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Biomedical subjects

M G Dunn

Publications and source records attributed to M G Dunn.

At least 37 records · Page 2Linked to original sources
PubMed1993

Mechanical and histological evaluation of amorphous calcium phosphate and poorly crystallized hydroxyapatite coatings on titanium implants.

The effect of amorphous calcium phosphate (Ca/P) and poorly crystallized (60% crystalline) hydroxyapatite (HA) coatings on bone fixation to "smooth" and "rough" (Ti-6A1-4V powder sprayed) titanium-6Al-4V (Ti) implants was investigated. Implants were evaluated histologically, mechanically, and by scanning electron microscopy (SEM) after 4 and 12 weeks of implantation in a rabbit transcortical femoral model. Histological evaluation of amorphous vs. poorly crystallized HA coatings showed significant differences in bone apposition (for rough-coated implants only) and coating resorption (for smooth- and rough-coated implants) that were increased within cortical compared to cancellous bone. The poorly crystallized HA coatings showed most degradation and least bone apposition. Mechanical evaluation, however, showed no significant differences in push-out shear strengths between the two types of coatings evaluated. Differences between 4 and 12 weeks were significant for coating resorption and push-out shear strength but not for bone apposition. Significant enhancement in interfacial shear strengths for bioceramic coated as compared to uncoated implants were seen for smooth-surfaced implants (3.5-5 times greater) but not for rough-surfaced implants at 4 and 12 weeks. Rough implants showed greater mean interfacial strengths than uncoated smooth implants at 4 and 12 weeks (seven times greater) and to coated smooth implants at 12 weeks only (two times greater). Mechanical failure of the bone/coating/implant interface consistently occurred within the bone, even in the case of the poorly crystallized HA coatings, which had almost completely resorbed on rough implants. These results suggest that once early osteointegration is achieved biodegradation of a bioactive coating should not be detrimental to the bone/coating/implant fixation.

Alloys↗
PubMed1993

Optimization of extruded collagen fibers for ACL reconstruction.

Collagen fibers used in a scaffolding device for ligament reconstruction must be thin, strong, and degradable. The purpose of this study was to determine the effects of fiber diameter (20, 50, or 90 microns), crosslinking agent (uncrosslinked, dehydrothermal-cyanamide, or glutaraldehyde), and hydration on the initial mechanical properties, biocompatibility, and subcutaneous degradation rates of fibers extruded from an acidic dispersion of insoluble type I collagen. The wet tensile strength of extruded collagen fibers was significantly improved by decreasing the fiber diameter. Low-diameter, crosslinked fibers had wet tensile strengths ranging from 75-110 MPa. In contrast, high diameter fibers had wet strength values of about 30 MPa. The degradation rate of the implanted fibers, in contrast, was not significantly prolonged by changing the initial fiber diameter. This result is important because prolonged degradation of the fibers can lead to implant encapsulation instead of neoligament formation. By minimizing the diameter, fiber strength can be increased without prolonging the fiber degradation rate. Low-diameter, dehydrothermal-cyanamide crosslinked fibers have greater tensile strength and a more rapid degradation rate than medium-diameter, glutaraldehyde crosslinked fibers, and are therefore more suitable for use in a degradable ligament reconstruction device.

Animals↗
PubMed1992

The prevalence of carpal tunnel syndrome in patients with basal joint arthritis of the thumb.

Basal joint arthritis of the thumb and carpal tunnel syndrome are common conditions with an acknowledged coexistence. This article attempts to quantify the prevalence of carpal tunnel syndrome in patients with basal joint arthritis and to examine some of the etiologic factors that affect the coexistence of the two disorders. Of 246 patients who had surgery about the basal joint, 95 patients (39%) were identified by chart review as having carpal tunnel syndrome. Eleven of 122 remaining patients contacted had symptomatic carpal tunnel syndrome confirmed by nerve-conduction studies, bringing the total to 106 (43%). The prevalence was higher in worker's compensation patients and those with diabetes mellitus. The prevalence was lower in men than in women, and patients with inflammatory joint disease were at less risk than those with osteoarthritis. Given this high association, great care should be taken to diagnose or exclude coexistent carpal tunnel syndrome in patients scheduled for basal joint surgery so that, if present, it can be treated at the same time, diminishing the risk of postoperative morbidity and delayed symptoms.

