Extensively Porous-Coated Cylindrical Uncemented Femoral Stems in Revision Total Hip Arthroplasty: A Review

RECONSTRUCTION 2023, Jan, 26 | 12:00 am

Extensively Porous-Coated Cylindrical Uncemented Femoral Stems in Revision Total Hip Arthroplasty: A Review

Extensively porous-coated cylindrical uncemented femoral stems are a type of implant used in revision total hip arthroplasty. This article reviews the indications, contraindications, techniques, and outcomes of using extensively porous-coated cylindrical uncemented femoral stems in revision total hip arthroplasty.

INDICATIONS

Despite the overwhelming success and long-term reliability of total hip arthroplasty, several situations necessitate the revision of a femoral component. The most frequent indications for femoral revision include aseptic loosening, recurrent instability from a malpositioned component, component fracture, periprosthetic fracture, delayed infection, and the need for improved acetabular exposure. Relative indications for femoral component removal include progressive distal femoral osteolysis or revision of a femoral component with a poor track record encountered during acetabular revision.

A successful surgical result following femoral reconstruction requires the extraction of the previous component with minimal bone loss, the insertion of a new component with resistance against rotational and axial stresses, and a stable articulation following reduction. The use of an extended trochanteric osteotomy is often recommended to extract a well-fixed stem, to insert a femoral component into a femur that has exhibited varus remodeling, or to facilitate the removal of distal cement. This surgical technique allows preservation of proximal bone stock and allows concentric reaming of the remaining isthmic femur.

The type of femoral component utilized during revision surgery ultimately depends on the quality of remaining host bone. The femoral defect classification of Paprosky et al. is helpful in preoperative planning and in assisting the surgeon with reconstructive options (Table 27-1) (1). The majority of femoral revisions encountered in daily practice have an intact diaphyseal segment with a compromised metaphyseal region (Paprosky type II) (Fig. 27-1) or more severe involvement of the proximal diaphysis (Paprosky type IIIA) (Fig. 27-2). In these patients, the metaphyseal region cannot be relied upon to provide stable component fixation. High failure rates have been observed when metaphyseal fixation has been attempted in this patient population (2). It is the author's preference to always use an implant that obtains distal fixation for type II and greater defects.

Distal fixation can be achieved through either a cemented or cementless implant. A cementless implant is recommended in the majority of patients due to the high rate of osseointegration and long-term clinical results. There are two philosophies regarding the optimal distal femoral geometry

in order to obtain distal femoral fixation. A tapered stem with splines will obtain axial stability through the tapered distal segment, while the splines will provide rotational stability (Fig. 27-3). An extensively fully porous-coated cylindrical stem relies upon a press fit between the distal portion of the cylindrical stem and the reamed diaphyseal bone (Fig. 27-4). Excellent clinical results have been observed with both modular and nonmodular tapered as well as extensively coated cylindrical stems.

Diaphyseal fixation with a monoblock extensively coated cylindrical stem remains the authors preferred implant choice in Paprosky type II and IIIA defects. A monoblock extensively porous-coated cylindrical stem avoids the potential for modular junction fractures (Fig. 27-5) and, unlike a tapered stem design, allows the ability to use a trephine to remove the distal segment of a well-fixed stem should extraction be required in the future.

TABLE 27-1 Paprosky Classification

Type of Defect Description of Femur

Minimal defects. Similar to primary total hip arthroplasty.

Metaphyseal damage. Minimal diaphyseal damage.

IIIA

Metadiaphyseal bone loss. 5-cm scratch fit can be obtained at the isthmus.

IIIB

Metadiaphyseal bone loss. 5-cm scratch fit unable to be obtained.

Extensive metadiaphyseal damage. Thin cortices, widened canals.

FIGURE 27-1 AP femur radiograph demonstrating a Paprosky type II femoral defect. Note the compromised metaphyseal bone and the intact isthmus. The black lines indicate the remaining bone of the isthmus where a cylindrical stem can obtain a press fit.

