# Orthopaedic Implants and Instruments

> Orthopaedic implants and instruments for OT Technology: cortical and cancellous screws, DHS, IM nails, PMMA bone cement, C-arm safety and implant tracing.

- Canonical URL: https://prepelephant.com/topics/allied/operation-theatre-technology/orthopaedic-implants-instruments
- Exam / course: Allied Health · Subject: Operation Theatre Technology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Orthopaedic Implants and Instruments", PrepElephant, https://prepelephant.com/topics/allied/operation-theatre-technology/orthopaedic-implants-instruments

## 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.