Major Bone Defect Management

FIGURE 46-1. A: Preoperative radiograph of a failed TKA due to severe polyethylene wear and periprosthetic osteolysis (arrows).B: Postoperative radiograph following revision TKA using a large metaphyseal sleeve (arrow) to fill a massive osteolytic defect of the proximal tibia.

A cadaveric study by Parks and Engh⁹ suggests that cancellous structural allograft either does not revascularize, or revascularizes at a very slow rate. A segment of allograft bone supporting the implant remains as dead bone for an indefinite time interval. This dead bone is subject to microfracture if excessive physiologic loads pass through the allograft. Should microfracture occur, the allograft has no potential for healing and remodeling. The stem of the component helps to protect the allograft from microfracture.

AORI Bone Defect Classification Over the past 15 years, the Anderson Orthopaedic Research Institute (AORI) Bone Defect Classification has been used at our institution to classify revision TKA cases based on the bone deficiency encountered at revision surgery. A Type 3 defect of either the distal femur or proximal tibia is considered a major bone defect and as such is the subject of this chapter. By definition, a Type 3 bone defect is deficient bone that requires either the use of an allograft, a custom component, or a hinged implant to replace or repair much of the metaphyseal segment of either the distal femur or proximal tibia.

The extent of bone loss encountered at revision surgery often is underestimated from the radiographic appearance of osteolytic lesions. Whenever an osteolytic lesion is visible on prerevision radiographs, options for managing bone loss must be considered that compliment implant options.

Revision cases can be stratified based on the extent of bone damage encountered intraoperatively. Revision of knees with Type 3 defects are more difficult to manage and should be reported separately as these knees are more likely to result in a less satisfactory outcome. In outcome studies of revision knee arthroplasty, revisions that are managed with primary components, polyethylene insert exchanges, or patellar components should not be compared to revision procedures that necessitate bone repair/reconstruction.

Allografts—Biology, Physiology, and Biomechanical Considerations Allografts used to repair bone defects with revision TKA are primarily dead cancellous bone. The most common source of allograft bone is the retrieved femoral head from a patient undergoing total hip arthroplasty for degenerative arthritis. The recipient’s biologic response to the allograft appears to be dictated by the host response to dead bone and marrow elements as well as to the foreign body nature of an allograft. Healing of the graft is compromised by an immune response and the recruitment of inflammatory cells that block revascularization of the graft. Rapidly progressive necrosis of the graft ensues.¹⁰

The biologic response of the host bone to the necrotic cancellous bone of the allograft is to replace the trabecular bone of the allograft with new bone formation through osteoblastic activity. New bone forms in direct apposition to the dead trabeculae. This new bone formation is followed by the gradual resorption of the underlying trabeculae through osteoclastic activity.

The host biologic response to the foreign body (allograft) is encapsulation by fibrous tissue. Parks and Engh⁹ found that allograft femoral heads heal rapidly to host bone particularly at the margins of the graft in areas adjacent to vascular soft tissue. Graft union occurs across the entire graft-host junction. The main body of the graft then becomes encapsulated in dense fibrous tissue. This fibrous tissue creates a barrier that prevents further revascularization of the graft. Therefore, large structural allografts appear to remain mostly as dead bone either indefinitely or for an undetermined length of time.

Of concern as to the long-term strength of the allograft is the failure of allograft bone to revascularize and be replaced by living bone. The potential for dead trabeculae to undergo fatigue failure remains long after the reconstruction. Dead bone cannot remodel in response to increasing stress as living bone typically does. In other words, the strength of the reconstructed bone segment will not change through graft remodeling. It is important to consider the mechanical loads that the allograft must support and use a stemmed component of appropriate length and diameter to permanently offload as much stress from the graft as is possible.

The biologic response to morsellized allograft used with impaction grafting has not been validated by cadaveric retrievals. However, biopsies from areas of impaction grafting are encouraging as the grafted areas demonstrate new bone formation.¹¹ The quality of the new bone and its capabilities of supporting an implant in the long-term have not been established.

