Medial Implantable Shock Absorber (MISHA) to Treat Medial Knee Arthritis in an Active Patient

Abstract

Background:

Osteoarthritis is a leading cause of disability worldwide and most commonly affects the knee. Conservative management has limited long-term efficacy, and traditional surgical options present inherent challenges for the high-demand patient population, creating a treatment gap. Medial implantable shock absorber (MISHA) technology has been recently approved by the US Food and Drug Administration (FDA) for active patients with symptomatic medial compartment arthritis who are either unable or unwilling to undergo high tibial osteotomy (HTO) or arthroplasty. Select advantages include early return to full weightbearing without bracing, reversibility without “burning bridges,” and favorable patient-reported outcomes.

Indications:

We present a 58-year-old female, active military officer with a 9-month history of symptomatic medial knee osteoarthritis who had conservative measures that failed. Following informed consent regarding surgical options, the patient underwent left knee arthroscopy followed by open implantation of the MISHA Knee Implant.

Results:

Medial internal shock absorbers can serve to shield the medial compartment from abnormal loads, reduce pain, and improve function without altering the native knee joint or lower extremity anatomy. In a prospective nonrandomized cohort study supervised by the FDA, a comparison between MISHA and HTO demonstrated the favorable outcomes of the MISHA group in terms of pain, function, adverse events, implant integrity, and conversion surgery at the primary end point of 24 months.

Discussion/Conclusion:

In this surgical technique, we outline the key steps for implantation of the medial internal shock absorber to treat symptomatic medial compartment osteoarthritis in an active patient. We also review technical pearls and pitfalls and postoperative care, including rehabilitation and return to activity timelines and expectations.

Patient Consent Disclosure Statement:

The author(s) attests that consent has been obtained from any patient(s) appearing in this publication. If the individual may be identifiable, the author(s) has included a statement of release or other written form of approval from the patient(s) with this submission for publication.

Graphical Abstract

This is a visual representation of the abstract.

Video Transcript

Overview

In this video, we describe the surgical technique for medial implantable shock absorber (MISHA Knee System; Moximed) implantation to treat medial knee arthritis in a high-demand patient. The authors’ disclosures can be found on the American Academy of Orthopaedic Surgeons website. MISHA has been recently approved by the US Food and Drug Administration (FDA) as a novel alternative for active patients with medial knee osteoarthritis who are unable or unwilling to undergo osteotomy or arthroplasty. We review indications, surgical technique, technical pearls and pitfalls, and postoperative care, including rehabilitation and return to play (RTP).

Background

Osteoarthritis is a leading cause of disability worldwide and most commonly affects the knee.^1,6,8,10 Conservative management has limited long-term efficacy. Traditional surgical options such as unicompartmental knee arthroplasty (UKA) present challenges for younger, active individuals, requiring activity modification and posing risks of early revision or conversion to total knee arthroplasty (TKA).^2,4,9 High tibial osteotomy (HTO) is a time-tested option but has the risk of surgical morbidity, relies on bone healing, requires slow rehabilitation with weightbearing limitations, and has inherent difficulty when converting to eventual arthroplasty. This treatment gap has led to the development of an alternate surgical treatment option: MISHA. This extra-articular implant spans the joint and is positioned along the medial collateral ligament (MCL). Its purpose is to load share with the medial compartment during weightbearing, thereby reducing pain and improving function without altering native knee joint or lower extremity anatomy. ⁷

Indications

In this video, we present a 58-year-old female active military officer with a 9-month history of left medial-based knee pain. The pain is worse with high-impact activities, including running and hiking. She also endorses intermittent mechanical symptoms, reports activity-related swelling, and has poor baseline patient-reported functional outcomes. Conservative management failed, including activity modification, medications (nonsteroidal anti-inflammatory drugs, acetaminophen), extensive physical therapy, and injections. On physical examination, she has an effusion, medial joint line tenderness, positive McMurray test, and a stable ligamentous examination. Left knee radiographs demonstrated mild to moderate medial joint space narrowing as well as early osteophytes with neutral mechanical alignment. Magnetic resonance imaging demonstrates a complex medial meniscus tear, medial compartment chondrosis with subchondral edema, intact ligaments, and early chondrosis in the other compartments. Overall, clinical and imaging findings are consistent with a complex chronic medial meniscus tear in the setting of medial knee osteoarthritis.

