Orthopaedic Implants and Instruments
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Direct answer
Bone is fixed with a small alphabet of hardware — screws that bite cortex or cancellous bone, plates that bridge or compress, intramedullary nails that splint long bones from within, and arthroplasty components cemented or press-fit into place — and the OT technician must know every letter by size and by name. Cortical screws, about 3.5 mm with fine threads, hold diaphyseal cortex; cancellous screws, about 6.5 mm with coarse threads, grip metaphyseal sponge; the 135-degree dynamic hip screw glides a lag screw down the femoral neck; and polymethyl methacrylate bone cement cures exothermically, so mixing is vacuum-assisted and implant lot numbers are recorded for life. Add power tools, reamers and the image intensifier, and the specialty's instrument discipline — and radiation discipline — becomes the exam's real subject.
What you must remember
- Screw logic: cortical screws fully threaded, fine pitch, about 3.5 mm (or 4.5 mm in large bones) needing overdrilled glide holes for compression; cancellous screws partially threaded, coarse pitch, about 6.5 mm, engaging metaphyseal bone.
- Plates: neutralisation, compression (DCP), buttress for tibial plateau, and locking plates (LCP/LISS) whose threaded screw heads lock into the plate — biological fixation for osteoporotic bone.
- Hip hardware: dynamic hip screw at a 135-degree barrel-plate angle for intertrochanteric fractures; cancellous lag screws or cannulated screws for femoral neck; cephalomedullary nails for unstable patterns.
- Intramedullary nails: reamed or undreamed femoral, tibial and humeral nails with interlocking screws — load-sharing splints for diaphyseal fractures.
- Wires and frames: Kirschner wires for small-fragment and paediatric fixation; external fixators spanning open fractures and damage control; tension band wiring for patella and olecranon.
- Bone cement (PMMA): methyl methacrylate polymerises exothermically, reaches temperatures that can threaten tissue, and is vacuum-mixed to reduce porosity; antibiotic-loaded cement is standard in revision and infection settings.
- Arthroplasty: cemented versus uncemented components; sizes, trials and the real implant staged before cement mixing begins — cement waits for no one.
- Power and imaging safety: drills and saws checked with battery charged; C-arm use governed by lead aprons (about 0.25–0.5 mm lead equivalent), distance, collimation and dosimeters under ALARA; implant lot numbers and sizes documented for traceability.
Setting up for a dynamic hip screw fixation
The fracture table is the first instrument: the patient positioned with the affected limb tractioned and the C-arm able to swing between anteroposterior and lateral without moving the patient — position confirmed on image before prepping. The technician stages the DHS set: awl and T-handle, guidewires, reamers over the guide, the 135-degree plate with its barrel, the lag screw and its tap, and the screwdriver set — each laid in the order the surgeon will ask.
Cement is not on this list, but in the arthroplasty theatre it rules the clock: monomer and polymer mixed under vacuum when the surgeon says so, not before; the dough packed pressurised into a clean, dry, bleeding-free bed; and the real component seated on its trial-confirmed size within the cement's working window. The technician also runs the tracing layer: K-wires and guidewires counted like swabs, and every implant's lot sticker into the operation note. When the C-arm runs, the team stands back, leaded, the beam returning only on request — orthopaedics accumulates dose faster than any other list.
Where students slip
The pairs that decide marks: cortical versus cancellous screws (fine full threads for dense cortex versus coarse partial threads for sponge — candidates swap them), and reamed versus undreamed nails or locking versus non-locking plates without the biological rationale. The cement question fails on physics: PMMA is not "glue" — it interdigitates mechanically, cures with an exotherm that can exceed safe tissue temperatures, and mixing early wastes the working time surgeons depend on. On C-arm safety, answers drift into vagueness; the specifics — lead apron thickness, distance and the inverse square law, collimation, dosimeter badges, and stepping back during exposure — are what examiners want named. And the Indian exam favourite: traceability. Implant lot numbers, sizes and manufacturers belong in the record, because a future revision surgeon's first question is what is in the bone.
Frequently asked questions
How do cortical and cancellous screws differ?
Cortical screws are fully threaded with fine threads (commonly 3.5 or 4.5 mm) engaging dense diaphyseal cortex; cancellous screws are partially threaded with coarse threads (commonly 6.5 mm) gripping porous metaphyseal bone.
What is a dynamic hip screw?
A 135-degree barrel-plate device whose sliding lag screw allows controlled collapse across an intertrochanteric fracture as it heals — impaction in place of rigid beam-loading.
Why is bone cement vacuum-mixed?
Vacuum mixing reduces cement porosity, improving mechanical strength and longevity of the cement mantle in joint replacement.
Which plate suits osteoporotic bone?
Locking plates (LCP/LISS), whose screws thread-lock into the plate to create a fixed-angle construct that does not depend on screw-bone purchase alone.
What radiation precautions govern C-arm use?
Lead aprons of about 0.25–0.5 mm lead equivalent, thyroid shields, dosimeter badges, maximum practical distance (inverse square law), collimation, and exposure only on request — ALARA in practice.