Implantable Shock Absorber for Medial Compartment Osteoarthritis

Abstract

Background:

Treating younger patients with isolated medial compartment osteoarthritis (OA) remains challenging, as these patients have often failed nonoperative treatment and wish to remain active, while surgeons are cautious using unicompartmental knee arthroplasty in young patients.

Indications:

Patients (1) aged between 25 and 65 years, (2) with medial compartment OA Kellgren-Lawrence grades 1 to 4, (3) who have failed a minimum of 6 months of nonoperative treatments, and (4) who do not have large medial osteophytes.

Technique Description:

The patient is positioned supine with a foot positioner for full extension, 90° of flexion, and deep flexion. A medial incision is made, and the medial collateral ligament is exposed. With standard instrumentation, Kirschner-wires are drilled in the tibia and femur, and a trial component is placed on the medial side of the knee. Then, in full extension, the absorber prevents varus collapse, whereas in flexion, the absorber does not engage, allowing for full flexion. The final implant is then placed with 6 locking screws, and closure is performed in layers. Rehabilitation consists of weightbearing as tolerated and a progressive range of motion.

Results:

Three trials with the implantable shock absorber have shown that the implant is generally safe and leads to clinical improvement in nearly all cases. Compared with a historical cohort of patients undergoing high tibial osteotomy, those in the present study have an earlier time to weightbearing (13 versus 59 days) and show greater improvement in functional and pain outcomes. Survivorship of the first 171 cases has been estimated to be 90% at 3 years and 85% at 5 years.

Discussion/Conclusion:

An implantable shock absorber is a suitable treatment option for younger patients with medial OA who wish to remain active. Rehabilitation typically involves weightbearing as tolerated. Short- to midterm outcomes in the literature are promising in this challenging patient cohort; however, longer follow-up is needed before recommendations can be made for this procedure to be a definitive surgery.

Level of Evidence:

Level 3.

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.

Download video file ^{(170.6MB, mp4)}

DOI: 10.1177/26350254251359675.M1

Video Transcript

We present our study titled "Implantable Shock Absorber for Medial Compartment Osteoarthritis." In this video, we will review the background, preoperative planning, patient positioning, and the key steps of the implantable shock absorber procedure, along with potential complications, rehabilitation, return to sports, and patient outcomes in the literature.

Background

We present a 50-year-old female patient who presents with 5 years of progressive medial pain without trauma that has worsened by running. She enjoys activities such as swimming, walking her dogs, and gardening. Previous treatments have failed and included physical therapy, cortisone and hyaluronic acid injections, and an unloader brace that relieved her symptoms but was difficult to wear. At the time of surgery, her body mass index was 34 kg/m² after treatment with Wegovy. Radiographs reveal isolated Kellgren-Lawrence grade 3 medial compartment osteoarthritis (OA) with well-preserved other compartments, no large osteophytes, and 8° of varus on standing alignment films.

Magnetic resonance imaging of T2 sagittal, coronal, and axial slices shows isolated medial compartment wear with mild medial meniscus extrusion and degeneration without tear or posterior root tear.

Indications

First, the eligibility criteria are reviewed, which include age between 25 and 65 years, medial compartment OA on anteroposterior and/or flexion views, failure of nonoperative treatment for at least 6 months, and no large medial osteophytes. Furthermore, the maximum varus alignment is 10°, and most of the pain must be present when the patient is weightbearing. The indication partially overlaps with unicompartmental knee arthroplasty (UKA). However, it is somewhat different, as these patients mostly experience weightbearing pain and are often somewhat younger. In contrast, UKA is better indicated for osteoarthritic pain that is more pronounced at rest or at night in slightly older or less active patients. This procedure can be seen as a procedure to delay or potentially prevent knee replacement surgery.

It is important to have received training with the implant, and fluoroscopy must be available. Knee arthroscopy can be performed if needed, for procedures such as concomitant medial meniscus root repair or procedures in other compartments. ³

Technique Description

Patient positioning is supine with a post to stabilize the leg. A foot positioner is used to have reproducible full extension, 90° of flexion, and deep flexion as shown. Fluoroscopy comes from the ipsilateral.

