Biomedical Engineering Reference
In-Depth Information
Rapid prototyping is a very effective technique for verification as it aids com-
munication between design engineers and orthopedic surgeons. Models of joint
replacement implants created using computer-aided design software can be pro-
totyped in plastic, using techniques such as three-dimensional printing, within a
few hours. The plastic models can then be used to size against a skeleton and to
check for functionality. The interactions of surgical instruments with the prototype
implant can also be investigated.
Risk analysis is an important stage in the design process to consider all the
hazards associated with a joint replacement implant (e.g., wear, lack of strength,
incorrect materials, etc.) and to reduce the risk as far as possible. A common
technique used in the risk analysis of joint replacement implants is failure
mode and effect analysis, which is a bottom-up approach where each part, and
then the assembly (including packaging, sterilization, and labeling), is consid-
ered [62]. In the risk analysis of possible failure modes, the effect of each
failure mode and the possible cause of the failure are identified. The occur-
rence, severity, and detection of each failure are then rated on a scale from 1
to 10. A risk priority number is then calculated by multiplying the three rat-
ings together. The design team must then decide if a possible failure mode is
acceptable or if ways need to be found to reduce it. Table 8.2 shows the re-
sults of a partial risk analysis for a Swanson finger implant that is described in
Section 8.4.2.
Once the final design has been produced, manufacture of the final implants for
preclinical testing is undertaken.
Table 8.2
Results of a risk analysis for the Swanson finger implant.
Possible hazard
Effect of hazard
Cause of hazard
O S D RPN
or failure mode
or failure
or failure
Fractures
Device does not
function as
intended
Fatigue failure
10
6
5
300
Crack initiated from sharp
edges of bone
10
6
5
300
Crack initiated from
abrasion with bone
10
6
5
300
Damaged during
implantation
265 0
Incorrect implantation
technique
265 0
Wear particles of
silicone
Silicone synovitis
Implant rubbing against
bone
8
6
7
336
Occurrence (O), severity (S), detect (D), risk priority number (RPN).