Preoperative Planning

Given the patient’s desire to return to intense physical activity and failure of conservative measures, surgical intervention was offered. Options discussed included MISHA implantation, HTO, and medial UKA. After careful consideration and informed consent, the patient opted for left knee arthroscopy with partial medial meniscectomy and chondroplasty followed by open implantation of the MISHA device. This procedure was preferred due to its quicker recovery time and ability to return to daily activities earlier without weightbearing restriction or bracing. Additionally, the patient was not a candidate for osteotomy given her neutral mechanical axis and was deemed too active for arthroplasty at this time. Candidates for MISHA implantation are patients with moderate medial knee arthritis with neutral alignment, have a body mass index of less than 35, are young or have high activity levels, and are unwilling to undergo an arthroplasty procedure (UKA or TKA) or to accept the extended recovery associated with an HTO.

Technique Description

The patient is transferred to a radiolucent operating room table and placed in the supine position. The bony prominences of the nonoperative limb are padded, and the leg is secured to the table. A thigh-high tourniquet is placed on the operative extremity. A foot rest is attached to the table to allow the knee to rest at 90° of flexion as well as hyperflexion with the foot engaged against the foot rest. A leg holder is also attached to the bed to allow for intraoperative lateral radiographs in full extension and also to assess the isometry of the implant position. An examination under anesthesia is then performed. The patient is then prepared and draped in the standard sterile fashion. A 10-cm longitudinal incision is demarcated from the medial epicondyle proximally to the posterior border of the proximal tibia at the level of the pes anserine tendons. The skin and subcutaneous tissue are sharply incised down to the level of the sartorial fascia. The pes anserine tendons are identified, and the sartorial fascia is incised distally in line with these tendons at their insertion. The sartorial fascia is then freed from the underlying tissue and split longitudinally along the anterior border of the superficial MCL (sMCL) up to the inferior aspect of the vastus medialis oblique muscle. At this point, a subvastus pocket is developed to allow for later implant positioning. The sMCL, joint line, and the subvastus pocket are clearly identified before moving forward with the remainder of the case.

Diagnostic arthroscopy is then performed through standard anterolateral and anteromedial portals. The sMCL is trephinated for access using a spinal needle. Chondoplasty is performed for loose and unstable cartilage flaps, and meniscectomy is performed to stabilize the complex medial meniscus tear. Arthroscopic equipment is then removed, and the knee is drained of excess fluid.

The medial joint line is localized through the incision with a spinal needle using direct palpation and fluoroscopic guidance. This will allow for appropriate identification of the start point for the implant positioning. An 18-gauge hypodermic needle is then placed 3 mm distal to the joint line and 3 mm posterior to the anterior edge of the sMCL fibers in the proximal tibia. The position of this tibial starting point is assessed fluoroscopically with an intraoperative anteroposterior (AP) radiograph. The ruler is then placed parallel to the sMCL so that the proximal tibial needle rests at the notch labeled “S.” Proximally, a 1.6-mm K-wire is then placed through the ruler into the medial femoral condyle at the hole labeled “0” parallel to the spinal needle. At this point, the knee is flexed to 90° and the ruler should move 4 mm from the “S” notch to the “L” notch. It is important to note that the ruler positioning should not change in hyperflexion if the K-wire and needle are placed in the appropriate anisometric positions. The needle and ruler are then removed, and the spacer is placed over the femoral K-wire. With the spacer parallel to the sMCL, a K-wire is inserted into the proximal tibia through the spacer parallel to the first K-wire. The spacer is then removed, and the trial implants are inserted over the K-wires. The femoral base should be positioned in the previously created subvastus pocket. The blue collars are inserted over the K-wires down to the trial implant and fastened to maintain the implant position against the medial knee structures. At this stage, the knee is taken through a range of motion from full extension to 90° of flexion and deep flexion to ensure the device functions properly. In full extension, the trial implant should demonstrate sufficient compression, with only the wide cut-out box visible. In both 90° of flexion and deep flexion, the trial compression system should display only the narrow cut-out box, indicating implant lengthening. The bases of the trial are checked to ensure they are seated against the bone. Then a second K-wire is inserted through each base. Then, two 2.4-mm Steinmann pins are inserted into each base, and the initial K-wires, collars, and trial implants are removed. With the knee at 90° of flexion, the final implant is assembled and inserted over the Steinmann pins. The feeler gauge is once again used to ensure adequate clearance of the medial structures. The locking drill guides are then inserted into the most distal and anterior hole on the tibial side and the most anterior hole on the femoral side. Each hole is drilled with a 4.3-mm drill bit and subsequently filled with 5.0-mm × 46-mm locking screws. The remaining 4 holes are filled in the same manner. All screws are finally fastened with a torque-limiting handheld screw driver. The Steinmann pins are then removed. Final AP and lateral fluoroscopic views are obtained to confirm final implant position. After final implantation, the MISHA implant utilizes anatomic and biomechanical feedback to compress in knee extension and varus load and lengthen in knee flexion and valgus load as demonstrated here.

