Biomaterials for orbital implants and ocular prostheses: Overview and future prospects

Biomaterials for orbital implants and ocular prostheses: Overview and future prospects
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  Review Biomaterials for orbital implants and ocular prostheses: Overviewand future prospects Francesco Baino a, ⇑ , Sergio Perero a,b , Sara Ferraris a , Marta Miola a , Cristina Balagna a , Enrica Verné a ,Chiara Vitale-Brovarone a , Andrea Coggiola c , Daniela Dolcino c , Monica Ferraris a a Institute of Materials Physics and Engineering, Applied Science and Technology Department, Politecnico di Torino, Corso Duca degli Abruzzi 24, Torino, Italy b Istituto Superiore Mario Boella, Torino, Italy c S.O.C. Oculistica, Azienda Ospedaliera Nazionale SS. Antonio e Biagio e Cesare Arrigo, Via Venezia 16, Alessandria, Italy a r t i c l e i n f o  Article history: Received 19 July 2013Receivedinrevisedform29November 2013Accepted 9 December 2013Available online 14 December 2013 Keywords: Orbital implantOcular prosthesisEnucleationPorous biomaterialsAntibacterial properties a b s t r a c t The removal of an eye is one of the most difficult anddramatic decisions that a surgeon must consider incase of severe trauma or life-threatening diseases to the patient. The philosophy behind the design of orbitalimplantshasevolvedsignificantlyoverthelast60years,andtheuseofevermoreappropriatebio-materials has successfully reduced the complication rate and improved the patient’s clinical outcomesand satisfaction. This review provides a comprehensive picture of the main advances that have beenmade in the development of innovative biomaterials for orbital implants and ocular prostheses. Specifi-cally, the advantages, limitations and performance of the existing devices are examined and criticallycompared,andthepotentialofnew,smartandsuitablebiomaterialsaredescribedanddiscussedindetailto outline a forecast for future research directions.   2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. 1. Introduction Datingbackthousandsofyears,thereisevidencethattheSume-riansandEgyptianswereabletosurgicallyremovetheocularglobeas well as to make artificial eyes; however, it was not until the late1500s that enucleation procedures were reported in detail in themedical literature [1]. The advances in this field progressed rela-tivelyslowly,andonlyin1885wastheuseofawell-definedorbitalimplant, a glass sphere, to restore the socket volume documented[2]. Improvements in surgical techniques, anesthesia, implantmaterials and design over the last decades have significantly im-proved clinical outcomes and patient satisfaction. Furthermore,the ability to more effectively deal with the long-term complica-tions of the anophthalmic socket, such as enophthalmos, exposureandlowerlidlaxity(ectropion),havegreatlyimproved.Today,mostpatients can confidently return to their daily activities with goodcosmetic results following the removal of an eye.Thisarticlechroniclestheevolutionoforbitalimplantsandocu-larprostheses,givesacomprehensiveoverviewofthecurrentstateoftheartandprovidesapictureforprospectiveresearch.Otherde-vices used in oculo-orbital surgery, such as the biomaterials fororbital floor repair, have been recently reviewed elsewhere [3–5]and are not included in the present work. Medical details are oftengiven, so that the reader can well understand the key problemsrelated to the use and applications of the described devices,the suitability and limitations of existing solutions, and thepotential of some novel approaches suggested at the end of thearticle.Thisreviewisdividedintothreeparts, devotedtopresentinganessential medical background, a comprehensive materials/im-plants review, and some indications for prospective research andfuture challenges, respectively. The first part, Section 2, gives thereader a concise overview of the surgical approaches that can beadopted to remove a diseased eye, as well as the basic informationrelated to orbital implants and ocular prostheses. In this context,Table 1 provides a short glossary of the medical terms that arenot explaineddirectly in the text and which may be unclear or un-known to non-specialist readers. In the second part, the differenttypes of biomaterials and devices used as orbital implants (Sec-tion 3) and ocular prostheses (Section 4) are extensively reviewed. At the end of Section 3, an organized and critical comparisonamong the several existing types of orbital implants is provided.The third part, Section5, presents the future challenges in the fieldand particularly highlights the potential of new experimental bio-materials with advanced properties (e.g. angiogenetic ability, con-trolled resorption, antiseptic functionality). 