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Biomedical subjects

D R Pedersen

Publications and source records attributed to D R Pedersen.

At least 55 records · Page 3Linked to original sources
PubMed1991

Interstitial bone stress distributions accompanying ingrowth of a screen-like prosthesis anchorage layer.

Recent development of screen-like bonded weaves of titanium wire for orthopaedic implant anchorage affords a unique opportunity for analytic studies of porous ingrowth micromechanics. The regular geometry of individual wires and the periodicity of the mesh weave are exploited in a series of two-dimensional finite element models, mapping interstitial bone stress fields as a function of ingrowth depth and wire size, shape, and spacing. When the depth of bone ingrowth was less than one wire diameter, peak bone stresses always occurred at the leading (i.e. deepest) edge of bone ingrowth, immediately adjacent to the wire. As ingrowth depth approached a full wire diameter, peak local bone stresses were 2-9 times the nominal applied host bone stress, with greater stresses occurring for lower screen weave densities. Within multiple screen layers, the top layer consistently experienced the peak stress and transmitted most of the applied load, regardless of the number of underlying screen layers surrounded by bone. Neither wire size variations nor partial wire flattening substantially affected general trends in stress predictions.

Bone and Bones↗
PubMed1991

A model to predict canine pelvic limb musculoskeletal geometry.

We developed a model to predict the three-dimensional canine pelvic limb muscular geometry (i.e., all muscle moment arms during any instant in gait). Forty-one muscle origins and insertions, as well as external landmarks (to obtain anthropometric dimensions) were marked on both pelvic limbs of five dogs and digitized on biplanar radiographs. Reference frames in the pelvis, femur, and tibia established the three-dimensional coordinates of each origin, insertion, and landmark. A set of dimensionless 'scaled coordinates' was created by dividing the actual origin and insertion coordinates by selected anthropometric dimensions of each animal. Combining scaled coordinates from all ten limbs produced an averaged 'template' of scaled coordinates. To provide limited validation of the scaling procedure, we measured the anthropometric dimensions between externally palpable landmarks of two additional pelvic limbs. The anthropometric dimensions were multiplied by the averaged template coordinates to calculate two new sets of hindlimb muscle coordinates within the three bony reference frames. The two limbs then were dissected, muscle endpoints were marked, and biplanar radiographs of each of the limb segments were digitized. The actual coordinates so obtained were similar to those predicted by the template and anthropometric measures.

Animals↗
PubMed1990

Toward an identification of mechanical parameters initiating periosteal remodeling: a combined experimental and analytic approach.

The ability of bone to adapt to its mechanical environment is well recognized, although the specific mechanical parameters initiating or maintaining the adaptive responses have yet to be identified. Recently introduced mathematical models offer the potential to aid in the identification of such parameters, although these models have not been well validated experimentally or clinically. We formulated a complementary experimental/analytic approach, using an animal model with a well-controlled mechanical environment combined with finite element modeling (FEM). We selected the functionally isolated turkey ulna, since the loading could be completely characterized and the periosteal adaptive responses subsequently monitored and quantified after four and eight weeks of loading. Known loads input into a three-dimensional, linearly elastic FEM of the ulna then permitted full-field mechanical characterization of the ulna. The FEM was validated against a normal strain-gaged turkey ulna, loaded in vivo in an identical fashion to the experimental ulnae. Twenty-four candidate mechanical parameters were then compared to the quantified adaptive responses, using statistical techniques. The data supported strain energy density, longitudinal shear stress, and tensile principal stress/strain as the mechanical parameters most likely related to the initiation of the remodeling response. Model predictions can now suggest new experiments, against which the predictions can be supported or falsified.

Adaptation, Physiological↗
PubMed1988

Contact stress aberrations following imprecise reduction of simple tibial plateau fractures.

