BIOLOGICALPLATINGA New ConceptTo Foster Bone Healing

Original Instruments and Implants of the Association for the Study of Internal Fixation—AO ASIF

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Early Temporary Porosis Beneath Bone Plates

Observed patterns of boneloss beneath a plate do notcorrespond to the stress patterns of the correspondingbone segment.

Disturbance of blood supply demon-strated with disulphine blue as anindicator of blood perfusion.

A comparative histological sectionshows remodelling located in the areaof previous avascularity.

Osteoporosis beneath a plate is the direct result of damage to the bloodsupply and not the result of mechanical unloading of the bone [2].

The disturbance of the bloodsupply and porosis arestrongly correlated [2].

Situation under a plate: Under weight-bearing conditions, stresses in the bone(represented by black lines) diminishtowards the plate (shielding effect of a plate [1]).

Situation under a plate: The area ofobserved porosis is sharply defined andunrelated to the pattern of stress shielding (as represented at left).

Bone loss in the vicinity of a plate has always been explained on the basisof Wolff’s law as a reaction of living bone to mechanical unloading of the plated bone segment (stress protection). Although experiments havedemonstrated that flexible plastic plates do not improve the situation, the rigidity of a plate is generally still considered the prime factor induc-ing porosis.

Is there a correlation between bone loss beneath the plate and“stress protection”?

AO ASIF observation

Limited Contact Concept

Reduced vascular damage

Reduction of surface contactbetween plate and boneresults in a reduced distur-bance of blood supply [3].

Histological investigation brings a new understanding of vascular damagein relation to different contact surfaces and enlightens the etiology of temporary porosis of bone.

Improved bone consolidation with reduced porosis

Extensive area of bone loss as observedbeneath a full-contact plate.

Reduced area of bone loss (porosis)as observed beneath a limited-contact 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.

Favorable blood perfusionconditions promote betterbone quality.

Area of disturbed circulation in bone cross section

mm2

20

10

Full Contact Limited Contact

Minimal Contact Achieved Without Impaired Implant Strength

Uniform bending and torsional stiffness

To achieve a uniform stiffness with limited bone contactalong the plate length, CAD/CAE tools and mathematicalanalysis are used for a reliable functional design [4].

The Finite Element Analysis of a plate under bendingload conditions (see picture below) shows bending stresses evenly distributed along the plate.

Finite Element Analysis grid of the optimized plate viewedfrom above

Improved Mechanical Performance

Improved contouring of the plate

Contouring the LC-DCP™: The plate holes preserve their shape.Conventional plate: Kinks at holes impair the mechanical performance.

Exact contouring is essential for good force transmissionand successful fracture treatment.

The uniform stiffness of the LC-DCP™ enables continuouscurvature, allows a good fit of the screw head in theplate hole, and preserves the mechanical features of the plate.

Plate protected from localized high stresses

The LC-DCP deforms over a long distance, resulting in an evenstress distribution (exaggerated situation).

When loaded, the uniform stiffness of the LC-DCPresults in an even distribution of stresses over a long distance along the plate, protecting the plate holes fromlocalized high stresses. Therefore, the LC-DCP is lessprone to fatigue, especially where plates span a widebony defect or in comminuted fractures. In contrast, a conventional plate deforms mainly at the hole, andstresses concentrate at the smallest cross section.

Stra

in

The Material of Biological Choice: AO ASIF Pure Titanium

High corrosion resistance of conventionalimplant materials

Standard implant materials are protected by a thin passive layer.

The conventional implant materials used today arehighly corrosion-resistant and generally well tolerated.They are protected by a spontaneously forming sub-microscopic thin oxide layer on their surface whichprevents the metal from further oxidation and corrosion. This protective oxide layer is known as passive film.

Fretting effects on conventional implantmaterials

Typical congruent zones where fretting occurs between screwhead and plate hole in stainless steel implants [5].

