Application of a microcalorimetric method for determining drug susceptibility in mycobacterium species.

Mycobacterium tuberculosis is a global public health concern, particularly with the emergence of drug-resistant strains. Immediate identification of drug-resistant strains is crucial to administering appropriate treatment before the bacteria are allowed to spread. However, developing countries, which are most affected by drug resistance, are struggling to combat the disease without the facilities or funds for expensive diagnostics. Recent studies have emphasized the suitability of isothermal microcalorimetry (IMC) for the rapid detection of mycobacteria. In this study, we investigate its suitability for rapid and reliable M. tuberculosis drug susceptibility testing. Specifically, IMC was used to determine the MICs of three drugs, namely, isoniazid, ethambutol, and moxifloxacin, against three mycobacteria, namely, Mycobacterium smegmatis, Mycobacterium avium, and Mycobacterium tuberculosis. The Richards growth model was used to calculate growth parameters, namely, the maximum bacterial growth rate and the lag phase duration from integrated heat flow-versus-time results. For example, MICs of isoniazid, ethambutol, and moxifloxacin were determined to be 1.00, 8.00, and 0.25 µg/ml, respectively. IMC, as described here, could be used not just in industrialized countries but also in developing countries because inexpensive and sensitive microcalorimeters are now available.

Real-time measurements of human chondrocyte heat production during in vitro proliferation.

Isothermal microcalorimeters (IMC) are highly sensitive instruments that allow the measurement of heat flow in the microwatt range. Due to their detection of minute thermal heat, IMC techniques have been used in numerous biological applications, including the study of fermentation processes, pharmaceutical development, and cell metabolism. In this work, with the ultimate goal of establishing a rapid and real-time method to predict the proliferative capacity of human articular chondrocytes (HAC), we explored to use of IMC to characterize one of the crucial steps within the process of cartilage tissue engineering, namely the in vitro expansion of HAC. We first established an IMC-based model for the real-time monitoring of heat flow generated by HAC during proliferation. Profiles of the heat and heat flow curves obtained were consistent with those previously shown for other cell types. The average heat flow per HAC was determined to be 22.0 ± 5.3 pW. We next demonstrated that HAC proliferation within the IMC-based model was similar to proliferation under standard culture conditions, verifying its relevance for simulating the typical cell culture application. HAC growth and HAC heat over time appeared correlated for cells derived from particular donors. However, based on the results from 12 independent donors, no predictive correlation could be established between the growth rate and the heat increase rate of HAC. This was likely due to variability in the biological function of HAC derived from different donors, combined with the complexity of tightly couple metabolic processes beyond proliferation. In conclusion, IMC appears to be a promising technique to characterize cell proliferation. However, studies with more reproducible cell sources (e.g., cell lines) could be used before adding the complexity associated with primary human cells.

Evaluation of a low-cost calorimetric approach for rapid detection of tuberculosis and other mycobacteria in culture.

AIMS: The objective of this study was to evaluate the effectiveness of microcalorimetry in rapid detection of mycobacterium species using an inexpensive Isothermal microcalorimetry (IMC) instrument. In addition, we compared microcalorimetry with conventional monitoring techniques. METHODS AND RESULTS: Isothermal microcalorimetry measures heat production rate and can provide rapid detection of living mycobacteria in clinical specimens. Using liquid medium showed that bacterial activity measured by IMC using a TAM Air® agreed with the triphenyl tetrazolium chloride (TTC) assay. Using solid medium to enhance growth, fast-growing mycobacteria detection was achieved between 26 and 53 h and slow-growing mycobacteria detection was achieved between 54 and 298 h. In addition, the calorimetric data were analysed to estimate the growth rate and generation time of the mycobacteria monitored. SIGNIFICANCE AND IMPACT OF THE STUDY: Infections caused by mycobacteria are severe and difficult to treat. With 9·27 million new cases of tuberculosis in 2007, developing countries experience severe health and economic consequences owing to the lack of an affordable, fast detection method. Research-grade IMC instruments are too expensive to use in developing countries. Our study demonstrates that less-expensive instruments such as the TAM air® are adequate for mycobacteria detection and therefore establishes a clear proof of concept.

