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REVIEW |
Department of Oral and Maxillofacial Surgery, University Medical Center Groningen, University of Groningen, Hanzeplein 1, PO 30.001, 9700 RB Groningen, the Netherlands
* corresponding author, g.j.buijs{at}kchir.umcg.nl
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ABSTRACT |
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 TOP ABSTRACT (1) INTRODUCTION(2) GENERAL ASPECTS OF...(3) CONTROLLED CLINICAL STUDIES-...(4) GENERAL DISCUSSION(5) SUMMARY AND CONCLUSIONSREFERENCES |
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KEY WORDS: biodegradable • osteofixation • treatment • stability • morbidity • systematic review
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(1) INTRODUCTION |
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TOPABSTRACT(1) INTRODUCTION(2) GENERAL ASPECTS OF...(3) CONTROLLED CLINICAL STUDIES-...(4) GENERAL DISCUSSION(5) SUMMARY AND CONCLUSIONSREFERENCES |
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Titanium fixation systems can be used safely and effectively (Matthews et al., 2003; Stoelinga and Borstlap, 2003). The intrinsic mechanical properties ensure that the device dimensions are kept within acceptable limits. The handling characteristics of titanium systems are simple and efficient (Bos, 2005). However, titanium devices also have disadvantages. These systems interfere with radiotherapy (Rozema et al., 1990; Peltoniemi et al., 1997; Goldstein, 2001) and imaging techniques. Besides, titanium implants have been associated with complications such as growth restriction and brain damage (Yaremchuk and Posnick, 1995; Yerit et al., 2005), infection, and possible mutagenic effects (Penman and Ring, 1984).
A second intervention to remove the implants implies additional surgical discomfort, risks, and associated socioeconomic costs (Bostman, 1994; Juutilainen et al., 1997; Rokkanen et al., 2000; Yerit et al., 2005). A plate removal percentage of 11.1% in Le Fort I osteotomies, due to infection and plate exposure, has been reported (Schmidt et al., 1998). In a retrospective study of 279 patients with isolated mandibular fractures, a plate removal percentage of 11.5% has been reported (Tuovinen et al., 1994). In another study (Matthew and Frame, 1999), 23 oral and maxillofacial surgeons were interviewed regarding removal of mini-plates. The authors concluded that the plate removal percentage varies between 5% and 40%. Nevertheless, the exploration of the feasibility of biodegradable ‘disappearing’ materials has been suggested (Goldstein, 2001; Cheung et al., 2004).
Biodegradable osteofixation systems have the obvious possibility to degrade, thus preventing the need for a second intervention (Ylikontiola et al., 2004; Kallela et al., 2005). Another advantage of biodegradable devices is their radiolucency, implying good compatibility with radiotherapy and imaging techniques (Rozema et al., 1990; Disegi, 1992; Eppley et al., 1993). Besides, osteoporosis can be prevented due to the gradual transfer of functional forces to the healing bone during the disintegration process of biodegradable devices (Laftman et al., 1989; Jainandunsing et al., 2005).
Since the introduction of biodegradable devices in 1966 (Kulkarni et al., 1966), the development of their mechanical properties and degradation characteristics has been extensive (Turvey et al., 2002). Numerous in vitro, animal, and clinical studies have been published about positive (Eppley and Sadove, 1995; Eppley and Prevel, 1997; Edwards and Kiely, 1998; Kallela et al., 1999; Voutilainen et al., 2002; Ashammakhi et al., 2004; Eppley et al., 2004) as well as negative results (Bostman et al., 1990; Bostman, 1991; Friden and Rydholm, 1992; Bergsma et al., 1993). Despite the supposed advantages of biodegradable osteofixation devices, these systems did not replace the titanium systems and are currently applied in only limited numbers (Enislidis et al., 2005; Yerit et al., 2005). The mechanical properties are less favorable, and ultimate resorption has not been proven (Cordewener and Schmitz, 2000). Another significant factor in their limited use is surgeons’ resistance to modify the conventional treatment techniques with which they have the most experience (Eppley, 2000). The major drawback to the general use of biodegradable devices is the lack of clinical evidence.
