Modular Versus Nonmodular Femoral Necks for Primary Total Hip Arthroplasty
In total hip arthroplasty (THA), proximal femoral neck stem modularity (PFNSM) has theoretical advantages over nonmodular stems, including the ability to more closely reconstruct anatomy and improve stability. However, risks of metallosis and breakage at the junction must be considered.
In this study, we compared the head centers of a modular neck system with that of its nonmodular counterpart. Of 463 primary THAs with a modular stem, 261 (56%) had a head center equivalent to that of its nonmodular counterpart, and an additional 132 (29%) had a head center within 4 mm in length and 2 mm of offset. Thus, only 70 stems (15%) had a head center that was more than 4 mm in length and more than 2 mm in offset different from the nonmodular stem. Only 12 stems had a verted neck.
These findings suggest that, in a majority of primary THAs, use of a modular stem results in head center positions also achievable with a nonmodular stem. Given the risks of modularity, PFNSM should be used with caution. We recommend PFNSM in cases that cannot be reconstructed with the nonmodular option.
A second limitation is that a significant number of Kinectiv stems (132) had a head center within 4 mm in length and 2 mm of offset of the nearest M/L Taper stem. We carefully template every primary THA to determine the plan that will optimize component size and position and restore length and offset. More options for achieving these goals are available when templating with the intention of using the Kinectiv modular neck. The neck cut and position of the stem proximally or distally in the proximal femur may not need to be so exact, as the additional options may be able to accommodate minor inaccuracies. Thus, the reported percentage of clinically indistinguishable head centers (12%) may underestimate the actual number of modular stems that could have been replaced with a nonmodular stem.
Third, this study did not evaluate the effect of the modular junction on ease of irrigation and débridement with head/neck and polyethylene exchange in cases of infection, or on ease of head/neck and polyethylene exchange for revision. In addition, the study did not evaluate other cases of instability involving a nonmodular stem that otherwise could have been solved with simple revision of the head/neck combination, avoiding revision of the entire stem and/or the acetabular component. We reported revisions for infection and for instability, but comprehensive assessment and comparison were beyond the scope of this study. Certainly ease of revision of the head and neck is a factor that could favor use of the modularity.
Fourth, this was not a clinical outcome study comparing 2 different femoral stems. We sought only to determine how often a modular neck was chosen that resulted in a head center that would have been unavailable to the non-modular stem suggesting that the patient was receiving a reconstructive benefit in exchange for the modularity. However, 2 recent reports have noted no clinical benefit at 2-year follow-up with use of the modular neck compared with the nonmodular stem.22,23
Though the M/L Taper with Kinectiv technology has, thus far, performed well, PFNSM should be used with caution in light of recently reported failures at the neck–stem junction.14,16-18 Our study results suggest that most (≥56%) of the modular stems used could have been reconstructed as acceptably with a nonmodular stem, and therefore a reconstructive benefit was not realized in trade for the potential risks of proximal modularity. Only 2 of the 9 revision cases saw a clear advantage in being able to change the modular neck geometry in the revision setting. Given the recently reported failures and the high-profile recall of a modular stem,14 we recommend restricting the modular stem to cases that cannot be adequately reconstructed with the nonmodular option.
