Driving comes back in stages, and researchers have timed the braking stage.

The studies measure brake reaction time

Most of the research uses a driving simulator. A patient sits with a foot on the accelerator and a signal appears. The machine times how long it takes to move to the brake and press it. Each patient is tested before surgery and again at intervals afterward, and the question is when braking returns to that patient's own baseline ¹.

Reviews of this work treat return to baseline as a floor, not a licence. Braking is one part of driving, and the reviews say so plainly. Reaction time is a surrogate, and surgeons must weigh it with the other factors that determine fitness to drive ¹².

After knee replacement, the right knee takes longer

A 2018 systematic review pooled nine studies with 330 patients. Braking measures returned to preoperative levels by about 2 weeks after a left knee replacement and by about 4 weeks after a right one ¹. The difference is mechanical. In a car with an automatic transmission the right foot works the brake, so the operated right leg is the one being tested.

A larger 2021 meta-analysis of 12 studies and 368 patients used finer measurements and found a more mixed picture. Total brake reaction time was not significantly different from baseline at any point up to 3 months. Movement time, the part spent shifting the foot to the brake, was slower early and no longer different from baseline from 6 weeks on. The authors concluded that simple reaction time is not a reliable indicator on its own ².

So the two reviews agree that braking recovers over weeks, and disagree on how finely it can be timed. Neither claims that every patient is ready at a fixed week.

Opioid pain medication and the law

The AAOS patient guide states that driving is safe to resume when a patient is no longer taking opioid pain medication. Strength and reflexes should have returned to a more normal state, and the physician sets the timing ³.

This is a legal matter as well as a medical one. The CDC states that driving while impaired by any substance is dangerous and illegal. Prescription and over-the-counter medicines can cause sleepiness, impaired vision and impaired coordination ⁴.

None of this is a reason to change a prescription on your own. How and when pain medication is reduced is a decision for the prescribing team. The driving question follows from it, not the other way round.

Why the surgeon's clearance is what counts

Every review on this topic ends the same way. Driving is a complex skill, and reaction time is a surrogate to be weighed with the other factors that determine fitness to drive ¹². Your surgeon holds the facts about you, including your car's transmission, the operated side and the medication you take; a study holds averages.

For knee replacement, the AAOS guide gives 4 to 6 weeks as the point when most people resume driving ⁵. The condition is that the knee bends enough to get into the car and sit comfortably ⁵. That figure describes most patients; it is not a date for any one of them.

What to ask your surgeon before the first drive

• Which side was operated on, and does my car's transmission change the advice? ¹
• Am I fully off opioid pain medication, and does anything else I take affect alertness? ³⁴
• Can I get in and out of the car and sit for the length of a normal trip without pain? ⁵
• Should the first drive be short, in daylight, on familiar roads, with someone else in the car?
• What would make you want me to stop driving again and call: new pain, swelling, or a reaction that feels slow?

A test drive in an empty car park, once cleared, tells both of you more than a calendar does.