Higher strains in the inner region of the meniscus indicate a potential source for degeneration.

Complex structural properties of menisci can be characterized in part by their inhomogeneous strain response under compression. This pilot study explored the feasibility to quantify characteristic strain distributions on meniscus cross-sections subjected to static compression using electronic speckle pattern interferometry (ESPI). Cross-sectional specimens of 5-mm thickness were harvested from eight human menisci. After application of 20% pre-strain, strain maps in response to 10µm compression were captured with ESPI. The 10µm compression induced an aggregate strain of nominally 0.14% and resulted in highly non-uniform strain distributions. Local compressive strain captured by ESPI ranged from 0.03% to 0.7%. The highest strain was in the central region of meniscus cross-sections, and the lowest magnitude of strain was at the femoral surface of the meniscus. After stratifying for age, peak compressive strain in older menisci (71±6 years, n=4) was 0.33%±0.09, compared to 0.25%±0.06 in younger menisci (34±9 years, n=4). In conclusion, this study captured for the first time continuous strain distribution maps over entire meniscus cross-sections. The non-uniform strain distributions demonstrated inhomogeneous structural properties. Age-related differences in characteristic strain distributions likely represent degenerative changes. As such, ESPI provides a novel strategy of further characterize meniscal function and degeneration.

Visible light photoinitiation of mesenchymal stem cell-laden bioresponsive hydrogels.

Biological activity can be added to synthetic scaffolds by incorporating functional peptide sequences that provide enzyme-mediated degradation sites, facilitate cellular adhesion or stimulate signaling pathways. Poly(ethylene glycol) diacrylate is a popular synthetic base for tissue engineering scaffolds because it creates a hydrophilic environment that can be chemically manipulated to add this biological functionality. Furthermore, the acrylate groups allow for encapsulation of cells using photopolymerization under physiological conditions. One complication with the addition of these peptides is that aromatic amino acids absorb light at 285 nm and compete with the ultraviolet (UV)-sensitive photoinitiators such as IrgacureTM 2959 (I2959), the most commonly used initiator for cytocompatible photoencapsulation of cells into synthetic scaffolds. In this study we define non-toxic conditions for photoencapsulation of human mesenchymal stem cells (hMSC) in PEGDA scaffolds using a visible light photoinitiator system composed of eosin Y, triethanolamine and 1-vinyl-2-pyrrolidinone. This visible light photoinitiator produced hydrogel scaffolds with an increased viability of encapsulated hMSCs and a more tightly crosslinked network in one-third the time of UV polymerization with I2959.

Operative stabilization of flail chest injuries: review of literature and fixation options.

BACKGROUND: Flail chest injuries cause significant morbidity, especially in multiply injured patients. Standard treatment is typically focused on the underlying lung injury and involves pain control and positive pressure ventilation. Several studies suggest improved short- and long-term outcomes following operative stabilization of the flail segments. Despite these studies, flail chest fixation remains a largely underutilized procedure. METHODS: This article reviews the relevant literature concerning flail chest fixation and describes the different implants and techniques available for fixation. Additionally, an illustrative case example is provided for description of the surgical approach. RESULTS: Two prospective randomized studies, five comparative studies, and a number of case series documented benefits of operative treatment of flail chest injuries, including a decreased in ventilation duration, ICU stay, rates of pneumonia, mortality, residual chest wall deformity, and total cost of care. Historically, rib fractures have been stabilized with external plates or intramedullary implants. The use of contemporary, anatomically contoured rib plates reduced the need for intraoperative plate bending. Intramedullary rib splints allowed less-invasive fixation of posterior fractures where access for plating was limited. CONCLUSION: Operative treatment can provide substantial benefits to patients with flail chest injuries and respiratory compromise requiring mechanical ventilation. The use of anatomically contoured rib plates and intramedullary splints greatly simplifies the procedure of flail chest fixation.

Biomechanical rationale and evaluation of an implant system for rib fracture fixation.

BACKGROUND: Biomechanical research directed at developing customized implant solutions for rib fracture fixation is essential to reduce the complexity and to increase the reliability of rib osteosynthesis. Without a simple and reliable implant solution, surgical stabilization of rib fractures will remain underutilized despite proven benefits for select indications. This article summarizes the research, development, and testing of a specialized and comprehensive implant solution for rib fracture fixation. METHODS: An implant system for rib fracture fixation was developed in three phases: first, research on rib biomechanics was conducted to better define the form and function of ribs. Second, research results were implemented to derive an implant system comprising anatomical plates and intramedullary rib splints. Third, the functionality of anatomic plates and rib splints was evaluated in a series of biomechanical tests. RESULTS: Geometric analysis of the rib surface yielded a set of anatomical rib plates that traced the rib surface over a distance of 13-15 cm without the need for plate contouring. Structurally, the flexible design of anatomic plates did not increase the native stiffness of ribs while restoring 77% of the native rib strength. Intramedullary rib splints with a rectangular cross-section provided 48% stronger fracture fixation than traditional intramedullary fixation with Kirschner wires. CONCLUSION: The anatomic plate set can simplify rib fracture fixation by minimizing the need for plate contouring. Intramedullary fixation with rib splints provides a less-invasive fixation alternative for posterior rib fracture, where access for plating is limited. The combination of anatomic plates and intramedullary splints provides a comprehensive system to manage the wide range of fractures encountered in flail chest injuries.

