Commentary Article - (2022) Volume 7, Issue 1
The ideal articulating surface in total joint design is difficult to determine. In total hip arthroplasty, the variability of bearing surfaces used and the grandiosity of their subsequent failures has generated much discussion, press, litigation, and catastrophic patient impact. The original hip designs used natural surfaces such as ivory. The Charnley total hip arthroplasty started with the use of Teflon which failed rapidly due to erosion, failing mechanical properties, and associated granulomas [1,2]. High density polyethylene was initially researched against Sir John Charnley’s wishes, who was then convinced after data showing wear of less than 1/2000 of an inch over the first two days, which was far less than previously used Teflon [3]. Modern highly crosslinked polyethylene shows wear rates even better than that, with radio stereometric analysis showing yearly wear of less than 0.06 and as low as 0.005 mm. Even though most wear studies are performed in the hip, some of the data extrapolates the findings for the knee.
Polyethylene has been the gold standard for the articulating surface in total knee arthroplasty implants since their original design in the 1970s following principles outlined by Freeman et al. [4]. Unlike the hip where a lot of attention has been focused on the mechanical properties of polyethylene, the geometry of the articulation is of significant importance in the total knee arthroplasty design. From its conception to the modern day, there has been significant evolution in the geometry of the polyethylene insert. In the late 20th century early stages of design such as the duocondylar and geometric prosthesis, polyethylene was used as a flat tibial component [5]. The introduction of anterior and posterior lips of the polyethylene tibial component came about with the design of the total condylar implant, and the basic mechanics of this early prosthetic are now utilized in modern implants [6].
Continued optimization of the polyethylene insert seeks to better replicate native knee kinematics as well as provide constraint. The use of posterior stabilized symmetric inserts, such as the Sigma Curved (Cvd) design, have been successful in mimicking native knee movement for patient’s retaining their posterior cruciate ligament (PCL). These symmetric inserts have shown high patient satisfaction and longevity following total knee arthroplasty (TKA) [7]. Despite the success of these symmetric inserts, increasing revision numbers have prompted further research of alternate insert designs to address instability and range of motion issues. Asymmetric inserts, which differ geometrically from symmetric inserts in their proximal surface came about with the development of the mobile bearing TKA implant. These asymmetric polyethylene inserts were introduced to improve stability for patients following TKA [8].
To address issues of instability in patients with deficient or deteriorated PCLs, the development of the ultracongruent insert occurred. Originally posterior stabilized implants were utilized in these cases, but due to the nature of their design there was a high risk of polyethylene wear on the cam-mechanism [9]. The ultracongruent insert is characterized by a high anterior wall and a deeper trough which allows for posterior stability without the cam-mechanism, ultimately reducing wear [10]. Decreased wear of ultracongruent inserts was also made possible with the creation of highly cross-linked polyethylene. Conventional polyethylene has been credited as a common cause of revision TKA due to insert particle wear [11]. Subsurface cracking of conventional polyethylene inserts due to compressive and tensile loads release particles into the joint space inciting periprosthetic inflammation. Over time this can lead to osteolysis and ultimate implant failure [12,13] Studies have indicated that highly crosslinked polyethylene has greater wear resistance, making it ideal for use in ultracongruent inserts [14].
The Sigma Curved Plus (Cvd+) insert became commercially available in 2004 and follows the design features of other ultracongruent inserts [15]. Our study looked at the difference in outcomes for patients who received either the Sigma Cvd or Cvd+ polyethylene inserts during TKA. 68 patients were retrospectively reviewed during the study with SF-12 MCS, SF-12 PCS, KOOS, KOOS-Pain and KOOS-ADL data assessed both pre-and postoperatively. For each outcome measure the absolute improvement (AI) along with the percentage of maximal possible improvement (%MPI) were calculated and the difference in primary outcomes, AI, and %MPI between CVD and CVD+ inserts determined using two-sample t-tests. The results of the study indicated that the only statistically significant difference between the CVD and CVD+ designs was that the CVD design had a greater improvement in KOOS-pain scores for both AI and %MPI. The increased level of pain for the ultracongruent design could indicate a kinematic disparity. Despite this finding, further investigation is required as the bias of a single surgeon retrospective study could have led to ultracongruent insert designs being utilized for higher complexity cases [16].
It is well documented in the literature that the use of ultracongruent polyethylene inserts in PCL-sacrificing TKA provides statistically similar outcomes to patients who underwent PCL-sparing knee replacement [17]. The improved wear profile of the ultracongruent highly crosslinked polyethylene insert compared to the posterior stabilized insert indicates that the prior is the preferred choice in PCL-sacrificing TKA.
Despite the annual number of total knee replacements predicted to increase 40% by 2040, roughly 20% of patients are not satisfied with their surgical outcome [18,19]. While the design and kinematic movement of implants have improved with innovations like the ultracongruent insert, these statistics demonstrate a need for continued development of implants which better mimic the native knee. Instability of the knee joint and reduced range of motion are common complaints following TKA and most likely stem from a difference in kinematics between the implant and patient’s native knee [20]. Studies have indicated that posterior stabilized inserts are associated with a “paradoxical motion” during active motion and this phenomenon has been linked to post-surgical complications including instability, pain, and poor range of motion [21].
Current attempts to address this “paradoxical motion” and better mimic the native physiology in implant design have focused on the anatomy of the medial and lateral compartments. Medial pivot polyethylene inserts have been designed with a more concave and congruent medial compartment and a shallower lateral compartment that displays less conformity [22]. The deeper medial compartment transfers a greater percentage of the patient’s body weight to the medial side, improving stability between full extension and deep flexion. While some research has indicated that medial pivot inserts provide patients with improved post-surgical outcomes, further investigation comparing medial pivot polyethylene inserts with other designs is needed to provide significant evidence [23,24].
With over one hundred and fifty total knee implants on the market today, companies and surgeons continue to optimize implant design for better functional outcomes following surgery. There is strong interest focused on modifications to the polyethylene insert of the implant, with recent advancements in wear properties (highly cross-linked polyethylene) and geometric design (ultracongruent and medial pivot inserts). Ultimately as developments are made, the focus will remain on creating a total knee implant which best replicates the kinematics of a patient’s native knee joint.
Citation: Barrett C, Barrett T, Testa EJ, et al. A Review of Knee Polyethylene Geometry Design - Progression to Constraining. Med Rep Case Stud. 2022, 07 (1), 003-004.
Received: 11-Jan-2022 Published: 27-Jan-2022
Copyright: 2022 Barrett C, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