Despite the well-recognized association between poorly reduced intraarticular fractures and late degenerative changes, current guidelines regarding the reduction precision necessary to avoid excessive cartilage pressures are based largely on anecdotal clinical observations. To gain a quantitative appreciation of the relation between local pressure elevations and fracture reduction imprecision, a simplified laboratory cadaver model of minimally displaced tibial plateau fractures was developed. Cartilage contact stress distributions were measured as a function of depressed fragment malreduction in seven knees, using high-resolution (100 pixels/mm2) digital image scans of Fuji-film stain patterns. The contact stress data showed a general trend of increases of peak local pressure with increasing fracture site incongruity, and in a few isolated instances the effect was very pronounced. Across the whole series, however, statistically significant departures from anatomic pressure levels did not occur until the fragment stepoff was greater than 1.5 mm. Even at the 3-mm stepoff level, for which the depressed fragment usually no longer made contact with the femoral condyle, the peak local pressure values on the intact side of the fracture line averaged only approximately 75% greater than those prevailing anatomically. Given the successful clinical outcomes normally achieved for conservatively managed simple tibial plateau fractures having stepoff magnitudes (5-10 mm) clearly sufficient to insure fragment articular noncontact, the present laboratory results suggest that nominally factor-of-two peak local pressure elevations, provided that they occur over only small portions of the cartilage surface, are probably within the long-term overall tolerance range of an articular joint.

Aged↗
PubMed1988

Global mechanical consequences of reduced cement/bone coupling rigidity in proximal femoral arthroplasty: a three-dimensional finite element analysis.

Component loosening in total hip arthroplasty is often accompanied by substantial bony remodelling, associated initially with reduced stiffness of the cancellous bone bordering the cement, and eventually with the formation and proliferation of a compliant fibrous membrane at the bone/cement interface. An anatomically based three-dimensional finite element model has been developed to explore the salient stress changes occurring with progressive degradation of the stiffness of the cancellous bone in a thin zone bordering the cement. This border zone, modelled as a distinct linearly elastic and isotropic material layer, assumed a geometry and a range of mechanical properties inferred from eventual membrane thickness apparent in recent animal studies of component loosening. The major variables considered were: stiffness (elastic modulus) and compressibility (Poisson ratio) of the border zone, stiffness changes in the outlying cancellous bone, resultant hip contact force, and trochanteric muscle loadings. The results for the limiting case of a non-degraded border zone compared reasonably with previous studies of femoral reconstructions having rigid bone-to-cement attachment. Progressive decay of border zone stiffness produced complex changes in load transmission. Foremost among these were a generalized increase in stress levels (especially of transverse-plane tension) in the proximo-lateral cancellous bone, and a corresponding generalized decrease in stress levels in the proximo-medial cancellous bone. There were also large bending moment increases in the prosthesis and its cement mantle, especially at mid-stem. At almost all sites, the critical stress levels were those developed for peak stance-phase loading, rather than for the lower loads (and different resultant contact force directions) occurring elsewhere in the gait cycle. The elevated proximo-lateral cancellous bone stresses occurring with eventual membrane development, consistent with localized bony hypertrophy seen in recent animal studies, may be a response to hoop stresses occurring during pistoning of the tapered cement mantle.

Bone Cements↗
PubMed1988

Total hip acetabular component position affects component loosening rates.

Loosening is the most common long-term problem following total hip arthroplasty. Many factors, including patient selection, cement technique, femoral component placement, and prosthesis design reportedly affect the incidence of loosening. Theoretically, the location of the hip center of rotation substantially affects the load on the hip, and superior and lateral hip center location will result in higher loads than medial and inferior placement. Long-term follow-up studies (average, 9.1 years after surgery) using logistical regression analysis demonstrate significantly higher rates of femoral loosening with acetabular components placed in a superior and lateral (i.e., nonanatomic) position, compared with acetabular components placed in a nearly anatomic position.

Acetabulum↗
PubMed1987

Direct comparison of muscle force predictions using linear and nonlinear programming.

