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Biomedical subjects

D D Robertson

Publications and source records attributed to D D Robertson.

At least 19 recordsLinked to original sources
PubMed1992

Comparison of helical and serial CT with regard to three-dimensional imaging of musculoskeletal anatomy.

To test the hypothesis that helical computed tomographic (CT) scans provide three-dimensional images as good as or better than those provided by serial CT, two objects were used to study the effects of helical CT: an angled cylindrical bone phantom and a human cadaveric femur specimen with a simulated fracture 1 mm wide. Both objects were immersed in a water bath, and a series of helical and serial CT scans were obtained with various parameters. Volumetric rendering was applied to the resultant data sets to create three-dimensional images, which three radiologists reviewed in a blinded manner to rate their fidelity, accuracy, and diagnostic usefulness. As expected, the images obtained with thin collimation and small intersection spacing or slow table movement were considered superior. Helical and serial CT data acquired with similar parameters were similar in quality, but helical CT is approximately five times faster than serial CT; hence, it is possible to use thinner collimation and obtain more sectional data with helical CT.

Femur↗
PubMed1990

An investigation of a compliant interface for press-fit joint replacement.

Earlier in vitro studies showed that a compliant layer between a metal surface and trabecular bone improved the load distribution. In this study, the behavior of a compliant layer of Dacron velour was investigated in vivo using a patella resurfacing in a sheep as a model. Bilateral cases were used to compare the velour interface with a direct metal-to-bone interface. For the metal patellas, a fibrous layer developed adjacent to the metal while the underlying bone formed a new subchondral-like layer. With the velour interface, fibrous tissue invaded the velour, followed later by bone, which sometimes reached the metal surface. For follow-ups of 8 months or more, the load across the interface was transferred over localised patches, for both the press-fit and velour interfaces. There was no significant difference in the areas of contact. There was evidence that this was due to the irregularity of the bony surface beneath the fibrous layer, or to bone nodules actually growing up to the metal. It was concluded that in this in vivo model, the velour layer did not retain a more uniform load distribution compared with the press-fit joint, due to the nature of the bone and fibrous tissue that formed at the interfaces.

Animals↗
PubMed1990

Effect of press-fit femoral stems on strains in the femur. A photoelastic coating study.

A photoelastic coating method was used to study the strain patterns on the surface of the femur, before and after insertion of femoral stems. The anatomically shaped stems were press-fit without a collar, press-fit with a collar, and press-fit with proximal cementing to approximate a bone ingrowth situation. The strain patterns of the intact femurs were consistent with bending. The pattern changed considerably after stem insertion, probably due to the stiffening effect of the stems. Nevertheless, the collarless press-fit stem preserved 64% of the magnitude of the proximal medial strain. The collar restored the average to normal, but the surface strains varied because of the localized regions of contact between the collar and the bone. Proximal cementing gave results similar to those of the collarless press-fit. The press-fit stems often showed local patches of high strain, probably reflecting local endosteal contact points. Local high stresses at the level of the distal tip were seen in only a few instances.

Cadaver↗
PubMed1990

Bone loss in the distal anterior femur after total knee arthroplasty.

Bone loss in the distal anterior femur in asymptomatic total knee arthroplasty (TKA) patients has been noted roentgenographically and during revision surgery. A retrospective roentgenographic review of 147 TKA cases was carried out to document bone loss. The influence that the mode of fixation (porous coated and cemented) and the implant design have on bone loss was examined. The time of onset and the progression of bone loss were studied. Bone loss occurred in the distal anterior femur in the majority of cases reviewed (68%). The prevalence of bone loss was independent of the mode of fixation and the implant design. By qualitative observation, roentgenographically detectable bone loss occurred within the first postoperative year and did not progress further. Previously three-dimensional finite element analysis demonstrated that the replacement of the bearing surface of the femur with a stiff metallic implant reduces the stress in the distal anterior femur by at least one order of magnitude. It is therefore speculated that the observed bone loss results from stress shielding. The apparent lack of progression may reflect the development of a new remodeling equilibrium under the altered stress conditions. The bone loss in the distal anterior femur described has not been implicated as a source of failure. However, since the bone strength in the femoral region is compromised as it becomes osteopenic, bone failure may occur with longer periods of cyclic loading. Furthermore, as a result of bone loss, revision arthroplasty may be more difficult.

Aged↗
PubMed1989

A mathematical model for the evaluation of the behaviour during flexion of condylar-type knee prostheses.

