An instrumented pendulum system for measuring energy absorption during fracture insult to large animal joints in vivo.

For systematic laboratory studies of bone fractures in general and intra-articular fractures in particular, it is often necessary to control for injury severity. Quantitatively, a parameter of primary interest in that regard is the energy absorbed during the injury event. For this purpose, a novel technique has been developed to measure energy absorption in experimental impaction. The specific application is for fracture insult to porcine hock (tibiotalar) joints in vivo, for which illustrative intra-operative data are reported. The instrumentation allowed for the measurement of the delivered kinetic energy and of the energy passed through the specimen during impaction. The energy absorbed by the specimen was calculated as the difference between those two values. A foam specimen validation study was first performed to compare the energy absorption measurements from the pendulum instrumentation versus the work of indentation performed by an MTS machine. Following validation, the pendulum apparatus was used to measure the energy absorbed during intra-articular fractures created in 14 minipig hock joints in vivo. The foam validation study showed close correspondence between the pendulum-measured energy absorption and MTS-performed work of indentation. In the survival animal series, the energy delivered ranged from 31.5 to 48.3 Js (41.3±4.0, mean±s.d.) and the proportion of energy absorbed to energy delivered ranged from 44.2% to 64.7% (53.6%±4.5%). The foam validation results support the reliability of the energy absorption measure provided by the instrumented pendulum system. Given that a very substantial proportion of delivered energy passed--unabsorbed--through the specimens, the energy absorption measure provided by this novel technique arguably provides better characterization of injury severity than is provided simply by energy delivery.

A new sensor for measurement of dynamic contact stress in the hip.

Various techniques exist for quantifying articular contact stress distributions, an important class of measurements in the field of orthopaedic biomechanics. In situations where the need for dynamic recording has been paramount, the approach of preference has involved thin-sheet multiplexed grid-array transducers. To date, these sensors have been used to study contact stresses in the knee, shoulder, ankle, wrist, and spinal facet joints. Until now, however, no such sensor had been available for the human hip joint due to difficulties posed by the deep, bi-curvilinear geometry of the acetabulum. We report here the design and development of a novel sensor capable of measuring dynamic contact stress in human cadaveric hip joints (maximum contact stress of 20 MPa and maximum sampling rate 100 readings/s). Particular emphasis is placed on issues concerning calibration, and on the effect of joint curvature on the sensor's performance. The active pressure-sensing regions of the sensors have the shape of a segment of an annulus with a 150-deg circumferential span, and employ a polar/circumferential "ring-and-spoke" sensel grid layout. There are two sensor sizes, having outside radii of 44 and 48 mm, respectively. The new design was evaluated in human cadaver hip joints using two methods. The stress magnitudes and spatial distribution measured by the sensor were compared to contact stresses measured by pressure sensitive film during static loading conditions that simulated heel strike during walking and stair climbing. Additionally, the forces obtained by spatial integration of the sensor contact stresses were compared to the forces measured by load cells during the static simulations and for loading applied by a dynamic hip simulator. Stress magnitudes and spatial distribution patterns obtained from the sensor versus from pressure sensitive film exhibited good agreement. The joint forces obtained during both static and dynamic loading were within ±10% and ±26%, respectively, of the forces measured by the load cells. These results provide confidence in the measurements obtained by the sensor. The new sensor's real-time output and dynamic measurement capabilities hold significant advantages over static measurements from pressure sensitive film.

A novel impaction technique to create experimental articular fractures in large animal joints.

