Surgical Technique,Tips,and Tricks: Medial Implantable Shock Absorber for Medial Compartment Knee Osteoarthritis

Abstract

Background:

Isolated medial compartment knee osteoarthritis (OA) presents a challenging problem to treat for knee surgeons, with a multitude of options from conservative management, including injections and unloader braces, meniscal procedures, osteotomies, and unicompartmental knee arthroplasty (UKA). A new medial implantable shock absorber (MISHA) allows for offloading 142 N of the medial compartment during stance phase of gait.

Indications:

US Food and Drug Administration approval was obtained on April 10, 2023, with the following indications: isolated medial knee OA (Kellgren-Lawrence grades I-IV) that failed 6 months of conservative management, ages 25 to 65 years, body mass index <35 or body weight <300 lbs, <15° of varus, no flexion contracture >10°, and no significant medial osteophytes or medial meniscal extrusion.

Technique Description:

A longitudinal medial knee incision is made 1 cm proximal to the medial epicondyle to the pes insertion, around 3 cm medial to the tibial tubercle, exposing the superficial medial collateral ligament, and medial joint line. The establishment of the femoral anisometric point is critical to provide a 4-mm posterior condylar offset in 90° of flexion compared to full extension. A trial implant can be placed to confirm appropriate anisometry, implant loading in extension, and relaxation in flexion. The final implant is placed and the titanium femoral and tibial baseplates are fixed with 3 unicortical titanium locking screws each.

Results:

Expected results per recommended postoperative protocol are immediate weightbearing without range of motion restrictions. From author experience, patients typically feel improvement and return to sport more rapidly than with anterior cruciate ligament reconstructions. Recovery and return to work are faster than osteotomies or UKAs based on comparative short-term studies. While long-term results are lacking given the novelty of the procedure, prospective studies have demonstrated 100% arthroplasty-free survival at 2 years and 85% survival at 5 years.

Discussion/Conclusion:

The MISHA is a viable option for isolated medial compartment knee OA that provides a joint-preserving alternative to arthroplasty and a less morbid alternative to osteotomy. This treatment can be technically difficult to perform, but several pearls and techniques can offer a reproducible, minimally invasive surgery and good functional results.

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 Background

In this video, we will present the utilization of the medial implantable shock absorber (MISHA) for medial compartment knee osteoarthritis (OA), including implant design, surgical technique, and technical tips and tricks.

The implant is a cylindrical polycarbonate urethane shock absorber that is implanted in the medial knee, overlying the superficial medial collateral ligament (MCL). It is fixed to the distal femur and proximal tibia using titanium base plates and unicortical titanium locking screws and articulates through a carbon-fiber reinforced polyether ether ketone (PEEK) and cobalt chrome ball-and-socket articulation. ³

It passed US Food and Drug Administration (FDA) approval on April 10, 2023, and functions by reducing peak forces on the medial compartment by 32% during stance phase of gait by providing an opposing force of up to 142 N. The shock absorber compresses in knee extension, allowing for the transfer of load during the stance phase, and relaxes during knee flexion during the nonweightbearing swing phase. ³

Indications

Our representative case is a 55-year-old man with 1 year of worsening bilateral knee pain, left greater than right. It is localized to the medial joint line and is primarily during ambulation. There is no pain at night or with prolonged sitting. He has trialed nonsteroidal anti-inflammatory drugs, physical therapy, and injections in both knees with 2 months of relief. He had temporary improvement in pain with a medial unloader brace but did not tolerate the brace well.

Radiographs demonstrate bilateral knee OA primarily in the medial compartment, with joint space narrowing, subchondral sclerosis, and mild osteophyte formation on the medial plateau. The patellofemoral joint on the left has mild lateral joint narrowing and osteophyte formation with a mostly preserved lateral compartment. Not shown here, but long leg alignment films demonstrate 3° of bilateral hip knee angle (HKA) varus.

Magnetic resonance imaging (MRI) demonstrates preserved chondral surfaces of the patellofemoral and lateral compartments, chondral loss, and mild medial meniscal extrusion. The evidence of bony edema, mostly of the medial plateau but some in the medial femoral condyle, is an excellent indication for offloading since unloading that stress reaction will provide pain relief.

Treatment options include nonoperative and operative measures, which we will highlight with MISHA.

Indications and contraindications are critical. The FDA approval trial indications included age range from 25 to 65 years and isolated medial compartment knee OA from Kellgren-Lawrence grades I to IV that had failed 6 months of nonoperative management. Patients over 300 lbs total or body mass index (BMI) of 35 kg/m² were excluded from the trial.