(1.2) Objectives
The use of biodegradable osteofixation devices should be evidence-based if uncomplicated bone healing is to be achieved (Tiainen et al., 2004). Numerous studies describe and claim the advantages of biodegradable over titanium devices as a bone fixation method (Suuronen et al., 2000; Eppley et al., 2004). In the present study, the currently available literature regarding the clinical efficacy and safety of biodegradable osteofixation devices compared with titanium osteofixation devices in oral and maxillofacial surgery was systematically reviewed. The research question was phrased as follows: "Is there a difference in stability and morbidity regarding the fixation of bone segments with biodegradable or titanium fixation devices in orthognathic and trauma surgery?" The available literature regarding current relevant aspects of bone surgery will also be discussed.
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(2) GENERAL ASPECTS OF BONE SURGERY |
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(2.1) Mechanism of Bone Healing
Fractured bone or locally damaged bone causes disruption of many blood vessels. This disruption results in local hemorrhage, followed by the formation of a blood clot. Osteocytes at both sides of the fracture die due to deprivation of blood perfusion. Restoration of the fracture area starts with the clearance of the blot clot, dead cells, and bone matrix as a result of revascularization. Periosteum, endosteum, and surrounding tissues respond by cell proliferation. The tissue that arises between both fracture ends, and serves as a temporary bridging, is called ‘callus’. Its composition varies with site and circumstances (Frost, 2004). Cartilage is formed in parts of the callus that are not sufficiently saturated with blood. Subsequently, cartilage is transformed into bone by endochondral bone formation. If sufficient blood saturation occurs, a direct network of bars of plexiform bone is formed by primary bone formation. As the strength of the bony callus increases, it can be subjected to normal tension and compression forces (Junqueira et al., 1996).
Resorption and formation of bone are dynamic and continuously changing processes, with an equilibrium defined by internal factors (mainly hormones) and external factors (mainly mechanical forces). Inadequate immobilization during the healing process causes disruption of the revascularization process. This results in the formation of a fibrous callus, followed by an incomplete healing of the fracture. Too rigid fixation, in contrast, may also cause problems. Lack of normal functional stimuli in the final stages of bone healing will inhibit the formation of new bone, but the resorption of bone at the fracture site continues (Kennady et al., 1989a,b). This could result in local osteoporosis (Paavolainen et al., 1978; Frost, 1989a,b).
(2.2) Mechanical Aspects
Various muscles of the maxillofacial skeleton exert a wide variety of forces in different directions. This implies that it is difficult to estimate the required mechanical properties of a fixation system. Decisions regarding the utilization of plates and screws are rarely evidence-based (Edwards and David, 1996).
The primary mechanical strength and stiffness of biodegradable osteofixation devices are less favorable compared with their conventional titanium counterparts. This is inherent because of the use of biodegradable polymers. However, the question is whether their mechanical properties are sufficient for resisting local deforming forces (Cox et al., 2003).
The main objective in orthognathic and trauma surgery is fast, anatomical, and painless functional reunion of bone segments (Bozic et al., 2001). Revascularization plays an essential role in this process (Brons and Boering, 1970; Junqueira et al., 1996). Titanium plates and screws are intrinsically small, strong, and biocompatible (Goldstein, 2001). As a result, the main objectives regarding fixation management can be met. The rigidity of titanium fixation systems might also be disadvantageous. The system probably inhibits the transfer of functional forces to healing (or healed) bone, which may result in osteoporosis (see above) (Paavolainen et al., 1978; Laftman et al., 1989; Jainandunsing et al., 2005). By contrast, the strength and stiffness of biodegradable fixation systems decrease with time, because of the disintegration of the polymer chains, resulting in a progressive loading during subsequent stages of bone healing. To compensate for the less favorable primary mechanical strength and stiffness of biodegradable devices, manufacturers increase their dimensions. This may interfere with tension-less wound closing, making the wound area more prone to infection. Enlarged dimensions restrict easy application in small areas that are difficult to access (e.g., in pediatric surgery) (Bos, 2005). These factors imply that the field of application of biodegradable devices, in particular regarding bone fixation in the maxillofacial area, is restricted (Yerit et al., 2005), whereas titanium systems may be applied almost anywhere.