1742-7061/$ - see front matter   2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +39 011 090 4668; fax: +39 011 090 4624. E-mail address: (F. Baino).Acta Biomaterialia 10 (2014) 1064–1087 Contents lists available at ScienceDirect Acta Biomaterialia journal homepage:  2. Need for eye removal: aetiology and surgery  The removal of an eye or the orbital contents is one of the mostserious and difficult decisions that a patient and a surgeon mustconsider. The patient facing the loss of an eye has often undergonemultiple ophthalmic/orbital surgeries, experienced severe oculartrauma or been diagnosed with a potentially life-threatening dis-ease, such as an eye tissue tumour. Therefore, psychological sup-port before and after surgery is fundamental to these patients,who often feel depressed and overwhelmed [6].At present, the removal of a diseased eye can be carried out byone of a number of different surgical approaches, according to theparticular pathology and the medical history of each patient. Evis-ceration involves the removal of the intraocular contents of the eyewhile the sclera, Tenon’s capsule, conjunctiva, extraocular musclesand optic nerve are left intact [7]. Enucleation is another optioninvolving the removal of the globe from the orbital socket, togetherwith the scleral envelope and a portion of the optic nerve, while, aswith evisceration, the conjunctiva, Tenon’s capsule and extraocularmuscles are spared [8,9]. It has long been believed that eviscera-tion is superior to enucleation with regard to motility and cosme-sis; however, modern enucleation procedures, which involve thecareful attachment of extraocular muscles to the implant, actuallyrival those of evisceration in the preservation of motility of theartificial eye and cosmetic outcome. In the final stage of surgery,an orbital implant is placed within the scleral envelope after evis-ceration or within the Tenon’s capsule after enucleation; an ocularprosthesis will be then worn by the patient to restore an appropri-ate aesthetic appearance (Fig. 1). Unfortunately, recovery of thevisual function of the eye by implantation of what we mightideallyterm a ‘‘seeing artificial device’’ still remains a dream; nonetheless,the present surgical strategies are fully able to restore an accept-able cosmetic appearance and life-like motility to the prostheticeye.Removal of an eye can be necessary in the cases of intraocularmalignancy (e.g. retinoblastoma, which can develop especially inchildren), blind painful eye, prevention of sympathetic ophthalmiain a blind (or even seeing) eye, severe trauma, cosmesis and infec-tions not responsive to pharmaceutical therapy. In some caseseither approach can be adopted: from a general viewpoint, eviscer-ation is less invasive and less surgically complex than enucleationand can even be performed under local anesthesia, but some re-ports have demonstrated that the complication rate for eviscera-tion, specifically implant extrusion, may be significantly higher[10]. Evisceration is indicated in the treatment of active, uncon-trolled endophthalmitis and in all cases when there may be a dan-ger of intraocular infection spreading back along a cut optic nervesheath; however, enucleation may be indicated if the infection hasspread to the sclera. Evisceration is also recommended in patientswho cannot tolerate general anesthesia or have bleeding disorderssince it is a faster, easier procedure and damages fewer blood ves-sels than enucleation. Evisceration is absolutely contraindicated inthe presence of intraocular malignancy as it does not allow foreradication of tumour cells that have spread to the sclera. Enucle-ation is generally indicated for tumours that are confined to theocular globe; the exenteration procedure, which involves the re-moval of the entire orbit and surrounding structures and tissues,should be performed if the malignancy has spread to the  Table 1 Medical glossary (terms listed alphabetically). Term ExplanationConjunctiva Clear mucous membrane composed of stratified columnar epithelium that covers the sclera and lines the inside of the eyelids. Itcontributes to eye lubrication by producing mucus and tears, although in a smaller amount with respect to the lachrymal glands. Inaddition, it prevents the entrance of pathogenic agents and foreign bodies into the eyeEctropion Turning out of the eyelid (usually the lower eyelid) so that its inner surface is exposedEndophthalmitis