Robert M. Pilliar

The optimum pore size for the fixation of porous-surfaced metal implants by the ingrowth of bone.

For a study on the effects of pore size variation on the rate of bone growth into porous-surfaced metallic implants and on the strength of fixation resulting from this ingrowth, 4 distinct pore size ranges were prepared on cobalt-base alloy implants with cobalt-base alloy powder particles of different dimensions. The porous implants were placed into canine femurs for periods of 4, 8, and 12 weeks. Mechanical tests were performed to measure the shear strength of fixation of the implants to cortical bone. For implants with powder-made porous surfaces, a pore size range of approximately 50 to 400 microns provided the optimum or maximum fixation strength (17 MPa) in the shortest time period (8 weeks).

Observations on the effect of movement on bone ingrowth into porous-surfaced implants.

Although porous-surfaced orthopedic implants have been designed for fixation by bone ingrowth, there is clinical evidence that this does not always occur. Initial implant movement relative to host bone can result in attachment by a nonmineralized fibrous connective tissue layer. The ranges of movement that result in either bone or fibrous connective tissue fixation are observed in dogs in two independent studies. Experimentally, bone ingrowth can occur in the presence of some movement, albeit very small (up to 28 μ), while excess movement (150 μ or more) can result in attachment by mature connective tissue ingrowth.

Porous calcium polyphosphate scaffolds for bone substitute applications -- in vitro characterization.

Porous structures were formed by gravity sintering calcium polyphosphate (CPP) particles of either 106-150 or 150-250 microm size to form samples with 30-45 vol% porosity with pore sizes in the range of 100 microm (40-140 microm). Tensile strength of the samples assessed by diametral compression testing indicated relatively high values for porous ceramics with a maximum strength of 24.1 MPa for samples made using the finer particles (106-150 microm). X-ray diffraction studies of the sintered samples indicated the formation of beta-CPP from the starting amorphous powders. In vitro aging in 0.1 M tris-buffered solution (pH 7.4) or 0.05 M potassium hydrogen phthalate buffered solution (pH 4.0) at 37 degreesC for periods up to 30d indicated an initial rapid loss of strength and P elution by 1 d followed by a more gradual continuing strength and P loss resulting in strengths at 30d equal to about one-third the initial value. The observed structures, strengths and in vitro degradation characteristics of the porous CPP samples suggested their potential usefulness as bone substitute materials pending subsequent in vivo behaviour assessment.

Tumor Necrosis Factorα Modulates Matrix Production and Catabolism in Nucleus Pulposus Tissue

Study Design. This study examines changes in the production of extracellular matrix molecules as well as the induction of tissue degradation in in vitro formed nucleus pulposus (NP) tissues following incubation with tumor necrosis factor (TNF)&agr;. Objective. To characterize the response of NP cells to TNF-&agr;, a proinflammatory cytokine present in herniated NP tissues. Summary of Background Data. TNF-&agr; is a proinflammatory cytokine expressed by NP cells of degenerate intervertebral discs. It is implicated in the pain associated with disc herniation, although its role in intervertebral disc degeneration remains poorly understood. Methods. In vitro formed NP tissues were treated with TNF-&agr; (up to 50 ng/mL) over 48 hours. Tissues were assessed for histologic appearance, proteoglycan and collagen contents, as well as proteoglycan and collagen synthesis. Reverse transcriptase polymerase chain reaction was used to determine the effect of TNF-&agr; on NP cell gene expression. Proteoglycan degradation was assessed by immunoblot analysis. Results. At doses of 1–5 ng/mL, TNF-&agr; induced multiple cellular responses, including: decreased expression of both aggrecan and type II collagen genes; decreases in the accumulation and overall synthesis of aggrecan and collagen; increased expression of MMP-1, MMP-3, MMP-13, ADAM-TS4, and ADAM-TS5; and induction of ADAM-TS dependent proteoglycan degradation. Within 48 hours, these cellular responses resulted in NP tissue with only 25% of its original proteoglycan content. Conclusions. Because low levels of TNF-&agr;, comparable to those present physiologically, induced NP tissue degradation, this suggests that TNF-&agr; may contribute to the degenerative changes that occur in disc disease.

