Thanks for the query to H.C.M. Forum.
Let me explain you that injection outside duramater is known as epidural injection and usually analgesic
injections are given and this is a symptomatic relief i.e. not treatment but for pain.
Now on main complaint, you mentioned that there is fusion of L 4 , L 5 S 1 bones in vertebral column.
The fusion may be due to ,
1 Ankylosing spondylitis
2 Ossification of posterior longitudinal ligament
( union of or fusion of these ligaments)
i.e ankylosis may unite these bones .
As there is fusion so the nerve passing from (through ) the canal of these bones will compress and there develops pain .
if compression more excruciating pain.
In my opinion consult orthopaedician / neurologist
and get their opinion and treatment after M R I and C T scan.