Double-Tapered Primary Stems

 

 

 

INTRODUCTION

 

Tapered femoral stems can be described by their geometry: single taper, double taper, and triple taper. Single-taper stems have a reduction (taper) in medial-lateral dimension in the frontal plane but have a constant anteroposterior dimension. Double-taper stems have tapers in the frontal plane and sagittal plane. Triple-taper stems include a reduction in anteroposterior dimension through the cross section of the stem from lateral to medial (the third taper), in addition to tapers in the frontal and sagittal planes (Fig. 17-1). Tapered femoral stems can also be described by their surface finish or coating: grit-blasted, hydroxyapatite (HA), plasma spray, or porous ingrowth. The stems vary in the degrees of tapering as well as their length. There are also differences in neck-shaft angle and/or offset. Virtually, all cementless tapered stems are made from titanium alloy because of its biocompatibility and relatively low stiffness. While there are debates regarding the benefits and trade-offs of various design variables, the tapered stem class has been clinically successful over three decades.

 

INDICATIONS

Absent severe osteoporosis, the intercortical geometry of the proximal femur has a tapered shape in the frontal plane, which allows a satisfactory fit in most femurs with a similarly shaped stem. Tapered stems load the endosteal bone in compression, and the “wedge fit” offers good initial stability, which generally leads to long-term fixation through osseointegration: bone ongrowth or ingrowth. The surgical preparation of the proximal femur for a tapered stem is straightforward. For these reasons, stems with a tapered geometry are currently utilized more than any other type.

A tapered cementless stem is indicated for end-stage osteoarthritis, rheumatoid arthritis, avascular necrosis of the femoral head, femoral neck fracture, and conversion of a hip resurfacing to a total hip. Primary tapered stems can be used in revision cases when there is minimal damage to the femoral metaphysis (Paprosky type I). There are tapered stems for revisions, which are addressed in Chapter 28.

 

 

CONTRAINDICATIONS

A tapered stem should not be used if there is deficiency or deformity of the proximal femur that precludes seating or does not provide sufficient support of the implant. Axial and rotational stability must be achieved at the time of implantation for osseointegration to occur. Osteoporosis is not, in and of itself, a contraindication to a cementless tapered stem (1,2,3). The shape of the femur may be more important than bone density, and it may be difficult to get adequate initial stability in some femurs with a true “stovepipe” geometry. If the bone in the femoral metaphysis is not viable, such as with postirradiation necrosis, a cementless tapered stem should not be used.

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FIGURE 17-1 Single-, double-, and triple-taper femoral stem geometries.

 

 

 

PREOPERATIVE PREPARATION

Preoperative radiographs need to show all of the femur that will be occupied by the stem in both the AP and lateral planes. It is recommended that templating be performed on a low AP pelvis x-ray to facilitate assessment of limb length and offset (Fig. 17-2). Acetabular templating is done first since the acetabular component establishes the center of rotation of the arthroplasty. The primary goal is obtaining adequate bony support for the component. For most cases, a target of 45 degrees of abduction is satisfactory. The acetabular tear drop is a consistent reference for the medial wall. An adequately sized acetabular component will approach the medial wall and be supported by superolateral bone.

For femoral templating, separately consider the part of the component inside the bone (the fit of the stem), from the part outside the bone (the neck and head), that determine the biomechanics of the reconstruction. An appropriately sized tapered femoral component may or may not be canal filling but fits the metadiaphyseal region (Fig. 17-2). The vertical position of the template on the radiograph influences the location of the neck osteotomy. There are essentially two ways to make limb length adjustments: the vertical position or height of the stem inside the canal and modular femoral heads. The “neck length” of a modular femoral head influences both length (vertical component) and offset (horizontal component), while the vertical position of the stem only influences length. The vertical position of the stem determines the location of the neck osteotomy.

 

 

 

FIGURE 17-2 Templating. The acetabular template is at 45 degrees lateral opening and sized to approach the medial wall and be supported by bone superolateral. The red dot is the acetabular center of rotation. The femoral template is sized to fit. The vertical position determines the neck resection level (red line). This femoral template shows two offset options for the neck (standard and high); both have a 130-degree neck-shaft angle. For this case with mild dysplasia, the standard offset neck is appropriate. The modular head center that overlies the acetabular center of rotation will not change the limb length or hip offset, if executed surgically.