The surgical site is then copiously irrigated and closed in the standard layered fashion. A dry surgical dressing is applied.

Results

Our postoperative rehabilitation follows a 4-phase, 12- to 16-week protocol, modifiable for additional procedures like medial meniscus root repair. Progression is based on functional criteria and not time. Patients use a knee immobilizer until they regain quadriceps control if a nerve block is used without the need for bracing. Range of motion and weightbearing are advanced as tolerated, typically achieving full weightbearing within 2 weeks. Rehabilitation transitions from low impact to linear activities, then to lateral/plyometric movements, and finally to sports-specific activities and return-to-sport testing.

Discussion/Conclusion

Although RTP outcomes following MISHA implantation have not been investigated, other patient-reported outcomes have been documented in the literature. In a prospective nonrandomized FDA-supervised study comparing MISHA and HTO in patients with mild to moderate medial knee osteoarthritis, the MISHA group demonstrated favorable outcomes. ³ Specifically, 95.8% of the MISHA group reported pain relief compared to 87.9% in the HTO group, and 91.7% showed improved function versus 81.3% in the HTO group. ³ MISHA patients returned to full weightbearing on an average of 13.4 days compared to 58.0 days for the HTO group. ³ There were no mechanical device malfunctions in the MISHA group, which also had fewer secondary operations for implant removal and only 1 case of conversion arthroplasty. ³

A pooled analysis of 171 patients from the Phantom, Atlas, and Calypso trials, with a mean follow-up of 3.2 years, showed a 90.6% conversion-free survival rate. ⁵ A subgroup analysis from the Calypso trial indicated a 97.3% three-year survival rate, reflecting improvements in the implant design. ⁵ Mean Western Ontario and McMaster Universities Arthritis Index (WOMAC) Pain scores decreased by 71%, and WOMAC Function scores improved by 69%. ⁵

According to Gomoll et al., ⁵ the implant survival rate at an average follow-up of 3.2 years is approximately 91%. Serious adverse events reported in a prospective cohort study were about 16% for the internal shock absorber, with causes primarily related to infection and symptomatic hardware. ³

Pearls and Pitfalls

For optimal results, the incision should be positioned over the medial epicondyle and the posterior border of the proximal tibial cortex, ensuring it is 10 to 14 cm in length. Additionally, blunt dissection should be performed to elevate a subvastus medialis pocket, which is critical for proper proximal implant positioning. The accurate positioning of the starting K-wires is essential. If visual cues are insufficient, the femoral origin of the MCL can be identified under fluoroscopy. It is crucial to ensure sufficient compression in knee extension and varus load across the knee joint, as well as appropriate implant lengthening during knee flexion and valgus load. Last, maintaining a 2-mm clearance around the implant will prevent soft tissue impingement and avoid dislodgement.