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BIOLOGICAL PLATING A New Concept To Foster Bone Healing R Original Instruments and Implants of the Association for the Study of Internal Fixation—AO ASIF Early Temporary Porosis Beneath Bone Plates Is there a correlation between bone loss beneath the plate and “stress protection”? Observed patterns of bone loss beneath a plate do not correspond to the stress patterns of the corresponding bone segment. Situation under a plate: Under weightbearing conditions, stresses in the bone (represented by black lines) diminish towards the plate (shielding effect of a plate [1]). Situation under a plate: The area of observed porosis is sharply defined and unrelated to the pattern of stress shielding (as represented at left). Bone loss in the vicinity of a plate has always been explained on the basis of Wolff’s law as a reaction of living bone to mechanical unloading of the plated bone segment (stress protection). Although experiments have demonstrated that flexible plastic plates do not improve the situation, the rigidity of a plate is generally still considered the prime factor inducing porosis. AO ASIF observation The disturbance of the blood supply and porosis are strongly correlated [2]. Disturbance of blood supply demonstrated with disulphine blue as an indicator of blood perfusion. A comparative histological section shows remodelling located in the area of previous avascularity. Osteoporosis beneath a plate is the direct result of damage to the blood supply and not the result of mechanical unloading of the bone [2]. Limited Contact Concept Reduced vascular damage Area of disturbed circulation in bone cross section mm2 Reduction of surface contact between plate and bone results in a reduced disturbance of blood supply [3]. 20 10 Full Contact Limited Contact Histological investigation brings a new understanding of vascular damage in relation to different contact surfaces and enlightens the etiology of temporary porosis of bone. Improved bone consolidation with reduced porosis Favorable blood perfusion conditions promote better bone quality. Extensive area of bone loss as observed beneath a full-contact plate. Reduced area of bone loss (porosis) as observed beneath a limitedcontact plate. Besides supplying the energy for the body’s regenerative mechanisms, an intact blood supply is essential as a defense against infection and as a medium to reduce early porosis, possible necrosis, and/or sequestration. Minimal Contact Achieved Without Impaired Implant Strength Uniform bending and torsional stiffness Finite Element Analysis grid of the optimized plate viewed from above To achieve a uniform stiffness with limited bone contact along the plate length, CAD/CAE tools and mathematical analysis are used for a reliable functional design [4]. The Finite Element Analysis of a plate under bending load conditions (see picture below) shows bending stresses evenly distributed along the plate. Improved Mechanical Performance Plate protected from localized high stresses Contouring the LC-DCP™: The plate holes preserve their shape. Conventional plate: Kinks at holes impair the mechanical performance. The LC-DCP deforms over a long distance, resulting in an even stress distribution (exaggerated situation). Exact contouring is essential for good force transmission and successful fracture treatment. When loaded, the uniform stiffness of the LC-DCP results in an even distribution of stresses over a long distance along the plate, protecting the plate holes from localized high stresses. Therefore, the LC-DCP is less prone to fatigue, especially where plates span a wide bony defect or in comminuted fractures. In contrast, a conventional plate deforms mainly at the hole, and stresses concentrate at the smallest cross section. Strain Improved contouring of the plate The uniform stiffness of the LC-DCP™ enables continuous curvature, allows a good fit of the screw head in the plate hole, and preserves the mechanical features of the plate. The Material of Biological Choice: AO ASIF Pure Titanium High corrosion resistance of conventional implant materials Fretting effects on conventional implant materials tissue passivelayer metal Standard implant materials are protected by a thin passive layer. Typical congruent zones where fretting occurs between screw head and plate hole in stainless steel implants [5]. As long as the protective oxide layer on the implant surface is intact, the material remains passive. Relative motion between metallic implants leads to fretting effects, and alloying components can be released into the tissues. The conventional implant materials used today are highly corrosion-resistant and generally well tolerated. They are protected by a spontaneously forming submicroscopic thin oxide layer on their surface which prevents the metal from further oxidation and