Effect of concentration and molecular weight on the rheology of hyaluronic acid/bovine calf serum solutions.

Rheological measurements were conducted on hyaluronic acid (HA)/bovine calf serum (BCS) solutions that can be used as analogues of synovial or periprosthetic fluid. The presence of HA affected profoundly the rheological properties of the solutions, with increased concentrations and higher molecular weight of HA leading to higher viscosities as well as more pronounced shear thinning in steady-shear flow. In dynamic oscillatory flow, the elastic character of the solutions became more pronounced and the cross-over frequency decreased upon increasing concentration and molecular weight of HA. The relevance of these results in the context of appropriate selection of a lubricating environment that mimics physiological conditions is discussed.

Rheological properties of synovial fluids.

Synovial fluid is the joint lubricant and shock absorber [Semin. Arthritis Rheum. 32 (2002), 10-37] as well as the source of nutrition for articular cartilage. The purpose of the present paper is to provide a comprehensive review of the rheological properties of synovial fluid as they relate to its chemical composition. Given its importance in the rheology of synovial fluid, an overview of the structure and rheology of HA (hyaluronic acid) is presented first. The rheology of synovial fluids is discussed in detail, with a focus on the possible diagnosis of joint pathology based on the observed differences in rheological parameters and trends. The deterioration of viscoelastic properties of synovial fluid in pathological states due to effects of HA concentration and molecular weight is further described. Recent findings pertaining to the composition and rheology of periprosthetic fluid, the fluid that bathes prosthetic joints in vivo are reported.

Development of a rheological analogue to periprosthetic fluid.

The purpose of this work is to design a rheological analogue to periprosthetic fluid, for potential use in wear testing of orthopaedic implants. Polymer solutions of sodium carboxymethyl cellulose (CMC), xanthan and mixtures thereof were prepared, and the experimentally determined viscosity-shear rate dependence was analyzed using the three-parameter modified Cross model. A mixture containing 0.185 wt% CMC and 0.075 wt% xanthan was identified as being the closest rheological match for periprosthetic fluid in both steady shear and oscillatory modes of deformation.

A suite of objective biomechanical measurement tools for personal load carriage system assessment.

For application to military and civilian needs, Defence Research and Development Canada--Toronto contracted Queen's University, Kingston to develop a suite of biomechanical assessment and analytical tools to supplement human-based load carriage system assessment methods. This suite of tools permitted efficient objective evaluation of biomechanical aspects of load-bearing webbing, vests, packs and their components, and therefore contributed to early system assessment and a rapid iterative design process. This paper is a summary of five assessment and analytical tools. A dynamic load carriage simulator was developed to simulate cadence of walking, jogging and running. The simulator comprised a computer-controlled pneumatic platform that oscillated anthropometrically weighted mannequins of varying dimensions from which measures of skin contact pressure, hip reaction forces and moments and relative pack-person displacements were taken. A stiffness tester for range of motion provided force-displacement data on pack suspension systems. A biomechanical model was used to determine forces and moments on the shoulders and hips, and validated using a static load distribution mannequin. Subjective perceptual rating systems were used gather soldier feedback during a standardized mobility circuit. Objective outcome measures were validated by means of other objective measures (e.g., Optotrak, video, Instron, etc.) and then compared to subjective ratings. This approach led to development of objective performance criteria for load carriage systems and to improvements in load carriage designs that could be used both in the military and in general.

Short-term load bearing capacity of osteochondral autografts implanted by the mosaicplasty technique: an in vitro porcine model.

