Modularity in femoral revision evolved to address the specific weaknesses in the execution and results of the early Wagner SL stem, namely dislocations and subsidence. With modularity, distal canal fit can be achieved independently, and the proximal geometry can be created to re-establish the leg length and offset. The benefits of modularity relate specifically to being able to modify a plan intra-operatively based on the conditions that are encountered in mid battle. Inherent in this concept is the principle of predictability. The extent to which the conditions of operation may change requires alternatives to manage those changes. More importantly we need to be able to predict how an implant will sit in the bone. At the inception and with subsequent manifestations of modular fluted stems, our ability to predict where the final implant will seat based on the trial options that existed was poor. For this reason, some modular stem designs offered no trial. This was part of the imperative for modularity, so that if the implant set too high it could be easily removed with reaming a little deeper and put back in. If the stem sat more deeply than had been anticipated, the change could be compensated by an alteration in the proximal modular segment. Reproducible mid- to long-term results have been published with this type of stem. Potential negatives of the modular junction include stem breakage, fretting and corrosion, cost, and the need to accommodate a large sized proximal segment within the proximal femur. The most important feature in modern non-modular implants will be predictability. We need to be able to predict that the final reamer will sit at a particular level in the femoral bone, and the trial will reproduce this level, and the final implant will reproduce this level. More importantly, we need to be able to predict that implants will remain where they are put, and not subside. Subsidence has been causally associated with implant under-sizing, which is an error in surgical execution. As such, design features that optimise the ability to achieve intimate and broad endosteal contact between the implant and the bone can help reduce subsidence. These include precise, sharp reamers, implants in 1 mm increments, and trials that reproduce the position of the final implant. A larger implant is less likely to break, and we recommend preparation for the largest implant that the diaphysis can accommodate, often evident in the tactile feedback from the reamer, and the quality of the reamed bone being removed. Reaming is performed eccentrically in the proximal femur, so as to engage the diaphysis optimally. The need for a kink in the stem is important for modular stems, which have bulky proximal segments that can create conflict with the peritrochanteric bone in smaller patients.
The “keel” is the relatively short part of the undersurface of the tibial component that extends into the medullary canal. Most knee replacement systems have the capacity to attach modular stem extensions for enhanced intra-medullary fixation for revision. Diaphyseal length, large diameter stems may also guide positioning of trial components and are ideal for accurate surgical technique, even if fully cemented stems are eventually implanted. Smaller diameter
Modern modular revision stems employ tapered conical (TCR) distal stems designed for immediate axial and rotational stability with subsequent osseo-integration of the stem. Modular proximal segments allow the surgeon to achieve bone contact proximally with eventual ingrowth that protects the modular junction. The independent sizing of the proximal body and distal stem allows for each portion to obtain intimate bony contact and gives the surgeon the ability precisely control the femoral head center of rotation, offset, version, leg length, and overall stability. The most important advantage of modular revision stems is versatility - the ability to manage ALL levels of femoral bone loss (present before revision or created during revision). Used routinely, this allows the surgeon to quickly gain familiarity with the techniques and instruments for preparation and implantation and subsequently master the use for all variety of situations. This also allows the operating room staff to become comfortable with the instrumentation and components. Additionally, the ability to use the stem in all bone loss situations eliminates intra-operative shuffle (changes in the surgical plan resulting in more instruments being opened), as bone loss can be significantly under-estimated pre-operatively or may change intra-operatively. Furthermore, distal fixation can be obtained simply and reliably. Paprosky 1 femoral defects can be treated with a primary-type stem for the most part. All other femoral defects can be treated with a TCR stem. Fully porous coated stems also work for many revisions but why have two different revision stem choices available when the