IMPORTANT POINTS
No real contraindications exist for the use of uncemented components in total hip arthroplasty. With the introduction of improved uncemented components, the traditional contraindications for the use of uncemented components in patients with inflammatory arthritis, metabolic disease, and poor bone are changing. In the authors’ opinion, the main relative contraindications apply to the use of the femoral stem and include extreme osteoporosis and wide femoral canal, such as that seen in elderly patients with fractures.
CLINICAL/SURGICAL PEARLS
- 1
Uncemented total hip arthroplasty is successful as long as components with a proven track record are used. Some uncemented components have done poorly. These include acetabular components with a thick metal back that reduce the thickness of polyethylene (e.g., Universal I, Exeter Total Hip System, Exeter, U.K.) and stems with a poor surface for osseointegration (e.g., PCA, Howmedica, Rutherford, NJ).
- 2
Patients with extremely poor bone stock, such as elderly patients with fracture, should be excluded.
- 3
Preoperative templating should be performed to ensure proper sizing of the component.
- 4
Basic surgical techniques in performing the surgery should be observed.
INTRODUCTION
Total hip arthroplasty (THA) is one of the most successful and effective surgical procedures to relieve pain and improve function in patients with various pathologies of the hip, including degenerative joint disease, rheumatoid arthritis, osteonecrosis, and degenerative changes from developmental dysplasia. During the past 3 decades implants have undergone multiple changes in design and/or material to improve fixation. Although the cemented Charnley prosthesis provides excellent long-term results, press-fit fixation provides an advantageous alternative to cemented THA in the younger and active population. The number of primary THAs performed and resource utilization are expected to increase tremendously during the next decade. Knowledge of the various implants, designs, indications, and survivorship will enable the physician to choose among the diverse assortment of stems and cups available.
FEMORAL COMPONENT
Material, Design, and Coating
The shape and material of the femoral stem determine its flexional and torsional stability. Cobalt- and titanium-based alloys currently are the most commonly used materials for producing femoral stems. These two materials have advantages and disadvantages ( Table 11-1 ). Stem stiffness is proportional to the fourth power of its diameter and depends on the modulus of elasticity of its material. Therefore large-diameter stems made of cobalt chrome predispose the construct to a phenomenon known as stress shielding, or the resorption of the bone around the calcar. This stress shielding can be alleviated by using titanium that has a 50% smaller modulus of elasticity than cobalt chrome. Mechanical fixation to bone can be improved by roughening the surface by grit blasting, surface etching, titanium plasma spraying, and application of layers of beads.
| Material of THA | Advantages | Disadvantages |
|---|---|---|
| Titanium (and TMZF * ) |
|
|
| Cobalt chrome |
|
|
Three different types of uncemented stems are used: anatomic, cylindrical, and tapered. Anatomic stems have a slight proximal posterior bow of the metaphysis, a few degrees of neck anteversion, and a distal anterior bow of the femur to mimic the anatomy of the canal. Straight, or cylindrical, stems are symmetric throughout their cross-section and allow close contact between the cortical bone of the proximal diaphysis. Conversely, wedge-shaped, tapered stems with a wide proximal body and a rectangular shape achieve contact with metaphyseal bone and give excellent rotational and torsional stability.
Press-fit fixation is required to achieve initial implant stability, allowing osseointegration and stable bonding of the prosthesis to the bone. Micromotion (less than 50 μm) allows bony ingrowth, whereas greater motion leads to failure of osseointegration and fibrous tissue ingrowth. Circumferentially coated stems can be either proximally, extensively, or fully coated. Although no general consensus exists regarding the degree of coating, most authors agree that circumferential proximal coating is preferable. Circumferential coating and proximal osseointegration are believed to help prevent debris from migrating to the distal femur between the bone-implant interface, causing osteolysis and minimizing stress shielding. However, some cementless stems have been manufactured without porous coating and lack the capacity for bone ingrowth ; they have roughened surfaces that allow locking to bone.
