Biomechanical comparison of three second-generation reconstruction nails in an unstable subtrochanteric femur fracture model.

The purpose of this study is to determine if a biomechanical difference exists in dynamic stiffness, fatigue life, and fracture site displacement by comparing three cephalomedullary reconstruction nails. An examination was made of the Biomet Uniflex reconstruction nail, the Biomet Vector nail, and the Stryker Howmedica Long Gamma nail in the fixation of an unstable subtrochanteric femur fracture model, using a synthetic bone model. Mean stiffness for each nail was initially determined in control specimens (i.e. no fracture and no instrumentation). The nail stiffness values were 1764.0 N/mm (controls), 373.61 N/mm (Uniflex), 294.27 N/mm (vector), and 656.36 N/mm (Gamma). The Gamma was statistically stiffer than the Uniflex and Vector (p < 0.002). Mean fatigue life measurements were: Uniflex at 52,891 cycles, failing at the most distal of the proximal two screw holes; Vector at 45,344 cycles, failing in the nail at the level of the fracture site; Gamma at 88,748 cycles failing at the lag screw hole. The p value between the Gamma and Vector was less than 0.01 and between Gamma and Uniflex was less than 0.05. The mean maximal axial displacement at the fracture site was 2.448 mm, 2.305 mm, and 0.790 mm for the Uniflex, Vector and Gamma, respectively. The p value between the Gamma and the other nails was < 0.01. The mean maximal transverse displacement at the fracture site was 1.223 mm, 1.197 mm, and 0.280 mm respectively. The p value between the Long Gamma and the other two nails was < 0.01. In conclusion, the Long Gamma nail demonstrated statistically significant fixation superiority in stiffness, resistance to fatigue, and fracture site displacement compared to the Uniflex and Vector nails. This biomechanical information may aid in choosing implants for fixation of unstable, subtrochanteric femur fractures.

Anatomic investigation of the deep posterior compartment of the leg.

The purpose of this study is to critically investigate the anatomy of the deep posterior compartment of the leg. Specifically, the relationship of the deep posterior compartment to the superficial posterior compartment and their insertion onto the posteromedial border of the tibia are assessed. Cross-sectioning of 10 fresh-frozen cadaver legs was performed at 2-cm increments. The inferior surface of each section was photographed. The photographs were visually analyzed, and the fascial separation between the posterior compartments along with their relationship to the posteromedial border of the tibia were recorded for each specimen. Magnetic resonance images in the axial plane of 10 healthy, normal volunteers' lower extremities at 2-cm increments were obtained and analyzed. All specimens and images demonstrated that the medial fascial attachment of the deep posterior compartment was along the posteromedial aspect of the tibia in the proximal third of the leg and was not superficially accessible. In the proximal third of the leg, the superficial posterior compartment fascial attachment overlapped the deep posterior compartment by inserting medial and anterior to the deep posterior compartment fascial attachment. In the middle and distal thirds of the leg, the medial fascial attachment of the deep posterior compartment shifted medially and anteriorly, making the deep posterior compartment superficially accessible. The surgeon must appreciate the change in the anatomic relationships along the medial side of the leg while performing double-incision four-compartment fasciotomy release to obtain a complete release of the muscular portion of the deep posterior compartment.

Triple arthrodesis: a biomechanical evaluation of screw versus staple fixation.

The subtalar, talonavicular, and calcaneocuboid joints were internally fixed to simulate triple arthrodesis was performed on eight matched pairs of human cadaver feet. Feet were randomly assigned such that one specimen in each pair was internally fixed with cobalt-chrome staples and one specimen with stainless steel screws. Liquid metal strain gauges were placed in a perpendicular fashion across the three joints of each specimen. Each foot was then secured to the Shore Western Materials Test System where a series of 10 increasing eversion stresses across the foot was created. Displacement was measured at each joint with every increase in eversion stress. Our results show that there is no statistical difference in fixation strength between screws and staples at the talonavicular, calcaneocuboid, or subtalar joints (P = 0.862). Although many studies determining the strength to failure of different implants have been performed, shear stress and micromotion at the joint surfaces have not been evaluated, to our knowledge, and no single implant in this study has shown superior immobilization characteristics to recommend its use as the implant of choice in triple arthrodesis.

Analysis of cryotherapy penetration: a comparison of the plaster cast, synthetic cast, Ace wrap dressing, and Robert-Jones dressing.