Despite the disadvantages of the enlarged dimensions of biodegradable systems, as mentioned above, several patient series have been published regarding the successful use of biodegradable fixation systems applied in different (e.g., heavy-load-bearing) situations (e.g., mandibular fractures and bilateral sagittal split osteotomies). The treatment of 1883 patients, in whom craniomaxillofacial deformities were fixed with the biodegradable LactoSorb® fixation system (W. Lorenz Surgical, Jacksonville, FL, USA), was evaluated in a recent study (Eppley et al., 2004). With respect to the rapidly growing cranial vault, the authors noted that fewer potential complications occurred with the biodegradable system compared with the titanium plates and screws. The BioSorbTM FX biodegradable fixation system has been found to be an appealing alternative for titanium fixation systems in orthognathic, trauma, and cancer surgery, corrective cranioplasty, and fixation of bone grafts (Suuronen et al., 2004).
Thus, when one considers the biomechanical aspects, the selection of plates and screws is not always that straightforward. The surgeon should consider (1) the local deforming forces, and (2) which system (biodegradable or titanium) could optimally resist the deforming forces (Rohner et al., 2002), and in what configuration (number of screws in both fracture ends).
(2.3) Biocompatibility and Resorption Aspects
Biocompatibility refers to how a material elicits a host response in a specific situation. Tissue responses to implanted material are numerous and complex. The term ‘biocompatibility’ also describes aspects of interactions between implanted material and the host (Vince et al., 1991; Hunt et al., 1993). The process of removal of a material by cellular activity and/or dissolution in a biological environment is called ‘resorption’ (Pinkhof et al., 1998). Degradation is the disintegration of material into smaller parts. Biocompatibility, resorption, and degradation are closely interrelated.
The biocompatibility of biodegradable internal fixation devices is strongly influenced by the degradation and resorption behavior of the polymers used (Vert et al., 1992; Bostman and Pihlajamaki, 2000a). These systems are made of different polymers [e.g., poly(L-lactide), poly(D-lactide), polyglycolide, polydioxanone, trimethylene carbonate]. These materials degrade and resorb in two phases (Pietrzak et al., 1997). During the first phase, water molecules hydrolyze the long polymer chains into shorter fragments. The molecular weight and the polymer strength decrease during this process. The second phase consists of the body’s physiological response, in which macrophages phagocytize and metabolize the short fragments, which subsequently enter the citric acid cycle (Brandt et al., 1984; Bostman et al., 1992; Vert et al., 1994). Water and carbon dioxide result and are subsequently excreted from the body, mainly through respiration. The mass of the biomaterial rapidly disappears during phase two (Kulkarni et al., 1966; Williams, 1992). In addition, enzymes are supposed to play a considerable role in the degradation (Williams and Mort, 1977; Li et al., 2002).
The degradation and resorption processes of biodegradable polymers frequently elicit adverse tissue responses. This represents an inherent biologic tissue response (Bostman and Pihlajamaki, 2000a), as occurs with every implanted material (Bostman et al., 1990). Regarding orthopedic surgery, the general incidence of adverse tissue responses from the use of fixation devices made of polyglycolide varies from 2.0 to 46.7% (Bostman and Pihlajamaki, 2000a). The incidence of adverse tissue responses is generally lower for plates and screws made of polylactide (Bostman and Pihlajamaki, 2000a). The time between implantation and appearance of adverse tissue responses varies, being from 10-12 wks (Bostman et al., 1990; Partio et al., 1992; Rokkanen et al., 2000) to 4–5 yrs (Bergsma et al., 1993, 1995; Bostman and Pihlajamaki, 1998) for polyglycolide and polylactide, respectively.
The clinical characteristics of the adverse tissue responses vary, from a local swelling without signs of inflammation (Bergsma et al., 1993) to a suddenly emerging painful, erythematous, fluctuating papule which reveals a sinus discharge of liquid remnants of disintegrated implant materials (Bostman and Pihlajamaki, 2000a). Radiographs obtained at the time of manifestation show osteolytic changes around the implanted material in 50% of the patients (Bostman, 1991; Fraser and Cole, 1992). The histopathologic picture reveals an abundant polymeric debris, surrounded by mononuclear phagocytes and multinucleated foreign-body giant cells (Bostman et al., 1990; Bostman, 1991, 1992; Hovis and Bucholz, 1997).
The possible risk factors for developing adverse tissue responses seem to be associated with the extent of vascularization, which inherently depends on the site of implantation. Moreover, the implant design appears to affect the response rate. Cylindrical pins and rods show a lower incidence of adverse tissue responses than do screws. Foreign-body response rates seem to be independent of patients’ age and gender, as well as of the implanted polymer volume.