A 3D knee model was developed in order to evaluate the mechanical behaviour during flexion of condylar-type knee prosthesis. Based on the total energy minimization principle, it takes into account the articular surfaces (the tibial surface being deformable), the body weight, and the patello femoral joint. It generates the kinematics of the joint, the motion of the centre of contact, the quadriceps forces, the pressure distribution on the tibial plateau, and ligament lengths and forces between 0 and 120 degrees of flexion. The results for ten digitized knees and the commercially available prostheses are presented. They are in general agreement with experimental results published in the literature. It is concluded that this computer program may be, within its limitations, a useful tool in the preliminary evaluation of new condylar-type knee prosthesis designs.

Biomechanical Phenomena↗
PubMed1989

Enhanced computed tomographic techniques for the evaluation of total hip arthroplasty.

Computed tomography (CT) has revolutionized the evaluation of musculoskeletal pathology. Until recently, however, CT of the postoperative orthopaedic patient has been severely limited by its inability to provide useful information in the vicinity of acetabular and femoral implants. Typically the hardware produces extensive artifacts that can markedly degrade the whole image. Methods are now available to reduce the metal artifact. Following hip arthroplasty, these methods have been used to plan for revision arthroplasty and to evaluate the contralateral side for avascular necrosis.

Evaluation Studies as Topic↗
PubMed1988

The effects of knee brace hinge design and placement on joint mechanics.

A computer model of 23 knees was obtained by embedding, slicing and digitizing the bone outlines and ligament co-ordinates. Using co-ordinate transformations, various three-dimensional motions were imposed on the knees, and calculations made of femoral-tibial contact error, contact point locations and ligament lengths. Significant deviations in these parameters were noted for abnormal motions including the elimination of internal-external rotation and a-p displacement and the misplacement of a hinge producing correct motion. The resulting mismatch could result in shear in soft tissues, cuff-to-skin slippage and inaccurate ligament length patterns.

Biomechanical Phenomena↗
PubMed1988

Improving the fit of press-fit hip stems.

The implant-bone fit is critical to the longevity of press-fit total hips. Maximum contact on the internal cortical bone surface, especially proximally, produces more normal strain values and reduces micromotion and sinkage. Stem design computer software was developed to design both individualized hip stems that maximize implant-bone contact for a unique anatomy, and average anatomic hip stems in which the mean error of fit is minimized for many patients. Cadaver femurs were used to test the following two hypotheses: (1) Average anatomic stems, designed using the optimal-fit software for an average femoral geometry, improve implant-bone fit when compared with standard stems; and (2) Individualized optimal-fit stems provide an even better fit than average anatomic stems. Results showed that the average anatomical stems fit better than the standard stems. However, the individualized hip stems, designed from individual three-dimensional geometry data, fit the best.

Aged↗
PubMed1988

Strategies for improving fixation of femoral components in total hip arthroplasty.

As strategies are considered for improving fixation of femoral components in total hip arthroplasty (THA), one is challenged to exceed the standard set by contemporary cement procedures. However, despite the improved ten- to 15-year clinical results anticipated with current cementing techniques, the limited fatigue strength of polymethylmethacrylate warrants continued investigation of alternative systems, particularly for younger patients and in revision arthroplasty. Design considerations for femoral stems for cementless THA include (1) initial mechanical stability afforded by the stem shape, (2) strength and stiffness of the stem, and (3) surface features relating to biocompatibility and attachment to bone. In one approach a fit-and-fill algorithm has been implemented to design stems that maximize contact between prosthesis and cortex in priority areas to achieve stability. Titanium is recommended for the fabrication of such stems because of its corrosion resistance, its biocompatibility, and its modulus, which is lower than that of cobalt-chromium alloy. Long-term fixation of these implants will be dependent upon the maintenance of normal strain patterns in the host bone. Achievement of this goal will require additional strategies that combine optimal fit and optimal material properties of the prosthesis.

Biocompatible Materials↗
PubMed1988

Design and fabrication of cementless hip stems.

Theoretical and experimental studies before and after stem insertion demonstrated that stresses and strains were closer to normal for uncemented stems than for cemented stems. The values were affected by relative tightness of fit in the proximal and distal regions and by the presence or absence of a collar. For designing an optimal fit-stem, the average femoral geometry was first determined. The stem was used in a photoelastic coating study that showed a continuous strain field over the bone surface. The average proximomedial bone strains for intact, press-fit, loose press-fit with collar, and proximally cemented (to simulate ingrowth) designs were 100%, 65%, 101%, and 54%, respectively. Localized patches of high strain were seen on the proximal bone surface and beneath a collar, indicative of localized stem-bone contact points.

Biomechanical Phenomena↗
PubMed1986

Metallic hip implants: CT with multiplanar reconstruction.

