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Biomedical subjects

A S Greenwald

Publications and source records attributed to A S Greenwald.

At least 19 recordsLinked to original sources
PubMed1995

Reconstructive strategies for radioulnar dissociation: a biomechanical study.

Radial head fracture combined with disruption of the soft-tissue restraints to proximal radial migration is defined as radioulnar dissociation. This injury often involves irreparable radial head fracture. When excised, management has emphasized the use of silicone implants to prevent proximal migration. However, concerns regarding silicone use led to the search for reconstructive alternatives. The purpose of this study was to compare reconstructive methods for radioulnar dissociation. Biomechanical testing to measure radial displacements under physiologic loads was performed on fresh-frozen cadavers after radial head resection. The central interosseous membrane (IOM) was disrupted to simulate radioulnar dissociation. This was followed by an orderly reconstructive sequence including radial head replacement with both silicone and titanium implants as well as a newly described flexible IOM reconstruction. Calculated stiffness values were used for comparison. Silicone implants provided no statistical improvement in forearm stiffness compared with the disrupted membrane state. Significant improvements were achieved with titanium implants and flexible IOM reconstruction, which restored stiffness to 89% and 94% of normal, respectively. No statistical differences were identified between these successful methods. Titanium implants combined with flexible reconstruction increased stiffness to 145% of normal. Further doubt is cast on the role of silicone implants in the management of radioulnar dissociation. Titanium implants and flexible IOM reconstruction appear to be wiser reconstructive choices.

Aged↗
PubMed1994

Patellar resurfacing or retention in total knee arthroplasty. A prospective study of patients with bilateral replacements.

Patellofemoral problems are a common cause of morbidity and reoperation after total knee arthroplasty. We made a prospective study of 52 patients who had bilateral arthroplasty (104 knees) and in whom the patella was resurfaced on one side and not on the other. A movable-bearing prosthesis with an anatomical femoral groove was implanted on both sides by the same surgeon using an otherwise identical technique. The mean follow-up was 5.24 years (2 to 10). In the 30 available patients (60 knees) there was no difference between the two sides in subjective preference, performance on ascending and descending stairs or the incidence of anterior knee pain. Radiographs showed no differences in prosthetic alignment, femoral condylar height, patellar congruency or joint line position. The use of an appropriate prosthetic design and careful surgical technique can provide equivalent results after knee arthroplasty with or without patellar resurfacing. Given the indications and criteria, which we discuss, retention of the patellar surface is an acceptable option.

Adult↗
PubMed1993

A comparison of the disassociation strength of modular acetabular components.

Five short-term in vivo disassembly of two-piece acetabular cup designs have been reported. This study evaluates the liner retention strengths of eight contemporary cup systems. Both push-out (663 +/- 65.5 pounds force to 29 +/- 1.4 pounds force) and lever-out (684 +/- 114 inch-pounds to 43 +/- 1.5 inch-pounds) test modes show a wide variation in retention strength. Repeat liner separation testing demonstrates a 26% and 32% respective decrease in locking mechanism integrity. These findings indicate that reseating modular liners at the time of surgery or reassembling a previously separated liner should be avoided.

Acetabulum↗
PubMed1993

Difficulties with bearing dislocation and breakage using a movable bearing total knee replacement system.

In 1980, the low contact stress, or New Jersey, system was introduced with movable high-density polyethylene bearings between the metallic femoral and tibial components. This system was developed to deal with two principal concerns of conventional knee replacement systems: loosening and wear. A federal Drug Administration review was required, and the study was performed in two phases. From 1981 until 1984, components were cemented in 40 knees, which then were evaluated for an average of seven years after surgery. From 1984 until 1986, 16 knees were selected for cementless fixation and were followed for an average of five years. The cemented group had a failure rate of 10% (four knees), two from tibial component loosening, one from bearing dislocation, and one from infection. Failure was determined by the necessity for revision. The uncemented group had a failure rate of 31% (four knees from bearing fracture and one from knee pain of uncertain source). Bearing failure was attributable to the entrapment of the subluxed lateral bearing. Sacrifice of the anterior cruciate ligament predisposes the system to bearing subluxation and failure.

