Robotic-Assisted Isolated Polyethylene Exchange: A Novel Surgical Technique

Abstract

An isolated polyethylene exchange (IPE) is a revision procedure with mixed results in the literature, typically indicated for well-aligned total knee arthroplasties with instability. Surgeons often rely on subjective feedback and experience to determine whether an IPE or a more involved revision procedure is necessary. The technique of robotic-assisted IPE offers the potential for more precise decision-making in revision knee arthroplasty. Robotic systems can provide intraoperative data that may help identify ideal candidates for IPE and guide the selection of the appropriate PE component to address instability. Future studies comparing robotic-assisted IPE with traditional methods are needed to investigate patient-reported outcomes and functional results.

Introduction

Total knee arthroplasty (TKA) has been the mainstay treatment for end-staged osteoarthritis since its widespread adoption in the 1970s. Since then, the techniques and tools used to perform TKA and revision surgeries have evolved. According to the American Joint Replacement Registry, aside from infection, the primary factors contributing to TKA failure include mechanical loosening and instability [1]. Many of these issues may be influenced by surgical technique and the gap balance achieved during the primary surgery.

Robotic-assisted surgery enables the surgeon to make intraoperative decisions based on objective, real-time data. Recent robotic innovations during primary TKA have demonstrated consistent results with achieving more accurately aligned implants and improved medial and lateral flexion and extension gap symmetry [2,3]. Robotics continue to gain popularity during primary TKA, but recently there have been several studies employing their use in revision scenarios [[4], [5], [6]].

An isolated polyethylene exchange (IPE) is a revision surgery that has been described in the literature with mixed outcomes, depending on the indications and patient population. Specific criteria for IPE have yet to be fully defined, despite multiple authors providing their “expert opinions” in different revision scenarios [[7], [8], [9]]. IPE can be used during instability cases where a larger polyethylene (PE) component can be implanted to help retension the collateral ligaments to improve knee stability [10]. Conversely, downsizing a PE component in cases of postoperative stiffness can be used to help improve knee motion in some instances, but the results have been mixed [11,12]. A third indication for IPE is when an older PE liner has worn down, leading to joint instability, where a new PE can be reinserted [13]. Finally, an IPE can be performed as part of the surgical management of an acute periprosthetic joint infection, where the PE is replaced after thorough soft tissue debridement.

In general, an IPE is a revision option for patients with instability whose TKA is well aligned, has acceptable flexion and extension gap symmetry, and whose components are well fixed to the bone [14,15]. Additionally, the tibial locking mechanism must also remain intact for the procedure to be successful.

To the best of our knowledge, robotic-assisted IPE has yet to be investigated. The authors believe the use of robotic assistance provides objective data to assist with intraoperative decision-making regarding gap assessment and range of motion (ROM) when various PE liners are trialed to address cases of instability. If acceptable knee balance cannot be achieved with an IPE, conversion to a more involved revision TKA procedure may then be considered. We present 3 cases of robotic-assisted IPE performed for instability (Figure 1, Figure 2). Written informed consent was obtained from all subjects.

Figure 1.

Demographics. Depuy Attune (Warsaw, IN); Smith & Nephew Legion (Memphis, TN); Stryker Triathlon (Mahwah, NJ).

Figure 2.

Anteroposterior (a, e, i) and lateral (b, f, j) preoperative radiographs and anteroposterior (c, g, k) and lateral (d, h, l) postoperative radiographs for case 1, case 2, and case 3, respectively.

A 54-year-old male sustained a complex left bicondylar tibial plateau fracture in a motorcycle accident initially managed with open reduction and internal fixation and eventually converted to a TKA 2 years ago. The patient continued to have pain and flexion instability exacerbated by repetitive weight-bearing activities particularly when climbing stairs. After failing conservative treatment including therapy, weight loss, and bracing, he elected to undergo revision surgery. Infection workup was negative.

A 70-year-old male with complaints of knee pain and instability, 5 years status after left femoral-only revision TKA. These issues significantly impaired his ability to navigate stairs and walk on uneven surfaces, despite therapy, bracing, and activity modifications. The patient had obvious flexion instability with tenderness noted over the pes anserine region. After a negative infection workup, he wished to proceed with revision surgery.