FIGURE 27-2 AP femur radiograph demonstrating a Paprosky type IIIA femoral defect. Note that greater than 5 cm of diaphyseal bone remain available to achieve implant stability.

FIGURE 27-3 The distal splined tapered stem of a modular revision hip system. Note the splines that provide rotational stability while the taper provides axial stability.

FIGURE 27-4 An extensively porous-coated cylindrical stem. Note the cylindrical porous coating relies upon a scratch fit in order to obtain axial and rotational stability.

FIGURE 27-5 AP femur radiograph demonstrating implant fatigue fracture at the modular stem junction. Note the radiographic signs of osseointegration of the distal cylindrical stem segment.

CONTRAINDICATIONS

Extensively porous-coated cylindrical stems rely upon the interference “press fit” between the remaining host diaphysis and the femoral component. It has been suggested that a minimum of 4 to 5 cm of diaphyseal

bone be available and engaged by the extensively porous-coated cylindrical implant in order to provide adequate rotational and axial stability to minimize micromotion and to facilitate bone ingrowth (3). The authors recommend alternative reconstructive options when there is less than 5 cm of diaphyseal bone remaining (Paprosky type IIIB or IV defect) or when the patient has compromised biologic activity such as a tumor or radiation that would prevent bone ingrowth. Additionally, patients with a type IIIA or IIIB femur (Fig. 27-6) and an associated endosteal diameter greater than 19 mm have been shown to have a higher incidence of subsidence and a decreased likelihood of achieving boney ingrowth (4). In these patients, age, medical comorbidities, and activity level are important factors to consider when choosing a femoral component. A modular tapered stem can be used successfully in the majority of these large femoral defects

(5). The author advises against the use of a monoblock extensively coated stem when the metaphysis is compromised and the femoral canal size is less than 13 mm. In these situations, the femoral component is at a relatively high risk of cantilever bending and catastrophic fracture (Fig. 27-7) (6).

1

FIGURE 27-6 AP femur radiograph demonstrating failure of bone ingrowth into an extensively porous-coated cylindrical stem. This patient had a type IIIB femoral defect and required a 22-mm femoral stem. Alternative methods of fixation are advised in type IIIB and IV femoral defects.

FIGURE 27-7 AP femur radiograph demonstrating fracture of a 12.5-mm extensively porous-coated stem. Note the femoral component is well fixed distally, and there is poor proximal metaphyseal bone support.

TECHNIQUE

Preoperative Preparation

Similar to other surgical procedures, adequate preoperative planning is essential for a successful surgical

outcome. An extended trochanteric osteotomy should be considered if the femoral component appears to be well fixed or if there is significant femoral varus remodeling. The length of the osteotomy should be long enough to reach the apex of notable deformity or allow safe removal of the retained implant yet be short enough to allow a minimum of 5 cm of press fit with the chosen extensively porous-coated cylindrical stem length (Fig. 27-8).

Additionally, it is helpful to have an oscillating saw, pencil-tip burr, wide osteotomes, cylindrical trephines, Gigli saw, metal-cutting burrs, and cerclage wires available to complete the osteotomy and to safely extract the retained component.

FIGURE 27-8 AP femur radiograph demonstrating an 8-inch femoral trial. Note the trial component provides greater than 5 cm of diaphyseal fixation.

Exposure

The surgical approach in the revision setting may be directed by previous surgical incisions. In general, the author prefers a posterior lateral approach because it may be extensile and it allows visualization of both the femur and the posterior column of the acetabulum. A lateral surgical skin incision is made in line with the femur based over the posterior one-third of the greater trochanter. The tensor fascia lata and the fascia of the gluteus maximus are then split in line with the surgical incision and retracted with a Charley bow. The posterior pseudocapsule and the short external rotators are then elevated as a posteriorly based flap. A portion of the gluteus maximus insertion is released to allow mobilization of the femur anteriorly if required. The femoral head is dislocated posteriorly, and the hip is placed in internal rotation with the knee flexed. The stability of the femoral component can be assessed. If the stem is grossly loose and the greater trochanter is not preventing extraction, the component is removed. However, if the trochanter is preventing safe component extraction or if the stem is well fixed, an in situ extended trochanteric osteotomy should be considered. A prophylactic cerclage cable should immediately be placed distal to the osteotomy site in order to minimize the risk of distal fracture propagation during subsequent bone preparation trialing and component insertion (Fig. 27-9).

