Harold Alexander

Canine bone response to tyrosine‐derived polycarbonates and poly(L‐lactic acid)

Tyrosine-derived polycarbonates are a new class of degradable polymers developed for orthopedic applications. In this study the long-term (48 week) in vivo degradation kinetics and host bone response to poly(DTE carbonate) and poly(DTH carbonate) were investigated using a canine bone chamber model. Poly(L-lactic acid) (PLA) served as a control material. Two chambers of each test material were retrieved at 6-, 12-, 24-, and 48-week time points. Tyrosine-derived polycarbonates were found to exhibit degradation kinetics comparable to PLA. Each test material lost approximately 50% of its initial molecular weight (Mw) over the 48-week test period. Poly(DTE carbonate) and poly(DTH carbonate) test chambers were characterized by sustained bone ingrowth throughout the 48 weeks. In contrast, bone ingrowth into the PLA chambers peaked at 24 weeks and dropped by half at the 48-week time point. A fibrous tissue layer was found surrounding the PLA implants at all time points. This fibrous tissue layer was notably absent at the interface between bone and the tyrosine-derived polycarbonates. Histologic sections revealed intimate contact between bone and tyrosine-derived polycarbonates. From a degradation-biocompatibility perspective, the tyrosine-derived polycarbonates appear to be comparable, if not superior, to PLA in this canine bone chamber model.

A phalangeal fracture model—Quantitative analysis of rigidity and failure

Nine types of internal fixation techniques were tested in 4-point bending using a pig metacarpal model for phalangeal fractures. Levels of bending rigidity and bending moments at failure were determined, and the modes of failure are described. Plate and screw fixation afforded the greatest rigidity, and epiphyseal fractures occurred, leaving intact the test section. Flexible wire loop fixation failed by wire cutting into bone when a square knot was used. Twisted wire unraveled when placed in tension. Depending on the fracture type and the wire placement. Kirschner wires failed either by slipping in the bone, twisting in the bone cortex, or bending at the bone cortex interface. Rigidity varied widely depending on the way in which the wires were employed.

Classes of Materials Used in Medicine

Publisher Summary There is a diverse range of materials and methods available for the immobilization of biomolecules and cells on or within biomaterial supports. This chapter explains various classes of materials used in medicine. Metallic implant materials have a significant economic and clinical impact on the biomaterials field. Apart from orthopedics, there are other markets for metallic implants and devices including (1) oral and maxillofacial surgery, for example, dental implants, craniofacial plates, and screws and (2) cardiovascular surgery, for example, the parts of artificial hearts, pacemakers, balloon catheters, valve replacements, and aneurysm clips. The chapter also introduces the concepts of polymer characterization and property testing as they are applied to the selection of biomaterials. It provides a table that compares some of the biomolecule immobilization techniques—physical and electrostatic adsorption, cross linking, entrapment, and covalent binding.

AN APPLICATION OF THE EXTENDED KANTOROVICH METHOD TO THE STRESS ANALYSIS OF A CLAMPED RECTANGULAR PLATE

SummaryThe extendedKantorovich method discussed recently, byA. D. Kerr, is used to analyze a clamped rectangular plate subjected to a uniform lateral load. It was found that the generated one term solution approximates very closely, throughout the plate region, not only the deflections but also the bending moments and shearing forces. It is shown that the final form of the solution is independent of the initial choice, and that the convergence of the iterative procedure is very rapid. Because of the lack of a closed form exact solution in the technical literature, the coefficients occurring in the obtained solution were evaluated for various plate side ratios and are presented in graphs in order to simplify the utilization of the obtained results in engineering practice.ZusammenfassungDie erweiterte methode vonKantorowitsch, die kürzlich vonA. D. Kerr diskutiert wurde wird benützt, um die eingespannte Rechteckplatte unter Gleichlast zu untersuchen. Es ergab sich, daß die erzielte eingliedrige Lösung nicht nur die Durchbiegung, sondern auch die Biegemomente und Querkräfte im Bereich der Platte sehr gut approximiert. Es wird gezeigt, daß die Endgestalt der Lösung unabhängig ist von der anfangs getroffenen Wahl und daß die Konvergenz des Iterationsverfahrens sehr rasch fortschreitet. Da in der technischen Literatur eine exakte Lösung in geschlossener Form fehlt, wurden die in der erhaltenen Lösung auftretenden Koeffizienten für verschiedene Seitenverhältnisse der Platte ausgerechnet und in Schaubildern dargestellt, um die Benützung der erhaltenen Resultate in der Ingenieurpraxis zu erleichtern.