A 73-year-old female underwent a right TKA at an outside facility in 2012. She did well with her knee until 2020 when she sustained a fall on the anterior aspect of the knee and started to experience feelings of instability especially when descending stairs and uneven ground. Two rounds of physical therapy, bracing, and modalities did not adequately address her instability symptoms, so she elected to undergo revision surgery after infection was ruled out.

Surgical technique

Cases were performed with the Smith & Nephew Cori Surgical System (Memphis, TN). This system is currently approved for robotic revision surgeries [16]. Surgeries were performed through the existing anterior midline incision followed by a standard medial parapatellar arthrotomy. The tibial array was placed through stab incisions in the mid-tibial diaphyseal region, at least 150 mm from the joint line, to allow for a potential conversion to a more complex revision surgery if necessary. The femoral array was positioned intraincisional in the distal femoral metadiaphyseal region, approximately 50 mm proximal to the femoral flange. The mechanical axis of the operative extremity was acquired via registration of the required landmarks: hip center, distal femoral intercondylar notch, center of the tibial component (analogous to the anterior cruciate ligament tibial insertion), and ankle malleoli. Data points were then collected along the existing femoral component and surrounding distal femur, as well as the proximal tibia and existing tibial component, to satisfy the required areas for an accurate digital knee model.

Next, the “special points” feature was utilized to register the cement–implant interface of the existing femoral and tibial implants. Collectively, these “special points” are interpreted as the existing implants on the subsequent planning screens (Fig. 3). The virtual knee implants are then positioned relative to the “special points” to provide a more accurate spatial representation of the existing implant within the coronal, sagittal, and transverse planes.

Figure 3.

Example of intraoperative “Implant Planning Screen” depicting current femoral and tibial implants (blue) positioned relative to the “special points” (Pink dots) to provide accurate spatial representation of the existing implant within the coronal, sagittal, and transverse planes. Note the relatively balanced flexion and extension gaps both medially and laterally at bottom of the figure.

The digital tensioner was used to collect collateral ligament tension throughout the knee ROM. The resultant graph provides the surgeon with a view of the existing laxity along the medial and lateral gaps during flexion and extension. No bony cuts are performed; therefore the femoral and tibial burring sequences are bypassed.

After collecting baseline stress data, knee ROM is collected to generate a graph depicting the postoperative gaps corresponding to the inserted PE trial size. By exchanging the PE trial and repeating the knee ROM screens, the surgeon can objectively compare the resulting graphs for different PE thickness noting changes in graph amplitude (Fig. 4). The goal is to reduce both the medial and lateral gaps to approximately 1 mm throughout the mid-flexion arc, between 30° and 90° of knee motion. It is essential the knee achieves near full extension and flexion. If the stability and ROM targets cannot be met, a more extensive revision procedure may be warranted. Once appropriate stability is confirmed, the trial PE component is replaced with the final implant, followed by standard wound closure.

Figure 4.

Knee ROM graphs with different PE components. Case 1: (top) 5 mm PS component (middle) 7 mm PS component, and (bottom) 8 mm PS component. Case 2: (top) 13 mm PS component (middle) 15 mm PS component, and (bottom) 15 mm PS constrained component. Case 3: (top) 9 mm cruciate retaining PE component (middle) 11 mm CS component, and (bottom) 13 mm CS component. Note the amplitude of the mid-flexion portion of the graph (red box) decreases with the increase in PE size while the peak difference between the medial and lateral compartments remains relatively unchanged. CS, cruciate stabilized; PS, posterior stabilized.

Follow-up

At the last follow-up (average 11.6 months), all subjects had considerable improvement in pain, flexion stability, and functional status compared to their preoperative condition (Fig. 1). No complications were reported for the subjects. Long-term follow-up is ongoing.

