A clinically realistic large animal model of intra-articular fracture that progresses to post-traumatic osteoarthritis.

OBJECTIVE: Translation of promising treatments for post-traumatic osteoarthritis (PTOA) to patients with intra-articular fracture (IAF) has been limited by the lack of a realistic large animal model. To address this issue we developed a large animal model of IAF in the distal tibia of Yucatan minipigs and documented the natural progression of this injury. DESIGN: Twenty-two fractures were treated using open reduction and internal fixation with either an anatomic reduction or an intentional 2-mm step-off. Pre-operatively, and 3 days, 1, 2, 4, 8, and 12 weeks post-operatively, animals were sedated for synovial fluid draws and radiographs. Limb loading was monitored at the same time points using a Tekscan Walkway. Animals were sacrificed at 12 weeks and the limbs were harvested for histological evaluation. RESULTS: All animals achieved bony union by 12 weeks, facilitating nearly complete recovery of the initial 60% decrease in limb loading. TNFα, IL1ß, IL6, and IL8 concentrations in the fractured limbs were elevated (P < 0.05) at specific times during the 2 weeks after fracture. Histological cartilage degeneration was more severe in the step-off group (0.0001 < P < 0.27 compared to normal) than in the anatomic reconstruction group (0.27 < P < 0.99 compared to normal). CONCLUSIONS: This model replicated key features of a human IAF, including surgical stabilization, inflammatory responses, and progression to osteoarthritic cartilage degeneration, thereby providing a potentially useful model for translating promising treatment options to clinical practice.

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.

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.

Technical feasibility of personalized articulating knee joint distraction for treatment of tibiofemoral osteoarthritis.

BACKGROUND: Knee osteoarthritis is a highly prevalent degenerative joint disorder characterized by joint tissue damage and pain. Knee joint distraction has been introduced as a joint preserving surgical procedure to postpone knee arthroplasty. An often used standard externally fixation device for distraction poses a burden to patients due to the absence of joint flexion during the 6weeks treatment. Therefore, a personalized articulating distraction device was developed. The aim of this study was to test technical feasibility of this device. METHODS: Based on an often applied rigid device, using equal bone pin positions and connectors, a hinge mechanism was developed consisting of a cam-following system for reproducing the complex joint-specific knee kinematics. In support, a device was developed for capturing the joint-specific sagittal plane articulation. The obtained kinematic data were translated into joint-specific cam shapes that were installed bilaterally in the hinge mechanism of the distraction device, as such providing personalized knee motion. Distraction of 5mm was performed within a range of motion of 30deg. joint flexion. Pre-clinical evaluation of the working principle was performed on human cadaveric legs and system stiffness characteristics were biomechanically evaluated. FINDINGS: The desired range of motion was obtained and distraction was maintained under physiologically representative loading. Moreover, the joint-specific approach demonstrated tolerance of deviations from anatomical and alignment origin during initial placement of the developed distraction device. INTERPRETATION: Articulation during knee distraction is considered technically feasible and has potential to decrease burden and improve acceptance of distraction therapy. Testing of clinical feasibility is warranted.

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.

Indentation assessment of biphasic mechanical property deficits in size-dependent osteochondral defect repair.

The apparent biphasic material properties of 10-month osteochondral defect repair tissue were determined for a series of full thickness defects of 1, 3, or 5 mm diameter, created in weight-bearing regions of 48 canine femoral condyles. Load cell recordings from indentation tests were compared with resultant contact forces computed using a corresponding linear biphasic finite element model. The spread of cartilage engagement by a spherical ended indentor was modeled by successively imposing an impenetrability kinematic boundary condition at cartilage surface nodes for which incipient indentor surface penetration was detected. For each indentation test, a least-squares-error curve fitting procedure was used to identify a set of biphasic coefficients (aggregate modulus, permeability, Poisson ratio) that closely modeled experimental behavior. In the near neighborhood of best-fit, the finite element solutions were found to be much more sensitive to aggregate modulus perturbations than to permeability permutations, suggesting that perceived permeability increase may be of lesser value as a discriminant of repair tissue inadequacy. Compared to surrounding cartilage, the repair tissue for all defect sizes had statistically significant decreases in aggregate modulus and in Poisson ratio (much more so for 3 and 5 mm defects than for 1 mm defects). The two larger defect diameters had significant increases in permeability, whereas the 1 mm defects did not. While the material property deficits were consistent, substantial and comparable to those in other recent animal models of osteochondral defect repair, the size-dependence per se of the observed constitutive differences was only modest.

