Pre-clinical evaluation of novel socket materials This paper will present the results of mechanical testing of novel rapid manufacture materials which we are assessing for possible use in prosthetic socket production. A comparison with the results obtained from testing of traditional socket materials will be drawn. This paper begins by discussing the advantages and disadvantages of traditional socket manufacture and the potential of CAD-CAM techniques. Brian McLaughlin, David Simpson & Arjan Buis

Traditional socket production Wrap casting Appling pressure to predetermined areas As you are all aware plaster of Paris bandages are used to capture the shape of the residual limb as the first stage of socket produced.

Traditional socket production Remove the cast from the patient Fill wrap cast with plaster of Paris slurry Remove bandages from the cast ~ destroys the original data Rectify cast

Traditional socket production This process creates a model which is not the same shape as the stump but which has been modified by artisan techniques ~ no record is available of these changes.

Traditional socket production Remove plaster model from socket Destroys the modified data.

Traditional socket production This method of design and fabrication is able to produce comfortable sockets. However, the process has drawbacks. No permanent record of the patient’s stump geometry nor the rectified cast. both the wrap cast and the positive mould are destroyed during the process.

CAD-CAM Scanning with a variety of scanners A digital model of the stump shape is stored On-screen rectification

CAD-CAM The file is usually sent to a central fabrication facility. Blank is cut by a milling machine. BUT the socket is still produced in the usual way using artisan methods and techniques. This traditional means of socket production over the computer generated model is still time consuming and costly in terms of the equipment, facilities and labour required.

But Considerable variability in the quality of sockets produced by different central fabrication facilities. Sanders et al CAD/CAM transtibial prosthetic sockets. JRRD, 44, 3 2007 Sanders et al 2007 ordered three different prosthetic sockets from 10 different central fabrications facilities (CFF). The sockets were then measured by a custom made digitiser and dimensions of each model were compared to the original data. Four of the CFFs consistently made sockets within +/- 1.1% volume of the original electronic shape file. Six of the CFFs showed consistent under sizing or over sizing of their sockets.

Rapid Manufacture RM is a relatively new class of manufacturing technology Has the potential to create a prosthetic socket directly from the CAD data.

Rapid Manufacture Material is deposited in thin horizontal sections to form the finished component.

Selective Laser Sintering 3D Printing Fused Deposition Modelling Rapid Manufacture Stereolithography Selective Laser Sintering 3D Printing Fused Deposition Modelling A number of Rapid Manufacturing technologies have been developed and some attempts have been made to use them to produce prosthetic sockets. Stereolithography is the selective setting of a liquid resin using laser technology. SLS is the selective fusing of powdered plastic materials using laser technology. 3D Printing uses a powdered material which is selectively bonded using inkjet printing technology to form the component. - This technology has been assessed at the University of Strathclyde for dimensional accuracy, mechanical strength and to ISO standards for trans-tibial prostheses. - It was found to be inadequate for commercial lower limb prosthetic socket production. FDM is selective deposition of plastic materials which are extruded in a semi-molten form, from a computer controlled nozzle. - This is the technology we are proposing to assess.

Fused Deposition Modelling Price of this equipment has fallen and it is becoming affordable. Strength of ABS is claimed to be similar to some materials that are currently used for socket fabrication. Our intention was to test this material against existing socket materials.

Specimen Preparation ABS Dimension Elite ABS M30 Polycarbonate Pre & Post draped Copolymer polypropylene Fibre reinforced acrylic resin (Blatchford trans-tibial lay-up) These are the FDM materials that were made available to us from which specimens were prepared. ABS+, ABS-M30, Polycarbonate. They were compared with pre and post-drape copolymer polypropylene and fibre reinforced acrylic resin.

Tensile Testing Five samples of each material were tested in an Instron tensile testing machine. Strain rate of 5mm/minute for all specimens. Stress was calculated using the dimensions at the point of fracture as measured prior to testing.

This slide shows the three FDM samples on the left all of which fractured during the testing. The draped and undraped polypropylene samples did not fracture and the tests were stopped when the samples had increase in length by 50% and the level of stress was not increasing. The materials contracted back to nearly their original length when removed from the testing machine. The final specimen, made using the typical Blatchfords trans-tibial lay-up of carbon fibre, stockinet, nyglass and 80:20 acrylic resin, fractured during testing and you can see the CF strands at the fracture site.

Results Material Maximum Strain Maximum Stress (Nmm-2) ABS – M30 0.12 30.2 ABS+ Dim EL 0.06 28.7 Polycarbonate 0.09 60 Undraped Copolymer 0.5 27.3 Draped Copolymer 28.1 Laminated Resin 0.07 262.5 This slide shows the averaged tabulated results for each of the materials tested. Points to note: The 0.5 strain for the copolymer which determined the end of the test. Draping the copolymer material does not appear to effect the mechanical properties of the material.

Click 1: This is the fibre reinforced resin which has an maximum tensile stress of almost 280Nmm-2. Click 2: This is the undraped copolymer with a maximum tensile stress of around 27Nmm-2. Click 3: This shows that the copolymer maximum tensile stress remains the same at around 28Nmm-2 when the material is draped. This graph shows the properties of the traditional socket materials. Please note the considerable difference between the copolymer and the fibre reinforced resin.

Click 1: Undraped copolymer Click 2: Draped copolymer Click 3: ABS+ Dimension Elite, Note: It is very similar to the copolymer samples. Click 4: ABS M30, Note: Is also very similar to the copolymer. Click 5: Polycarbonate, Note: Has a maximum tensile stress approaching 60 Nmm-2, which is double the copolymer samples and the other Rapid prototype samples. This graph shows the comparison between the FDM materials and the draped and undraped copolymer.

Summary The laminated resins are considerably stronger than the other materials tested – are they too strong? The draping process appears not to affect the strength of copolymer polypropylene. ABS+ and ABS-M30 materials have been shown to be of similar strength to copolymer. Polycarbonate seems to be twice as strong as copolymer and therefore the most appropriate material to continue evaluating.

Future Work Further evaluation of polypropylene as a suitable socket material Produce polycarbonate sockets for testing to the ISO standards Produce sockets for clinical trials, subject to ethical approval

The End Thanks to Laser Lines for supplying the FDM test samples