Hydroxyapatite (HA)-coated implants are designed to help with formation of biologic fixation between the host bone and implant. HA can stimulate bone ingrowth and bridge the gap between the host bone and implant soon after implantation, thereby preventing the formation of an intervening fibrous tissue layer. It can be applied to either roughened or porous surfaces to improve bonding. A thin (50 to 150 μm) coating must be applied to porous surfaces to leave the pores open. Table 11-2 describes the advantages and disadvantages of each coating technique.
| Degree of Coating | Advantages | Disadvantages |
|---|---|---|
| Proximally coated |
|
|
| Extensively coated |
|
|
| HA coated |
|
|
SURVIVORSHIP OF STEMS
Porous-coated titanium stems with an anatomic design have shown to have excellent long-term survivorship and can improve quality of life. Cobalt chrome and titanium versions of this design have shown comparable results in terms of survivorship and clinical and functional outcome in young patients (45 years or younger). The Harris-Galante Porous I stem (Zimmer, Warsaw, Ind.) was among the first-generation designs of proximally porous-coated femoral stems introduced. Unfortunately, short-term survivorship and performance have been suboptimal and long-term results have been somewhat disappointing, with a survivorship free of revision of 82% at 15 years of follow-up. New second-generation cementless stems with a proximal circumferential porous coating have a survival rate of more than 90% at 10 years and good to excellent clinical results. The AML prosthesis (DePuy, Warsaw, Ind.) is an example of a straight, cylindrical stem that can be coated up to five eighths of its length; it has shown a 96% rate of good to excellent clinical results at long-term follow-up. Similarly, Reikeras et al reported only one revision of 323 HA fully coated titanium, grit-blasted stems in a young patient population (mean age, 48 years) with a follow-up of 8 to 10 years. Several studies have reported encouraging results with HA-coated stems in young, active patients, with a survivorship of 90% to 100% at 10 to 12 years.
The S-ROM stem (DePuy) was first introduced to facilitate the management of complex THA in patients with developmental dysplasia of the hip or after fracture or corrective osteotomy. Christie et al reported excellent mid-term follow-up (average, 5.3 years) of the S-ROM stem, with 98% stable bone ingrowth and 7% bone lysis restricted to above the sleeve. Longer follow-up (average, 101 months) of 58 patients with the S-ROM stem revealed adequate bone ingrowth and no revisions, although 42% of the patients had proximal osteolysis.
ACETABULAR COMPONENT
Material, Design, and Coating
The acetabular component originally consisted of a cemented all-polyethylene cup. Most studies have shown that this design is plagued by a high rate of loosening, especially in young patients. Therefore renewed interest in cementless acetabular components has surfaced in an attempt to improve initial fixation and secondary osseointegration. Long-term stability is determined by the osseous ingrowth and depends on the acetabular coating.
Initial stabilization can be achieved by using screws inserted through screw holes in the cup, pegs, or spikes driven into preformed recesses or an enlarged rim without the need for further fixation devices. Won et al documented that adding screws to press-fit cups does not increase primary stability. However, concerns regarding inadequate seating in the acetabulum and early loosening have been raised with the screwless designs. In general, excellent mid- and long-term survivorship with adequate bone ingrowth have been reported in the different screwless designs.
Cups currently used have grit-blasted or plasma-sprayed rough surfaces that can be either porous coated or have additional HA coating for additional bone ingrowth. The surface of the cup can be smooth or microstructured with grit blasting or roughened (macrostructured) with the metal coating arc deposited. The roughened or macrostructured surface is believed to have a higher surface area for ingrowth and allows more bone ingrowth than the microstructured (smooth) cup.
The porous-coated acetabular component (Harris-Galante) has shown excellent survivorship at 10 years of follow-up, although osteolysis of the acetabulum and femur remains prominent. Similarly, the majority of porous-coated cups have shown consistent and reproducible mid- to long-term survivorship with good to excellent clinical outcomes. Conversely, HA-coated acetabular components have not consistently shown as successful results as the HA-coated femoral components. Multiple studies have reported high mid- to long-term failure rates (10% to 12%) of HA-coated cups compared with the porous-coated implants (6% to 8%). Polyethylene wear and osteolysis remain the major nemesis of cementless acetabular components.
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