In areas where implants are in contact, the protectiveoxide layer may be destroyed by relative motion; evenmicromotion 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 thealloy itself can be released during the abrasive processof mechanical wear. Usual metals used in alloys, likenickel, chromium, cobalt, aluminum, and vanadium,can thus find their way into the body tissues. Theingestion and transport of metallic degradation products in the body tissues is complex, and differentmechanisms might operate simultaneously dependingon the form in which the metal is released [5]. Themetallic degradation products may be in the form of wear particles, oxidized compounds, or ionizedspecies.

As long as the protective oxide layer on the implantsurface is intact, the material remains passive.

Relative motion between metallic implants leads tofretting effects, and alloying components can bereleased into the tissues.

tissue

passivelayer

metal

Alloying elements with biological activity

Corrosion resistance vs. biocompatibility of some pure metalsand alloys [6].

The biocompatibility of metallic materials is closelyrelated to their corrosion resistance and to the trans-portability of the corrosion products. The extent towhich the wear products from alloys further corrodein tissues is uncertain, but biological activity of alloy-ing components does occur, and occasional reac-tions cannot be ruled out.

Nickel is a known allergen, and contact alone can provoke an allergic reaction (i.e., no particular cor-rosive process has to take place). Speculations on thesystemic effects of nickel and other metals abound.Various published and unpublished data from cell and organ cultures show that nickel and vanadiumhave cytotoxic effects at considerably lower concen-trations than other metals used in implants [7,8] (see diagram above).

Pure titanium is biologically inert

Tissue impregnated with pure titanium wear particles does notshow adverse reactions.

In contrast to the above-mentioned metals, pure titanium displays very little biological activity, and itsunmatched tissue tolerance has been scientifically and clinically demonstrated.

In cases of unstable internal fixation with tissuesstained by abrasion particles, no accompanying corrosion has been observed [5].

In tissue fluids, the pure titanium mechanical wearproducts are practically insoluble and are chemicallynon-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).

Pure titanium displays excellent biocompatibility.

The AO chose not to add any element to pure titanium which could cause anadverse biological response.

Pure titanium and its wear products remain passiveand do not affect the tissue.

Increasing biocompatibility

Co

rro

sio

n r

esis

tan

ce

toxic andcorroding

increasingacceptance

excellentacceptance

V

Co

Cu

Ni

Mo

Fe

Al

Ag Au

Co-AlloysSt Steel

Titanium

Pt Ta Nb

AO ASIF Pure Titanium

Broad Clinical Experience

Since 1965, the AO ASIF has gained clinical experi-ence with nearly 5,000 cases treated with AO ASIFPure Titanium implants. The outstanding tissue compatibility of AO ASIF Pure Titanium has beenrepeatedly 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 choicefor implants to be used in patients suffering frommetal allergy. Furthermore, if it is advantageous toavoid explantation surgery (e.g., distal humerus),implants made of inert materials, such as pure titanium, are ideal.

Better Vascularized Tissues

Well vascularized tissue around AO ASIF Pure Titaniumimplants

Clinical experience shows that the tissues whichenvelop pure titanium implants are better vascular-ized and show a reduced tendency towards capsuleformation [9,10,11]. There is better tissue adherenceto the pure titanium plate than when other standardimplant 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 adequatestrength 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 collaboratinglaboratories, and its exclusive producers have devel-oped special manufacturing and thermic treatmentmethods. With these methods, the required strengthand ductility are achieved without the addition of alloying elements of proven toxicity such as vanadium. Thus, the AO can take uncompromisedclinical advantage of the outstanding biocompati-bility of pure titanium.

Distinct Mechanical Properties

The diagram shows the range of tensile strength characteristicof commercially available pure titanium and AO ASIF PureTitanium.

Depending on the type of implants and their func-tion, the appropriate strength level is chosen withinthis range.

