Ординатура / Офтальмология / Английские материалы / Biomaterials and regenerative medicine in ophthalmology_Chirila_2010

17

Orbital enucleation implants: biomaterials and design

D.A. Sami, Children’s Hospital of Orange County, USA; S. R. Young, California Pacific Medical Center, USA

Abstract: New biomaterials and design evolutions have improved rehabilitation, and reduced complication rates after enucleation. This evolution of thought and practice is the subject of this chapter. Topics covered include: orbital anatomy and physiology after enucleation, the current state of knowledge on implant motility, developments in the use of porous implants, and considerations for pediatric enucleation. A

comprehensive review of implant types, in relation to implant motility and biocompatibility, is included.

Key words: enucleation, orbital enucleation implants, socket implants, orbital implant motility, orbital biomaterials.

17.1Introduction

New biomaterials and design evolutions have improved rehabilitation and reduced complication rates after enucleation. The evolution of thought and practice, in relation to orbital enucleation implants is the subject of this chapter. A brief summary of the important themes and conclusions of the chapter are outlined here.

Animal studies and modern imaging techniques have changed our understanding of the physiology and anatomy of the post-enucleation orbit. Previous opinions (Soll 1982; Soll 1986) about the contribution of fat atrophy to orbital volume loss after enucleation have been challenged (Manson et al. 1986; Kronish et al. 1990a; Kronish et al. 1990b; Smit et al. 1990b). Following enucleation, there is a redistribution of intraorbital fat downward and forward in the anophthalmic orbit, with associated inferior displacement of the superior rectus-levator complex. (Soll 1986; Smit et al. 1990b). Placement of a spherical implant within Tenon’s capsule counteracts this change. This is true even when an implant is placed late after enucleation. (Smit et al. 1991b).

Since their introduction in the late 1980s, porous implant have become widely adopted by surgeons who perform enucleations in North America (Hornblass et al. 1995; Su & Yen 2004). Are the porous implants truly superior to non-porous implants? In general, it appears that the incidence of implant extrusion and socket infection is lower with porous implants. (Chuah et al.

434 Biomaterials and regenerative medicine in ophthalmology

2004). When muscles are imbricated over the surface of a sphere, implant migration occurs substantially more frequently with non-porous implants. (Allen 1983; Trichopoulos & Augsburger 2005). This supports theoretical considerations that vascular ingrowth helps to anchor the implant and permits immune surveillance. The rough surface of unwrapped hydroxyapatite implants appears to be associated with a higher exposure rate when compared with buried non-porous implants. Overall, donor sclera-covered hydroxyapatite implants appear to have higher late exposure rates than sclera-covered silicone implants. (Nunery et al. 1993b; Custer et al. 2003). Exposure rates for porous polyethylene implants wrapped in absorbable material appear to be similar to those for unwrapped porous polyethylene. (Li et al. 2001; Blaydon et al. 2003). Excellent outcomes have been reported by suturing the rectus muscles independently to a 20 mm spherical silicone implant reinforced with autogenous fascia or preserved sclera, with no cases of implant migration (an extrusion rate of 0.84% (1 of 119) over a 10-year study period) (Nunery et al. 1993a).

The basic science of the major factors that determine implant and ocular prosthesis motility is poorly understood. To date, no objective difference has been documented for implant or prosthetic motility with respect to porous and spherical alloplastic implants (Custer et al. 2003). Placement of a peg may improve horizontal excursions (Guillinta et al. 2003) but is associated with a significant rate of complications (chronic discharge, pyogenic granuloma formation and peg extrusion) (Jordan et al. 1999b). When enucleation is performed in infancy, implant exchange may be necessary to stimulate adequate orbital growth. As such, porous implants in young children (less than 2–3 years of age) are controversial, in that implant exchange is difficult after a porous implant has vascularized. Although there are only a handful of studies in children, scleral wrapped porous implants appear to have a reduced exposure rate compared with unwrapped implants (De Potter et al. 1994; Karcioglu et al. 1998; Christmas et al. 2000; Lee et al. 2000; Nolan et al. 2003).

17.2Historical perspective on enucleation

In the process of mummification, the ancient Egyptians would remove the eyes, fill the orbit with wax, and use precious stones to simulate the iris.