 

 

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The limb length and offset change for the proposed reconstruction can be assessed on the templates by

comparing the location of the center of rotation of the acetabulum to the location of the center of rotation of the

selected modular femoral head. For the templating exercise, consider the acetabular center of rotation as a fixed reference and the femoral head center as variable. When the selected head center directly overlies the acetabular center, there is no change in the limb length or hip offset when such a plan is executed surgically. If the acetabular center of rotation is medialized, then the femoral offset will be increased by a corresponding amount. When the head center on the templating is superior to the acetabular center, limb length will be gained when the hip is reduced at surgery. When the head center is medial to the acetabular center on the template, then relative femoral lateralization (increased offset) will occur when such a plan is executed surgically.

Many tapered stems are available with more than one offset, that is, standard and high. Some systems accomplish this by changing the neck-shaft angle, with the higher offset option having a lower neck-shaft angle. Other systems maintain the same neck-shaft angle and increase offset by medializing the take off point of the femoral neck (Fig. 17-3). Together with the modular heads, these options enable the surgeon to better achieve both the desired limb length and offset for a given hip.

 

 

 

FIGURE 17-3 Stem offset options. Standard offset (left) and high offset (right). In this example, the neck-shaft angle is the same 130 degrees. The increased offset is attained by direct lateralization of the stem. (Summit; Courtesy of DePuy, Warsaw, IN.)

 

TECHNIQUE

 

Obtain adequate exposure of the proximal femur, and protect the adjacent soft tissues.

 

The neck resection level is determined from the preoperative templating. The lesser or greater trochanter are commonly used references.

 

Some distinction has been made between systems that ream the canal before progressively rasping or broaching the proximal femur and “broach-only” systems. A rasp can remove bone moving both distally and proximally in the femur, while a broach removes or compacts bone only while moving distally.

 

The need for any canal reaming is determined by the internal geometry of the femur and the length of the stem. When the femoral cortices are thick and the canal is small, there is a large metaphyseal flare and a relative mismatch between the size of the metaphysis and the canal (Dorr type A femur). Such proximal-distal mismatch can present a challenge to proper fitting of a cementless stem. This is less of an issue for short stems that barely engage the diaphysis, compared to longer stems designed to be centered in, and stabilized by, the diaphysis.

 

 

Some systems are designed to first ream the canal to the depth of the intended stem and then progressively rasp the proximal femur to the corresponding size. Other systems are designed to prepare the femur with a progressively enlarging rasp- or broach-only technique. However, in

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cases with a proximal-distal mismatch, the tight canal and dense distal bone can lead to proximal (metaphyseal) undersizing of the stem. Undersized cementless stems have an increased risk of not

osseointegrating. The surgeon must be aware of proximal-distal mismatch and consider this variable when preparing the femur. In such cases, some reaming of the canal may be necessary, even with a so-called broach-only system, to avoid undersizing of the stem.

 

While preoperative templating will suggest a target stem size, the best-fit stem size is determined at surgery. Using progressively enlarging rasps or broaches, the surgeon evaluates the progressive resistance to

insertion and the axial and rotational stability of each rasp or broach.

 

The templated stem size is an estimation, but the best-fit stem may be a little smaller or a little larger. There is a learning curve for all surgical techniques, and experience is helpful in determining when the fit is tight enough. While undersizing should be avoided, the risk of going too tight is creating a linear crack in the proximal femur from excessive hoop stresses generated by the tapered geometry or the rasp, broach, or stem.

 

If in doubt about the sizing, or any other reconstructive parameter, an intraoperative radiograph with the intended rasp or broach in situ should be obtained.

 

Depending on the proximal cross-sectional shape of the tapered stem, there is some opportunity to adjust femoral version (Fig. 17-4). In general, hips with dysplasia have an increase in femoral anteversion, and some other pathologies, such as hips with slipped capital femoral epiphysis or cam impingement, can have little anteversion or may be retroverted. The version of a cementless stem does not have to match that of the native femur, but the amount of adjustment may be 5 to 10 degrees at most. Version adjustment can improve the range of motion and stability of the reconstruction. The version adjustment is accomplished by progressively broaching obliquely to the native axis. Because the version of the stem will be oblique to the native axis, the stem that fits in this envelope may be one size smaller than the same stem with an anatomic fit.

