Hip Arthroplasty: Primary Component Considerations and Reconstruction Techniques

Disclosures

Medscape Orthopaedics & Sports Medicine eJourn. 2000;4(2) 

In This Article

Primary Femoral Stem Results

Good clinical results have been reported with the use of both cemented and cementless components for total hip arthroplasty. And although within each category there are a variety of design options from which to select, criteria for determining the most appropriate femoral stem for a primary total hip arthroplasty continue to elicit debate. Among the topics of discussion are:

  1. What is the preferred material, geometry, and shape for cementless femoral stems?

  2. What is the optimal porous coating for a cementless femoral stem?

  3. What is the most appropriate surface finish for a cemented femoral stem?

  4. Do clinical results favor polished cemented stems rather than the rough surface-cemented stems?

  5. What are the long-term outcomes of a matte surface-collared straight stem with a rectangular cross-section?

These and other questions were addressed at the 8th Annual North American Hip and Knee Symposium.

Cementless stems can be divided into 4 categories:

  1. anatomic (eg, PCA, Howmedica, Rutherford, NJ);

  2. cylindrical, extensively coated (eg, AML, DePuy, Warsaw, IN);

  3. modular (eg, S-ROM Total Hip System, J&J Orthopaedics, Warsaw, IN); and

  4. tapered titanium (eg, Mallory Head, Biomet, Warsaw, IN).

Reliable stem fixation is obtainable with all of these designs, and the rate of fixation failure requiring revision is generally very low. There is, however, variability in the incidence of thigh pain and stress shielding with these various designs. Thigh pain incidence for the PCA

[1]

has been reported to be over 20%. A lower incidence has been reported with the AML,

[2]

but thigh pain remains a concern in a large number of these cases. Stress shielding is also of concern with the AML, especially when using larger sizes or in females with osteoporosis. Modular titanium stems have a low reported incidence of thigh pain; however, there appears to be a potential for debris generation and other complications related to the modular interface. Dr. Cecil Rorabeck from London, Ontario, reported that his group has had excellent results with a tapered titanium stem. They previously reported good clinical results with the Mallory Head prosthesis

[3]

: a low incidence of thigh pain and low revision rate. On the acetabular side, however, they observed a high rate of polyethylene wear, pelvic osteolysis, and acetabular revision. This was attributed to a suboptimal acetabular component design, but there is also a concern that the plasma spray cementless surface coating and the relatively low stem offset may also have contributed to the high rate of wear and osteolysis. Based on these results, a second-generation tapered titanium stem was developed. Features of the Synergy stem (Smith & Nephew, Memphis, Tenn) include increased offset, simple dual offset design with same neck shaft angle, an optimized neck geometry to improve range of motion and minimize impingement, proximal fins to increase rotational stability, and a centered bead porous coating to address concerns of possible debris generation from the plasma spray surface (Figure 1). Excellent short-term clinical results at 2 years were reported, including a very low incidence of thigh pain (less than 5%) and no cases of early osteolysis. Radiographically, offset was restored significantly more often than with the Mallory-Head stem. These results are preliminary due to the short follow-up, but they are indeed promising.

Preoperative (left) and postoperative (right) radiographs of high-offset Synergy tapered titanium stems.

Preoperative (left) and postoperative (right) radiographs of high-offset Synergy tapered titanium stems.

The tapered titanium cementless design has been a market leader in Europe for a number of years. Examples of this design are the CLS and Zwymuller. This stem design is becoming increasingly popular in North America because the technique is simple and the insertion is easy. The low reported incidence of thigh pain and the low stem stiffness based on the modulus of the titanium and the tapered geometry should theoretically decrease the potential for stress shielding.