corrosion. This protective oxide layer is known as passive film. In areas where implants are in contact, the protective oxide layer may be destroyed by relative motion; even micromotion is enough. In this situation, called fretting, abrasive action takes place, and fretting corrosion can occur because the passive film is disrupted. Alloying elements less tolerated by the tissues than the alloy itself can be released during the abrasive process of mechanical wear. Usual metals used in alloys, like nickel, chromium, cobalt, aluminum, and vanadium, can thus find their way into the body tissues. The ingestion and transport of metallic degradation products in the body tissues is complex, and different mechanisms might operate simultaneously depending on the form in which the metal is released [5]. The metallic degradation products may be in the form of wear particles, oxidized compounds, or ionized species. Alloying elements with biological activity toxic and corroding increasing acceptance Pure titanium is biologically inert excellent acceptance Corrosion resistance Titanium Co-Alloys St Steel Pt Ta Nb Ag Au Ni V Cu Co Mo Al Fe Increasing biocompatibility Corrosion resistance vs. biocompatibility of some pure metals and alloys [6]. Pure titanium displays excellent biocompatibility. The biocompatibility of metallic materials is closely related to their corrosion resistance and to the transportability of the corrosion products. The extent to which the wear products from alloys further corrode in tissues is uncertain, but biological activity of alloying components does occur, and occasional reactions cannot be ruled out. Nickel is a known allergen, and contact alone can provoke an allergic reaction (i.e., no particular corrosive process has to take place). Speculations on the systemic effects of nickel and other metals abound. Various published and unpublished data from cell and organ cultures show that nickel and vanadium have cytotoxic effects at considerably lower concentrations than other metals used in implants [7,8] (see diagram above). Tissue impregnated with pure titanium wear particles does not show adverse reactions. Pure titanium and its wear products remain passive and do not affect the tissue. In contrast to the above-mentioned metals, pure titanium displays very little biological activity, and its unmatched tissue tolerance has been scientifically and clinically demonstrated. In cases of unstable internal fixation with tissues stained by abrasion particles, no accompanying corrosion has been observed [5]. In tissue fluids, the pure titanium mechanical wear products are practically insoluble and are chemically non-transportable. Besides this, the body seems to be saturated with titanium, and this suggests that no additional soluble titanium can become active [6]. These properties and the extraordinary corrosion resistance of pure titanium help to explain why adverse tissue reactions are not observed (inertness). The AO chose not to add any element to pure titanium which could cause an adverse biological response. AO ASIF Pure Titanium Broad Clinical Experience Better Vascularized Tissues Since 1965, the AO ASIF has gained clinical experience with nearly 5,000 cases treated with AO ASIF Pure Titanium implants. The outstanding tissue compatibility of AO ASIF Pure Titanium has been repeatedly confirmed and is well established; no documented case of metal sensitivity (allergy) or adverse tissue reaction has ever been related to pure titanium implants.* Today, pure titanium is already the material of choice for implants to be used in patients suffering from metal allergy. Furthermore, if it is advantageous to avoid explantation surgery (e.g., distal humerus), implants made of inert materials, such as pure titanium, are ideal. Well vascularized tissue around AO ASIF Pure Titanium implants Clinical experience shows that the tissues which envelop pure titanium implants are better vascularized and show a reduced tendency towards capsule formation [9,10,11]. There is better tissue adherence to the pure titanium plate than when other standard implant materials are used. These biologically favorable conditions help to reduce the spread of bacteria and increase resistance to infection. * In cases of prosthetic-related wear, adverse reactions to titanium alloy (Ti-6AI-4V) have indeed been documented. Titanium alloys have different properties from pure titanium. It may be mentioned that titanium alloys are often addressed as titanium. This inaccurate expression causes confusion and must be avoided. Highly Advanced Manufacturing Methods Pure titanium is a relatively soft metal. In order to maintain reduction and to withstand anatomical loads, the implant requires a minimum adequate strength which depends on the type of implant and its function. The strength of pure titanium can easily be increased by adding alloying