 

Rotational stability can be assessed by manually torquing the broach handle. There should be no motion between the broach and the endosteal bone.

 

Version adjustment cannot compromise the axial or rotational stem stability. If there is any concern about initial stem stability, keep sizing up until sufficient stability is achieved. Assess the resultant broach version, and place the acetabular component in a complementary position. A reasonable target for combined anteversion is 35 degrees for a posterior approach and 5 to 10 degrees less for approaches that dislocate the hip anteriorly

(4). The hip should be put through a trial range of motion test and adjustments made, if needed.

 

Many systems have a calcar planer that will ream the femoral neck resection level evenly down to the top of the broach. The resultant “planed calcar” is a good reference for the stem insertion level. Most tapered stems fit a little tighter than the corresponding broach, and the “planed calcar” is a good target for stem seating with sufficient initial stability, given that the rasp or broach was stable.

 

 

 

FIGURE 17-4 Femoral version adjustment. Surgeon's perspective of femoral preparation with a mini-posterior approach. The extremity is internally rotated 90 degrees with the patients leg held vertical (perpendicular to the floor). The surgical assistant is in the background (blue gown), and the cobra retractor held by the assistant is on the calcar. The view of the proximal femur is enlarged and inserted over and above. The relatively horizontal yellow line is parallel to the posterior femoral cortex and the vertical yellow line is parallel to the patient's leg, demonstrating relatively neutral version of the native femur. The white rectangle illustrates the potential for version adjustment from placement of the femoral component.

 

 

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Similar to the learning curve for femoral rasping or broaching, there is a learning curve for stem insertion. Again, most tapered stems fit a little tighter than the corresponding broach. It should also be recognized that the rasps and broaches get a little smaller with use as the edges wear down, which will result in a tighter stem fit. This is more of a factor in dense bone. Tapered stems will easily insert to within 1 to 2 cm of full seating and then get progressively tighter as they wedge into the prepared femur. It is recommended to use multiple medium-force hits to impact the stem (Video 17-1).

 

Overly vigorous impaction risks creating a longitudinal crack in the proximal femoral cortex. If the stem is axially and rotationally stable, being 2-mm proud is clinically inconsequential. In this setting, the next shorter modular head could be utilized if stability is not compromised.

 

PEARLS AND PITFALLS

 

There are many tapered cementless stems, and they vary in shape and length.

 

Tapered stems may not fit well in the femurs with severe osteoporosis and a “stovepipe” intercortical geometry.

 

The importance of templating on quality AP and lateral x-rays cannot be overemphasized.

 

While preoperative templating will suggest a target stem size, the best-fit stem size is determined at surgery by the axial and rotational stability of the rasp or broach.

 

In Dorr type A bone, some reaming of the diaphysis can reduce proximal-distal stem size mismatch, even in some so-called broach-only systems.

 

Undersized cementless stems have an increased risk of not osseointegrating.

 

The risk of cracking the proximal femoral cortex increases in dense bone, with smaller femurs (the impaction force per unit area is higher), and when the rasps or broaches have become dull from use.

 

If in doubt about the sizing, or any other reconstructive parameter, an intraoperative radiograph with the intended rasp or broach in situ should be obtained.

 

Multiple medium-force hits should be used to impact the stem (Video 17-1).

 

Overly vigorous impaction risks creating a longitudinal crack in the proximal femoral cortex.

 

POSTOPERATIVE MANAGEMENT

A well-fit tapered cementless stem is stable under axial and rotational load. On this basis, immediate full weight bearing is generally allowed. However, if a postoperative x-ray shows an undersized stem, or a crack in the proximal femur, partial weight bearing with crutches is recommended for 4 to 6 weeks. Repeat radiographs should be taken, and weight bearing can be advanced if the stem position has stabilized. Tapered stems can subside and stabilize; however, pain with weight bearing more than 3 months postoperative suggests lack of osseointegration of a subsided stem.