Direct cytotoxicity evaluation of 63S bioactive glass and bone-derived hydroxyapatite particles using yeast model and human chondrocyte cells by microcalorimetry.

In this study, the cytotoxicity evaluation of prepared 63S bioactive glass and bone-derived hydroxyapatite particles with yeast and human chondrocyte cells was carried out using isothermal micro-nano calorimetry (IMNC), which is a new method for studying cell/biomaterial interactions. Bioactive glass particles were made via sol-gel method and hydroxyapatite was obtained from bovine bone. Elemental analysis was carried out by XRF and EDXRF. Amorphous structure of the glass and completely crystalline structure of HA were detected by XRD analysis. Finally, the cytotoxicity of bioactive glass and bone-derived HA particles with yeast and cultured human chondrocyte cells was evaluated using IMNC. The results confirmed the viability, growth and proliferation of human chondrocyte cells in contact with 63S bioactive glass, and bone-derived HA particles. Also the results indicated that yeast model which is much easier to handle, can be considered as a good proxy and can provide a rapid primary estimate of the ranges to be used in assays involving human cells. All of these results confirmed that IMNC is a convenient method which caters to measuring the cell-biomaterial interactions alongside the current methods.

Quantification of vital adherent Streptococcus sanguinis cells on protein-coated titanium after disinfectant treatment.

The quantification of vital adherent bacteria is challenging, especially when efficacy of antimicrobial agents is to be evaluated. In this study three different methods were compared in order to quantify vital adherent Streptococcus sanguinis cells after exposure to disinfectants. An anaerobic flow chamber model accomplished initial adhesion of S. sanguinis on protein-coated titanium. Effects of chlorhexidine, Betadine®, Octenidol®, and ProntOral® were assessed by quantifying vital cells using Live/Dead BacLight™, conventional culturing and isothermal microcalorimetry (IMC). Results were analysed by Kruskal-Wallis one-way analysis of variance. Live/dead staining revealed highest vital cell counts (P < 0.05) and demonstrated dose-dependent effect for all disinfectants. Microcalorimetry showed time-delayed heat flow peaks that were proportioned to the remaining number of viable cells. Over 48 h there was no difference in total heat between treated and untreated samples (P > 0.05), indicating equivalent numbers of bacteria were created and disinfectants delayed growth but did not eliminate it. In conclusion, contrary to culturing, live/dead staining enables detection of cells that may be viable but non-cultivable. Microcalorimetry allows unique evaluation of relative disinfectant effects by quantifying differences in time delay of regrowth of remaining vital cells.

Micro- and nanomechanical analysis of articular cartilage by indentation-type atomic force microscopy: validation with a gel-microfiber composite.

As documented previously, articular cartilage exhibits a scale-dependent dynamic stiffness when probed by indentation-type atomic force microscopy (IT-AFM). In this study, a micrometer-size spherical tip revealed an unimodal stiffness distribution (which we refer to as microstiffness), whereas probing articular cartilage with a nanometer-size pyramidal tip resulted in a bimodal nanostiffness distribution. We concluded that indentation of the cartilage's soft proteoglycan (PG) gel gave rise to the lower nanostiffness peak, whereas deformation of its collagen fibrils yielded the higher nanostiffness peak. To test our hypothesis, we produced a gel-microfiber composite consisting of a chondroitin sulfate-containing agarose gel and a fibrillar poly(ethylene glycol)-terephthalate/poly(butylene)-terephthalate block copolymer. In striking analogy to articular cartilage, the microstiffness distribution of the synthetic composite was unimodal, whereas its nanostiffness exhibited a bimodal distribution. Also, similar to the case with cartilage, addition of the negatively charged chondroitin sulfate rendered the gel-microfiber composite's water content responsive to salt. When the ionic strength of the surrounding buffer solution increased from 0.15 to 2 M NaCl, the cartilage's microstiffness increased by 21%, whereas that of the synthetic biomaterial went up by 31%. When the nanostiffness was measured after the ionic strength was raised by the same amount, the cartilage's lower peak increased by 28%, whereas that of the synthetic biomaterial went up by 34%. Of interest, the higher peak values remained unchanged for both materials. Taken together, these results demonstrate that the nanoscale lower peak is a measure of the soft PG gel, and the nanoscale higher peak measures collagen fibril stiffness. In contrast, the micrometer-scale measurements fail to resolve separate stiffness values for the PG and collagen fibril moieties. Therefore, we propose to use nanostiffness as a new biomarker to analyze structure-function relationships in normal, diseased, and engineered cartilage.