Clinical Evaluation of Laser Microtexturing for Soft Tissue and Bone Attachment to Dental Implants

Introduction:A tapered dental implant (Laser-Lok [LL] surface treatment) with a 2 mm wide collar, that has been laser micromachined in the lower 1.5 mm to preferentially accomplish bone and connective tissue attachment while inhibiting epithelial downgrowth, was evaluated in a prospective, controlled, multicenter clinical trial. Materials:Data are reported at measurement periods from 1 to 37 months postoperative for 20 pairs of implants in 15 patients. The implants are placed adjacent to machined collar control implants of the same design. Measurement values are reported for bleeding index, plaque index, probing depth, and crestal bone loss. Results:No statistical differences are measured for either bleeding or plaque index. At all measurement periods there are significant differences in the probing depths and the crestal bone loss differences are significant after 7 months (P < 0.001). At 37 months the mean probing depth is 2.30 mm and the mean crestal bone loss is 0.59 mm for LL versus 3.60 and 1.94 mm, respectively, for control implant. Also, comparing results in the mandible versus those in the maxilla demonstrates a bigger difference (control implant − LL) in the mean in crestal bone loss and probing depth in the maxilla. However, this result was not statistically significant. Discussion:The consistent difference in probing depth between LL and control implant demonstrates the formation of a stable soft-tissue seal above the crestal bone. LL limited the crestal bone loss to the 0.59 mm range as opposed to the 1.94 mm crestal bone loss reported for control implant. The LL implant was found to be comparable with the control implant in safety endpoints plaque index and sulcular bleeding index. There is a nonstatistically significant suggestion that the LL crestal bone retention superiority is greater in the maxilla than the mandible.

Ligament replacement with an absorbable copolymer carbon fiber scaffold--early clinical experience.

Eighty-two patients, average age 29 years, were surgically treated (during the period from April 1981 to July 1983) for both acute (8%) and chronic (92%) knee ligament instabilities. An absorbable copolymer-carbon fiber ligament prosthesis was used as a tissue scaffold. Seventy-five percent had anterior cruciate ligament reconstructions, 6% had anterior cruciate and posterior cruciate ligament reconstructions, 6% had just posterior cruciate ligament reconstructions, and 9% had other combinations of anterior cruciate ligament, medical collateral ligament, and lateral collateral ligament reconstructions. Preoperative and postoperative evaluation, consisting of questionnaires, physical examinations, and isoskinetic testing, revealed significant improvements in categories of stability, pain, function, and strength persisting to the end of the study at 24 months. Arthroscopic examination and histologic studies of retrieved specimens demonstrated well-vascularized reconstructions with collagenous tissue ingrowth into the carbon-copolymer implants.

Osteoconductive Composite Grouts for Orthopedic Use

A composite of HA particulate and calcium sulfate hemihydrate has been developed. When this dry material is mixed with either sterile saline or water, the resulting mixture begins to gel. While in the gel or dough state, the composite can be manually inserted or injected under pressure into a bone defect where it sets in situ. The calcium sulfate acts as a binder to hold the HA particles in place. The subsequent resorption of the calcium sulfate leaves controlled porosity for bone ingrowth and attachment to the nonresorbable HA particulate. The combination of calcium sulfate and HA results in a composite with handling properties superior to those of HA alone. Calcium sulfate has a long history of medical use as an implant material. The biocompatibility of the material has been clearly established. The combination of HA and calcium sulfate is likewise extremely compatible as demonstrated in the present studies. Bone ingrowth concomitant with resorption occurs rapidly with efficient conduction of bone from particle to particle. Based on the experiments presented herein, the composite of HA and calcium sulfate may be a useful alternative or adjunct to autogenous bone grafting. Many questions, however, remain regarding long-term tissue response and bone remodeling as well as the efficacy of this material relative to bone grafting.

