Bioresorbable polymers in trauma and bone surgery

During the last few decades interest in resorbable polymeric materials has been steadily increasing. As with other materials for implantable devices, they have to satisfy several biological and technical requirements. Implants should maintain adequate mechanical properties in vivo for the time required and degrade at an effective rate. The conditions of polymer synthesis, further processing into implants and the sterilization process determine the mechanical properties of resorbable implants. Degradation of implants is manifested by implant fragmentation, strength loss and the decrease of polymer molecular weight. The rate of degradation and the tissue reaction are strongly affected by the material chemical composition and to some extent also by the mechanical properties. Potentially, devices made from bioresorbable polymers can overcome problems associated with metal implants like stress protection, potential for corrosion, wear and debris formation, as well as the necessity of implant removal. Resorbable polymers have proven to be good materials for a range of devices in trauma surgery. However, modifications and optimizations are still required. Three-dimensional porous scaffolds in various geometrical forms offer a good potential for the manufacture of tissue-engineered implants in the future.     

New resorbable bone fixation. Biomaterials in craniomaxillofacial surgery: present and future

Metallic implants have been extensively used for osteofixation in craniomaxillofacial surgery (CMF). Although they are mainly inert, the disadvantage of not being resorbable remains. Advances in biomaterials technology led to the development of resorbable polymers composed by monomeric units that are naturally present in the body. Among them are polylactide (PLA), polyglycolide (PGA) and their copolymers [PLGA and P(L/DL)LA]. After in-depth studies of the structure of these materials, the self-reinforced (SR) technology has merged to solve their mechanical limitations. SR is based on reinforcing elements within the same material, increasing their strength. Ultra-high strength implants were then manufactured by the SR technique. In CMF surgery, SR devices have been used for over 10 years without reported complications. As advantages, SR devices have shown to be biocompatible, reliable, easily handled, and to have favorable degradation properties. At present, copolymeric SR devices [P(L/DL)LA, BioSorb FX, and PLGA, BioSorb PDX] represent an advance in the clinical application of absorbable devices in CMF surgery. The aim of this review is to give an overall understanding of the ultimate advances in biomaterial research for CMF reconstruction, with emphasis in self-reinforced resorbable devices. We also intend to give insight into new devices, such as SR, tacks and their applicability.     

Coralline bone graft substitutes.

Coralline porous ceramics are biocompatible and osteoconductive implants. They have proven to be effective as bone graft substitutes in large animal models and in humans. Bone and supporting soft tissue grow into and throughout their porosity if the implant is placed in direct apposition to viable bone and the interfaces are stabilized. The bone within the implant remodels in response to Wolff's law. Both the implant properties (chemistry and porosity) and the biologic environment modulate the rate of implant resorption. Composite technology with resorbable polymers can improve the mechanical properties of these ceramics.     

Healing of subcapital femoral osteotomies fixed with self-reinforced poly-L-lactide screws: an experimental long-term study in sheep

Subcapital femoral osteotomies of ten young adult sheep were fixed with two bioabsorbable, self-reinforced, poly- L-lactide (SR-PLLA) lag screws of 4.5 mm in diameter. At 3 weeks radiographs were taken to check the reduction and fixation achieved. After follow-up periods of 12 weeks, 1 year and 3 years with three sheep in each group, and of 7 years and 4 months with one sheep, the sheep were killed, and the healing of the osteotomies, degradation and tissue response of the implants were examined radiographically, histologically and microradiographically. All osteotomies healed with a firm bony union. There was no dislocation or wound infection. Histologically, there was no marked tissue response in the bone tissue. At 12 weeks the implants were grossly intact, at 1 year granulation tissue and new bone formation had started to penetrate into the implant, and at 3 years the implant area was mostly replaced by connective tissue and new bone, but implant material was still seen as little islands surrounded by some lymphocytes. At 7 years and 4 months, the implant material had been degraded and replaced by tight bone. Self-reinforced poly- L-lactide lag screws seem to possess adequate mechanical properties and good biocompatibility for this demanding fixation.     

Biodegradable implants in sports medicine: the biological base.

Biodegradable implants are increasingly used in the field of operative sports medicine. Today, a tremendous variety of implants such as interference screws, staples, sutures, tacks, suture anchors, and devices for meniscal repair are available. These implants consist of different biodegradable polymers that have substantially different raw material characteristics such as in vivo degradation, host-tissue response, and osseous replacement. Because these devices have become the standard implant for several operative procedures, it is essential to understand their biological base. The purpose of this report is to provide a comprehensive insight into biodegradable implant biology for a better understanding of the advantages and risks associated with using these implants in the field of operative sports medicine. In particular, in vivo degradation, biocompatibility, and the osseous replacement of the implants are discussed. A standardized classification system to document and treat possible adverse tissue reactions is given, with special regard to extra-articular and intra-articular soft-tissue response and to osteolytic lesions.     

