Brief Answer:
INABILITY to move any part of the leg at this point- Call 911

Detailed Answer:
If it has been 12+ hrs. since the injection and you are either UNABLE to FULLY EXTEND the leg straight out due to muscle weakness, unable to bear weight due to spontaneous collapsing or buckling of the knee to include falls or near falls, your ankle easily rolls (not doing this prior), or you cannot do any type of LEFT TOE or HEEL raises that would be similar to the right side then, this is likely an UNEXPECTED complication of the injection which should be looked at immediately and preferably in an emergency setting. Ifyou are experiencing any type of incremental or escalating back pain that is worsening instead of getting better at this point then, again, get to an ER. There could be a a small bleed developing and the formation of a blood clot in the facet regions that were injected that is compressing the L4, L5, or S1 nerve roots.

If you have developed or seem to be developing increased numbness or tingling in any type of regional distribution below the knee on the left or even on the front or back of the thigh and this does not resolve or seem like it's getting significantly better within the next 12 hrs. then, again, this may represent an UNEXPECTED DIRECT injection of anesthetic and/or steroid during into a NERVE roots of either L4, L5, or S1. There are also many other branches of nerves coming off from those nerve roots that could've been accidentally contacted or injected by the needle.

In a study of a modest series of patients (600+) undergoing various facet injections from the neck to lumbar spine published in 2012 found that among the various SIDE EFFECTS of such injections was limb weakness and numbness with tingling. However, the average duration of this weakness was approximately 70 minutes. However, the range of time to recovery for these patients went from 10 minutes to 2 months therefore, it's hard to say at this time exactly how your symptoms will pan out in the long haul except to say that going for help and treatment after 24 hrs. of persistent symptoms IS NOT UNREASONABLE at all and the doctor's office should be VERY WILLING to get you in and examine you or at the very least get a neurologist involved right away to assess your left leg function.

If symptoms of weakness and numbness (heavy) or tingling (significant) continue into tomorrow then, I would definitely be calling the doctor's office and ask to be re-examined right away. The doctor may wish to get a NEUROLOGIST involved to perform a detailed motor and sensory examination on the left leg and if symptoms do not improve over the course of the next few days it would be reasonable to submit for an electrical study called an EMG/NCV evaluation.

In the mean time, if you have some type of ambulatory aid such as a cane, walker, or chair then, using that over the next 48-72 hrs. instead of trying to go places without the assistance is going to improve your chances of not falling and hurting yourself.

Take a look at the following short movie clip on FACET JOINT INJECTIONS which is rather sweet and to the point. Perhaps, you've already seen such a video prior to the procedure but take a closer look and see if you might understand why the injection needle COULD'VE accidentally crossed over or even into nerves since there are a TON of them crisscrossing and hanging around the area where FACET INJECTIONS are typically given. Unfortunately, the FLUOROSCOPE cannot "see" the multitude of networking nerves and branches that are in the vicinity of the facet joints. Also, anatomical variations are VERY COMMON abound and therefore, it is not surprising at all to think the needle could've penetrated something in the area of the facet joint. This is a somewhat blind procedure all those branches of nerves and blood vessels simply don't show up on the fluoroscope as the needle is placed into the facet joint area and injected.

https://www.summitortho.com/services/back-neck-spine/treatments/injections/lumbar-facet-joint-block/

Here is a 2nd decent summary of LUMBAR FACET INJECTIONS and what to expect following the procedure:

https://www.spine.org/KnowYourBack/Treatments/Injection-Treatments-for-Spinal-Pain/Lumbar-Zygapophysial-Joint-Injections

If I've provided useful and helpful information to your question could you do me the utmost of favors by CLOSING THE QUERY and TAKING A MOMENT to provide a few kind words of feedback with perhaps even a stamp of a 5 STAR rating if you feel so inclined?

Do not forget to contact me in the future at: www.bit.ly/drdariushsaghafi for additional questions, comments, or concerns having to do with this topic or others.

This query has utilized a total of 70 minutes of professional time in research, review, and synthesis for the purpose of formulating a return statement.

---