elements like vanadium, aluminum, etc., but this may have negative effects. Ductility (workability) is reduced, and the addition of toxic elements compromises biocompatibility. To endow pure titanium with a strength that is similar to that of medium-hard stainless steel while preserving its ductility, the AO ASIF, its collaborating laboratories, and its exclusive producers have developed special manufacturing and thermic treatment methods. With these methods, the required strength and ductility are achieved without the addition of alloying elements of proven toxicity such as vanadium. Thus, the AO can take uncompromised clinical advantage of the outstanding biocompatibility of pure titanium. Distinct Mechanical Properties Ultimate tensile strength N/mm2 Commercially available Pure Titanium AO ASIF Pure Titanium 900 800 700 600 500 400 300 200 100 0 The diagram shows the range of tensile strength characteristic of commercially available pure titanium and AO ASIF Pure Titanium. Depending on the type of implants and their function, the appropriate strength level is chosen within this range. Advantages in Surgical Technique Compression can be achieved in either longitudinal direction A lag screw can be inserted at greater angulation 40° 40° LC-DCP hole Longitudinal cross section of LC-DCP hole and feasible angulation of a lag screw The basic spherical gliding principle of the screw in the DCP ™ hole is now implemented at both ends of the plate hole in the LC-DCP plate. This enables compression in either direction along its longitudinal axis. The redesigned geometry of the hole also adds more flexibility to the plating technique and eases handling of complex situations. With conventional DCP plates, the maximum screw angulation in the longitudinal axis is about 20 degrees. Greater angulation of the lag screw could not be achieved without impeding the gliding of the lag screw in the gliding hole. The LC-DCP hole offers the possibility of safe insertion of the lag screw up to 40 degrees through the plate in both directions. This gives the surgeon more possibilities to achieve interfragmental compression through the plate [12,13]. The plate hole can be used for interfragmental compression of multifragmentary fractures. Due to the regular hole spacing along the axis of the plate—no midsection without holes—the surgeon has more options to reposition a plate of different length using the same predrilled holes. To achieve additional interfragmental compression, correct angulation of the lag screw is required. To secure a loose fragment through the plate, an angle greater than 24 degrees (standard plating systems) is occasionally required. Implant removal is facilitated Standard plate cross section LC-DCP plate trapezoidal cross section reduces the risk of generating stress risers. The normally thin bone lamella lining the plate may impede removal of conventional plates. The frail lamella is susceptible to damage upon removal and will act as a stress riser [14] which could eventually result in refracture of the bone. Because of the trapezoidal cross section of the LC-DCP, the bone lamella attached to the plate is flatter and less fragile, allowing easier plate detachment and reducing the risk of refracture. References [1] Cordey, J. and S.M. Perren. “Stress Protection in Femora Plated by Carbon Fiber and Metallic Plates: Mathematical Analysis and Experimental Verification.” Biomaterials and Biomechanics 1983. Eds. P. Ducheyne, G. Van der Perre, and A.E. Aubert. Amsterdam: Elsevier Science Publisher B.V., 1984. 189-194. Excerpt—“Bone refracture after plate removal has been attributed to the structural adaptation of bone (loss of bone) to reduced stress (stress protection). The analysis of the stress pattern in plated bones seems to be a prerequisite for the assertion that bone loss is stress-related. The strain distribution at the surface of plated human femoral shaft has been analyzed using the composite beam theory and verified experimentally using strain gauges. Plates made of carbon, titanium and stainless steel were investigated. The difference between the reduction of stress obtained using the less stiff plate materials and that obtained by using thinner stainless steel plates is astonishingly small. The reduction of the rigidity of the plate does not result in a proportional improvement of the strain in bone under the combined axial and flexural load.” [2] Gautier, E., J. Cordey, R. Mathys, B.A. Rahn, and S.M. Perren. “Porosity and Remodelling of Plated Bone After Internal Fixation: Result of Stress Shielding or Vascular Damage?” Biomaterials and Biomechanics 1983. Eds. P. Ducheyne, G. Van der Perre, and A.E. Aubert. Amsterdam: Elsevier Science Publisher, 1984. 