Material Properties and Composition of Soft-Tissue Fixation

Surgical interference screws and suture anchors for attaching soft tissue, such as ligaments and tendons, to bone are routinely used in arthroscopic surgery and sports medicine. Interference screw fixation provides a press fit between bone, graft/tendon, and screw and is frequently used to attach replacement ligaments in tunnels drilled for anterior and posterior cruciate ligament reconstruction. Suture anchors are used in surgical procedures wherein it is necessary for a surgeon to attach (tie) tissue to the surface of the bone, for example, during joint reconstruction and ligament repair or replacement. The composition of these implants ranges from metals to polymers and composites. Typically, because of the relatively large amount of torque that must be applied during insertion, these screws are constructed from metal. However, interference screws and suture anchors have also been constructed from bioabsorbable polymers and composites. The ideal material would (1) provide adequate mechanical fixation, (2) completely degrade once no longer needed, and (3) be completely replaced by bone. Because no material has been shown to be superior for all applications, the surgeon must weigh the advantages and disadvantages of each to evaluate the optimum material for a given application and patient. The purpose of this article is to present a comprehensive review of the commercially available interference screws and suture anchors, with an emphasis on implant composition, interaction, and design. This article provides the orthopaedic surgeon with a background on biomaterials, specifically those used in interference screws and suture anchors. Because there is no material that is perfect for all surgical situations, this review can be used to make educated decisions on a case-by-case basis.     

Biodegradable implants in traumatology: a review on the state-of-the-art

Up to now the internal fixation of fractured bones and joints has been managed by metal implants. There are certain associated disadvantages: the mechanical properties of the metals are stronger than those of cortical bone (stress-protection); the removal of the implants requires a second operation; an increasing number of patients are confronted with problems of sensitivity to metal components of the implants, especially nickel. About 40 different biodegradable polymers, copolymers and composites have been developed as substitutes for metal implants in internal fracture fixation. The early experimental and clinical results demonstrate their limitations. From the current point of view, it is not possible to transfer the designs and assembling principles of metal implants in orthopaedic surgery to biodegradable polymers. The attempt to simply mimic metal implants in polymers is condemned to fail from the very beginning. This is a review of the literature and of our first 100 patients operated on using implants made of self-reinforced polyglycolide acid and polydioxanone. The main difficulty with the material is the loss of stiffness in a time interval which is not long enough to guarantee bone healing. The development of a sterile sinus over the site of implantation is a problem also reported by other groups. Certain additives have to be inserted into the polymers to make them visible on conventional X-radiographs. Despite these drawbacks, however, there are indications for the isolated or adjuvant implantation of biodegradable materials. They could be employed in the treatment of osteochondral fractures and other defined injuries. The available literature on these indications will be discussed. A standardized set of possible indications for the use of different biodegradable devices in orthopaedic surgery is presented as are clear contraindications for their use at the present time. For the future, fixation devices adapted to the characteristic properties of polymers and adjusted to specific therapeutic problems will have to be developed.     

Plates, screws, and children: their relationship in craniomaxillofacial trauma

The ideal modality for fixation of pediatric craniomaxillofacial fractures remains elusive for a number of reasons. Surgeons who manage these injuries have replaced wiring techniques with the introduction of some form of reconstructive implant. The most commonly used implants are either resorbable or semi-rigid titanium. This presentation is a synopsis of the past 30 years of the English-speaking scientific literature including plastic and reconstructive surgery, otolaryngology, head and neck surgery, oral and maxillofacial surgery, pediatric, trauma, craniofacial, materials, and biomaterials publications. While no consensus on ideal management was observed, various implant treatment options are discussed, including their indications, contraindications, considerations, and consequences after implant placement.     