Articular surface congruency and graft stability are considered essential factors in the success of osteochondral grafting; however, quantitative measures of short-term load bearing capacity of grafts implanted by the mosaicplasty technique have not been reported. The purpose of this study was to develop a live tissue in vitro model to examine short-term fixation strength of mosaicplasty autografts immediately after and 1 week following graft implantation. Cylindrical osteochondral autografts were implanted in vitro by the mosaicplasty technique on five pairs of porcine femoral condyles within one and a half hours of animal sacrifice. Immediately following the surgical procedure, graft push-in and pull-out strength tests as well as indentation tests to determine modulus of the surrounding cancellous bone were performed on half of the specimens from the distal femurs of each animal. The remaining specimens, matched for location in the contralateral leg, were incubated in culture medium for 7 days prior to performing the same set of mechanical tests. Averaged push-in and pull-out graft fixation strength decreased 44% from 135.7 to 75.5N over the 7-day period, while no change in modulus was detected in the surrounding cancellous bone. These in vitro results demonstrate a substantial deterioration of short-term fixation strength of mosaicplasty grafts from the immediate post-operative state. Such a reduction in short-term graft load bearing capacity may pose a threat to the surgically established articular surface congruency and blood vessels formed during the early stages of the healing response.

An in vitro investigation of the acetabular labral seal in hip joint mechanics.

Labrum pathology may contribute to early joint degeneration through the alteration of load transfer between, and the stresses within, the cartilage layers of the hip. We hypothesize that the labrum seals the hip joint, creating a hydrostatic fluid pressure in the intra-articular space, and limiting the rate of cartilage layer consolidation. The overall cartilage creep consolidation of six human hip joints was measured during the application of a constant load of 0.75 times bodyweight, or a cyclic sinusoidal load of 0.75+/-0.25 times bodyweight, before and after total labrum resection. The fluid pressure within the acetabular was measured. Following labrum resection, the initial consolidation rate was 22% greater (p=0.02) and the final consolidation displacement was 21% greater (p=0.02). There was no significant difference in the final consolidation rate. Loading type (constant vs. cyclic) had no significant effect on the measured consolidation behaviour. Fluid pressurisation was observed in three of the six hips. The average pressures measured were: for constant loading, 541+/-61kPa in the intact joint and 216+/-165kPa following labrum resection, for cyclic loading, 550+/-56kPa in the intact joint and 195+/-145kPa following labrum resection. The trends observed in this experiment support the predictions of previous finite element analyses. Hydrostatic fluid pressurisation within the intra-articular space is greater with the labrum than without, which may enhance joint lubrication. Cartilage consolidation is quicker without the labrum than with, as the labrum adds an extra resistance to the flow path for interstitial fluid expression. However, both sealing mechanisms are dependent on the fit of the labrum against the femoral head.

Diagnostic ultrasound artifacts during imaging of two-body interfaces: part 1--wavelength resonance.

A diagnostic ultrasound technique is to be developed for measuring surface contact areas at the tibio-femoral interface of a total knee replacement in an in vitro industrial engineering setting as a design tool. As a first step, a previous study mathematically characterized the ultrasound behaviour expected at a two-body circular-on-flat interface of known geometry. In the current investigation, a series of test objects was constructed and imaged to experimentally validate the theoretical contact models. Specifically, several unique metal-on-polymer test objects, whose interfaces were point, non-point, and circular contact areas, were ultrasonically imaged. The effects of interface geometry, ultrasound resonance wavelength, lambda/2, and compressive load were studied.

Diagnostic ultrasound artifacts during imaging of two-body interfaces: part 2--beam thickness artifact.

A diagnostic ultrasound method is being developed for measuring surface contact areas at the tibio-femoral interface of a total knee replacement in a non-clinical industrial setting as an engineering design tool. As an initial step towards this, a previous study mathematically predicted the effect of ultrasound beam thickness on contact area measurements at a two-body interface. In the current study, a novel metal-on-polymer acoustic test object was constructed to create circular two-body interfaces of known geometry. The object was ultrasonically imaged, contact areas measured, and the results compared with the theoretical model previously developed.

A digital image analysis method for diagnostic ultrasound calibration.