A new articulated elbow external fixation technique for difficult elbow trauma.

Articulated external fixation of the elbow allows aggressive elbow range of motion while protecting the joint and periarticular structures from excessive forces. A technique for aligning a monolateral-hinged fixator to the rotational axis of the elbow without the use of an invasive axis pin has been developed. Thirteen patients with acute and chronic post-traumatic elbow problems were treated over a four year period with this technique. An average arc of motion of 84 degrees was achieved in the frame. Frames were removed at an average of 7.6 weeks. Complications were confined to pin tract infections. In 11 patients followed for an average of 35 weeks the average arc of motion was 81 degrees. Further experience is required to determine the role of this device and to identify which elbows achieve the most benefit compared to conventional techniques.

Radiographic determinants of the elbow rotation axis: experimental identification and quantitative validation.

This study identifies new radiographic indices to approximate the location of the elbow rotational axis. With use of electromagnetic motion tracking source data, the average rotational axis of the ulnohumeral articulation was calculated in seven cadaveric specimens. Quasi-lateral radiographs of the elbow specimens were then analyzed to identify radiographic landmarks of the elbow axis in the lateral view. The spatial relationships of these landmarks with the elbow aligned on-axis were contrasted with their relationships in eight distinct off-axis alignments. Elbow axis orientation in the transverse plane (internal/external rotation) was identified by the location of a dense intramedullary cortical line, appearing in the projection of the distal humerus in relation to the periosteal surface of the posterior cortex of the humerus. This intramedullary line corresponds to the posteromedial cortex of the distal humerus. Correct alignment occurred when this line laid 27.1+/-3.7% of the anteroposterior humeral diameter anterior from the periosteal surface of the posterior cortex. Axis orientation in the coronal plane (abduction/adduction) was identified by the concentric appearance of radiographic arcs formed by the capitellum, trochlear sulcus, and medial trochlear flange. Using these radiographic indices, three orthopaedic surgeons were able to fluoroscopically align the elbow along the axis of rotation with an accuracy of 3.7+/-1.8 degrees. These results are immediately applicable to fluoroscopic identification of the elbow axis. This technique can be used to increase the accuracy of hinge placement during application of hinged external fixation or distraction arthroplasty.

Assessment of elbow joint kinematics in passive motion by electromagnetic motion tracking.

This research provides a detailed analysis of the kinematics of passive elbow motion. It quantifies how closely humeroulnar kinematics approximates rotation around a fixed axis. The results are clinically relevant for emerging treatment modalities that impose an artificial hinge to the elbow joint, such as total elbow arthroplasty and articulated external fixation. In a cadaveric study of seven specimens, we quantified ulnar rotation around the humerus in terms of instantaneous screw displacement axes calculated from electromagnetic motion-tracking source data. This methodology enabled description of the complex excursion of the elbow axis in terms of translation and orientation changes of the screw displacement axes over the range of motion. Furthermore, we analyzed the envelope of joint laxity for elbow motion under applied small varus and valgus moments. In addition, radiographic landmarks of clinical utility for axis location were evaluated by visualizing the elbow's radiographic appearance when viewed from along the calculated best-fit (average) rotation axis. Over the normal range of elbow motion, the screw displacement axis varied 2.6-5.7 degrees in orientation and 1.4-2.0 mm in translation. All instantaneous rotation axes nearly intersected on the medial facet of the trochlea. The breadth of the envelope of varus-valgus joint laxity was greatest within the initial 40 degrees of flexion and decreased by a factor of approximately two for flexion angles exceeding 100 degrees.

Hinged external fixation of the elbow: optimal axis alignment to minimize motion resistance.

OBJECTIVE: To establish an optimal single hinge axis position for application of hinged external fixation to the elbow joint. DESIGN: Cadaveric biomechanical investigation. SETTING: A customized motion transducer applied passive elbow motion to six cadaveric upper extremities. The instant rotation axis of the humero-ulnar articulation was determined from three-dimensional kinematic data acquired by an electromagnetic motion tracking system. For each specimen, an optimal fixator hinge position was calculated from these motion data. INTERVENTION: A prototype articulated external fixator was applied to the elbow, first with its hinge aligned along the computed optimal position. Then the fixator was mounted in sixteen distinct off-axis positions. MAIN OUTCOME MEASURE: Additional resistance to joint motion (in terms of energy) corresponding to deliberately introduced amounts of relative malalignment between the optimal elbow axis and the actual fixator hinge axis. RESULTS: Aligning the fixator hinge along the optimized axis position resulted in a minimal amount of energy (0.15 joules) needed to rotate the elbow through a prescribed range of motion. Malpositioning the hinge by ten millimeters caused up to ten times that amount of motion resistance. CONCLUSIONS: An optimal fixator hinge position can be determined to minimize the increase in motion resistance due to fixator application. The severely increased motion resistance associated with small amounts of malalignment between the fixator hinge and the anatomic elbow axis suggests the need for highly accurate fixator hinge application.