However, there are no recorded techniques of enucleation or evisceration in the living until the late sixteenth century in Europe (Kelley 1970; Luce 1970). This early technique was known as extirpation, which was, essentially, subtotal exenteration without anesthesia.

The first written record of the operation is credited to George Bartisch, published in his ‘Augendienst’ in Dresden, 1583 (Luce 1970). The operation was so painful and dangerous that it was rarely used. In preparation, the

Table 17.1 Materials used for enucleation implants

Agar

Aluminum

Asbestos

Bone

Cartilage

Cat gut

Cellulose

Charred bone

Coral (hydroxyapatite)*

Cork

Fascia lata

Fat*

Glass (single hollow ball or beads)

Gold

Ivory

Paraffin

Peat

Plastic

Platinum

Poly(methyl-methacrylate) (PMMA)*

Polyvinyl sponge

Rubber

Silicone (solid or inflatable)*

Silk

Silver

Stainless steel

Tantalum

Vaseline

Vitallium

Wool

*Biomaterials that are still in common use.

by human radiographic studies (Smit et al. 1990b). The associated inferior displacement of the superior rectus–levator complex produces a hollow and sunken appearance of the superior lid sulcus (superior sulcus deformity) (Culler 1952).

The net effect of volume loss from enucleation (inferior displacement of the superior rectus–levator complex, downward and forward distribution of intraorbital fat) is a rotation of intraorbital contents. (Smit et al. 1990b) (Fig. 17.3). The large ocular prosthesis necessary to replace volume has a characteristic shape: thin on the inferior edge, becoming thicker superiorly and temporally. Despite its compensatory shape, the ocular prosthesis is often tilted: depressed superiorly and pushing against the lower eyelid inferiorly (Fig. 17.3). Superiorly, the ocular prosthesis does not adequately support the SR–levator complex. This translates into variable amounts of ptosis and deepening of the superior sulcus. These features are responsible for the

often develops, which further compromises the fornix available to support the ocular prosthesis. This sets up a vicious cycle. As the lower lid and sulcus stretch inferiorly, the ocular prosthesis begins to sink, requiring a larger ocular prosthesis to re-establish normal appearance, which in turn places more weight on the lower eyelid.

Fat atrophy has previously been cited as a contributor to volume loss in the anophthalmic orbit (Soll 1982). This was based on thermogram studies showing a colder post-enucleation orbit (compared with the contralateral normal orbit), suggesting a presumed decrease in orbital metabolism, leading to fat atrophy (Soll 1982; Soll 1986). Alternatively, the change in the orbital thermal image may simply reflect removal of the eye, which has a rich uveal circulation. (Kronish et al. 1990a). More recent evidence based on animal and radiographic (human) studies suggest that accelerated fat atrophy is not a sequel of enucleation. (Manson et al. 1986; Kronish et al. 1990a; Kronish et al. 1990b; Smit et al. 1990b). Still, deepening of the superior sulcus over time, which is out of proportion to the contralateral normal eye, is well known. It may be that with age the normal atrophy of fat associated with both orbits becomes more apparent in an already volume-deficient socket.

Volume loss appears to be the major determinant of post-enucleation anatomic changes (Smit et al. 1990a; Smit et al. 1990b; Smit et al. 1991b).

Human radiographic studies have confirmed that placement of a spherical implant within Tenon’s capsule counteracts the post-enucleation rotation of intraorbital contents (and associated back-tilt of the ocular prosthesis). This is true even when the implant is placed late after enucleation (Smit et al. 1991b). Partial volume replacement permits a thinner ocular prosthesis, thus relieving weight on the lower eyelid and minimizing associated ectropion formation.

Traditionally, enucleation is thought to produce about a 7.0 ml loss of orbital volume, based on an average ocular axial length of 24 mm (Table 17.2). Recent studies suggest the average volume loss is higher: about 7.5–8.0 ml (Custer & Trinkaus 1999), emphasizing that there is substantial variability (5.5–9.0 ml) (Thaller 1997). A 20 mm spherical implant has a volume of 4.2 ml (Table 17.2). The remaining volume (about 3–4 ml) must be replaced by the ocular prosthesis. However, the physical dimensions of the palpebral fissure and conjunctival cul-de-sac, as well as problems associated with lower lid laxity produced by a heavy ocular prosthesis, limit the practical maximum size and volume of the ocular prosthesis. The average ocular prosthesis volume is 2.0–2.5 ml. A recent study suggested that the upper limit of prosthetic volume is about 4.2 ml (in the presence of a small implant). Interestingly, among patients with implant sizes of 14–22 mm and optimal prosthetic fit as judged by an ocularist, the average ocular prosthesis volumes were remarkably similar: 2.2–2.3 ml (Kaltreider 2000).

