Component selection for revision total hip arthroplasty is important for creating a stable hip, providing offset to maximize joint mechanics, and restoring appropriate leg lengths. On the femoral side, fully porous coated stems, modular tapered stems, and proximal femoral replacements can be used depending on the level of bone loss. For the acetabulum, smaller defects can be contained using second-generation porous coating hemispherical cups, whereas larger acetabular defects can be contained with cup cages, cages, or custom triflange implants. In addition, acetabular liners can improve stability through altered cup version, dual mobility, or constraint of the femoral head.
Key points
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Paprosky acetabular and femoral classification systems are important for diagnosis, prognosis, and treatment of bone loss when choosing revision total hip arthroplasty implants.
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Revision femoral stems provide diaphyseal fixation using either fully porous coated cylindrical stems or modular tapered stems for almost all bone deficiencies.
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For complete proximal bone loss, proximal femoral replacements may be used or an allograft-prosthetic composite can be grafted to existing host bone.
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Most acetabular defects can be addressed with second-generation porous coating hemispherical cups with or without the addition of metal augments.
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The preferred method for addressing severe acetabular defects is the cup-cage construct, in which a second-generation porous coating cup is held in place with a cage and secured with screws into the ischium and ilium. Bone loss rarely requires the use of a custom triflange implant.
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Acetabular liners can be used to change acetabular version, use effectively larger femoral heads through dual mobility liners, or constrain the femoral head.
Introduction
Component selection for revision total hip arthroplasty (THA) is dictated by choosing components that create a stable hip, restore appropriate leg lengths, and provide offset to maximize joint mechanics. Multiple factors go into the decision-making process for which components to use, including the amount of bone loss present and accounting for the components that are currently in place. For the femur, determining the geometry of the remaining bone helps to guide orthopedic surgeons to use fully porous coated stems for metaphyseal bone loss, modular stems that provide various stem versions, and proximal femoral replacements (PFRs) for reconstructions in which there is great proximal bone loss and no attachment for the abductors. Smaller acetabular defects can be contained by using hemispherical cups with second-generation porous coatings with or without metal augments or bone graft, whereas larger acetabular defects can be contained with cages or custom triflange implants. In addition, specific acetabular liners can be placed to alter the version of the cup by a set number of degrees, provide articulation through dual mobility liners, or constrain the femoral head. This article provides guidance as to which components to use in THA revisions.
Introduction
Component selection for revision total hip arthroplasty (THA) is dictated by choosing components that create a stable hip, restore appropriate leg lengths, and provide offset to maximize joint mechanics. Multiple factors go into the decision-making process for which components to use, including the amount of bone loss present and accounting for the components that are currently in place. For the femur, determining the geometry of the remaining bone helps to guide orthopedic surgeons to use fully porous coated stems for metaphyseal bone loss, modular stems that provide various stem versions, and proximal femoral replacements (PFRs) for reconstructions in which there is great proximal bone loss and no attachment for the abductors. Smaller acetabular defects can be contained by using hemispherical cups with second-generation porous coatings with or without metal augments or bone graft, whereas larger acetabular defects can be contained with cages or custom triflange implants. In addition, specific acetabular liners can be placed to alter the version of the cup by a set number of degrees, provide articulation through dual mobility liners, or constrain the femoral head. This article provides guidance as to which components to use in THA revisions.
Revision femoral components
Choosing femoral components in revision THA depends on the type of fixation needed to stabilize a prosthesis within a femur with bone loss. Although metaphyseal fixation is often used in primary THA stems with wedge and taper stems, most stems used in revision THA are designed for diaphyseal fixation, because the metaphysis can be weakened or even absent in the revision setting. If too much proximal femoral bone is missing, then a PFR may need to be used. The options for femoral components in revision THA are described later.
