Driving is an important part of a modern life style. ACL injury is the most common ligamentous injury of the knee. However, there is a paucity of information about the pre and post-operative ability of an ACL injured knee to respond to stimuli for specific situation such as braking reaction in an emergency. Does an ACL unstable knee affect braking reaction time? If it does, is there a difference between left and right injured knee? When is it safe to resume driving after an ACL reconstructive surgery? Is there any simple clinical test to assess patient’s recovery after surgery?

Braking reaction time of 73 patients who underwent arthroscopic ACL reconstruction and 25 normal controls was prospectively studied using a computer-link automobile simulator. Majority of these patients had autologous hamstring tendon graft. Every patients and controls were tested pre-operatively, and every 2 weeks after surgery up to 8 weeks. At each time point, two clinical tests namely stepping and standing test were also performed.

The pre-operative results did not differ significantly between controls, left ACL group and right ACL group for the braking reaction time and the two clinical tests. Post-operatively, it took 6 weeks for braking reaction time of the right ACL group to be equivalent to that of the controls, compared to 2 weeks for the left ACL group. There were a strong corelation between the stepping and standing test and the braking reaction time at each time point.

Conclusion: an ACL unstable knee does not affect patient’s braking reaction time. After a right ACL reconstruction, patient should delay at least 6 weeks before resuming driving. However, patient may resume driving as early as 2 weeks after a left ACL reconstruction. The stepping and standing test can be used at follow-up to assess patient’s recovery after surgery and to suggest appropriate time to resume driving.