Contraindications include significant medial joint line osteophytes or medial meniscal extrusion, as these can lead to impingement on the actual implant during range of motion (ROM) and cause implant dislocation. There is no set size of osteophyte or extrusion, but a general rule is any osteophyte or meniscus that protrudes beyond 3 mm, or 4 or more mm, past the medial femoral condyle/medial tibial plateau border. This is due to the implant sitting at a minimum 2 mm off the medial soft tissues as intraoperatively, a 2-mm spacer is used to confirm positioning, but there is some leeway as the implant can be positioned slightly off bone with the locking screws to prevent impingement. For coronal alignment, the FDA trial limited HKA to <15°, but most expert consensus would recommend only performing this procedure in those with <10° of mechanical varus, and lastly, flexion contractures >10° are contraindicated. Relative contraindications include individuals with low BMIs, as those without at least a moderate soft tissue envelope medially are at risk for implant prominence and wound complications. Here are radiographic and MRI examples of 2 contraindications, such as the implant sitting over the MCL and the osteophyte or extruded meniscus impinging on the implant during ROM.

Technique Description

Patient positioning is essential, and full extension, 90° of knee flexion, and hyperflexion should be able to be held without assistance intraoperatively, especially for surgeons who plan to perform the procedure with either 0 or 1 assistant. The use of a stepped foot positioner and preoperative goniometer can aid in precise knee positioning.

Following positioning including the use of a thigh tourniquet, an intraoperative timeout is performed, confirming the surgeon’s initials on the operative extremity. A 10- to 15-cm longitudinal incision, depending on patient size, is made from 1 cm proximal to the medial epicondyle to the pes insertion, around 3 cm medial to the tibial tubercle. Dissection is carried through the subcutaneous tissue to the sartorius fascia, and if the saphenous vein and infrapatellar branch of the saphenous nerve are encountered, they can be ligated or preserved. The sartorius fascia is then incised the entire length of the incision. This step is key for both exposure of the superficial MCL (sMCL) and pes insertions, which can be transected for later end-to-end repair, and ideally, the incision and fascial split is centered over the anterior one-third of the sMCL. The next critical portion of the exposure is creation of the sub-vastus medialis obliquus (VMO) pocket. The VMO muscle belly is identified, and dissection is carried beneath it, such as in a subvastus approach, until the surgeon’s entire finger can fit underneath the VMO—this will be the placement for the femoral plate later in the procedure.

If adequate exposure has been performed, the surgeon should easily see all relevant anatomy—proximally from the sub-VMO pocket, the entire length of the sMCL, especially the anterior border, and distally to the pes insertions, which do not have to be released overlying the distal sMCL.

The next step is tibial needle placement to determine the anisometric point of the distal femur. A needle from the implant kit (or any 18-gauge needle as an alternative) is placed 3 to 4 mm posterior to the anterior border of the sMCL and 3 to 4 mm distal from the medial joint line on a fluoroscopic anteroposterior plateau view. The needle should be parallel to the joint line and is malleted into the proximal tibia.

We return to the surgical video, which shows palpation for the joint line and insertion of the tibial needle, which is confirmed fluoroscopically. Next, in full extension, the proprietary ruler is placed with the “S” laid over the tibial needle, and a K-wire is placed in the center hole of the distal femur. A K-wire replaces the tibial needle, keeping both K-wires parallel, and then the knee is brought into 90° of flexion. In this case, at 90°, the distance between the 2 pins remains at “S,” which is incorrect.

In full extension, for most patients, the distance between the tibial and femoral points should be from the center femoral hole to the S marking. In 90° of flexion, the tibial needle should sit in the “L” notch, which is 4 mm longer than the “S” notch due to the 4-mm increase in flexion gap given the posterior condylar offset of the femur. In hyperflexion, the tibial needle should sit in the L notch ±4 mm. When done properly, this will ensure that the implant will be compressed in extension, thereby offloading the joint (due to the 4-mm shorter distance) and relaxed in flexion (due to the 4-mm longer distance).

In our case, in both extension and 90° of flexion, the ruler sits in the “S” notch, which would put our implant at risk for overcompression in flexion. In reality, we want the tibial pin to sit in the L notch in 90° of flexion, as demonstrated in the bottom right figure, which would mean a +4-mm flexion distance due to the normal posterior condylar offset of the distal femur. So how do we fix it? The ruler has several slots to allow for 2- to 4-mm adjustments, so in this case, we move the femoral pin 4 mm anterior, which the ruler demonstrates would be at the far end of the first oval hole from the center pin hole with the knee in 90° of flexion. This would increase our flexion gap 4 mm while leaving our extension gap distance unchanged. We will now demonstrate this in real time.

A new femoral pin is placed 4 mm anterior in the distal femur in 90° of flexion, and the ruler is adjusted to place the center hole on the new pin location, confirming the new +4-mm L notch distance between pins at 90° of flexion. The knee is brought back to extension, where we confirm our extension distance has remained unchanged at the “S” notch.

The last example was overcompression in 90° of flexion. The next scenario would be undercompression in 90° of flexion. In this scenario, in extension, the length is set at S, but in 90° of flexion, the gap is L +4 mm, indicating too large of a flexion gap. In this scenario, the implant would be at risk of dislocating as the piston mechanism pulls out in flexion. To adjust, the femoral K-wire is moved 4 mm posterior while in 90° of flexion, toward the tibial pin.