Inflammatory condition of the intraocular cavities containing the aqueous/vitreous humour, usually caused by infection; panophthalmitisis the inflammation of all coats of the eye, including intraocular structures. Endogenous endophthalmitis results from the haematogenousspread of organisms from a distant source of infection; exogenous endophthalmitis results from direct inoculation of bacteria/fungi fromthe outside as a complication of ocular surgery, foreign bodies or penetrating ocular traumaEnophthalmos Recession of the eyeball or orbital implant inserted in the anophthalmic socket within the orbit; it may be a congenital anomaly, acquiredas a result of trauma (e.g. blow-out fracture of the orbit) or related to postoperative complications of oculo-orbital surgeryExposure Break in the tissue overlying the orbital implant, which in severe cases may lead to extrusion of the entire implant. Poor surgical technique,excessively large implant size and infection may all contribute to this postoperative complicationExtraocular muscles Group of six muscles, attaching to the sclera/orbital implant, which control the movements of the eye/implantHypo-ophthalmos Downward displacement of the ocular globe or orbital implant. Its causes and symptoms are similar to those observed for enophthlamosMigration Change in position of the implant following placement within the anophthalmic socketPegging Surgical procedure that can be optionally performed after some months from orbital implant placement in the anophthalmic socket(primary surgery). In this procedure, a hole is drilled into the front surface of the implant and a polymeric or metal peg is inserted into thishole. The peg articulates with a cavity in the back surface of the prosthetic eye, thereby providing improved motility. Pegging is usuallyonly adopted for porous implants, as fibrovascularization decreases the risk of infections that might follow the pegging procedurePyogenic granuloma Overgrowth of tissue due to irritation or physical trauma. Its appearance is usually a colour ranging from red/pink to purple and can besmooth or lobulated. Younger lesions are more likely to be red because of the high number of blood vessels, whereas older lesions begin tochange to a pink colour. It can be painful, grow rapidly and often bleed profusely with little or no traumaRetinoblastoma Rare type of eye cancer that affects the retina and usually develops in early childhood, typically before the age of 5 (it is typically diagnosedin children aged 1–2 years). Retinoblastoma is due to mutation of the RB1 gene, which can be inherited (in this case the tumour typicallydevelops in both eyes) or can occur in the early stages of foetal developmentSclera Opaque, fibrous, protective, outer layer of the eye. Primarily composed of collagen, it maintains the shape of the globe, offers resistance tointernal and external forces, and provides an attachment for the extraocular muscle insertions. The thickness of the sclera varies from1 mm at the posterior pole to 0.3 mm just behind the rectus muscle insertions. It is commonly referred to as the ‘‘white of the eye’’Superior sulcus deformity Result of the loss of orbital volume and relaxation of the soft tissues within the orbit. It is seen as a deep groove between the upper eyelidand the orbital rimSympathetic ophthalmia Bilateral diffuse granulomatous uveitis (a kind of inflammation) of both eyes following ocular trauma. It is quite rare but can leave thepatient completely blind; early symptoms (e.g. pain, photophobia) may develop from days to several years after a penetrating eye injuryTenon’s capsule Sheet of connective tissue that lines the eyeball and provides a smooth socket that allows the free movement of the ocular globeWrapping Preoperative strategy involving the wrapping of an orbital implant within a foil of a smooth material with the aim of facilitating itsplacement within the soft tissue of the eye socket, diminishing tissue drag and helping the precise fixation of the rectus muscles to theimplant surface. Wrapping is particularly recommended for porous implants in order to provide a physical barrier over their slightlyirregular porous surface. Suitable wrapping materials include scleral autografts and allografts, bovine pericardium and synthetic polymerfoils F. Baino et al./Acta Biomaterialia 10 (2014) 1064–1087   1065  extraocular tissues and structures (e.g. adjacent sinuses, craniumbone, face muscles and skin, conjunctiva and eyelids) [11]. Exen-terations vary in the amount of tissue removed and, apart frombeing indicated for the eradication of extended tumours, can be ap-plied in the case of otherwise unmanageable rhino-orbital infec-tions and, less commonly, severe orbital pain and deformity [12].After the socket has healed, silicone or acrylic custom-made pros-thetic devices can be constructed and attached to the orbit or skinwith various types of adhesives to provide an excellent cosmeticresult [13]. The use of osteointegration techniques, which involvethe permanent placement of bone-anchored titanium implants,can also be used to successfully support maxillofacial prostheticdevices [14]. 