ABSTRACT
Purpose Chronic knee pain affects 25% of adults and significantly impairs functionality.1Total knee arthroplasty (TKA) is a common treatment modality for patients with chronic knee pain unresponsive to conservative management. Although studies have shown that, on average, patients who have undergone TKA experience improved quality of life, there is considerable variation in patient outcomes.2The persistence of pain after TKA suggests that the pain is not purely mechanical, as TKA corrects underlyingmechanical dysfunction. Rather,this pain is suspected to be multifactorial in origin, including a neuropathic component.3 Treatments for chronic knee pain post-TKA include pharmacologic management, with neuropathic medications and opioid analgesics, and interventional options such as radiofrequency ablation (RFA) of the genicular nerves. Problems with medical management include efficacy and negative side effect profiles. RFA is an effective option, however, up to 48% of patients do not experience 50% or greater relief of pain from RFA.4,5As a result, patients are left with few remaining treatment options that are safe and efficacious. Recently, spinal cord stimulation (SCS) has become an increasingly popular treatment modality for persistent pain. One case study has shown efficacy of SCS in reducing pain in a patient with chronic regional pain syndrome after TKA.6However, more evidence is needed to evaluate the efficacy of SCS in patients who have persistent chronic knee pain after TKA and multiple other interventions. Additionally, studies have not simultaneously evaluated opioid use in this population. Therefore, the goal of this case series is to evaluate the effect of SCS in patients who have persistent chronic knee pain after total knee arthroplasty (TKA) by analyzing the reduction in their daily opioid usage, subjectively reported pain and functional status. Methods Three patients received SCS placement for chronic knee pain following TKA. Prior to permanent SCS placement, each patient underwent medical management, diagnostic genicular nerve blocks, and possible radiofrequency ablation without improvement in their pain. Prior to permanent SCS placement, each patient underwent a one-week SCS trial, with improvement in their pain. Patients' charts were abstracted for demographic data, diagnoses, treatment history, and pain medications. Patients were evaluated at the following time points (when applicable): pre-SCS trial, one-week post-SCS trial, post-permanent SCS placement (short-term), and post-permanent SCS placement (long-term). Short-term was defined as a period of three months or less since permanent SCS placement, and longterm was defined as greater than three months since permanent SCS placement. To quantify daily opioid usage, morphine milligram equivalents (MME) were determined at each of the above time points. MME were calculated by applying conversion factors to each patient's medication list based on the Center for Disease Control and Prevention (CDC) Guidelines for MME. Patient 1 was a 56 year old female with a 10 year history of bilateral knee osteoarthritis who underwent bilateral TKA. Her pre-operative pain failed to improve following TKA, prompting a referral to outpatient pain management. Her pain continued despite pharmacologic management and a failed diagnostic genicular nerve block. Patient 2 was a 59 year old male with a history of bilateral knee osteoarthritis who underwent bilateral TKA. His knee pain of greater than 12 years returned two months after TKA, prompting a referral to outpatient pain management. He had significant chronic pain despite pharmacologic management, successful diagnostic genicular nerve block, and subsequent RFA. Patient 3 was a 41 year old female with chronic knee pain secondary to pigmented villonodular synovitis since 2005 who underwent bilateral TKA and left knee revision four years later. She continued to suffer from significant pain following TKA and revision and was seeking non-operative pain reduction. Her pain continued despite pharmacologic management and mult ple RFAs that became less efficacious with time. Results For patient 1, prior to the SCS trial, her total daily opioid use calculated to 80 MME/day. She described her pain as 10/10, required a cane for ambulation, and had difficulty getting around her house. At the end of the one-week SCS trial, the patient was not requiring any pain medications, totaling to 0 MME/day, and had substantial improvement in pain and functional status. After permanent SCS placement, in short-term assessment, the patient had a maximum of 15 MME/day. Additionally, she no longer required a cane to walk. At longterm evaluation, the patient had a maximum of 5 MME/day. Moreover, she described her pain as a 5/10 with 70% relief, and continued to ambulate independently without a cane. For patient 2, prior to the SCS trial, his opioid use was calculated to be 30 MME/day. He described his pain as 10/10 and stated that pain was affecting his sleep and daily activities. At the end of the SCS trial, the patient was no longer taking pain medication, resulting in 0 MME/day, and noted significant functional improvements. At short-term follow-up visits, following permanent SCS placement, the patient had reduced his opioid use, totaling to 20 MME/day. He endorsed 50% relief of pain, rated his pain as 7/10, and reported that he was able to exercise and lose weight due to reduced pain. At long-term follow up visits, the patient had not changed his opioid use, yielding 20 MME/day. He rated his pain as 6/10, felt 50% relief in his pain compared to before SCS placement, was exercising, and staying active. Prior to the SCS trial, patient 3 had to be weaned off of long-acting opioid medications, as her total daily opioid use calculated to 240 MME/day. She rated her pain as 10/10 and required a cane for ambulation. At the end of the one week trial, the patient was not taking any opioid medications and reduced to 0 MME/day, and she endorsed significant improvements in pain. Following permanent SCS placement, at shortterm follow up, the patient's daily opioid use totaled to 40 MME/day. However, she did not feel that she was having adequate coverage with the stimulator, rated her pain was 7/10, and noted trouble with stairs. Conclusions Prior to SCS placement, all three patients took opioids daily, reported subjective pain scores of 7-10/10, and had difficulty with ambulation and ADLs due to their pain. Following SCS placement, all three patients had lower daily MME values at short-term (<3 months) assessment, and patient 1 and patient 2 had lower MME values at long-term assessment (>3 months). Patient 1 had a reduction in MME by 93.7%, patient 2 by 33.3%, and patient 3 by 83.3%. Patient 3 had a delay in her SCS placement due to the COVID-19 pandemic and thus only has short-term data available presently. Nonetheless, all three patients reported less pain and greater functional status following SCS placement as compared to before. This case series suggests that SCS could serve as a promising treatment modality for patients with chronic knee pain after TKA. This treatment modality can help reduce daily opioid use, reduce subjective pain, and improve functionality.