OBJECTIVE: A novel impaction fracture insult technique, developed for modeling post-traumatic osteoarthritis in porcine hocks in vivo, was tested to determine the extent to which it could replicate the cell-level cartilage pathology in human clinical intra-articular fractures. DESIGN: Eight fresh porcine hocks (whole-joint specimens with fully viable chondrocytes) were subjected to fracture insult. From the fractured distal tibial surfaces, osteoarticular fragments were immediately sampled and cultured in vitro for 48 h. These samples were analyzed for the distribution and progression of chondrocyte death, using the Live/Dead assay. Five control joints, in which "fractures" were simulated by means of surgical osteotomy, were also similarly analyzed. RESULTS: In the impaction-fractured joints, chondrocyte death was concentrated in regions adjacent to fracture lines (near-fracture regions), as evidenced by fractional cell death significantly higher (P < 0.0001) than in central non-fracture (control) regions. Although nominally similar spatial distribution patterns were identified in the osteotomized joints, fractional cell death in the near-osteotomy regions was nine-fold lower (P < 0.0001) than in the near-fracture regions. Cell death in the near-fracture regions increased monotonically during 48 h after impaction, dominantly within 1 mm from the fracture lines. CONCLUSION: The impaction-fractured joints exhibited chondrocyte death characteristics reasonably consistent with those in human intra-articular fractures, but were strikingly different from those in "fractures" simulated by surgical osteotomy. These observations support promise of this new impaction fracture technique as a mechanical insult modality to replicate the pathophysiology of human intra-articular fractures in large animal joints in vivo.

Elevated tibiofemoral articular contact stress predicts risk for bone marrow lesions and cartilage damage at 30 months.

OBJECTIVE: As cartilage loss and bone marrow lesions (BMLs) are associated with knee joint pain and structural worsening, this study assessed whether non-invasive estimates of articular contact stress may longitudinally predict risk for worsening of knee cartilage morphology and BMLs. DESIGN: This was a longitudinal cohort study of adults aged 50-79 years with risk factors for knee osteoarthritis. Baseline and follow-up measures included whole-organ magnetic resonance imaging score (WORMS) classification of knee cartilage morphology and BMLs. Tibiofemoral geometry was manually segmented on baseline magnetic resonance imaging (MRI), and three-dimensional (3D) tibiofemoral point clouds were registered into subject-specific loaded apposition using fixed-flexion knee radiographs. Discrete element analysis (DEA) was used to estimate mean and peak contact stresses for the medial and lateral compartments. The association of baseline contact stress with worsening cartilage and BMLs in the same subregion over 30 months was assessed using conditional logistic regression. RESULTS: Subjects (N = 38, 60.5% female) had a mean ± standard deviation (SD) age and body mass index (BMI) of 63.5 ± 8.4 years and 30.5 ± 3.7 kg/m2 respectively. Elevated mean articular contact stress at baseline was associated with worsening cartilage morphology and worsening BMLs by 30 months, with odds ratio (OR) [95% confidence interval (CI)] of 4.0 (2.5, 6.4) and 6.6 (2.7, 16.5) respectively. Peak contact stress also was significantly associated with worsening cartilage morphology and BMLs {1.9 (1.5, 2.3) and 2.3 (1.5, 3.6)}(all P < 0.0001). CONCLUSIONS: Detection of higher contact stress 30 months prior to structural worsening suggests an etiological role for mechanical loading. Estimation of articular contact stress with DEA is an efficient and accurate means of predicting subregion-specific knee joint worsening and may be useful in guiding prognosis and treatment.

Physical performance characteristics of high-level female soccer players 12-21 years of age.

Performance assessment has become an invaluable component of monitoring player development and within talent identification programs in soccer, yet limited performance data are available for female soccer players across a wide age range. The aim of this study was to describe the physical performance characteristics of female soccer players ranging in age from 12 to 21 years. High-level female soccer players (n=414) were evaluated on linear sprinting (36.6 m with 9.1 m splits), countermovement jump (CMJ), and two agility tests. Separate one-way ANOVAs were used to compare performance characteristics between (1) each year of chronological age and (2) three age groups: 12-13 years, n=78, 14-17 years, n=223, and 18-21 years, n=113. Mean linear sprint speed over 9.1 m was similar across all chronological ages, however sprint speed over the final 9.1 m, CMJ height and agility scores improved until approximately 15-16 years. Outcomes from the group data indicated better performance on all tests for the 14-17-year-old group compared with the 12-13-year-old group. Additionally, sprint speed on the second and fourth 9.1 m splits and 36.6 m sprint speed as well as performance on the Illinois agility test was better in the 18-21-year-old group compared with the 14-17-year-old group. The findings from this study indicate that marked improvements of high intensity short duration work occur up until 15-16 years. Smaller gains in performance were observed beyond 16 years of age as evidenced by better performance on 36.6 m sprint speed, several sprint splits and the Illinois agility test in the college aged players (i.e., 18-21-year-old group).