Aged↗
PubMed1992

The biomechanical effects of geometric configuration of bone-tendon-bone autografts in anterior cruciate ligament reconstruction.

This study provides biomechanical support for a new technique of autograft anterior cruciate ligament reconstruction featuring circular bone plugs and endosteal interference fit fixation. Six matched pairs of fresh frozen human knees were utilized. Femoral interference fit pull-out strength was determined from material-testing-machine-generated oscillograph recordings at a strain rate of 100%/s. Circular bone plugs, obtained with a circular oscillating saw, provided 19.9% greater interference fit pull-out strength compared with identically fixed trapezoidal bone plugs. Different geometric defects were compared in three- and four-point bending on an Instron machine with frozen patellae and an artificial bone composite. Circular defects have 107% greater strength than matched trapezoidal patellar defects in three-point bending. In a bone composite, circular defects are 53% stronger than triangular and 25% stronger in four-point bending than trapezoidal defects. A new technique of harvesting bone plugs with endosteal interference fit fixation is described and biomechanically supported. To date, this technique has been performed on over 500 cases clinically without evidence of patellar fracture or fixation failure. This study demonstrates the efficacy of this simple and reproducible technique compared with previously reported procedures.

Aged↗
PubMed1991

External fixator pin insertion techniques: biomechanical analysis and clinical relevance.

A series of identically matched pairs of fresh-frozen canine femora (approximating human radii in size and dimension) were used to mechanically compare pull-out strength between 4 mm predrilled, self-tapping, half-pins and 4 mm self-drilling, self-tapping half-pins with drill bit-like cutting flutes. A second biomechanical and videotape analysis was done comparing the differences of pin insertion by power versus hand drilling. Results indicated a mean 22% reduction in bone purchase of self-drilling, self-tapping pins compared with that of predrilled pins and a marked increase in depth of insertion required of the self-drilling pins for comparable pin purchase (10 mm). It was also observed that a visible "wobble factor" exists, which tends to weaken the pin-bone interface when hand drilling is performed.

Animals↗
PubMed1991

Repair of the tibiofibular syndesmosis with a flexible implant.

Fractures of the adult ankle with disruption of the tibiofibular syndesmosis require adequate stabilization of the ankle mortise to ensure satisfactory healing of the syndesmotic ligaments. Numerous internal fixation techniques for stabilization of the syndesmosis have been used. However, most techniques require partial device removal before weight bearing can be initiated. The "flexible syndesmosis repair" uses simple, inexpensive, readily available synthetic materials to restore distal tibiofibular stability. Once early fracture healing has been obtained, weight bearing is begun (average 6 weeks). Biomechanical testing on paired cadaver ankles demonstrated a suture tensile strength of 60 lbs and consistent suture-button strength of 49 lbs, whereas tricortical screw fixation was found to have a higher 82 lbs average pull-out strength, but demonstrated a wide variability depending on bone quality. Twelve patients have been managed with flexible syndesmosis repair and followed for 2-4 years. All fractures have healed without deformity and there are no cases of mortise instability. Subsequent analysis of devices removed 8-12 months following implantation has shown that all have remained intact without failure. The flexible syndesmosis repair is a reliable, stable way to restore syndesmosis integrity, allowing early weight bearing without need for interim surgery.

Adolescent↗
PubMed1991

Use of survivorship and contact stress analyses to predict the long-term efficacy of new generation joint replacement designs. A model for FDA device evaluation.

Long-term investigational device exemption (IDE) clinical trial evaluation, as currently monitored by the Food and Drug Administration (FDA) regulatory process, is time-consuming, burdensome, and extremely costly to the product developers and ultimately to the consumer. Thus, almost all devices introduced in the past decade have not been clinically tested; they have been introduced under a "510k" grandfather provision that does not address the safety and efficacy of the product. As a result, joint replacement devices have been sold and used with serious failure consequences for the consumer. When 10-year survivorship studies are available for specific joints that demonstrate strong evidence of favorable clinical performance with suitable supporting analysis, they can be used as a standard for comparison with newer, similar designs. Additionally, contact stress analysis of the bearing surfaces of these joint replacements can be used to determine their long-term wear characteristics under normal loading conditions and compared to retrieved devices used for 10 years or more. Devices that demonstrate a 90% survivorship over a 10-year interval and have low enough contact stresses to minimize wear failure during the same period can be used as "standard designs" to which newer designs should be compared. Using this methodology, the developers of any new device should cite the standard design that they have improved on and document an improvement in early (1 to 3 years) survivorship studies with a contact stress analysis demonstrating bearing contact stresses equal to or lower than the standard design. Any device that fails more often or wears out faster than a standard design should not be released until design modifications have proved it superior or equal to the standard designs.