The long-term ultimate biocompatibility and resorption of biodegradable plates and screws have frequently been investigated, yet remain to be established (Ignatius and Claes, 1996; Bostman and Pihlajamaki, 2000a). Researchers have reported various in vivo results. A recent histologic study (Heidemann et al., 2003) reported complete resorption of Resorb® X (KLS Martin, LP, Tuttlingen, Germany) and LactoSorb® screws after 12 and 14 mos, respectively, as determined by fluorescence microscopy. However, bone remodeling was not completed after 26 mos. The degradation process of biodegradable implants has also been investigated by Magnetic Resonance Imaging (Pihlajamaki et al., 1997). The authors concluded that no complete resorption had occurred after 34 mos.
Based on these findings, large-scale, long-term controlled clinical trials are needed to verify the ultimate biocompatibility and resorption characteristics of biodegradable implants and to establish evidence-based treatment methods.
(2.4) Characteristics of ‘Ideal’ Osteofixation Devices
Based on the issues discussed in the previous sections, an ideal osteofixation device should: (1) be fabricated and designed with appropriate initial strength to meet the biomechanical demands, (2) not cause tissue responses necessitating removal of the device, (3) be easy to use and handle, (4) be cost-effective, and (5) be compatible with radiotherapy and imaging techniques (Edwards and David, 1996; Pietrzak et al., 1997). In addition, biodegradable osteofixation devices should: (6) degrade in a predictable fashion and allow for safe progressive loading during each stage of bone healing, and (7) disappear completely.
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(3) CONTROLLED CLINICAL STUDIES—A SYSTEMATIC REVIEW |
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. We contacted several experts in the field of biodegradable osteofixation devices to ensure that eligible studies were not overlooked. Moreover, we screened leading oral and maxillofacial journals for missing articles. To complete the search, we checked reference lists in the obtained literature for additional relevant articles. No language and time restrictions were included in the search strategy.
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(3.1.2) Study Selection
The relevance of studies was evaluated by a first selection based on title and abstract. Since the research question focused on the efficacy and safety of biodegradable osteofixation devices in comparison with titanium devices, only controlled clinical trials (CCT) were considered for inclusion in the systematic analysis.
The review focused on studies concerning the treatment of fractures and the performance of osteotomies of the maxillofacial skeleton (i.e., Le Fort I, Le Fort II, and Le Fort III fractures and osteotomies, cranial fractures, malar fractures, mandibular fractures, and sagittal split osteotomies of the mandible). Studies involving children were also considered for inclusion. Disagreement about whether a study should be included was resolved by a consensus discussion. We retrieved full-text documents of all relevant articles. The study selection procedure is outlined in the Fig.
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Studies meeting the abovementioned criteria were subjected to further methodological appraisal.
(3.1.4) Quality Assessment of Studies
We performed a quality assessment of the remaining studies to control the influence of bias in a systematic analysis, to gain insight into potential comparisons, and to guide interpretation of findings (Higgins and Green, 2005). A registered methodologist and oral and maxillofacial surgeon (BS), as well as a PhD resident (GJB), assessed the methodological quality with the ‘quality of study tool’ developed by Sindhu et al.(1997). The ‘quality of study tool’ consists of 53 items in 15 dimensions (Table 2). Each dimension has a specific weighting (W). Using the 15 dimensions (range, 0–100), the two observers independently generated a score for the included articles. Agreement regarding the weighting of the individual sub-dimensions and the required minimum ‘methodological’ values for each dimension was reached in a consensus meeting. Based on these minimum values, summation yielded a threshold value, which, in this study, was 54.
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(3.1.5) Statistical Analysis
The pre-consensus degree of agreement between the two observers regarding eligible studies was expressed as a percentage of agreement of unweighted Cohen’s kappa. Where applicable, we used Cochrane Review writing software (RevMan computer program, version 4.2 for Windows, The Cochrane Collaboration: Oxford, England, 2002) to calculate the overall effect sizes by means of the random-effects model.