Acoustic test objects are commonly used for quality assurance testing of diagnostic ultrasound machines. However, the accompanying calibration protocols rely heavily on the judgment of the sonographer, are dependent on machine settings and are semi-quantitative. In the current study, two unique test objects and protocols were designed to quantitatively determine diagnostic ultrasound parameters, namely axial resolution and geometric uniformity, and lateral resolution and geometric uniformity of the ultrasound field. The effect of focal zone, signal gain, and distance from the ultrasound probe on these parameters was assessed. The investigation was performed using a typical low-frequency diagnostic unit equipped with a 7.5 MHz linear pulse-echo probe. Results underline the need to ensure that sensitivity of routine testing regimes is adequate for the measurements to be made. This study is a preliminary part of a larger project developing an ultrasound technique to be used as an engineering design tool in a non-clinical industrial setting for quality assurance testing of total knee replacements immersed in water.

A computational model of postoperative knee kinematics.

A mathematical model for studying the passive kinematics of total knee prostheses can be useful in computer-aided planning and guidance of total joint replacement. If the insertion location and neutral length of knee ligaments is known, the passive kinematics of the knee can be calculated by minimizing the strain energy stored in the ligaments at any angular configuration of the knee. Insertions may be found intraoperatively, or may come from preoperative 3D medical images. The model considered here takes into consideration the geometry of the prosthesis and patient-specific information. This model can be used to study the kinematics of the knee joint of a patient after total joint replacement. The model may be useful in preoperative planning, computer-aided intraoperative guidance, and the design of new prosthetic joints.

The material properties of the bovine acetabular labrum.

The compressive and tensile material properties of the bovine acetabular labrum were measured. Confined compression testing was used to determine the aggregate compressive modulus and the permeability of the labrum. The compressive modulus of the labrum (0.157 +/- 0.057 MPa) is comparable to that of the morphologically similar meniscus, and approximately one-quarter to one-half that of the adjoining acetabular cartilage. The permeability of the labrum (4.98 +/- 3.43 x 10(-16) m4/N s) was lower than that of the meniscus and cartilage. With a significantly higher resistance to interstitial fluid flow across the acetabular rim than along the rim. Specimens from the posterior and superior regions of the labrum were tested to failure in uniaxial tension. The maximum stress at failure (11.9 +/- 6.1 MPa), maximum strain at failure (26.5 +/- 7.6%) and tangent modulus (74.7 +/- 44.3 MPa) were similar to those reported for the bovine meniscus, and to other tissues composed of highly oriented collagen fiber bundles. In tension, the labrum is much stiffer (10-15 x) than the adjoining articular cartilage, and the posterior region of the labrum is significantly stiffer (45%) than the superior region. The labrum's low permeability may contribute to sealing of the hip joint. The high circumferential tensile stiffness of the labrum, together with its ring structure, reinforce the acetabular rim and may contribute to joint stability.

Three-dimensional metacarpophalangeal joint kinematics using two markers on the phalanx.

A protocol for analysing three-dimensional metacarpophalangeal (MCP) joint motion in vivo using two markers on the proximal phalanx is described. The analysis uses an assumption that the rotation of the phalanx about its own long axis is zero. In an experimental study 24 volunteers had surface markers applied to the dorsal surfaces of their hands and index and long finger proximal phalanges, with three-dimensional marker positions recorded in two hand and finger postures in an incomplete box design using a test-retest protocol. Kinematic parameters from the optoelectronic system were compared with those obtained from three-dimensional reconstruction of bone landmarks and of the marker positions identified on stereoradiographs. Pronation/supination angles obtained from bone landmarks showed high test-retest variability, reflecting the difficulty in obtaining reliable pronation/supination data in small bones without the use of implanted markers. Changes in MCP joint extension and deviation angles determined using two surface markers agree with those obtained from bone landmarks. The results indicate a reproducible protocol for tracking MCP joint motion using only two phalangeal markers, suggesting that the 'no-rotation assumption' can be applied without affecting measures of extension and deviation motion in the normal joint.

Fuji film and ultrasound measurement of total knee arthroplasty contact areas.