Subchondral bone remodeling is related to clinical improvement after joint distraction in the treatment of ankle osteoarthritis.

OBJECTIVE: In osteoarthritis (OA), subchondral bone changes alter the joint's mechanical environment and potentially influence progression of cartilage degeneration. Joint distraction as a treatment for OA has been shown to provide pain relief and functional improvement through mechanisms that are not well understood. This study evaluated whether subchondral bone remodeling was associated with clinical improvement in OA patients treated with joint distraction. METHOD: Twenty-six patients with advanced post-traumatic ankle OA were treated with joint distraction for 3 months using an Ilizarov frame in a referral center. Primary outcome measure was bone density change analyzed on computed tomography (CT) scans. Longitudinal, manually segmented CT datasets for a given patient were brought into a common spatial alignment. Changes in bone density (Hounsfield Units (HU), relative to baseline) were calculated at the weight-bearing region, extending subchondrally to a depth of 8mm. Clinical outcome was assessed using the ankle OA scale. RESULTS: Baseline scans demonstrated subchondral sclerosis with local cysts. At 1 and 2 years of follow-up, an overall decrease in bone density (-23% and -21%, respectively) was observed. Interestingly, density in originally low-density (cystic) areas increased. Joint distraction resulted in a decrease in pain (from 60 to 35, scale of 100) and functional deficit (from 67 to 36). Improvements in clinical outcomes were best correlated with disappearance of low-density (cystic) areas (r=0.69). CONCLUSIONS: Treatment of advanced post-traumatic ankle OA with 3 months of joint distraction resulted in bone density normalization that was associated with clinical improvement.

Reliability of the CFTM and GPA methods for strain analysis at ultra-thin layers.

The reliability of the Computational Fourier Transform Moiré (CFTM) and Geometric Phase Analysis (GPA) techniques for strain analysis at ultra-thin layers has been investigated using computer-generated images. Our results revealed that the leakage effect creates error that is linearly dependent on the mask size used for Fourier filtering. Error due to the leakage effect has a significant impact on the analysis of strain for small-mismatched systems with low resolution in the original image. We demonstrate that the error due to the leakage effect can be minimized with improved resolution of the original image. In order to obtain a measurement of the reliability of the CFTM and GPA methods on ultra-thin layers, we systematically quantify the error due to the leakage effect as a function of image resolution and applied strain value for the original image. The presence of the leakage effect and the resulting limitations of the CFTM and GPA methods are demonstrated using a high-resolution transmission electron microscopy (HRTEM) image of an ultra-thin heterointerface from a strained layer superlattice.

Acute articular fracture severity and chronic cartilage stress challenge as quantitative risk factors for post-traumatic osteoarthritis: illustrative cases.

Novel biomechanical methods have been developed to objectively measure acute fracture severity (from inter-fragmentary surface area) and chronic contact stress challenge (from patient-specific finite element analysis) in articular fractures. These new methods help clarify the pathomechanics of the development of post-traumatic osteoarthritis, and can contribute directly to the clinical care of patients. In this manuscript, the value of these two new measures is demonstrated in three illustrative tibial plafond fracture cases, in which both metrics are correlated with cartilage status and with patient outcomes at a minimum of two years after injury. These clinical cases demonstrate the utility of new biomechanical variables to advance clinical research and patient care, by providing a basis to predict outcome and select treatment.