Estimating forces in muscles and joints during locomotion requires formulations consistent with available methods of solving the indeterminate problem. Direct comparisons of results between differing optimization methods proposed in the literature have been difficult owing to widely varying model formulations, algorithms, input data, and other factors. We present an application of a new optimization program which includes linear and nonlinear techniques allowing a variety of cost functions and greater flexibility in problem formulation. Unified solution methods such as the one demonstrated here, offer direct evaluations of such factors as optimization criteria and constraints. This unified method demonstrates that nonlinear formulations (of the sort reported) allow more synergistic activity and in contrast to linear formulations, allow antagonistic activity. Concurrence of EMG activity and predicted forces is better with nonlinear predictions than linear predictions. The prediction of synergistic and antagonistic activity expectedly leads to higher joint force predictions. Relaxation of the requirement that muscles resolve the entire intersegmental moment maintains muscle synergism in the nonlinear formulation while relieving muscle antagonism and reducing the predicted joint contact force. Such unified methods allow more possibilities for exploring new optimization formulations, and in comparing the solutions to previously reported formulations.

Algorithms↗
PubMed1986

Hemodynamics after autogenous, interpositional grafting in small arteries.

This investigation initiates the quantitative, hemodynamic assessment of interpositional grafts in small rat arteries. The left femoral arteries in 16 Sprague-Dawley rats were transected at two levels, and were then repaired using interrupted suturing technique. This effectively provided an ideally sized and histologically matched interposition graft. The 20-MHz PUDVM method was used to measure blood velocities, and derive values for vessel lumen area, temporal mean of the spatial mean velocity (Vsm), and volumetric flows. Measurements were completed distal to the interposed graft. Variables were quantitated in the preoperative and at the 5-, 15-, and 30-minute postoperative intervals. Statistical analysis of data indicated that the interpositional grafting procedure resulted in increased vessel lumen area and decreased Vsm, but, importantly, volumetric flow (about 8.20 ml/min) remained unchanged. This study demonstrated that, although hemodynamic characteristics are altered, volumetric flow can be restored after an experimental, interpositional grafting procedure in small arteries.

Animals↗
PubMed1986

The sensitivity of muscle force predictions to changes in physiologic cross-sectional area.

The mechanical effects of a muscle are related in part to the size of the muscle and to its location relative to the joint it crosses. For more than a century, researchers have expressed muscle size by its 'physiological cross-sectional area' (PCSA). Researchers mathematically calculating muscle and joint forces typically use some expression of a muscle's PCSA to constrain the solution to one which is reasonable (i.e. a solution in which small muscles may not have large forces, and large muscles have large forces when expected or when there is significant electromyographic activity). It is obvious that muscle mass (and therefore any expression of PCSA) varies significantly from person to person, even in individuals of similar weight and height. Since it is not practical to predict the PCSA of each muscle in a living subject's limb or trunk, it is important to generally understand the sensitivity of muscle force solutions to possible variations in PCSA. We used nonlinear optimization techniques to predict 47 muscle forces and hip contact forces in a living subject. The PCSA (volume/muscle fiber length) of each of 47 lower limb muscle elements from two cadaver specimens and the 47 PCSA's reported by pierrynowski were input into an optimization algorithm to create three solution sets. The three solutions were qualitatively similar but at times a predicted muscle force could vary as much as two to eight times. In contrast, the joint force solutions were within 11% of each other and, therefore, much less variable.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1986

How definition of "loosening" affects the incidence of loose total hip reconstructions.

Loosening is the most common long-term problem following total hip reconstruction. For many reasons, it is important to identify the loosened total hip reconstruction. For some purposes, radiographic interpretation is the sole or principal determinant in loosening. However, there are inherent problems with such interpretations, and these problems have neither been explored in any detail in the literature nor emphasized. This article is a review of the literature and one of the problems of defining loosening. Interpretations of radiographic criteria of loosening are matters of judgment, but this judgment is not reflected in the descriptions of precisely how radiographs were interpreted. Definitions in the literature, based on lucencies and position changes, influenced the loosening rate by a factor of at least two, and influenced the apparent importance of risk factors affecting the long-term results by as much as two to three times. Thus by merely altering a definition one can alter which factors seem more or less important. The selection of a definition of loosening must be designed to a specific purpose, but data and a rationale should be provided as a basis for its interpretation. In general, descriptions of radiograph interpretation need to be precise and less subjective.

