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What's New in Orthopaedic Research

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Specialty Update has been developed in collaboration with the Council of Musculoskeletal SpecialtySocieties (COMSS) of the American Academy of Orthopaedic Surgeons.

Specialty Update

What’s New in Orthopaedic Research

BY SUZANNE A. MAHER, PHD, CHISA HIDAKA, MD, MATTHEW E. CUNNINGHAM, MD, PHD, AND SCOTT A. RODEO, MD

To address the challenges of translating discoveries from the basic sciences into clinical therapeutics, the National Institutes of Health recently launched a new program of institutional Clinical and Translational Science Awards (http://nihroad-map.nih.gov/). Designed to foster environments that can effi-ciently translate laboratory findings into clinical therapies, the program aims to create educational environments specifically for the purposes of training a new generation of clinician-researcher interdisciplinary teams. In the orthopaedic sci-ences, funding to help combine expertise from surgeons, re-searchers in the life sciences, and engineers should lead to an acceleration in the already exciting advances that have been made in molecular medicine and regenerative medicine.

The most recent developments in the basic, transla-tional, and applied sciences in orthopaedics are reviewed in this article. Based in part on information presented at the 2006 annual meetings of the American Academy of Ortho-paedic Surgeons (AAOS) and the Orthopaedic Research Soci-ety (ORS), the significant advances made in understanding the mechanisms of tissue degradation and in applying this knowledge to the development of molecular therapies for tis-sue repair are outlined. Novel developments in total joint ar-throplasty and recent progress in understanding the plasticity of stem cells and in applying this knowledge to the design of constructs to repair tissue are also described.

Mechanisms of Cartilage and Tendon DegenerationSeveral studies have begun to elucidate the mechanisms that underlie degenerative processes in soft tissues such as cartilage and tendon. These studies have improved our understanding of how factors such as mechanical stress, inflammatory fac-tors, degradative enzymes, and genetics may converge, re-sulting in the clinical manifestations that are commonly encountered in overuse syndromes and/or osteoarthritis. They also provide important new insight into potential novel tar-gets for therapeutic intervention.

Two recent studies demonstrated the critical importance of a relatively new family of matrix-degrading enzymes, the ADAMTS (a disintegrin and metalloprotease with thrombo-spondin-like repeat), in the pathogenesis of degenerative and inflammatory arthritides. First discovered in 1997, the AD-AMTS family of proteases includes approximately nineteen members, including aggrecanase-1 and 2, also known as AD-AMTS-4 and 5, respectively. Observations that the synovial fluid of arthritic patients contained aggrecan cleavage prod-ucts whose sequence indicated specific enzyme cleavage by proteases other than the other known matrix metalloprotein-ases led to the initial discovery of the “aggrecanases.” Subse-quent studies revealed that these “aggrecanases” were in fact ADAMTS-4 and 5. Two studies from independent laboratories showed that, at least in mice, the deletion of ADAMTS-5 is specifically protective in experimental models of arthritis. Glasson et al.1 reported that mice lacking ADAMTS-5 (AD-AMTS-5-/-) had development of significantly less severe arthri-tis than genetically normal (wild-type) control mice or ADAMTS-4-/- mice. Arthritis was assessed by means of histo-morphometry four or eight weeks after surgical transsection of the medial meniscotibial ligament. Prior to surgery, at the age of eighteen weeks, the ADAMTS-5-/- mice had no ob-servable skeletal abnormalities, suggesting that development occurred normally and that the observed arthritis did not oc-cur as the result of a developmental predisposition. Similar studies in ADAMTS-4-/- and ADAMTS-5-/- mice by Stanton et al.2 showed the protective effect of specifically deleting ADAMTS-5 in in vitro and in vivo models of inflammatory arthritis in which interleukin-1 (IL-1) was used to induce car-tilage degradation. Those studies strongly supported the pos-sibility that specific blockade of ADAMTS-5 may be effective for treating osteoarthritis and/or rheumatoid arthritis.