Biodegradable Films in Trauma and Orthopedic Surgery

Abstract Resorbable materials have gained a considerable position in the daily routine of all surgical disciplines. Natural products like catgut and collagen have been used historically. Since the development of synthetical macromolecules more than 40 years ago, the range of indications for the use of such materials has widened significantly in our daily routine. Suture materials, mesh, tissue pads, clips, screws and anchors are in use. More recent developments in the field of orthopedic and trauma surgery include screw-plate systems, wound dressing materials and films for the prevention of adhesions and ossifications. This appears to be the beginning of an era of new materials as these implants not only fulfil a temporary biomechanical role but in theory also can release a controlled amount of biologically active substances at a set timeframe. Also they are potential carriers for transplants on a cellular level. This aspect will be of importance in the orthopedic field, where resorbable films only play a minor role so far. At the moment there are six resorbable or biodegradable films or foils on the market that are used or can be used in the field of orthopedic and trauma surgery. These are foils and films made of following materials: carboxy-methyl-cellulose and hyaloronic acid (Seprafilm^?), oxydized regenerated cellulose (Interceed^?), polydioxanon (PDS^?) and copolymers of lactid und caprolacton (Topkin^?, Oprafol^?, Mesofol^?). Main indications for their use are wound dressing, especially after split skin graft and thermal wounds, prophylaxis of adhesions and prevention of the formation of synostoses and heterotopic ossifications. The results of clinical trials are promising and the increasing number of publications in the last 5 years in this field is an expression of increasing demand of these materials. However, it could also be an expression of the growing interest in drug delivery techniques as well as in tissue engineering which are possible with these materials.     

MacroPore resorbable devices in craniofacial surgery

Resorbable polymer implants have become a compelling option in the treatment of acquired and congenital craniofacial deformities. The resorbable polylactide (PLa) and polyglycolide (PGa) polymers in particular have demonstrated excellent safety profile sin multiple in vitro, animal, and clinical studies and are currently being used in a wide variety of craniofacial applications. In this article, the authors discuss the biomaterial properties of PLa and PGa resorbable implants and provide an overview of the use of these polymers in craniofacial surgery. They conclude by relating their experience with an ongoing clinical series using MacroPore PLDLa and FRP implants for various applications,including Le Fort osteotomies, midface/monobloc internal distraction, and craniosynostosis reconstruction.     

The use of BoneWelding? technology in spinal surgery: an experimental study in sheep

The innovative BoneWelding^? technology, where ultrasound energy bonds bioresorbable implants to bone, was tested for its feasibility in spine surgery and its local thermal effects. The three tested concepts consisted of implementation of a resorbable plating system, two converging polymer pins and suture anchors to the cervical vertebral bodies. Bioresorbable polylactide implants (PLDLLA 70/30) were inserted ventrally into the third and fourth vertebral body of seven sheep, of which six were sacrificed at 2 months and one sheep immediately after temperature measurements during implant insertion. Polymer screws were used as controls. Qualitative, semi-quantitative histological, and quantitative histomorphometrical evaluation showed excellent anchorage of the implants, new mineralized bone at the implant-bone interface, no inflammatory cell reaction or thermal damage to the adjacent bone in response to the novel insertion technology. The application of two converging pins, parallel inserted polymer pins, or fusion of the implant to the polymer plates did not affect the overall excellent tissue tolerance of the technology. Temperature increase during insertion was noticed but never exceeded 47°C for less than 1 s. The BoneWelding^? technology was proven to be safe and easy to apply.     

Osteointegration of hydroxyapatite-titanium implants coated with nonglycosylated recombinant human bone morphogenetic protein-2 (BMP-2) in aged sheep

Osteointegration of metal implants into aged organisms can be severely compromised due to reduced healing capacity of bone, lack of precursor cells for new bone formation, or osteoporosis. Here, we report on successful implant healing in a novel model of aged sheep in the presence of nonglycosylated bone morphogenetic protein 2 (BMP-2). Ewes of 8 to 12 years with significant radiologic and histologic signs of osteoporosis and adipocytic bone marrow received a cylindrical hydroxyapatite-titanium implant of 12 x 10 mm. BMP-2 has been produced as a bacterial recombinant fusion protein with maltose-binding protein and in vitro generation of mature BMP-2 by renaturation and proteolytic cleavage. A BMP-2 inhibition ELISA was developed to measure the in vitro release kinetics of bioactive human BMP-2 from immersed solid implant materials by using Escherichia coli expressed and biotinylated recombinant human BMP-2 receptor IA extracellular domain (ALK-3 ECD). The implants were placed laterally below both tibial plateaus, with the left leg implant carrying 380 microg BMP-2. Both implant types became integrated within the following 20 weeks. The control implant only integrated at the cortical bone, and little new bone formation was found within the pre-existing trabecular bone or the marrow cavity. Marrow fat tissue was partially replaced by unspecific connective tissue. In contrast, BMP-2-coated implants initiated significant new bone formation, initially in trabecular arrangements to be replaced by cortical-like bone after 20 weeks. The new bone was oriented towards the cylinder. Highly viable bone marrow appeared and filled the lacunar structures of the new bone. In mechanical tests, the BMP-2-coated implants displayed in average 50% higher stability. This animal model provided first evidence that application of nonglycosylated BMP-2 coated on solid implants may foster bone healing and regeneration even in aged-compromised individuals.     