3. Orbital implants Over the centuries, a wide variety of materials has been used tomanufacture more or less rudimental orbital fillers with the aim of replacing the anophthalmic socket volume and restoring anacceptable aesthetic appearance to the patient’s face. The use of metals (e.g. gold, silver, platinum, stainless steel), substances of vegetal (e.g. wool) or animal (e.g. cork, ivory) origin and evenrock-derived materials (asbestos) has been documented [15]. Sincethe late nineteenth century, surgical procedures and materials tobe implanted have progressively moved to more defined standards,in order to avoid, or at least limit, the negative outcomes of a trial-and-error approach. Therefore, the term ‘‘orbital implant’’ has beenemployed to denote a properly designed, reproducible, often man-made device which is able not only to replace orbital volume butalso, hopefully, to ensure adequate motility to an aesthetic ocularprosthesis (artificial eye); in this review, particular emphasis willbe dedicated to recently developed solutions (approximately inthe last two decades) and related studies. The earliest orbital im-plants were simple spheres buried within the Tenon’s capsule[2]; the extraocular muscles were disinserted from the globe andleft to contract within the socket. Due to the limited movementof the overlying ocular prosthesis, surgeons began to perform mus-cle attachment to the implant to better anchor it, thereby reducingextrusion rates, and to allow conjugate movement with the contra-lateral normal eye.The attachment of the extraocular muscles to the implant, theimplant incorporation within the surrounding orbital tissues andthe implant–prosthesis interlocking have all become sources of confusion in terminology over the years. For instance, someauthors referred to ‘‘implant integration’’ as the simple attachmentof the extraocular muscles to the implant, whereas other research-ers defined integration as the mechanical contact between implantand ocular prosthesis. In order to solve this controversy, Sami et al.[15] recently suggested a three-type categorization (buried, ex-posed-integrated and buried-integrated implants) based on theassumption that integration specifically refers to the nature of fitbetween the ocular prosthesis and the implant, whereas attach-ment of the extraocular muscles to the implant does not implyintegration. In the present work, the authors propose a seven-typegeneral classification of orbital implants (Table 2) in the attempt totake into account all of the currently available implants, includingthe porous ones with their own peculiarities. According to thisclassification, some overlapping among the classes is unavoidable,depending on the context of use; for instance, a solid siliconesphere may be a simple non-integrated device but, if wrappedwithin a foil of biological tissue, it will become a non-integratedand biogenic implant. As research continues and new materialsare developed, in the next future even an eighth class (bioactiveimplants) might be added to Table 2, as shortly discussed at theend of the article in Section 5.2.1.The orbital implants developed over the years – available on themarketplace or currently abandoned – are collected in Table 3 withessential information for the reader’s benefit. To provide a shortoverview of the complex issues related to the design and selectionof suitable orbital implants, we must mention that an ideal implantshould display a number of characteristics, including biocompati-bility, adequate volume replacement, adequate support for theocular prosthesis, accessible cost for the patient, ease of implanta-tion, good motility transmitted to the ocular prosthesis and a lowrate of complications (e.g. postoperative infections). The use of non-toxic materials should be a mandatory precondition to pro-duce biocompatible implants. In order to fit the anatomic needsof each specific patient, including children, implants of differentsize are today available on the marketplace; the prices