Some questions regarding the role of ADAMTS-4 and 5 in human arthritides remain, however. For example, the ADAMTS-5 protein has not been found in mouse cartilage, al-

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though the messenger ribonucleic acid (mRNA) of ADAMTS-5 as well as specific cleavage products produced by the enzyme are easily detected1. Furthermore, in human cartilage explants, ADAMTS-4 is the enzyme that is induced in chondrocytes stimulated by factors such as IL-1 or tumor necrosis factor (TNF), which are known to stimulate matrix degradation by chondrocytes (chondrolysis). In contrast, ADAMTS-5 is con-stitutively active. Despite these questions, it is clear that the ADAMTS family of matrix degradation enzymes will likely be important targets for therapeutic blockade in the treatment of arthritis.

Investigators studying tendon abnormalities have fo-cused on the cellular and molecular mechanisms of tendon re-sponse to both stress deprivation and stress overload. These studies have implications for improving our understanding of the basic mechanism (or mechanisms) underlying tendon de-generation. Particular attention has been focused on the role of mechanical stress on the development of tendinopathy (simulating clinical overuse injuries). Investigators in the lab-oratory of Steve Arnoczky, DVM, at Michigan State Univer-sity, are studying the response of tendon cells to strain and their interaction with the pericellular matrix. This group re-cently reported upregulation of integrin α1 and α2 expression following stress deprivation in rat tail tendon3. Integrins are cell-surface receptors that mediate cell-matrix interactions. The changes in integrin expression were accompanied by loss of contact between tendon cells and their pericellular matrix. These findings shed further light on the mechanisms by which tendon cells sense strain in their microenvironment and how tendon cells interact with and regulate the surrounding ma-trix. Tendon explants that were subjected to excessive levels of cyclic loading (up to 18 MPa) released increased levels of col-lagenase and the inflammatory mediator prostaglandin E2 (PGE2)4. However, the role of inflammation in the develop-ment of tendinosis is unclear as inflammatory cells are rarely seen in biopsy specimens of degenerative human tendon. There is evidence that mechanical loading can even have an anti-inflammatory effect on cells cultured under “inflamma-tory conditions” (such as in the presence of the inflammatory cytokine IL-1)5. Additional studies are required to elucidate the complex interactions between mechanical load and cellu-lar responses.

Although explant models allow precise control of load-ing conditions, improved understanding of tendon overuse injury will come from animal models that replicate the typical microstructural changes seen in tendinosis. A rat model has been developed to apply repetitive, controlled loading of the patellar tendon in vivo, resulting in a significant loss of me-chanical properties (tendon modulus and failure stress) and histological changes consistent with tendinosis6. Models such as this one will be useful for examining the cellular repair re-sponses to subfailure matrix damage. In a rat model of over-use injury of the supraspinatus tendon that was used to study microstructural and biomechanical changes that occur sec-

ondary to overuse, there was increased expression of genes that are highly expressed in cartilage in these animals, includ-ing type-II collagen and aggrecan7. Increased accumulation of type-III collagen protein also occurred in this overuse model. Those findings were supported by data from biopsies of de-generative Achilles tendon specimens that showed increased levels of aggrecan and biglycan, indicating that there is in-creased compression or shear stress in the abnormal tendon8.

Molecular MedicineAdvances in understanding the molecular mechanisms of tis-sue degradation have contributed to new attempts to develop targeted therapeutics aimed at eliciting a reparative response from within musculoskeletal tissues. A burgeoning technology in the area of molecular medicine is the use of antisense ribo-nucleic acid (RNA) strategies for therapeutic gene silencing. In the past decade, RNA has been shown to have functions far beyond its role in providing a template for protein transcrip-tion. Specifically, RNAs have been shown to inhibit or modify gene expression and immunological reactions through their interactions with deoxyribonucleic acid (DNA) or other RNAs or through their ability to act as autocatalytic enzymes (ri-bozymes). The use of RNA to silence specific genes has been used to address many disease processes, including cancer and inflammation. One RNA product is clinically available for the treatment of retinitis, and others are in development. A recent study showed the effectiveness of this strategy in suppressing inflammation in a rodent model of rheumatoid arthritis. In addition to being a treatment strategy, the use of RNA for gene silencing is rapidly becoming established as a powerful tool for drug discovery.