Arthritis↗
PubMed1991

Minisatellite binding protein Msbp-1 is a sequence-specific single-stranded DNA-binding protein.

Msbp-1 is a minisatellite-specific DNA-binding protein. Using synthetic binding substrates, we now show that Msbp-1 binds not to double-stranded DNA, but exclusively to single-stranded DNA. Binding is specific to the guanine-rich strand of the minisatellite duplex, interactions with the cytosine-rich strand being undetectable by southwestern analysis. Furthermore, the binding site required for successful DNA-protein interactions appears to be two or more minisatellite repeat units. We have also isolated, by whole-genome PCR and cloning, one Msbp-1 binding site from the human genome. Again, the binding strand of this molecule contains a repetitive G-rich structure equivalent to that of a small minisatellite. These observations are discussed with respect to other single-stranded DNA-binding proteins known to play a role in recombination processes.

Animals↗
PubMed1991

Anterior cruciate ligament replacement: a review.

The anterior cruciate ligament (ACL) is the major intra-articular mechanical element that limits motion of the tibia with respect to the femur. It is a multi-fasciculated structure composed of crimped aligned collagen fibers. The purpose of this paper is to review the literature on ACL structure and mechanical properties in an effort to stimulate the development of a new generation of more effective replacement devices. Replacement of the ACL is achieved using biologic and synthetic grafts. Biologic grafts include illiotibial band, semitendinosus and gracilis tendons, patellar tendon, and meniscus. Bone-patellar-bone complexes used to replace the ACL are revascularized and ultimately replaced by neo-ligament. Synthetic implants including the Integraft, Leads-Keio ligament, Gore-Tex¿ ligament and Kennedy Ligament Augmentation Device (LAD) have either not been approved or approved by the FDA for limited use as a replacement for the ACL. The Kennedy LAD has been found to increase the strength of autogenous tissue during revascularization. Based on the success of autografts and the Kennedy LAD, we conclude that the next generation of ACL replacement devices will consist of a scaffold and a biodegradable augmentation device. The scaffold will have a structure that mimics the normal ACL as well as stimulates revascularization and healing. A biodegradable augmentation device will be employed to mechanically reinforce the scaffold without stress shielding the neo-ligament. By combining the advantages of autografts and a biodegradable augmentation device, a new generation of ACL replacements will be achieved.

Anterior Cruciate Ligament↗
PubMed1991

Hydroxyapatite-coated orthopedic implants.

Hydroxyapatite (HA)-coated metal implants were developed in order to capitalize on the excellent biological properties of HA, and minimize the potential for mechanical failure of the HA in vivo. Results of implantation studies suggest that synthetic HA is osteoconductive (enhances local bone healing) and becomes osteointegrated (forms an intimate bond with bone). HA-coated prostheses are currently being evaluated for use in cementless total joint arthroplasty. The purpose of this article is to review the experimental studies leading to the development of HA-coated orthopedic devices, and to summarize the current status of clinical studies. Finally, the concerns and future directions for HA-coated implants are addressed.

Animals↗
PubMed1991

Regeneration of Achilles tendon with a collagen tendon prosthesis. Results of a one-year implantation study.