Woven bone formation around implants and the effect of bacterial infection.

Several implant materials used in dental and orthopedic surgery were placed in rat tibial bones to study their effects on mineralization. The implants consisted of bone bonding and non-bonding materials. Changes in mineralization were defined by morphometric analysis of matrix vesicle distribution at the implant interface and in normal bone healing following marrow injury. Bone-bonding materials induced an increase in matrix vesicle activity. This finding was supported by study of the biochemical changes in the same model that manifested high correlations to the morphometrical observations with regard to enhancement or delay of primary mineralization. In addition, the study of healing using nuclear methods indicated that implants alter bone healing as shown by the different uptakes of 99mTc and 32P in the different bone compartments. Decreased 32P uptake by the organic phase in the presence of bone-bonding implants suggested that cleavage of 99mTc-MD32P into its technetium and methylene diphosphonate moieties was inhibited by administration of implants. Further studies on the effect of bacterial infection on the peri-implant tissues revealed a decrease in woven bone formation due to infection.     

Anorganic bovine bone and analogs of bone mineral as implants for craniofacial surgery: a literature review

Hydroxyapatite compounds and a new anorganic bovine bone mineral are materials that offer an alternative to autologous bone grafting. Ceramic hydroxyapatite implants are limited in their usefulness due to difficulty in contouring the cortical form and the tendency for the granular form to exist outside the reconstruction site. These limitations have been overcome by the advent of a hydroxyapatite cement paste exhibiting good biocompatibility, contour manipulation, implant stability and osseoinduction. Preliminary studies indicate that a new resorbable anorganic bovine bone mineral with a chemical composition and structure similar to human bone promotes initial bony healing more readily than its synthetic hydroxyapatite counterpart, and therefore may serve as an excellent alternative for craniofacial reconstruction.     

Degradable synthetische Implantatmaterialien

In the last decade biodegradable synthetic implant materials have been established for various clinical applications. Ceramic materials such as calcium phosphate, bioglass and polymers are now routinely used as degradable implants in the clinical practice. Additionally these materials are now also used as coating materials or as microspheres for controlled drug release and belong to a series of examples for applications as scaffolds for tissue engineering. Because immense local concentrations of degradation products are produced during biodegradation, this review deals with the question whether allergic immune reactions, which have been reported for classical metallic and organic implant materials, also play a role in the clinical routine for synthetic biodegradable materials. Furthermore, possible explanatory theories will be developed to clarify the lack of clinical reports on allergy or sensitization to biodegradable synthetic materials.     

Combining dissimilar metals in orthopaedic implants: revisited

The use of metals as implant materials has become common practice in the field of orthopaedics. A wide variety of conditions are treated with metallic implants, and designers have used an assortment of materials to meet the unique mechanical demands of each application. The majority of implants used today, whether pins, plates, screws, or total joints, are made of cobalt-chrome alloy, stainless steel, or titanium. Common metallurgic wisdom cautions against bonding dissimilar metals in a biologically active environment. Surgeons have therefore shied away from combining dissimilar metal implants because of the fear of inciting corrosion that could potentially compromise the implants and lead to aseptic loosening, implant failure, or adverse biological reaction in host tissue. As surgical reconstruction and arthroplasty options expand with the advent of newer implants and expanded operative techniques, the orthopaedic surgeon will increasingly be faced with weighing the risks and benefits of combining implants made of dissimilar metals in a patient. Here, the authors examine the origins of the concern over using mixed metals, discuss mechanisms of corrosion as they relate to surgical implants, and review both in vitro and in vivo studies concerning the most common combinations of dissimilar metals in order to guide the surgeon in choosing implants.     