Long-term intermittent shear deformation improves the quality of cartilaginous tissue formed in vitro

The formation of cartilaginous tissue in vitro is a promising alternative to repair damaged articular cartilage. However, recent attempts to tissue‐engineer articular cartilage that has similar properties to the native tissue have proven to be difficult. The in vitro‐formed cartilaginous tissue typically has a similar proteoglycan content to native cartilage, but has a reduced collagen content and only a fraction of the mechanical properties. In this study, we investigated whether the intermittent application of cyclic shearing forces during tissue formation would improve the tissue quality. Chondrocyte cultures were stimulated at a 2% shear strain amplitude at a frequency of 1 Hz for 400 cycles every 2nd day. At one week, both collagen and proteoglycan synthesis increased (23 ± 6% and 20 ± 6%, respectively) over the unstimulated, static controls. At four weeks, an increased amount of tissue formed (stimulated: 1.85 ± 0.08, unstimulated: 1.58 ± 0.07 mg dry wt.). This tissue contained approximately 40% more collagen (stimulated: 511 ± 23, unstimulated: 367 ± 24 μg/construct) and 35% more proteoglycans (stimulated: 376 ± 21, unstimulated: 279 ± 26 μg/construct). Tissues that formed in the presence of shearing forces also displayed a 3‐fold increase in compressive load‐bearing capacity (stimulated: 16 ± 5, unstimulated: 5 ± 1 kPa max. equilibrium stress) and a 6‐fold increase in stiffness (stimulated: 112 ± 36, unstimulated: 20 ± 6 kPa max. equilibrium modulus) compared to the static controls. These results demonstrate that intermittent application of dynamic shearing forces over a four‐week period improves the quality of cartilaginous tissue formed in vitro. Interestingly, low amplitudes of shear stimulation for short periods of time (6 min of stimulation applied every 2nd day) produced these changes.

Porous calcium polyphosphate scaffolds for bone substitute applications in vivo studies

Porous rods (6 mm in length and 4 mm in diameter) of calcium polyphosphate (CPP) made by gravity sintering of particles in the size ranges of 45-105, 105-150. and 150-250 microm and with initial volume percent porosity in the range of 35-45% were implanted in the distal femur of New Zealand white rabbits. In an initial experiment, four rabbits implanted with rods made from coarse particles (150-250 microm) were sacrificed at each of the following time points: 2 days, 2 weeks, 6 weeks and 12 weeks. In a subsequent experiment, 10 rabbits were implanted with rods made by sintering 45-105 microm particles and another 10 were made by using particles of 105-150 microm. These rabbits were sacrificed at 6 weeks (five rabbits) and 1 year (five rabbits). No adverse reaction was found histologically at any time point in either experiment. These experiments show that CPP macroporous rods can support bone ingrowth and that between 12 weeks and 1 year, the amount of bones formed is equivalent to the natural bone volume found at similar sites. The degradation of the CPP material is inversely proportional to the original particle size and is rapid initially (within the first 6 weeks) and slows down thereafter. In conclusion, this material seems to promote rapid bone ingrowth and can be tailored to degrade at a given rate in vivo to some degree through appropriate selection of the starting particle size.

The effect of sol–gel-formed calcium phosphate coatings on bone ingrowth and osteoconductivity of porous-surfaced Ti alloy implants

Ti-6Al-4V implants formed with a sintered porous surface for implant fixation by bone ingrowth were prepared with or without the addition of a thin surface layer of calcium phosphate (Ca-P) formed using a sol-gel coating technique over the porous surface. The implants were placed transversely across the tibiae of 17 rabbits. Implanted sites were allowed to heal for 2 weeks, after which specimens were retrieved for morphometric assessment using backscattered scanning electron microscopy and quantitative image analysis. Bone formation along the porous-structured implant surface, was measured in relation to the medial and lateral cortices as an indication of implant surface osteoconductivity. The Absolute Contact Length measurements of endosteal bone growth along the porous-surfaced zone were greater with the Ca-P-coated implants compared to the non-Ca-P-coated implants. The Ca-P-coated implants also displayed a trend towards a significant increase in the area of bone ingrowth (Bone Ingrowth Fraction). Finally, there was significantly greater bone-to-implant contact within the sinter neck regions of the Ca-P-coated implants.