Discussion

The most common reasons for revision TKA within the first 5 years are instability and component wear with IPE representing a possible alternative to full revision in certain patients [7,8,10]. However, the most frequently reported complications following IPE are continued pain and instability [7,8,17]. Historically, studies have highlighted unsatisfactory outcomes associated with IPE. Engh et al reported 14% of patients required further revision surgery within 5 years following PE exchange [18]. Similarly, Fehring et al found only 50% of patients experienced satisfactory outcomes regarding stability and pain relief after an IPE [7]. In contrast, recent literature suggests an IPE can yield favorable outcomes when performed in appropriately selected patients. Studies by Duensing et al (87%) and Green et al (93%) both investigated survivorship free of reoperation after IPE in select patient populations, with an average follow-up period of nearly 3.5 years after IPE surgery [8,19].

In a recent large retrospective study of 280 cases, Cheng et al reported that 10% of patients who underwent IPE required further treatment for instability, compared to only 3% of those who underwent full revision TKA [20]. The authors concluded that IPE is less reliable than full revision in addressing instability. Unfortunately, the authors did not assess implant rotation or its relationship to knee balance, nor did they report whether robotic assistance was utilized in their cohort of primary TKAs [20]. An IPE may be an effective option for correcting sagittal plane laxity but is not designed to fully address issues in the axial plane, which are typically caused by misaligned or improperly rotated components [21,22].

TKA biomechanics are highly complex, but the primary goal is to optimize stability in the mid-flexion range. From 0° to 30° of flexion, the knee joint is relatively stable due to the screw-home mechanism, as well as the tension in the collateral ligaments and joint capsule [23,24]. Between 30° and 100°, the collateral ligaments guide motion, while the conformity of the articulating joint surfaces is primarily responsible for stability [23]. Beyond 120° of flexion, excessive femoral rollback and posterior femoral translation can occur, leading to abnormal knee mechanics and gap asymmetry [23,24].

Being able to decrease flexion instability is key in activities requiring flexion moments such as stair climbing and navigating sloped surfaces. This provides a stable surface and increases the mechanical advantages of muscles to function as primary movers of the knee joint instead of secondary stabilizers which is often the case with instability cases [24].

Multiple studies have demonstrated proper femoral rotation is integral to establishing a balanced TKA [2,21,22]. Femoral component rotation is directly related to sagittal and coronal plane alignment based on implant geometry. These relationships are difficult to visualize with conventional techniques. Today, robotic-assisted TKA allows a greater appreciation of how femoral component sizing and orientation, particularly rotation, intimately affect coronal and sagittal balance throughout knee ROM [22]. While a larger PE component can address flexion laxity, it may also restrict full knee extension, therefore an IPE should be approached with caution. A potential advantage of robotic-assisted IPE is that it enables the surgeon to visualize the resulting changes in ROM intraoperatively, helping to determine whether a given PE thickness will improve or hinder knee function.

Implant companies manufacture an assortment of PE components with variable amounts of constraint—with cruciate retaining, cruciate substituting, posterior stabilized, constrained, and medial congruent options available. IPE can help restore function in patients experiencing moderate wear of their initial liner and those experiencing instability during mid-flexion [13,15,25]. Oftentimes a thicker PE component is implanted in these scenarios to address any residual laxity that may have developed [10,15].

In some instances, trialing a more constrained or medial congruent PE may address subtle differences between the medial and lateral compartments. However, large differences (>3 mm) between the compartments usually implies a component malposition that cannot be addressed with an IPE [7,9,15]. Traditionally, surgeons have relied on their prior experience and the “feel” of the knee to determine the PE size during an IPE. This method is highly subjective and difficult to teach due to the lack of objective data [7,9]. The authors believe that the gap assessment graph generated by the robotic system enables the surgeon to make a more informed decision, with objective data, during revision surgery.

Single-use pressure sensors have been utilized to measure force in the medial and lateral compartments of a TKA [26]. In theory, these devices may also be used in revision scenarios to help determine whether an IPE is the appropriately indicated procedure. It is important to note that tension differs from force; proper ligamentous tension is essential for achieving stability throughout knee motion [23,24]. In contrast, the digital tensioner used in this surgical technique provides objective feedback on ligamentous tension throughout the ROM during gap assessment, allowing the surgeon to identify gap imbalances.