(3.2) Results
The MEDLINE, EMBASE, and CENTRAL search identified 124, 29, and 87 publications, respectively. Systematic assessment of these 240 articles according to the specified ‘eligibility’ criteria revealed 7 possible eligible publications. Inclusion of a ‘titanium control group’ appeared to be the limiting criterion in this selection; however, it was essential for answering the research question. Inclusion of a control group and, preferably, random assignment are major aspects for the control of unknown influences and possible confounders (Bhanot et al., 2002; Higgins and Green, 2005). Checking references in relevant articles and contacting experts did not yield additional articles. Methodological assessment of the 7 eligible publications revealed 5 methodologically ‘acceptable’ articles. Two articles were excluded because of inadequate reporting of the methods and results (Bohm et al., 1998) and the absence of a prospective control group (Landes and Ballon, 2006). Inter-assessor agreement on the methodological quality of each study was 95% (unweighted kappa, 0.90; 95% CI, 0.85 to 0.96). Disagreements were generally caused by slight differences in interpretation and were easily resolved in a consensus meeting.
Three studies used randomization to allocate patients to the treatment groups (Cheung et al., 2004; Norholt et al., 2004; Ueki et al., 2005). Two studies allocated patients consecutively (Ferretti and Reyneke, 2002; Ueki et al., 2006). LactoSorb® plates and screws were used to fix bone segments in two studies (Ferretti and Reyneke, 2002; Norholt et al., 2004). The LactoSorb® fixation system has a copolymer composition of 82% L-lactide and 18% glycolide. Ferreti and Reyneke (2002) studied mandibular splits fixed with 3 bi-cortical screws, whereas Norholt et al.(2004) investigated the stability and relapse of Le Fort I osteotomies. One other methodologically ‘acceptable’ study (Cheung et al., 2004) investigated the fixation of different osteotomies using BioSorbTM FX plates and screws (Linvatec Biomaterials Ltd., Tampere, Finland). The BioSorbTM FX fixation system is made of self-reinforced (70% L-lactide, 30% DL-lactide) polylactic acid. The most recent studies (Ueki et al., 2005, 2006) used 100% poly-L-lactic acid plates and screws (Fixorb®-MX, Takiron Co., Osaka, Japan). Ueki et al.(2005) investigated the changes in condylar long axis and skeletal stability after bilateral sagittal split ramus osteotomy. The latest study discussed the maxillary stability of Le Fort I osteotomy in combination with sagittal split ramus osteotomy (SSRO) and intra-oral vertical ramus osteotomy (IVRO) (Ueki et al., 2006).
Because of the different effect-sizes used in the methodologically ‘acceptable’ studies, it was impossible for us to perform a meta-analysis. Therefore, the major effects regarding the stability and morbidity of fracture fixation are qualitatively described in the subsequent sections.
(3.2.1) Stability
Stability of fixed bone segments is an important outcome measure, since the aim of fixation systems is to establish a functional, anatomical, and pain-free reunion of bone segments. In the five included articles, the stability of the osteotomized segments was assessed by different methods.
In four of the five included studies, cephalometric analysis was used for the accurate assessment of skeletal stability (Ferretti and Reyneke, 2002; Norholt et al., 2004; Ueki et al., 2005, 2006). The 2006 study suggested that significant differences existed between the titanium and biodegradable groups for the vertical component of A point after Le Fort I osteotomy and SSRO, and the vertical component of the posterior nasal spine (PNS) after Le Fort I osteotomy in both combinations with SSRO and IVRO (Ueki et al., 2006). The authors concluded, however, that sufficient skeletal and occlusal stability was obtained in both groups without complications (Ueki et al., 2006). Regarding bilateral sagittal osteotomies (Ueki et al., 2005), the outcome measures angles sella nasion A point (SNA), sella nasion B point (SNB), and A point nasion B point (ANB) did not significantly differ for the titanium and poly L-lactide acid (PLLA) groups. The inter-incisor angle, occlusal plane angle, mandibular length, overbite, overjet, and convexity were also similar in both groups. The location of the pogonion also did not show a significant difference. In the third study (Norholt et al., 2004), Le Fort I osteotomies fixed with biodegradable plates and screws revealed a significant difference in vertical dimension of the upper jaw (mean difference, 0.6 mm) 6 wks post-operatively. The osteotomies fixed with titanium plates and screws did not present a significant difference. The authors concluded that the statistically significant difference in the vertical dimension in the biodegradable group (LactoSorb®) was not clinically relevant (Norholt et al., 2004). Ferretti and Reyneke (2002) evaluated the relapse (skeletal stability) of bilateral sagittal osteotomies. The mean transposition of the mandible fixed with 3 bi-cortical screws was 4.7 (SD = 1.3) and 5.5 (SD = 1.7) mM, respectively, for the titanium and biodegradable groups. The mean relapse was 0.25 (SD = 1.25) and 0.83 (SD = 1.25) mM, respectively (not statistically significant).