A number of RNA-based strategies, including strategies involving oligodeoxynucleotides, RNA interference, and ri-bozymes, are currently used for gene silencing. The oligode-oxynucleotides are “antisense” DNAs that are the mirror-image sequence of RNAs that encode proteins, or “sense RNAs.” Because of Watson-Crick binding, antisense oligode-oxynucleotides are able to bind specifically to their “sense” targets, forming double stranded DNA-RNA complexes that prevent association with ribosomes, and, therefore, protein translation, through steric effects. RNA interference (RNAi) occurs when double stranded RNAs (dsRNAs) are processed in the cells by specific exonuclease enzymes known as Dicers within a complex called the RNA-induced silencing complex (RISC) that cleave dsRNAs into 21-to-24-nucleotide single stranded RNA (ssRNA) molecules known as small interfering RNAs (siRNAs). The siRNAs, in turn, can bind target normal messenger RNAs encoding specific genes of interest, inhibit-ing normal protein translation. This type of gene silencing can be accomplished either by delivering dsRNAs or 21-to-24-mer siRNAs that mimic those produced by Dicer/RISC cleavage. Finally, ribozymes are RNAs that have both a com-plementary component that can recognize and bind to spe-cific mRNA targets and an autocatalytic component that, on

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target binding, can cleave the target RNAs into fragments that are not functional.

As with many other molecular medicine strategies, de-livery of the nucleic acids to the target tissue remains a diffi-cult challenge in the development of these strategies for clinical use. The oligodeoxynucleotides and dsRNAs require modifications to facilitate entry across the cell membrane as well as survival in the in vivo milieu that normally favors rapid degradation of free nucleic acids. As opposed to oligodeoxy-nucleotides and dsRNAs, siRNAs and ribozymes can be intro-duced to and expressed in target cells through gene transfer with use of currently available vectors such as a retrovirus or adenovirus. The specificity of gene targeting is another area of concern when RNA antisense strategies are employed.

Despite these technical challenges, RNA-mediated gene silencing is a promising area for molecular therapy and drug discovery. In a recent study, Inoue et al.9 silenced the expres-sion of tumor necrosis factor-α (TNF-α) in the joint by means of intra-articular injection of polyamine-conjugated siRNA followed by electroporation by means of application of an electric pulse generator on the skin around the knee joint in a collagen-induced model of arthritis in rats. A significant de-crease in paw swelling was achieved in association with siRNA injection on days 3, 7, 13, and 16 after the induction of arthri-tis, although injections on days 7 and 10 alone were not effec-tive. Decreased swelling correlated with improved histological scores. Reverse transcription polymerase chain reaction was used to confirm the ablation of TNF-α expression in the syn-ovial tissue. Four different sequences of anti-TNF-α siRNA were compared. While all were effective, one particular se-quence was most effective, underscoring the importance of se-quence specificity when using this strategy.

Localized gene delivery clearly remains an important goal, whether for gene silencing or overexpression. To this end, Maloney et al.10 recently reported on an innovative method that results in the effective transfer and expression of genes in a particularly challenging tissue: articular cartilage. In that study, from the laboratory of Edward Schwarz, PhD, at the University of Rochester, ultraviolet (UV) light was used to activate the expression of marker genes (green fluores-cent protein [GFP] or β-galactosidase [LacZ]) in articular cartilage defects in rabbits with use of recombinant adeno-associated virus (rAAV)-mediated gene transfer. The use of ultraviolet light enhanced the level of rAAV transgene expres-sion, which is often otherwise limited by the relative ineffi-ciency of target cells to convert the single stranded AAV DNA into the double stranded form normally used by mammalian cells. Additionally, it ensured a very specific localization of gene expression that was limited to the area of ultraviolet light treatment. The study showed that the specific spectrum of ul-traviolet light used (UV-A at fluencies of <6000 J/m2) did not induce DNA damage but resulted in a temporary induction of reactive oxygen species. Effective gene transfer with use of this method was shown in human chondrocytes and synovial fi-

broblasts in culture as well as in the superficial and middle zones of articular cartilage in rabbits. That study presented the intriguing possibility that rAAV vectors, in conjunction with ultraviolet light, could be used to silence or overexpress thera-peutic genes within articular cartilage tissue with use of tech-niques that are highly compatible with clinical arthroscopic methods.

