Medical Policy

This document addresses the use of an implantable shock absorber device (eg., MISHA^™ Knee System [Moximed, Inc., Fremont, CA]) for the treatment of osteoarthritis of the knee.

Note: For a high-level overview of this document, please see “Summary for Members and Families” below.

Investigational and Not Medically Necessary:

Use of an implantable shock absorber device for treatment of osteoarthritis of the knee is considered investigational and not medically necessary.

This document describes clinical studies and expert recommendations, and explains whether use of an implantable shock absorber device is appropriate. The following summary does not replace the medical necessity criteria or other information in this document. The summary may not contain all of the relevant criteria or information. This summary is not medical advice. Please check with your healthcare provider for any advice about your health.

Key Information

An implantable shock absorber device, like the MISHA Knee System, is a treatment being studied for people with osteoarthritis in the inner (medial) part of the knee. Osteoarthritis is a disease that causes damage to the cartilage that cushions joints. The shock absorber device is implanted under the skin outside the knee joint and connects the thigh bone to the shin bone using plates and screws. It helps reduce pressure on the damaged part of the knee. This device is intended for people who have tried other treatments without relief and who may not want or qualify for knee replacement surgery. The implant may wear out or fail over time, and because it is placed on one side of the knee, it could cause uneven pressure and wear in the joint. While early studies show some promise, researchers have not yet done the kind of high-quality studies needed to know for sure whether this device works better than other options or improves long-term health.

What the Studies Show

One study compared the implant to a surgery called high tibial osteotomy. After 2 years, more people who got the implant met goals like pain relief and avoiding further surgery. They also could walk sooner after surgery. However, the study had limits. It was not randomized and used people from past records as the comparison group, which could affect the study results. Another small study looked at people with a type of knee damage called subchondral insufficiency fracture (SIFK). It found that none of the people who got the implant needed knee replacement after 2 years, while many in the non-surgery group did. Another study also found lower rates of knee replacement with the implant, but it was based on past data and had design issues. A third report followed people for 5 years. It showed that 9 out of 10 people did not need further surgery during that time and that pain and physical function improved. But this study did not include a group who did not get treatment with the device to compare to. Overall, while early results look positive, better studies are needed to know if this treatment improves health and how it compares to standard surgery or other treatments over the long term.

Is this clinically appropriate?

This treatment is not appropriate because it has not been proven to improve health. Most studies so far have been small or used weak study methods, such as lacking a comparison group, comparing people from different time periods, or no random assignment to groups. These types of studies can be biased and provide results that do not reflect the true effects of the use of a device. Also, there are concerns about the long-term safety of the device. For example, it uses screws that may fail and can cause uneven stress on the knee. Better studies are needed to know whether this type of device leads to long-term health improvement, including less pain and fewer surgeries.

(Return to Description/Scope)

Summary

The implantable knee shock absorber device has been studied as a less invasive alternative to high tibial osteotomy and knee arthroplasty for individuals with medial compartment knee osteoarthritis. The implantable shock absorber is implanted outside the knee capsule and extends from the distal femur to the proximal tibia. It employs a shock absorbing mechanical system and is stabilized by plates and screws. The use of screws may lead to failure over time. Also, since the device is anchored on one side of the knee joint it may lead to asymmetric wear and tear on the knee. The currently available literature addressing the safety, efficacy and clinical utility of implantable shock absorbers for knee osteoarthritis consists predominantly of retrospective matched control studies and a lack of head-to-head comparisons between treatment modalities. Further study is necessary to assess improvement in net health outcomes of this type of device.

Discussion

Osteoarthritis is caused by damage or breakdown of joint cartilage between bones. Treatment may be non-surgical or surgical. Surgical treatment can include high tibial osteotomy or total knee arthroplasty. Individuals, particularly young individuals, may not be eligible for, or do not want to have knee arthroplasty. An implantable shock absorber device, the MISHA Knee System, has been developed and is being studied as an alternative to knee arthroplasty. The device is placed subcutaneously alongside the knee joint to help reduce the amount of load carried by part of the joint.