This article describes tibiofemoral contact area measurement results from tests on 1 commercial total knee arthroplasty (TKA) using 2 experimental methods-fuji film and diagnostic ultrasound. The study presents a novel diagnostic ultrasound technique developed specifically for measuring TKA contact areas. Because most experimental investigations have been concerned with interimplant comparison, this article is one of few parametric TKA studies in the literature. Fuji film and ultrasound provide lower and upper bound contact area measurements based on their physical operating principles; this implies that no single measurement method can be relied on exclusively to glean contact area data. Designers should be cautious in using contact area and contact stress as the exclusive predictors of TKA failure.

The effect of kinematic conditions on the wear of ultra-high molecular weight polyethylene (UHMWPE) in orthopaedic bearing applications.

It is known that wear mechanisms differ between the ultra-high molecular weight polyethylene (UHMWPE) components of total hip replacement (THR) and total knee replacement (TKR). The difference in relative contact position or 'kinematic conditions of contact' between the metal and polymer components is thought to contribute to the contrast in observed wear mechanisms. A reciprocating wear tester was used to evaluate three basic kinematic contact conditions: sliding, in which the relative contact position on the polymer remains stationary; gliding, where the contact position on the polymer reciprocates; and rolling, where the contact position on the polymer varies and the relative velocities of both components are equal. All static load tests used cast Co-Cr alloy and irradiated Chirulen UHMWPE in a 37 degrees C environment lubricated with bovine serum albumin. UHMWPE test sample wear was measured gravimetrically at intervals of 600,000 cycles. The results indicated a difference in wear factor (volume lost due to wear per unit load per unit sliding distance) between the three groups with varying relative motion. The study indicates that screening tests which evaluate wear properties of new materials for total joint replacement should reflect the different kinematic contact conditions.

The influence of the acetabular labrum on hip joint cartilage consolidation: a poroelastic finite element model.

The goal of this study was to investigate the influence of the acetabular labrum on the consolidation, and hence the solid matrix strains and stresses, of the cartilage layers of the hip joint. A plane-strain finite element model was developed, which represented a coronal slice through the acetabular and femoral cartilage layers and the acetabular labrum. Elements with poroelastic properties were used to account for the biphasic solid/fluid nature of the cartilage and labrum. The response of the joint over an extended period of loading (10,000s) was examined to simulate the nominal compressive load that the joint is subjected to throughout the day. The model demonstrated that the labrum adds an important resistance in the flow path of the fluid being expressed from the cartilage layers of the joint. Cartilage layer consolidation was up to 40% quicker in the absence of the labrum. Following removal of the labrum from the model, the solid-on-solid contact stresses between the femoral and acetabular cartilage layers were greatly increased (up to 92% higher), which would increase the friction between the joint surfaces. In the absence of the labrum, the centre of contact shifted towards the acetabular rim. Subsurface strains and stresses were much higher without the labrum, which could contribute to fatigue damage of the cartilage layers. Finally, the labrum provided some structural resistance to lateral motion of the femoral head within the acetabulum, enhancing joint stability and preserving joint congruity.

The acetabular labrum seal: a poroelastic finite element model.

OBJECTIVE: The aim of the study is to investigate the labrum's ability to seal a pressurised layer of synovial fluid within the joint, and to study the influence of this sealing mechanism on cartilage deformation, interstitial fluid pressure and collagen solid matrix stresses. BACKGROUND: Cartilage degeneration has been observed in conjunction with labrum pathology. However, little is known about the function of the labrum. Experimental observations have been reported which are consistent with a sealing function of the labrum. METHODS: The model was an axisymmetric geometric approximation of the acetabular and femoral cartilage layers and the surrounding labrum. A poroelastic formulation was used to account for the solid and fluid components of these hydrated tissues. A sensitivity analysis of the labrum material properties was carried out. RESULTS: With a compressive load of 1200 N applied across the joint model, the labrum could seal a layer of pressurised fluid between the femur and acetabulum, thus preventing contact of the articulating surfaces. With this sealing effect, loads were transferred across the joint predominantly by uniform pressurisation of the interstitial fluid of the cartilage layers. In the absence of this sealing, strains within the solid matrix of the cartilage layers were higher (e.g. 20% vs. 3%). CONCLUSIONS: The labrum can seal against fluid expression from the joint space. This sealing function protects the cartilage layers of the hip. RELEVANCE: Current treatments for labrum damage and early arthrosis may compromise the sealing function of the labrum. With continued study of the function and importance of the labrum, new surgical repair strategies can be developed to maintain the overall function of the hip joint.