In summary, regardless of whether siRNA technology can be developed to be clinically applicable, it will clearly sup-port the development of gene-based medicine. The ability to locally deliver genes and to silence candidate genes efficiently and specifically will enable researchers to identify novel gene targets for effective therapeutic intervention.

Regenerative MedicineScaffoldsRegenerative medicine has been described as being “at the in-terface of the medical implant industry and the biological rev-olution.”11 For the orthopaedic sciences, regenerative medicine encompasses efforts to develop a tissue capable of withstand-ing physiological loads by controlling a combination of scaf-fold design features, cellular phenotype, and biological/mechanical stimulation.

The effect of material type, morphology, and biological factors on matrix generation within cell-seeded scaffolds con-tinues to be a central research theme in musculoskeletal tissue engineering. Recent technological advances that enable com-ponents to be manufactured with controllable and well de-fined structural morphologies in the nanometer (10-9 m) range present a unique opportunity to optimize scaffolds for the purposes of tissue engineering at a scale that was previ-ously unimaginable. As summarized in a symposium on “Nano-Technologies” held at the recent annual meeting of the ORS, geometric features such as fiber diameters and grain size, biological features such as the distribution and density of ligands incorporated within scaffolds, and scaffold mechanical characteristics are powerful modulators of cellular response. For example, by mimicking collagen fibril diameters (in the 60-nm range) in synthetic scaffolds, robust cellular adhesion and extracellular matrix generation of chondrocytes12 and preosteoblasts13 have been demonstrated. However, under-standing cell-scaffold interactions is vital for optimizing in vivo scaffold performance. Recently, fluorescence resonance energy transfer imaging techniques were used to monitor the movement of adhesion peptides by preosteoblasts seeded onto alginate gels, and the data suggested that cellular proliferation and differentiation are regulated in part by the traction forces exerted by the cells on the adhesion ligands14.

The importance of the meniscus in the preservation of knee cartilage has been well established, and recent efforts have focused on developing novel biomaterials for the pur-poses of meniscal regeneration and repair. Tienen et al.15, for example, developed a biodegradable porous polymer implant made of polycaprolactone-polyurethane (PCLPU) that is geo-

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metrically similar to that of the native meniscus and serves as a scaffold to support meniscal tissue formation in vivo. Pre-vious studies in their laboratory determined the optimal po-rosity and compression modulus. Although the implant supported the formation of fibrocartilaginous tissue, it did not prevent progressive articular cartilage degradation in a dog lateral meniscectomy model. Another group used a sheep model to evaluate a porous composite of polycaprolactone and hyaluronic acid as a meniscal scaffold16. This material sup-ported neotissue formation and was well integrated with the joint capsule. However, giant cells accumulated throughout the implant. Alginate is another material that is being evalu-ated for meniscal tissue engineering. Meniscal fibrochondro-cytes can be suspended in alginate and can produce glycosaminoglycan when cultured in vitro17. Improvements in material properties will require mechanical stimulation of the construct in vitro prior to implantation.

Stem CellsThe use of “stem-like” or progenitor cells with a large capac-ity for cellular proliferation as well as plasticity and multilin-eage differentiation continues to be a compelling area of research for regenerative medicine and tissue engineering. A great deal of controversy has surrounded research employing embryonic stem cells in the past several years, but the fervor over the issue is well deserved because of the tremendous po-tential of these special primordial cells to deliver tissue repair and healing where healing potential has been lost or im-paired. Early efforts to assess and describe cell fates that are possible with human embryonic stem cells demonstrated that embryonic stem cells required differentiation queues and guidance toward a particular tissue lineage (e.g., bone, cartilage, muscle, etc.) to be utilized for medical applications. Furthermore, it has become apparent that the population of treated human embryonic stem cells cannot be assumed to be uniform with regard to their differentiation responsiveness to specific treatments.

