Comment on: The use of strain energy as a convergence criterion in the finite element modelling of bone and the effect of model geometry on stress convergence.
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Biomechanical Phenomena↗
Biomedical subjects
H B Skinner
Publications and source records attributed to H B Skinner.
Symposium: the national hip registry: a new resource for measuring and improving the quality of health care.
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Hip Prosthesis↗
Knee joint proprioception in below-knee amputees.
This study determined knee joint proprioception in below-knee amputees using two methods of measurement of knee joint proprioception. The threshold for detection of passive motion and the ability to reproduce specific limb positions were recorded in nine below-knee amputees. A significant difference was observed between the prosthetic and sound limbs in the amputees in the threshold of detection test. The large angles (5 degrees to 25 degrees) involved in position reproduction tests may have provided cues through the cuff suspension. Different strategies are used by below-knee amputees to discern large and small movements of the knee.
Aged↗
Gait initiation of persons with below-knee amputation: the characterization and comparison of force profiles.
The purpose of this study was to characterize gait initiation of persons with leg amputation and determine whether prosthetic alignment was a critical parameter in the initiation process. Gait initiation was chosen for study because of the difficult neuromuscular demands placed on the body in negotiating the transition from stance to ambulation. In this investigation, ground reaction force data were collected on seven persons with below-knee amputation. These subjects underwent a series of gait initiation trials while varying prosthetic alignment. An analysis of the data demonstrated key elements in the gait initiation process, including the motion of the center of gravity in preparation for steady-state walking. Significant asymmetries in the force profiles of the residual and nonamputated limbs were also found; gait initiation forces were consistently higher for the prosthetic limb and the timings of maxima and minima were indicative of an intact limb preference. Relatively small changes in prosthesis alignment proved not to have statistically significant effects on generalized force parameters. This result is consistent with the findings of other studies that gait initiation is a motor program with certain invariant characteristics.
Aged↗
The prediction of metabolic energy expenditure during gait from mechanical energy of the limb: a preliminary study.
Measurements of metabolic energy consumption and free-walking velocity were recorded for four persons with trans-femoral amputation with variations of prosthesis mass and mass distribution. Hot-film anemometers, rate gyroscopes, and a force platform were used to measure prosthetic limb segment velocities and ground reaction forces. Metabolic energy consumption for the nine configurations of mass and mass distribution averaged 1.177 cal/kg/m with a standard deviation of +/- 0.052 cal/kg/m. Two measures of mechanical work of the amputated extremity, one based on power developed across joints (W1) and the other based on changes in energy of the body segments (W2), were computed to be 0.162 +/- 0.014 and 0.175 +/- 0.025 cal/kg/m, respectively. A linear regression model led to rejections of both W1 and W2 as predictors of metabolic energy expenditure of the amputee at a significance level of 0.05.
Aged↗
Effects of variation of prosthesis size on cement stress at the tip of a femoral implant.
With the resurgence of the use of bone cement in total hip arthroplasty, a renewed interest in techniques or designs that may reduce cement fixation failure has arisen. Analysis of the stresses at the tip of the prosthesis may suggest strategies to reduce loosening. This study analyzed stresses in the cement near the tip of a femoral component as a function of cement thickness, using a three-dimensional finite element model. A section of an idealized circular femoral shaft with implanted prosthesis and cement was modeled with loading conditions representing the stance phase of gait. Increasing cement thickness from 2 to 5 mm by reducing the prosthesis diameter from 15 to 9 mm is predicted to reduce stress significantly in the cement mantle of a femoral implant. Peak tensile stresses are reduced 45%, whereas peak von Mises and shear stresses are reduced 40%. Such a reduction in stress can increase fatigue life by an order of magnitude. Peak interface tensile stresses occur on the medial side at the tip of the prosthesis in a transverse direction, indicating likelihood of failure due to debonding. The shear and tensile stresses predicted by our model greatly exceed the fatigue endurance limit values for both bulk cement and the cement-prosthesis interface, indicating the likelihood of premature fatigue failure, even allowing for considerable uncertainty. These analytical results suggest that the surgeon should adopt a strategy of selecting a prosthesis that permits a 5-mm cement mantle near the tip of the prosthesis.
Bone Cements↗
Correlations between orthogonal mechanical properties and density of trabecular bone: use of different densitometric measures.
