Inclusion Criteria:

• Subject shall have a unilateral amputated lower limb for no less than 12 months. If the amputation needed revision within 12 months, patient could be enrolled if investigator documents that the amputation site has healed and subject’s symptoms have stabilized.
• Post-amputation pain shall be chronic (persistent over 6 months) and resistant to pain medications with a documented history within the subject’s medical records.
• Subject shall have frequent and recurring pain defined as no less than 4 episodes of pain ≥ 5 (NRS) per week on average (to be confirmed with baseline pain diary).
• Subject’s typical pain episode should last no less than 60 minutes.
• Subject shall demonstrate response to two injections, one regional nerve block and the other saline. Response to the regional nerve block is defined as greater than or equal to a 50% pain reduction by NRS at 20 minutes from administration of Lidocaine. An allowable, non-therapeutic response to saline is defined as less than 30% pain reduction by NRS 15 minutes after administration. NRS must be ≥ 5 before first injection.
• Subject’s regimen of drug therapy for pain shall be stable for no less than 4 weeks prior to implant and shall not change without approval of investigator until after their Month- 3 visit. Subject shall sign a pain medication “contract” to confirm acceptance of guidelines for the use of pain medication.
• Subject agrees not to replace or alter their prosthetic (if applicable) until after their Month-3 (primary endpoint) visit.
• Subject is able to independently read and complete all questionnaires provided in English and use electronic diary during study.
• Subject is willing and able to provide informed consent and comply with all procedures and assessments required by study protocol.
• Subject, and caregiver if applicable, is able and willing to be available for study visits throughout the duration of the study; e.g., no planned relocation of residence or extended vacation during the study that would prevent compliance with study visit schedule.
• Subject shall be 21 years of age or older (FDA definition of non-pediatric) and legally able to provide written informed consent.

Exclusion Criteria:

• Implanted with an active implantable medical device (i.e., pacemaker).
• Confounding source of pain that interferes with reporting of limb pain.
• Uncontrolled diabetes.
• Spasticity preventing full range of motion of involved side .
• Extremely short stump; sits on end.
• Untreated psychological condition (i.e., borderline personality).
• Condition requiring MRI studies or diathermy after device implant.
• Life expectancy of less than 24 months.
• Progressive neurological disease (i.e, multiple sclerosis).
• Subjects with active local or systemic infection or immunocompromised.

Exclusion Criteria:

• Subject is currently implanted with any active implantable device including but not limited to: pacemaker, implantable cardiac defibrillator, implantable neurostimulator (e.g., peripheral or spinal cord stimulator), or implantable drug pump.
• Subject has a source of pain other than post-amputation pain (incl. dysesthesia, cancer- related, visceral, angina, migraine, causalgia) which in the opinion of the investigator may interfere with the reporting of post-amputation pain.
• Subject has medical contraindications to surgery, including but not limited to cardiovascular, pulmonary, renal, liver or hematological disorders, active inflammation, medical contraindication for general anesthesia (e.g., severe cardiopulmonary disease), compromised immune state (due to concomitant disease or medications such as chemotherapy or immunosuppressants), or anticoagulant medication that cannot be discontinued for perioperative period.
• Uncontrolled diabetes as defined by HbA1c > 8.0.
• Spasticity in their residual limb such that the subject cannot achieve volitional full range of motion (ROM) of joints on involved side.
• Subject has skin graft or severe scarring over targeted implant site or any anatomical conditions that would prevent placement of the Altius System components.
• Subject demonstrates an inability to discern differences in pain severity, report pain intensity and related information, or complete a pain diary.
• Subject has a suspected or known allergy to any materials of the Altius System in tissue contact or Lidocaine (necessary for injection screen).
• Subject has received therapeutic regional nerve block (e.g., anesthetic with steroid, and/or opioids) for post-amputation pain within 30 days prior to baseline visit.
• Subject’s usual seated posture includes sitting on the end of their stump.
• Subject is a woman who is not using adequate contraception, is pregnant or breastfeeding, or intends to become pregnant during the course of the study.
• Subject is currently participating or intends to participate in another investigational drug or device clinical study that may influence or interfere with the data that will be collected for this study.
• Subject has a condition requiring MRI studies or diathermy after device implantation.
• Subject has a history of any alcohol or substance abuse or dependence which has required prior medical treatment or intervention. Subject has active alcohol or substance abuse.
• Subject has a condition that, in the opinion of the investigator, would interfere with study compliance (incl. unresolved issues of secondary gain) or subject’s safety.
• Subject has a life expectancy of less than 24 months.
• Subject is diagnosed with or has untreated psychological conditions: borderline personality disorder, major depression disorder characterized by hospitalization within the prior year for a major depressive episode.
• Subject has current diagnosis of any progressive neurological disease such as multiple sclerosis, chronic inflammatory demyelinating polyneuropathy, rapidly progressive diabetic peripheral neuropathy, or any tumor of the nervous system.
• Subjects with active local or systemic infection, prior recurrent bacterial infection, those who are immunocompromised or have high risk of infection due to other comorbidities.