COMPLICATIONS

The categorical complications of tapered cementless stems include failure of osseointegration, proximal femur fracture, thigh pain, and stress shielding. Cementless femoral fixation is generally classified as osseointegrated (bone ongrowth or ingrowth), fibrous stable, or fibrous unstable. Cementless femurs that are diagnosed with “aseptic loosening” did not have sufficient initial stability to allow osseointegration.

Undersizing of the stem is a significant risk factor for failure of osseointegration and aseptic loosening. A stem that is not osseointegrated is more likely to have thigh pain, especially with more vigorous activities. Radiographs may initially appear normal, other than undersizing of the component. Over time, a radiolucency and sclerotic line may develop around the implant. The stem may axially subside or rotate into retroversion. In this setting, pain and loss of function are the indications for revision.

Because of the wedge shape, axial impaction at surgery or postoperatively transmits hoop stresses into the proximal femur that can cause a longitudinal fracture. Depending on the amount of energy transferred, the fracture may be displaced or undisplaced. One study evaluated 1,039 hips with tapered stems from three prospective studies. There were 58 periprosthetic fractures (5.6%): 38 intraoperative (3.7%) and 20 postoperative (1.9%). No intraoperative fractures extended below the lesser trochanter. Twenty-five of these fractures were treated with a single cable or cerclage wire. The remaining received no specific treatment. Of the 20 postoperative fractures, 5 were treated nonoperatively. All stems osseointegrated (5).

 

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Thigh pain with well-fixed stems occurs more often with cementless stems than cemented stems and can occur to some degree in up to 25% of cases. In most cases, the thigh pain is transient and rarely necessitates revision of a well-fixed stem (6). Stress shielding is a function of stem stiffness (size and material) and the level of the stem where bone ingrowth occurs. Stress shielding does not, in and of itself, necessitate revision. Proximally porous-coated tapered cementless femoral components demonstrated less bone density reduction in Gruen zone 7 than fully porous-coated components (7).

Excessive fretting and corrosion of the modular head-stem taper can cause an adverse local tissue reaction and necessitate revision surgery (8). It has been more recently reported that corrosion at the modular neck-body junction in dual-tapered stems with a modular cobalt-chromium-alloy femoral neck can lead to release of metal ions and particles resulting in local soft tissue destruction. In these cases, the serum cobalt level is disproportionately elevated compared to serum chromium, and MARS MRI can be helpful in establishing this diagnosis. Adverse local tissue reaction should be considered as a potential cause for new-onset pain in patients with such modular components (9). While the etiology of taper fretting and corrosion is debatable, taper assembly is a variable. The concern for femoral fracture from overly vigorous impaction of a tapered stem may result in less secure modular taper assembly in some cases.

 

RESULTS

An early triple-taper cementless stem is the CLS Spotorno, developed by Prof. Lorenzo Spotorno, first implanted in 1984 and in use today (Fig. 17-5). Characterized by multiple sharp ribs proximally on the anterior and posterior sides and a grit-blasted surface finish, this stem has one of the highest survival rates in the Swedish National Hip Arthroplasty Registry. A consecutive series of 92 hips implanted between 1986 and 1991 with the Spotorno stem were assessed at a minimum of 21 years. The mean age at surgery was

59.6 years (39 to 77) (M:F, 43:49). The 23-year Kaplan-Meier survival rate was 91.5% (95% confidence intervals [CIs] 85.4% to 97.6%; 55 hips at risk) with revision of the femoral stem as the end point. At the time

 

of this review, 76 patients without stem revision were assessed clinically and radiologically (mean follow-up 24.0 years [21.5 to 26.5]). For the 76 unrevised hips, the mean Harris hip score was 87.1 (65 to 97).

Femoral osteolysis was detected in five hips (6.6%) only in Gruen zone 7. Undersized stems were at higher risk of revision for aseptic loosening ( p = 0.0003). Patients implanted with the stem in a varus position were at higher risk of femoral cortical hypertrophy and thigh pain ( p = 0.0006 and p = 0.0007, respectively) (10).