The clinical mobility of the bone segments was evaluated in four included studies (Cheung et al., 2004; Norholt et al., 2004; Ueki et al., 2005, 2006). The most recent studies (Ueki et al., 2005, 2006) reported that no bone instability or non-union had occurred. The third study (Norholt et al., 2004) reported a slight mobility during the first 6 wks (6 in the biodegradable group and 3 in the titanium group), whereas one case presented mobility in the biodegradable group 1 yr post-operatively. The fourth study (Cheung et al., 2004) reported that the clinical stability improved gradually over time. No difference in this respect was revealed between titanium and biodegradable fixation. In all patients, the mobility was very mild and was present in the maxilla. The mobile maxillae became stable and firm in the sixth week, and no further mobility could be detected during the follow-up period.
(3.2.2) Morbidity
The morbidity of osteofixation devices was evaluated in all of the included studies (Ferretti and Reyneke, 2002; Cheung et al., 2004; Norholt et al., 2004; Ueki et al., 2005, 2006).
Ueki et al.(2005) evaluated different aspects regarding morbidity: pain on chewing (using a Visual Analogue Scale [VAS]), maximum mouth-opening range (measuring the distance between the edges of the upper and lower incisors), and temporomandibular disorder (TMD) symptoms based mainly on sound (click and crepitus) on movement. Pain on post-operative chewing revealed lower VAS scores compared with pre-operative chewing in both groups. The between-group VAS scores were nearly similar. Maximum mouth-opening range did not reveal a significant difference. The number of symptomatic joints in the titanium group was significantly less compared with that in the PLLA group. General clinical aspects (infection, wound dehiscence, plate exposure, and palpability of plates and screws) were objectively assessed in four included studies (Cheung et al., 2004; Norholt et al., 2004; Ueki et al., 2005, 2006). The inflammatory responses gradually decreased with time. The two most recent studies (Ueki et al., 2005, 2006) reported that no patients experienced complications such as wound infection or dehiscence. The third study (Norholt et al., 2004) reported wound dehiscence in one patient in the biodegradable group, whereas the fourth study (Cheung et al., 2004) revealed wound dehiscence in three patients in the titanium group (10%) and in two patients in the biodegradable group (6.7%). No complications occurred as a result of dehiscence. The palpability of biodegradable plates and screws decreased with time in both studies, while the palpability of titanium plates and screws increased. In the study by Cheung et al.(2004), plate exposure affected 1.02% and 1.21% of the patients in the titanium and biodegradable groups, respectively, whereas Norholt et al.(2004) discussed one patient in the biodegradable group with plate exposure (4.2%). One included study (Cheung et al., 2004) reported the removal of 3 titanium (1.53%) and 6 (3.36%) biodegradable plates (as a percentage of all plates and screws used). Ferreti and Reyneke (2002) briefly reported on the clinical appearance of the surgical ‘sites’, which appeared to be abnormal with respect to the evaluation criteria (swelling, discharge, pain, or discoloration of the mucosa and skin) during the post-operative 12 mos. The general characteristics, results, and conclusions of the included studies are summarized in Table 3.
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(4) GENERAL DISCUSSION |
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Besides the initial mechanical characteristics of osteofixation systems, the torsion strengths and stiffnesses of the screws are important. The screws fix the osteofixation plate against the bone segments and prevent sliding of the bone segments and the fixation system relative to each other. This ensures adequate stabilization of the bone segments. Screws also generate inter-fragmentary compression to stabilize mandibular splits, which will enhance fracture healing. The torsion strengths and stiffnesses of the biodegradable screws are less favorable (Shetty et al., 1997) compared with those of titanium screws, and this has been reported by several authors as a disadvantage (Shetty et al., 1997; Bahr et al., 1999). Moreover, biodegradable polymeric screws relax when a force is continuously applied (Shetty et al., 1997). These aspects may result in decreased fracture stability and possible compromised fracture healing.