AO ASIFPure Titanium

CommerciallyavailablePure Titanium

UltimatetensilestrengthN/mm2

900

800

700

600

500

400

300

200

100

0

Advantages in Surgical Technique

Compression can be achieved in either longitudinal direction

The basic spherical gliding principle of the screw in the DCP™ hole is now implemented at both ends ofthe plate hole in the LC-DCP plate. This enables compression in either direction along its longitudinal axis. The redesigned geometry of the hole also addsmore flexibility to the plating technique and easeshandling of complex situations.

A lag screw can be inserted at greaterangulation

Longitudinal cross section of LC-DCP hole and feasibleangulation of a lag screw

With conventional DCP plates, the maximum screwangulation in the longitudinal axis is about 20degrees. Greater angulation of the lag screw couldnot be achieved without impeding the gliding of thelag screw in the gliding hole. The LC-DCP hole offersthe possibility of safe insertion of the lag screw up to40 degrees through the plate in both directions. Thisgives the surgeon more possibilities to achieve inter-fragmental compression through the plate [12,13].

LC-DCP hole

The plate hole can be used for interfragmental compression of multifragmentary fractures. Due to the regular hole spacing along the axis of theplate—no midsection without holes—the surgeon has more options to reposition a plate of differentlength using the same predrilled holes.

To achieve additional interfragmental compression,correct angulation of the lag screw is required. Tosecure a loose fragment through the plate, an anglegreater than 24 degrees (standard plating systems) isoccasionally required.

40° 40°

Implant removal is facilitated

The normally thin bone lamella lining the plate mayimpede removal of conventional plates. The fraillamella is susceptible to damage upon removal andwill act as a stress riser [14] which could eventuallyresult in refracture of the bone.

LC-DCP plate trapezoidal cross section reduces the risk of generating stress risers.

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 detach-ment and reducing the risk of refracture.

Standard plate cross section

References

[1] Cordey, J. and S.M. Perren. “Stress Protection in FemoraPlated by Carbon Fiber and Metallic Plates: MathematicalAnalysis and Experimental Verification.” Biomaterials andBiomechanics 1983. Eds. P. Ducheyne, G. Van der Perre, andA.E. Aubert. Amsterdam: Elsevier Science Publisher B.V., 1984.189-194.Excerpt—“Bone refracture after plate removal has been attrib-uted to the structural adaptation of bone (loss of bone) toreduced stress (stress protection). The analysis of the stress pat-tern in plated bones seems to be a prerequisite for the assertionthat bone loss is stress-related. The strain distribution at the sur-face of plated human femoral shaft has been analyzed using thecomposite beam theory and verified experimentally using straingauges. Plates made of carbon, titanium and stainless steel wereinvestigated. The difference between the reduction of stressobtained using the less stiff plate materials and that obtained by using thinner stainless steel plates is astonishingly small. Thereduction of the rigidity of the plate does not result in a propor-tional improvement of the strain in bone under the combinedaxial 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 andBiomechanics 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 followingplating of cortical bone is mainly due to porosis. The porosisaccompanies the internal remodelling of the diaphyseal bone. It is not clear whether the porosis is a reaction due to themechanical unloading of the bone, or whether, as seems moreprobable, it is a temporary stage in the remodelling of necroticbone. In an experimental study using sheep, plates of differentbending stiffness and different lower surface structure were fixedonto the medial aspect of intact tibiae. Changes in blood supplyand bone remodelling were assessed at four, ten and twentyweeks.

There is no difference in the amount of bone remodellingbetween groups with plates made of steel and groups with similar plates made of polyacetal with a thin metal core. It seemsnoteworthy that the extent of the remodelling differs towardsthe proximal and the distal ends of the plated bone. A correla-tion was found between plate contact to bone and the extent of the vascular damage at four weeks on the one hand, andbetween plate contact and the extent of the bone remodellingarea at twenty weeks on the other hand.