Three-dimensional computational reconstruction of mixed anatomical tissues following histological preparation.

The creation of geometrically accurate computer models of anatomical structures with complex shape and mixed tissue types can be difficult. A method for shape reconstruction based on digital images of polymer embedded, serially sectioned specimens is presented. The distortion of bone and soft tissue specimens during all stages of histological preparation was measured. Serial sections of one specimen were stained with common histological stains to enhance the contrast between different tissue types. High-resolution digital images of these sections were then processed into a three-dimensional solid model using commercial software. Preparations containing bone and cartilaginous tissues were dimensionally stable following fixation, dehydration and embedding (shrinkage/expansion less than 2%). Staining was necessary to identify anatomical features that otherwise could not be differentiated from their surroundings. Although time consuming, this method provides cross-section images of a higher resolution than those obtained from CT or MRI scanning, and with better soft tissue visualisation.

Mechanical properties of wrist extensor tendons are altered by the presence of rheumatoid arthritis.

The in vitro mechanical properties of 14 wrist extensor tendons salvaged at surgery from patients with inflammatory (rheumatoid) arthritis and noninflammatory arthrosis were measured in uniaxial tension and compared. The rheumatoid tendons had higher extensibility at low stresses, lower stiffness in the linear portion of the stress-strain curve, greater rates of stress relaxation, and lower ultimate strengths than did the nonrheumatoid tendons. Differences in tangent modulus, stress remaining at 100 seconds, and ultimate tensile strength were significant at the 95% confidence level. In vivo, mechanically impaired tendons may play an important role in destabilization of the wrist in patients with rheumatoid arthritis.

Dynamic contact stress and rolling resistance model for total knee arthroplasties.

Problems associated with premature failure of total knee replacements (TKR's) include: wear, creep, and oxidation of ultrahigh-molecular-weight polyethylene (UHMWPe) as well as adverse tissue reactions to polyethylene wear debris. These problems are associated in part with the mechanical behavior of UHMWPe. In TKR's, contact stress analyses have been performed on the UHMWPe tibial component; however, most have employed simplified material properties and not accounted for joint kinematics. A nonlinear viscoelastic rolling model was developed for TKR's to predict the contact stress and rolling friction for varying rolling speed, conformity, applied load, and tibial plateau thickness. Results indicated that the contact stress increased and rolling friction decreased with increasing rolling speed. Effects of conformity, applied load, and tibial plateau thickness were consistent with previous models. At large rolling speeds, predicted peak contact stresses were almost twice their static value, resulting in a compound fatigue problem in UHMWPe components due to normal cyclic loading, moving point of contact, and velocity-dependent stresses.

Surface degradation features and microstructural properties of ultra-high molecular weight polyethylene (UHMWPe).

Surface degradation of UHMWPe is recognized as a leading clinical concern, limiting the long-term performance in total knee replacements. Eight retrieved tibial plateaux and six wear screening test samples were evaluated for surface degradation features and microstructural features. The surface degradation features were assessed using stereomicroscopy and scanning electron microscopy. Microstructural features were evaluated using optical microscopy of thin cross-sections and a permanganate etching technique. The pitting mechanism of wear was observed on all eight retrieved TKR and covered an average of 12.6% of the surface area. The size of the pits were similar to the size of grains observed in the microstructural evaluation - approximately 100 to 200 microm. The presented observations of pitting in retrieved knee implants have shown that the post-processing microstructure may influence this mechanism of surface degradation and hence the wear products.