**8. Future perspectives**

We verified that treatment of bone defects associated with knee prosthetic surgery include many different surgical techniques and options, each with specic disadvantages and complications. Generally, in the elderly bone loss should be accomplished using artificial materials, while in younger patients, therapy should be addressed to regeneration of new bone as a foundation for future revision procedures. So that, an ideal bone substitute should possesses osteogenic, osteoinductive and osteoconductive potential.

In revision total knee arthroplasty, the mechanical stability of the femoral and tibial components is increased by the addition of intramedullary stems. Whatever solution is indicating to compensate bone loss, the use of an extended femoral and tibial stems reduce the stress forces on the metaphyseal region and bone-implant interface. Stem extensions, with or without offset, can supplement fixation, decrease stress at the bone-implant interface, and help address asymmetric bone defects. Offset stems can assist with implant alignment on the metaphysis, reducing the incidence of coronal or sagittal components malalignment and helping balance the flexion and extension spaces by effectively

The stem ability to protect the proximal tibia or distal femur has been demonstrated in a laboratory setting using finite element model and cadaver models. Brooks et al used stems in conjunction with various bone augmentation techniques for defect in the proximal tibia: they suggested that a 70-mm stem carried from 23% to 38% of the axial load (Brooks

A finite element analysis has revealed that the predicted bone loss is even greater in stemmed components compared to stemless ones; this may have consequences to discouraged routine use of stems in revision TKA (van Lenthe et al., 2002). Nevertheless Stern and Insall (Stern & Insall, 1992) advocates routine use of stemmed components in revision TKA. Engh et al used femoral stems mainly to protect large structural grafts in revision TKA (Engh et al., 1997). Meneghini et al. advise the use of cemented stemmed extension to maximize early implant xation and allow for successful biologic ingrowth of the TM cones into the remaining part of the bone (Meneghini et al., 2008). We recommend to add stem extensions when using metal augments to decrease stress at the bone-implant

Cemented stems allow for intraoperative adjustment with unusual anatomy and achieve fixation in large canals and osteopenic bone (Murray et al., 1994). The main disadvantages are that they are difficult to remove if revision is necessary and since they are not canal filling, they do not guarantee alignment (Parsley et al., 2003). Cementless press-fit stem extensions are easy to use and facilitate component alignment, and diaphyseal engaging stems ensure fixation (Radnay & Scuderi, 2006). In our practice with stemmed components of TKA, we prefer to use hybrid fixation in both femur and tibia, with proximal cementation just in the metaphyseal area as usual. In fact, Jazrawi et al. demonstrated in a cadaveric study that a press-fit modular cementless stem could achieve equivalent stability than a somewhat shorter fully cemented stem (Jazrawi et al., 2001), whereas Albrektsson et al. showed with radiostereogrammetic analysis that a long cementless stem provide optional

We verified that treatment of bone defects associated with knee prosthetic surgery include many different surgical techniques and options, each with specic disadvantages and complications. Generally, in the elderly bone loss should be accomplished using artificial materials, while in younger patients, therapy should be addressed to regeneration of new bone as a foundation for future revision procedures. So that, an ideal bone substitute should

possesses osteogenic, osteoinductive and osteoconductive potential.

**7. Intramedullary stems** 

translating the components.

stability (Albrektsson et al., 1990).

**8. Future perspectives** 

et al., 1984).

interface.

The ''osteogenic'' potential of the graft corresponds to capacity of cells living within the donor graft to survive during transplantation, then proliferate and differentiate to osteblasts and eventually to osteocytes. ''Osteoinduction'' on the other hand is the stimulation and activation of host mesenchymal stem cells from the surrounding tissue, which differentiate into bone-forming osteoblasts. This process is mediated by a cascade of signals and the activations of several extra and intracellular receptors the most important of which belong to the TGF-beta family (Cypher & Grossman, 1996). "Osteoconduction" describes the facilitation and orientation of blood-vessel and the creation of the new Haversian systems into the bone scaffold. Finally these three properties together allow ''osteointegration'' between the host bone and the grafting material surfaces (Giannoudis et al., 2005).

The gold standard for regeneration of new bone is autologous bone graft, which contains a scaffold, osteoblasts, and the necessary signalling proteins and molecules. However, autograft is of limited availability and may be insufficient due to poor quality (eg, osteoporosis). Furthermore, it may fail in clinical practice as most of the cellular (osteogenic) elements do not survive transplantion (Sandhu et al., 1999).