The results of the experiment suggest that porosis of the boneis 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 InternalFixation Implants: A Reaction to Necrosis, Not to Stress Protec-tion?” Clinical Orthopaedics and Related Research 232 (1988):139-151.Excerpt—“Stabilization of the fracture using implants requirescontact surfaces between implant and bone. Such contact hasbeen observed to induce bone porosis first seen at one month

after surgery. Bone loss in the vicinity of implants has hithertobeen explained as being induced by mechanical unloading ofthe bone (stress protection). Experiments in sheep, dogs, andrabbits combining intravital staining of blood circulation andpolychrome fluorescent labelling of bone remodelling leads tothe conclusion that early bone porosis in the vicinity of theimplants is the result of internal remodelling of cortical bone andis induced by necrosis rather than by unloading. This theory isfavored by the evidence that 1) the bone is of a temporarynature, an intermediate stage in internal bone remodelling; 2)the pattern of the remodelling zone is closely related to that ofthe disturbed circulation, and not to that of unloading; 3) plasticplates may produce more porosis than steel plates; and 4)improved blood circulation using modified plates resulted inreduced porosis. The clinical relevance of these findings is relatedfirst to temporary weakening of the bone, and second to thepossibility of sequestration. Sequestration may be the result ofintensified remodelling activity in the presence of inflammationor infection.”

[4] Gasser, B., S.M. Perren, and E. Schneider. ParametricNumerical Design Optimization of Internal Fixation Plates.Transactions of the 7th Meeting. Aarhus, Denmark: EuropeanSociety of Biomechanics, 1990.Excerpt—“Disturbance of blood supply due to contact betweenplate and bone has been made responsible for bone remodellingin the early postoperative phase. Based on the experience withthe Dynamic Compression Plate (DCP), the design of a new LowContact-DCP (LC-DCP) was optmized.

Design optimization is a multiparametrical problem, and thegoal of this study was to identify by mathematical means theoptimal design for a new plate with respect to the following criteria: a reduced bone/plate contact by means of a lateral recessin the undersurface of the plate; a symmetrical hole geometrywith an oblique undercut at both ends to increase tilting of lagscrews; a trapezoidal plate cross section to facilitate removal. Forevaluation, particular emphasis has been put on preserving thecontinuity of bending and torsional stiffnesses along the plateand the maintenance of plate strength under different loadings.

Tools and Method: Finite Element Analysis (FEA) to study andcompare geometrical and mechanical properties of differentplate types.

Four load cases (simulated screw load, bending and/or torsion)were compared with the DCP. The homogeneity of the bendingstiffness was improved by 49%, without reduction of thestrength 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 OrthopaedicImplants.” Biomaterials in Reconstructive Surgery. Ed. L. Rubin.St. Louis: The C.V. Mosby Co., 1983. 158-227.Excerpt—“Metal degradation can be evoked through dissolutionand corrosion as well as through the action of mechanical forces.The latter lead to wear, fatigue, and overload failures. Particularlydetrimental are combinations of mechanical and chemical/electrochemical attack, which can cause, for example, frettingcorrosion and corrosion fatigue. Orthopaedic implants can suffer

those forms of material destruction through interaction with the body.

Mechanical and chemical/electrochemical damage should bedistinguished sharply. Through fretting, wear debris and corro-sion products are generated, and local and systemic effectsmight be considered.

A thorough section of the paper concentrates on the descrip-tion of the destructive mechanisms found typically on the dif-ferent implant materials, mainly based on the investigation ofimplants for fracture treatment.

In general, the biological tolerance in the in vivo tests is higher compared to that in culture tests. It is characteristic thatthe nickel- and cobalt-containing alloys are very well tolerated as long as they do not disintegrate. Only when, through frettingand fretting corrosion, the individual metals are released in active form, they unfold their specific metabolic effects.”

[6] Steinemann, S.G. “Corrosion of Surgical Implants—in vivoand 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 shownthat such data can be related to pH shifts and metal ion con-centrations in tissue by solving a realistic transport equation. The results go beyond a symptomatic connection between cor-rosion and tissue reaction and, with the aid of electrochemicalequilibria, explain conditions for interaction.

But surgical implants are also prone to special forms of corro-sion, e.g. crevice attack and fretting, which lead to a drasticenhancement of local ion concentration and can then inducetoxic reactions.”

