Abstract
Introduction
In major orthopaedic departments, typically several total knee systems are used. Each system requires several sets of instruments, each set with many trays of complicated and expensive parts. The logistics and costs of maintainance are considerable. Our overall goal is to investigate the feasibility of autoclavable single-use 3D printed instruments made from a polymeric material, used for any type of total knee design. The procedure will be standardized and adjustments easy to implement. Each set will be packaged individually, and used for a single case. There are many aspects to this study; in this part, the aims are to identify suitable materials for autoclavability and strength, and then to compare the accuracy of a novel design of 3D printed tibial cutting guide with a current metallic guide.
Methods
Test samples were designed to simulate shapes in current instruments, such as mating pegs and holes, threaded screws, and slotted blocks. Each set was produced in biocompatible materials, ABS-M30i, VeroClear (MED610), Ultem1010, and Nylon 12. Each part was laser scanned, and then imaged virtually using a reverse engineering software (GeoMagic). Manual measurements of key dimensions were also made using calipers. The parts were autoclaved using a standardized protocol, 30 minutes at 250° F. All parts were re-scanned and measured to determine any changes in dimensions. To test for strength and abrasion resistance, the slotted blocks were pinned to sawbones model tibias, and an oscillating saw used to cut through the slot. A compact 3D printed tibial cutting guide was then designed which fitted to the proximal tibia and allowed varus-valgus, tibial slope and height adjustments. A small laser attached to the guide projected to a target at the ankle. Tests were made on 20 sawbones, and compared with 20 with a standard metal cutting guide. Digitization was used to measure the angles of the cuts.
Results
Prior to autoclaving, the mating parts of all parts were congruent, except for Nylon 12 which had processing debris in slots and screw threads. The ABS-M30i shapes became grossly deformed after autoclaving. The other materials experienced only small changes in dimensions without loss of overall shape, but the slot of the Nylon 12 block was stenotic, 1.4 mm compared to 0.9 mm before autoclaving. In saw blade testing, the VeroClear block fractured through the corner of the slot, while the Nylon 12 block deformed due to heating. The Ultem1010 block produced a small amount of debris, but maintained its shape without any structural damage. In the tests of the tibial cutting guide the 3D printed laser-guided tibial cutting guide resulted in a mean absolute error of 1.72°±1.31° and 1.19°±0.93°, for the tibial slope and varus-valgus respectively. For the conventional guides, these values were 3.78°±1.98° and 2.33°±0.98°, respectively. These measurements were found to be statistically significant with p values of 0.004 and 0.001, respectively.
Conclusions
Thus far, apart from patient specific cutting guides and trial components, 3D printing has had limited applications in total knee surgery. As cost containment remains prominent, the use of 3D printing to produce standardized instruments may become viable. These instruments would not require pre-op planning such as CT or MRI, yet allow patient-specific angular settings. Our results indicated that Ultem1010 is a promising material, while a novel tibial cutting guide showed higher accuracy than standard, as well as being quicker to use. These initial tests indicated the viability of 3D printed instruments, but further work will include design and evaluation of the other cutting guides, manufacturing logistics such as in-house or company- based, and economics.