The tibial pin is removed and replaced using the S, or small spacer, at the location closest to the original tibial pin, parallel to the femoral pin. A key portion here is that the center mechanism should be rotated so that the flag cutout is visible medially to the surgical team, which can be done with a K-wire through a hole in the ball-bearing joint of the femoral side. The trial is then placed over the pins, which can sometimes be easiest by placing it separately and engaging them once on the pins. The femoral and tibial baseplate trials are positioned, and a 2-mm spacer is placed under the implant to ensure there is no bony or soft tissue impingement on the implant. The femoral and tibial baseplates are fixed with K-wires, and a repeat check for impingement is performed. Next comes the final anisometric check of knee ROM. Stoppers are placed on the K-wire to keep the trial on bone, and the knee is taken through a ROM. In full extension, the trial should demonstrate overlap anywhere in the wider portion of the cutout, indicating adequate implant compression in extension. As the knee is brought into 90° of flexion, the trial should overlap anywhere in the entire flag cutout—wider or thinner portion of the cutout—indicating either no change or implant relaxation in flexion.

Here we will explain more clearly. Notice the flag-shaped cutout in the trial indicating level of implant compression. When the overlap is toward the wider cutout, there will be more implant compression and more offloading of the joint. In full extension, the trial should demonstrate increased compression and overlap in the larger cutout. When the overlap is in the thinner cutout, there will be more implant relaxation and less offloading of the joint. If there is too much relaxation/distraction of the trial, then the implant disengages and can dislocate.

So, in 90° of flexion to deep flexion, the trial overlap should either not move, indicating no change, or move into the thinner portion of the cutout, indicating implant relaxation. However, the overlap should not move past the cutout completely in flexion, as this would indicate that the final implant would be at risk for disarticulation in flexion.

Last, we want to reemphasize that at all ranges of motion, a 2-mm spacer should be able to slide between the implant and medial soft tissues, as any impingement of the tissue on the implant can lead to implant displacement medially and possible increased risk for implant dislocation.

Back to the case, once happy with the trial position, 4 Steinman pins are then placed into the trial for final implant positioning. One last check is performed, showing excellent implant compression in extension and no change/relaxation in flexion. The trial is removed, and the final implant is placed over the Steinman pins, and again the spacer is used to ensure there is no impingement. Six unicortical locking titanium screws are then placed through locking towers. Tips for this portion include placing 1 screw in both the femoral and tibial baseplate before placing the remaining screws. All screws should be finished by a hand torque–limiting screwdriver, and only once all 6 screws are placed should final torque-limiting tightening be performed for all 6 screws to avoid implant rotation. The Steinman pins are then removed, and final ROM check is performed. Now instead of the trial cutout, attention should be paid to the polyurethane shock absorber. Notice in extension how it bulges as it compresses and will take 142 N of force off the medial compartment in stance phase. As the knee is brought into flexion, the polyurethane absorber relaxes and thins out, indicating no compression and joint offloading during the swing phase. A final impingement check is performed, and then closure is performed, sliding the VMO over the femoral baseplate and closing the sartorius fascia over the implant. The skin is closed with 3-0 Monocryl demonstrating no obvious implant prominence.

Final fluoros (which are optional) demonstrate well-positioned implants. The postoperative protocol involves immediate weightbearing as tolerated (WBAT) without immobilization and ROM as tolerated. At the most recent follow-up, our patient is currently 6 months postoperative with continued pain relief and ambulating without an assistive device. He is interested in the same procedure for his right knee, but an ongoing discussion is being held for this surgery versus unicompartmental knee arthroplasty given the more severe OA and osteophytes on that side. His ROM is 0° to 135° with minimal implant prominence on palpation and no pain.

Results

Pearls and pitfalls of this procedure are intertwined with avoiding complications. Indications are crucial to avoid overloading the implant, having medial bony or soft tissue impingement on the implant, having not enough soft tissue coverage, or having bones that are too small for the implant dimensions as it only comes with 46-mm-length locking screws. A meticulous exposure allows for both precision of implantation and an intact soft tissue sleeve of sartorius fascia to close over the implant to avoid implant prominence, and the pes tendons can be preserved. In cases of future surgery, such as conversion to unicompartmental or total knee arthroplasty, the implant can be removed in a concomitant or staged manner through the separate medial incision.

Discussion/Conclusion

Several studies on the procedure involved in the FDA approval process have been published over the past year. Pareek et al ⁴ demonstrated, in a retrospective case-control study of 21 implantable shock absorbers in patients with a mean age of 52 years, a 0% conversion rate to arthroplasty at a 2-year follow-up compared to a 55% conversion rate in 21 matched control patients. Diduch et al ¹ performed a multicenter prospective comparative cohort study of 81 implantable shock absorbers to 81 historical high tibial osteotomy (HTO) controls and demonstrated a more rapid return to weightbearing and increased rate of meeting primary outcome “success” criteria. Gomoll et al ² performed a multicenter prospective cohort study of 171 implantable shock absorbers with a mean 3.2-year follow-up and demonstrated an 85% 5-year arthroplasty-free survival and significant improvements in pain and function scores on the Western Ontario and McMaster Universities Osteoarthritis Index. Last, a recent study by Pareek et al ⁵ demonstrated the feasibility of MISHA in preventing progression to arthroplasty for cases of subchondral insufficiency fractures of the knee.