are quitevariable (from a few tens to several hundreds of euros in Europe),and mainly depend on the material employed and the implantstyle. Surgical implantation can be facilitated by wrapping the im-plant within a foil of a smooth material; this procedure is particu-larly recommended for porous implants characterized by a slightlyirregular porous surface. Over the years, different strategies havebeen developed to suture the extraocular muscles to the implantin order to improve motility; for instance, the muscle can be di-rectly and independently attached to the implant or sutured to-gether in front of it (imbrication). Different approaches have alsobeen tried to improve the motility of the ocular prosthesis, includ-ing pegging procedures and the use of magnets to guide the pros-thesis movement in accordance to that of the orbital implant.Infections following implant exposure are more amenable to Fig. 1.  Sagittal sections of a human orbit after enucleation surgery followed byplacement of a spherical implant that replaces the volume deficit created by eyeremoval. In these pictures, the extraocular muscles are sutured directly to theimplant. The ocular prosthesis is designed to fit in between the eyelids and theconjunctiva/implant in order to mimic the normal appearance of a healthy eye. Theconnection between orbital implant and ocular prosthesis can be indirect, due tothe interposition of the conjunctiva (a), or direct, by the use of a peg (b). Peggingprocedures are normally performed only in porous orbital implants. After somemonths from primary surgery, a hole can be drilled into the anterior section of theimplant; a peg is then inserted in the hole. Use of pegged implants leads to a greatertransmission of movement of the implant to the artificial eye, giving a more life-likeappearance.1066  F. Baino et al./Acta Biomaterialia 10 (2014) 1064–1087   treatment in porous implants, as vascular in-growth helps to an-chor the implant and permits immune surveillance. Therefore,the use of a porous implant is a good option in adults but is gener-ally discouraged in children, since the substitution of one implantwith another one of larger size may be necessary to stimulate ade-quate orbital growth. All of these issues will be critically discussedin the following sections to give the reader a comprehensive pic-ture about the features, advantages and limitations of each implanttype, as well as some criteria for implant choice.  3.1. Non-integrated implants 3.1.1. Glass In 1885, Mules placed the first orbital implant after evisceration[2], and one year later orbital implant placement after enucleationsurgery was described [16,17]. The Mules implantwas essentially ahollow blown glass sphere, and was commonly used until WorldWar II (WWII). Volume replacement by the Mules implant withinthe Tenon’s capsule was a significant advance, reducing socketretraction, intra-orbital fat redistribution and superior sulcusdeformity. Implants of different sizes were experimented withfor better fitting to the patient’s anatomy; it was also noted thatthe use of smaller and lighter devices led to decreased stress onthe lower lid and associated ectropion formation. Initially, theMules implant had high extrusion rates (50–90%), but the progres-sive improvement of surgical techniques led to reductions in thiscomplication, although still high compared to modern standards:Verrey reported an extrusion rate of 21% in 343 cases receivingthe Mules implant up to 1898, and in 1944 Burch reported failuresin less than 10% of 52 operations [18,19]. The major drawbacks of the Mules implant were the brittleness, as the implant could breakdue to trauma, and the risk of implosion due to sudden tempera-ture changes.Today, the use of glass spheres as orbital implants has been al-most totally abandoned, as other implants are available that gener-ally ensuring better outcomes. Glass has occasionally been used inrecent years: for example, in the late 1980s Helms et al. [20] im-planted a glass sphere (one patient) that underwent posteriorintracranial migration, and in the 1990s Christmas et al. [21] useda glass implant in a single patient without reporting any complica-tion after a 2 year follow-up.  3.1.2. Silicone For more than 50 years, silicone has been extensively proposedas a suitable material for various surgical applications due to itsattractive properties, including biological/chemical inertness, flex-ibility, ease of handling and low cost. For instance, episcleral im-plants made of solid or porous silicone are still today the onlydevices clinically