Bone Loss Classification
The most useful classification system is the Paprosky classification, which is both prognostic and helps to direct treatment options. For type I defects, there is minimal bone loss in the metaphyseal region and the diaphysis is intact. In this setting, a primary THA stem may be used. For type II defects, the femur has extensive metaphyseal cancellous bone loss, but the diaphysis is still intact. Type III defects are characterized by severe bone damage and are classified into 2 groups. Type IIIA defects have severe compromise of the metaphyseal bone, but have greater than 4 cm of bone remaining in the diaphysis. Type IIIB defects also have an unsupportive metaphysis, but have less than 4 cm of bone in the diaphysis to provide for fixation. In addition, type IV defects have extensive metaphyseal and diaphyseal bone loss with an unsupportive isthmus and a widened femoral canal.
Fully Porous Coated Cylindrical Stems
Fully porous coated stems require adequate diaphyseal bone present for fixation. These stems are cylindrical and achieve an interference fit in the diaphysis, and studies have shown the need for greater than 4 cm of intact diaphyseal bone for secure fixation. The severity of the defect determines the long-term fixation and survivorship of these stems, because Paprosky type I and II defects can be treated with fully porous coated stems as long as the femoral canal is less than 19 mm in width. Long-term results show that using fully porous coated stems leads to long survivorship and high rates of osseointegration determined by radiographic studies, but one of the side effects is increased thigh pain and proximal stress shielding, because most of the stress transfer to bone occurs in the diaphysis.
Modular Tapered Stems
Modular cementless stems are commonly used in revision THA ( Fig. 1 ), because the proximal and distal femur are individually prepared to accommodate a prosthesis that fills bone defects. These implants can provide good diaphyseal fixation, limb length discrepancies can be restored, and the version of the implant can be set to provide joint stability and maximize joint mechanics. Although these modular stems can vary the distal stem configuration, the tapered conical stem is now favored and has the best results. Studies show that modular tapered stems had lower failure rates, better osseointegration, and decreased revision rates compared with cylindrical stems. These modular, tapered stems can be used to treat any level of bone damage, but are particularly effective (compared with cylindrical fully porous coated stems) for Paprosky type III femoral defects, with demonstrated 75% to 94% survivorship at 4.5 to 10 years. Using proper technique, these implants have a low failure rate, but can subside if not fitted tightly, or can fracture at the interface between the neck and the stem of the prosthesis if that junction is not supported by bone.
Allograft-Prosthetic Composite
Although modular stems can also be used to treat Paprosky type IV femoral defects, allograft-prosthetic composites (APCs) are another option for replacing bone loss by using a proximal femoral allograft and cementing or press fitting a stable implant into the allograft. Distal fixation can be achieved by providing step cuts for fixation, or tunneling the graft bone into host bone (intussusception) for Paprosky type IV femurs. APCs were first used to reconstruct limbs in dogs with osteosarcoma who underwent limb-sparing surgery, and they were subsequently used in revision THA to treat circumferential defects of greater than 3 cm from the calcar. More current studies recommend using APCs in Paprosky type III and IV defects, with 69% survivorship at 10 years. This method of fixation is beneficial because it restores bone stock, allows for load sharing of the revision construct with host bone, provides a biological anchor for abductor attachment, and improves the ease of reoperation if bone has incorporated by future revision surgery. However, implanting an APC is a technically demanding procedure, and because allograft is used there is a risk of fracture because the construct is not as strong as metal implants, there is a risk of disease transmission, and the graft may not incorporate and may lead to nonunion or resorption.