The shock absorber was compared to high tibial osteotomy in a non-randomized study, with results reported by Diduch (2023). In this prospective, open-label cohort study, 81 participants with osteoarthritis of the medial compartment of the knee and a failure of ≥ 6 months of nonsurgical treatments received implantation of the shock absorber and were compared to 81 participants in a historical control arm who received high tibial osteotomy surgery. Participants were followed for 24 months. The primary endpoint was the proportion of individuals in each arm that met 5 criteria at 24 months: responder for Western Ontario and McMaster Universities Arthritis Index (WOMAC) pain score, responder for WOMAC function score, no deep infection requiring surgical intervention, maintained integrity of the implant or hardware, and no requirement of conversion to arthroplasty or other joint-modifying surgery. At the 24-month follow-up, there were 85.6% of participants in the implantable shock absorber arm and 65.5% of participants in the high tibial osteotomy group who achieved the primary endpoint. Time to full weightbearing in the implantable shock absorber group was 13.4 days (± 10.12 [SD]) and 58.0 days (± 39.91) in the high tibial osteotomy group. Baseline mean WOMAC pain rating was 60.4 (± 12.37 [SD]) in the implantable shock absorber group and 52.9 (± 13.1) in the high tibial osteotomy group. At 24 months, the mean pain rating was 14.2 in the implantable shock absorber arm and 19.9 in the high tibial osteotomy arm. Baseline mean functional WOMAC score at baseline was 59.9 in the implantable shock absorber arm and 48.4 in the high tibial osteotomy arm. The 24-month scores were 15.4 in the implantable shock absorber arm and 20.1 in the high tibial osteotomy arm. There were no mechanical device malfunctions in either arm. There were 11/81 implant removals in the implantable shock absorber arm and 61/81 removals in the high tibial osteotomy arm. Reasons for removal of the shock absorber included infection, catching/pulling sensation, pain, scar formation, and dissatisfaction. In the high tibial osteotomy group, reasons for implant removals included bone consolidation, pain, swelling, and nerve injury/neuropathy. Limitations of the study include the open-label, non-randomized design and use of historical controls which could have biased outcomes.

Subchondral insufficiency fracture of the knee (SIFK) is associated with high rates of knee osteoarthritis and arthroplasty. A 2023 study by Pareek compared freedom-from-arthroplasty in those with medial compartment knee osteoarthritis and SIFK who received an implantable shock absorber to a matched cohort of individuals treated non-surgically. The authors also reported on the ability of SIFK scores to predict the 2-year rate of arthroplasty in both groups. The participant population in the implantable shock absorber arm were selected from the enrollees in the Diduch 2023 study discussed above. The control group of participants were taken from a retrospective review of charts and were matched with a 1:1 ratio. SIFK scoring uses radiographic findings of individuals with SIFK to predict the likelihood of progression to arthroplasty. An SIFK score is calculated based on 5 clinical parameters: (1) lateral meniscus extrusion, (2) lateral meniscus root tear, (3) Kellgren-Lawrence (K-L) Grade 4, (4) SIFK on the medial femoral condyle, and (5) medial meniscus extrusion. The maximum possible SIFK score (highest risk) is 11 points. This study was comprised of 21 individuals in each arm. At the 2-year follow-up, 19% (8/21) of participants progressed to knee arthroplasty, all from the control group. For those in the control group, the median 2-year arthroplasty-free survival probability was 55% vs. 100% in the implantable shock absorber group. This study is limited by the lack of a prospective control group, blinding, and other methodological factors.

Another study by Pareek (2024) evaluated whether the use of an implantable shock absorber altered likelihood that an individual with SIFK will progress to knee arthroplasty. This was a retrospective case-control (2:1) study comparing those with SIFK who received the implanted shock absorber (n=19) compared to those who did not receive surgery (n=38). In the control group, there were 34% of participants who progressed to knee arthroplasty, with 16% of participants progressing by 2 years. In the implantable shock absorber group, none of the participants progressed to knee arthroplasty. Overall survival rates were 77% at 2 years and 52% at 5 years. In this study, the majority of the participants had received prior surgical procedures including partial meniscectomy and chondroplasty. This previous surgical management may have had an effect on the natural history of progression to arthroplasty.