The optimal surface for a cemented femoral stem has been the subject of debate for the past several years. Surface finish is measured with a profilometer, and the most common parameter reported is the Ra, which is defined as the arithmetic average of all departures from the centerline of the roughness profile. The unit of measurement is either micro-inches or microns. One micro-inch is equivalent to approximately 40 microns. Polished stems have an Ra of 2 to 5 micro-inches; matte finish stems, approximately 15 to 40 micro-inches; and grit blast stems, approximately 80 to 120 micro-inches. There is, as yet, no consensus on the terminology.

Interest in rough surface stems developed from the research of the late 1980s, which emphasized the importance of the stem-cement bond. Initially, the failure of cemented stems was thought to begin at the bone-cement interface. Autopsy retrieval studies that examined well-cemented stems in service for many years demonstrated a stable bone-cement interface but deterioration of the stem-cement interface.

Debonding of the stem from the cement seemed to be an important initiating factor in failure of cemented stems. Therefore, increasing the strength of this bond seemed logical. Increasing the surface roughness substantially increased the shear strength of the cement implant interface, but if micromotion does occur, the amount of wear debris generated would also increase dramatically.[4] This is consistent with the massive lysis observed in a recent series of rough-surface cemented stems that debonded early (Figure 2). But other series of rough-surface stems such as the HD2, Spectron, and Lubinus have not experienced early debonding or osteolysis. It is thought that the surface finish alone does not explain these early, dramatic failures. Early debonding is associated with other suboptimal design parameters, such as a circular cross-sectional geometry and inadequate length.

Postoperative radiograph of a rough (grit blast, Ra ~100) cemented stem (left). Early failure rate at less than 3 years with extensive lysis (right).

Postoperative radiograph of a rough (grit blast, Ra ~100) cemented stem (left). Early failure rate at less than 3 years with extensive lysis (right).

Results of Polished Stems

The case for collarless, polished stems was presented by Dr. James Benjamin from Tucson, Arizona. The first part of his discussion included the potential negatives of a collar and a rough surface finish. The potential disadvantages of a collar include lack of contact, failure to transmit load, and generation of particulate debris from stem micromotion leading to calcar resorption and focal lysis. This can also serve as a pilot site for varus tilt as described by Gruen.[5]

A notable advantage of the polished stem is that if debonding does occur, a polished stem continues to seal the stem-cement interface, whereas rough surface stems do not. Clinically, debonding of Charnley stems with subsidence of up to 2 mm was not found to affect stem survival.[6] Conversely, it has been demonstrated that once debonding of rough stems occurs, loosening and osteolysis rapidly follow in a high percentage of cases (Figure 2).

Results of the Charnley and Exeter polished stems at 20 years are as good as any in the literature. There is a notable absence in the literature of reports of osteolysis associated with the use of a polished stem.[7,8]

Long-Term Outcomes of the Cemented Stem

Paul DiCesare, MD, of the Hospital for Joint Diseases, New York, reported the 10-year results with the Spectron (Smith & Nephew, Memphis, Tenn) cemented total hip arthroplasty (THA). One hundred fifty-two hips were implanted in 136 patients with a mean follow-up of 9.6 years reported in 132 hips (87%), which have an intermediate (matte) surface roughness (Ra = 20 to 40). The average age of the patients was 68 years, and there were more females (2/3) than males (1/3). The 10-year survival (Kaplan-Meier) was 96%. Stem loosening of 2.3% was seen at 8 years; an additional 2% were radiographically loose. Nine percent required acetabular revisions. The use of 32-mm heads may have contributed to the higher acetabular revision rate. Focal osteolysis was seen in only 5% of cases. The authors attributed the good results to the stem length, geometry, and modulus, which they believed were optimal for lowering stresses in the cement. The stems were relatively long and the cross-section was rectangular the entire length of the stem. There was also an anteroposterior groove to improve rotational stability.

Nine surgeons contributed the data for this study, although 3 surgeons did the majority of the surgery. These results rival those of the single-surgeon series of polished stems and were achieved with a stem of moderate (matte) surface roughness.

Made possible through an unrestricted educational grant from Smith & Nephew.

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