TCR stems work for ALL defects?. The most critical advantage is the ability to separate completely the critical task of fixation from other important tasks of restoring offset, leg length, and stability. Once fixation is secured, the surgeon can concentrate on hip stability and on optimization of hip mechanics (leg length and offset). The ability to do this allows the surgeon to maximise patient functionality post-operatively. Modular tapered stems have TWO specific advantages over monolithic stems in this important surgical task. The proximal body size and length can be adjusted AFTER stem insertion if the stem goes deeper than the trial. Further, proximal/distal bone size mismatch can be accommodated. The surgeon can control the diameter of the proximal body to ensure proper bony apposition independent of distal fitting needs. If the surgeon believes that proximal bone ingrowth is important to facilitate proximal bone remodeling, modular TCR stems can more easily accomplish this. The most under-appreciated advantage is the straightforward instrumentation system that makes the operation easier for the staff and the surgeon, while enhancing the operating room efficiency and reducing cost. Also, although the implant itself may result in more cost, most modular systems allow for a decrease in inventory requirements, which make up the cost differential. One theoretical disadvantage of modular revision stems is modular junction fracture, which can happen if the junction itself is not protected by bone. Ensuring proximal bone support can minimise this problem. Once porous ingrowth occurs proximally, the risk of junction fracture is eliminated. Even
Corrosion in modular taper connections of total joint replacement has become a hot topic in the orthopaedic community and failures of modular systems have been reported. The objective of the present study was to determine in vivo titanium ion levels following cementless total hip arthroplasty (THA) using a modular neck system. A consecutive series of 173 patients who underwent cementless modular neck THA and a ceramic on polyethylene bearing was evaluated retrospectively. According to a standardized protocol, titanium ion measurements were performed on 67 patients using high-resolution inductively coupled plasma-mass spectrometry. Ion levels were compared to a control group comprising patients with non-modular titanium implants and to individuals without implants. Although there was a higher range, modular-neck THA (unilateral THA: 3.0 μg/L (0.8–21.0); bilateral THA: 6.0 μg/L (2.0–20.0)) did not result in significant elevated titanium ion levels compared to non-modular THA (unilateral THA: 2.7 μg/L (1.1–7.0), p = 0.821; bilateral THA: 6.2 μg/L, (2.3–8.0), p = 0.638). In the modular-neck THA group, patients with bilateral implants had significantly higher titanium ion levels than patients with an unilateral implant (p < 0.001). Compared to healthy controls (0.9 μg/L (0.1–4.5)), both modular THA (unilateral: p = 0.029; bilateral p = 0.003) and non-modular THA (unilateral: p < 0.001; bilateral: p < 0.001) showed elevated titanium ion levels. The data suggest that the present modular stem system does not result in elevated systemic titanium ion levels in the medium term when compared to
Wear debris associated with CoCr bearings has been implicated in the development of adverse soft tissue reactions and pseudotumors following THA with large metal heads and following hip resurfacing. Additional concerns have been raised regarding trunion fretting and corrosion. Most recently, the neck-stem junction of some modular femoral stem designs have come under additional scrutiny. We undertook a review of patients who had undergone THA with a proximal modular junction stem design in order to ascertain the state of the junction in early follow up. We examined the records of all patients in our practice who had undergone uncomplicated, unilateral THA with the ARC stem (OmniLife Science, East Taunton, MA, USA) between April 2010 and April 2012. Office records, radiographs and laboratory data were included. Serum or blood cobalt and chromium ion levels were obtained at the one-year post-op visit or later or if the patient had unexpected pain. The test obtained (serum or blood) was dependent on the lab performing the study. In the study period 100 patients met the inclusion criteria and had metal ion levels available for review. No patient required revision for adverse soft tissue reaction or elevated metal ion levels. Cobalt levels fell with the normal lab ranges in the majority of patients with a very small percentage demonstrating levels slightly above the normal range. Chromium levels all fell within the expected normal range. One patient had a neck exchange for mechanical reasons at 8 weeks following primary THA. This patient went on to develop elevated serum cobalt levels and a large hip effusion. The hip was revised at one year to a
