ABSTRACT
Despite the effectiveness of external fixation in the treatment of complex wrist fractures, the complication rate for this modality ranges from 20% to 62%. Common complications are related to the use of percutaneous metal pins and result in an average reoperation rate of 16%. In addition, external fixation is unable to prevent dorsal collapse of the radius or maintain the normal palmar tilt of the radiocarpal joint surface. This complication may predispose to posttraumatic wrist instability and arthritis. The problems with external fixation have prompted a search for a better treatment option. An internal fixator placed through limited incisions on the dorsal aspect of the radius and spanning the fracture site can, in theory, provide the benefits of external fixation without the associated morbidity. This study determined the biomechanical efficacy of internal fixators compared with external fixators using a standardized model for an unstable wrist fracture. Two commercially available metal plates were used as internal fixators. Biomechanical testing of the devices was done, and stiffness was determined. Results showed that the internal fixators were significantly stiffer than were the external fixators in axial loading. Failure in axial loading, specifically compression, is a consistent reason for loss of reduction in intraarticular distal radius fractures. The clinical implications of these results suggest that an internal fixator theoretically can prevent loss of reduction in the axial plane and maintain palmar tilt by acting as a rigid dorsal buttress. In addition, the use of an internal fixator potentially decreases the high morbidity associated with external fixation. Additional investigation into the clinical application of internal fixators for distal radius fractures is needed.
Subject(s)
Internal Fixators , Radius Fractures/surgery , Biomechanical Phenomena , External Fixators , Fracture Fixation, Internal , Humans , Materials TestingABSTRACT
The effect of loading rate on specimen calibration was investigated for an implantable force sensor of the two-point loading variety. This variety of sensor incorporates a strain gage to measure the compressive load applied to the sensor due to tensile loading in a soft tissue specimen. The Achilles tendon in each of four human cadaveric lower extremities was instrumented with a force sensor and then loaded in tension using a materials testing machine. Each specimen was tensile tested at three different displacement rates, 0.25, 2.5 and 12.7 cm s(-1), corresponding with mean loading rates of 33.8, 513.2, and 2838.6 N s(-1), respectively. A calibration curve relating the force sensor signal and applied tendon tension was generated for each specimen/ displacement rate combination. For each specimen, calibration curves were compared by calculating an RMS error for the entire data set (eRMS = 1.6% of the full load value) and a coefficient of determination, R2, of a curve fit through all of the data (R2 = 99.6%). Over the range of rates tested, no measurable change in sensor sensitivity due to loading rate was observed. Hysteresis for all displacement rates was on the order of 2.4%.
Subject(s)
Achilles Tendon/physiology , Tensile Strength/physiology , Transducers, Pressure , Arthroscopy , Calibration , Equipment Design , Humans , In Vitro Techniques , Prostheses and Implants , Sensitivity and Specificity , Weight-Bearing/physiologyABSTRACT
Prompted by common observations, we investigated why physiologic bowing occurs in infants and what purpose is served by its characteristic varus to valgus to neutral movement of the mechanical axis of the knee which we term "pendulum-like swing" in the frontal plane. Anthropomorphic data were readily available on age-related increase in weight, femoral and tibial lengths, and changes in lower extremity to allow construction of an "average" leg representing "normal" limb alignment of a growing infant male aged 1-8 years. This enabled us to calculate the bending moments on five anatomic levels of the limb about the knee; more important, during some growth periods the bending moments are minimal and may be used to diagnose pathologies of the lower limb. This form of analysis may prove useful in pathologies in which mechanisms serve to "mask" problems diagnosed on the basis of knee angle alone. When the child begins to stand, the pendulum mechanism is needed to equalize physeal growth about the knee.
Subject(s)
Knee Joint/physiology , Leg/physiology , Movement/physiology , Biomechanical Phenomena , Bone Development/physiology , Humans , Infant , Male , Models, Biological , Tibia/physiologyABSTRACT
An experimental apparatus was assembled that permitted measurement of the vertical and lateral ground reaction forces as the hip is abducted, resulting in foot separations ranging from 0.25 to 71 cm, with the knee in 0 degree flexion. Twelve healthy volunteers (8 men and 4 women) were tested. The hip joint was located by means of center of rotation measurements on each subject's legs, and the location of the knee joint was determined using anatomical measurements. It was observed that the mediolateral force was nonzero and directed toward the body midline, even when the subject's feet were placed together. With the feet placed at shoulder width, the population mean mediolateral force was 3% of body weight. It was determined that simplifying assumptions based upon either "zero lateral force," or "zero hip moment," produced errors, when compared with our measured values, over various ranges of foot separation, with the zero hip moment assumption providing accuracy over a broader range. The inclination of the tibial plateau, with respect to the long axis of the tibia, that would produce minimal mediolateral shear at the knee is presented. Research and clinical applications of our results and techniques are discussed.
Subject(s)
Hip Joint/physiology , Knee Joint/physiology , Leg/physiology , Weight-Bearing , Adult , Biomechanical Phenomena , Female , Femur/physiology , Foot/physiology , Humans , Male , Regression Analysis , Tibia/physiologyABSTRACT
Eight human and nineteen pig unembalmed proximal left anterior descending and circumflex coronary arteries were subjected to linear volume changes (2 s ramp time) at three fixed axial extensions while immersed in a physiological saline bath at body temperature. Measured parameters included: lumen pressure, outside diameter, axial force, and axial extension. The deformations were measured using a video dimensional analyzer. The arteries were inflated to pressures well above the physiological range at each axial extension. A latex inner tube was placed inside of each specimen to prevent leakage, and its effects upon the measured stresses were corrected analytically. With this method, the average circumferential and axial stresses could be computed directly from the experimental data. In both directions the average stresses measured displayed two distinct regions: stresses occurring for small diameter changes (physiological pressures) and stresses occurring for large diameter changes (high pressures). The resulting average small strain and large strain stress components were curve-fit separately and, when reassembled, provided a piece-wise model of the stress response of coronary arteries over a wide range of inflation pressures and axial extensions.
Subject(s)
Coronary Vessels/physiology , Animals , Coronary Vessels/anatomy & histology , Elasticity , Humans , Models, Cardiovascular , Pressure , Stress, Mechanical , Swine , Video Recording , ViscosityABSTRACT
Gait analysis was used to compare the ground reaction forces, ankle and foot rotations in the sagittal plane, and the center of pressure pattern beneath the right feet of seven normal subjects walking barefoot, with and without their right ankles taped in the neutral position. Instrumentation included a force plate, ankle goniometer, and two accelerometers mounted on top of the foot. The ground reaction forces showed no changes between the same ankle, taped and untaped. Taping served to reduce the range of ankle rotations in the sagittal plane by approximately 20%, with a subsequent increase in the rotation about the metatarsal heads during heel-up. Heel-up occurred earlier in stance when the ankle was taped than with no taping. The vertical force graph was integrated over time when the center of pressure was located beneath the heel and the ball, resulting in two impulse measurements. The heel impulse decreased for each of the 7 subjects and 6 of the 7 subjects displayed an increase in the ball impulse due to taping, indicating that taping served to shift the load-time history away from the heel and toward the ball. The results of this study may apply to fused ankle patients, who may suffer forefoot abnormalities subsequent to ankle fusion surgery.