Madreporische Hydroxylapatitgranulate zur Füllung ossärer Defekte

Two macrocrystalline madreporic granular hydroxyapatite implants of different size range (single crystal size within both implants 1-3 microns) were implanted for 7, 28, 84 and 168 days into the trabecular bone of the distal femur epiphysis of rabbits. Both materials were investigated histologically. For testing of granular materials a new animal model has been developed. The drill hole was closed by reimplantation of an autologeous chondrocortical tissue slice to prevent loss of particles into the knee-joint. Both of the granular materials tested developed increasing bone bonding from the 7th day on to outer surfaces and pore surfaces. The degradation of both of the materials affected the superficial implant layers in soft-tissue interfaces exclusively and was mainly due to passive processes, e.g. leaching, fragmentation of granules after crack-production, particulate degradation and subsequent phagocytosis of liberated implant particles by macrophages and foreign body giant cells. Zones of superficial implant degradation were bonded partially to bone a second time. A possible low-degree, active superficial degradation by foreign body giant cells is discussed. Osteoclasts of typical morphology as being observed on other hydroxyapatite implant surfaces were not demonstrated. This was related to the low degradation rate of the implants. Both of the granular materials tested are useful in filling bone defects. A guided tissue regeneration due to partial implant degradation and subsequent bone formation seems to be impossible since the degradation rate of the materials is too low.     

Mesh implants: An overview of crucial mesh parameters

Hernia repair is one of the most frequently performed surgical interventions that use mesh implants. This article evaluates crucial mesh parameters to facilitate selection of the most appropriate mesh implant, considering raw materials, mesh composition, structure parameters and mechanical parameters. A literature review was performed using the Pub Med database. The most important mesh parameters in the selection of a mesh implant are the raw material, structural parameters and mechanical parameters, which should match the physiological conditions. The structural parameters, especially the porosity, are the most important predictors of the biocompatibility performance of synthetic meshes. Meshes with large pores exhibit less inflammatory infiltrate, connective tissue and scar bridging, which allows increased soft tissue ingrowth. The raw material and combination of raw materials of the used mesh, including potential coatings and textile design, strongly impact the inflammatory reaction to the mesh. Synthetic meshes made from innovative polymers combined with surface coating have been demonstrated to exhibit advantageous behavior in specialized fields. Monofilament, largepore synthetic meshes exhibit advantages. The value of mesh classification based on mesh weight seems to be overestimated. Mechanical properties of meshes, such as anisotropy/isotropy, elasticity and tensile strength, are crucial parameters for predicting mesh performance after implantation.     

Serial magnetic resonance imaging evaluation of operative site after fixation of patellar tendon graft with bioabsorbable interference screws in anterior cruciate ligament reconstruction.

Magnetic resonance imaging (MRI) is accepted as the imaging procedure of choice for showing internal derangement of the knee. In contrast to metal implants, bioabsorbable interference screws do not produce an artifact and provide an opportunity to expand the evaluation of the postoperative anterior cruciate ligament (ACL) ligament repair. There is the potential to evaluate the implant, the graft, the adjacent tissue, and the surgically created bone tunnels. The purpose of this study was to evaluate with MRI the postoperative site of ACL patellar tendon autografts in which bioabsorbable screws were used for fixation. It was hypothesized that a time line of bone tissue changes resulting from this type of surgery could be developed based on the expanded evaluation of MRI. From January 1993 through October 1997, 270 patients underwent surgical repair of a disrupted native ACL. There were 173 men 97 women; the average age was 25.1 years, (range, 17 to 50 years). There were 155 right knees and 115 left knees. In addition to the conventional postoperative clinical assessment and plain film radiographs, opportunistic MRIs were obtained with the patient's permission. The examinations were performed at different postoperative intervals from the third postoperative day to 4 years postoperatively. A total 206 MRIs from various time intervals were available for study. The study protocol was designed to look for loss of integrity of the screws, adjacent fluid collection, tunnel widening, and tunnel healing or narrowing. The hypothesis was substantiated in this study. The use of MRI provided observations not available by other imaging methods. The absence of metal implants for fixation provided an opportunity to examine the adjacent tissue in detail and to form a time line of the tissue response in this type of surgery.     

Relative effects of wound healing and mechanical stimulus on early bone response to porous-coated implants

We hypothesized that early bone adaptation to well fixed porous-coated implants is influenced more by wound healing than by mechanical loading. To test this hypothesis, two groups of dogs with identical, hydraulically controlled porous-coated implants interference fit within distal femoral trabecular bone were used. One group had no load; the other had 35 N of load applied to the implants. At 5 weeks after surgery, the resulting adaptation of bone around the implants was quantified on a cellular basis by cytochemical analysis of type-I procollagen synthesis and on a structural basis using three-dimensional micro-computed tomography imaging. The percentage of trabecular surfaces covered by osteoblasts expressing type-I procollagen was significantly increased in bone surrounding the implant in both groups compared with contralateral control bone tissue. There was no difference between the groups with no load or 35 N of load. In addition, measures of trabecular bone structure did not differ significantly between the load and no-load groups. Taken together, these results suggest that wound healing plays a much greater role in the early response of bone to well fixed porous-coated implants than does mechanical stimulus.