 

In another study, 354 Spotorno stems were implanted between January 1985 and December 1989 with minimum follow-up evaluation at 20 years (mean, 22 years; range, 20 to 25 years). Survivorship

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at 22 years with revision of the femoral component for any reason as the end point was 86% (95% CI, 81% to 90%). Survivorship for femoral revision for aseptic loosening as the end point was 93% at 22 years (95% CI, 90% to 96%). Undersized stems (canal fill index less than or equal to 80%) and stems in hips with cup revision were at higher risk for aseptic loosening (hazard ratio, 4.2 and 4.3, respectively). The performance of a stem can be considered independently of other components in the arthroplasty. In this series, there was a high rate of acetabular revision (38%) of the smooth-threaded, cementless socket (11).

 

 

 

FIGURE 17-5 Spotorno stem. (Courtesy of Zimmer, Warsaw IN.)

In an effort to increase fixation and stress transfer in the proximal femur, many tapered stems have coatings of a plasma spray, HA, or a porous coating to the proximal stem. The Taperloc (Biomet, Warsaw, IN) femoral component is made of forged titanium alloy (Ti-6Al-4V) and has a tapered rectangular shape designed to achieve fixation mediolaterally within the proximal femur. The proximal 47% of the implant has a plasma spray coating of the same titanium alloy. The distal noncoated portion of the stem has a satin surface finish (Fig. 17-6).

One hundred and forty-five consecutive total hip arthroplasties in 138 patients were performed between 1983 and 1985, by a single surgeon, with this stem. At a mean of 20 years (range, 18 to 22.6 years) postoperatively, 58 patients (65 hips) were living. Of the 80 patients (80 hips) who had died, 5 hips had undergone revision of the femoral component. Only 1 stem had been revised for aseptic loosening, and no femoral component was definitely loose by radiographic criteria. In the remaining 65 hips in the 68 living patients, 8 femoral components were revised. No femoral component underwent revision for aseptic loosening, although one was radiographically loose (12).

In a prospective study, five surgeons at five sites followed 189 patients (216 hips) for an average of 10.3 years (range, 10 to 12.4 years) comparing alumina ceramic bearings (144 hips) with cobalt-chrome-on-polyethylene bearings (72 hips). All hips had the same tapered stem with a thin HA coating proximally (Omnifit HA; Stryker Orthopaedics, Mahwah, NJ) (Fig. 17-7). There was no difference between the metal-on-polyethylene and the alumina-bearing couple cohorts with

 

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regard to bearing-related failures (98.9% vs. 99.1%). Revisions for any reason occurred in 10.5% of the metal-on-polyethylene hips and 3.1% of the hips with alumina bearings. All femoral implants remained well fixed (100%); one acetabular component (1%) was unstable in the control group. The role of other components in the arthroplasty is again illustrated: osteolysis occurred in 26% of the metal-on-polyethylene hips and in none of the hips with alumina bearings. Squeaking occurred in 2 of 144 hips (1.4%) of the patients with ceramic bearings (13).

 

 

 

FIGURE 17-6 Taperloc femoral component. (Courtesy of Biomet, Warsaw, IN.)

 

 

 

FIGURE 17-7 Omnifit HA femoral stem. (Stryker Orthopaedics, Mahwah, NJ.)

National joint registries pool survivorship data from the experience of multiple surgeons. The 2014 Australian National Joint Registry (AOA NJR) includes 280,522 total hip replacements with 2,362 different stem and acetabular combinations for primary total hip replacement. The large experience facilitates stratification for stem survival by acetabular component and bearing. As an example, the 9-year survival of one tapered cementless stem (Summit; DePuy, Warsaw, IN), when combined with a cementless socket (Pinnacle; DePuy, Warsaw, IN) and without a metal-metal bearing, was 98.1%. Dislocation (12), fracture (8), and infection (7) accounted for the majority of the 40 revisions. Only 5 of 2,826 hips have been revised for aseptic loosening (0.18%).