Biomechanical Phenomena↗
PubMed1990

Biomechanical analysis of pin placement and pin size for external fixation of distal radius fractures.

A series of biomechanical analyses were performed to explain the recent reduction in treatment-related complications of external fixation of distal radius fractures using a limited open approach for pin placement and larger 4-mm self-tapping half pins. A comparison of pull-out strength, stress concentration effect, and inherent bending strength of 3- and 4-mm half pins was performed. The effect of proximal pin placement in the radius or in the ulna and the effect of distal pin placement in four, six, or eight metacarpal cortices were determined. These analyses demonstrate that the 4-mm self-tapping half pins result in a significantly higher pull-out strength and only a small decrease in torsional load strength of the bone. They also demonstrate that proximal pin fixation in the radius produces the most stable fixation and that distal pin fixation into six metacarpal cortices produces a strong configuration that does not violate the interosseous muscles of the second intrinsic compartment. The rate of treatment-related complications in the external fixation of distal radius fractures (specifically, pin loosening, bending and breakage, fracture through pin sites, collapse at the fracture site, and intrinsic contracture) are addressed in this study. Such complications can be minimized by using 4-mm pins after central predrilling, with proximal placement in the radius and distal placement through six cortices of the bases of the second and third metacarpals.

Animals↗
PubMed1988

Total articular replacement arthroplasty.

Surface replacement, popularized in the early 1970s has fallen into disfavor by much of the orthopedic community. To date, the authors' experience with 29 cemented total articular replacement arthroplasties (TARAs) in 25 patients has been quite favorable. Factors associated with these favorable results include: a physiologic age less than 60, good bone quality, viability of the remaining femoral head, and preservation of the acetabular subchondral plate. The mean age of cases in this study was 52 years, with patient follow up ranging between 3.5 and 7 years; there were two failures. Today, although conventional cementless total hip arthroplasty seems to be in vogue, real and potential problems exist. These concerns, in conjunction with the good results reported in this series, should encourage the orthopedic surgeon to take a second look at resurfacing arthroplasty, especially in the select group of patients defined in this article. Pending the long-term results of non-cemented prostheses the surgeon may even consider using cementless TARA components.

Adult↗
PubMed1988

Evaluation of Dacron-covered and plain bovine xenografts as replacements for the anterior cruciate ligament.

Surgical repair of the anterior cruciate ligament often involves the use of a suitable autograft. As alternatives to sacrificing these normal structures, various allografts, xenografts, and synthetic materials have been investigated as ligament replacement materials. This study investigates Dacron fabric-covered and plain bovine xenograft tendon as such materials in the canine knee. The implants were tested to failure in an MTS machine following 13 weeks of implantation in a canine knee. Dacron woven fabric-covered implants became more firmly attached than those covered by Dacron mesh fabric or plain xenografts. The implants were also analyzed according to their method of attachment (fixation staples or sutures). Overall, the sutured implants failed at slightly higher forces than did the stapled ones. Histologically, limited vascular invasion of the xenograft was observed. No host fibrous or osseous tissue could be identified within the graft. Fibrous tissues did form between the bone and xenograft. The implants exhibited extreme intraarticular wear, which suggests a low potential for intraarticular ligament replacement.

Animals↗
PubMed1986

Use of tricalcium phosphate or electrical stimulation to enhance the bone-porous implant interface.

