The 16-Year Evolution of Proximal Modular Stem Design – Eliminating Failure of Modular Junction

Abstract

Background: The complexity of hip reconstruction has been and continues to be a perplexing problem with restoring leg length, femoral offset, joint stability and overall hip implant fixation. These were contributing factors that lead to the development of a novel proximal femoral component design “Apex Modular Stem” (Omni, Raynham, MA). The basic stem geometry features a straight stem with a metaphyseal fit and fill cone, a medial triangle and a modular neck junction that allows for version and offset adjustment. In recent years, there has been great concern with the use of modularity in total hip arthroplasty. The goals of this study are (1) to identify complications with the use of a proximal modular design and (2) demonstrated factors that have eliminated those complications. Methods: This is a retrospective study of a single surgeon series (Design A and Design B) of using the same cementless stem and proximal modular neck body (Apex Modular Stem and Omni Mod Hip Stem) from 2000 to 2016 totaling 2,125 stems. 483 stems were the Design A and 1,642 stems, were of the Design B style. Results: Design A, 483 stems were implanted between 2000 and 2004. 31 alignment pins sheared resulting in a revision rate of 6.4%. Design B, 1,642 stems have been implanted between 2004 and 2016 all by the same surgeon, with no failures of the modular junction. Conclusion: All implant devices entail a multitude of risks and benefits. The Apex Modular Stem (Design A), provided excellent fixation, minimal risk of modular junction  corrosion, and simple control of anteversion and femoral offset. The limitation was found to be the risk of the alignment pin shearing (6.4%). The pin was enlarged to make it 225% stronger in torsional resistance, and in a subsequent series of over 1,600 femoral stems in a single surgeon series, there were no pin failures over a 12 year duration.