To compare the numerous modulus-density and strength-density relations that have been found for human trabecular bone from the proximal tibia, correlations between various measures of density were sought. Hydrated and dry apparent density, ash density, and density from quantitative computed tomography (QCT) were determined for cubic trabecular specimens taken from the proximal portion of human tibiae, and correlations between these measures were found (r > 0.99, P < .001). Orthogonal moduli and strengths of the specimens were measured mechanically, and were significantly correlated with ash density according to power relations (r > or = 0.85, P < .001). The strong correlation between density from QCT and ash density indicates that these measures can be used with nearly equal precision in estimating modulus and strength of tibial trabecular bone. Equations between mechanical properties and density reported in previous studies were converted to use a common density measure and, after considering the effects of specimen size, were in general agreement with results of the present study.
Adult↗
Femoral prosthesis implantation induces changes in bone stress that depend on the extent of porous coating.
The objective of this study was to evaluate the effect of implantation of porous-coated anatomic medullary fitting prostheses on stress in the proximal femur. Three-dimensional finite element models of a cadaveric femur before and after implantation were used to evaluate the resulting changes in stress in the bone. Models of the femur were generated automatically from computed tomographic scan data with use of an innovative mesh-generation technique. The models were analyzed for three levels of porous coating (proximal, 5/8, and full), with the assumption of ideal ingrowth (perfect bonding) over porous areas and a frictionless, tension-free surface on smooth areas. All models were loaded and restrained to represent conditions of normal gait. The stresses predicted in the implanted femur are consistent with clinical observations of proximal cortical atrophy (normal stress reduced to 6-9% of normal at the calcar and 50-55% at mid-prosthesis) and of hypertrophy at the porous coating junctions (normal stress at the 5/8-coating junction, 123% of stress proximal to the junction) and hypertrophy near the distal tip of the prosthesis (anterior and posterior normal stresses 200-800% of normal). The fully coated prosthesis induced stresses in the bone near the tip of the prosthesis that were most like stresses in the normal femur (medial and lateral normal stress 105 and 102% of the stress in the normal femur). Below the collar, the normal stress associated with the proximally coated prosthesis was 6% greater than that produced with the other two levels of coating but still was only 2% of normal. The 5/8-coated prosthesis appeared to combine the worst features of the fully coated and proximally coated prostheses--greater stress-shielding at the calcar and higher stress near the tip of the prosthesis.
Artificial Limbs↗
Constrained acetabular components.
Between May 1987 and October 1990, 21 constrained acetabular components were used in revision total hip arthroplasty at the University of California, San Francisco. In 18 patients, the device was placed for a chronically dislocating total hip arthroplasty. In the remaining three, intraoperative instability during revision total hip arthroplasty necessitated its use. At the minimum 2-year follow-up evaluation (average, 31 months; range, 24-64 months), 15 patients (71%) experienced no further dislocations or subluxations. There were eight dislocations in the remaining six patients (29%). The average Harris hip score at the follow-up evaluation was 76 points (range, 32-100 points). For those patients who redislocated (n = 6), an increased acetabular abduction angle of the metallic acetabular cup, averaging 70 degrees, was the only predictive factor of failure of the constrained cup (P < .05). No radiographic or clinical evidence of loosening in the 19 porous ingrowth acetabular component was observed. This device will relieve the complication of severe hip instability in the majority of patients, but is not universally successful.
Acetabulum↗
A phosphatidylinositol transfer protein controls the phosphatidylcholine content of yeast Golgi membranes.
SEC14p is required for protein transport from the yeast Golgi complex. We describe a quantitative analysis of yeast bulk membrane and Golgi membrane phospholipid composition under conditions where Golgi secretory function has been uncoupled from its usual SEC14p requirement. The data demonstrate that SEC14p specifically functions to maintain a reduced phosphatidylcholine content in Golgi membranes and indicate that overproduction of SEC14p markedly reduces the apparent rate of phosphatidylcholine biosynthesis via the CDP-choline pathway in vivo. We suggest that SEC14p serves as a sensor of Golgi membrane phospholipid composition through which the activity of the CDP-choline pathway in Golgi membranes is regulated such that a phosphatidylcholine content that is compatible with the essential secretory function of these membranes is maintained.
Carrier Proteins↗
A phosphatidylinositol/phosphatidylcholine transfer protein is required for differentiation of the dimorphic yeast Yarrowia lipolytica from the yeast to the mycelial form.