196-200. Excerpt—“The bone loss observed in the five months following plating of cortical bone is mainly due to porosis. The porosis accompanies the internal remodelling of the diaphyseal bone. It is not clear whether the porosis is a reaction due to the mechanical unloading of the bone, or whether, as seems more probable, it is a temporary stage in the remodelling of necrotic bone. In an experimental study using sheep, plates of different bending stiffness and different lower surface structure were fixed onto the medial aspect of intact tibiae. Changes in blood supply and bone remodelling were assessed at four, ten and twenty weeks. There is no difference in the amount of bone remodelling between groups with plates made of steel and groups with similar plates made of polyacetal with a thin metal core. It seems noteworthy that the extent of the remodelling differs towards the proximal and the distal ends of the plated bone. A correlation was found between plate contact to bone and the extent of the vascular damage at four weeks on the one hand, and between plate contact and the extent of the bone remodelling area at twenty weeks on the other hand. The results of the experiment suggest that porosis of the bone is related to internal remodelling, which in turn is related to vascular damage due to plate contact.” [3] Perren, S.M., J. Cordey, B.A. Rahn, E. Gautier, and E. Schneider. “Early Temporary Porosis of Bone Induced by Internal Fixation Implants: A Reaction to Necrosis, Not to Stress Protection?” Clinical Orthopaedics and Related Research 232 (1988): 139-151. Excerpt—“Stabilization of the fracture using implants requires contact surfaces between implant and bone. Such contact has been observed to induce bone porosis first seen at one month after surgery. Bone loss in the vicinity of implants has hitherto been explained as being induced by mechanical unloading of the bone (stress protection). Experiments in sheep, dogs, and rabbits combining intravital staining of blood circulation and polychrome fluorescent labelling of bone remodelling leads to the conclusion that early bone porosis in the vicinity of the implants is the result of internal remodelling of cortical bone and is induced by necrosis rather than by unloading. This theory is favored by the evidence that 1) the bone is of a temporary nature, an intermediate stage in internal bone remodelling; 2) the pattern of the remodelling zone is closely related to that of the disturbed circulation, and not to that of unloading; 3) plastic plates may produce more porosis than steel plates; and 4) improved blood circulation using modified plates resulted in reduced porosis. The clinical relevance of these findings is related first to temporary weakening of the bone, and second to the possibility of sequestration. Sequestration may be the result of intensified remodelling activity in the presence of inflammation or infection.” [4] Gasser, B., S.M. Perren, and E. Schneider. Parametric Numerical Design Optimization of Internal Fixation Plates. Transactions of the 7th Meeting. Aarhus, Denmark: European Society of Biomechanics, 1990. Excerpt—“Disturbance of blood supply due to contact between plate and bone has been made responsible for bone remodelling in the early postoperative phase. Based on the experience with the Dynamic Compression Plate (DCP), the design of a new Low Contact-DCP (LC-DCP) was optmized. Design optimization is a multiparametrical problem, and the goal of this study was to identify by mathematical means the optimal design for a new plate with respect to the following criteria: a reduced bone/plate contact by means of a lateral recess in the undersurface of the plate; a symmetrical hole geometry with an oblique undercut at both ends to increase tilting of lag screws; a trapezoidal plate cross section to facilitate removal. For evaluation, particular emphasis has been put on preserving the continuity of bending and torsional stiffnesses along the plate and the maintenance of plate strength under different loadings. Tools and Method: Finite Element Analysis (FEA) to study and compare geometrical and mechanical properties of different plate types. Four load cases (simulated screw load, bending and/or torsion) were compared with the DCP. The homogeneity of the bending stiffness was improved by 49%, without reduction of the strength for any of the load conditions investigated. The area of the underside of the Low Contact-DCP was reduced to 50%, compared with the DCP.” [5] Pohler, O.E.M. “Degradation of Metallic Orthopaedic Implants.” Biomaterials in Reconstructive Surgery. Ed. L. Rubin. St. Louis: The C.V. Mosby Co., 1983. 