A medial straight incision of approximately 10 to 12 cm is made from just above the medial epicondyle (marked with a circle) to the level of the pes tendons. The superficial fascia is opened to approach the medial side of the knee. Space is created under the vastus medialis obliquus large enough for a finger, and an army or navy will be placed here in the case. The deep fascia and first layer are opened to expose the medial collateral ligament (MCL). Then, the anterior border of the MCL is identified as shown here. A 1.6-mm K-wire is drilled 3 mm underneath the joint line and 3 mm posterior to the anterior border of the MCL, perpendicular to the joint line, and this is confirmed on fluoroscopy. With the knee in extension and the foot in neutral rotation, the ruler is placed over the tibial pin in the S notch, and a femoral K-wire is drilled in the middle hole and aimed at the lateral femoral condyle. Flexion and extension are then checked to ensure that the ruler has the tibial pin at the L notch in flexion and the S notch in extension.

The spacer is then placed over the medial femoral condyle K-wire, the leg is kept at extension and neutral rotation, and a third K-wire is drilled through the spacer hole perpendicular to the femoral condyle wire. The first tibial K-wire may be removed if it is in the way. The initial K-wire in the tibia is removed if it was not removed before. The femoral trial component and the tibial trial component are placed over the pins and are connected as shown here. The trial is then fixed to the pins using collars that are displayed at the top right of the slide. One should ensure that the femoral and tibial components are flush over the bone and not levered anteriorly or posteriorly. A 2-mm Feeler Gauge reveals that there is initially insufficient space between the trial and the MCL. However, after the collars are adjusted and the trial is moved off the bone, sufficient space is created. In extension, the edge of the distal absorber should be in the broad top window of the trial, as shown in the right top of the slide. Then, in flexion and deep flexion, the edge of the distal absorber should be positioned in the lower, narrower part of the window, as shown at the bottom right of the slide. The trial is fixed to the femur and tibia using 1.6-mm K-wires. The 2-mm Feeler Gauge can be left in situ to ensure the components are not fixed too tightly to the bone, and one should pay attention to not levering the components anteriorly or posteriorly. Also, 2.4-mm Steinmann pins are now used to fix the trial. These should first be placed by hand into the slots, and then 2 femoral and tibial pins are drilled into the bone. The K-wires are then removed, and the trial is removed over the Steinmann pins.

The definitive implant is then placed back over the Steinmann pins, and fixation is performed with a total of 6 locking screws of 5 mm in diameter and 46 mm in length. First, 1 femoral screw and 1 tibial screw are placed after drilling with a 4.3-mm drill. Once these are in place, the remaining 2 femoral and 2 tibial screws can be drilled and placed. One should ensure not to fully insert the screw with power and leave the last few threads for the torque-limited screwdriver, as can be seen at the end. The final range of motion confirms that at flexion, there is a gap between the absorber and piston flange of <4 mm. At extension, it should be confirmed that the absorber compresses, and that varus stress is not possible, as is shown here. The Feeler Gauge should fit between the absorber and the knee at all angles. Standard closure in layers is performed after irrigation. Another view confirms that at 90° of flexion, the absorber does not engage and can spin freely, whereas in extension, the absorber engages and cannot spin, indicating the engaged shock absorber function.

Several potential complications should be avoided. This procedure should be avoided in patients with large tibial osteophytes or those who lack sufficient soft tissue coverage for this implant, as it can lead to pain, prominent hardware, or wound healing issues. Furthermore, detailed attention should be paid to the tightness in flexion and extension to ensure that the shock absorber function is working in extension, and the knee is not caught in deep flexion.

Results

The first 2 weeks of rehabilitation are focused on wound healing, range of motion until 90°, and weightbearing as tolerated. Over the next 4 weeks, the range of motion is extended to 120°, and advanced strengthening and cardiovascular conditioning are started. From 6 weeks on, a full range of motion and more demanding activities are started, and the return to sports program is started at 3 months postoperatively.

Discussion/Conclusion

Three large cohorts have been published in the literature. The PHANTOM trial was the first case series conducted in 4 centers in Poland and South Africa, involving 26 patients.^4,5 It showed the procedure was safe; 1 patient required implant removal, and 96% of patients had at least 20% improvement in Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) scores. Then, a prospective study was conducted at 10 centers in Europe and the United States, involving 81 patients who were compared with a historical cohort of patients undergoing high tibial osteotomy (HTO). ¹ The study showed that the primary endpoint of improvement without further surgeries was better with the implant compared with HTO, and there was a significantly shorter time to weightbearing. There was also relatively more improvement in WOMAC pain and function. However, there were 4 cases of deep infection, 1 case of revision to UKA, and in total, 14% of implants were removed for these reasons. Finally, 3 large cohorts were combined to assess survivorship in 171 patients, with a mean age of 51 years, and the authors found a 3-year survivorship of 90% and a 5-year survivorship of 85%, and similar WOMAC improvement as the other studies. ²