The effect of anterior-posterior shear load on the wear of ProDisc-L TDR.

The current wear-testing standard (ISO18192-1) for total disc replacement (TDR) requires only four degrees of freedom (DOF) inputs: axial load, flexion-extension, lateral bending and axial rotation. The study aim was to assess the effect of an additional DOF, anterior-posterior (AP) shear on the wear of the ProDisc-L TDR. A 5DOF simulator was used to test ProDisc-L implants under 4DOF and 5DOF conditions. The 4DOF conditions were defined by ISO18192-1 whilst the 5DOF used ISO18192-1 conditions with the addition of an AP load of +175 and -140 N (anterior and posterior, respectively), extrapolated from in vivo data. The implants were mounted such that the polyethylene insert could be removed for gravimetric measurements. Tests were run using bovine serum (15 g/l protein concentration) as a lubricant for five million cycles (MC), with measurements repeated every 1 MC. The mean wear rate in the 4DOF test was 12.7 +/- 2.1 mg/MC compared to 11.6 +/- 1.2 mg/MC in the 5DOF test. There were marked differences in the wear scars between 4DOF and 5DOF simulations. With 4DOF, wear scars were centralised on the dome of the insert, whilst 5DOF scars were larger, breaching the anterior rim of the dome causing deformation at the edge. The 4DOF wear test showed similar gravimetric wear rates to previously published ISO-tested TDRs. The addition of AP load was found to have no significant effect on the overall wear rate. However, there were pronounced differences in the respective wear scars, which highlights the need for more research in order to understand the factors that influence wear of TDR.

Wear simulation of ultra-high molecular weight polyethylene hip implants by incorporating the effects of cross-shear and contact pressure.

The effect of multi-directional cross-shear (CS) motion and contact pressure on ultra-high molecular weight polyethylene (UHMWPE) wear was investigated in this study, based on an integrated experimental and computational approach. The wear factor as a function of CS was determined experimentally from a multi-directional pin-on-plate wear tester under a nominal contact pressure of 1 MPa. A computational wear model was developed which included the effects of CS as well as the load and sliding distance imposed on the hip joint employing a UHMWPE cup against a metallic femoral head under both gait and Leeds ProSim hip joint simulator conditions. The CS ratios were quantified over the articular surface of the UHMWPE cup and the CS-dependent wear factors derived from multi-directional pin-on-plate studies were applied in the computational wear model. Outputs from the computational wear model were validated independently against an experimental hip simulator study. Comparisons of linear and volumetric wear were made between the computational wear model and the hip simulator testing for a nominal conventional (0 MRad) UHMWPE cup of 28mm diameter and a highly cross-linked (10 MRad) UHMWPE cup. The difference between the computed and experimental volumetric wear was approximately 30 per cent for the 0 MRad UHMWPE, although the worn areas between the prediction and the measurement were similar. For the 10 MRad UHMWPE, the discrepancy was reduced to 16 per cent. In both cases, the computational model predicted a lower wear rate than the experimental simulator testing. The effect of using alternative wear factors under a different nominal contact pressure of 3MPa was also considered. The input wear factor to the computational model, derived from a constant loaded pin-on-plate test configuration, may underestimate the dynamic effect due to the variation in the load in the hip joint simulator.

The effect of incongruity and instability on contact stress directional gradients in human cadaveric ankles.

OBJECTIVE: Measure incongruity and instability-associated changes in transient contact stress directional gradients in a human cadaveric ankle model. METHODS: Seven cadaveric ankles were subjected to quasi-physiologic forces and motion under intact conditions and with a stepoff incongruity of the anterior one-third of the distal tibia. Anterior/posterior forces were modulated to create incongruous specimens that either maintained a stable articulation between the talus and distal tibia or developed gross instability during motion. Real-time contact stresses were measured using a custom-designed ankle stress transducer at 132 Hz. Contact stress data were differentiated using a central-differencing formula to calculate transient contact stress directional gradients over the entire ankle articulation. RESULTS: Transient 95th percentile contact stress directional gradient values increased by 30 and 100%, respectively, in stable-incongruous and unstable-incongruous conditions compared to intact conditions. Compared to stable-incongruous conditions, transient contact stress directional gradients increased by 60% in unstable-incongruous conditions. CONCLUSIONS: Instability resulted in greater percentage increases in transient contact stress directional gradients compared to incongruity. Pathologic increases in contact stress directional gradients potentially play an important role in the etiology of post-traumatic arthritis.