Four groups were studied to determine the cooling effect at skin level of ice application over common ankle bandages and casts. The plaster cast, synthetic cast, and Ace wrap dressing groups had 18 participants and the Robert-Jones dressing group had four participants. In a standardized fashion, ice bags were applied over each bandage or cast and skin temperatures were measured with a thermocouple over a 90-min period. The average final skin temperature was 16.5 degrees in the plaster cast, 18.8 degrees in the synthetic cast, 21.2 degrees in the Ace wrap dressing, and 28.7 degrees in the Robert-Jones dressing. The temperature curves showed that ice application effectively decreased skin temperatures through the plaster cast, synthetic cast, and Ace wrap dressing. The cooler temperature did not adequately penetrate the Robert-Jones dressing.

Alkyl cyanoacrylates as space filling bone adhesives.

Higher chained alkyl cyanoacrylates have potential to function efficiently as space filling, hydrophobic, viscoelastic, biocompatible, and rapidly polymerizing bone adhesives. Hence they may be useful in applications where a void has to be filled, such as replacement of the intervertebral disc. To assess their applicability as space filling material in such an application, three alkyl cyanoacrylates; methoxyethyl cyanoacrylate, isobutyl cyanoacrylate, and isoamyl cyanoacrylate have been evaluated in this study. The bonding strength of these cyanoacrylates to bone in a space filling situation have been measured. The results indicate the bond strength of isoamyl cyanoacrylate (0.13 MPa) to be significantly (p less than 0.05) lower compared with methoxyethyl cyanoacrylate (0.33 MPa) and isobutyl cyanoacrylate (0.37 MPa). There was no significant difference in the bond strengths of isobutyl cyanoacrylate and methoxyethyl cyanoacrylate.

Ligament strain and ankle joint opening during ankle distraction.

To determine the efficacy of ankle distraction and to investigate possible complications of the procedure, the strain on four ankle ligaments and the tibiotalar joint opening resulting from distraction force and various foot positions were studied. We mounted strain gauges on the deltoid, calcaneofibular, tibiofibular, and anterior talofibular ligaments of six fresh human cadaver ankles. An Acufex ankle distractor was used to apply forces of 45, 90, 135, and 180 N at 20 degrees dorsiflexion, neutral, and 10 degrees plantar flexion. The ankle distractor proved to be effective in opening the joint space for better visualization, but complications of pin bending, excessive ligament strain, and bony destruction did occur within the clinically recommended range. Based on the observed results, the safest method of distraction was to use forces < 135 N in the neutral position.

Material and structural characterization of human saphenous vein.

Saphenous vein patch rupture after carotid endarterectomy is an infrequent but devastating complication. This study was undertaken to evaluate the material and structural properties of fresh human saphenous veins to understand the causes of this complication. Segments of saphenous veins were obtained from 22 patients from vein harvested during coronary artery bypass surgery. Ninety-three specimens, oriented in both circumferential (n = 45) and longitudinal (n = 48) directions, were prepared from the available vein segments for testing. Specimens were mounted on specially designed grips and then subjected to uniaxial tension testing. For each specimen the following material and structural parameters were determined: vessel diameter, tensile stiffness, failure and ultimate forces, and tensile modulus, failure stress, and strain. The physical properties of specimens evaluated in longitudinal orientations and thus limit the inherent strength of the vein. The physical properties of circumferentially tested vein specimens were negatively correlated to age, female gender, diabetes, and hypertension. The data obtained in this investigation suggest that age, hypertension, as well as diabetes and gender may adversely influence the circumferential tensile strength of human saphenous veins used as patch grafts.

Fatigue life improvement of nitrogen-ion-implanted pedicle screws.

An experimental study of the fatigue life of pedicle screws and their nitrogen-ion-implanted counterparts was conducted. Nitrogen-ion implantation parameters were varied in the first part of the study to obtain the best implantation characteristics. Using this optimal parameter for N(+)-implantation, the performance of two matched groups were compared at two bending moments near the "knee" of the S-N curves. At 3.96 N-m and 5.09 N-m of applied bending moments, the increase in the mean fatigue life for the former was 98% and for the latter 20%. The 98% prolongation was statistically significant at P less than 0.005; however, the 20% increase was statistically insignificant (P less than 0.1). The implantation depth is about 0.1 mu in the near surface of the screw. This appears to be thick enough to inhibit crack initiation at 3.96 N-m, but only marginally at 5.09 N-m. Increasing the implantation depth has the potential of increasing the fatigue life at the higher bending moments.

