Convert to ICD-10-CM: 
721.91 converts approximately to:
- 2015/16 ICD-10-CM M47.10 Other spondylosis with myelopathy, site unspecified
 ICD-9-CM Volume 2 Index entries containing back-references to 
721.91:
- Arthritis, arthritic (acute) (chronic) (subacute) 716.9  deformans (see also Osteoarthrosis) 715.9 deformans (see also Osteoarthrosis) 715.9 
  degenerative (see also Osteoarthrosis) 715.9 degenerative (see also Osteoarthrosis) 715.9 
  hypertrophic (see also Osteoarthrosis) 715.9 hypertrophic (see also Osteoarthrosis) 715.9 
  nodosa (see also Osteoarthrosis) 715.9 nodosa (see also Osteoarthrosis) 715.9 
  senile or senescent (see also Osteoarthrosis) 715.9 senile or senescent (see also Osteoarthrosis) 715.9 
 
- Compression spinal (cord) (old or nontraumatic) 336.9 spinal (cord) (old or nontraumatic) 336.9   by displacement of intervertebral disc - see Displacement, intervertebral disc by displacement of intervertebral disc - see Displacement, intervertebral disc
  nerve nerve   root NEC 724.9 root NEC 724.9     postoperative 722.80 postoperative 722.80 
    traumatic - see Injury, nerve, spinal traumatic - see Injury, nerve, spinal
 
   traumatic - see Injury, nerve, spinal traumatic - see Injury, nerve, spinal
 
  spondylogenic 721.91 spondylogenic 721.91
  traumatic - see also Injury, spinal, by site traumatic - see also Injury, spinal, by site   with fracture, vertebra - see Fracture, vertebra, by site, with spinal cord injury with fracture, vertebra - see Fracture, vertebra, by site, with spinal cord injury
 
 
 
- Myelopathy (spinal cord) 336.9  due to or with due to or with
  spondylogenic NEC 721.91 spondylogenic NEC 721.91
 
- Spondylosis 721.90  with with  disproportion 653.3 disproportion 653.3    affecting fetus or newborn 763.1 affecting fetus or newborn 763.1 
   causing obstructed labor 660.1 causing obstructed labor 660.1     affecting fetus or newborn 763.1 affecting fetus or newborn 763.1 
 
 
  myelopathy NEC 721.91 myelopathy NEC 721.91