FIGURE 27-9 AP femur radiograph demonstrating a femoral revision with an extensively porous-coated cylindrical stem. Note the prior extended trochanteric osteotomy and the prophylactic cable placed distal to the osteotomy site.

Bone Preparation

It is imperative that the surgeon be able to concentrically ream the remaining diaphyseal bone when an extensively porous-coated cylindrical stem is utilized. An extended trochanteric osteotomy at the apex of the deformity is required in order to avoid perforation of the lateral cortex if the femur has undergone varus remodeling (Fig. 27-10). The femoral canal is sequentially reamed with straight cylindrical reamers to the depth of the templated stem length until cortical resistance is encountered. It is advised that a minimum of 5 cm of diaphyseal bone, “scratch fit,” be engaged when utilizing a fully porous-coated stem (Fig. 27-11). The femoral canal is reamed solely with straight reamers if a straight 6 or 8-inch extensively porous-coated prosthesis is chosen preoperatively. Templating on both the anteroposterior and lateral projections is critical to identify the potential for anterior cortical perforation with longer straight stems. An 8- or 10-inch curved stem may be required if the femoral bow is encountered. In these situations, flexible reamers are used to sequentially ream the distal diaphysis. Typically, the femoral canal is underreamed by 0.5 mm compared to the implant diameter. This amount of mismatch between the host bone and implant will allow adequate axial and rotational stability for bone ingrowth.

A femoral trial can be placed upon completion of the femoral reaming. Trialing is recommended to assure appropriate leg length, offset, combined anteversion, and overall stability of the hip. The optimal position of the femoral stem (depth of insertion and anteversion) should be marked prior to trial removal. If an 8- or 10-inch bowed stem is utilized, the bow of the femur and the prosthesis may direct the amount of femoral anteversion. If the hip is not stable in this configuration, alternative methods of reconstruction such as a modular stem should be considered.

FIGURE 27-10 AP femur radiograph demonstrating femoral varus remodeling. Note the inability to place a straight extensively porous-coated cylindrical stem without the use of an extended trochanteric osteotomy.

FIGURE 27-11 A: Cylindrical reamer advanced by hand demonstrating greater than 5 mm of endosteal contact. B: Distal cylindrical reamer after final reaming demonstrating removal of endosteal bone during the final bone preparation.

Prosthesis Implantation

The placement of an extensively porous-coated cylindrical stem in the revision situation is similar to that used during primary arthroplasty. A hole gauge should be utilized to verify that the manufacturing process has resulted in the appropriate distal femoral diameter. Ideally, there should be a 0.5 mm mismatch between the diameter of the last cylindrical reamer used and the femoral implant (Fig. 27-12) with the reamer diameter undersized compared to the component. If the component is slightly oversized compared to its stated diameter due to the bead sintering process, the femoral canal can be reamed an additional 0.5 mm to avoid femoral fracture. When inserting the femoral component, the implant should be able to be seated by hand to within 5 cm of the location of the trial (Fig. 27-13). The femur should be reamed an additional 0.5 mm if the desired depth of insertion is greater than 5 cm. Multiple impacts with a 2-pound mallet are used to fully seat the implant while taking care to maintain the desired amount of anteversion. The femoral stem should advance slowly with each strike of the mallet. Sudden advancement of the stem should raise concern for distal fracture.

FIGURE 27-12 A hole gauge may be used to compare the relative mismatch between the femoral component and the last cylindrical femoral reamer.