approved and commercially available worldwidefor scleral buckling in retinal detachment surgery [22].Regarding orbital implants, a non-porous silicone sphere,as-such (bare) or wrapped, centred within the muscle cone andattached to the four rectus muscles, was the most common alter-native to Allen and Universal implants before porous implantswere introduced on the market at the end of 1980s. The use of anon-porous silicone sphere is still now a good option if peggingis discouraged or cannot be performed; however, although pros-thetic movement occurs, it is not as much as is seen with mounded(i.e. quasi-integrated) devices or pegged porous implants. In theview of many surgeons, a standard silicone sphere simply placedinto the orbit, without a wrap and without connection to the rectusmuscles, is the least desirable choice as it offers little movementand the implant is prone to migrate with time [23].Non-porous silicone spheres could also be preferred dependingon the patient’s age. In infants and preschool-aged children, awrapped silicone sphere centred within the muscle cone and con-nected to the four rectus muscles and inferior oblique muscle is of-ten recommended; implant exchange with a porous orbitalimplant that may potentially be pegged is then considered at a la-ter age (>15 years). Some surgeons also prefer to implant awrapped non-porous silicone sphere in aged patients (>65 years);porous implants are not used routinely in this age group, as expe-rience has suggested that these patients are often poor candidatesfor pegging because of their gradually failing health and the diffi-culty in maintaining regular follow-up visits [23].Excellent outcomes have been reported by independentlysuturing the recti to a 20 mm spherical silicone implant reinforcedwith autogenous fascia or preserved sclera: an extrusion rate of only 0.84% (1/119 patients over a 10 year follow-up period) andno cases of implant migration were reported [24]. This is an inter-esting achievements as, when muscles are imbricated over the sur-face of a spherical implant, implant migration tends to occur morefrequently with non-porous implants [25,26].  Table 2 Seven-type classification of orbital implants proposed in this article. Type Features ExamplesI: Non-integrated These implants do not usually contain any specific apparatus for attachment to theextraocular muscles, do not allow fibrovascular in-growth (they are non-porous) and have nodirect attachment to the ocular prosthesis (there is an uninterrupted conjunctival lining overthe implant)Silicone sphere, poly(methylmethacrylate)sphereII: Quasi-integrated(or, more rarely,semi-integrated)These implants are characterized by a specific apparatus for attachment to the extraocularmuscles and there is no interruption of conjunctival lining, but their irregular anterior surfaceallows the translation of movement to ocular prosthesis. There is no direct contact betweenorbital implant and ocular prosthesisCutler implant I, Allen implant, Iowa implants Iand II, Universal implantIII: MagneticallyintegratedThese implants are characterized by a magnet incorporated in the frontal part which allowsmovement transfer to the ocular prosthesis, which has another magnet placed on its posteriorsurface; the conjunctiva is sandwiched between the implant and the prosthesisRoper-Hall implantIV: MechanicallyintegratedThese implants are characterized by a specific apparatus for attachment to the extraocularmuscles and the conjunctival lining is interrupted in order to allow direct coupling of theimplant to the ocular prosthesis (e.g. by a peg)Cutler implant IIV: Porous These implants allow fibrovascular tissue in-growth and may or may not have direct couplingwith the ocular prosthesis, depending on the use of a peg systemHydroxyapatite, polyethylene and aluminiumoxide spherical implantsVI: Porous quasi-integratedThese implants are quasi-integrated devices made of porous materials, potentially allowingfibrovascular tissue in-growthGuthoff implant, polyethylene ‘‘quad’’ implantVII: Biogenic The implant as a whole is totally or partially composed of a biological graft (autograft,allograft, xenograft) or tissue. In the broadest sense, this class also comprises implantswrapped within or partially coated by biological tissue sheets (e.g. sclera, dermis), andtherefore can be interpreted as transversal to non-integrated and porous typesCancellous bone, implants of other type thathave been wrapped in biological material F. Baino et al./Acta Biomaterialia 10 (2014) 1064–1087   1067
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