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D R Pedersen

Publications and source records attributed to D R Pedersen.

65 records · Page 4Linked to original sources
PubMed1983

An analysis of tibial component design in total knee arthroplasty.

An axisymmetric finite element model of the proximal tibia and cemented tibial component subject to nonaxisymmetric loading is presented. Model variations included polyethylene components and steel reinforced polyethylene components both with and without a central fixation post. Central fixation posts of 35 and 70 mm were modeled. A vertically oriented load applied unilaterally to the tibial component was found to generally cause the largest magnitude peak stresses within the various components of the structure. The addition of steel reinforcement to tibial components without central fixation post is predicted to significantly reduce stress levels within the polymethylmethacrylate and underlying cancellous bone. Although to a lesser extent, the addition of a relatively short central fixation post to the steel reinforced tibial component further reduced these stress levels. The longer steel central fixation post can appreciably reduce proximal cement and bone stress levels. The tibial component condylar width is predicted to have little effect on polymethylmethacrylate and cancellous bone stresses, with the exception that proximal tibial cancellous bone compressive stresses are reduced with wide steel reinforced components.

Biomechanical Phenomena↗
PubMed1983

Pathologic ligamentous constraint of the hip.

A mathematic model of the hip capsule and lower extremity musculature was utilized to predict the forces present in the hip ligaments during locomotion. The results demonstrate principles and trends (rather than absolute results) in hip mechanics, the details of which are affected by the associated modeling assumptions. The active stretching of a hip joint capsule tightened by scarring or surgical transfer may appreciably increase the hip contact force. Capsular elements that prevent hip flexion and adduction play a major role in hip contact force exaggeration during common activities. The positive effect of maintaining the hip capsule to reduce total hip component dislocation contrasts with the potential negative effects of restricting joint motion and increasing the joint contact force. Increased joint loading due to capsular restriction may contribute to prosthetic component loosening.

Hip Joint↗
PubMed1983

A stress analysis of acetabular reconstruction in protrusio acetabuli.

UNLABELLED: We are reporting the results of a finite-element analysis of acetabular reconstruction for total hip replacement in the presence of protrusio acetabuli. In a protruded acetabulum, cortical bone stresses on the medial part of the pelvic wall increase with medial placement of the acetabular component, while normal placement of the component (more lateral placement) reduces these stresses. Metal backing of a polyethylene acetabular component causes a reduction in the peak cement and trabecular-bone stresses. A metal protrusio ring about only the periphery of the acetabular component increases stress levels within the lateral part of the pelvic cortex and has little effect on stresses in the medial part of the pelvic wall. A complete metal protrusio cup increases stresses in the lateral part of the pelvic cortex while decreasing substantially the stresses in the medial part of the cortex and the trabecular bone. Prosthetic reinforcement of the medial part of the acetabular wall has little effect on stress patterns in the acetabular region. CLINICAL RELEVANCE: The major long-term problem with cemented total hip prostheses is loosening. Loosening is probably related in part to the stress state in the cement and surrounding bone. The protruded acetabulum is particularly difficult to reconstruct in a manner that ensures longevity of the total hip replacement. In patients with protrusio acetabuli, the prosthetic acetabulum should be placed in a normal and not in a protruded position. A metal-backed acetabular component or a complete metal cup incorporated within the cement reduces stress levels within the medial aspect of the pelvic bone and thus may reduce the incidence of loosening.

Acetabulum↗
PubMed1982

An axisymmetric model of acetabular components in total hip arthroplasty.

An axisymmetric finite element model with nonaxisymmetric loading of an acetabular arthroplasty and the surrounding pelvic bone is presented. Model variations include ultra high density polyethylene acetabular components of varying wall thicknesses and metal backed ultra high density polyethylene components. Each of the two component types is modeled as implanted within an acetabulum with intact subchondral bone and within an acetabulum without subchondral bone. Thin wall polyethylene acetabular prostheses are predicted to increase, relative to thick wall components, maximum stresses in the cement-bone composite. Trabecular bone stresses are predicted to increase with the removal of subchondral bone. Stiffer metal-backed acetabular components are predicted to reduce maximum cement and bone stresses and to abridge the effects of altered component wall thickness and of subchondral bone removal.

Acetabulum↗
PubMed1982

A model of lower extremity muscular anatomy.

The mathematical prediction of muscle and joint force requires a quantitative knowledge of muscle origins and insertions. A model is presented based upon marking the origins and insertions in three cadavers (six limbs). Right-to-left biological variations and/or making errors are sometimes significant, but they rarely result in moment arm calculation variations of greater than 20 percent and usually the variations are less than 10 percent. The muscle origin and insertion differences between small and large cadavers is great, as would be expected, and the use of single specimen or average data will result in large errors in muscle force predictions. A scaling scheme is presented which substantially reduces those errors. The inherent limitations of developing a straight line muscle model include: 1) right-to-left biological variations and/or marking errors; 2) difficulties in establishing "effective" origins or insertions when the locations of the actual origin or insertion do not accurately reflect muscle function; and 3) intersubject variability which cannot be accounted for by simple scaling schemes.

Female↗
PubMedDate not supplied

Interobserver variability in interpreting radiographic lucencies about total hip reconstructions.

Radiographs are commonly used to identify the loosened total hip reconstruction (THR), to plan revision surgery, and to evaluate long-term follow-up patients for study purposes. Interobserver variability was studied in an attempt to interpret 63 random follow-up films in a group of 93 patients eight to 12 years post-THR. Nine observations were made on the acetabular and femoral sides of the reconstructions on all films for a total of 567 observations. There was considerable disagreement in observations of the acetabular lucencies; a consensus of all three observers was obtained only 46% of the time. There was more uniform agreement among observers (87%) in analyzing the femoral side, although agreement was most frequent when there was no lucency. There was no difference in observer variables when greater lucencies were reported by one or more observers than when there were smaller lucencies. It was concluded that a single observer can reliably record a femoral observation of no lucency and, making allowances for the Mach effect, can reliably distinguish the presence or absence of a lucency. However, a single observer may not be able to make observations of acetabular lucencies with great reliability and, in contrast to femoral lucencies, a single observer may be less reliable in recording acetabular lucencies over 2 mm as compared with those under 2 mm.

Acetabulum↗