Immunologic Response. Femoral heads and other cancellous allografts contain marrow elements that remain even after meticulous graft preparation. These cells have the potential to be highly immunogenic.¹² Proper graft preparation should remove as much of the marrow elements as possible while retaining the structural integrity of the graft. The importance of removing marrow elements and spores is emphasized in the current literature.¹³ As a further precaution, the surgeon should thoroughly flush the prepared allograft with sterile solution prior to implantation. The goal of allograft sterilization processes and additional flushing of the graft by the surgeon is to diminish the recipient’s exposure to cellular elements. Although necrotic marrow elements are clearly evident in cadaveric examination of allografts, the clinical significance of the host immunologic response remains speculative.

In addition, the bone matrix consisting mostly of collagen has the potential to produce a humoral response. Anticollagen antibodies have been identified in human recipients of freeze-dried bone. The immunologic response, however, to the noncellular components of an allograft are not significant enough to result in graft rejection.¹⁴

Biomechanical Factors. Repairing a metaphyseal bone defect with cancellous bone replaces the lost bone with a material that has similar structural characteristics and mechanical strength. Intact structural bone behaves as an anisotropic material. As such, allograft bone is capable of withstanding compression loads. The graft must be properly oriented and placed such that its optimum strength is appropriate for the loads that will pass through it. By maintaining the structural integrity of the allograft, the load can be shared with the surrounding host bone.

The use of femoral heads for allograft material is supported by studies that tested the mechanical strength of the heads. Pelker and Friedlander¹⁵ reported that even when the donor patient was in their 70s, graft strength is largely maintained. In addition, the major difference between bone from males and females was related to the size of the bone, as both were equivalent in strength. Femoral heads are harvested from patients with osteoarthritis, not from patients with femoral neck fractures, thus largely eliminating most femoral heads with poor mechanical strength.

Bone harvested for allograft use is preserved by freezing at -70°C. Freezing has little effect on the strength of bone under compression, torsion, and bending loads. Freezing may alter the immunogenicity of allografts, although animal studies have failed to substantiate changes.¹⁶ Freeze drying may also be used for bone preservation but is rarely used for large structural bone grafts. Although the compression strength of bone is largely preserved with freeze drying, torsional and bending strength are adversely altered.¹⁵

Principles of Bone Banking Although allograft bone is frequently used in numerous orthopaedic procedures, quite often the surgeon and hospital personnel who are responsible for obtaining the allograft tissue are not familiar with the actual procurement and processing that occurs before the allograft is delivered on the day of surgery. To ensure the safety of the patient, it is the responsibility of the surgeon to know where the hospital obtains the allograft bone and the method by which the tissue bank processes the donor tissue.

The American Association of Tissue Banks (AATB), founded in 1976, is the national standard-setting organization for tissue banks (www.aatb.org). The AATB began to provide certification in 1986 to those tissue banks which voluntarily pursue this accreditation. The AATB developed Standards for Tissue Banking that the tissue bank must follow if the bank is to become accredited in the retrieval, processing, storage, and/or distribution of tissues. AATB accreditation extends for a 3-year period after which the tissue bank must be reinspected in order to continue its certification. In 2002, approximately 10% of musculoskeletal allografts were processed by banks not accredited by the AATB.¹⁷ At the present time, not all tissue banks are required to be accredited by the AATB.

Tissue banks with AATB accreditation that follow the Standards for Tissue Banking are similar in their donor screening process. Specially trained personnel will review the potential donor’s medical history to screen for infections, cancer, autoimmune, and some neurologic disorders. The donor’s family is interviewed to determine if there is any history of drug or alcohol abuse, or any other high-risk behavior that would prohibit the safe use of the donor tissue. Most bone banks accredited by the AATB, recommend that the age of the potential donors is between 12 and 70 years. Age limitations are set to ensure the maximum weight-bearing potential and the structural integrity of the graft.

Once a potential donor has been initially screened, the medical records are reviewed also by the medical director or the director of technical operations of the facility. If the donor tissue is deemed acceptable for processing, the tissues are recovered in an aseptic environment in accordance with the AATB recommendations. Donor tissue often is recovered by an organ procurement organization (OPO) and subsequently processed by another organization.

