Titanium Back Plate for the PMMA Titanium Back Plate for the PMMA Keratoprosthesis – Clinical OutcomesKeratoprosthesis – Clinical Outcomes

Claes H Dohlman, Amit Todani, Jared D Ament, James Chodosh, Claes H Dohlman, Amit Todani, Jared D Ament, James Chodosh, Joseph B Ciolino, Kathryn A Colby, Roberto Pineda, Joseph B Ciolino, Kathryn A Colby, Roberto Pineda, Michael W Belin, James V Aquavella, John M GraneyMichael W Belin, James V Aquavella, John M Graney

The Boston Keratoprosthesis is manufactured under the auspices The Boston Keratoprosthesis is manufactured under the auspices of the Massachusetts Eye and Ear Infirmary. of the Massachusetts Eye and Ear Infirmary.

Dr. Dohlman, Dr. Chodosh, Dr. Colby, and Dr. Pineda are Dr. Dohlman, Dr. Chodosh, Dr. Colby, and Dr. Pineda are full-time employees of this hospital.full-time employees of this hospital.

Mr. Graney is the machinist. Mr. Graney is the machinist. (J.G.Machine Shop, Woburn, MA)(J.G.Machine Shop, Woburn, MA)

IntroductionIntroduction

Up until recently, the Boston Keratoprosthesis (BKPro) has been manufactured Up until recently, the Boston Keratoprosthesis (BKPro) has been manufactured exclusively of medical grade poly (methylmethacrylate) (PMMA). This material exclusively of medical grade poly (methylmethacrylate) (PMMA). This material was introduced to the budding field of keratoprosthesis already some sixty years was introduced to the budding field of keratoprosthesis already some sixty years ago and results have been satisfactory with out overt toxicity. ago and results have been satisfactory with out overt toxicity. 1-41-4

The BKPro is shaped like a collar button, modified from previous designs.The BKPro is shaped like a collar button, modified from previous designs.4-54-5

In the constant search for improvements, it can be questioned whether other 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 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 still be made from a transparent material and likewise the front plate, to allow for inspection of the central carrier cornea.inspection of the central carrier cornea.

The back plate, on the other hand, which presents a large surface area facing 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 the anterior chamber, does not have to be transparent. Here, among several possibilities, titanium comes to mind considering its widespread successful use possibilities, titanium comes to mind considering its widespread successful use in joint replacement, tooth transplants, pacemakers, etc. Titanium has a proven 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 high tissue tolerance and it can be easily machined to thin, flexible plates with still extraordinary strength. still extraordinary strength.

Titanium is not new to the keratoprosthesis field. Thus Russian surgeons have Titanium is not new to the keratoprosthesis field. Thus Russian surgeons have used a titanium plate for instrastromal positioning in patients.used a titanium plate for instrastromal positioning in patients.66 Also, in a more Also, in a more recent rabbit study from Finland, instrastromal haptics of the same material was recent rabbit study from Finland, instrastromal haptics of the same material was employed.employed.77

In-vitro toleranceIs 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).

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.

MTT Assay: HCLE Cell Growth with PMMA vs. Titanium

Clinical ImpressionClinical Impression

The Boston keratoprosthesis with titanium back plate was introduced at The Boston keratoprosthesis with titanium back plate was introduced at the Massachusetts Eye and Ear Infirmary, Boston , in 2005. Since then a the Massachusetts Eye and Ear Infirmary, Boston , in 2005. Since then a total of 145 such devices have been implanted. The titanium back plate total of 145 such devices have been implanted. The titanium back plate has not yet been approved by FDA for general distribution.has not yet been approved by FDA for general distribution.

The overall clinical impression has been favorable. It has been the The overall clinical impression has been favorable. It has been the unanimous opinion of all collaborators that titanium seems to cause less unanimous opinion of all collaborators that titanium seems to cause less postoperative reaction than PMMA. Only three devices had to be replaced postoperative reaction than PMMA. Only three devices had to be replaced – in autoimmune patients.– in autoimmune patients.