This concept of responsive subpopulations has been ad-dressed in several studies in which fluorescence activated cell sorting (FACS) has been employed to purify the subpopula-tion of interest. Barberi et al.18 reported mesenchymal differ-entiation of human embryonic stem cells with use of a co-culturing paradigm in tissue culture. They found that after forty days of co-culture of human embryonic stem cells on mouse-derived cells, approximately 5% of the human embry-onic stem cells were positive for CD73 (an immunological mesenchymal marker) and could be efficiently separated from the rest of the nonresponsive population. Once separated, this mesenchymal subpopulation could be differentiated into cells with chondrocyte-like or osteoblast-like phenotypes. Al-though a great deal of progress has been made toward explor-ing the potential application of human embryonic stem cells, the true complexity of utilizing these cells for the purposes of repairing tissues is only emerging. Research on mouse embry-

onic stem cells from the laboratory of Eric Lander at the Whitehead Institute recently revealed critical aspects of the mechanism by which embryonic stem cells differentiate19. That study described two specific patterns of DNA modifica-tion—specifically, two different patterns of histone methyla-tion—in which one pattern resulted in gene silencing and another resulted in a looser state of “protection” that allowed gene expression much more readily. In contrast to mature cells, where these patterns were discretely arranged, stem cells showed a “bivalent pattern” with both types of modifications overlapping. More important, this “bivalent pattern” occurred specifically at highly conserved sites within DNA, many of which encode for transcription factors that are important de-velopmentally (so-called master genes). As such, that study suggested that in stem cells, but not in mature cells, master genes that determine the fate of a cell are in a “bivalent” state of being silenced and yet also poised for expression.

Stem cells have been discovered in several adult tissues, including adipose tissue, muscle, and bone marrow, and are thought to represent a dormant reservoir of cells that can be called on to repopulate or repair these tissues as needed while avoiding the ethical issues of utilizing human embryonic stem cells. Bone marrow-derived stem cells, also referred to as bone marrow mesenchymal cells or marrow stromal cells, have re-ceived a great deal of attention because of their ability to offer regenerative potential for multiple different tissues, including bone, blood, heart, kidney, fat, inner ear, and skin. The ability of murine bone marrow stem cells to differentiate down a skeletal muscle phenotype pathway via signaling through the canonical Wnt/beta-catenin pathway and to differentiate to-ward a cardiac muscle pathway via signaling through the non-canonical Wnt/Ca2+ pathway was demonstrated by Bedada et al.20. Those authors also found that treatments with fibroblast growth factor-2 or hepatocyte growth factor led to cell pheno-type changes consistent with the adoption of neuron-like or hepatocyte-like differentiation states, respectively.

Minguell et al.21 attempted to better explain the cellular mechanisms underpinning the multipotentiality of human bone marrow mesenchymal stem cells by examining the na-ture of uncommitted precursors. They found that human bone marrow mesenchymal stem cells, when placed in culture, could be divided grossly into two groups, uncommitted and committed, as determined by their morphology and division rate. Uncommitted human bone marrow mesenchymal stem cells were spindly and divided slowly, whereas committed cells showed more abundant cytoplasm and faster division rates (6% compared with 27% in S or G2/M, respectively). Both cell populations stained positively with markers for mesenchymal phenotypes (alpha-smooth muscle actin, beta actin, vimen-tin), as well as osteoblastic phenotypes (Cbfa1 and Msx-2) and chondrocytic phenotypes (Sox-9). Differences appeared in the cellular localization of the markers; specifically, undif-ferentiated cells had Msx-2 and Sox-9 in the nucleus only, whereas committed cells had these factors both in the nucleus

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and in the cytoplasm. It was suggested that the altered local-ization of the factors allowed their activation in the cytoplasm, resulting in further progress toward a differentiated pheno-type. Perhaps more dramatically, the myogenic marker Myf-5 and neuronal markers NeuroD, beta III-tubulin, and NeuN were only detected in committed cells, whereas myogenic markers desmin and MyoD and neuronal marker nestin were detected in both cell populations. Collectively, these observa-tions imply that the resting “uncommitted” human bone mar-row mesenchymal stem cells express markers for each of the different potential tissue lineages, despite their being obtained from bone marrow, and that when they become “committed” and begin dividing, their phenotypes are altered to make them more receptive to differentiation signals.