With the general success of tapered cementless stems, the concept has evolved. Based on the goals of being less invasive and reducing complications, shorter tapered stems have been developed. One study compared 389 hips with standard-length tapered stems to 269 hips with short stems. There was a higher rate of intraoperative fractures with the standard-length stems (3.1%; three trochanteric avulsions, nine femoral fractures) compared with the shorter stems (0.4%; one femoral fracture). There were no differences

in implant survival, Harris hip score, and Lower Extremity Activity Scale score between groups. Fewer

intraoperative fractures attest to the easier insertion of the short stem. Longer follow-up is needed to determine the durability of fixation and function (14).

In summary, cementless tapered stems vary by geometry, surface finish, and length. As a class, they are the most utilized type of cementless femoral component and have been clinically successful for more than 30 years. Long-term stable fixation can be achieved in greater than 95% of cases. While preparation of the femur may differ from system to system, undersizing is associated with an increased risk of clinical failure. Tapered stems will continue to evolve with improvements in bone conservation, instrumentation, ease of insertion, biomechanics, and reduced surgical variability.

 

 

REFERENCES

1. Rhyu KH, Lee SM, Chun YS, et al.: Does osteoporosis increase early subsidence of cementless double-tapered femoral stem in hip arthroplasty? J Arthroplasty 27(7): 1305-1309, 2012.

    

    

2. Kelly SJ, Robbins CE, Bierbaum BE, et al.: Use of a hydroxyapatite-coated stem in patients with Dorr Type C femoral bone. Clin Orthop Relat Res 465: 112-116, 2007.

    

    

3. Dalury DF, Kelley TC, Adams MJ: Modern proximally tapered uncemented stems can be safely used in Dorr type C femoral bone. J Arthroplasty 27(6): 1014-1018, 2012.

    

    

4. Schmalzried TP: The importance of proper acetabular component positioning and the challenges to achieving it. Oper Tech Orthop 19: 132-136, 2009.

    

    

5. Capello WN, D'Antonio JA, Naughton M: Periprosthetic fractures around a cementless hydroxyapatite-coated implant: a new fracture pattern is described. Clin Orthop Relat Res 472(2): 604-610, 2014. doi: 10.1007/s11999-013-3137-x

    

    

6. Wixson RL, Stulberg SD, Mehlhoff M: Total hip replacement with cemented, uncemented, and hybrid prostheses. A comparison of clinical and radiographic results at two to four years. J Bone Joint Surg Am 73(2): 257-270, 1991.

    

    

7. MacDonald SJ, Rosenzweig S, Guerin JS, et al.: Proximally versus fully porous-coated femoral stems: a multicenter randomized trial. Clin Orthop Relat Res 468(2): 424-432, 2010.

    

    

8. Svensson O, Mathiesen EB, Reinholt FP, et al.: Formation of a fulminant soft-tissue pseudotumor after uncemented hip arthroplasty. A case report. J Bone Joint Surg Am 70(8): 1238-1242, 1988.

    

    

9. Cooper HJ, Urban RM, Wixson RL, et al.: Adverse local tissue reaction arising from corrosion at the femoral neck-body junction in a dual-taper stem with a cobalt-chromium modular neck. J Bone Joint Surg Am 95(10): 865-872, 2013.

    

    

10. Evola FR, Evola G, Graceffa A, et al.: Performance of the CLS Spotorno uncemented stem in the third decade after implantation [Review]. Bone Joint J 96-B(4): 455-461, 2014. doi: 10.1302/0301-620X.96B4.32607

     

     

11. Streit MR, Innmann MM, Merle C, et al.: Long-term (20- to 25-year) results of an uncemented tapered titanium femoral component and factors affecting survivorship. Clin Orthop Relat Res 471(10): 3262-3269, 2013.

     

     

12. McLaughlin JR, Lee KR: Total hip arthroplasty with an uncemented tapered femoral component. J Bone Joint Surg Am 90(6): 1290-1296, 2008.

     

     

13. D'Antonio JA, Capello WN, Naughton M: Ceramic bearings for total hip arthroplasty have high survivorship at 10 years. Clin Orthop Relat Res 470(2): 373-381, 2012.

     

     

14. Molli RG, Lombardi AV Jr, Berend KR, et al.: A short tapered stem reduces intraoperative complications in primary total hip arthroplasty. Clin Orthop Relat Res 470(2): 450-461, 2012.