• The success of revision surgery of the hip can be divided into three parts: the prosthesis design, the surgical technique, and the patient. Surgical technique is probably the most important factor.

• The surgeon should understand the concept of “press-fit” fixation.

• For successful implantation and fixation of the stern, the importance of primary stability and secondary stability and their relevance to uncemented tapered, fluted, modular implants must be appreciated.

• The surgical team should undertake adequate preoperative planning (defined as three distinct steps: radiologic analysis of the femur, surgical strategy, and templating) to identify potential problems and alternative solutions.

• Do not hesitate to perform an osteotomy to allow for a safer revision; this can make a difficult operation much easier and can facilitate a good press-fit construct.

Broadly speaking, cementless revision stems can be divided into stems that are designed to gain fixation in the proximal or in the distal femur. The advantage of achieving fixation in the distal femur is that the bone stock of the proximal femur in the revision setting is often poor. Long-term uncemented fixation in the diaphysis can be achieved by different methods, including extensively porous-coated stems and fluted, tapered grit-blasted stems. Uncemented, extensively porous-coated stems have long been considered the gold standard of uncemented revision femoral stems in North America1–13 and are able to gain rotational stability from a scratch fit in the diaphysis. Although this stem design has been successful in the revision setting,^2,5,7 stress shielding remains a concern because of the relative stiffness of the implant.

An alternative method of uncemented diaphyseal stem fixation, used frequently in Europe, requires the use of fluted, tapered, grit-blasted stems. These stems are tapered to gain axial and rotational stability in the diaphysis of the femur. The femur is reamed to a tapered cone. Then a fluted, tapered implant is impacted into the tapered cone of the prepared diaphysis. Essentially, the distal part of the implant, which is cone shaped, is wedged into the diaphysis to achieve primary stability. The purpose of this chapter is to discuss use of these implants, so-called uncemented modular fluted implants, in femoral revision during revision total hip arthroplasty.

Design Rationale

For fluted, tapered revision femoral stems, Wagner¹⁴ proposed that the contact zone between the implant and the bone needed to measure between 70 and 100 mm for primary stability. However, Beguec and associates¹⁵ believed that a good press fit depends on the quality of the wedging and a contact zone of just 30 mm in excellent bone, but 40 to 50 mm if bone quality is poor. The contact zone between implant and bone can be limited in other ways to help reduce stiffness. Blaimont¹⁶ demonstrated that during hip flexion, tensile forces at the level of the diaphysis occur in the anterior half of the femur and compressive forces in the posterior half. As one moves through the femur in the diaphyseal region from anterior to posterior, these forces gradually become less tensile and more compressive. At a point in this plane, forces are neither compressive nor tensile, and this “zone” is termed the neutral zone. An implant that establishes contact with bone in this neutral zone should allow the bone to react and remodel as it would normally, at least partially and theoretically. This can be achieved by using a quadrangular stem or a stem with flutes that contact the femur in the coronal plane, that is, in the neutral zone.

The cross-sectional shape of a press-fit, uncemented stem is also important for rotational stability. A circular cross-section would create a large contact surface while offering little resistance to rotational loading. An implant with a quadrangular cross-section (such as a tapered stem) has been shown to be very successful in resisting rotational torque and achieving good primary fixation.^17,18 A stem with cutting flutes offers further advantages not only in neutralizing any rotational forces but also in preserving a space between implant and bone, which may aid revascularization and subsequent bone regeneration. This also allows the use of a less stiff implant and reduces the stress shielding seen with fully porous-coated cylindrical implants.

These theoretical advantages have been shown in clinical practice using the Wagner stem (a monoblock fluted, tapered titanium stem), as has been reported by several authors.^19–21

Because these stems depend on a tapered geometry, it is often difficult to predict when they will stabilize as they are being wedged into the femur. For this reason, modularity of the stem, whereby proximal bodies of different lengths to accommodate for the lack of predictability of the exact wedging location of the distal implant, has been developed and is now considered the standard of care for tapered, fluted revision stems. This modularity allows the surgeon to implant the distal segment in the diaphysis for optimal stem rotational stability, with different options for the proximal body that optimize leg length, femoral offset, femoral version, and ultimate stability. However, this poses certain engineering challenges because these stems require a modular junction at a high-stress location of the femoral component. Postak and associates²² examined the modular junction and found that the structural endurance limit was compatible with good long-term performance.

Preoperative Planning

Before embarking on a revision of the femoral component, the surgeon should evaluate the existing bone stock and anticipate the quality of remaining bone after the implants and cement, if present, are removed. Paprosky²³ described a useful classification system for evaluating femoral bone stock in revision total hip arthroplasty. This classification enables an algorithmic approach as a guide to femoral reconstruction. Although this topic is covered in greater detail elsewhere, a brief summary is provided here: