Transcript Titanium Back Plate for the PMMA Keratoprosthesis

Titanium Back Plate for the PMMA
Keratoprosthesis – Clinical Outcomes
Claes H Dohlman, Amit Todani, Jared D Ament, James
Chodosh, Joseph B Ciolino, Kathryn A Colby, Roberto Pineda,
Michael W Belin, James V Aquavella, John M Graney
The Boston Keratoprosthesis is manufactured under the
auspices of the Massachusetts Eye and Ear Infirmary.
Dr. Dohlman, Dr. Chodosh, Dr. Colby, and Dr. Pineda are
full-time employees of this hospital.
Mr. Graney is the machinist.
(J.G.Machine Shop, Woburn, MA)
Introduction
Up until recently, the Boston Keratoprosthesis (BKPro) has been manufactured
exclusively of medical grade poly (methylmethacrylate) (PMMA). This material
was introduced to the budding field of keratoprosthesis already some sixty
years ago and results have been satisfactory with out overt toxicity. 1-4
The BKPro is shaped like a collar button, modified from previous designs.4-5
In the constant search for improvements, it can be questioned whether other
materials than PMMA might be superior. The stem of the device must obviously
still be made from a transparent material and likewise the front plate, to allow for
inspection of the central carrier cornea.
The back plate, on the other hand, which presents a large surface area facing
the anterior chamber, does not have to be transparent. Here, among several
possibilities, titanium comes to mind considering its widespread successful use
in joint replacement, tooth transplants, pacemakers, etc. Titanium has a proven
high tissue tolerance and it can be easily machined to thin, flexible plates with
still extraordinary strength.
Titanium is not new to the keratoprosthesis field. Thus Russian surgeons have
used a titanium plate for instrastromal positioning in patients.6 Also, in a more
recent rabbit study from Finland, instrastromal haptics of the same material was
employed.7
In-vitro tolerance
Is titanium more “tissue friendly” than PMMA in vitro? A recent study by JD Ament,
S Spurr-Michaud, CH Dohlman, and I Gipson has sought to answer that question.8 It was
shown in tissue culture using a corneal epithelial cell line that growth over PMMA occurred
much slower than over titanium even though neither material caused any distant toxicity in
the culture medium (see following graph).
MTT Assay: HCLE Cell Growth with PMMA vs. Titanium
Tissue culture: growth of corneal epithelial cells over PMMA
(bottom curve) and titanium (middle curve) (top curve control).
Cells grow better on titanium than on PMMA.
Clinical Impression
The Boston keratoprosthesis with titanium back plate was introduced at
the Massachusetts Eye and Ear Infirmary, Boston , in 2005. Since then a
total of 145 such devices have been implanted. The titanium back plate
has not yet been approved by FDA for general distribution.
The overall clinical impression has been favorable. It has been the
unanimous opinion of all collaborators that titanium seems to cause less
postoperative reaction than PMMA. Only three devices had to be replaced
– in autoimmune patients.
A measure of the degree of postoperative inflammation is the formation of
a retroprosthesis membrane (RPM). If it becomes sufficiently dense, vision
becomes impaired and the membrane will have to be opened with YAG
laser, rarely surgically. Here titanium and PMMA back plates have been
compared by their rate of triggering a retroprosthesis membrane. It is clear
that the use of a titanium back plate results in a statistically significant
reduction of RPM formation compared to a PMMA back plate, indicating
less postoperative inflammatory reaction (Todani, et al, unpublished data)
(see table below).
Retroprosthesis membrane formation with PMMA or
Titanium back plate at 6 month follow-up
Disease Category
Total cases
Total no. of RPM
Autoimmune
Chemical Burns
Others
6
8
25
2 (33.3%)
5 (62.5%)
11 (44.0%)
Overall
39
18 (46.1%)
Autoimmune
Chemical Burns
Others
3
2
11
2 (66.6%)
1 (50.0%)
2 (18.1%)
Overall
16
5 (31.2%)
Autoimmune
Chemical Burns
Others
3
1
19
1 (33.3%)
1 (100%)
1 (05.2%)
Overall
23
3 (13.0%)
GRAND TOTAL
78
26 (33.3%)
Group 1: Threaded PMMA
Group 2: Threadless PMMA
Group 3: Threadless Titanium
A: Titanium back plate B: PMMA back plate C: Assembled BKPro with
titanium back plate (corneal graft not included)
The titanium back plate can be made much thinner than the PMMA: 0.25
mm edge thickness for titanium vs. 0.9 mm for PMMA
Pre- and post-operative (3 years) appearance after Boston
Keratoprosthesis with titanium back plate.
The only disadvantage inherent in the use of titanium may be
cosmetic – it can result in a certain metallic sheen. Heavy
sandblasting during the manufacturing can diminish this
impression, however. Also, tinting or painting of the soft contact
lens that must be present anyway, can restore a more natural look.
There have been no patient complaints over the appearance of the
titanium.
Conclusion
Boston Keratoprosthesis back plates made of
titanium are more “tissue friendly” than those
made of PMMA (Ament, et al).
Clinically, by gross observation, titanium appears
to cause less postoperative reaction. The
incidence of retroprosthesis membrane
formation is less with titanium than with PMMA
back plates (Todani, et al).
Bibliography
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