Other issues related to bone marrow stem cells are the stability of the cells when placed in culture for expansion and the effect that the age of the donor has on the quality and character of the stem cells that are obtained. Mareschi et al.22 found that donors who were less than eighteen years old had marrow with a significantly increased population doubling (growth rate) as compared with donors who were more than eighteen years old (p < 0.05). However, the numbers of cellu-lar passages possible, the immunological phenotypes, and the stability of telomeres with the cells from the two donor pools were similar. These findings indicate that bone marrow stem cells can be isolated from both pediatric and adult popula-tions, stably expanded, and potentially utilized for therapeu-tic interventions.

Several studies that were presented at the annual meeting of the ORS demonstrated the existence of cells (specifically, cells in tendon and ligament) with multilineage differentiation potential. Traditionally, it has been assumed that differentiated cells in tendon and ligament have a stable phenotype. However, recent work has shown that cells from these tissues have the potential to change phenotype, de-pending on environmental factors. This would have impor-tant implications for ligament and tendon healing as well as for understanding the development of the structural and metabolic changes seen in tendon degeneration. Lee et al. reported that cells derived from synovial fluid in knees with anterior cruciate ligament injury can differentiate into os-teoblasts, adipocytes, and chondrocytes23. Steinert et al. re-ported that cells derived from culture specimens of the anterior cruciate ligament obtained at the time of anterior cruciate ligament reconstructive surgery also have multilin-eage differentiation potential24. In that study, the investiga-tors were careful to remove the synovial covering over the torn anterior cruciate ligament in order to obtain anterior cruciate ligament cells; nonetheless, the possibility remains that synovial-derived cells had infiltrated the torn anterior cruciate ligament after injury. De Mos et al. reported that cells derived from human tendon also have multilineage dif-ferentiation potential25, which may explain the findings of increased glycosaminoglycan deposition, calcifications, and

lipid accumulation in degenerative tendon.Investigators in the laboratory of Rocky Tuan, PhD, at

the National Institutes of Health, examined the multipotenti-ality of meniscal fibrochondrocytes derived from different re-gions of the meniscus (the outer vascular area, the inner avascular area, and the horn attachment area)26. Those investi-gators reported that meniscal cells from all regions have a multilineage differentiation potential, with cells from the outer region of the meniscus having a greater differentiation range. The more limited differentiation range of cells from the inner, avascular area of the meniscus likely contributes to the poorer healing potential in this area. Additional support for the potential of stem cells to improve meniscal healing was provided by a study involving the injection of synovium-derived stem cells labeled with green fluorescent protein into the knees of wild-type rats that had a full-thickness defect in the meniscus27. The transplanted cells remained in the menis-cal defect and promoted healing in comparison with un-treated meniscal defects. Platelet-rich plasma is a rich source of autologous growth factors that stimulates DNA synthesis and extracellular matrix protein synthesis in meniscal cells and promotes healing in vivo in animal models.

Regenerative Medicine in the Clinical SettingDuring a symposium on tissue engineering that was held at the combined Research Day of the ORS and AAOS, the pre-liminary results of a phase-II clinical trial for the treatment of long-bone fractures with allogenic stromal cells were pre-sented by Matthew Jimenez, MD. The study group included six patients with atrophic tibial nonunions. Cells were isolated from patient aspirates and were expanded with use of pat-ented processes (Tissue Repair Cells; Aastrom Biosciences, Ann Arbor, Michigan). The cells were suspended in an elec-trolyte solution, combined with human serum albumin, and mixed with demineralized corticocancellous allograft (Muscu-loskeletal Transplant Foundation, Edison, New Jersey). When treatment with these cells was used in combination with in-ternal fixation, osseous healing was found in all six patients within six months. Although the study was not a randomized, prospective clinical trial and the number of patients was lim-ited, the study nonetheless represented an important advance in the use of multipotential cells to heal nonunions.