Biomedical use of isothermal microcalorimeters.

Isothermal microcalorimetry is becoming widely used for monitoring biological activities in vitro. Microcalorimeters are now able to measure heat production rates of less than a microwatt. As a result, metabolism and growth of relatively small numbers of cultured bacteria, protozoans, human cells and even small animals can be monitored continuously and extremely accurately at any chosen temperature. Dynamic effects on these organisms of changes in the culture environment--or of additions to it--are easily assessed over periods from hours to days. In addition microcalorimetry is a non-destructive method that does not require much sample preparation. It is also completely passive and thus allows subsequent evaluations of any kind on the undisturbed sample. In this review, we present a basic description of current microcalorimetry instruments and an overview of their use for various biomedical applications. These include detecting infections, evaluating effects of pharmaceutical or antimicrobial agents on cells, monitoring growth of cells harvested for tissue eingineering, and assessing medical and surgical device material physico-chemical stability and cellular biocompatibility.

Isothermal micro calorimetry--a new method for MIC determinations: results for 12 antibiotics and reference strains of E. coli and S. aureus.

BACKGROUND: Antimicrobial susceptibility testing of microorganisms is performed by either disc diffusion or broth dilution tests. In clinical use, the tests are often still performed manually although automated systems exist. Most systems, however, are based on turbidometric methods which have well-known drawbacks. RESULTS: In this study we evaluated isothermal micro calorimetry (IMC) for the determination of minimal inhibitory concentrations (MICs) of 12 antibiotics for five micro-organisms. Here we present the data for the 12 antibiotics and two representative microorganisms E. coli (a Gram-) and S. aureus (a Gram+). IMC was able to determine the MICs correctly according to CLSI values. Since MICs require 24 hours, time was not reduced. However, IMC provided new additional data - a continuous record of heat-producing bacterial activity (e.g. growth) in calorimetry ampoules at subinhibitory antibiotic concentrations. Key features of the heatflow (P) and aggregate heat (Q) vs. time curves were identified (t delay and Delta Q/Delta t respectively). Antibiotics with similar modes of action proved to have similar effects on t delay and/or Delta Q/Delta t. CONCLUSION: IMC can be a powerful tool for determining the effects of antibiotics on microorganisms in vitro. It easily provides accurate MICs - plus a potential means for analyzing and comparing the modes of action of antibiotics at subinhibitory concentrations. Also IMC is completely passive, so after evaluation, ampoule contents (media, bacteria, etc.) can be analyzed by any other method desired.

A comparison between ((3)H)-thymidine incorporation and isothermal microcalorimetry for the assessment of antigen-induced lymphocyte proliferation.