Gomoll (2023) reports the 5-year outcomes of three single-arm studies of 171 individuals who had insertion of an implantable shock absorber for treatment of medial knee osteoarthritis after a failure of ≥ 6 months of conservative treatment. Primary outcome was treatment survival rate without a conversion to knee arthroplasty or high tibial osteotomy. Secondary outcomes included changes in WOMAC pain and function scores after implantable shock absorber insertion. Overall treatment survivorship (freedom from conversion to arthroplasty) was 90.6% (155/171). The mean time to conversion was 2.1 years. Mean baseline WOMAC pain score was 57.6 with the latest follow-up score of 16.0. Mean baseline WOMAC function score was 55.7 with the latest follow-up score of 16.9. Limitations include the single-arm study design.

Osteoarthritis is the most common type of arthritis. It occurs most frequently in the hands, hips, back, and knees. It is estimated that osteoarthritis affects over 33 million adults in the United States (Centers for Disease Control and Prevention [CDC], 2024). Symptoms of osteoarthritis include joint pain, stiffness, and swelling of the joint. Treatment can include physical therapy, medications, and surgery.

On April 10, 2023, the United States Food and Drug Administration (FDA) authorized marketing of the MISHA Knee System as a medial knee implanted shock absorber for treatment of knee osteoarthritis for those who have failed surgical or non-surgical treatment modalities. The FDA clearance was based on a determination of substantial equivalence to previously classified devices. This device is implanted under the skin alongside the knee joint and is intended to provide physical support, decrease joint load and pain, and slow or stop progression of osteoarthritis.

Osteoarthritis: A chronic degenerative joint disease characterized by the breakdown of cartilage within a joint. It may be due to previous injury, trauma or surgery; age-related changes in the joint; wear and tear due to being overweight, joint malalignment, gout, and other conditions.

Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC): A set of validated questionnaires used objectively to assess the condition of individuals with osteoarthritis of the knee or hip. The result is reported as a total score, pain score, stiffness score, and physical functioning score. Higher scores indicate worse pain, stiffness or physical functioning.

The following codes for treatments and procedures applicable to this document are included below for informational purposes. Inclusion or exclusion of a procedure, diagnosis or device code(s) does not constitute or imply member coverage or provider reimbursement policy. Please refer to the member's contract benefits in effect at the time of service to determine coverage or non-coverage of these services as it applies to an individual member.

When services are Investigational and Not Medically Necessary:
For the following procedure codes, or when the code describes a procedure indicated in the Position Statement section as investigational and not medically necessary.

When services are also Investigational and Not Medically Necessary:

Peer Reviewed Publications:

1. Diduch DR, Crawford DC, Ranawat AS, et al. Implantable shock absorber provides superior pain relief and functional improvement compared with high tibial osteotomy in patients with mild-to-moderate medial knee osteoarthritis: a 2-year report. Cartilage. 2023; 14(2):152-163.
2. Gomoll AH, Diduch DR, Flanigan DC, et al. An implantable shock absorber yields an 85% survival-from-arthroplasty rate through 5 years in working-age patients with medial compartment knee osteoarthritis. Knee Surg Sports Traumatol Arthrosc. 2023; 31(8):3307-3315.
3. Pareek A, Parkes CW, Gomoll AH, Krych AJ. Improved 2-year freedom from arthroplasty in patients with high-risk SIFK scores and medial knee osteoarthritis treated with an implantable shock absorber versus non-operative care. Cartilage. 2023; 14(2):164-171.
4. Pareek A, Parkes CW, Slynarski K, et al. Risk of arthroplasty in patients with subchondral insufficiency fractures of the knee: a matched study of the implantable shock absorber using a validated predictive model. J Knee Surg. 2024; 37(1):73-78.

Government Agency, Medical Society, and Other Authoritative Publications:

1. U.S. Food and Drug Administration (FDA) De Novo classification. MISHA Knee System. DEN220033. April 10, 2023. Available at: https://www.accessdata.fda.gov/cdrh_docs/pdf22/DEN220033.pdf. Accessed on October 17, 2025.

1. Centers for Disease Control and Prevention. Osteoarthritis (OA). January 26, 2024. Available at: https://www.cdc.gov/arthritis/osteoarthritis/. Accessed on October 17, 2025.

MISHA knee system
Osteoarthritis

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