Dear Dr. Corenman,
Have you ever encountered HO in ADR patients that protrudes into the spinal canal? I had 2 level C5-7 ADR 3 years ago and a recent CT shows a central 3mm HO protrusion at the top of C6 that contacts, but does not compress the cord. Relative stenosis of 10mm at that point and the foremen are clear with the exception of some right facet arthrosis at C8/T1 that is little changed from a 4 y.o. MRI. None of the follow up X-rays show the HO protrusion so no idea on its progression. The literature on ADR & HO often shows posterior vertebral endplate HO, but rarely mentions the effect on the spinal canal. Symptoms are bilateral non-dermatomal arm/hand numbness/pain, bilateral leg/foot numbness/pain L5&L4 dermatomes mostly but not exclusively, heavy legs after brief walk and “wandering” right leg. I also had lumbar ADR with vertebroplasty and have cement extravasation in the center of the L4 & L5 vertebral bodies about 5mm wide and protrude 3-4mm indenting the thecal sac but not impinging on any nerve roots. Symptom free until 6 mo ago so the cement may be a red herring, but worse with lumbar flexion. Could a 3mm HO cervical protrusion compress the anterior cord tracts or artery enough to cause my symptoms? My herniations were larger and caused fewer symptoms.
The problems with ADRs is that these devices allows motion and motion can induce new bone formation. The heterotopic ossification may or n=may not affect the spinal cord. It really depends upon the motion of the segment. With greater motion (translation), the spur could abut the cord and cause myelopathy. See https://neckandback.com/conditions/cervical-central-stenosis-and-myelopathy/
The best way to determine if this is a possibility is with flexion/extension X-rays. Make sure you fully flex and extend when the images are acquired.