Bi-modular stem designs offer surgeons an intraoperative choice of neck version and length independently of the stem size, which has led to their worldwide popularity. There are important differences in femoral neck length, shaft diameter, caput-column-diaphysis (CCD) angle, neck version, and neck offset, and modularity enables the orthopaedic surgeon to adapt to those differences . Excellent results with titanium-titanium alloy made stems and modular necks were reported in single-centre studies . Despite these advantages, several studies reported an increase in titanium alloy modular femoral neck failures [2, 6, 7]. In a study of two groups of THA patients, Duwelius et al. looked for differences in anatomical hip restoration and revision rates between nonmodular and modular stems . A total of 284 patients with nonmodular stems (Zimmer M/L Taper) and 594 patients with modular stems (Zimmer M/L Taper Kinectiv) were followed up with a mean of 2.4 years (maximum 5.9 years). Clinical and radiographic measurements of leg length and offset were done before and after surgery. Clinical evaluations included Harris Hip and SF-12 scores, respectively. With no differences in outcome scores, the authors concluded that modular stems offered no added value over standard non-modular stems .
Different specific types of corrosion at the stem-neck junction were found responsible for exchangeable neck failures. In 2014, Mumme et al. submerged standardized alloys (TiAl6V4, CoCr29Mo, FeCrNiMoMnNbN and pure titanium) in human serum and measured in vitro serum levels of metal ions . Elevation of in vitro ions concentration shows that electrochemical corrosion occurs without the need for mechanical load . Types of corrosion that can develop in orthopaedic implants include uniform corrosion attack, galvanic corrosion, fretting corrosion, crevice corrosion, pitting corrosion, intergranular corrosion, leaching, and stress-corrosion cracking . MACC at stem-neck junction was shown to release metal debris that caused local tissue reactions like those found in failed THA-s [11,12,13,14].
Titanium alloy is suitable for non-modular prosthesis due to its passive oxide layer that protects it from uniform corrosion. Fretting corrosion is caused by oscillating micro motion between two surfaces . In modular stem prosthesis, micro movements at the stem-neck junction occur and the oxide layer falls off, exposing titanium alloy to bodily fluids  Preventing micromotion is thus the best way to avoid damage within modular junctions. Since Co-Cr alloy has a twice as high module of elasticity compared to titanium alloy, there should be less micro movement than with titanium alloy exchangeable necks. However, with neck fracturing already at 2 years after implantation because of an added galvanic corrosion, Co-Cr necks have not proven to be a safe alternative .
Other factors that may escalate fretting corrosion include patient BMI, lateral offset, varus femoral stem positioning, longer necks and larger heads, time since implantation and inconsistency in the assembly of modular heads including the force of impaction, the vector of the applied force, and contamination of the interface . A crack can start on the medial proximal side of the neck-stem taper surface . After the initial crack is made, the corrosion can continue without additional external loads on the femoral neck. This type of autocatalytic corrosion is a result of chemical changes within the crevice fluid .
Several studies report complications after implantation of bi-modular femoral stems with symmetrically oval Morse taper joints in primary THA. Grupp et al. followed 5000 bi-modular Metha Short Hip stems (Aesculap AG, Tuttlingen, Germany) implanted between 2004 and 2006 and found that 1.4% of the titanium alloy necks failed after 2 years . Bernstein et al. reported 86% clinical failure rate of the Rejuvenate bi-modular stem implant with Co-Cr neck (Stryker Orthopaedics, Mahwah, New Jersey) at 4.2 ± 0.6 years mean final follow up . Pour et al. followed 277 patients after Profemur (successor of An.C.A. Fit prosthesis with the same oval Morse taper neck-stem junction) bi-modular series stem implantation and found 6% neck fractures at 50 months mean follow up . Finally, Kovač et al. studied the long-term behaviour of the Profemur Z modular stem on a national basis (2767 hips followed) and found out that the mean time for bi-modular femoral neck fracture (0.83%) was 4.7 years (SD ± 2.2 years) . Furthermore, long neck, Co-Cr modular neck, which was introduced by the producer (Wright Medical Technology, now MicroPort Orthopedics Inc.) in 2010 to reduce the failure rate of titanium alloy made necks, and male gender represent the independent risk factors for modular neck fracture . In contrast, Pelayo-de-Tomás et al. found only 1 modular neck fracture in their cohort of 317 consecutive patients followed for 6.1 (range, 2–8) years after bi-modular stem (H-MAX M, Lima, San Danielle, Italy) THA . The authors attributed the discrepancy between the H-MAX M model and other commercially available modular stems to the elliptical dual radius neck-stem coupling causing fever micro-movements .
