Cemented Femoral Components

General Considerations

Shen, an engineer, suggested that cemented femoral components can be divided into two basic types ( Fig. 65.3 ): taper-slip and composite beam . Taper-slip (also called force-closed) stems are exemplified by the original polished flatback Charnley (Zimmer, Warsaw, IN) and the polished Exeter (Stryker Inc., Mahwah, NJ) stems that routinely subside within the cement mantle without fracturing it. With these stems, stability is maintained by a balance of forces across the stem-cement interface with no form of bond between the two. In contrast, composite-beam (or shape-closed) stems cannot, by definition, subside within the cement, and stability is maintained by the bond that arises when the cement gains mechanical interlock against a roughened implant with irregular surface features. A recent systematic review and meta-analysis of radiostereometric assay and survival studies showed that there was a clinically relevant association between early subsidence of composite-beam stems at the stem-cement interface and late revision for aseptic loosening, and that subsidence regularly occurred with some composite-beam stems, although to a lesser extent than with taper-slip stems, thus calling into question the whole concept of how these composite-beam stems can function without damage occurring at the stem-cement interface.

Good results can be obtained using composite-beam stems, but they are less forgiving in terms of surgical technique. The basic surgical techniques described here for contemporary femoral cementing are valid for both types of stems. If a composite-beam stem is to be used, it is essential that a complete cement mantle of adequate thickness is established, as well as solid fixation at both the stem-cement interface and the cement-bone interface. Scheerlinck and Casteleyn point out that, although in vivo both concepts of stem fixation have proved effective, they cannot work together, and it is important to understand on which principle a particular stem relies.

In contrast with the variable results of composite-beam components, most taper-slip stems have been demonstrated to yield excellent results in all existing national joint registries, as well as in individual publications from multiple centers. In engineering practice, the taper is one of the strongest and most reliable methods of transmitting not only axial, but also torsional forces between one component and another. In the context of the cemented stem, subsidence of the tapered stem is analogous to engagement of the taper; if the stem is to function effectively as a taper, it must not in any way be fixed to the cement or be end-bearing. Shen and Howie et al. point out that the taper-slip system is more forgiving in terms of surgical technique, and the success of the concept is demonstrated by the fact that all major manufacturers of hip implants now have such stems in their portfolio. In contemporary practice, they are by far the most widely used cemented stems throughout the world.

To function by the taper-slip principle, a stem must be tapered in shape and must have a polished surface. There also must be no feature on the device, such as a collar, that could prevent the stem from subsiding within the cement mantle. A stem with these characteristics can take advantage of the viscoelastic behavior of acrylic bone cement as movement is allowed at the stem-cement interface without damage to the internal surface of the cement mantle. This subsidence has three important effects:

• Because a force on the stem produces subsidence at the stem-cement interface, it does not induce damaging shear forces at the biological (bone-cement) interface. This is in contradistinction to a composite-beam stem in which shear forces exerted on the cement by the stem are transmitted directly to the biological interface.

• As the stem subsides, it generates hoop tensile forces and radial compression within the cement ( Fig. 65.4 ). Radial compressive forces serve to protect the cement-bone interface from any shear forces; they also load the bone. During periods of rest, tensile strain persists and stress relaxation of the cement occurs, dissipating hoop tensile stresses. This mechanism protects the cement against fatigue fracture, and the whole loading regimen is dominated by compressive forces. Crowninshield et al. noted that, “Load transmission by compression across the stem-cement boundary may be the only reliable mechanism of loading in vivo.”

  

• Subsidence of the polished taper within the cement increases the torsional stability of the stem.

With contemporary cementing, observed subsidence of the stem within the cement is approximately 1 mm at 22 years of follow-up. It continues very slowly throughout the life of the implant, but with contemporary cementing, this subsidence is not of clinical importance with regard to leg length.

Indications and Contraindications of Cemented Femoral Components

Cemented femoral stems can be considered for any patient who requires a hip arthroplasty. Cemented, taper-slip devices are recognized as delivering the gold standard in terms of long-term fixation. This includes when they are used in young, active patients. The use of a collarless, polished, tapered, cemented stem confers significant advantages over cementless designs in that stem size, stem offset, leg length, and version all are independently variable, allowing accurate recreation of hip biomechanics no matter what the original deformity.

Over the longer term, the use of a taper-slip stem also confers an advantage if an additional operation is required to address problems with the hip other than femoral fixation. The device is in effect modular at the stem-cement interface. After clearing the shoulder of the implant of any cement that would prevent its removal, the stem can be knocked out of a well-fixed cement mantle and another stem of similar design cemented into the existing cement mantle at the end of the procedure. This technique allows for the correction of leg length, offset, and version in the revision situation. It has also been shown to confer excellent results in terms of clinical outcome and is a profound advantage of using this type of stem, especially in younger patients. The use of cemented stems confers an advantage over uncemented stems with regard the incidence of periprosthetic fracture and this decrease in risk is most profound in elderly patients.

In complex cases in which distortion of the anatomy occurs, femoral shortening procedures and derotation osteotomies can be carried out. The osteotomy site can be protected from cement intrusion by the use of autograft or allograft bone chips impacted into the endosteal surface. Cement fixation confers an advantage in cases of previous septic arthritis of the hip, because acrylic bone cement can be loaded with an appropriate antibiotic to reduce the risk of recrudescence of infection.