[7] Gerber, H.W. and S.M. Perren. “Evaluation of Tissue Com-patibility of in-vitro Cultures of Embryonic Bone.” Evaluation ofBiomaterials. Eds. G.D. Winter, J.L. Leray, and K. de Groot. NewYork: 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 responsecurves could be established which are well-suited for statisticalanalysis. Miniature solid metal implants serve to determine thegrowth inhibition due to the complex corrosion product. The histological appearance of the tissue at different distances fromthe 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 ofCorrosion Products by Organ Culture. Proceedings of theEuropean Society of Artificial Organs. Vol. 1. Davos, Switzerland: Laboratory for Experimental Surgery, AO ASIF, 1975. 29-34.Excerpt—“Surgical implants are manufactured from metals ofgood mechanical strength and corrosion resistance. However,every metal implant releases, in vivo, metal ions into the tissuepermanently. Clinical requirements lead to the search for newmetal alloys, which then necessitates a method of comparativetesting tissue toxicity.”

[9] Rüedi, T.P., S.M. Perren, O. Pohler, and U. Riede. Titan undStahl und deren Kombination in Knochenchirurgie. LangenbeckArch. Suppl. Chir. Forum, 1975.Excerpt—“The combined application of titanium plates withstainless steel screws appeared interesting. Three different com-binations of titanium and stainless steel implants were tested inthe animal and on humans. A morphometric evaluation of thesoft tissue (animal) gave similar good results for stainless steelimplants as for the combination of titanium and steel, whilepure 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 metaldeposits in the soft tissue than primary bone healing. The com-bination of the two metals appears possible and temporarilywithout danger.”

[10] Matter, P. and H.B. Burch. Titanium Implants and LimitedContact 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 forusing pure titanium as an implant material. The first prospec-tively controlled clinical series of implants made of this materialdates back to 1966 and was reported to be most successful.Pure titanium became the material of choice for implants to beused in patients suffering from metal allergy. Today a long-termand well-documented experience with pure titanium implantsexists which proves that this material fulfills the requirements ofoptimal biocompatibility. For this reason it was integrated in thebiological concept of the limited-contact-plating system, whichaims to preserve the biointegrity of the affected area as much as possible by means of a less aggressive approach to treat thealready 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 57plates have so far been removed. The preliminary results aremost favorable. They especially confirm the effects of the preserved cortical blood flow and the outstanding biocom-patibility of the pure titanium.”

[11] Simpson, J.P., V. Geret, K. Merritt, and S.A. Brown. RetrievedFracture 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 in-vestigate the suitability of different materials for bone plates forosteosynthesis. A total of 80 plates were retrieved together withclinical 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 cobalt-chrome-nickel-tungsten-iron alloy as used in the AO ASIF com-pression plates and screws. The protocols used and the results of these first 80 cases are presented. The histologic analysisrevealed the greatest differences occurring in the accumulationof 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 dis-cussed. The examination of plates and screws revealed thatstainless 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 osteo-synsthesis plates has been presented along with a scheme foranalyzing tissue responses and obtaining clinical data. The number of cases presently available for analysis does not permitany definite conclusions as to the advantages of one of thealloys tested over another. It is hoped that with the addition ofmore cases this will be possible. We recommend that the type ofprotocol we have described here be used so that comparisoncan be made between different studies.”

[12] Klaue, K. The Dynamic Compression Unit (DCU) for StableInternal Fixation of Bone Fractures. Davos, Switzerland: TheLaboratory for Experimental Surgery, AO ASIF.Excerpt—“In the internal fixation of fractures, compressionbetween the fragments is often applied by the use of inter-fragmentary lag screws together with plates. Plate screws may,in certain circumstances, also function simultaneously as lagscrews. If conventional interfragmentary lagged plate screws areused in the inclined position, they permit more efficient com-pression. However, problems may arise, due to the possible interference of the thread of the screw with the edge of theplate hole and/or with the bone within the ‘gliding hole.’ To take advantage of inclined lagged plate screws, the system wasmodified to provide more efficient compression. The systemreported here provides symmetrical interfragmentary compres-sion using modified implants. This is achieved by using inclinedshaft screws inserted through longitudinal slots in the plate andcrossing the fracture line. Implantations performed on sheep,after preliminary investigations in vitro, seem to confirm the predicted compression effects. Although a high incidence ofexcellent reduction was achieved, further investigation will berequired to determine the exact conditions in which the bonewill support the forces applied.”