In 2009, Blakey et al. conducted a study on 319 patients that were treated with primary THA using uncemented hydroxyapatite-coated An.C.A. Fit modular femoral stem due to osteoarthritis . Five years after implantation, the authors clinically and radiographically examined 212 males and 107 females with a mean age of 64.4 years. Oxford Hip Score got significantly better (from mean 41 points to 20 points). There were two cases of aseptic loosening. They concluded that there were no clinical or radiographic complications due to modularity . In 2017, Toni et al. reported on clinical and radiographic outcomes of 235 patients 13–18 years after An.C.A. Fit prosthesis implantation with ceramic-on-ceramic (CoC) bearing and noted no modular neck fracture; 93.2% of the implanted THA were still functioning well . However, this case report demonstrates that titanium-titanium alloy modular-neck implants in primary THA are not so uniformly good and complication-free.
Lex et al. recently published a systemic review of 14 studies (12 case series and 2 joint registry analyses) on current-generation primary THA using titanium-titanium alloy modular-neck implants, including An.C.A. Fit . The mean follow-up duration of the studies was 5.7 years and they included 591,025 patients, of which 21,841 underwent modular neck THAs and 569,184 received a fixed-neck prosthesis. Even though the authors have found the overall mean revision rate (3.95%) and the mean revision rate for fracture of the modular neck component (0.43%) for modular prosthesis acceptable, they consider modular prostheses to be a viable management option only in patients with considerable anatomical hip deformities that cannot be corrected with standard fixed-neck implants. The review also points out two national registry reports, which revealed that modular neck prostheses had a higher revision rate compared to traditional, fixed-neck prostheses in patients with osteoarthritis .
To our knowledge, the present is the first reported case with bilateral bi-modular femoral neck fracture in a single patient. The modular neck fracture in the presented case occurred relatively early on the left and later on the right side. Beside the variation in the size of acetabular and stem components, the only difference between the implants was in the bearing couple, which was ceramic-on-polyethylene (CoP) on the left side and metal-on-polyethylene (MoP) on the right side. Together with slight variation between implant position and offset, these dissimilarities could be responsible for different timing at which modular neck fracture occurred. However, Frisch et al. have shown that the shape of the neck and stem tapers deviate from ideal design dimensions, contributing to relative motions between the neck and the stem and may place the implants at higher risk for failure . This means that such catastrophic failures can occur unpredictively in otherwise normally functioning implant(s).
The patient in this report was a younger active male with a BMI in the obese range. He had a long modular neck implanted in combination with a long head – all of which are known risk factors for modular neck fracture and therefore make the patient at risk for neck failure [5, 7, 10, 19]. The fatigue strength of TiAl6V4 is about 400 MPa. In heavier and more active patients this threshold can be exceeded because very high tensile stresses occur on long necks in combination with long femoral heads [10, 25, 26]. This stress can open microcracks on the surface and stress fields inside the prosthesis add to crack propagation after its initiation. The patient was hospitalized for 208 days altogether due to bilateral bi-modular prosthesis failure, which profoundly impacted his social life and exposed him to other health problems and threats that arise with more prolonged hospitalizations. Since higher septic complications after THA revision surgeries are well established, modular neck fracture per se could not be blamed for septic complications that developed after revisions of both sides in presented patient [27, 28]. However, more complications also mean higher costs of treatment, which is not beneficial for the health care system. If a high percentage of bi-modular stems started to fail, this could become a major public health issue. Continued usage of bi-modular stems for primary THA is therefore not only a problem associated with more treatment complications and patient suffering but also an economic problem and a threatening public health concern.
In conclusion, the patient could have avoided all the unnecessary complications and had a better quality of life if he were primarily treated with monoblock stems instead.