Comparison of the biomechanics and histology of two soft-tissue fixators composed of bioabsorbable copolymers

The purpose of this study was to assess the dynamic in vitro and in vivo characteristics of two different bioabsorbable copolymer soft-tissue fixation devices and to determine their efficacy in reattaching soft tissue to bone. Suretac fixators (Smith & Nephew/Acufex MicroSurgical Inc., Northwood, MA), made of polyglyconate (2:1 glycolic acid:trimethylene carbonate), and Pop Rivets (Arthrotek, Warsaw, IN), made of LactoSorb (82% poly L-lactic acid, 18% polyglycolic acid), were anchored into synthetic bone, and their pull-out strengths were evaluated. The devices were also evaluated with the use of an in vivo goat model in which the medial collateral ligament (MCL) was elevated from the tibia and directly reattached. In the in vitro biomechanical study, the Suretac fixators had negligible strength remaining by four weeks, whereas the Pop Rivets retained 50% of their strength at 4 weeks, 20% at 8 weeks, and negligible strength at 12 weeks. The in vivo strength of MCL repairs affected by each implant was not statistically different at any of the time points. Histologically, both implants were absorbed by 52 weeks, and there was no appreciable adverse tissue response. In conclusion, both copolymer fixators were found to be biocompatible. The Pop Rivet fixators demonstrated in vivo performance comparable to the Suretac fixators, although the Pop Rivets retained strength longer in vitro. Our results suggest that both devices provide adequate strength of fixation before degrading to allow the healing soft tissues to reach or surpass their native strength.

Resorbable polymer fixation for craniomaxillofacial surgery: development and engineering paradigms

Advances in medical technology continue to present clinicians with new treatment options for their patients. Frequently, however, a "settling-in" period occurs after initial introduction, during which continued use and experience present new considerations. Absorbable internal fixation has been available in the orthopedic arena for more than a decade, but has been widely available in the craniomaxillofacial arena for only the past few years. Although the major considerations governing its use were obvious initially, since then a set of essential paradigms has presented itself. The authors summarize these principles, providing numerous clinical examples, with the goal of illustrating the current potential of this technology as well as establishing a baseline from which future developments can issue.

Principles of development and use of absorbable internal fixation

Absorbable internal fixation implants, by virtue of their temporary in situ residence and variable load sharing with the healing tissues, have the potential to yield a clinical outcome that approaches the native state. Despite the growing availability and acceptance of absorbable fixation, however, many surgeons continue to rely upon metal fixation for their patients, due, in part, to unfamiliarity with the technology. Although many of the principles of metallic internal fixation also apply to absorbable internal fixation, significant differences exist as well. This paper presents basic background in absorbable technology and proposes a set of principles that may help govern the development and use of absorbable fixation devices in clinical practice.

Calcium sulfate bone void filler: a review and a look ahead

Bone grafting to augment skeletal healing has become one of the most common techniques in surgical practice. However, the morbidity and limited availability associated with autografts, and the potential for disease transmission, immunogenic response, and variable quality associated with allografts, have engendered a plethora of alternative materials. Such alternatives range from the simple, such as calcium sulfate and calcium phosphate materials, to the complex that contain allograft extracts, bone morphogenetic proteins, or other agents. Calcium sulfate has the distinction of being the alternative that is both one of the simplest as well as that which has the longest clinical history as a synthetic bone graft material--spanning more than 100 years. This article reviews the structure and function of calcium sulfate as a synthetic bone void filler and speculates on its future surgical role. It is anticipated that this foundation will also help assist in the understanding of how other bone graft alternatives may operate.

Critical concepts of absorbable internal fixation

Absorbable internal fixation implants, by virtue of their temporary in situ residence and variable load sharing with the healing tissues, have the potential to yield a clinical outcome that approaches the native state. Despite the growing availability and acceptance of absorbable fixation, however, many surgeons continue to rely on metal fixation for their patients, because, in part, of unfamiliarity with the technology. Although many of the principles of metallic internal fixation also apply to absorbable internal fixation, significant differences exist as well. This article presents basic background in absorbable technology and proposes a set of principles that may help govern the development and use of absorbable fixation devices in clinical practice.

Effect of simulated intraoperative heating and shaping on mechanical properties of a bioabsorbable fracture plate material

Heating bioabsorbable fracture fixation plates to above their glass transition temperature renders them temporarily malleable, thus facilitating their adaptation to the underlying bone geometry, although the consequence of heating is not well understood. Poly (L-lactide-co-glycolide) copolymer specimens were heated under various conditions, and the effects on specimen mechanics were assessed. Heating temporarily increased toughness while slight reducing flexural modulus. No lasting effects on in vitro material degradation were seen.