[13] Klaue, K. and S.M. Perren. Interfragmentary CompressionUsing Inclined Lag Screws in Self-Compressing Plate Holes:Problems and Solutions. Transactions of the 35th AnnualMeeting. Davos, Switzerland: Laboratory for ExperimentalSurgery, Orthopaedic Research Society, 1989.Excerpt—“Fully threaded lag screws, inclined 20° towards thefracture and inserted through self-compressing plate holes across the fracture plane, yield only about 49% of their possiblecompressive effect along the fracture plane. This loss of com-pressive effect is due to anchorage observed histologically afterthe screw has glided towards the fracture. Modifications of theplate 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 whichserve simultaneously as lag screws. The term ‘Unit’ was coinedto indicate the combined effect of screw and plate. The com-bination can be used to function as a tension band; it can

also function as a protecting or buttressing splint. The newlydesigned plate holes are elongated and flared. They aredesigned to avoid the screw abutting against the inner edge of the plate hole. With this design, simultaneous compressionalong the bone axis (axial compression) as well as along thescrew axis (interfragmentary compression) is achieved with onlyone bone screw. After tests in 20 sheep, a first series of morethan 100 DCU plates have been implanted in humans; evalua-tion awaits their removal.

ConclusionsA substantial improvement of the stabilizing interfragmentarycompression of lag screws used in self-compressing holes hasbeen achieved. Clinical tests underway will show whether it issafe to reduce the total number of screws in plate fixation andso take advantage of the new design.”

[14] Klaue, K. and S.M. Perren. Unconventional Shapes of thePlate 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 biologicalresponse of bone to an implant include: implant material (biocompatibility), contact area between the implant and thebone (blood supply), and stiffness of the implant (mechanicalload). The goal of the present study was to determine the biological and mechanical effect of four different plates on theunderlying bone. These plates consisted of: 1) steel plates ofconventional AO dimension and rectangular shape; 2) steelplates of trapezoidal cross section to reduce area of contact but similar stiffness; 3) carbon-polysulfone thermoplastic fiberplates of conventional shape and dimension; 4) thinned con-ventional steel plates of similar stiffness to the carbon plates.

The carbon fiber composite plates did not reveal any specificadvantage in respect to bone stiffness. There is no correlationbetween the stiffness of the plate and the stiffness of the boneafter 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 markedlyhigher and remodelling more intense when compared to thestainless steel plates.

While the results of the carbon plates were discouraging, thetrapezoidal plates provided not only a better bone structure butwere also easier to remove. The chances to produce stress risersby defects of the side laminae was minimized in this group aswell. The experiment revealed the mechanical importance of theintactness of the cortical bone lining the plates. The trapezoidalplate provides increased bone cross section and increased stiff-ness of bone when compared to conventional plates.”

Holzach, P., and P. Matter. “The Comparison of Steel and Titanium DynamicCompression 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 toOsteolysis 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 withthe limited contact DCP plate of titanium–a prospective study of 504 cases].” [Articlein 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 contactdynamic compression plate in the upper extremity: an analysis of 114 consecutivecases.” Injury 1995 Dec.; 26(10):661-6.

Perren, S.M. “Basic Aspects and Scientific Background of Internal Fixation.” ScientificBulletins 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 andNails.” 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 RemodelingFollowing Medullary Nailing of Long Bones: Experimental Study in the Sheep Tibia.”Akt. Traumatol. 9 (1979): 191-195.

J3046 Rev 2/00

Additional Literature

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