Lymphocyte transformation tests (LTT) are time-consuming radioactive assays used in the clinic for the determination of allergic drug reactions and extensively in basic immunological research. In the present study we propose an alternative method in the monitoring of T-cell responses by isothermal microcalorimetric (IMC) measurements of overall cellular heat production as a function of time. For mitogen-induced lymphocyte proliferation, we analyzed a concentration dependent effect of phytohemaglutinin (PHA) and both tests showed a good correlation. This was also the case for specific antigenic stimulation with Varidase(R) or tetanus toxoid. On the other hand, antigen-induced lymphocyte proliferation analyzed by pre and post influenza vaccine (Inflexal(R) V) samples, showed no such correlation. Our study suggests that IMC measurements, despite the advantages of simplicity, on-line recording of metabolic activity and no use of radioactivity, may be limited to monitoring mitogen-induced lymphocyte proliferation.

Rapid differentiation of methicillin-susceptible Staphylococcus aureus from methicillin-resistant S. aureus and MIC determinations by isothermal microcalorimetry.

In this study, the use of isothermal microcalorimetry (IMC) for differentiation between methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-susceptible S. aureus (MSSA) and MIC determination was evaluated. It was possible to differentiate between MRSA and MSSA within 4 h, whereas the standard method required 24 h. The MICs of cefoxitin were successfully determined for MRSA and MSSA by using IMC.

Cartilage tissue engineering by expanded goat articular chondrocytes.

In this study we investigated whether expanded goat chondrocytes have the capacity to generate cartilaginous tissues with biochemical and biomechanical properties improving with time in culture. Goat chondrocytes were expanded in monolayer with or without combinations of FGF-2, TGF-beta1, and PDGFbb, and the postexpansion chondrogenic capacity assessed in pellet cultures. Expanded chondrocytes were also cultured for up to 6 weeks in HYAFF-M nonwoven meshes or Polyactive foams, and the resulting cartilaginous tissues were assessed histologically, biochemically, and biomechanically. Supplementation of the expansion medium with FGF-2 increased the proliferation rate of goat chondrocytes and enhanced their postexpansion chondrogenic capacity. FGF-2-expanded chondrocytes seeded in HYAFF-M or Polyactive scaffolds formed cartilaginous tissues with wet weight, glycosaminoglycan, and collagen content, increasing from 2 days to 6 weeks culture (up to respectively 2-, 8-, and 41-fold). Equilibrium and dynamic stiffness measured in HYAFF M-based constructs also increased with time, up to, respectively, 1.3- and 16-fold. This study demonstrates the feasibility to engineer goat cartilaginous tissues at different stages of development by varying culture time, and thus opens the possibility to test the effect of maturation stage of engineered cartilage on the outcome of cartilage repair in orthotopic goat models.

Pulsatile dynamic stiffness of cartilage-like materials and use of agarose gels to validate mechanical methods and models.

Stiffness is a fundamental indicator of the functional state of articular cartilage. Reported test modes include compressive incremental strain to determine the equilibrium modulus, and sinusoidal strain to determine the dynamic modulus and stress/strain loss angle. Here, initial development is described for a method recognizing that gait is pulsatile. Agarose gels have been used by others for validation or comparison of mechanical test methods and models for cartilage and proteoglycan aggregate. Accordingly, gels ranging from 0.5 to 20% agarose were prepared. Pulsatile stiffness in both indentation and unconfined compression were closely reproducible. Stiffness as a function of agarose concentration rose exponentially, as found using other methods. Indentation stiffness was higher than for unconfined compression and ranged from approximately 2.0 kPa for 0.5% gel to approximately 3,800 kPa for 20% gel. Pulsatile dynamic stiffness appears to be a useful method, although further development is needed. Agarose gel stiffness values obtained by other methods were reviewed for comparison. Unfortunately, reported values for a given agarose concentration ranged widely (e.g. fourfold) even when test methods were similar. Causes appear to include differences in molecular weight and gel preparation time-temperature regimens. Also, agarose is hygroscopic, leading to unintended variations in gel composition. Agarose gels are problematic materials for validation or comparison of cartilage mechanical test methods and models.

Experimental model for observation of micromotion in cell culture.

