Addressing Femoral Bone Loss in Revision Total Hip Arthroplasty
In this article, the authors provide a summary of bone loss classification systems and an overview of revision stem selection for each stage of femoral bone loss.
Ian Duensing, MD, and Jeremy M. Gililland, MD
Total hip arthroplasty (THA) is one of the most successful operations in orthopaedics in terms of providing pain relief and functional improvement. With life expectancy rising and THA being performed at younger ages, arthroplasty components need to last longer and perform better than ever before.
When components fail – which happens for many reasons – one of the consequences may be bone loss. In revision THA, femoral bone loss is a challenge that requires careful deliberation and planning to address in an effective manner.
This article provides a summary of bone loss classification systems and an overview of revision stem selection for each stage of femoral bone loss.
Etiology and Classification of Femoral Bone Loss
The etiology of femoral bone loss includes:
- Aseptic loosening
- Periprosthetic fractures
- Recurrent dislocations or instability
- Osteolysis of the femur
- Osteonecrosis of the femur in patients with metallosis
- Primary tumor or metastatic bone disease,
- Removal of a previously placed implant
These etiologies can cause different patterns of bone loss, resulting in varying amounts of residual fixation options.
Several classification systems describe and define femoral bone loss. The classification scheme from the American Academy of Orthopedic Surgeons, for example, helps in identifying the location and pattern of bone loss: cavitary, segmental, or combined.  This is useful for communication between providers, but it is somewhat less helpful than alternative classification systems that guide treatment options in a clinical setting.
The classification system created by Paprosky et al  is treatment-oriented and has become the most widely used system in North America. It focuses on 3 components of femoral bone loss:
- Location of bone loss: metaphyseal versus diaphyseal
- Degree of remaining proximal metaphyseal cancellous bone stock
- Amount of remaining isthmic bone in the femoral diaphysis
This system stratifies femoral bone loss into 4 groups.
Type 1 defects show minimal metaphyseal bone loss, with preserved cancellous bone stock for proximal component fixation as well as an intact diaphysis. These defects often are not discussed in the same context as the rest of the Paprosky classification because revision components are usually not required; primary components will generally suffice.
Type 2 defects are the most commonly encountered defect patterns and differ from Type 1 defects by marked loss of metaphyseal bone. These patterns have poor proximal component stability but retain a robust, intact diaphysis for distal fixation.
Type 3 defects are separated into 2 sub-classifications – A and B – based on the degree/pattern of diaphyseal bone loss and isthmic bone remaining. Type 3 femurs have extensive metaphyseal bone loss. Type 3A defects are defined as having more than 4 cm of diaphyseal isthmic bone remaining for distal component fixation, while Type 3B defects have less than 4 cm of isthmic bone remaining.
Type 4 defects are the highest grade and are defined as profound canal ectasia with complete lack of metaphyseal or diaphyseal femoral support. No diaphyseal isthmus remains for distal fixation.
In short, as the Paprosky grade increases, the amount of support available for fixation decreases, as does the capacity for bone-component in-growth, while proximal femoral remodeling increases.
Component Options by Type of Bone Loss
Before proceeding with a revision procedure, the surgeon must obtain proper preoperative imaging, including a CT scan if necessary, to fully categorize the amount of bone loss. In addition, preoperative labs that include infectious markers and aspiration are needed to rule out an infectious etiology for femoral bone loss. Serum cobalt and chromium levels and a metal artifact reduction sequence MRI may be indicated in patients with metal-on-metal hip implants. The etiology and degree of bone loss may not always be clear, but a thorough preoperative workup is imperative for a successful revision outcome.
Type 1 Defects
As mentioned above, proximal femoral bone is relatively intact, with good cancellous support, in Type 1 defects and, therefore, revision components may not be needed. The surgeon can generally use a standard-length primary implant.
If revision components are necessary, proximally porous-coated femoral stems are the treatment option of choice. These stems have demonstrated favorable clinical results in much of the literature due to the generally robust proximal metaphyseal bone stock that remains in Type 1 defects. 
Type 2 and Type 3 Defects
Type 2 and 3A defects require similar solutions as they both rely on diaphyseal engagement. Metaphyseal fit provides no initial stability for these patterns. Proximal fixation options are limited, therefore, and the use of diaphyseal press-fit components becomes the method of salvage. Extensively porous-coated femoral stems and modular or monoblock fluted tapered stems are the most frequently used revision components. Modular fluted tapered stems are becoming more widely used than other stem options due to the relative ease of insertion and their modularity, which helps the surgeon control for version and limb length.
Extensively porous-coated stems were once regarded as the gold standard press-fit stem for revision THA. These stems rely on initial stability through the friction created by an interference fit that occurs when the femur is prepared 0.5 mm smaller than the diameter of the implant, allowing 3-point contact between the implant and bone. Lasting stability is achieved through extensive bony ingrowth into the porous coating along most of the length of the implant. 
Good long-term survivability with minimal loosening has been demonstrated for extensively porous-coated stems, [5-10] making them a viable component option. However, their use is limited. Intraoperative fracture is a common complication, as high as 20% in some literature, due to high insertional force and increasing stem length (Figure 1). Stress shielding is another common problem and, as expected, is seen more often in components with a larger diameter (Figure 2).
Figure 1. Intraoperative femoral periprosthetic fracture with fully porous-coated stem insertion.
Figure 2. Proximal femoral stress-shielding with a fully porous-coated femoral stem.
