Question 4
Which plastic is the most suitable for use as a medical screw?
Not quite. Nylon is strong and safe in the body, and the 2026 syllabus even lists medical implants under PA6. Against these four options, though, it does not break down, so it loses to polylactic acid, the only one absorbed as the bone heals. In 2024, 12.0% of Queensland students chose (A).
Not quite. Polypropylene is non-toxic and strong, which is why it tempts you. It never breaks down in the body, so a polypropylene screw stays until a surgeon takes it out. In 2024, 20.4% of Queensland students chose (B).
That is it. Step through the hints below to see why. In 2024, 60.6% of Queensland students chose (C).
Not quite. The 2019 syllabus listed PVC for car parts, electrical equipment, drainage pipe and furniture, not implants, and it does not break down in the body. In 2024, 6.6% of Queensland students chose (D).
A medical screw holds a broken bone together while it heals. It sits inside the body, so it must be non-toxic, and it must be strong enough to clamp the bone.
The best screw is one the body absorbs once the bone has healed, so the patient does not need a second operation to remove it.
The 2019 syllabus that 2024 students sat listed polylactic acid for “medical implants, e.g. screws, plates, pins, rods, mesh”. It listed PVC and polypropylene for car parts, electronics, pipes and laboratory equipment, never implants.
Neither PVC nor polypropylene breaks down in the body. That rules out (B) and (D).
Both are strong and safe in the body. Only polylactic acid is biodegradable, so only a PLA screw is absorbed as the bone heals.
QCAA’s 2020 marking guide listed these properties for PLA in medical implants: biodegradable, high tensile strength and non-toxic in solid form, along with resistant to chemicals. That is (C).
A medical screw must be non-toxic and strong, and the best one is absorbed once the bone heals. Polylactic acid is the only option that is biodegradable as well as strong and non-toxic, and the 2019 syllabus listed it for medical screws. The QCAA marking guide key is (C).
The most common wrong answer was (B), chosen by 20.4%. Polypropylene is non-toxic and strong, which makes it tempting, but it does not break down, so it cannot be the screw the body absorbs.
QCAA did not publish a band for this question. It reads as simple familiar because one recall step takes you from a medical job to its plastic. For 2026 the content has moved. Polylactic acid is not in the syllabus at all, polypropylene and PVC are only in Unit 2, which the external exam does not assess, and the syllabus now lists medical implants under polyamide, PA6. A 2026 question about an implant made from a Unit 4 plastic points to nylon, the distractor here. What transfers is matching the job’s demands to a listed property.
If I had to guess where you might lose marks, it would be mixing the two syllabuses. For 2026, medical implants sit under PA6, and the biodegradability argument belonged to PLA alone.
Only mark this done when you could do it without help. Reading the solution does not count.
Question wording and any figures are from the 2024 QCAA Engineering external assessment, © State of Queensland (QCAA) 2024, licensed under CC BY 4.0, and have been adapted, except where a third-party source is named. Tangent Tuition is not affiliated with the QCAA.
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