Currently, most tissue banks aseptically process the donor tissues. This means that the bone is processed in clean rooms (similar to the sterile setting in the operating room) with no contamination of the allograft during the processing. Sometimes the tissue processor labels the allograft as “sterile.” However, this is a dangerous assumption. The Center for Disease Control (CDC) reports that aseptic processing minimizes bacterial contamination but does not eradicate contamination from organisms or spores originating from the donor.¹⁸ In 2003, a case of Streptococcus pyogenes after allograft implantation was reported to the CDC.¹⁹

When a patient died as the result of Clostridium sordellii sepsis after allograft implantation,¹⁸ the CDC launched an investigation into the processing of the contaminated allograft. As of March 2002, the CDC had identified 26 cases of bacterial infections associated with musculoskeletal tissue allografts.²⁰ Subsequently, this investigation led to a report published in the New England Journal of Medicine of 14 patients in whom a Clostridium infection was traced to allograft implantation.¹³ The contaminated allografts were traced to one tissue bank. The tissue bank aseptically processed the allograft tissue and used antimicrobial solutions to reduce the risk of contamination by organisms and spores. Cultures were reported negative and the tissue was released for implantation.

Further examination of the tissue bank procedures revealed that the tissues were not cultured prior to placement in the antimicrobial solution. Residual antimicrobial solution may have led to the “negative” culture results that were used by the tissue bank in the screening for quality assurance. At the time of the report, current regulations did not require tissue banks to eliminate bacteria and spores present on the tissue at the time of recovery or to use processes that guaranteed sterility of the graft.¹³ These cases have heightened awareness of validating tissue-processing procedures.

Kainer et al.¹³ and the CDC¹⁹ have made recommendations to reduce the risk of allograft-associated infections. These recommendations for tissue processors include implementing a process for killing bacterial spores, and validating quality assurance methods for culturing tissues to avoid the risk of “false” negative results.¹³ They also caution that great care must be taken in the processing and culturing of fresh allografts, as sporicidal agents may not be suitable for use with these tissues. Unless a sporicidal method is used to kill bacteria, the tissue should not be considered sterile, and the Health Care providers and patients should be informed as to the possibility of bacterial infection. Additional recommendations include validation of tissue bank procedures and enhanced accountability with regard to the release of donor tissues for implantation.^13,17,19

After the CDC investigation,²⁰ the FDA released a Guidance Document for Tissue Banks that requires tissue processors to validate their processing and testing methods.²¹ As a result, the tissue banking industry is taking further steps to assure the safety and sterility of donor tissues. Methods of removing marrow elements, blood lipids, and endogenous materials are being developed, patented, and validated. At the present time, Regeneration Technologies, Inc. (RTI) has a patented sterilization technology (BioCleanse), which eliminates the risk of donor-to-recipient disease transmission while preserving biocompatibility and tissue strength.¹³ The Musculoskeletal Transplant Foundation (MTF) and LifeNet tissue banks are also validating their sterilization processes, the Allograft Tissue Purification Process and Allowash, respectively. We anticipate that allografts sterilized by these and similar processes will be used routinely.

Although the procedure by which the donor tissue is processed varies with the facility, it is important to use a tissue bank that is accredited by the AATB and follows the FDA regulations in the testing and processing of tissues. Tissue banks that are not licensed or accredited are not required to comply with external quality control requirements beyond screening donors for HIV and hepatitis.^18,22

MANAGEMENT OF TYPE 3 DEFECTS WITH LARGE STRUCTURAL ALLOGRAFTS

Preoperative Planning Osteolytic bone defects are often either unrecognized or underestimated by the interpreter of plain radiographs. Whenever planning a revision TKA because of implant wear, the surgeon should anticipate a bone defect. Osteolytic lesions may be difficult to identify, particularly with implants that substitute for the posterior cruciate ligament (PCL). In the PCL-substituting implant, the femoral box that accommodates the post of the insert may obscure a defect that is present in either femoral condyle. In the knee with a posterior cruciate-retaining component, a defect may not be evident on the lateral radiograph as the bone of the opposite condyle may mask the lesion. Defects that are caused by debris-generated osteolysis usually have a radiodense line of demarcation at the junction of the defect and the surrounding host bone. Despite the presence of large osteolytic lesions, the implant usually is not loose.

Aseptic component loosening with implant migration or multiple revision operations often result in bone loss. On the femoral side, joint line elevation can be recognized by locating the position of the femoral component relative to the epicondyles, and on the tibial side, by locating the tibial component relative to the fibular head and the tibial tubercle. Modular augments are not always suitable for managing defects that result from severe component migration. Allograft material should be available.

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