Basilar crescentic osteotomy. A three-dimensional computer simulation.

A three-dimensional computer simulation of the basilar crescentic osteotomy has been presented. The bunion deformity consists of hallux valgus, an increased first and second intermetatarsal angle, pronation of the great toe, and elevation of the first metatarsal head. Every foot is different and some may have more or less of each of the above noted components. Because the deformity is multiplanar, at least two roentgenograms are needed to evaluate the deformity. The weight-bearing, anterior-posterior roentgenogram is the principle radiograph used in preoperative planning. The use of a weight-bearing, sesamoid roentgenogram is recommended to quantify the anterior-posterior deflection and rotation of the first metatarsal head. A computer model (based on a cylinder) of the first metatarsal has been formulated. The osteotomy then was performed in a variety of scenarios in order to simulate the surgical correction. A great deal of flexibility is afforded by this osteotomy. The surgeon needs to be aware of the coupled motions that occur. That is, closure of the intermetatarsal angle may also cause head rotation, depression, or elevation. If the osteotomy is performed in an oblique multiplanar direction, then it is possible for the metatarsal head to elevate, pronate, and significantly shorten as the intermetatarsal angle is closed. If this scenario should occur, a poor surgical outcome will result. Excision of the medial eminence is recommended after the osteotomy has been completed and secured with stable fixation because of these rotational changes. The basilar crescentic osteotomy is an excellent method for correction of a marked metatarsus primus varus. It is important to pay close attention to a variety of anatomic considerations. The osteotomy must not be made in the diaphysis because of potential nonunion. There should be little dissection of the periosteum because of possible delayed union. As in any bunion surgery, it is essential to perform an adequate, distal, soft-tissue repair. Three dimensional preoperative planning is essential in obtaining correction of all components of a bunion. Specific guidelines, based on a three-dimensional computer model, are now available. An interactive computer program also is available to aid the surgeon in preoperative planning. We hope there will be better understanding of this technically difficult but highly versatile osteotomy.

Response of the ligamentous lumbar spine to cyclic bending loads.

The effect of a "pure" cyclic flexion bending moment on the three-dimensional load-displacement behavior of fresh ligamentous lumbar spine was investigated. The load-displacement behavior, for 11 L1-sacrum specimens, pre- and post-cyclic fatigue bending tests were quantified using a Selspot II system. A special fixture was designed to mount the specimen within the MTS system to administer "pure" cyclic flexion bending, under displacement control, for 5 hours. The testing was accomplished in a 100% humidity chamber at 0.5 Hz. The maximum cyclic bending moment, based on the literature dealing with loads experienced by the spine during activities involving lifting, was set at 3.0 Nm. An increase in motion of the order of 10% in the extension loading mode was observed. The increase in motion in other loading modes was not significant. In the extension loading mode, the increase in the anteroposterior displacement (retrodisplacement) in general was higher than the corresponding rotation component. The results suggest that the bending moment of low magnitude, usually experienced by the spine during activities of daily living, alone may not trigger the mechanical failure processes in the disc. The presence of high axial compressive loads on the disc seems to be the main contributing factor in this process. The presence of bending moments and axial twist along with axial compressive load may accelerate the unstable processes leading to low back pain.

Tin ear syndrome: rotational acceleration in pediatric head injuries.

An array of signs pathognomonic of child abuse is described for three young children. The eponym "tin ear syndrome" describes distinct unilateral ear bruising, radiographic evidence of ipisilateral cerebral edema with obliteration of the basilar cisterns, and hemorrhagic retinopathy. Rotational acceleration produced by blunt trauma to the ear is shown to be a necessary and sufficient cause for the production of this syndrome. An approximate value of 4,670 radians/s2 produced by a force of 309 to 347 N (70 to 80 lb) results in shear stress and tearing of the subdural veins, loss of cortical vasomotor tone, cerebral hyperemia, herniation, and death. Past history and environmental circumstances implicated child abuse in every case.

Bone-particle-impregnated bone cement: an in vitro study.

Bone-particle-impregnated bone cement specimens (up to 30% by weight) were characterized by various test methods. The experimental bone cement showed decreased crack propagation rates and increased Young's modulus, while the ultimate tensile strength and impact strength were decreased. The viscosity could be adjusted by adding initiators lost when substituting the PMMA powder with bone particles. The present study warranted further in vivo experiments on the possibility of tissue ingrowth for which the new bone cement was developed.

