Femoral Revision: Uncemented Tapered Fluted Modular Implants

Introduction

The objective of revising a loose femoral prosthesis is to restore the hip joint to a condition as close to normal as possible. This means a hip joint that is painless and stable, enabling the patient to resume a good quality of life. The technical goals of revision total hip arthroplasty (THA) are to (1) create a construct with axial and rotational stability at the bone-implant interface; (2) stabilize the hip joint by maintenance of length, version, and offset; and (3) optimize joint mechanics. Several concepts have been described for revision of femoral components of a failed hip arthroplasty. Stem stability in the revision setting can be achieved by cemented or cementless fixation. While cemented revision provides the advantage of excellent initial stem stability when there is enough bone for osseointegration, loosening of the primary implant may result in enlargement of the femoral canal and can cause the cortices to become thinner, with more endosteal sclerosis. A stable construct is then more difficult to achieve in this setting. Ideally, when cemented fixation is chosen, it should be combined with impaction grafting to manage bone loss and optimize long-term fixation.

Broadly speaking, uncemented revision hip stems can be divided into those that are designed to gain fixation in either the proximal (metadiaphyseal) or distal (diaphyseal) femur. Because bone stock of the proximal femur in the revision setting is often poor, achieving fixation in the distal femur is generally preferred. Long-term uncemented fixation in the diaphysis can be achieved by different methods, including extensively porous-coated stems and fluted tapered stems. The latter come in 2 distinct types: modular and monoblock (or nonmodular). At least in North America, uncemented, extensively porous-coated stems were historically the most commonly used. Vertical and rotational stability were achieved by scratch fit in the diaphysis. Although this stem design has been successful in the revision setting, stress shielding remains a significant concern because of the relative stiffness of the implant, which has ultimately resulted in a decreased frequency of use.

In North America currently, the preferred method of uncemented diaphyseal stem fixation involves the use of fluted tapered stems, which have been in use in Europe for the past 2 to 3 decades. These stems are tapered to gain cone-in-cone axial stability, with superadded flutes to engage the endosteal surface of the bone and impart rotational stability. The femur is reamed to a tapered cone. Then, a fluted tapered implant is impacted into the 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 uncemented modular fluted implants in femoral revision during revision THA.

Design Rationale

For fluted tapered revision femoral hip stems, Wagner proposed that the contact zone between the implant and the bone needed to measure between 70 and 100 mm for primary stability. However, Le Beguec and associates 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. Moreover, using a stem that is too long can produce less reliable 3-point fixation as opposed to the tight diaphyseal fit required to properly seat the cementless stem distally in order to promote bony regeneration in the proximal femur. The contact zone between implant and bone can be limited in other ways to help reduce stiffness. Blaimont demonstrated that during hip flexion, there is a gradient of tensile and compressive forces that occurs at the level of the diaphysis. Tensile forces are highest in the anterior cortex, with a transition to compressive forces which are highest in the posterior cortex. At a point in this plane, forces are neither compressive nor tensile; this zone is termed the neutral zone. Ideally, a revision stem implant establishes contact with bone in this neutral zone, which allows the bone to react and remodel as it would normally. This can be achieved by using a tapered fluted stem. The titanium alloy stem further promotes normal bone remodeling by having a modulus of elasticity near that of normal bone.

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. A stem with cutting flutes offers further advantages not only in neutralizing any rotational forces but also in preserving a space between the implant and bone, which may aid revascularization and subsequent bone regeneration, in a process that allows fixation through bone ongrowth. 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 fluted tapered titanium stems, as has been reported by several authors.

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, especially when remaining bone stock is poor. Modular stems have been developed with proximal bodies of different lengths to accommodate for the lack of predictability of the exact wedging location of the distal implant. This modularity allows the surgeon to implant the distal segment in the diaphysis for optimal stem axial and rotational stability, with different options for the proximal body that optimize leg length, femoral offset, femoral version, and stability. However, this poses certain engineering challenges because these stems require a modular junction at a high-stress location of the femoral component. This will be further discussed later in this chapter.

Preoperative Planning for Femoral Revision

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. Paprosky described a useful classification system for evaluating femoral bone stock in revision THA. It enables an algorithmic approach as a guide to femoral reconstruction.