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Surgical Technique
April 15, 2026
5 min read
Surgical Technique
Knee osteoarthritis is one of the leading causes of disability in the United States and worldwide.
Patients who have failed conservative treatments but are not ready or indicated for knee replacement are a challenging population to treat, especially in individuals with high activity demands.
Source: Seth L. Sherman, MD
Traditional surgical options such as unicompartmental knee arthroplasty may require activity modification and carry a risk for early revision or conversion to total knee arthroplasty. High tibial osteotomy presents concerns regarding surgical morbidity, osseous healing and prolonged rehabilitation with weight-bearing restrictions.
To address this treatment gap, an alternative surgical option has been developed: the medial implantable shock absorber (MISHA Knee System, Moximed Inc.). This extra-articular implant spans the joint along the medial collateral ligament, functioning as a load-sharing device for the medial compartment during weight-bearing. It helps alleviate pain and improve function by load-sharing without altering native knee anatomy.
Indications
Ideal candidates for this novel procedure include patients with moderate medial compartment knee arthritis, neutral or mild/moderate varus (less than 10°), a BMI less than 35 kg/m2 (average BMI, 28 kg/m2) and a stable ligamentous exam. It is particularly suited for younger or more active individuals (average age, 51 years) who are either unwilling to undergo arthroplasty (UKA or TKA) or are not ideal candidates for high tibial osteotomy. Surgical candidates should present with localized, load-dependent medial knee pain and associated dysfunction. In addition, patients must be committed to participating in the postoperative rehabilitation protocol.
Surgical technique
The patient is positioned in the supine position on a radiolucent table. A thigh-high tourniquet is recommended and tranexamic acid may be considered. During implant preparation, the operative extremity must remain in neutral rotation. A lateral post and footrest are used to facilitate controlled and consistent positioning of the limb, including full extension, 90° and occasional deeper flexion as these are required checkpoints for proper implantation. C-arm typically comes in from the operative side to obtain anteroposterior and lateral fluoroscopy that guides implant placement.
Source: Seth L. Sherman, MD
A 10-cm longitudinal incision is made, beginning 1 cm proximal to the femoral medial epicondyle and extending to three finger breadths medial to the tibial tubercle. The incision typically follows the posterior border of the tibia and runs the length of the MCL. In addition, this incision is posterior enough to have sufficient skin bridge to perform knee arthroplasty from an anterior midline approach if required in the future (Figure 1).
The sartorial fascia is incised distally in line with the pes tendons at their insertion and freed from underlying tissue. The sartorial fascia is split longitudinally along the anterior border of the superficial MCL up to the inferior aspect of the vastus medialis oblique muscle. A subvastus pocket is then developed to allow for later implant positioning (Figure 2). The superficial MCL, joint line and the subvastus pocket are clearly identified before proceeding with the remainder of the procedure.
Preparation
The goal of the surgical technique is to utilize anatomic and isometric principles to position the implant such that it provides approximately 30% load-sharing in full knee extension without introducing additional constraint during flexion. It is also critical to ensure implant placement avoids soft tissue impingement. The following steps outline this approach in detail.
Source: Seth L. Sherman, MD
The medial joint line is localized under fluoroscopic guidance, allowing for appropriate identification of a reference point for the implant positioning by placing an 18-gauge needle or K-wire 3-mm to 4-mm distal to the joint line and 3-mm to 4-mm posterior to anterior edge of the superficial MCL fibers parallel to the joint line (Figure 3).
A specialized ruler is then aligned parallel to the superficial MCL so that the 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” and directed toward the lateral epicondyle. Importantly, the ruler is pressed to bone at both the “0” and “S” locations to ensure the measured distance is at the bone level. At this point, the knee is flexed to 90° while in neutral rotation. The anisometric location of the K-wire in the femur should result in the ruler showing a 4-mm length change from the “S” notch to the “L” notch. The assessment is continued with the knee in deep flexion to confirm acceptability of the target femoral K-wire location. The needle and ruler are then removed, the knee is placed in extension and a specialized spacer instrument is placed over the femoral K-wire. With the spacer parallel to the superficial MCL, a K-wire is inserted into the proximal tibia through the spacer and parallel to the first K-wire in all planes. The spacer is then removed, and the trial implants are inserted over the K-wires.
Source: Seth L. Sherman, MD
The trial femoral base should be positioned in the previously created subvastus pocket, ensuring that the absorber window remains visible (Figure 4).
The blue collars are applied over the K-wires to secure positioning, and implant clearance is confirmed using a feeler gauge to ensure greater than 2 mm of space from the medial soft tissues. The knee is then taken through a full range of motion to verify proper function. In full extension, the trial implant should demonstrate sufficient compression within the horizontal cut-out box. In both 90° of flexion and deep flexion, the trial compression system should display any segment of the cut-out box, indicating implant lengthening. A second K-wire is then inserted through each base. Two 2.4-mm Steinmann pins are inserted into each base by hand and driven deep under power.
Implantation
Once proper positioning is confirmed, the initial K-wires, collars and trial implants are removed. With the knee at 90° of flexion and neutral rotation, the final implant is inserted over the Steinmann pins. Care is taken to ensure the femoral base of the implant is within the pocket under the vastus medialis oblique. The feeler gauge is once again used to ensure adequate clearance of the medial soft tissue. Fixed-angle locking drill guides are then used to insert 5-mm x 46-mm locking screws at the designated positions, securing the implant. Each screw is tightened with a torque-limiting screwdriver. The Steinmann pins are then removed. Final anteroposterior and lateral fluoroscopic views are obtained to confirm final implant position (Figure 5).
Source: Seth L. Sherman, MD
After final fixation, the shock absorber function is reassessed by confirming compression in extension and varus loading, as well as lengthening in flexion and valgus loading (Figure 6). Finally, the surgical site is copiously irrigated and closed in the standard layered fashion (Figure 7).
Source: Seth L. Sherman, MD
Rehabilitation
Postoperative rehabilitation follows a four-phase protocol spanning approximately 12 to 16 weeks, with progression determined mainly by functional milestones rather than a fixed timeline. If a nerve block is used, patients wear a knee immobilizer until quadriceps control is regained; otherwise, bracing is not required. Range of motion and weight-bearing are advanced as tolerated, with most patients achieving full weight-bearing within 2 weeks. In general, impact loading is avoided for the first 8 to 12 weeks to ensure adequate functional recovery prior to resumption of high-level activity.
Source: Seth L. Sherman, MD
Conclusions
An improved understanding of the biomechanical factors contributing to medial compartment OA, along with limitations in current treatment options, has led to the development of an implantable shock absorber device. Early clinical studies have shown promising results, with good survivorship and favorable clinical outcomes in patients with isolated medial compartment OA.
For more information:
Yazdan Raji, MD, is in the department of orthopedics at the University of Illinois Chicago in Chicago. Seth L. Sherman, MD, is in the department of orthopedic surgery at Stanford University in Redwood City, California. Raji’s email: yazdanrajimd@gmail.com. Sherman’s email: shermans@stanford.edu.
Surgical Technique
Sources/Disclosures
Source:
Expert Submission
References:
Disclosures:
Sherman reports being an educational consultant for and holding stock options with Moximed. Raji reports no relevant financial disclosures.
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