We previously reported on the short-term biocompatibility of a reconstituted type-I collagen prosthesis that had been tested in the Achilles tendons of rabbits. Preliminary results indicated that, by ten weeks after implantation, carbodiimide-cross-linked implants had been replaced by neotendon in a manner that was similar to that of autogenous tendon grafts that had been used as controls. Also by ten weeks after implantation, glutaraldehyde-cross-linked collagen implants were encapsulated and appeared to have caused an acute inflammatory response. In the present study, carbodiimide and glutaraldehyde-cross-linked collagen implants and autogenous grafts that served as controls were implanted for fifty-two weeks as a replacement for a three-centimeter section of the Achilles tendon of rabbits. The absence of a crimp in a cross-linked implant and the presence of a crimp in normal tendon and in tendon that formed after an implant had been resorbed made it possible to distinguish between a cross-linked implant and new host tendon that had replaced the implant after it was resorbed. New collagen that had replaced the implant and autogenous (control) tendon graft were compared with normal Achilles tendon with respect to the angle and length of the crimp. The autogenous grafts and the carbodiimide-cross-linked collagen implants had been completely resorbed and replaced by neotendon. The neotendon that was present fifty-two weeks after implantation was similar, but not identical, to normal tendon. In contrast, the glutaraldehyde-cross-linked implant was essentially inert, had not been resorbed, and was surrounded by a capsule of collagenous connective tissue. The neotendon in the capsule was also similar, but not identical, to normal tendon. There were more cells in the capsule than in the autogenous grafts or in the carbodiimide-cross-linked implants. The results of the present study indicate that rapid repair is achieved with a carbodiimide-cross-linked collagenous implant that has a structure and mechanical properties that are similar to those of an autogenous tendon graft and that biodegrades at a similar rate. Prolonged biodegradation of a glutaraldehyde-cross-linked collagenous implant results in formation of a capsule and only limited formation of neotendon.

Achilles Tendon↗
PubMed1989

Achilles tendon replacement by a collagen fiber prosthesis: morphological evaluation of neotendon formation.

Reconstituted type I collagen was processed into fibers which were subsequently severely dehydrated and cyanamide cross-linked. Fibers prepared by this method were stronger and more resistant to degradation than uncrosslinked fibers. When used as a tendon replacement prosthesis, morphological events occurred which were observed by light, scanning, transmission electron microscopy and electron histochemistry. Resorption was the initial host response to the prosthesis and involved gradual biodegradation. Formation of a host-replacement tendon was the second response. Increased collagen fibril diameters and a transition in the proteoglycan/collagen fibril interactions occurred in the newly developing connective tissue between 3 and 10 weeks postimplantation. These extracellular matrix transitions were major events occurring during wound healing and led to the assembly of a mature connective tissue. When used as a tendon prosthesis, these collagen fibers rapidly resorb while allowing simultaneous formation of aligned connective tissue. The fibers may have other applications in the fields of Orthopaedic Surgery, Neurosurgery and Biomaterials Research.

Achilles Tendon↗
PubMed1988

Wound healing using a collagen matrix: effect of DC electrical stimulation.

Rapid fibroblast ingrowth and collagen deposition occurs in a reconstituted type I collagen matrix that is implanted on full-thickness excised animal dermal wounds. The purpose of this study is to evaluate the effects of direct current stimulation on dermal fibroblast ingrowth using carbon fiber electrodes incorporated into a collagen sponge matrix. Preliminary results suggest that fibroblast ingrowth and collagen fiber alignment are increased in collagen sponges stimulated with direct currents between 20 and 100 microA. Maximum fibroblast ingrowth into the collagen sponge is observed near the cathode at a current of 100 microA. These results suggest that electrical stimulation combined with a collagen matrix may be a method to enhance the healing of chronic dermal wounds.

Animals↗
PubMed1988

Relationship between mechanical properties and collagen structure of closed and open wounds.

Mechanical properties and collagen structure of excisional wounds left open are compared with wounds closed by clips. In both wound models, collagen fiber diameter increases with time post-wounding and is related to tensile strength. Clipped wounds show a higher ultimate tensile strength and tangent modulus compared with open wounds. In clipped wounds, newly deposited collagen appears as a biaxially oriented network as observed in normal skin. In open wounds a delay in the organization of the collagen network is observed and parallel wavy-shaped ribbons of collagen fibers are deposited. At long term, the high extensibility observed in open wounds may be due to the sliding of ribbons of collagen fibers past each other.