For any patient in whom a hip arthroplasty is indicated, there are no specific contraindications for the use of cement fixation.

Preoperative Planning

The aim of preoperative planning is to determine the correct position of the components to allow restoration of the anatomic center of rotation of the joint, and to recreate the correct leg length and offset. Preoperative planning also helps the surgeon to predict the offset and size of prostheses before the start of the procedure as well as the very occasionally need for bone graft in the primary situation. A major advantage of scaled digital templating is that implant templates can be imported at the correct size rather than at the assumed 120% magnification, as is the case with acetates. The image taken for templating must be taken with a scale on the image. An ideal scale for hip replacement is a total hip replacement (THR) on the contralateral side, where the size of the femoral head is known. The next best system is for a marker of known size to be placed in the plane of the femoral head. We have established a convention to position a HipScaler ( www.hipscaler.com/templatinginstructions.html ) on all anteroposterior radiographs of the pelvis taken at our institution, thereby decreasing the need to take additional films for this purpose.

Clinical examination of the patient is an essential part of the preoperative assessment, so that the surgeon is aware of any true or apparent leg length discrepancy or fixed contracture of the hip, or the presence of fixed pelvic obliquity. In the absence of robotic assistance or a navigation system, it is the responsibility of the operating surgeon personally to position the patient on the operating table so that they are aware of the relative position of the pelvis to the horizontal and vertical planes.

Results

Survivorship data with the longest follow-up when a double-tapered, collarless, polished cemented stem has been used have been published by the Exeter team (using the Exeter stem). At 33 years of follow-up, stem survivorship with the end point of revision for aseptic loosening was 93.5% (95% confidence interval [CI], 90.0%–97.0%). In the “worst case scenario,” in which all cases lost to follow-up are regarded as failures from aseptic stem loosening, survivorship was 85.8% (95% CI, 81.3%–90.3%). Of 433 hips in the series, 14 have been revised for aseptic stem loosening (3.46%). Two of these were re-revisions, and two had previously undergone intertrochanteric osteotomy. None have been revised for aseptic loosening since year 20 of the survivorship study, and none have been lost to follow-up since that time. These results were achieved with first-generation cementing techniques and with surgeons of widely differing experience, illustrating that excellent results can be attained with a stem functioning by the taper-slip principle, despite relatively poor quality cementing by today's standards. The Exeter Universal stem, which had an identical surface finish and a very slight difference in stem geometry but functions in exactly the same way, was introduced in 1988. The 23-year survivorship with the end point of revision for aseptic stem loosening or lysis was 99% in the series published by the design center. In a separate study of results in patients younger than 50 years at the time of surgery (average age at surgery of 41.8 years), with no case lost to follow-up, stem survivorship at 22 years was 96.3% with revision for aseptic stem loosening or lysis as the end point, and 74.9% for all causes of stem revision, including periprosthetic fracture follow-up (mean, 21.6 years). There were no cases of aseptic loosening of the Exeter stem within this cohort. There was one case of localized femoral lysis on a patient with Gaucher's disease at 21.1 years and two cases of polyethylene wear and subsequent osteolysis at 14.1 and 17.0 years. In 2000, the V40 stem, with a modified neck diameter and trunnion was introduced, replacing the Universal design. The 13.5-year survivorship with revision for aseptic loosening or lysis as the end point was 100% and with the end point of all-cause stem revision, the survivorship was 96.8%, confirming that, despite the design changes to the trunnion, the V40 design was comparable to the Universal stem and performed well beyond 10 years.

Many reports from other centers have described results with stems that function by the taper-slip principle. Burston et al. reported results in patients younger than 50 and concluded that

the performance of polished tapered stems in patients younger than 50 years is excellent, with stem survival and subsidence equivalent to the older, standard hip arthroplasty population. They compare very favorably with other stem designs that have been reported among the younger patients. With ease of insertion and such predictable behavior, this type of stem has to be the benchmark for comparison to other stem designs, including uncemented stems and resurfacing implants, in this young age group.

All papers cited reported a consistent pattern of behavior with regard to femoral components, irrespective of the experience of the operating surgeon, and demonstrated similar results to those obtained in the Exeter with benign radiologic appearances, a small degree of subsidence within the cement mantle, and satisfactory clinical outcomes.

Survivorship analysis, as used in hip registries, is a powerful tool in the long-term assessment of arthroplasty because it gives a more realistic presentation of longevity than is provided by simple examination of failure rates. The behavior of both the polished Exeter Universal and V40 stems in the Swedish and Norwegian Hip Registries is in general in line with results from elsewhere. Furnes et al. reported that the Exeter stem had the lowest percentage revision rate at 15 years (3.0%) and the smallest increment in revision rate between 10 and 15 years (from 2.2% to 3.0%) among the 10 most commonly used cemented stems in Norway. A study from Finland of patients aged 55 years or older undergoing surgery for osteoarthritis found that the only femoral and acetabular component combination to have a survivorship of greater than 90% at 15 years was the Exeter Universal/Exeter All-Polyethylene couple. Junnila et al. published results at brand level from the combined Nordic Arthroplasty Register Association showing further improvement in results of the Exeter stem when combined with highly crosslinked polyethylene. Both the UK and Australian registries consistently report 10-year survival of the Exeter stem as greater than 95%, as per 2014 National Institute for Health and Care Excellence guidelines, irrespective of the combination of cup used, with encouraging early results with the highly crosslinked polyethylene RimFit cup.