(4.2) Biocompatibility and Resorption Aspects
Long-term ultimate biocompatibility, as is the goal of any implanted material, is difficult to establish. Despite considerable clinical experience of fracture fixation with biodegradable materials, long-term clinical studies are scarce. Moreover, studies reporting the long-term complications (Bostman et al., 1990; Bergsma et al., 1993; Bostman and Pihlajamaki, 1998) probably represent one end of a continuous spectrum of biological responses. The majority of the cases pass subclinically and remain unnoticed, despite the elicitation of a (small) biological host response, as is the case with every implanted material (Bostman et al., 1990).
The degradation and resorption characteristics, as well as the possible development of adverse tissue responses, depend largely on the nature of the implanted materials. Polylactide is a major component of biodegradable fixation devices, and the time for a considerable host response to be elicited is 4 to 5 yrs (Bergsma et al., 1993, 1995; Bostman and Pihlajamaki, 1998; Suuronen et al., 1998). Therefore, studies reporting the biocompatibility and degradation characteristics regarding this material should last for at least 5 yrs (Bostman and Pihlajamaki, 2000b). However, few laboratory animals live long enough, and, consequently, long-term biocompatibility experiments are difficult to design.
The development of adverse tissue responses seems to originate from several different physiologic and chemical processes. Crystalline remnants and a decrease of pH (Taylor et al., 1994) during degradation are probably responsible for the adverse effects of biodegradable polymers, although the local tissue tolerance and the local clearing capacity seem to be important aspects as well (Bostman et al., 1990; Pihlajamaki et al., 1992; Bergsma et al., 1995; Matsusue et al., 1995). The quantity of crystalline remnants and the rate of pH decrease are partly determined by the molecular structure of the biomaterial (Vert et al., 1992). Amorphous polymers degrade faster than crystalline polymers, resulting in a rapid decrease in pH. Crystalline polymers may remain in situ for decades (Pietrzak et al., 1997). A high blood flow rate is an essential prerequisite for successful implantation of biodegradable fixation materials, since adequate blood flow secures sufficient removal of degradation products, thereby preventing a decrease in pH (Bostman and Pihlajamaki, 2000b). Poly DL-Lactide acid (PDLLA) implants enriched with calcium phosphates to prevent a local decrease in pH have been investigated in rats (Heidemann et al., 2001). The control group received pure PDLLA implants. The PDLLA implants enriched with calcium phosphates showed a more severe tissue response after 72 wks. The authors concluded that the ‘enriched’ implants are not suitable for clinical use.
(4.3) Clinical Aspects
The major objective of this systematic review was to evaluate the clinical efficacy and safety of biodegradable osteofixation devices in comparison with titanium osteofixation devices used in oral and maxillofacial surgery. Unfortunately, we cannot draw any firm conclusions regarding the fixation of traumatically fractured bone segments, owing to the lack of controlled clinical trials. Studies with two randomized treatment groups are difficult to design and not (yet) available. Regarding the fixation of bone segments in orthognathic surgery, only a few (randomized) controlled clinical studies (Ferretti and Reyneke, 2002; Cheung et al., 2004; Norholt et al., 2004; Ueki et al., 2005, 2006) are available. There does not appear to be a significant difference in outcome between titanium and biodegradable fixation systems. Definite conclusions regarding the long-term performance of biodegradable fixation devices used in maxillofacial surgery cannot be drawn.
The methodologically ‘acceptable’ studies contain much heterogeneity. The studies individually defined the outcome measures for stability and morbidity. Moreover, the treatment modalities performed in these studies were different (Le Fort I, sagittal split osteotomies, and various osteotomies). The biodegradable fixation systems used—LactoSorb® (Ferretti and Reyneke, 2002; Norholt et al., 2004) and Fixorb®-MX (Ueki et al., 2005, 2006)—were similar in only two studies. Because of heterogeneity, the pooling of outcome measures was not meaningful.