158-227. Excerpt—“Metal degradation can be evoked through dissolution and corrosion as well as through the action of mechanical forces. The latter lead to wear, fatigue, and overload failures. Particularly detrimental are combinations of mechanical and chemical/ electrochemical attack, which can cause, for example, fretting corrosion and corrosion fatigue. Orthopaedic implants can suffer those forms of material destruction through interaction with the body. Mechanical and chemical/electrochemical damage should be distinguished sharply. Through fretting, wear debris and corrosion products are generated, and local and systemic effects might be considered. A thorough section of the paper concentrates on the description of the destructive mechanisms found typically on the different implant materials, mainly based on the investigation of implants for fracture treatment. In general, the biological tolerance in the in vivo tests is higher compared to that in culture tests. It is characteristic that the nickel- and cobalt-containing alloys are very well tolerated as long as they do not disintegrate. Only when, through fretting and fretting corrosion, the individual metals are released in active form, they unfold their specific metabolic effects.” [9] Rüedi, T.P., S.M. Perren, O. Pohler, and U. Riede. Titan und Stahl und deren Kombination in Knochenchirurgie. Langenbeck Arch. Suppl. Chir. Forum, 1975. Excerpt—“The combined application of titanium plates with stainless steel screws appeared interesting. Three different combinations of titanium and stainless steel implants were tested in the animal and on humans. A morphometric evaluation of the soft tissue (animal) gave similar good results for stainless steel implants as for the combination of titanium and steel, while pure titanium gave the best result. Atomic absorption test (human) showed that in the case of the titanium/steel mixture, only the stainless steel screws did corrode. Delayed fracture healing or mechanical instability always gave risk to more metal deposits in the soft tissue than primary bone healing. The combination of the two metals appears possible and temporarily without danger.” [6] Steinemann, S.G. “Corrosion of Surgical Implants—in vivo and in vitro Tests.” Evaluation of Biomaterials. Eds. G.D. Winter, J.L. Leray, and K. de Groot. New York: John Wiley & Sons Ltd., 1980. 1-34. Excerpt—“In vivo and in vitro corrosion data for pure metals and alloys for surgical implants are reviewed, and it is shown that such data can be related to pH shifts and metal ion concentrations in tissue by solving a realistic transport equation. The results go beyond a symptomatic connection between corrosion and tissue reaction and, with the aid of electrochemical equilibria, explain conditions for interaction. But surgical implants are also prone to special forms of corrosion, e.g. crevice attack and fretting, which lead to a drastic enhancement of local ion concentration and can then induce toxic reactions.” [10] Matter, P. and H.B. Burch. Titanium Implants and Limited Contact DCP-System: Clinical Experience. Bern, Switzerland: AO-Documentation Center, 1990. Excerpt—“AO ASIF with its collaborating laboratories has developed special methods to obtain the required strength for using pure titanium as an implant material. The first prospectively controlled clinical series of implants made of this material dates back to 1966 and was reported to be most successful. Pure titanium became the material of choice for implants to be used in patients suffering from metal allergy. Today a long-term and well-documented experience with pure titanium implants exists which proves that this material fulfills the requirements of optimal biocompatibility. For this reason it was integrated in the biological concept of the limited-contact-plating system, which aims to preserve the biointegrity of the affected area as much as possible by means of a less aggressive approach to treat the already damaged bone. Pilot clinics have started to implant titanium LC-DCP in 1987. Today 271 plates have been implanted mainly for the treatment of fresh fractures, and 57 plates have so far been removed. The preliminary results are most favorable. They especially confirm the effects of the preserved cortical blood flow and the outstanding biocompatibility of the pure titanium.” [7] Gerber, H.W. and S.M. Perren. “Evaluation of Tissue Compatibility of in-vitro Cultures of Embryonic Bone.” Evaluation of Biomaterials. Eds. G.D. Winter, J.L. Leray, and K. de Groot. New York: John Wiley & Sons Ltd., 1980. 307-314. Excerpt—“Organ-cultured embryonic rat femurs were used as an experimental model to evaluate metal tolerance. Using a variety of metal chlorides, individual dose response curves could be established which are well-suited for statistical analysis. Miniature solid metal implants serve to determine the growth inhibition due to the complex corrosion product. The histological appearance of the tissue at different distances from the metal is reported. The model seems to be sensitive and easily standardized.” [8] Gerber, H.W., M. Bürge, J. Cordey, W.J. Ziegler, and S.M. Perren. Quantitative Determination of Tissue Tolerance of Corrosion Products by Organ Culture. Proceedings of the European Society of Artificial Organs. Vol. 1. Davos, Switzerland: Laboratory for Experimental Surgery, AO ASIF, 1975. 