It is known that the micromotion between implant and bone inhibits direct bone growth either on or into implant surfaces in vivo. Nevertheless, biocompatibility tests in vitro of biomaterials for bone/implant interfaces are mainly performed under static conditions. This work describes a dynamic, in vitro experimental simulation of the effect of mutual, small-scale implant surface-tissue displacement on adhered cells. Disks of simulated tissue (PVP hydrogel) were subjected to cyclic micromotion ranging from 0 at the center to 1000 microm at the periphery at approximately 13 Hz, relative to biomaterial surfaces or tissue culture polystyrene controls populated with human osteoblasts in standard tissue culture plate wells. The effect of the interfacial micromotion on the number of cells remaining attached was quantitated by XTT assay. The activity level of the remaining cells was determined by an alkaline phosphatase assay, and cell stress was evaluated by nitrogen assay. Significantly more cells (ANOVA) became detached from similarly prepared surfaces of titanium, hydroxyapatite, and alumina compared to the polystyrene control, and detachment from alumina was greater than for the other two materials. The activity of the remaining attached cells was lower as compared to the static (no micromotion) control but not significantly different among the biomaterials. All nitrogen assays were negative, suggesting minimal cell stress occurred. The method is proposed as a useful and discriminating in vitro tool for biocompatibility studies focused on cell adhesion to biomaterials under conditions related to those which exist at the implant/bone interface in vivo, and it allows subsequent studies of the still-viable cells by other methods.

Clinical development and current status: Europe.

This article reviews the development and current status of cemented fixation in total hip replacement in Europe. Key points include the wide country-to-country variation in use of cemented vs. non-cemented fixation and the largely overlooked importance of the choice of bone cement as a factor highly correlated with clinical outcome. Laboratory studies by the authors are also reviewed. Results suggest that the type of acrylic bone cement used affects wear phenomena at the implant/cement interface. Further studies by microcalorimetry suggest that certain aspects of acrylic starting materials (low molecular weight and use of radiation sterilization) affect long-term physico-chemical stability and may thus influence clinical outcomes.

Rectal balloons: complications, causes, and recommendations.

Nine previously unreported rectal injuries caused by barium-enema examinations have been reviewed. In each case, the injury occurred in conjunction with inflation of a rectal balloon. Analysis of the clinical material suggested that certain mechanical properties of balloon catheter tips might transfer mechanical stress to the rectal wall and contribute to the observed injuries. Careful manometric evaluation of in vivo rectal balloons suggest that significant anatomic differences in patients may be clinically important. Further experimental bench studies revealed undesirable mechanical properties in many commercially available rectal balloon catheters. These mechanical problems include low compliance, asymmetrical inflation, strong lateral and anterior displacement of a firm catheter tip into the restraining wall, and self-obstruction of the balloon deflation outlet by the inflated baloon. Many of these problems were clinically confirmed by careful in vivo observations and by evidence collected from the nine cases of rectal injury. A series of practical prophylactic procedures are recommended.

Isothermal microcalorimetry: an analytical technique for assessing the dynamic chemical stability of UHMWPE.

In this work, isothermal microcalorimetry (IMC) was utilized to measure the exothermic heat flow from specimens of ultra-high-molecular-weight polyethylene (UHMWPE), that had been sterilized by various standard methods, under simulated shelf storage (air at 25 degrees C, 30% r.h.) and simulated implantation (phosphate buffered saline, PBS, at 37 degrees C) conditions. Gamma-radiation sterilized UHMWPE yielded initial heat flow rates approximately 7-10 times higher in simulated shelf storage and 2-3 times higher in simulated implantation (even after 1 month in PBS) than specimens which were unsterilized or sterilized using either ethylene oxide gas (ETO) or gas plasma (GP). These results show that gamma sterilization of UHMWPE produces many more unstable bonds in the polymer than is the case when ETO or GP is used, and that the net exothermic physico-chemical change proceeds steadily in a diffusion-limited manner in air or saline. In addition, gamma sterilization in nitrogen rather than in air did not prevent the creation of unstable bonds, but did defer physico-chemical change until the UHMWPE was exposed to oxygen. These results demonstrate the usefulness of IMC as a viable method for studying the stability of polymeric implant materials.