A primary way to establish whether a fixation system has functioned successfully is to assess the extent of clinical mobility. However, objective mobility measurements in the maxillofacial skeleton are difficult to perform. One study reported stability according to a nominal scale [none, slight mobility, and gross mobility (Norholt et al., 2004)], while another study reported mobility according to a binary scale [immobility vs. mobility (Cheung et al., 2004)]. Two studies (Ueki et al., 2005, 2006) did not report the method by which clinical stability was evaluated, but reported only the results. One methodologically ‘acceptable’ study did not even report the extent of mobility (Ferretti and Reyneke, 2002). In our opinion, it is essential that researchers report the extent of mobility when investigating the clinical efficacy and safety of biodegradable osteofixation systems. Therefore, we advise the use of a binary scale. The aim of an osteofixation device is to achieve functional, pain-free re-union within a reasonable period of time (6 wks) (Juniper and Awty, 1973). Compromised healing or slight mobility after 6 wks should be defined as non-union. In the most recent studies (Ueki et al., 2005, 2006), post-operative inter-maxillary fixation (IMF) was applied for 2 wks to prevent adverse alterations of the post-operative occlusion. The authors did not know whether the PLLA plates were strong enough to stabilize the bone segments. Today, the use of IMF is not the state of the art and, thus, in our opinion, IMF should not be used to compare the skeletal stability of SSROs or IVROs fixed with titanium or PLLA plates.
One of the major drawbacks of the reviewed literature is the lack of sufficient follow-up. Four of the included studies (Ferretti and Reyneke, 2002; Norholt et al., 2004; Ueki et al., 2005, 2006) followed their patients for only 1 yr postoperatively. Another included recent study (Cheung et al., 2004) followed a few patients for 2 yrs (six from the titanium group and seven from the biodegradable group), and 24 patients in both groups were evaluated for 1 yr. In our opinion, the follow-up periods are too short to lead to definite conclusions as to whether these biodegradable implants could serve as a safe and reliable fixation method over the long term. Many authors (Eppley et al., 2004; Suuronen et al., 2004; Enislidis et al., 2005) have reported patient series with longer follow-up periods. As mentioned earlier, since these patient series lack a control group, an adequate comparison with titanium fixation devices could not be made in these studies. Future clinical trials should, from a biocompatibility and resorption point of view, evaluate patients for at least 5 yrs [see section (4.2), above].
The onset of infections seems to differ for the fixation of fractures with titanium or biodegradable devices. One included study (Cheung et al., 2004) reported that the infections in the biodegradable group were diagnosed after 6 wks, 3 mos, and 6 mos, while those in the titanium group were diagnosed after 2 wks, 6 wks, and 3 mos. Another included study (Norholt et al., 2004) reported that 1 infection in the titanium group was diagnosed after 1 wk, whereas 2 infections in the biodegradable group were diagnosed after 6 mos. These clinical findings suggest that the onset of infections tended to occur later in the biodegradable groups. The authors could not explain this tendency, although one (Norholt et al., 2004) suggested that it could be caused by the ongoing degradation of the plates and screws.
The known causes of infection are loosened screws and wound dehiscence (Cheung et al., 2004). In one of the included trials, the authors report the infection percentages in terms of individual plates (1.53% in the titanium group and 1.82% in the biodegradable group), and in terms of individual patients (10% in each group) (Cheung et al., 2004). In the discussion, the authors advocate the plate and screw as the more reasonable unit for calculation, because an infection will occur if any single component fails. However, in our opinion, it is more reasonable to use the individual ‘patient infection percentages’ to calculate the percentage of infection. After all, infection percentages in terms of individual patients will provide more insight into the extent of actual re-operating procedures. Moreover, cost-effectiveness analyses are more meaningful based on infection percentages in terms of individual patients. However, cost-effectiveness analyses regarding the use of biodegradable bone fixation devices were not reported in any of the included trials (Ferretti and Reyneke, 2002; Cheung et al., 2004; Norholt et al., 2004; Ueki et al., 2005, 2006).
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(5) SUMMARY AND CONCLUSIONS |
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TOPABSTRACT(1) INTRODUCTION(2) GENERAL ASPECTS OF...(3) CONTROLLED CLINICAL STUDIES-...(4) GENERAL DISCUSSION(5) SUMMARY AND CONCLUSIONSREFERENCES |
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Since this systematic review has some implications for future research, there is an urgent need for sufficiently powered, high-quality, and appropriately reported randomized controlled trials with respect to biodegradable osteofixation devices vs. non-degradable osteofixation devices for well-defined maxillofacial fractures and osteotomies. Future studies should include a cost-effectiveness analysis in which hospital admission costs, surgical costs (material), and the costs associated with sick leave of the patients should be analyzed.
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ACKNOWLEDGMENTS |
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Received November 1, 2005; Accepted May 9, 2006
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REFERENCES |
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