A measure of the degree of postoperative inflammation is the formation of A measure of the degree of postoperative inflammation is the formation of a retroprosthesis membrane (RPM). If it becomes sufficiently dense, vision a retroprosthesis membrane (RPM). If it becomes sufficiently dense, vision becomes impaired and the membrane will have to be opened with YAG becomes impaired and the membrane will have to be opened with YAG laser, rarely surgically. Here titanium and PMMA back plates have been laser, rarely surgically. Here titanium and PMMA back plates have been compared by their rate of triggering a retroprosthesis membrane. It is clear 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 that the use of a titanium back plate results in a statistically significant reduction of RPM formation compared to a PMMA back plate, indicating reduction of RPM formation compared to a PMMA back plate, indicating less postoperative inflammatory reaction (Todani, et al, unpublished data) less postoperative inflammatory reaction (Todani, et al, unpublished data) (see table below).(see table below).

Retroprosthesis membrane formation with PMMA or Retroprosthesis membrane formation with PMMA or Titanium back plate at 6 month follow-upTitanium back plate at 6 month follow-up

Disease CategoryDisease Category Total casesTotal cases Total no. of RPMTotal no. of RPM

Group 1: Threaded PMMAGroup 1: Threaded PMMA

Autoimmune Autoimmune 6 6 2 (33.3%)2 (33.3%)Chemical Burns Chemical Burns 8 8 5 (62.5%) 5 (62.5%) Others Others 25 11 (44.0%) 25 11 (44.0%)

OverallOverall 3939 18 (46.1%)18 (46.1%)

Group 2: Threadless PMMAGroup 2: Threadless PMMA

Autoimmune Autoimmune 3 3 2 (66.6%)2 (66.6%)Chemical Burns Chemical Burns 2 2 1 (50.0%)1 (50.0%)Others Others 11 11 2 (18.1%)2 (18.1%)

OverallOverall 1616 5 (31.2%)5 (31.2%)

Group 3: Threadless TitaniumGroup 3: Threadless Titanium

Autoimmune Autoimmune 3 3 1 (33.3%)1 (33.3%)Chemical Burns Chemical Burns 1 1 1 (100%)1 (100%)Others Others 19 19 1 (05.2%) 1 (05.2%)

OverallOverall 2323 3 (13.0%)3 (13.0%)

GRAND TOTALGRAND TOTAL 7878 26 (33.3%)26 (33.3%)

A: Titanium back plate B: PMMA back plate C: Assembled BKPro with A: Titanium back plate B: PMMA back plate C: Assembled BKPro with titanium back plate (corneal graft not included)titanium back plate (corneal graft not included)

The titanium back plate can be made much thinner than the PMMA: 0.25 The titanium back plate can be made much thinner than the PMMA: 0.25 mm edge thickness for titanium vs. 0.9 mm for PMMAmm edge thickness for titanium vs. 0.9 mm for PMMA

Pre- and post-operative (3 years) appearance after Boston Pre- and post-operative (3 years) appearance after Boston Keratoprosthesis with titanium back plate.Keratoprosthesis with titanium back plate.

The only disadvantage inherent in the use of titanium may be The only disadvantage inherent in the use of titanium may be cosmetic – it can result in a certain metallic sheen. Heavy cosmetic – it can result in a certain metallic sheen. Heavy sandblasting during the manufacturing can diminish this sandblasting during the manufacturing can diminish this impression, however. Also, tinting or painting of the soft contact impression, however. Also, tinting or painting of the soft contact lens that must be present anyway, can restore a more natural lens that must be present anyway, can restore a more natural look. There have been no patient complaints over the appearance look. There have been no patient complaints over the appearance of the titanium.of the titanium.

ConclusionConclusion

Boston Keratoprosthesis back plates made of Boston Keratoprosthesis back plates made of titanium are more “tissue friendly” than those titanium are more “tissue friendly” than those made of PMMA (Ament, et al).made of PMMA (Ament, et al).

Clinically, by gross observation, titanium Clinically, by gross observation, titanium appears to cause less postoperative reaction. appears to cause less postoperative reaction. The incidence of retroprosthesis membrane The incidence of retroprosthesis membrane formation is less with titanium than with PMMA formation is less with titanium than with PMMA back plates (Todani, et al).back plates (Todani, et al).

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