The SEC14SC gene encodes the phosphatidylinositol/phosphatidylcholine transfer protein (PI/PC-TP) of Saccharomyces cerevisiae. The SEC14SC gene product (SEC14pSC) is associated with the Golgi complex as a peripheral membrane protein and plays an essential role in stimulating Golgi secretory function. We report the characterization of SEC14YL, the structural gene for the PI/PC-TP of the dimorphic yeast Yarrowia lipolytica. SEC14YL encodes a primary translation product (SEC14YL) that is predicted to be a 497-residue polypeptide of which the amino-terminal 300 residues are highly homologous to the entire SEC14pSC, and the carboxyl-terminal 197 residues define a dispensible domain that is not homologous to any known protein. In a manner analogous to the case for SEC14pSC, SEC14pYL localizes to punctate cytoplasmic structures in Y. lipolytica that likely represent Golgi bodies. However, SEC14pYL is neither required for the viability of Y. lipolytica nor is it required for secretory pathway function in this organism. This nonessentiality of SEC14pYL for growth and secretion is probably not the consequence of a second PI/PC-TP activity in Y. lipolytica as cell-free lysates prepared from delta sec14YL strains are devoid of measurable PI/PC-TP activity in vitro. Phenotypic analyses demonstrate that SEC14pYL dysfunction results in the inability of Y. lipolytica to undergo the characteristic dimorphic transition from the yeast to the mycelial form that typifies this species. Rather, delta sec14YL mutants form aberrant pseudomycelial structures as cells enter stationary growth phase. The collective data indicate a role for SEC14pYL in promoting the differentiation of Y. lipolytica cells from yeast to mycelia, and demonstrate that PI/PC-TP function is utilized in diverse ways by different organisms.
Amino Acid Sequence↗
Three-dimensional finite element modeling of a cervical vertebra: an investigation of burst fracture mechanism.
Finite element modeling was used to study the mechanical behavior of a cervical vertebra under axial compressive loading. A three-dimensional (3-D) finite element (FE) model of a mid-cervical vertebra using inhomogeneous material properties was generated from quantitative computed tomographic (CT) scan data. This model improved upon previous vertebral FE models by using a highly refined mesh to represent the 3-D variation in material properties of vertebral bone. Traumatic loading of the vertebra was simulated by applying an axial compressive displacement through linear spring elements. Bone strength was computed from the CT scan data and compared with predicted stress. Based on the maximum shear stress theory of failure, the model predicts initiation of failure in the central cancellous region of the vertebral body. The type of fracture pattern predicted by the model is consistent with the typical cervical burst fracture that is seen clinically after compressive loading of the cervical spine. As such, we have developed a tool that can be useful for validating proposed fracture mechanisms in the cervical spine.
Cervical Vertebrae↗
Functional redundancy of CDP-ethanolamine and CDP-choline pathway enzymes in phospholipid biosynthesis: ethanolamine-dependent effects on steady-state membrane phospholipid composition in Saccharomyces cerevisiae.
It has been established that yeast membrane phospholipid content is responsive to the inositol and choline content of the growth medium. Alterations in the levels of transcription of phospholipid biosynthetic enzymes contribute significantly to this response. We now describe conditions under which ethanolamine can exert significant influence on yeast membrane phospholipid composition. We demonstrate that mutations which block a defined subset of the reactions required for the biosynthesis of phosphatidylcholine (PC) via the CDP-choline pathway cause ethanolamine-dependent effects on the steady-state levels of bulk PC in yeast membranes. Such an ethanolamine-dependent reduction in bulk membrane PC content was observed for both choline kinase (cki) and choline phosphotransferase (cpt1) mutants, but it was not observed for mutants defective in cholinephosphate cytidylyltransferase, the enzyme that catalyzes the penultimate reaction of the CDP-choline pathway for PC biosynthesis. Moreover, the ethanolamine effect observed for cki and cpt1 mutants was independent of the choline content of the growth medium. Finally, we found that haploid yeast strains defective in the activity of both the choline and ethanolamine phosphotransferases experienced an ethanolamine-insensitive reduction in steady-state PC content, an effect which was not observed in strains defective in either one of these activities alone. The collective data indicate that specific enzymes of the CDP-ethanolamine pathway for phosphatidylethanolamine biosynthesis, while able to contribute to PC synthesis when yeast cells are grown under conditions of ethanolamine deprivation, do not do so when yeast cells are presented with this phospholipid headgroup precursor.
Carrier Proteins↗
Correlation of computed finite element stresses to bone density after remodeling around cementless femoral implants.