29-34. Excerpt—“Surgical implants are manufactured from metals of good mechanical strength and corrosion resistance. However, every metal implant releases, in vivo, metal ions into the tissue permanently. Clinical requirements lead to the search for new metal alloys, which then necessitates a method of comparative testing tissue toxicity.” [11] Simpson, J.P., V. Geret, K. Merritt, and S.A. Brown. Retrieved Fracture Plates: Implant and Tissue Analysis (NBS SP-601). Eds. A. Weinstein, D. Gibbons, S. Brown, and W. Ruff. Washington: National Bureau of Standards, 1980. 423-448. Excerpt—“A study was undertaken by the AO ASIF to investigate the suitability of different materials for bone plates for osteosynthesis. A total of 80 plates were retrieved together with clinical data, histologic data, and chemical analysis. The materials used were stainless steel 316 LVM, commercially available pure titanium, titanium (Ti-6AI-4V) alloy, and a cobaltchrome-nickel-tungsten-iron alloy as used in the AO ASIF compression plates and screws. The protocols used and the results of these first 80 cases are presented. The histologic analysis revealed the greatest differences occurring in the accumulation of lymphocytes, macrophages, and giant cells which were greatest with the cobalt alloy. The quantity of debris in the tissue was greatest with pure titanium. The chemical analysis revealed a wide scattering of values and the results are discussed. The examination of plates and screws revealed that stainless steel suffers fretting corrosion and that the amount of metal loss is less than on the cobalt alloy, titanium, and the titanium alloy, although significant corrosion was observed at the plate screw contact area for the cobalt alloy. In this paper, a protocol for the evaluation of metal osteosynsthesis plates has been presented along with a scheme for analyzing tissue responses and obtaining clinical data. The number of cases presently available for analysis does not permit any definite conclusions as to the advantages of one of the alloys tested over another. It is hoped that with the addition of more cases this will be possible. We recommend that the type of protocol we have described here be used so that comparison can be made between different studies.” [12] Klaue, K. The Dynamic Compression Unit (DCU) for Stable Internal Fixation of Bone Fractures. Davos, Switzerland: The Laboratory for Experimental Surgery, AO ASIF. Excerpt—“In the internal fixation of fractures, compression between the fragments is often applied by the use of interfragmentary lag screws together with plates. Plate screws may, in certain circumstances, also function simultaneously as lag screws. If conventional interfragmentary lagged plate screws are used in the inclined position, they permit more efficient compression. However, problems may arise, due to the possible interference of the thread of the screw with the edge of the plate hole and/or with the bone within the ‘gliding hole.’ To take advantage of inclined lagged plate screws, the system was modified to provide more efficient compression. The system reported here provides symmetrical interfragmentary compression using modified implants. This is achieved by using inclined shaft screws inserted through longitudinal slots in the plate and crossing the fracture line. Implantations performed on sheep, after preliminary investigations in vitro, seem to confirm the predicted compression effects. Although a high incidence of excellent reduction was achieved, further investigation will be required to determine the exact conditions in which the bone will support the forces applied.” [13] Klaue, K. and S.M. Perren. Interfragmentary Compression Using Inclined Lag Screws in Self-Compressing Plate Holes: Problems and Solutions. Transactions of the 35th Annual Meeting. Davos, Switzerland: Laboratory for Experimental Surgery, Orthopaedic Research Society, 1989. Excerpt—“Fully threaded lag screws, inclined 20° towards the fracture and inserted through self-compressing plate holes across the fracture plane, yield only about 49% of their possible compressive effect along the fracture plane. This loss of compressive effect is due to anchorage observed histologically after the screw has glided towards the fracture. Modifications of the plate holes and screws allow full efficiency of the lag screw compression to be retained. The Dynamic Compression Unit (DCU) of lag screw and self-compressing plate has been developed to optimize stabilization using plate screws which serve simultaneously as lag screws. The term ‘Unit’ was coined to indicate the combined effect of screw and plate. The combination can be used to function as a tension band; it can also function as a