Tibial plafond fractures treated by articulated external fixation: a randomized trial of postoperative motion versus nonmotion.

OBJECTIVES: Assess whether postoperative ankle motion after fixation of a fracture of the tibial plafond, treated with articulated external fixation, leads to a better outcome when compared with similar treatment without postoperative ankle motion. DESIGN: Multicenter randomized trial. SETTING: Three Level I trauma centers. PATIENTS/PARTICIPANTS: Fifty-five patients were enrolled and entered into a Web-based database and randomized into 1 of 2 groups. Forty-one patients were evaluated at a 1-year follow-up visit, and 31 were seen at 2 years or longer after injury. INTERVENTION: Patients were treated with a hinged external fixator and limited internal fixation of the articular surface. They were divided postoperatively into two groups, 1 of which had a locked hinge and the other had a mobile hinge and a motion protocol. MAIN OUTCOME MEASUREMENTS: A general health status questionnaire, the SF-36 (short-form 36); a joint-specific ankle questionnaire, the Ankle Osteoarthritis Score (AOS); and range of motion (ROM) of the ankle joint. RESULTS: There were no significant differences between the two groups at either follow-up interval in the ankle ROM measurement, the AOS pain and disability scale, or the SF-36 physical component summary (PCS) and mental component summary (MCS) scales. CONCLUSIONS: These results indicate that treatment protocols that use long periods of cross-joint external fixation that immobilizes the ankle as definitive treatment result in similar patient outcomes compared to otherwise identical treatment protocols that incorporate and use an articulated hinge for ankle motion. However, the results should be interpreted with caution because the patient numbers were too small to detect potentially meaningful differences in outcomes and the follow-up was too short to assess for differences in the development of arthrosis.

Positional characteristics of physical performance in Division I college female soccer players.

AIM: The purpose of this investigation was to determine and compare positional characteristics (physical and physiological) of Division I college female soccer players. METHODS: Sixty-four university soccer players volunteered to participate and were evaluated at the end of their spring season. Test items included height and body mass, acceleration (9.14 m), speed (18.28 and 36.58 m), agility (Pro-agility and Illinois), lower body power (countermovement jump), and estimated aerobic capacity (20 meter beep test). RESULTS: Mean (+/-SD) height and body mass were 168.4+/-5.9 cm and 64.8+/-5.9 kg, respectively. No significant differences were observed between positions, however defenders and keepers tended to be taller and heavier compared to forwards and midfielders. Positional differences did not appear for any of the other performance tests. Yet, defenders tended to show slightly slower times for the speed and agility tests while keepers tended to be slower on the agility tests compared to forwards and midfielders. Aerobic capacity was similar across the four positions. CONCLUSIONS: Similar physical and physiological characteristics were found within this sample of Division I female college soccer players.

Stance-phase aggregate contact stress and contact stress gradient changes resulting from articular surface stepoffs in human cadaveric ankles.