Fundamental to the development of a durable, uncemented femoral component is an understanding of the remodeling process that goes on after implantation. Predicting the bone remodeling that results from the use of a given hip implant would facilitate the design of a prosthesis that would optimize bone adaptation. This report combines the results of finite element stress analysis of the AML prosthesis implanted in vitro into a proximal femur with quantitative bone mineral density measured in vivo in the medial and lateral aspects of human femora at periods after implantation. Unimplanted femora were also analyzed for comparison purposes. Bone density measurements were obtained using dual energy x-ray absorptiometry. Absolute values of the maximum principal stress and maximum shear stress calculated in the femur at the time of implantation accurately predict bone density resulting from remodeling caused by the prosthesis. The calculated initial strain was not found to correlate with resultant bone density. These findings suggest that the results of stress analyses using three dimensional models of femora implanted in vitro can predict bone remodeling around prostheses and may be used to quantitate appropriate design criteria for total hip replacements.
Absorptiometry, Photon↗
Current concepts in orthopaedic biomaterials and implant fixation.
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Biocompatible Materials↗
Phospholipid transfer activity is relevant to but not sufficient for the essential function of the yeast SEC14 gene product.
To investigate several key aspects of phosphatidylinositol transfer protein (PI-TP) function in eukaryotic cells, rat PI-TP was expressed in yeast strains carrying lesions in SEC14, the structural gene for yeast PI-TP (SEC14p), whose activity is essential for Golgi secretory function in vivo. Rat PI-TP expression effected a specific complementation of sec14ts growth and secretory defects. Complementation of sec14 mutations was not absolute as rat PI-TP expression failed to rescue sec14 null mutations. This partial complementation of sec14 lesions by rat PI-TP correlated with inability of the mammalian protein to stably associate with yeast Golgi membranes and was not a result of rat PI-TP stabilizing the endogenous sec14ts gene product. These collective data demonstrate that while the in vitro PI-TP activity of SEC14p clearly reflects some functional in vivo property of SEC14p, the PI-TP activity is not the sole essential activity of SEC14p. Those data further identify an efficient Golgi targeting capability as a likely essential feature of SEC14p function in vivo. Finally, the data suggest that stable association of SEC14p with yeast Golgi membranes is not a simple function of its lipid-binding properties, indicate that the amino-terminal 129 SEC14p residues are sufficient to direct a catalytically inactive form of rat PI-TP to the Golgi and provide the first evidence to indicate that a mammalian PI-TP can stimulate Golgi secretory function in vivo.
Amino Acid Sequence↗
Effect of reconstruction of the anterior cruciate ligament on proprioception of the knee and the heel strike transient.
Abnormal proprioception of the knee joint has been documented after rupture of the anterior cruciate ligament (ACL) and may result in the loss of muscular reflexes. Excessive loading from the lack of muscular control may predispose the joint to osteoarthrosis. To investigate this problem, 10 patients were studied at an average of 31.6 months after ACL reconstruction. Three tests of joint proprioception and measurements of the vertical component of heel strike force during normal gait were used. A normal control group also was studied. For two of the proprioception tests (reproduction of passive motion and relative reproduction), there were no statistical differences among the uninjured (control) limbs, the normal contralateral limb of patients with a reconstructed ACL, and the extremity with a reconstructed ACL. In the third test (threshold of detection of motion), which previously has been shown to be adversely affected by ACL injury, the measurements for both extremities of patients with a reconstructed ACL were more accurate than those for the control group. The reconstructed extremity performed less accurately than the contralateral extremity (p < 0.05). The heel strike transient (vertical component of ground reaction force at heel strike) for uninjured and ACL-reconstructed limbs was not significantly different. In fact, the extremity with the reconstructed ACL had a lower transient than the uninjured extremity. Heel strike transients in patients with a reconstructed ACL were higher than those in the controls, but the differences were significant only when corrected for velocity of gait.(ABSTRACT TRUNCATED AT 250 WORDS)
Adult↗
Validation of an automated method of three-dimensional finite element modelling of bone.
This study validated an automated method of finite element modelling of bone from CT scan data. After a fresh-frozen cadaveric femur was modelled, strain gauges were attached to the bone at 11 locations and the femur was mechanically tested by applying a load to the femoral head. Linear regression analysis was used to correlate the strains predicted by the model with the experimentally measured strains. The regression results were significant (P < 0.001), indicating that the strain calculated by the FE model is a valid predictor of the measured strain. Verification of the surface strains also supports the validity of the strains and stresses predicted inside the bone. The present study provides a strong rationale for use of this modelling method as a research tool and in possible clinical applications.
Biomechanical Phenomena↗