An algorithm for approximate crinkle artifact compensation in pressure-sensitive film recordings.

An objective, empirically based image-processing technique was devised to compensate for the presence of crinkle artifact in Pressensor pressure-sensitive film recordings. A spherical indentor was used to produce film stains which deliberately included radially directed artifact streaks, superimposed upon otherwise smooth, nearly axisymmetric stain recordings. An interactive, threshold-based search algorithm was developed to delineate explicitly the perimeters of specific artifacts present within manually (cursor) circumscribed regions where crinkle features were visually apparent. Three mathematical artifact transformation operators were parametrically evaluated in terms of their ability to approximate objectively the corresponding artifact-free axisymmetric pressure fields. All three operators were found to reduce substantially the quantitative deviation from the idealized distributions. When appropriately tuned transformation operators were applied to typical in vitro intraarticular contact stains, the visual prominence of crinkle artifact features was markedly reduced.

The effects of freezing or freeze-drying on the biomechanical properties of the canine intervertebral disc.

The transplantation of spinal allografts to correct defects that include disc, body, or segments of both is currently under experimental investigation. A method of graft preservation that will least compromise the biomechanical integrity of the bodies or discs is required. Using a five-mode biomechanical analysis, the authors compared the change in stiffness of ten preserved canine spines. The freeze-dried specimens lost a significantly greater amount of stiffness in compression, flexion, extension, and torsion than did the frozen specimens. Therefore, from a biomechanical viewpoint, deep-freezing is superior to freeze-drying for spinal allograft preservation.

Vertebral column allografts for the treatment of segmental spine defects. An experimental investigation in dogs.

Vertebral column allografts, with their intervertebral discs, were implanted into thoracic spine defects (T7-T9) in 11 dogs in an attempt to re-establish spinal stability and preserve spinal biomechanics. Before implantation, the allografts were harvested under sterile conditions from similar-sized dogs and deep frozen at -80 C. The animals were followed for 18 months postoperatively. Radiographs demonstrated gradual loss of intervertebral disc height. Biomechanical analysis showed that the dogs with allografts had no significant difference in spine stiffness compared with normal spines in compression, flexion, and extension testing. Control spines that had been fused were significantly stiffer than the allograft spines in all modes tested (P less than 0.05). Histologic analysis showed incorporation of the allograft but with incomplete revascularization of the allograft's eighth thoracic body. This investigation found that vertebral body allografts with intervertebral discs can function successfully for 18 months in a canine model. This research may assist in the development of physiologic treatment for spinal deficiencies in humans.

Peripheral tears of the meniscus. The effect of open versus arthroscopic repair on intraarticular contact stresses in the human knee.

The mechanical properties of the repaired meniscus may affect its ability to heal and to protect the articular surface against degenerative changes. We compared the immediate biomechanical consequences of open versus arthroscopic repair in the human cadaver knee. Additionally, having measured postoperative stresses at various degrees of knee flexion, we have addressed the effect of tethering of the meniscus, a question relevant to both meniscus repair and replacement. Peak stresses were measured by the Pressensor system. Fresh human cadaver knees were subjected to loading in an Instron unit, on an unconstrained base. Instantaneous loads were applied with the knee in 0 degree, 30 degrees, or 60 degrees of flexion, and stress distributions were measured after repair of a 2 cm peripheral tear, by an open or arthroscopic approach. The results of loading experiments on five knees revealed no statistically significant differences between stresses after the two repairs. Similarly, there was no statistically significant difference between the normal and repaired menisci. In our model, this suggests that the immediate biomechanical consequences of open and arthroscopic repair are equivalent and that the "tethered" meniscus distributes loads as well as the normal meniscus.

A clinical and comparative biomechanical evaluation of proximal femoral osteotomy fixation in children.

This study compared the biomechanical characteristics of a 90 degrees infant blade plate construct and anteriorly applied 120 degrees angled plate construct in response to an axial compression load. The blade plate construct stiffness was three times greater than the angled plate construct (238.9 vs 85.7 N/deg). Four degrees failure load was also significantly higher for the blade plate. Despite inferior biomechanical characteristics, the 120 degrees angled plate functioned very satisfactorily in a retrospective clinical review of 21 proximal femoral varus osteotomies. The authors conclude that the 120 degrees angled plate has practical benefits in terms of ease of insertion and intraoperative adjustability which may outweigh its biomechanical inferiority.

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.

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.