FIGURE 27-13 Intraoperative image of an extensively porous-coated cylindrical stem prior to final seating. The femoral component should be able to be advanced by hand to within 4 to 5 cm of the desired position.

Wound Closure

The surgical wound is closed in a routine manner once the femoral component has been placed. A minimum of two cables are recommended to secure the greater trochanter if an extended trochanteric osteotomy was utilized. Our preference is to repair the posterior capsule and short external rotators to the posterior aspect of the gluteus medius. The gluteus maximus fascia and the iliotibial band are closed with a nonabsorbable #1 suture, while the subcutaneous tissue is closed with an absorbable 2-0 suture.

POSTOPERATIVE MANAGEMENT

The postoperative weight-bearing status is dependent upon the severity of the femoral defect. In general, patients who have undergone a femoral revision with an extensively porous-coated cylindrical stem are initially touchdown weight bearing on the operative leg. Patients with type II defects are made weight bearing as tolerated at 2 weeks postoperatively, while patients with type IIIA defects or those with an extended trochanteric osteotomy are made fully weight bearing at 6 weeks.

PEARLS AND PITFALLS

A minimum of 5-cm “scratch fit” is required to provide component stability. If this cannot be obtained, alternative methods of reconstruction should be considered.

During the preparation of the femur, it is imperative that the diaphyseal portion of the femur can be reamed centrally. Inability to ream concentrically will either result in cortical perforation or undersizing of the implant.

An extended trochanteric osteotomy will allow direct access to the femoral canal with little associated morbidity in patients with retained cement, a well-fixed stem or varus remodeling. In general, the osteotomy should be performed if it is contemplated by the surgeon.

If an 8- or 10-inch stem is planned, the magnitude of the anterior femoral bow must be assessed preoperatively with appropriate templates. A curved stem should be utilized if templating would indicate anterior cortical perforation.

A cerclage cable placed distal to the site of an extended trochanteric osteotomy can minimize the risk of inadvertent fracture during the preparation and insertion of an extensively coated stem.

The femoral component should be removed if an intraoperative fracture occurs and the implant is unstable. Distal cerclage cables should be placed around the femur extending beyond the extension of the fracture. While a longer extensively porous-coated cylindrical stem can be placed in this scenario, the author's preference is to use an modular tapered implant.

The extensively porous-coated cylindrical stem should be able to be advanced to within 5 cm of the desired insertional depth by hand prior to striking it with a mallet. Failure to advance the implant to this level before insertion results in a greater risk of component incarceration or fracture.

COMPLICATIONS

Perioperative complications can occur with all femoral revision implants. One concern that has been raised repeatedly with the use of extensively coated devices surrounds the concept of stress shielding (Fig. 27-14). When a rigid femoral stem with distal fixation is used, the stresses are directly transferred to the distal

part of the femur, thereby shielding the proximal bone. The proximal bone responds according to Wolff's law and results in atrophy of the unloaded portions. Stem sizes greater than 13.5 mm demonstrate a fivefold increase in stress shielding. Radiographic evidence of bone ingrowth into an extensively coated stem results in a 2.5-fold increase in proximal bone resorption (7). The stiffness of the bone relative to the implant appears to be a dominant risk factor for stress shielding. Because the bending stiffness of implants is

proportional to the radius4, a small change in the stem diameter can have a marked impact on the stiffness of the stem. Stress shielding appears to occur within the first 2 years following components insertion and after that time does not appear to be progressive (8).

Extensively coated stems used in femoral revision rely upon distal fixation in the remaining intact isthmus. As a result, the proximal bone is rarely supportive, and bone ingrowth into the proximal coating is rare.

Consequently, the implant is subjected to cantilever bending. An extensively coated cobalt-chromium implant has a relatively high modulus of elasticity, and physiologic loads are generally well below the endurance limit. However, in some situations, fatigue fracture of the stem may

occur above the area of ingrowth. This has been observed most commonly in extensively coated stems of 12 mm diameter or less (6). Fortunately, most femoral revisions have an enlarged endosteal diameter, and the revision stem is greater than 12 mm.