Adult↗
PubMed1985

A hemodynamic evaluation of cuffed microarteriorrhaphy.

A cuffing technique and a standard interrupted suturing technique of microarteriorrhaphy were compared in the rat femoral artery. The comparison was based on hemodynamic variables, which were measured by 20-MHz pulsed ultrasound Doppler velocity meter (PUDVM) method. The hemodynamic variables included measured blood velocity and vessel lumen diameters, and calculated volumetric flows. Measurements were completed at 5-, 10-, and 20-minute intervals after vessel repair. Data from 26 experimental animals were suitable for analysis, with 15 animals in the standard repair group and 11 animals in the cuffed group. All variables were statistically similar prior to the surgical procedure, indicating that the study groups were appropriate for comparison. The measured lumen diameters were not statistically different at any time interval (P = .06). However, the velocities and the volumetric flow were statistically different (P less than .05) between groups at all time intervals. These values returned toward normal preoperative values over time. The cuffing technique used in this study was not as efficacious as the standard technique of microarteriorrhaphy, when evaluated by 20-MHz PUDVM-derived hemodynamic criteria.

Animals↗
PubMed1985

Microarteriorrhaphy: blood flow after wound healing.

The present study tests the hypothesis that early wound healing effects blood flow after experimental microarteriorrhaphy. Hemodynamic variables were measured in the rat femoral artery prior to and 3 weeks after both interrupted and continuous microarteriorrhaphy techniques. The hemodynamic variables (blood velocities, lumen geometry, and calculated blood flow) were measured by 20-MHz pulsed ultrasonic Doppler velocity meter (PUDVM) methods. The control values (N = 22) and the 3 weeks postoperative values, for both the interrupted (N = 11) and continuous groups (N = 11), were not statistically different (P greater than .01). The average calculated blood flows were as follows: control group 10.85 +/- 1.45 cc/min, interrupted group 12.01 +/- .92 cc/min, and continuous group 8.50 +/- 1.45 cc/min. Three weeks of wound healing after microarteriorrhaphy did not significantly change blood flow variables compared to preoperative values.

Animals↗
PubMed1985

Some characteristics of laminar flow velocity spectra detected by a 20 MHz pulsed ultrasound Doppler.

For the purpose of improving accuracy of noninvasive flow measurements in small (1-2 mm diameter) blood vessels, an existing 20 MHz pulsed ultrasound Doppler velocimeter (PUDVM) has been augmented to allow fast Fourier transformation (FFT) of its Doppler shift signal. The modified instrument was used to collect velocity spectra for a benchtop test section delivering precise Poiseuille flows at velocities in the range of physiological interest. The velocity spectra demonstrated a substantial degree of broadening, much of which was attributable to the geometry of the finite sample volume size. Several spectral indices were studied as a function of flow field variables. Results showed that the intensity-weighted mean Doppler shift frequency, when converted to its corresponding velocity vM, agreed very closely with the theoretically predicted local fluid velocity. Measurement linearity and repeatability were evaluated for a number of system variables, indicating that FFT performance was essentially unaffected by several parameters capable of causing major degradation of (phasic) Doppler shift signals produced by conventional zero-crossing-counter circuitry. As presently configured, the augmented PUDVM instrument is fully capable of detailed flow field mapping in small subcutaneous vessels such as human digital arteries.

Blood Flow Velocity↗
PubMed1985

A three-dimensional computational simulation of some sources of measurement artifact in microvascular pulsed ultrasound Doppler velocimetry.

Recent applications of 20 MHz pulsed ultrasound Doppler velocimetry (PUDVM) in microsurgical research have necessarily employed piezoelectric crystals whose diameter is not negligible compared to the lumen size (1-2 mm) of many vessels of interest. A three-dimensional numerical model was developed to explore relationships between actual and detected flow field parameters, for (steady) Poiseuille flow, when appreciable velocity gradients exist within the PUDVM sample volume. Validation studies showed that highly accurate velocity profiles could be obtained in the limiting case of a very small sample volume (0.1 mm radius), but that for currently employed crystals (approximately equal to 0.5 mm radius) there was appreciable underestimation of the centersteam velocity, and appreciable overestimation of the flow stream diameter. Errors in perceived velocity and flow rate were found to be relatively insensitive to perturbations in the sample volume thickness, in the size of the sampling range increment, or in the angle of insonation beam divergence. By contrast, these apparent flow parameters were found to be very sensitive to perturbations of sample volume diameter or of the Doppler angle. Small variations in the degree of partial sample volume overlap of the flowstream periphery were shown to be capable of causing large fluctuations in apparent flow stream diameter.