OBJECTIVE: Determine how stepoff incongruities of the distal tibia affect aggregate (whole-cycle) contact stresses and contact stress gradients for a complete motion cycle in human cadaveric ankles. METHOD: Ten human cadaveric ankles were subjected to quasiphysiologic forces during stance-phase range of motion. Each specimen was loaded intact, with anatomic reduction of the anterolateral quarter of the distal tibia, and with increasing stepoffs of the anterolateral fragment up to 4.0mm. Transient contact stresses were measured using a custom-built, real-time stress transducer that sampled stresses at 132Hz at 1472 separate foci (sensels). Aggregate stresses were calculated by summing the sequential transient stress values multiplied by the transient sampling duration for the complete motion cycle at each sensel. Transient contact stress gradients were calculated at each sensel using a central-differencing formula applied to adjacent transient stress measurements. Aggregate contact stress gradients were calculated by vector summation of sequential transient stress gradients multiplied by the sampling duration. RESULTS: Compared to the intact configuration, anatomic reduction of the fragment caused minimal changes in aggregate contact stresses and stress gradients (30% increase compared to intact values). In contrast, stepoffs caused substantial increases (200% increase compared to intact values) in peak and mean whole-cycle stresses and gradients. CONCLUSIONS: Aggregate contact stresses and stress gradients quantify loading history for the complete motion cycle. Incongruity-associated changes in aggregate stresses and gradients are a surrogate for "accumulated" damage over a motion cycle in stepoff specimens. These loading abnormalities may be important determinants of posttraumatic arthritis.

Activity-dependence of the "safe zone" for impingement versus dislocation avoidance.

Presently, the basis for optimal cup positioning to minimize the likelihood of dislocation rests upon subjective clinical impressions. To help elucidate optimal cup positioning more objectively, and to clarify the distinction between impingement avoidance and dislocation avoidance, kinematic and kinetic inputs for seven dislocation-prone activities were applied to finite element models of a contemporary 22-mm modular total hip reconstruction. Twenty-five cup placement positions (combinations of 30, 40, 50, 60, and 70 degrees of abduction with 0, 10, 20, 30, and 40 degrees of anteversion) were chosen to include the conventional 'safe zone' of 30-50 degrees of tilt and 5-25 degrees of anteversion. Activities studied were: rising from a low seat (toilet) and from a normal height chair, leg-crossing while seated, tying a shoe from a seated position, bending at the hip from an erect stance to retrieve an object on the floor (stooping), a standing pivot maneuver, and rolling over in bed. Neck-on-cup impingement occurred during one or more of the dislocation-prone activities at all 25 cup positions. Of the 175 combinations of cup position and kinetic challenge, dislocation and impingement events both occurred for 51 situations, while impingement occurred in 45 instances without dislocation, and dislocation occurred in 10 instances without impingement. Neither dislocation nor impingement was observed in the 69 other combinations of cup position and loading challenge. Kappa statistics showed dislocation and impingement, as outcome measures of activity-dependent challenges, exhibit little more than chance agreement. Therefore, the use of impingement range of motion (ROM) as a predictor of a given cup position's vulnerability to dislocation should be viewed cautiously.

The effect of accuracy of implantation on range of movement of the Scandinavian Total Ankle Replacement.

When performing the Scandinavian Total Ankle Replacement (STAR), the positioning of the talar component and the selection of mobile-bearing thickness are critical. A biomechanical experiment was undertaken to establish the effects of these variables on the range of movement (ROM) of the ankle. Six cadaver ankles containing a specially-modified STAR prosthesis were subjected to ROM determination, under weight-bearing conditions, while monitoring the strain in the peri-ankle ligaments. Each specimen was tested with the talar component positions in neutral, as well as 3 and 6 mm of anterior and posterior displacement. The sequence was repeated with an anatomical bearing thickness, as well as at 2 mm reduced and increased thicknesses. The movement limits were defined as 10% strain in any ligament, bearing lift-off from the talar component or limitations of the hardware. Both anterior talar component displacement and bearing thickness reduction caused a decrease in plantar flexion, which was associated with bearing lift-off. With increased bearing thickness, posterior displacement of the talar component decreased plantar flexion, whereas anterior displacement decreased dorsiflexion.

The effect of agility ankle prosthesis misalignment on the peri-ankle ligaments.