Gap junctions regulate responses of tendon cells ex vivo to mechanical loading.

Avian digital flexor tendons were used with a device to apply load ex vivo to study the effects on deoxyribonucleic acid and collagen synthesis when cell to cell communication is blocked. Flexor digitorum profundus tendons from the middle toe of 52-day-old White Leghorn chickens were excised and used as nonloaded controls, or clamped in the jaws of a displacement controlled tissue loading device and mechanically loaded for 3 days at a nominal 0.65% elongation at 1 Hz for 8 hours per day with 16 hours rest. Tendon samples were radiolabeled during the last 16 hours with 3H-thymidine to monitor deoxyribonucleic acid synthesis or with 3H-proline to radiolabel newly synthesized collagen. Cyclic loading of whole avian flexor tendons stimulated deoxyribonucleic acid and collagen synthesis, which could be blocked with octanol, a reversible gap junction blocker. Cells from human digital flexor tendon were used to populate a rectangular, three-dimensional, porous, polyester foam that could be deformed cyclically in vitro. Together, these results support the hypothesis that tendon cells must communicate to sustain growth and matrix expression and that an engineered three-dimensional construct can be used to study responses to mechanical load in vitro.

Articulated external fixation of the ankle: minimizing motion resistance by accurate axis alignment.

This study describes how an optimal single hinge axis position can be established for the application of articulated external fixation to the ankle joint. By deliberately introducing various amounts of relative mal-alignment between the optimal talocrural joint axis and the actual fixator hinge axis, it was possible to measure the corresponding amounts of additional resistance to joint motion. In a cadaveric study of six ankle specimens, we determined the instant axis of rotation of the talocrural joint from 3-D kinematic data. acquired by an electromagnetic motion tracking system. For each specimen, an optimal fixator hinge position was calculated from these motion data. Compared to the intact natural joint, aligning the fixator along the optimized axis position caused a moderate increase in energy (0.14 J) needed to rotate the ankle through a prescribed plantar/dorsiflexion range. However, malpositioning the hinge by 10 mm caused more than five times that amount of increase in motion resistance. While articulated external fixation with limited internal fixation can establish a favorable environment for the repair of severe injuries such as tibial pilon fractures, the large additional resistance to motion accompanying a malpositioned fixator axis suggests the development of untoward intra-articular forces that could act to disturb fragment alignment.

Loading paradigms--intentional and unintentional--for cell culture mechanostimulus.

Recent interest in the response of cells or tissues to mechanical stimuli has led to the introduction of a variety of laboratory devices designed to deliver quantified mechanical inputs to culture systems. Such devices commonly rely upon distention of a flexible culture substrate, achieved either by direct platen abutment or by transmural pressure differentials. Unfortunately, the substrate distentions in such systems are often unintentionally nonuniform, and typically also induce motions in the overlying liquid nutrient medium--motions which in turn exert unintended reactive stresses upon the culture layer. In order to characterize the nature of these reactive fluid stresses, computer models have been developed for the nutrient medium flow fields (ie, the velocity and pressure distributions) for three established contemporary cell culture mechanostimulus systems. Temporal and spatial distributions of reactive normal and shear stresses are reported for typical duty cycles in these respective instruments.

Factors influencing accuracy of screw displacement axis detection with a D.C.-based electromagnetic tracking system.

Recent technical improvements and cost reductions in electromagnetic motion tracking systems invite their application to motion axis determination in the surgical setting. After evaluation of the accuracy of a state-of-the-art D.C. electromagnetic tracking system, which generates complete three-dimensional kinematic outputs from just a single receiver, we calculated screw displacement axes (SDA's) from its source data. The accuracy of SDA determination from such source data was evaluated for various rotational increment sizes around a revolute joint. A novel smoothing procedure, customized for this type of source data, was developed, enabling SDA detection from incremental rotations of less than 1 deg, at an accuracy appropriate for intra-operative measurement of human joint motion. Examples of SDA determination are given for motion tracking of a ball joint and of the elbow articulation.

A cell strain system for small homogeneous strain applications.

A cell culture system has been developed that enables application of well characterized, homogeneously distributed cyclic strains to monolayer cell cultures. Optically clear silicone culture dishes atop Plexiglas base plates are deformed by four-point bending of flexible silicone culture wells driven in user specified strain cycle patterns using computer controlled electromagnetic linear actuators. Cyclic mechano-transduction can be induced in amplitudes of 0 to 3000 mustrain, in frequencies of 0 to 30 Hz and in any specified strain cycle pattern. The cell culture system, which contains six simultaneously driven culture wells, has been mechanically characterized by holographic interferometry, laser displacement sensor recordings of the dish surfaces, strain gauge monitoring of the base plates, and finite element modeling of the dishes on the base plates. The standard deviation of the strain amplitudes among the six simultaneously stimulated culture wells is less than 5%. The cell culture system allows accurate generation of small magnitudes of well characterized, homogeneous strain, easy handling of the culture wells, flexible setting of cyclic strain pattern parameters, simultaneous stimulation of 6 culture wells, and light microscopic observation of the cell cultures.