protecting or buttressing splint. The newly designed plate holes are elongated and flared. They are designed to avoid the screw abutting against the inner edge of the plate hole. With this design, simultaneous compression along the bone axis (axial compression) as well as along the screw axis (interfragmentary compression) is achieved with only one bone screw. After tests in 20 sheep, a first series of more than 100 DCU plates have been implanted in humans; evaluation awaits their removal. Conclusions A substantial improvement of the stabilizing interfragmentary compression of lag screws used in self-compressing holes has been achieved. Clinical tests underway will show whether it is safe to reduce the total number of screws in plate fixation and so take advantage of the new design.” [14] Klaue, K. and S.M. Perren. Unconventional Shapes of the Plate Cross-Section in Internal Fixation: The Trapezoid Plate. Long Term Study of Bone Reaction in Sheep Tibiae. Davos, Switzerland: Laboratory for Experimental Surgery, AO ASIF, 1990. Excerpt—“Plated cortical bone undergoes changes of its structure and shape. Generally, these changes are explained on the basis of stress relief. Factors that influence the biological response of bone to an implant include: implant material (biocompatibility), contact area between the implant and the bone (blood supply), and stiffness of the implant (mechanical load). The goal of the present study was to determine the biological and mechanical effect of four different plates on the underlying bone. These plates consisted of: 1) steel plates of conventional AO dimension and rectangular shape; 2) steel plates of trapezoidal cross section to reduce area of contact but similar stiffness; 3) carbon-polysulfone thermoplastic fiber plates of conventional shape and dimension; 4) thinned conventional steel plates of similar stiffness to the carbon plates. The carbon fiber composite plates did not reveal any specific advantage in respect to bone stiffness. There is no correlation between the stiffness of the plate and the stiffness of the bone after plate removal. Porosity of cortical bone under the plate was minimal with the trapezoidal plate. On the other hand, porosity underneath the polysulfone/carbon plate was markedly higher and remodelling more intense when compared to the stainless steel plates. While the results of the carbon plates were discouraging, the trapezoidal plates provided not only a better bone structure but were also easier to remove. The chances to produce stress risers by defects of the side laminae was minimized in this group as well. The experiment revealed the mechanical importance of the intactness of the cortical bone lining the plates. The trapezoidal plate provides increased bone cross section and increased stiffness of bone when compared to conventional plates.” Additional Literature Holzach, P., and P. Matter. “The Comparison of Steel and Titanium Dynamic Compression Plates Used for Internal Fixation of 256 Fractures of the Tibia.” 120 Injury 10 (1978): 120-123. Lombardi, A.V., Jr. et al. “Aseptic Loosening in Total Hip Arthroplasty Secondary to Osteolysis Induced by Wear Debris from Titanium-Alloy Modular Femoral Heads.” Journal of Bone and Joint Surgery 71-A.9 (October 1989). Matter, P., M. Schutz, M. Buhler, A Ungersbock, and S. Perren. “[Clinical results with the limited contact DCP plate of titanium–a prospective study of 504 cases].” [Article in German] Z Unfallchir Versicherungsmed 1994 Apr.; 87(1):6-13. McKee, M.D., J.G. Seiler, and J.B. Jupiter. “The application of the limited contact dynamic compression plate in the upper extremity: an analysis of 114 consecutive cases.” Injury 1995 Dec.; 26(10):661-6. Perren, S.M. “Basic Aspects and Scientific Background of Internal Fixation.” Scientific Bulletins of the AO Group. Davos, Switzerland: Laboratory for Experimental Surgery, AO ASIF, 1990. Perren, S.M. “The Biomechanics and Biology of Internal Fixation Using Plates and Nails.” Orthopedics 12.1 (1989): 21-34. Pfeiffer, K.M., J. Brennwald, U. Buchler, D. Hanel, J. Jupiter, K. Lowka, J. Mark, and P. Staehlin. “Implants of pure titanium for internal fixation of the peripheral skeleton.” Injury 1994 Mar.; 25(2):87-9. Pfister, U., B.A. Rahn, S.M. Perren, and S. Weller. “Blood Supply and Bone Remodeling Following Medullary Nailing of Long Bones: Experimental Study in the Sheep Tibia.” Akt. Traumatol. 9 (1979): 191-195. J3046 Rev 2/00 SYNTHES (U.S.A.) 1302 Wrights Lane East West Chester, PA 19380 Telephone: (610) 719-5000 To order: (800) 523-0322 Fax: (601) 251-9056 SYNTHES (CANADA) LTD. 2566 Meadowpine Boulevard Mississauga, Ontario L5N 6P9 Telephone: (905) 568-1711 To order: (800) 668-1119 Fax: (905) 567-3185 Printed in U.S.A. LCDCPTI1 3-91 © SYNTHES (USA) 1991 J3046-B 1/06 DCP, LC-DCP, SYNTHES, and ASIF are trademarks of SYNTHES (USA) and SYNTHES AG Chur.