Effect of Biomechanical Conditioning on Cartilaginous Tissue Formation in Vitro

Background: Although tissue engineering of articular cartilage is a promising approach for cartilage repair, it has been difficult to develop cartilaginous tissue in vitro that mimics the properties of native cartilage. Isolated chondrocytes grown in culture typically do not accumulate enough extracellular matrix, and the generated tissue possesses only a fraction of the mechanical properties of native cartilage. One potential explanation for this might be that the cells are grown in an environment that lacks the mechanical stimuli to which the chondrocytes are exposed in vivo. In this study, we compared the long-term effects of both dynamic compressive and shearing forces on cartilaginous tissue formation in vitro.Methods: Bovine articular chondrocytes were grown on the surface of porous ceramic substrates and were maintained under static, free-swelling conditions for a period of four weeks. Cultures were then subjected to six minutes of mechanical stimulation every other day, in either compression or shear, for an additional four-week period.Results: Cartilaginous tissues cultured in the presence of intermittent compression or shear were significantly thicker (p < 0.05) and had accumulated more extracellular matrix (p < 0.01) compared with the unstimulated controls. However, when normalized by the wet weight of the tissue, cultures stimulated in the presence of shearing forces contained more proteoglycans and collagen compared with compression-stimulated cultures. These cultures also displayed the largest increase in mechanical properties, with a threefold increase in equilibrium stress and a fivefold increase in equilibrium modulus.Conclusions and Clinical Relevance: The results of this study demonstrate that a brief application of mechanical forces applied periodically over a long duration can improve the quality of cartilaginous tissue formed in vitro. However, the changes in tissue composition and mechanical properties were dependent on the specific mode of the applied mechanical forces, with shear stimulation eliciting the greater effect. This finding suggests that chondrocytes may respond differently to different modes of applied forces.

Osseointegration of sintered porous-surfaced and plasma spray-coated implants : An animal model study of early postimplantation healing response and mechanical stability

The osseointegration and long-term success of bone-interfacing implants are dependent on mechanical stability of the implant relative to host bone during the early healing period. The geometric design of an implant surface may play an important role in affecting early implant stabilization, possibly by influencing tissue healing dynamics. In this study, we compared the early tissue healing response and resulting implant stability for two surface designs by characterizing the histological and mechanical properties of the healing tissue around Ti6Al4V sintered porous-surfaced and Ti plasma-sprayed implants. The implants were inserted transversely in rabbit femoral condyles and evaluated at 0, 4, 8, and 16 days postimplantation. At 4 and 8 days after implantation, the early healing tissue (fibrin and collagenous matrix) was more extensively integrated with the three-dimensional interconnected structure of the sintered porous surface than with the irregular geometry of the plasma-sprayed coating. In addition, histological examination indicated that initial matrix mineralization leading to osseointegration occurred more rapidly with the porous-surfaced implants. The more extensive tissue integration and more rapid matrix mineralization with the porous-surfaced implants were reflected in the mechanical test data, which demonstrated greater attachment strength and interfacial stiffness for the porous-surfaced implants 4 and 8 days postimplantation (p <.05). Sixteen days after implantation, both implant designs were osseointegrated and had comparable attachment characteristics. These data demonstrate that appropriate surface design selection can improve early implant stability and induce an accelerated healing response, thereby improving the potential for implant osseointegration.

Radiographic and morphologic studies of load-bearing porous-surfaced structured implants

Load-bearing porous-surfaced implants in dogs which failed were not incorporated by bone ingrowth. The implants were incorporated only by fibrous tissue ingrowth. Collagen fiber orientation within this fibrous tissue and Sharpeys fiber-like features at the interface between the fibrous tissue and a surrounding layer of dense bone suggest the adequacy of the fibrous tissue attachment for support of a loaded implant. The surrounding dense bone layer could be distinguished radiographically within three months of the operation. This radiopaque line is similar to that observed around some porous surfaced hip implants in humans. Observations of animal mobility up to six months postoperatively suggest that fibrous connective tissue growth into porous implants can provide adequate mechanical support for weight-bearing dogs.