Thus when a small implant is used, the overall volume deficit may be even greater.

Volume deficit = [orbital volume loss from enucleation – (implant + prosthetic volume)]

Placing an implant with diameter >22 mm carries a higher risk of exposure in the early post-operative period – as Tenon’s capsule must be closed with greater tension (Kim et al. 1994). At the extreme end, a large implant (usually

>24 mm) will prevent the ocularist from fitting an artificial eye with enough antero-posterior depth (4 mm) to create a realistic anterior chamber depth (Neuhaus & Shorr 1982; Tyers & Collin 1985; Thaller 1997; Custer et al. 1999). In addition, crowding of the conjunctival fornices could restrict ocular prosthesis movement. In 1967, Soll devised an inflatable silicone implant filled with silicone gel (Soll 1969). Using a 30 gauge needle, saline or antibiotic solution could be injected centrally through a self-sealing area. The implant was designed to preferentially expand superiorly, to address superior sulcus deficit (Soll 1969; Gougelmann 1970).

The smaller overall diameter of the implant as compared with the natural globe, alters the functional length and pivot point of the levator complex (Tyers & Collin 1982; Gale et al. 1985) with possible associated decreased levator function and ptosis (Vistnes 1976; Vistnes 1987). Clinically, the situation may be improved by adding to the superior margin of the ocular prosthesis, to restore functional length and create a more anatomic pivot point for the levator muscle (Allen & Webster 1969; Vistnes 1987) (Fig. 17.5).

17.4Motility implants

442 Biomaterials and regenerative medicine in ophthalmology

Attachment of the extraocular muscles to the implant has since become a source of some confusion in terminology, particularly over the meaning of

‘integration’. This question was brought to the 2002 scientific panel of the

ASO and AAO (American Society of Ocularists and American Academy of

Ophthalmology): ‘The definition of integrated implants does not appear to be consistent. Some refer to integration as the attachment of the extraocular muscles to the implant. Others define integration by the nature of physical contact between the implant and ocular prosthesis. What is the correct nomenclature?’ The panel’s consensus was that integration refers to the nature of fit between the ocular prosthesis and implant. Attachment of the extraocular muscles to the implant does not imply integration.

Based on the above consensus statement, implants are best categorized (Gougelmann 1970) as follows:

1.Buried – uninterrupted conjunctival lining. Smooth apposition between implant and ocular prosthesis. Some refer to implantation of a sphere without muscle attachment, as ‘simple buried’.

2.Exposed integrated – interrupted conjunctival lining allowing direct coupling of implant to ocular prosthesis.

3.Buried integrated – no interruption of conjunctival lining. Irregular anterior surface of implant to improve translation of implant movement to ocular prosthesis.

In efforts to address the problem of implant exposure and extrusion, surgeons began to tie together the rectus muscles over smooth spherical plastic implants (i.e. imbrication of muscles – see Fig. 17.6). In addition to providing an extra layer of tissue for anterior closure, the tension of the imbricated muscles held the implant back, decreasing pressure/tension on the wound. This seemed like a logical approach, especially since it provided some motility to the implant as well. However, it proved to be problematic: a smooth implant with the rectus muscles imbricated over its anterior surface can slip between the imbricated muscles (Bosniak et al. 1989). Typically, the sphere migrates supero-temporally into the space between the superior and lateral rectus. (Allen 1983) (Fig. 17.6). Lee Allen attributed this propensity of migration to the presence of Lockwood’s ligament inferiorly and the retracted obliques nasally – i.e. the implant migrates in the path of least resistance. In order to minimize this problem, later implants (Allen, Iowa and Universal implants) incorporated various types of grooves to receive the rectus muscles.

The first true motility implants were introduced in the early 1940s

(Gougelmann 1970; Danz 1990). Their evolution mirrored rapid advances in the fabrication of ocular prosthetics. The battle casualties of the Second

World War created a large demand for artificial glass eyes, which were mainly produced in Germany. The war-time shortage of glass eyes imported