In case 1, a more balanced knee was achieved using an IPE, without the associated morbidity of revising a well-fixed long-stemmed tibial component. Meanwhile, Case 2 utilized a manufacturer-specific constrained PE component, which significantly reduced mid-flexion laxity (Fig. 5). In addition to a broader post, the medial and lateral PE articular surfaces exhibit greater conformity compared to a standard posterior-stabilized PE component, thereby explaining the significant reduction in medial and lateral laxity during mid-flexion. In case 3, an attenuated posterior cruciate ligament contributed to the observed laxity. A larger cruciate-stabilized PE component was used to address flexion instability and compensate for slack in the attenuated soft tissues.

Figure 5.

Example of manufacturer-specific PE trials. Example of regular PS (Left) and constrained PS trials (Right). The constrained implant features a larger post increased conformity of the medial and lateral compartments, which limit coronal and sagittal knee laxity, respectively.

In all 3 cases, increasing the thickness of the PE component decreased the amplitude of the ROM graph to less than 1 mm. An amplitude of less than 1 mm is desirable, as higher values have been correlated with joint instability [3].

Larger longitudinal studies are needed to assess how robotic-assisted IPE compares to traditional IPE. Furthermore, the additional cost of robotic disposables and extra operating time required for robotic-assisted IPE may not be justified in all settings. The ideal candidate for an IPE has not yet been identified, but robotic assistance may help differentiate good candidates from poor candidates.

Summary

Robotic assistance provides surgeons with valuable intraoperative data, enabling more objective decision-making during TKA. By assessing implant orientation and measuring gap symmetry in real time, robotic systems offer potential for precise evaluation of knee alignment and balance throughout ROM. The data generated during trialing of different PE liners is critical for understanding the relationship between liner thickness, gap balance, and knee ROM. Additionally, this information may enable surgeons to assess whether an IPE is sufficient or if a full revision may yield better gap balance. Robotic-assisted IPE may represent a promising, less invasive alternative to full revision surgery in appropriately selected patients with instability. Future research is needed to further explore knee mechanics during ROM and to evaluate the accuracy, longevity, and patient-reported outcomes of this presented technique.

Conflicts of interest

Matthew Brown receives research support from Zimmer Biomet.

David Markel receives royalties from Smith & Nephew; is a paid consultant for Smith & Nephew and Stryker; owns Stock or stock options in HOPco, Arboretum Ventures, Plymouth Capital; receives Research support from Smith Nephew, HFH-Providence Hospital, OREF; has Medical/Orthopaedic publications at JOA, AT; and is a Board member/committee appointments for Michigan Ortho society, Mid-America Ortho Assoc, MARCQI.

Matthew Bullock is on the Speakers bureau/paid presentations and is a Paid consultant for Smith & Nephew; owns Stock or stock options in Stryker and Smith & Nephew; has Medical/Orthopaedic publications editorial/governing board Editorial Board and Journal of Arthroplasty and Arthroplasty Today Journal; is a Board member/committee appointments for West Virginia Orthopaedic Society and AAHKS Digital Health and Social Media Committee.

The other authors declare no potential conflicts of interest.

For full disclosure statements refer to https://doi.org/10.1016/j.artd.2025.101791.

CRediT authorship contribution statement

Liam Cleary: Writing – review & editing, Writing – original draft, Methodology, Investigation, Formal analysis. Alec McCann: Writing – review & editing, Conceptualization. Jake Peterson: Writing – review & editing, Writing – original draft. Caleb Pawl: Writing – review & editing, Methodology. Matthew Brown: Writing – review & editing, Methodology, Investigation, Conceptualization. Alex Caughran: Writing – review & editing, Supervision, Project administration, Methodology, Investigation, Conceptualization. David Markel: Writing – review & editing, Supervision, Project administration, Methodology, Investigation, Formal analysis, Conceptualization. Matthew Bullock: Writing – review & editing, Supervision, Project administration, Methodology, Investigation, Formal analysis, Conceptualization.