Animals↗
PubMed1985

Collagen fiber formation in repair tissue: development of strength and toughness.

Dermal repair tissue shows a progressive increase in collagen content which may be related to the wound tensile strength. Wound strength and extensibility are lower than those found in normal skin. In animals, wounds closed by metal clips are chosen as a model to study the proliferative and remodeling phases of healing from a mechanical and morphological point of view. During the proliferative phase the low wound strength is associated with formation of collagen fibers of small diameter, later, (days 28-45) an acute change appears corresponding to the remodeling phase, with increased collagen fiber diameters observed by scanning electron microscopy and light microscopy, and increased tensile strength and toughness. By 180 days, wound strength and collagen fiber morphology were close to that observed in the normal skin. These observations show a direct relationship between collagen fiber diameter and tensile strength. In addition, packing density of collagen fibrils (determined by the birefringence retardation per unit thickness under polarized light) was unchanged until day 90, although collagen fiber diameters increased during this time.

Animals↗
PubMed1985

Mechanical analysis of hypertrophic scar tissue: structural basis for apparent increased rigidity.

The mechanical behavior of normal human skin and hypertrophic scar tissue (HST) is compared using constant-strain-rate and successive stress-relaxation uniaxial loading programs in vitro. HST is less extensible, requires more energy to be stretched in the physiologic range, and stores strain energy less efficiently than normal skin. The explanations for the differences observed between the mechanical behavior of normal skin and HST are based on the differences in their composition and structure. We suggest that the collagen fiber network is partially "prealigned" in a crimped tendon-like organization in HST, which reduces its extensibility and raises the strain energy required to stretch it. It is further hypothesized that an incomplete elastic fiber network, an abnormal glycosaminoglycan content, and/or abnormal collagen fiber slippage are responsible for the reduced capacity to return strain energy in the hypertrophic scar tissue. The results of these studies indicate that although HST has been described as stiffer than normal skin, the maximum stiffness of skin and HST are similar. The "apparent" increased rigidity of HST is a result of reduced extensibility rather than a change in its stiffness. This inexensibility may manifest itself by limiting joint mobility in the patient with HST.

Adolescent↗
PubMed1985

Collagen deposition during wound repair.

Collagen fiber diameters, amount of birefringent collagen (brightness) and birefringence retardation were measured in implanted collagen-based sponges containing hyaluronic acid (HA) and fibronectin (FN). In the presence of HA and FN, increased number of fibroblasts and brightness were observed 6 days after wounding. Increased brightness in the presence of HA and FN reflected increased deposition of oriented collagen fibers. From days 9 to 12, increased fiber diameters were similar in implanted collagen-based sponges with or without HA and FN. Increased birefringence retardation in sponges containing HA and FN was consistent with increased packing density of collagen fibers observed by scanning electron microscopy. Our results suggest that HA and FN are effective in promoting fibroblast movement into a collagen sponge and deposition of collagen fibers during the early phases of wound healing. Use of a collagen-based sponge containing HA and FN may enhance collagen deposition in situations where healing is compromised as in the case of dermal ulcers.

Animals↗
PubMed1983

Viscoelastic behavior of human connective tissues: relative contribution of viscous and elastic components.

Stress-relaxation tests were performed at successive strain levels on strips of human aorta, skin, psoas tendon, dura mater, and pericardium. The elastic fraction, the equilibrium force divided by the initial force, was calculated at each strain increment. In the aorta, the elastic fraction decreased with strain and was modeled as the transfer of stress from elastic to collagen fibers, while in skin it increased with strain, probably due to the rearrangement of individual collagen fiber orientations, resulting in an aligned collagen network at high strains. The strain-independent elastic fractions for tendon, dura mater, and pericardium were similar, and approximately equal to the values found for aorta and skin at high strains. It was hypothesized that the elastic fraction is related to the type of fiber loaded, and the tissue geometry. This analysis may be useful in studying disease-induced changes in the mechanical properties of connective tissues.

Aged↗