PFR
In the case of massive proximal bone loss in which treatment with the aforementioned implants is not adequate for fixation, a PFR is an option that replaces the proximal femur with a metal prosthesis that allows for attachment of the abductors ( Fig. 2 ). These modular tumor megaprostheses can be used in patients with nonneoplastic conditions and are beneficial because they allow for version correction and leg length restoration. The survivorship of these implants has been reported as 87% at 5 years and 64% at 12 years. After undergoing PFR, many patients report improvement in quality of life, as measured by Harris hip scores, Western Ontario and McMaster (WOMAC) Universities Arthritis Index, Oxford scores, and Short-form 12 mental component scores. However, one study showed that, at a mean of 5 years’ follow-up, all patients were still ambulating with assistive walking devices. Complications after PFRs include dislocation and septic and aseptic loosening, which encourages the use of PFRs in low-demand, elderly patients who live sedentary lifestyles. Additional PFRs using tantalum components have been described in the literature and may provide a better bone ingrowth surface for host bone to incorporate into the prosthesis.
Custom Stems
In the revision THA setting, custom stems that have tantalum metal coatings can be used to improve fixation, especially in Paprosky type IV femurs with wide femoral canals.
Revision acetabular components
Acetabular component selection is mostly based on the amount of bone loss present, and the most useful classification system is another Paprosky classification, as described later. Determining the size of the existing implant is important for determining the minimum implant size that should be used in the revision setting. For implantation of the acetabular component, the options for components are listed later.
Bone Loss Classification
The Paprosky classification is the most commonly used, and it is also diagnostic, prognostic, and directs surgical management. Type I Paprosky acetabular bone loss has the presence of a supportive rim of bone without bone lysis or migration. Type II has a distorted acetabular hemisphere, but the anterior and posterior columns are intact and there is less than 2 cm of superomedial or superolateral migration. Type II is further divided into 3 groups: (1) type IIA, superomedial bone lysis with an intact superior rim; (2) type IIB, superolateral bone lysis with an absent superior rim; and (3) type IIC, medial wall bone lysis. Type III Paprosky acetabular bone loss is characterized by superior migration of greater than 2 cm and shows severe ischial and medial osteolysis. Type IIIA defects have bone loss between the 10 o’clock and 2 o’clock positions, and 30% to 60% of the bone is missing but the ilioischial line is intact. Type IIIB defects have bone loss between the 9 o’clock and 5 o’clock positions, and 60% of the bone is missing and the ilioischial line is not intact. This classification system has also been validated and is a reliable system, especially when individuals are taught about the classification system before looking at plain radiographs.
Revision Acetabular Cups
Almost all revision acetabular cups are hemispherical with a second-generation porous coating to promote bone ingrowth ( Table 1 ). This second-generation coating promotes osseointegration by providing a porous surface that is seemingly biocompatible with osteoblasts, and it also provides greater friction at the component-bone interface. These cups have shown osseointegration with good long-term results when there is minimal bone loss in the revision setting. Transacetabular screws can be used to augment fixation, whereas autograft or cancellous bone allograft can be used to fill acetabular defects. These hemispherical cups can only be used if there is a contained defect with minimal migration of the failed cup, which requires that both the anterior and posterior columns are largely intact, so hemispherical revision cups work well in Paprosky I and most Paprosky II acetabular defects.
| Implant Name | Company | Composition | Surface | |
|---|---|---|---|---|
| Hemispherical cups | Regenerex ® Revision Acetabular Shells | Biomet | Titanium | Porous titanium |
| Pinnacle ® Revision Acetabular Cup System | DePuy | Titanium |
| |
| R3 | Smith & Nephew | Titanium | STIKTITE porous coating | |
| Tritanium Acetabular Shell | Stryker | Titanium | Titanium matrix (Tritanium) | |
| Trabecular Metal™ Revision Shell | Zimmer | Titaniums | Trabecular metal | |
| Acetabular augments | Regenerex ® Acetabular Augments | Biomet | Titanium | Porous titanium |
| Gription TF ® Augments | DePuy | Titanium | Titanium foam | |
| Trabecular Metal™ Acetabular Augments | Zimmer | Titanium | Trabecular metal | |
| Restoration™ Acetabular Wedges | Stryker | Titanium | Titanium matrix (Tritanium) |
Acetabular Augments
To supplement acetabular fixation when there is segmental bone loss, acetabular augments can be used to fill specific defects. In the past, structural allografts served this purpose. The problems with structural allografts are nonunion, infection, aseptic loosening, graft resorption, immune reaction, disease transmission, and implant migration. More recently, porous coated metal augments have become available with different shapes that can be used to fill specifically shaped defects. These augments have essentially replaced the need for structural allograft ( Fig. 3 ). These augments can be used alone or in combination with other augments, and reamers are available to prepare the acetabular defect where augments will be placed. The augments are then impacted into the defect and secured with screws. Once the augments are fixed with screws, a second-generation porous coated hemispherical cup can be press fitted into the acetabulum and cement placed at the augment-cup interface. Studies evaluating the use of acetabular augments have shown radiographically stable fixation at 2-year follow-up when used alone, when combined with impaction grafting, and when used with a cage-cup construct. These augments extend the application of hemispherical revision cups to Paprosky type IIB and some type IIIA defects.