Blood Flow Velocity↗
PubMed1985

Prediction of muscle and joint loads after segmental femur replacement for osteosarcoma.

The authors designed a mathematical model that simulated 28 proximal and distal femoral resections and reconstructions for tumor and then predicted muscle and joint forces during gait and stair climbing. Such models will become increasingly useful as improvements are available, as tumor patient longevity increases, and as the design and manufacture of custom devices become more common. Inertial property changes with surgery were minor and can be ignored for most purposes. For most simulations, physiologically reasonable muscle forces were predicted, suggesting that patients would be able to walk in a normal or near-normal fashion. Large joint and muscle loads were predicted after extensive muscle resections, particularly during stair climbing, suggesting that patients would limp. The quadriceps muscle group was sufficiently strong to allow normal gait with partial excisions. With complete quadriceps excision and transfers, however, high joint loads were predicted. Furthermore, if an endoprosthesis had inherent varus-valgus stability in such cases, the moments to be satisfied by the prosthesis would predispose to high interface stresses and loosening. If, conversely, the endoprosthesis did not have inherent stability, muscle forces in the remaining muscles would be unrealistically high, suggesting that such a patient would limp or require ambulation assists.

Adult↗
PubMed1985

Hip arthrodesis. A long-term follow-up.

In a retrospective study, we examined twenty-eight patients who had had an arthrodesis seventeen to fifty years previously (average, thirty-five years). Hip and knee ratings were obtained, as well as anteroposterior and flexion-extension radiographs of the lumbar spine and standing anteroposterior radiographs of the knees and hips. About 60 per cent of the patients had pain in the ipsilateral knee (average time to onset, twenty-three years after arthrodesis), and a similar percentage had back pain (average time to onset, twenty-five years after the operation). Pain in the contralateral hip occurred in approximately 25 per cent of the patients (average time to onset, twenty years after arthrodesis). Only one patient was unemployed due to disabling pain in the back or knee. Seventy per cent of the patients could walk more than one mile (1.6 kilometers), and a similar percentage could sit comfortably for at least two hours. Seventy-five per cent of the patients had anteroposterior laxity of the ipsilateral knee, and 80 per cent had mediolateral laxity. The patients whose hip was fused in some abduction more frequently had pain in the ipsilateral knee and the back, and they had greater degenerative changes in the ipsilateral knee than the patients whose hip was fused in adduction or in the neutral position. Six patients had undergone total hip arthroplasty for pain in the back or the ipsilateral knee, or both, and all had marked relief of back pain, while two of four had relief of pain in the knee. Two patients had a total knee arthroplasty for relief of pain in the ipsilateral knee.

Activities of Daily Living↗
PubMed1984

Interrupted and continuous microarteriorrhaphy techniques: a hemodynamic comparison.

The purpose of this study was to compare interrupted and continuous suturing techniques in laboratory microarteriorrhaphy. The comparison was based on rat femoral artery hemodynamic variables, including blood velocities, lumen diameters, and calculated blood flow. These variables were calculated by an offline computer from Doppler frequency shifts that were measured by a 20-MHz pulsed ultrasonic Doppler velocity meter. No statistical difference was noted postoperatively between the interrupted and continuous techniques relative to any hemodynamic variable. Average blood flow after the interrupted (n = 12) and continuous (n = 12) techniques was 6.59 +/- 1.12 and 6.74 +/- 1.20 ml/min, respectively. The two microarteriorrhaphy techniques result in equal postoperative vessel hemodynamics in a laboratory model.

Animals↗