Bioabsorbable fixation devices: status for the craniomaxillofacial surgeon

With time, more and more types of medical devices become available to assist the surgeon in managing patients. Bioabsorbable fixation devices, which have been directed toward the orthopedic surgeon over the past 10 years, are but one example. One aim of this article is to present the current status of bioabsorbable devices in medical practice to the craniomaxillofacial surgeon who may not be aware of the inroads this technology has made. A bioabsorbable fixation system has become available for use by the craniomaxillofacial surgeon, which is described. A further aim of this article is to present concisely the testing rigor required of such devices before their introduction to the U.S. market. This has the added benefit of explaining the important role of the surgeon in developing and helping reduce to standard clinical practice the use of new technologies.

Bioabsorbable polymer science for the practicing surgeon

The structure and function relationships of polymers have long been the purview of engineers and polymer chemists. As bioabsorbable polymer implants continue to make inroads in the medical implant armamentarium, surgeons, long familiar with the properties and handling characteristics of metal implants, may find it advantageous to become aware of some of the unique characteristics of these types of materials so that an informed decision can be made regarding their usage. In this article, we present, in relatively nontechnical terms, the salient features of polymers in general and absorbable polymers in particular.

Bioresorbable implants--practical considerations

Traditional metal implants, primarily used for internal fixation, have been used by the orthopedic surgeon for years. Decades of development have produced such devices for almost every conceivable need. Despite their widespread use, a relatively consistent set of problems or issues have been identified. These include the potential for long term migration, breakage, stress shielding, reaction to the material, interference with standard imaging techniques, and growth restriction in young patients. A number of bioresorbable polymer devices have recently become available to create a viable alternative for some indications. As expected with an evolving technology, solving one set of problems has engendered another. One of the most limiting aspects of bioresorbable polymers is their inherently lower strength compared to metals. Although more of an issue with some materials and applications than others, significant tissue reactions have been observed in some cases as well. This paper discusses the field of synthetic bioresorbable polymers in general, but with specific reference to those materials and devices that can be used in place of metal implants for internal fixation.

In vitro comparison of parameters affecting the fixation strength of sagittal split osteotomies

PURPOSE

The goal of this study was to determine how different parameters affect the bending strength of human cadaver mandibles that have undergone a sagittal split osteotomy.

MATERIALS AND METHODS

The effects of screw material (titanium [Ti] vs polylactic acid/polyglycolic acid [PLA/PGA]), screw configuration (linear vs inverted L-shape), screw diameter (2.0 mm vs 2.7 mm), material into which screws were inserted (human mandible, bovine rib, synthetic polymer), and loading rate (1.0 mm/min vs 10.0 mm/min) were quantified. Also, biomechanical principles were used to model shear stress and displacement. Variable lever arms, screw material, screw diameter, screw configuration, distance between screws, and bone properties were all evaluated in this model.

RESULTS

Accounting for variable mandible geometries and differentiating between deflections (and shear stresses) due to bending and due to torsion, in vitro mechanical testing revealed that there was a statistically significant difference in total shear stress at 3 mm of deflection depending on screw material (Ti > PLA/PGA), screw diameter, and material into which screws are inserted (mandibles > ribs = synthetic polymer). There was no significant difference in total shear stress depending on screw configuration or strain rate.

CONCLUSION

Total shear stress and deflections are important and more viable parameters than load to assess parameters of clinical importance in osteotomy or fracture fixation.

Oxidative interactions between hemoglobin and egg lecithin liposomes

The oxidative interaction between hemoglobin and unsaturated egg lecithin liposomes, characterized by the consumption of oxygen and the production of methemoglobin over time, was measured and modelled. The oxygen and methemoglobin profiles were fit by a mathematical model, using numerical integration techniques. From the model, it was determined that the catalytic rate constants for the effect of hemoglobin on lipid peroxidation varied with hemoglobin type, as deoxyhemoglobin greater than oxyhemoglobin = methemoglobin. Under in vivo conditions of oxygen tension, reaction rates were oxyhemoglobin greater than deoxyhemoglobin greater than methemoglobin. Vitamin E had about 35 times the antioxidant activity of cholesterol, as determined by the model. Both additives, together, appeared to stabilize the liposomal membrane, as manifested by their ability to prevent oxidation of approximately 95% of the available lipid.