Reproducing the Proximal Femur Anatomy: Modular Femoral Component
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Stem modularity can be classified according to the coupling location: distal, mid-stem, and proximal . Mid-stem and proximal modularity have been more frequently used. Either the junction is located proximal or distal (mid-stem) to the neck osteotomy (Fig. 8.1). Proximal modularity with modular necks was introduced in 1987 by Cremascoli Ortho (Milan, Italy), in order to provide independent combinations of version, offset, and length .
KeywordsNeck Exchangeable Dysplasia Corrosion Titanium
8.1 Neck Modularity
Clinical results of modular necks. There is solid evidence that shows the reliability of well-designed modular neck implants regarding their capacity to restore the native proximal femoral anatomy and to provide good long-term clinical outcomes. Montalti et al.  reported good anatomical reconstructions in severe dysplastic hips using modular necks (AncaFit® stem, Cremascoli Ortho, Milan, Italy). In particular, the use of a high prosthetic hip center reconstruction, combined with modular necks, improved the biomechanics and the offset restoration, with good to excellent clinical results and only a case of cup aseptic loosening after a minimum follow-up of 10 years. Archibeck et al.  reported a comparison between 100 primary total hip arthroplasties (THAs) with modular neck stem design and 100 primary THAs without (respectively Kinectiv® and M/L Taper® stems, Zimmer-Biomet, Warsaw, USA). The offset and leg length were restored to within 1 mm in 85% of the modular hips and in 60% of the monoblock stems. On the contrary, Duwelius et al.  failed to demonstrate superior clinical outcomes at 2-years follow-up in a similar comparison involving the same implants. Nevertheless, a better reconstruction of leg length and offset characterized the modular neck cohort.
Our experience: The regional arthroplasty registry of Emilia Romagna. In the regional arthroplasty registry of Emilia Romagna, the 15-year cumulative implant survival rates were found to be similar between 16,575 modular implants (557 being exposed to risk at 15 years) and 35,620 monoblock implants (1781 being exposed to risk at 15 years) performed for primary osteoarthritis, at 90.8% and 91%, respectively . The rates of aseptic loosening were inferior in the modular group (0.4% vs. 0.7% for isolated acetabular implant loosening; 0.6% vs. 0.8% for isolated femoral stem loosening; and 0.1% vs. 0.4% for loosening of both the components), as well as the rate of revision due to polyethylene wear (0.04%, 7 implants out of 16,575 THAs), but none of those differences reached statistical significance. In terms of revisions due to prosthetic instability, no differences could be detected between the two groups. These data may suggest that modular necks allow for a better component interaction, enabling to reduce the mechanical stresses on component fixation and preserving them from aseptic loosening. Considering only the THAs performed due to congenital pathologies (e.g., developmental hip dysplasia), the results between the two groups were more striking. The modular neck implants (2805 cases with 238 being exposed to risk at 15 years) achieved a survival rate of 93.3% at 15 years, whereas conventional implants (3707 cases with 389 being exposed to risk at 15 years) had a lower performance with 89.6% of survivorship. Regarding the reasons for revision, the two groups had similar revision rates for early prosthetic instability (within the first 3 months), but modular neck THAs had an inferior revision rate for recurrent dislocations (0.5% vs. 0.8%). Revisions for aseptic cup loosening were significantly lower with modular necks (0.5% vs. 1.9%). This finding showed that, in the modular neck cohort, the revision rate for aseptic cup loosening was comparable to THAs implanted for primary osteoarthritis and was four times lower than conventional implants used for congenital pathologies. On the other hand, in the cohort of congenital pathologies, a rate of 0.5% of neck fractures was reported for the modular neck implants.
8.2 Head Modularity
Similarly to modular necks, modular heads were introduced in the 1980s, aiming to restore better prosthetic hip biomechanics . The success was outstanding and, in the 1990s, 90% of the implants had a head–neck modularity . Nowadays, head modularity is a capital element in THA as it allows the surgeon to use different bearing surfaces, to more accurately restore offset and leg length, to improve stability, and to facilitate revision procedures [16, 17, 18]. Usually, head modularity occurs at a Morse taper, resulting in a force-fit connection (taper locking) that resists the axial and torsional forces . Unfortunately, there is no standard taper. Tapers are fabricated with different configurations and angles, with several variations among manufacturers and hip devices [16, 17, 18]. Thus, the surgeons must carefully assess the compatibility between the new head and the well-fixed stem in case of partial revisions [16, 17, 18]. Although the benefits related to head modularity remarkably outweigh the cons, few drawbacks should be noticed: disassembling and excessive corrosion . Dissociation of the head is anecdotic in the modern implants, usually occurring after trauma and secondary to mismatches . Severe and clinically troublesome corrosion almost uniquely occurs with large metal heads, being ceramic balls involved in very few cases. The taper is subjected to mechanically assisted crevice corrosion due to oxidation and micromotion, similarly to the neck-stem junctions that are usually Morse tapers too . The main factors increasing corrosion at the head–neck interface occur with different metal combinations, larger heads (>32 mm), shorter tapers, high head offset (e.g., XL head), and active and/or obese patients . Despite those few concerns, the routine use of head–neck modularity in primary THAs is not challenged. Furthermore, the use of modular head–neck adapter systems seems particularly important in revision settings . Such systems allow the reduction of leg length discrepancy and the loss of offset, improving the biomechanics and the stability of the revised prosthetic hip. In a retrospective series including 95 patients, Hoberg et al.  described 95 revisions requiring the use of BioBall® system (Merete, Berlin, Germany). The survival rate was 92.8% at 8 years, two patients requiring a further procedure due to recurrent dislocations. No corrosion was noticed in the revised cases.
8.3 Femoral Component Modularity Contributes to Personalizing the Hip Reconstruction
Proximal femoral modularity is a useful tool to optimize prosthetic hip biomechanics and to potentially reduce complications related to poor component interaction (edge loading, prosthetic impingement, and related complications such as instability). Each patient has a unique hip biomechanics that may even vary with age. Three capital parameters of hip biomechanics, femoral offset and hip rotation center and combined anteversion of femur and acetabulum, are randomly and independently located even in conventional hip morphologies [1, 2, 3, 4]. Restoring the native hip anatomy when performing hip replacement is a sound option for optimizing prosthetic function and biomechanics, and overall clinical outcomes and patients’ satisfaction. However, as conventional implants, even with modular heads, may only address a few hip anatomies, most of the patients have their native hip anatomy altered after reconstruction. Thanks to the independent tuning of lengths and angles in the three planes, regardless the stem size, proximal modularity (head and neck) may effectively reproduce the constitutional hip biomechanics within millimeters [1, 2]. Thus, the muscle lever arms can be finely reconstructed and the combined anteversion optimized. A wider articular excursion, a more uniform distribution of joint forces, and an improved component interaction consequently result, highlighting that personalizing hip replacement using modularity is not without tangible outcomes . Proximal femoral modularity is even more desirable in case of outlier anatomies, when conventional implants with minimal modularity (head) grossly fail to reconstruct the hip biomechanics [1, 2, 5].
8.4 Modularity: Guidelines for Users
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