Determination of the activation energies of and aggregate rates for exothermic physico-chemical changes in UHMWPE by isothermal heat-conduction microcalorimetry (IHCMC).

Exothermic heat flow rates (Q=microW=microJ/s), as a function of elapsed time, were measured by isothermal heat-conduction microcalorimetry (IHCMC) in order to study the aggregate rate of physico-chemical change in specimens of unsterilized and sterilized ultra-high-molecular-weight polyethylene (UHMWPE). Standard protocols for performing the IHCMC tests were developed and are described. Use of the standard protocols yielded the desired results-data that were not significantly different among either replicate sets of unsterilized specimens or as a function of which calorimeter test well was used. Heat flow rates measured in air at 20 degrees C, 25 degrees C, 35 degrees C, and 45 degrees C yielded estimates of activation energies of 47, 11, and 41 kJ/mol for unsterilized, gamma-radiation sterilized, and ethylene oxide gas (EtO) sterilized polymer, respectively. These results support the ideas that (a). initial exothermic degradation takes place much more easily in the radiation-sterilized material, due to direct oxidation of readily available free radicals, and (b). the much slower degradation process in EtO-sterilized UHMWPE is not appreciably different than in unsterilized polymer. Comparison with other activation energy data suggests that the rate-limiting process in EtO- or un-sterilized polymer is oxygen diffusion into the polymer. For shelf storage in air, for periods up to 8 months, the mean exothermic heat flow in air, at 25 degrees C (Q(m)) [determined from the Q values averaged over the time period between 15 and 20 h after test start], from UHMWPE gamma-radiation sterilized in air was significantly higher than for unsterilized material (2.91+/-0.11 vs. 0.73+/-0.11 microW). The higher rate can be attributed to oxidation of radiation-induced free radicals in the polymer near its surface. For the gamma-irradiated polymer, the decline in Q(m) with shelf storage time suggests that, eventually, degradation might become oxygen diffusion limited in this case also. However, in vivo, surface wear of an UHMWPE articular component may continue to expose unoxidized free radicals, keeping the exothermic reaction rate high and, possibly, continuing to produce an oxidized UHMWPE surface prone to wear.

The effect of medial meniscectomy and coronal plane angulation on in vitro load transmission in the canine stifle joint.

The purpose of this study was to determine the in vitro load transmission characteristics of the canine stifle joint, paying particular attention to the positioning effect of the meniscus in the coronal plane. The intact joint was first loaded, and then tested under two different loading conditions after a complete medial meniscectomy. The first set of test conditions attempted to simulate those used by previous investigators, by ignoring the spacer effect of the meniscus and not repositioning the joint after its removal. The second set of tests was carried out after the joint was repositioned in the coronal plane to allow initial contact to occur in both tibiofemoral compartments. It is presumed that this occurs subsequent to a meniscectomy in vivo, following the application of any weight-bearing load. As with previous investigators, it was found that after meniscectomy the joints produced slightly larger displacements and lower stiffnesses than when intact (no significant differences from intact). However, repositioning the meniscectomized joint produced markedly smaller displacements (35-49%, p less than 0.01) and greater stiffnesses (47-123%, p less than 0.05) over the range of forces analyzed, compared with the intact joint. The ratio of dissipated to input energy was 42% for the intact joint, and rose following meniscectomy to 54% (p less than 0.05) with repositioning and 55% (p less than 0.05) without repositioning. Measured contact area decreased by 17% (p less than 0.05) following meniscectomy alone, and by 12% (p less than 0.05) following meniscectomy with repositioning. Since repositioning of the joint subsequent to meniscectomy (accounting for the loss of the meniscal spacer) resulted in an increase in structural stiffness, it was concluded that the medial meniscus decreases the structural stiffness of the intact stifle joint. In addition, the meniscus has a role in elastic energy storage and increasing contact area. This study is intended to serve as a baseline comparison for future in vivo studies on meniscectomy, meniscal repair, and meniscal replacement, in addition to more fully elucidating the mechanism of load transmission. A model is presented to explain both the decrease in stiffness after meniscectomy without repositioning and the increase in stiffness after meniscectomy with repositioning, employing linear springs of unequal length and different stiffnesses. After removal of the softer meniscal element and allowing joint approximation to occur, loading of the stiffer articular element results in an initially stiffer preparation.