Thus alternatives to autograft bone have emerged. Perhaps the most common bone substitute is cancellous allograft, which is osteoconductive only, and rely on a viable vascularized bone bed for incorporation. Moreover bone graft provided by musculoskeletal tissue bank could provoke immune response and transmission of viral disease; the processing of allograft tissue lowers this risk but can signicantly weaken the biologic and mechanical properties initially present in the bone tissue (Giannoudis et al., 2005).

Bone substitutes could be used to replenish lost bone stock during total knee arthroplasty. A bone-graft substitute to be useful should be: osteoconductive, osteoinductive, biocompatible, bio-resorbable, structurally similar to bone, easy to use, and cost-effective (Giannoudis et al., 2005).

Recombinant growth factors such as bone morphogenetic proteins (BMPs) have osteoinductive capacity (Greenwald et al., 2001). Nevertheless because they are powerful in small amounts, and they are expensive, their indication in knee prosthetic surgery is still limited. Contrary platelet rich plasma (PRP) is largely available as it is prepared from centrifugation of autologous blood; it is an osteopromotive adjunct with the ability to enhance natural bone formation by stimulatory signals. Both BMPs and PRP need a scaffold to support their bone regeneration properties.

Demineralized bone matrix (DBM) corresponds to portion of bone without the mineral phases, extracted by strong acids (Peterson et al., 2004). The demineralization process leaves behind the growth factors, the noncollagenous proteins and collagen, and therefore DBM is osteoconductive and osteoinductive. However, few prospective, randomized clinical studies delineate the efficacy of DBM, and the material can be costly.

Nowadays many synthetic substitutes are available, used either alone or in combination with other biologic adjuncts. Synthetic bone graft materials available include calcium sulphates, special glass ceramics (bioactive glasses) and calcium phosphates. Calcium phosphate-based implants have the most similar composition to human bone, in particular those made of hydroxyapatite (HA) (Paderni et al., 2009). HA-based substitutes provide an osteoconductive scaffold to which mesenchymal stem cells and osteoinductive growth factors can migrate and differentiate into functioning osteoblasts (Fujishiro et al., 2005). These materials can be sterilized and are moldable, but generally do not have sufficient mechanical properties to support full immediate weight bearing.

Management of Bone Loss in Primary and Revision Knee Replacement Surgery 407

prosthetic augment, it is imperative to use stemmed components to transfer stress away from the joint line. Reestablishment of well-aligned and stable implants is necessary for successful reconstruction, but this can't be accomplished without a sufficient restoration of

The authors did not receive any outside funding or grants in support of their research or for

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an eventual bone loss.

**10. Acknowledgment** 

preparation of this work.

**11. References** 

We thank Mrs Mariapia Cumani for graphical support.

1990), pp. 252-8, ISSN 0301-620X

12, ISSN 1528-1132

89, ISSN 1873-2763

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7817-3247-6, Philadelphia, PA

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Despite of being osteoconductive, the HA is not osteoinductive: in order to achieve a satisfying bone ingrowth, further factors could be added, such as multipotential stromal stem cells or PRP growth factors (Boyde et al., 1999), in order to favour and accelerate bone regeneration. Nowadays, combination of mature stromal cells and HA-based scaffolds represent an useful tissue regeneration approach to be used in a clinical setting: unfortunately, only few papers describe the clinical use of massive graft made of HA-based scaffolds (Marcacci et al., 2007) and long-term comparative studies are needed to evaluate their clinical effectiveness (Table 5).


Table 5. Different materials available as bone substitutes and their respective biologic properties.

### **9. Conclusions**

As the incidence of primary and revision total knee arthroplasty will continue to increase, proper management of femoral and tibial bone loss represents a common situation that have to be faced by the orthopedic surgeon. The choice between different surgical options depends on dimension and characteristics of bone defect but are also patient-related.

Whatever technique is used in the management of bone loss during knee arthroplasty, certain fundamentals must be applied and the remaining bone structure will guide treatment. For treatment of any periarticular defect requiring more than a minimal prosthetic augment, it is imperative to use stemmed components to transfer stress away from the joint line. Reestablishment of well-aligned and stable implants is necessary for successful reconstruction, but this can't be accomplished without a sufficient restoration of an eventual bone loss.