Implant stabilization by biologic ingrowth into a porous surface offers a durable method of prosthetic fixation. These systems, however, lack the immediate stability offered by the use of acrylic bone cement. The interface strength of porous coated Co--Cr--Mo in a canine model does not approach that of acrylic bone cement until two weeks postoperatively. It is expected that this would be a minimum time period in clinical applications. Both chemical and electrical means have been advocated as methods to affect tissue ingrowth. A study using a canine model was undertaken to determine tissue ingrowth rates utilizing examples of these two methods: (1) impregnation of the porous structures with tricalcium phosphate powder (TCP); or (2) the application of an electrical stimulator to the implant with the implant itself serving as the cathode. Ten implants were coated with TCP, two each at weekly intervals from 1 to 5 weeks. Plain porous rods were likewise implanted, serving as the controls. While histology did reveal a slightly more dense bony structure, the interface bond strength was not affected by TCP. Electrical stimulation of the implant was similarly investigated with an additional time period of 10 weeks. Compared to the controls, the electrically stimulated implants reveal no statistically demonstratable difference in interface strength. Histologic specimens indicate larger areas of calcification than are observed in the controls.

Animals↗
PubMed1986

Subchondral pathways to the superior surface of the human talus.

The suggestion that the blood supply to subchondral bone plays an active role in the nutrition of adult articular cartilage is a source of controversy. Morphological confirmation of the presence of subchondral defects extending through the basal cartilage layer of the adult human talus is presented. Perfusion of a fluorescent dye through these breaks offers the possibility that they may serve as active pathways for the transport of nutrients to the cartilage. The distribution patterns of the defects in areas of intermittent cartilage loading is noted and its significance is discussed.

Cartilage, Articular↗
PubMed1985

Finger joint contact areas and pressures.

Although the joints of the index finger are similar geometrically and kinematically, the occurrence of degenerative joint disease is more frequent and severe in the distal interphalangeal joint. Much circumstantial evidence exists to suggest a mechanical cause for the observed differences in frequency. This article presents the results of in vitro experiments designed to determine contact areas and average pressures in the joints of the human index finger for positions simulating tip pinch and power grasp. The results show that the highest average contact pressures do, in fact, occur in the distal interphalangeal joint. Average joint contact pressure correlates well with clinically observed patterns of frequency of degeneration and degenerative joint disease score. This correlation between clinical experience and experimental results indicates that mechanical stress is among the factors responsible for the initiation and/or propagation of degenerative joint disease in the joints of the finger.

Biomechanical Phenomena↗
PubMed1984

Fracture of long bones: rate effects under singular and combined loading states.

This study was undertaken to determine the effect of loading rate on bone failure under singular and combined loading states. Using the entire loading range of an Instron Materials Testing Machine, 190 pairs of canine radii were tested. One bone of each pair was subjected to either torsional or a combination of axial and torsional loads, and compared with a control specimen. Compressive loads of 4.6 and 11.5 N were utilized. Torque to failure and energy data versus loading rate were plotted. As loading rate increased, the torque and energy values were found to increase, reach a peak, and then decline at higher rates. Fractures produced under the combined state of loading are representative of those clinically found in human trauma situations.

Animals↗
PubMed1984

Biomechanical considerations in revision arthroplasty.

Accumulated knowledge about hip replacement arthroplasty has identified a number of mechanical factors that contribute to implant failure. The mechanics of component design, materials selection, implant fixation, and response of bone and soft tissue structures to an altered loading environment are variables that influence outcome. Revision arthroplasty presents additional problems of loss of bone stock, cortical perforation or fracture, and implant removal that must be appreciated to prevent recurrent failure.

Acetabulum↗
PubMed1983

Mathematical model of the human ankle joint.

It has been suspected that the mechanical environment in which a particular joint functions has an effect on the initiation or progression of degenerative joint disease. The objective of this study is to define the mechanical environment of the ankle joint, specifically, the contact areas and pressure distributions, through the development and analysis of a simplified mathematical model. Since the state of pressure across articular surfaces during function is influenced by joint incongruity, cartilage thickness profile and the geometry of the opposing surfaces, these factors have been incorporated into the model formulation. Mathematical analysis of the model has resulted in pressure distributions in both the anterior-posterior and medial-lateral directions and contact area growth plots which correlate well with observed ankle contact patterns obtained from in vitro investigations. The significance of joint incongruity to these pressure distributions and to the relative immunity of the ankle joint to primary osteoarthritis is discussed.

Ankle Joint↗