In the Agility total ankle replacement system, motion is constrained by the implant's articulating surfaces and the peri-ankle ligaments. The effects of plausibly occurring implant malpositioning on peri-ankle ligament functional extension during walking were explored in this study. The intent was to determine whether certain ligaments could serve as guides to assist in proper component positioning at implantation. Using a cadaver preparation with simulated physiologic motion and loading, we monitored change of ligament length of the anterior talofibular, posterior talofibular, calcaneofibular, and tibiocalcaneal ligaments resulting from controlled malpositioning of the tibial component relative to a neutral position. During a simulated walking cycle, effects of mediolateral and anterior/posterior translation, internal and external rotation, inversion and eversion, and elevation of the component were evaluated. In all cases, tibial component displacement from the neutral position caused atypical length change in one or more of the peri-ankle ligaments. In particular, anterior/posterior displacement significantly changed the lengthening behavior of all four tested ligaments. The anterior talofibular ligament was sensitive to transverse plane displacements, whereas the tibiocalcaneal ligament was sensitive to coronal plane displacements. For the Agility prosthesis, these two ligaments seem to be sensitive guides for tibial component positioning at implantation.

A novel cell entry pathway for a DAF-using human enterovirus is dependent on lipid rafts.

The glycosylphosphatidylinositol (GPI)-anchored complement regulatory protein decay-accelerating factor (DAF) is used by a number of enteroviruses as a receptor during infection. DAF and other GPI-anchored proteins can be found in cholesterol-rich ordered domains within the plasma membrane that are known as "lipid rafts." We have shown, by using drugs to specifically inhibit various endocytosis routes, that infection by a DAF-using strain of echovirus 11 (EV11) is dependent upon cholesterol and an intact cytoskeleton, whereas a non-DAF-using mutant derived from it was unaffected by these drugs. Using RNA transfection and virus-binding assays, we have shown that this requirement for cholesterol, the actin cytoskeleton, and the microtubule network occurs postbinding of the virus but prior to uncoating of the RNA, indicating a role during virus entry. Confocal microscopy of virus infection supported the role of cholesterol and the cytoskeleton during entry. In addition, [(35)S]methionine-labeled DAF-using EV11, but not the non-DAF-using EV11, could be copurified with lipid raft components during infection after Triton X-100 extraction. These data indicate that DAF usage by EV11 enables the virus to associate with lipid rafts and enter cells through this novel route.

Mobility and contact mechanics of a rotating platform total knee replacement.

Despite their increasing clinical use, mobile-bearing total knee replacements have not been well characterized biomechanically. An experimental and finite element analysis was done to assess the mobility and contact mechanics of a widely used rotating platform total knee replacement. Parameters that varied were axial load, condylar load allocation, flexion angle, and static versus dynamic loading. Similar results from the physical model and finite element model lend credence to the validity of the findings. The torque required to initiate rotation (static torque) was greater than that to sustain rotation (dynamic torque). At four times body weight axial load, peak resisting torque measured was 9.47+/-0.61 and 5.51+/-0.38 N-m, for static and dynamic torque, respectively. A 60-40 condylar load allocation produced slightly less resisting torque than the 50-50 load. For all practical purposes, the polyethylene insert rotated simultaneously with the femoral component, leading to maintenance of high contact area, desirable behavior clinically. Walking cycle simulations produced a total axial rotation range of motion of 6 degrees. The high frictional torques observed at the mobile interface may explain why a percentage of these mobile-bearings fail to rotate under routine functional load.

Experimental and computational simulation of total hip arthroplasty dislocation.

Other than fatal pulmonary embolism and deep infection, dislocation following total hip replacement remains probably the most vexing complication to patient and surgeon. Subluxation and dislocation are complex, poorly understood phenomena. Many important questions in this area unfortunately do not lend themselves well to clinical or registry study. Appropriate realistic laboratory models have been lacking. This article synthesizes new work undertaken independently by two groups of biomechanical investigators using very different, but complimentary, methodologies to study the mechanisms of dislocation, and especially the influence of specific design and surgical variables.