Robotic-Assisted and Computer-Navigated Arthroplasty

Degree of Robot Autonomy

• Active (autonomous): The robot performs the bone resection itself while the surgeon supervises. The prototype was ROBODOC for femoral canal preparation in THA. Concerns over soft-tissue injury, cost and inflexibility have made active systems largely historical.
• Semi-active (haptic / surgeon-guided): The surgeon holds and moves the cutting tool, but the robot constrains it to a pre-defined three-dimensional boundary (a "haptic envelope"), preventing cuts outside the plan. This is the dominant contemporary architecture (e.g. Mako).
• Passive (navigation): The computer plans and continuously displays alignment and resection data but imposes no physical constraint; the surgeon executes all cuts conventionally.

Image-Based vs Imageless

• Image-based: A pre-operative CT (occasionally MRI) builds a patient-specific 3D model on which the procedure is planned, and to which the patient is registered intra-operatively. Pros: detailed pre-planning. Cons: extra cost, logistics, and a radiation dose from the planning CT.
• Imageless: The anatomical model is generated intra-operatively by registering bony landmarks and kinematic axes (e.g. hip-centre by limb circumduction). Pros: no pre-op scan or radiation. Cons: accuracy depends on the quality of intra-operative registration.

Tracking and Registration

• Optical tracking: Infrared cameras follow reflective marker arrays mounted on bone-anchored pins - the most common method. Requires line-of-sight.
• Electromagnetic tracking: Avoids line-of-sight issues but is susceptible to metallic interference.
• Registration maps the patient's real anatomy onto the planning model; registration error is a key source of system inaccuracy and is operator-dependent.

Potential Advantages

• Alignment precision: Fewer mechanical-axis and component-position outliers, especially in deformed knees.
• Quantified soft-tissue balancing: Real-time gap and laxity data allow component position to be fine-tuned to the soft-tissue envelope - the likely mechanism behind any functional benefit.
• Enables modern alignment philosophies: Functional and kinematic alignment depend on the precise, reproducible bone cuts that robotics provides.
• Reduced soft-tissue releases: Planning component position to the native envelope can reduce the need for ligament releases.
• Education and reproducibility: Intra-operative data may flatten the variability between surgeons.

Limitations and Costs

• No proven outcome benefit: The decisive limitation - precision has not translated into better function or survivorship at high levels of evidence.
• Cost: Substantial capital outlay, disposables, and maintenance.
• Operative time: Randomised data consistently show longer operative duration, particularly during the learning curve.
• Learning curve: Registration, planning and workflow require dedicated training.
• Radiation: Image-based systems add a pre-operative CT dose.
• Registration error: System output is only as good as intra-operative registration.

Technology-Specific Complications

Beyond the usual risks of arthroplasty, tracker-pin fixation introduces distinct hazards that randomised trials have specifically monitored:

• Pin-tract infection
• Pin-site (stress-riser) fracture through a tracker-pin hole
• Peroneal nerve palsy and other neurovascular injury related to pin placement
• Patellar/extensor complications historically reported with some systems

These are uncommon but are the complications an examiner expects you to name as "specific to the technology."