Cages
In Paprosky III acetabular defects in which there is substantial superior migration, bone loss, and/or pelvic discontinuity (type IIIB), these patients may be more appropriately treated with a cage construct. A cage is a prosthesis with multiple screw hole options, so that the prosthesis can be supported by the ilium and ischium instead of the acetabular dome alone ( Table 2 ). This prosthesis is used to bridge bone loss as an antiprotrusio device, and can be used to contain allograft placed within the acetabular defect to allow for integration of the graft with host bone. Once adequate acetabular exposure is achieved, allograft can be reverse reamed and the cage can be fixed down through the dome screws first, then the superior flange, and then the inferior flange. An all-polyethylene cup or a polyethylene liner is then cemented into place to articulate with the femoral head. Although cages provide temporary fixation while bone graft incorporates into host bone, this method of treatment often lacks biological fixation and may lead to fatigue failure.
| Implant Name | Company | Composition | Surface | |
|---|---|---|---|---|
| Cages | Recovery ® Protrusio Cage | Biomet | Titanium | Grit blasted |
| Par 5™ | Biomet | Titanium Alloy | Porous Plasma Spray coating | |
| Protrusio Cage | DePuy | Titanium | None | |
| Contour | Smith & Nephew | Titanium | None | |
| Restoration Gap II | Stryker | Titanium | Grit blasted | |
| Burch-Schneider™ Reinforcement Cage | Zimmer | Titanium | None | |
| Cup-cages | Trabecular Metal™ Acetabular Revision System Cup-Cage Construct | Zimmer | Titanium | Trabecular metal |
| Custom Triflange | BIOMET ® Patient-Matched Implants (PMI ® ) | Biomet | Titanium | Porous Plasma Spray |
| Pinnacle™ Triflange Acetabular System | DePuy | Titanium | Porous- and/or hydroxyapatite-coated |
The preferred option to provide acetabular fixation is to use a cup-cage construct. In this method, a second-generation porous cup is fixed in the pelvis. If there is minimal stability of the implant, an acetabular cage can be placed on top of the cup and secured with screw fixation into the ilium and ischium. A polyethylene liner can then be cemented into the cup cage. This superior method of fixation allows the porous cup to achieve osseointegration, and one study showed that 88.5% of hips had no sign of clinical or radiographic loosening at 44.6 months.
Custom Implants
An alternative treatment in the case of pelvic discontinuity (Paprosky IIIB), especially in situations of massive bone loss, is the use of a custom triflange cup. These components are beneficial because they supposedly match the patient’s anatomy to try to minimize bone loss ( Fig. 4 ). However, creating these triflange cups requires a computed tomography scan of the pelvis to evaluate the patient’s anatomy and bone loss, and designing and manufacturing these implants is an expensive and time-consuming process. When these custom triflange implants are used in selected patients, the radiographic results have shown bridging bone formation in pelvic discontinuity and patients have reported improved Harris hip scores.
