A summary of the achievable dose release specification including the inside core position for the Cross-Linking of 55mm to 96mm Diameter UHMWPE Rods using THE 7-STATION TURNTABLE

Prepared by: Michael Eaton Position: QA Officer

Date:

Authorised by: John Harries Position: Quality Manager

Date:

METHOD

Orthoplastics Ltd's product consists of cylindrical Ultra High Molecular Weight Polyethylene rods, 500mm high with a diameter of 55mm to 96mm. The rods fit into aluminium irradiation containers, which have a height of 550mm and a diameter of 400mm. Between eight and sixteen rods fit in a circle in the irradiation containers (Figure 1), and there are 14 irradiation containers in 7 stations. The rods are held in place by an aluminium frame.



The irradiation containers were filled and placed onto the 7-station turntable in the continuous plant at Swindon. The containers were stacked with two layers (Figure 2), and half way through the processing time, the containers exchanged positions, with the bottom containers moving to the top, and the top containers moving to the bottom. Also at this point, all rods in every drum were rotated by 180 degrees. This reduced the dose variability across the height of the station and through the rods.



All rods in container one were dose mapped, using dosimeters placed as per figure 3. In all three validations the outside position was mapped on every rod. The core positions required the rods to be separated down the middle. Therefore the amount dose mapped was dependent upon the number sent in by the customer. This varied from two to eight rods. The dosimeters were arranged in three columns of three dosimeters. The first set of three dosimeters placed on the side of the rod facing the outside (A, B, C), the second set of three dosimeters placed on the inner facing side (D, E, F), and the third set of three dosimeters placed in the inside core region (H, I, G). As illustrated in Figure 3.



After the calculated mid point the dosimeters were removed and new ones put on in the same positions. This is required as the Red Perspex dosimeters being used have a dose range of 5-50kGy, and the current dose requirement is 75kGy.

During the Installation Qualification study it was concluded that all of the seven stations and 14 containers are statistically identical. Therefore validations were performed on one container only. Throughout the validation studies all the stations contained product, to ensure that the validation was representative of routine processing conditions.

The validation studies showed that the dose rate varies from between 0.39-0.44 kGy per hour. Due to this variability a dose check was introduced near to the end of each validation. This enables the delivered dose to be determined at that point. This allows an accurate estimation of process time to achieve the overall dose specification.

The top outside position is be used for the routine monitoring dosimeter. This plane displays the lowest variation in dosimeter values, it is also the most accessible for placement. A total of ten dosimeters should be place on separate rods. The mean of the ten dosimeters will be used to confirm if the overall dose specification has been met.

RESULTS

The results for dose received were averaged into top, middle and bottom for both the inside and outside positions. These average values were then used to calculate the minimum to maximum Ratio's and to calculate the most achievable release specification.

TABLE 1

The graph below illustrates the values in table 1. Between the yellow lines represents the variation of dose due to rod length and attenuation. This is constant and can't be improved. The area between the blue and yellow lines is the variation of the turntable. The area between the blue lines is the sum of all the variations. Outside of these lines is the area of achievable release specification.

SUMMARY

At a nominal specification of 75kGy, the positional variation in irradiation dosage for rods 55mm to 96mm ranges from +/-3.0 to 3.6kGy. This gives an achievable release specification of:

+/- 4.13kGy for the 55mm rods

+/- 4.88kGy for the 76mm rods

+/- 4.88kGy for the 96mm rods


The Operation Qualification and all validations have been completed in compliance with BS EN 552: 1994.