Dynamic performance characteristics of the liquid metal strain gage.

Performance characteristics of the liquid metal strain gage (LMSG) were evaluated by both static and dynamic bench testing. Statically, the devices were found to have outputs closely proportional to engineering strains, up to strain levels of 40%. While individual gage factors varied appreciably (up to 50%), each of the gages studied showed excellent reproducibility of behavior. Dynamically, the response to sinusoidal strain inputs was frequency-independent up to 50 Hz, and there was no detectable phase shift. Similarly, the LMSG response to constant-speed displacement inputs was velocity-independent over the range of nominal strain rates from 20 s-1 to 0.02 s-1. The devices proved capable of maintaining stable outputs when held stretched to fixed lengths, even if such tests were performed immediately following stepwise displacement inputs. Thermal artifacts were found to be modest (0.185% apparent strain per degree C), and there was no appreciable sensitivity to non-axial strains. When mounted on an in vitro ligament preparation, the LMSG measured apparent ligament strain similar to that detected by a video dimension analyzer. A protocol by which an implanted LMSG could be used to infer in vivo muscle forces was demonstrated, based on recordings of tendo-Achilles strains developed by a rabbit during slow hopping.

Torsional fatigue of the lumbar intervertebral joints.

The effect of cyclic torsional loads on the behavior of intact lumbar intervertebral joints was investigated. A total of 24 specimens, divided in two groups, were tested. In the first group, a specimen was subjected to a specified constant cyclic torque, and the resulting change in angular displacement with time was recorded. The specimens in the second group were subjected to a specified constant angular displacement and the resultant torque recorded. The results of the constant torque tests showed that fatigue failure was primarily a function of the averaged initial angular displacement and secondarily of the applied torque. The critical threshold values appear to be an initial angular displacement of +/- 1.5 degrees. All specimens, displaying an averaged initial angular displacement of less than +/- 1.5 degrees, did not fail in fatigue, while the others did. However, the specimens with angular displacements of less than +/- 1.5 degrees had lower applied torques (11.3 and 22.6 Nm), while specimens with higher applied torques (33.9 and 45.2 Nm) all exceeded the +/- 1.5 degrees threshold and failed in fatigue. Failure locations occurred in such diverse regions as end-plates, facets, laminae, capsular ligaments, etc. All specimens exhibited a synovial fluid discharge from the apophyseal joint capsule sometime during testing. Post-test examinations of all the cartilage surfaces showed fibrillation, whether or not the intervertebral joint failed.

The inertial and geometrical properties of helmets.

The center of gravity (CG) and the principal mass moments of inertia about the CG of Army aviator, American football, and bicycle helmets were experimentally determined by a variation of the classic differential weighing and torsional pendulum techniques. In the course of these experiments, an innovative method for three-dimensional (3D) digitization was found. An electronic caliper, which measured length, was used with a computer algorithm to achieve 3D digitization. The results of the above measurements show that the weight of the helmet and the distances from the CG to the orthogonal coordinate axes intercepts with the outer shell surface were highly correlated with its principal mass moments of inertia. A set of regression equations was derived on theoretical considerations and served to unify the experimentally obtained data. Our results indicate that the principal mass moments of inertia of helmets vary linearly with its mass but nonlinearly with size and shape. For a helmet, given its weight and certain geometrical distances, the regression equations estimate the principal mass moments of inertia to within 5% of its experimentally-determined values. For the helmets studied in this series, a modified linear-regression relationship between the principal mass moments of inertia and its mass was found. This result is reasonable because the mass distribution of the current generation of helmets are set primarily by the head size and secondarily by helmet size, shape, and materials.

Fatigue response of lumbar intervertebral joints under axial cyclic loading.

A low-cycle fatigue of 11 lumbar intervertebral joints under axial compression is reported. The magnitude of the maximum compressive load ranged from 37 to 80% of the failure load. The maximum deformation, as a function of the number of cycles, showed two distinct results: one group showed a gradual, stable increase, and the other an abrupt, unstable increase. The before- and after-test radiographs showed a one-to-one correspondence between unstable specimens and generalized bony failure. The radiographs of 5-mm thick transverse endplate slices show crack propagation from the periphery of the subchondral bone inward. Removal of the organic matrix from the cracked specimens produced its physical disintegration into small particles, while normal controls and stable specimens retained their size and shape.