Hernigou et al.28 percutaneously injected progenitor cells isolated from bone marrow aspirates into tibial nonunion sites and found healing (defined as definite radiographic evi-dence of fracture union and full weight-bearing without ten-derness) in fifty-three of sixty patients within six months. Interestingly, the authors found a correlation between the number of transplanted cells and outcome; if the number of progenitor cells was <70,000, an adverse outcome was likely. Of note, the relationship between the volume of callus formed and the number of progenitors in the graft was not examined.

Although the advantages of using bone morphogenetic proteins (BMPs) for reconstructive orthopaedic surgery are

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well recognized, their usefulness in augmenting osseous heal-ing of large critical-sized defects is less well established, in part because of their short half-life. An international team in-cluding the laboratory of Edward Schwarz, PhD, at the Uni-versity of Rochester, found that BMP expression did not differ when the robust healing of autografts was compared with the slower healing of allografts in a mouse femoral graft model29. On the basis of gene expression profiling studies, it was found that allografts were deficient in factors known to regulate angiogenesis (such as vascular endothelial growth factor [VEGF]) and osteoclastic bone resorption (receptor activator of nuclear factor kappa-B ligand [RANKL]). When recombi-nant adeno-associated viruses encoding RANKL and VEGF were freeze-dried onto the cortical surface of allografts, local allograft healing, vascularization, remodeling, and osseous union were observed. Although in vivo transduction effi-ciency was low and the formation of new bone was not uni-form on all allograft surfaces, the findings represent a new paradigm for simulating a beneficial autograft response in a processed allograft.

Total Joint ArthroplastySeveral studies that were published in the past year focused on the outcome of total disc replacement. Most of the studies from the United States have been positive, with good to excel-lent clinical results at two years after single and multiple-level implantations. Improvements in surgical technique have been suggested, including augmentation of vertebral bone stock in osteoporotic patients by means of open vertebroplasty to pre-vent implant subsidence. These early positive clinical out-comes are encouraging, but the need for long-term follow-up to establish the overall success of and indications for total disc replacements was highlighted in a study by Putzier et al.30. In that retrospective study of sixty-three disc replacements that were performed in fifty-three patients with use of the Charité Artificial Disc (DePuy Spine, Raynham, Massachusetts), poor long-term clinical and radiographic outcomes were reported. Subjective patient assessment according to the criteria of Odom revealed a 54% rate of good or excellent results. No de-generative segments were noted adjacent to functional total disc replacements. However, after seventeen years of follow-up, segmental motion as assessed with use of flexion-exten-sion radiographs revealed a 60% prevalence of spontaneous ankylosis, a high percentage for an implant intended to main-tain range of motion.

Although the Charité Artificial Disc was approved for use in the United States in 2004, in a recent landmark deci-sion, the Centers for Medicare and Medicaid Services issued a proposed national noncoverage determination for this lumbar artificial disc replacement. Few patients over the age of sixty-five years have been managed with the Charité disc technology in the United States, and the paucity of data appears to have contributed to the conclusion that the disc was not indicated for this population (www.cms.hhs.gov/mcd/viewdraftdeci-

sionmemo.asp?id=170). It is not clear what impact this deci-sion will have on the development and clinical use of other lumbar disc replacement technologies.

