck-stem THA patients when compared to MoM bearing THA patients [49]. In addition, a preferential elevation of cobalt relative to chromium, resulting in an increase in Co/Cr ratio, is also commonly observed in patients with symptomatic corrosion [56]. A management algorithm based on the consensus systematic risk stratification statement of the American Association of Hip and Knee Surgeons, the American Academy of Orthopaedic Surgeons, and the Hip Society stated that although metal ion levels alone should not be relied on as the sole parameter to determine revision surgery, threshold of cobalt level >5 μg/L and Co/Cr ratio >5 are useful clinical diagnostic adjuncts in the systematic clinical evaluation for taper corrosion-related adverse tissue reactions in patients with dual-modular taper THA [49].

The mechanism that leads to differential elevation of cobalt relative to chromium remains poorly understood [26]. It has been hypothesized that, in contrast to MoM bearing surface wear, the predominant ion release at modular taper junction may be chemical corrosion process that involves chromium precipitating as chromium orthophosphate and more soluble cobalt dissipating as free ions [57]. Metal ion levels have been reported to decline to near-normal levels within 3 months following revision surgery and removal of a dual-taper modular neck-stem THA [58]. In addition, metal ion levels can be difficult to interpret in patients with systemic renal disease, additional MoM bearings in conjunction with dual-modular neck-stem THAs, and bilateral dual-modular neck-stem THAs. Therefore, while metal ions are useful as an adjunct investigation, it must be noted that metal ion levels should not be used in isolation to determine clinical recommendation for surgery.

Radiographic and Cross-Sectional Imaging

Focused review of serial plain radiographs to identify loosening, osteolysis, trochanteric, and/or calcar erosion must be noted as they may be associated with taper corrosion (Fig. 16.1). Currently, ultrasound (US) or metal artifact reduction sequence (MARS) magnetic resonance imaging (MRI) is the preferred cross-sectional imaging modality of choice. Common MARS MRI abnormalities include the presence of a thickened capsule in association with an effusion (Fig. 16.2). Other typical findings include iliopsoas and abductor tendinopathy, peri-tendinous collections, and presence of metallic debris.

Both ultrasound and MARS MRI have been reported to be highly sensitive (92–100%) and specific for the detection of MoM THA-related pseudotumors (96–100%) [59]. Ultrasound is not affected by soft-tissue artifacts, and can discriminate solid from cystic lesions. Ultrasound assessment is operator dependent and does not allow the surgeon to perform preoperative planning nor longitudinal comparisons [60]. In addition, its inconsistency in evaluating the deep structures, especially in obese patients, is a potential disadvantage [61]. MARS MRI is a highly sensitive modality for detection of solid and cystic ALTR (pseudotumors); however, the potential disadvantages include an increased scanning duration, the obscuration of periprosthetic tissues by metal artifacts, and the costs [62].

Surgical Management

Management of a patient with a painful dual-modular neck-stem THA presenting with elevated metal ion levels and evidence of pseudotumor with bone or muscle damage warrants revision surgery. Revision surgery for dual-modular neck-stem THA necessitates a thoughtful approach to reduce intraoperative complications. Meticulous and careful debridement must be performed to remove the pseudotumor while protecting neurovascular structures. Removal of the well-fixed stem is technically challenging, requiring availability of instruments such as high-speed burrs, flexible thin osteotomes, and customized neck-stem extractors. Techniques such as simple and extended trochanteric osteotomies, high-speed needle-point burrs, and stack pin techniques have been described for stem removal [63, 64]. Stem removal may affect remnant proximal bone stock and fixation of the revision prosthesis stem.

Following removal of dual-modular femoral stem, acetabular component assessed to be loose intraoperatively is revised with a highly porous tantalum acetabular cup with multiple screws and highly cross-linked polyethylene liner. In the presence of reactive tissue necrosis, the area of necrosis is extensively debrided except in the close proximity of neurovascular structures . The use of modularity at the time of revision such as titanium modular tapered femoral stems is frequently required to optimize intraoperative stability in the setting of significant tissue necrosis. The largest diameter ceramic femoral head compatible with the acetabular component is used to maximize head-neck ratio in order to further optimize intraoperative stability.

The revision surgery may result in an increased complication rates and re-revision rates due to dislocations and recurrence of ALTR in the setting of well-fixed femoral stem requiring removal and adverse tissue reaction. The precise etiology of ALTR recurrence after revision for symptomatic ALTR remains largely unknown. The potential contributing factors for ALTR recurrence is likely to be multifactorial related to surgical, patient, or implant factors. Incomplete surgical debridement and inadequate removal of pseudotumor during the first revision may contribute to ALTR recurrence. However, extensive debridement needs to be performed safely juxtaposed to neurovascular structures. Inherent patient metal hypersensitivity may be a contributing factor. ALTR recurrence has also been suggested to be secondary to taper corrosion at head-neck taper junction of metal-on-poly bearings or at modular taper junction of revision femoral stem used at initial revision surgery.

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