Inclusion Criteria:

• Be between 35 and 80 years of age.
• Must demonstrate at the level to be treated (L2/3, L3/4 or L4/5) all three of the following:
  • Degenerative spondylolisthesis or retrolisthesis up to Grade I, as determined by the investigator based on flexion/extension X-rays, and;
  • At least moderate lumbar spinal stenosis, defined as greater than a 33% reduction in either the central canal, the lateral recess space, and/or the foramen when compared to an adjacent level, as determined by the investigator based on MRI, and;
  • Thickening of the ligamentum flavum and/or scarring of the facet joint capsule as identified by the investigator based on MRI.
• Have had at least six (6) months of failed conservative treatment prior to surgery (e.g., physical therapy, use of anti-inflammatory medications at maximum recommended dosage; administration of epidural/facet injections and/or nerve block).
• Have an Oswestry Disability Index (ODI) score of at least 40/100 at baseline.
• Have leg pain with a VAS score of at least 40/100 for at least one leg at baseline.
  • The leg with the higher pain score will be considered “Worst Leg.”
• Neurogenic claudication (as defined by worsening leg/buttock symptoms when walking or standing, which is reduced when sitting or bending forward).
• Demonstrate worse symptoms (e.g., pain, numbness, burning sensation, pin prick sensation, etc.) in the legs/buttock than in the lower back.
• Be psychosocially, mentally and physically able to fully comply with the clinical protocol.
• Be willing to adhere to the follow-up schedule and protocol requirements.
• The patient is willing and able to understand and sign the study-specific, IRB approved consent form.

Exclusion Criteria:

• More than one (1) motion segment involved in the degenerative pathology that requires a surgical procedure.
• Presence of free fragment disc herniation or prior discectomy at the index level or either adjacent level.
• Less than 4mm of disc height at the index level.
• Spondylolisthesis greater than Grade I.
• Traumatic or dysplastic spondylolisthesis.
• Lytic spondylolisthesis.
• Back or non-radicular leg pain of unknown etiology.
• Stenosis caused by an extruded spinal disc fragment (e.g., herniation) or where the etiology is considered to be congenital, iatrogenic, post-traumatic, or metabolic.
• Known allergy or sensitivity to PEEK, titanium, cobalt chrome, and/or polyurethane.
• Prior surgery at any lumbar vertebral level with instrumentation.
• Prior surgery at the index vertebral level or either adjacent lumbar vertebral level without instrumentation [exception – prior intervention of posterior elements at index level (e.g., rhizotomy, laminectomy, foraminotomy and/or facetectomy)].
  • Prior intervention of posterior elements that involve the lamina, foramen, or and/or facets, the extent of which must not be greater than the decompression that would be necessary to implant TOPS.
• Clinically compromised vertebral bodies at the affected level due to any traumatic, neoplastic, metabolic or infectious pathology.
• Scoliosis greater than ten (10) degrees by major Cobb angle (both angular and rotational).
• Morbid obesity defined as a body mass index > 40.
• Osteoporosis (lumbar spine T score < -2).
  • All subjects will be screened for osteoporosis using an osteoporosis risk score (SCORE). Subjects with a SCORE value greater than 6 will be referred for DEXA Scan. DEXA must be performed within the 6 months prior to surgery.
• Paget's disease, gout, osteomalacia, osteogenesis imperfecta, thyroid and/or parathyroid gland disorder and/or any other metabolic bone disease that has not been stabilized with ongoing medication for at least one year.
• Active infection - systemic or local.
• Active hepatitis.
• AIDS, HIV, Rheumatoid arthritis or other autoimmune disease.
• Tuberculosis - active or in the past 3 years.
• Active malignancy - history of any invasive malignancy (except non-melanoma skin cancer) unless prior treatment with curative intent and there have been no clinical signs or symptoms of the malignancy for at least 5 years.
• Any medical condition requiring treatment with any drug known to potentially interfere with bone/soft tissue healing or receiving radiation therapy that is expected to continue for the duration of the study.
• Cauda equina syndrome or neurogenic bowel/bladder dysfunction.
• Vascular claudication due to severe arterial insufficiency of the legs (Prospective subjects will be screened by physical examination for diminution or absence of dorsalis pedis or posterior tibialis pulses. If diminished or absent by palpation, then an arterial ultrasound is required with vascular plethysmography. If the absolute arterial pressure is below 50 mm Hg at the calf or ankle level, then the patient has severe arterial insufficiency and must be excluded).
• Sustained pathologic lumbar fractures of the vertebra or multiple lumbar fractures of the vertebra or hip.
• Significant peripheral neuropathy causing decreased sensation in a stocking-like or non-radicular and non-dermatomal distribution in the lower extremities.
• Insulin-dependent diabetes mellitus (unless well-controlled defined as HbA1c < 7%).
  • HbA1c value must be within 3 months of screening.
• Immunologically suppressed, receiving steroids > 1 month out of the past year.
• Currently taking anticoagulants other than aspirin unless the subject can be taken off the anticoagulant prior to and during surgery.
• Life expectancy less than 3 years.
• Currently experiencing an episode of major mental illness (psychosis, major affective disorder, or schizophrenia), or manifesting physical symptoms without a diagnosable medical condition to account for the symptoms, which may indicate symptoms of psychological rather than physical origin.
• History of or current chemical/alcohol dependency.
  • A maladaptive pattern of substance use leading to clinically significant impairment or distress, as manifested by one (or more) of the following, occurring within a 12-month period:
    • Recurrent substance use resulting in a failure to fulfill major role obligations at work, school, home (e.g., repeated absences or poor work performance related to substance use; substance-related absences, suspensions, or expulsions from school; neglect of children or household);
    • Recurrent substance use in situations in which it is physically hazardous (e.g. driving as automobile or operating a machine when impaired by substance use);
    • Recurrent substance-related legal problems (e.g. arrests for substance-related disorderly conduct);
    • Continued substance use despite having persistent or recurrent social or interpersonal problems caused or exacerbated by the effects of the substance (e.g. arguments with spouse about consequences of intoxication, physical fights).
• Smoking habit of more than 1 pack of cigarettes per week and/or frequent users (>1/week) of chewing tobacco.
• Pregnant or interested in becoming pregnant in the next 3 years (due to need for Xrays).
  • However, pregnancies occurring during the study will not be considered protocol deviations. Additionally, if a subject does become pregnant, no X-ray or MRI should be taken during the pregnancy.
• Currently involved in active spinal litigation.
• Currently having a workman's compensation claim.
• Currently incarcerated.
• Participation in any other investigational drug, biologic or medical device study within the 30 days prior to the study surgery.