Other motion-sparing spinal implants for degenerative disc disease include nucleus replacements and semirigid sta-bilization systems. Both of these interventions have been used and reported outside of the United States for many years, but their use and evaluation in the United States have only re-cently been initiated. Nucleus replacements, also referred to as partial disc replacements, are commonly performed through minimally invasive posterior approaches and in-volve annulotomy, evacuation of native nucleus pulposus, and implantation of a replacement nucleus. As reported by Bertagnoli et al.31, nucleus replacements have been made from polymethylmethacrylate, silicone, and stainless steel, but they currently are also being made from a variety of polymers, in-cluding hydrogels. The outcome of a worldwide multicenter trial of the prosthetic disc nucleus (PDN) device (Raymedica, Minneapolis, Minnesota) revealed that pain (as assessed with a visual analog scale) and disability (as measured with the Os-westry disability index) decreased dramatically postopera-tively. However, the rate of complications, including implant extrusion, averaged 25%. Implant design and surgical tech-niques are being refined to reduce the complication rate. The other popular motion-sparing implant concept is dynamic stabilization, which attempts to limit segmental motion in a degenerative level to a “safe zone” that protects against fur-ther degeneration. This methodology relies on specific place-ment of pedicle screws so that they can be connected by flexible connectors such as braided polyester bands (Graf ar-tificial ligament stabilization; SEM, Montrouge, France) or polyethylene terephthalate cord and polycarbonate urethane spacers (Dynesys; Centerpulse, Winterthur, Switzerland). These devices have been associated with successful results in short to intermediate-term follow-up studies, but adequately powered prospective studies are lacking. These interventions may offer a means to protect against the continued degenera-tion of a segment after microdiscectomy and may be an op-tion to allow protected healing of a degenerative segment instead of resorting to fusion.

The concept of controlling the dynamic motion of total knee replacements through mating surface geometry design, so called guided knee kinematics, was embodied in a novel implant (Journey; Smith and Nephew, Memphis, Tennessee) that was unveiled at the 2006 annual meeting of the AAOS. In-tended to recreate normal knee movement, the medial surface of the tibial plateau is concave to provide anteroposterior sta-bility and to promote a medial pivot. The lateral tibial insert is slightly concave and is posteriorly sloped to promote natural femoral external rotation during knee flexion. The posterior condylar surface is extended to allow for an increased area of contact at higher angles of flexion. Furthermore, an anterior cam limits anterior translation during early knee flexion to replicate anterior cruciate ligament function. Fluoroscopic

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What’s New in Orthopaedic Research

analysis of patients within one year after implantation demon-strated the screw-home motion of the total knee replacement. Another advance in the arena of knee arthroplasty was the de-velopment of a gender-specific knee implant. The Zimmer Gender Solutions Knee (Zimmer, Warsaw, Indiana) was de-signed on the basis of an analysis of 800 femora and patellae, which revealed that female patients had a narrower femoral width, a reduced anterior condylar height, and a tendency toward a more lateral patellar track. The implant was designed to reflect a sizing system based on specific differences in mediolateral and anteroposterior dimensions for male and female patients. 510(k) regulatory clearance for this implant is pending.

Earlier hip resurfacing implants were plagued with problems such as poor wear performance of the articulating surfaces and femoral neck fracture. The latter occurred as loads were carried by the metal cap and bypassed the trabecu-lae of the femoral head and neck, leading to bone resorption and eventual fracture due to stress-shielding. Through im-proved material characteristics (smaller, more uniform metal grain sizes), improved manufacturing geometric tolerances, and strict patient selection criteria, hip resurfacing with use of a metal-on-metal articulation is on the cusp of a revival32. This is in part because larger femoral heads can increase range of motion, increase stability, and require less osseous removal that might otherwise be necessary. Nonetheless, total hip re-surfacing prostheses are considered investigational implants by the United States Food and Drug Administration and thus are not currently approved for widespread use.

OverviewThe multidisciplinary approach to understanding the mecha-nisms of tissue degradation and the development of thera-peutics for eliciting a reparative response is leading to the development of novel therapeutic strategies. Furthermore, ex-citing advances in the manufacture and characterization of scaffolds, combined with the emerging availability of multipo-tential stem cells, likely will lead to important advances in ef-forts to engineer replacement musculoskeletal tissues. At the same time, developments to enhance the kinematics and to re-duce osseous resection needed for the implantation of tradi-tional implants continue to be vital for improving implant performance in younger, more active patients.

NOTES: The authors thank Dr. Timothy Wright and Joseph Lipman for their input.

Suzanne A. Maher, PhDChisa Hidaka, MDMatthew E. Cunningham, MD, PhDScott A. Rodeo, MDThe Hospital for Special Surgery, 535 East 70th Street, New York, NY 10021

The authors did not receive grants or outside funding in support of their research for or preparation of this manuscript. They did not receive pay-ments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.

doi:10.2106/JBJS.F.00688

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