Use of extensively porous-coated stems should be limited to Type 3A lesions and Type 3B lesions with canal diameters less than 19 mm, as demonstrated by Sporer and Paprosky in 2003.  They examined outcomes of 51 patients with Type 3 or Type 4 defects treated with extensively porous-coated stems and found no failures in the 3A group, 6.7% failure in 3B lesions with less than 19 mm diaphyseal canal diameter, 18% failure in 3B lesions with a canal greater than 19 mm, and 37.5% failure in the 4 group at 4.2 years after surgery.  These stems have generally fallen out of favor, especially as fluted tapered modular revision stems have become more widely available and have shown reliable early and mid-term results.
Modular fluted tapered stems have longitudinal ridges along the length of the stem that allow for rotational stability. These implants rely on a conically prepared canal that receives the matched tapered distal stem, rather than the friction-based scratch-fit interface of extensively porous-coated stems. This tapered ream provides an intimate engagement between the stem and the limited isthmic bone – much more than would be provided by a conical ream for a porous-coated stem – as well as axial stability of the stem. Modularity of the proximal aspect of these stems allows for adjustment of leg length and offset and alteration of version, which is helpful when bone loss involves the trochanteric profile and it become challenging to judge the version.
Titanium modular fluted tapered stems have shown excellent survivorship and outcomes [11-16] and have recently demonstrated superior results compared with non-modular implants, making them the implant of choice in high-grade femoral bone loss. [4,15,17-19] Pelt et al’s recent retrospective review of 75 patients with femoral bone loss who underwent revision surgery at a single institution showed good mid-term results, with significant Harris Hip Score improvements (52 +/- 14 to 86 +/- 11), at an average follow-up of 7 years.  The authors used modular splined tapered straight or porous-coated stems. All-cause failure rates were 11% and 3% for aseptic loosening, respectively.
However, modularity and press-fit components also carry the potential for complications. In the study by Pelt et al,  the authors noted a 13% intraoperative fracture rate, similar to previously reported rates (3%-18%). However, there were no cases of modular junction fracture, which have been previously described (Figure 3).
Figure 3. Modular stem with junctional fracture.
As the severity of bone loss increases to Type 3B, revision options become increasingly restricted. Given the higher rates of failure seen with porous stems in these defects, modular fluted tapered stems have been used in cases of limited diaphyseal isthmus available for distal fixation.
Type 4 Defects
The most severe bone loss pattern, Type 4, is characterized by canal ectasia, profound femoral remodeling, and a complete lack of bone for fixation points – which means standard revision components are not usually viable options. Arthroplasty surgeons must address these challenging defects with allograft augmentation or large tumor prostheses, neither of which show comparable survivorship to other implant options.
The 2 most frequently used allograft augmentation methods are impaction grafting and allograft-prosthetic composite.
Impaction grafting relies on tight-fitting, well-impacted cancellous bone chips to convert the smooth sclerotic endosteal bone of a Type 4 lesion to a coarse cancellous bone surface on which a revision stem is cemented. Complications with this procedure are subsidence, which has been reported as high as 38% and is likely secondary to poorly impacted allograft, and intraoperative fracture from over-aggressive impaction at a reported rate of 12%. 
Despite these high complication rates, successful implantation with impaction grafting has led to relatively promising mid- to long-term outcomes, with 10-year survival rates of 98% for aseptic loosening and 84% for reoperation for any reason.  One appeal of this technique is the potential for bone stock restoration in younger patients with high-grade femoral bone loss.
Allograft-prosthesis composite is another allograft option and consists of a long-stem prosthesis cemented into a bulk femoral allograft and subsequently press-fit into the femoral canal. This requires distal preparation of both canal and allograft by conical tapering of the distal end, which allows some degree of isthmic contact. Slightly lower survival rates are associated with this implant option, ranging from 77% to 83%, with a high incidence of periprosthetic bone resorption. [22,23]
Limb salvage in the form of tumor megaprostheses is reserved for the most severe bone loss or multiple failed revisions because of the high failure rate caused by periprosthetic loosening, shortening, and recurrent dislocations.
Management of revision THA in patients with femoral bone loss can be a challenge for arthroplasty surgeons, with poorer long-term results and higher complication rates than with primary procedures. As the number of hip replacements rises, so, too, does the number of revision surgeries.
In the last 2 decades, the development of classifications of bone loss and implant systems to manage the defects have led to successful revisions, with excellent 10-year survivorship. Two frequently used classification systems are the AAOS classification  and the Paprosky classification, [2,24] with the latter being the more widely used classification. Revision components include:
- Cemented options that rely on a pre-existing, intact cement mantle
- Uncemented long-stem components that rely on diaphyseal fit for axial and rotational stability
- Impaction of cancellous or bulk allograft augmentation
- Large tumor megaprostheses
Etiology of femoral bone loss is variable, and a thorough preoperative evaluation of the patient to determine a cause is paramount.
Revision THA is not without complications. Higher grades of bone loss are associated with poorer implant survivorship and lower revision success. However, advancements in component quality and design have led to improved implant survivorship and successful revision surgeries for even the most severe cases of femoral bone loss.
Ian Duensing MD, and Jeremy M. Gililland, MD, are from the Department of Orthopaedics, University of Utah, Salt Lake City.
Dr. Duensing has no disclosures relevant to this article. Dr. Gililland has disclosed that he has an equity interest in and/or consults for OrthoGrid Systems; that he serves as a consultant for DePuy Synthes, DJO Surgical, and Smith & Nephew; and that he receives institutional research support from Zimmer Biomet.
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