Thirty consecutive patients with metallic implants in the hip were evaluated with both standard transaxial computed tomography (CT) and multiplanar reconstruction (MPR) to assess the contribution of MPR. The metallic devices included total hip replacements (n = 6), one or more surgical plates with screws (n = 9), acetabular pins or screws (n = 9), and Knowles pins (n = 6). The metallic implants were on the side of interest in 25 hips and in the contralateral hip in five cases. One patient had bilateral hardware. MPR reformats axially acquired data into other planes. The reconstruction program weights the true signal over the randomly distributed artifacts by integrating adjacent axial images. Examination using standard transaxial imaging was graded as excellent in five cases, adequate in 15 cases, and inadequate in ten cases. After MPR, 19 studies were classified as excellent, 11 were classified as adequate, and none were classified as inadequate. It is concluded that MPR significantly reduced artifacts caused by metal present in transaxial images.

Acetabulum↗
PubMedDate not supplied

Design of custom hip stem prostheses using three-dimensional CT modeling.

Long life expectancy, demand for high activity levels, and bone loss at the time of revision motivate the search for reliable and successful noncemented hip stem designs. It is hypothesized that improved implant fit may increase the longevity of noncemented total joints. Quantitative X-ray CT has enabled the use of a computerized stem design program, which designs an optimal-fit hip stem for individual femurs. Computed tomography and interactive image processing methods are used to generate the individual three-dimensional femoral models, which are used by the stem design program. Optimal-fit design provides maximum stem-bone contact while satisfying the requirement of being surgically insertable. Previous methods of custom implant design, including those that use three-dimensional CT modeling, have not provided optimal stem-bone fit. Quantitative results of this new process are presented.

Aged↗
PubMedDate not supplied

Evaluation of CT techniques for reducing artifacts in the presence of metallic orthopedic implants.

Metallic intramedullary orthopedic implants generate artifacts that can markedly degrade transaxial CT images. The artifacts, typically seen as starburst streaking, result primarily from reconstructions involving missing projection data. Two approaches are clinically available to reduce the artifacts around orthopedic implants. These are (a) the imaging of implants with lower attenuation coefficients or smaller path lengths (less attenuating objects) and (b) the planar reformatting of image data. The sizing accuracy of these two approaches was quantified using phantoms and the efficacy using cadaveric femoral specimens. Results demonstrated that metal artifacts may be reduced and accurate bony dimensional data obtained.

Evaluation Studies as Topic↗
PubMedDate not supplied

Fidelity of three-dimensional CT imaging for detecting fracture gaps.

Simulated femoral neck fractures with varying amounts of distraction were studied using four modifications in a volumetric, three-dimensional CT reconstruction program to allow an objective comparison of volumetric versus surface (thresholded) rendering. Binary classification caused spurious fusion of gaps less than 2.0 mm or, at settings preserving these gaps, produced false holes in adjacent bone. Fracture gaps were visible to 0.0 mm using volumetric CT image rendering technique.

Femoral Neck Fractures↗
PubMedDate not supplied

Three-dimensional volumetric display of CT data: effect of scan parameters upon image quality.

Of the many steps involved in producing high quality three-dimensional (3D) images of CT data, the data acquisition step is of greatest consequence. The principle of "garbage in, garbage out" applies to 3D imaging--bad scanning technique produces equally bad 3D images. We present a formal study of the effect of two basic scanning parameters, slice thickness and slice spacing, on image quality. Three standard test objects were studied using variable CT scanning parameters. The objects chosen were a bone phantom, a cadaver femur with a simulated 5 mm fracture gap, and a cadaver femur with a simulated 1 mm fracture gap. Each object was scanned at three collimations: 8, 4, and 2 mm. For each collimation, four sets of scans were performed using four slice intervals: 8, 4, 3, and 2 mm. The bone phantom was scanned in two positions: oriented perpendicular to the scanning plane and oriented 45 degrees from the scanning plane. Three-dimensional images of the resulting 48 sets of data were produced using volumetric rendering. Blind review of the resultant 48 data sets was performed by three reviewers rating five factors for each image. The images resulting from scans with thin collimation and small table increments proved to rate the highest in all areas. The data obtained using 2 mm slice intervals proved to rate the highest in perceived image quality. Three millimeter slice spacing with 4 mm collimation, which clinically provides a good compromise between image quality and acquisition time and dose, also produced good perceived image quality. The studies with 8 mm slice intervals provided the least detail and introduced the worst inaccuracies and artifacts and were not suitable for clinical use. Statistical analysis demonstrated that slice interval (i.e., table incrementation) was of primary importance and slice collimation was of secondary, although significant, importance in determining perceived 3D image quality.

Data Display↗