Additional concerns about the use of an extensively coated stem relate to the perceived difficulty in removing a well-fixed implant. The technique of an extended trochanteric osteotomy is very useful in the event that removal becomes inevitable. Once the implant is exposed, the stem can be divided at the level of the cylindrical portion of the stem. Following extraction of the proximal piece with the use of a high-speed burr and Gigli saw, the distal portion can be removed with the use of trephines 0.5 mm larger than the stem (Fig. 27-15) (9).

FIGURE 27-14 AP femur radiograph demonstrating extensive proximal femoral stress shielding. Note the radiodensity and spot welds at the distal aspect of the femoral stem indicating stable bone ingrowth.

FIGURE 27-15 Intraoperative image demonstrating the use of a trephine to remove the distal portion of an extensively porous-coated cylindrical stem. Note the proximal segment was removed with the use of a Gigli saw following sectioning of the stem at the junction between the tapered and cylindrical portion.

RESULTS

Extensively porous-coated cylindrical stems have been the workhorse of revision femoral surgery. They have been shown to provide reliable and predictable long-term results in the majority of patients. Weeden et al. evaluated 188 patients with an extensively porous-coated cylindrical stem used for revision surgery at an average 14-year follow-up. The overall mechanical failure rate was 4.1%. The author of this series believed that extensive bone loss (Paprosky type IIIB and IV defects) and a large endosteal canal were predictive of a poor outcome (10). Similarly, Engh et al. (11) demonstrated a 3.4% prevalence of revision for loosening or progressive osteolysis among 204 femoral revision patients at an average 12.2 years postoperatively. The results of extensively porous-coated cylindrical stems remain superior to those encountered with either a cemented or a monoblock proximally porous-coated implant.

Extensively porous-coated cylindrical stems rely upon circumferential frictional interference in order to obtain axial and rotational stability. This initial stability becomes compromised as the severity of femoral bone loss increases and the ability to obtain a minimum of 5 cm of scratch fit is eliminated. Higher rates of subsidence, failure of bone ingrowth, and fracture have been observed when extensively porous-coated cylindrical stems are used in this difficult cohort of patients. Monoblock and modular tapered stem designs have been shown improved short-term results in these severe femoral defects (5).

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Meneghini RM, Hallab NJ, Berger RA, et al.: Stem diameter and rotational stability in revision total hip arthroplasty: a biomechanical analysis. J Orthop Surg Res 1: 5, 2006.
Sporer SM, Paprosky WG: Revision total hip arthroplasty: the limits of fully coated stems. Clin Orthop (417): 203-209, 2003.
Brown NM, Tetreault M, Cipriano CA, et al.: Modular tapered implants for severe femoral bone loss in THA: reliable osseointegration but frequent complications. Clin Orthop Relat Res 473(2): 555-560, 2014.
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Engh CA, Bobyn JD, Glassman AH: Porous-coated hip replacement. The factors governing bone ingrowth, stress shielding, and clinical results. J Bone Joint Surg Br 69(1): 45-55, 1987.
Engh CA Jr., Young AM, Engh CA Sr, et al.: Clinical consequences of stress shielding after porous-coated total hip arthroplasty. Clin Orthop (417): 157-163, 2003.
Kancherla VK, Del Gaizo DJ, Paprosky WG, et al.: Utility of trephine reamers in revision hip arthroplasty. J Arthroplasty 29(1): 210-213, 2014.
Weeden SH, Paprosky WG: Minimal 11-year follow-up of extensively porous-coated stems in femoral revision total hip arthroplasty. J Arthroplasty 17(4 Suppl 1): 134-137, 2002.
Engh CA Jr, Ellis TJ, Koralewicz LM, et al.: Extensively porous-coated femoral revision for severe femoral bone loss: minimum 10-year follow-up. J Arthroplasty 17(8): 955-960, 2002.