Comparison of bovine collagen xenografts to autografts in the rabbit.

The use of bovine tendon as a xenograft material in humans is attractive because of its ready availability and favorable mechanical characteristics. Previous research has shown that the fibroblasts and some extracellular proteoglycans and glycoproteins, not the collagen matrix itself, in bovine tendon are primarily responsible for its antigenicity. Various attempts have been made to decrease the antigenicity of these grafts. A chloroform/methanol (CM) extraction procedure has been developed that selectively removes the fibroblasts from bovine tendon without destroying the collagen matrix. The mechanical, immunologic, and local host tissue responses to these grafts were compared to autografts and to untreated and glutaraldehyde-treated bovine tendon xenografts. The humoral immune response to a purified bovine Type I collagen product was also studied. The central two-thirds of a rabbit Achilles tendon were replaced with a reversed autograft or an experimental graft. Histologic examination of one- and two- week specimens showed an acute inflammatory response to all grafts. Untreated grafts stimulated a severe inflammatory response and were almost completely resorbed by two weeks. Glutaraldehyde-treated grafts were encapsulated. Cellular repopulation was minimal and inflammatory response was more persistent than in the autograft and CM groups. Inflammatory response to CM-treated grafts was similar to that of autografts. The CM grafts repopulated rapidly with host cells. The mechanical strength of CM grafts was equal to autograft controls at 12 weeks. The mechanical strength of untreated and glutaraldehyde-treated grafts was significantly lower. Measurement of the humoral immune response to these grafts was conducted in an independent group of animals using an enzyme-linked immunosorbent assay. A significant antibody response to untreated, glutaraldehyde-fixed, and CM-treated grafts was detected at 30 days. Antibody titers to glutaraldehyde-fixed and untreated grafts remained elevated at 60 and 90 days. In the CM group, antibody titers decreased to the level of autograft controls by 90 days. No significant antibody response was detected toward purified bovine Type I collagen.

Mechanical analysis of Kirschner wire fixation in a phalangeal model

Transverse divisions of the mid-shaft of freshly frozen pig metacarpals were fixed with Kirschner wires of two sizes and using four different configurations. Compared to the usual cross-pin fixation using wires of 0.889 mm. four wires (0.712 mm) eccentrically placed and with their ends hooked provided a 300% improvement in bending rigidity and 170% in bending movement.

DEVELOPMENT OF NEW METHODS FOR PHALANGEAL FRACTURE FIXATION

Abstract Surgical intervention is warranted in the fixation of finger fractures only if it results in high mechanical stability. This allows early motion and avoids loss of range of motion and stiffness often associated with immobilization. To facilitate the rational design of finger fracture fixation devices an analytical and experimental program is presented to establish appropriate design criteria. The analytical scheme uses composite beam theory treating the device and bone as components of a system contributing to the strength and rigidity of fixation. The experimental program uses the pig metacarpal as an animal model for the proximal phalanx. K-wire fixation and intramedullary fixation with a polylactic acid polymer (PLA)-carbon fiber composite device are considered. Fixation rigidity is found to be limited by device slippage and device-bone hardness mismatch. However, both techniques are shown to be capable of sustaining the bending moments expected in normal pinch hand function. A method of fixation using four 0.712-mm ( 28 1000 - in. ) wires is shown to increase the maximum bending moment by 170% and bending rigidity by 300% over the conventional two crossed K-wire method. Its maximum bending moment capability provides a safety factor of 2.01 over expected tip pinch moments. A method using a 3.5-mm square cross-section PLA-carbon intramedullary (IM) device is shown to increase the maximum bending moment by 240% and bending rigidity by 297% over the two-wire method with a maximum bending moment safety factor of 2.53.