Can the Increased Loosening Rate of the Modular-Neck Hip Systems Be Attributed to Geometric Design Parameters? An Alternative Approach Using Finite Element Analysis

Abstract

Modular necks are a relatively new innovation in total hip arthroplasty (THA). They offer the surgeon the potential to effectively restore hip biomechanics with the ability to independently adjust offset, version, and limb length during operation. These should theoretically improve implant stability, decrease the dislocation and impingement rate, and assist in equalization of leg length (Grupp et al. 2010). Initially they were reserved for the severe torsional deformities of the developmental hip dysplasia (Umeda et al. 2003). However, due to their compelling rationale, the intraoperative flexibility and the fast learning curve, indications quickly expanded up to the younger and more active patients. To further serve the demands of these patients, several manufacturers currently combine modular necks with hard bearings, either metal on metal (MoM) or ceramic on ceramic (CoC) (Grupp et al. 2010). However, whether the advantages conferred by the neck modularity outweigh the additional complexity remains a question. Since the introduction of the modular-neck systems in the clinical practice, vulnerabilities of their application are being continuously revealed. Concerns for fretting corrosion and concomitant metal ion release from the additional MoM junction have been widely expressed (Gill et al. 2012; Jacobs and Hallab 2006; Grupp et al. 2010). Furthermore, corrosion, possibly influenced by the neck adapter material, may lead to fatigue component fracture at the modular site (Grupp et al. 2010). Dissociation at the stem–neck interface, even without trauma, has been also described (Kouzelis et al. 2011). The long-term success of adding a second modular junction has, therefore, yet to be established.