Effect of fibular plate fixation on rotational stability of simulated distal tibial fractures treated with intramedullary nailing.

BACKGROUND: The effect of an intact fibula on rotational stability after a distal tibial fracture has, to the best of our knowledge, not been clearly defined. We designed a cadaver study to clarify our clinical impression that fixation of the fibula with a plate increases rotational stability of distal tibial fractures fixed with a Russell-Taylor intramedullary nail. METHODS: Seven matched pairs of embalmed human cadaveric legs and sixteen fresh-frozen human cadaveric legs, including one matched pair, were tested. To simulate fractures, 5-mm transverse segmental defects were created at the same level in the tibia and fibula, 7 cm proximal to the ankle joint in each bone. The tibia was stabilized with a 9-mm Russell-Taylor intramedullary nail that was statically locked with two proximal and two distal screws. Each specimen was tested without fibular fixation as well as with fibular fixation with a six-hole semitubular plate. A biaxial mechanical testing machine was used in torque control mode with an initial axial load of 53 to 71 N applied to the tibial condyle. Angular displacement was measured in 0.56-N-m torque increments to a maximal torque of 4.52 N-m (40 in-lb). RESULTS: Initially, significantly less displacement (p < or = 0.05) was produced in the specimens with fibular plate fixation than in those without fibular plate fixation. The difference in angular displacement between the specimens treated with and without plate fixation was established at the first torque data point measured but did not increase as the torque was increased. No significant difference in the rotational stiffness was found between the specimens treated with and without plate fixation after measurement of the second torque data point (between 1.68 and 4.48 N-m). CONCLUSIONS: Fibular plate fixation increased the initial rotational stability after distal tibial fracture compared with that provided by tibial intramedullary nailing alone. However, there was no difference in rotational structural stiffness between the specimens treated with and without plate fixation as applied torque was increased.

The effect of test methodology on apparent compressive stiffness of tibiofemoral joint specimens.

Several investigators have analysed the compressive load bearing properties of the knee. Careful review of these force/displacement data showed considerable variation, with some investigators reporting displacements 12-15 x higher than others for nearly identical testing conditions using the same animal model. In this study, we sought to determine if this variability was inherent in the tibiofemoral joint or if differences in experimental methodology explained the variation. Compressive force/displacement curves were obtained from 39 normal canine tibiofemoral specimens mounted in a universal testing machine. Two commonly reported methods of measuring compressive displacement were used simultaneously. The testing machine crosshead displacement was used as one measure of displacement of the joint. The other method consisted of extensometers mounted to bone at the joint line. Resultant joint rotation in the parasagittal plane was also measured. Using either approach, we found comparatively little variation among the 39 specimens tested. However, the crosshead displacement measurements diverged from the extensometer measurements as the compressive load increased. At 770 N, the crosshead measurement was nearly twice the extensometer displacement. Further analysis showed that the compliances differed by a uniform amount. Parasagittal joint rotation, as measured by the extensometers, was minimal--less than one half of one degree. Although our loading fixtures were expected to be rigid under the loads used, these data suggest that the deformation of the bone and loading fixtures was responsible for the differences we observed, and may be responsible for the variation in compressive displacement results among several published studies. A model is presented which uses a simple elastic element to represent this deformation.