ICJR REWIND: Medial Pivot Implants for Total Knee Arthroplasty
Manufacturers began incorporating the “medial pivot” into knee implants in the late 1990s to mimic native knee kinematics. The goal: A relatively stable medial compartment through flexion and extension, with evidence of femoral rollback in the lateral compartment. What does the literature say about outcomes with these designs?
Dionisio Ortiz III, MD; Akash K. Shah, MD; James Slover, MD, MSc; Ran Schwarzkopf, MD, MSc
It is estimated that by 2030, roughly 3.48 million total knee arthroplasties (TKA) will be performed each year. This equates to approximately $27 billion in annual healthcare expenditures.  Although implant designs have improved since the 1950s, nearly 20% of TKA patients remain unsatisfied,  suggesting that there is significant room for improvement in implant design and/or surgical techniques.
Evolution of TKA Implant Design
Total knee implant design has undergone significant changes over the last 70 years. In the 1950s, Swedish surgeon Börje Walldius, MD, designed the first TKA implant. It was a hinged design that functioned in a single plane of motion, sacrificing the cruciate and collateral ligaments for implantation. This prosthesis was extremely stiff and had a high failure rate due to loosening.
In the 1970s, several designs were developed: the polycentric knee, the geomedic knee, the duocondylar knee, and the total condylar knee.
The polycentric knee, developed by Frank Gunston, MD, from Canada, was anatomic in its design. Cruciate and collateral ligaments were preserved, and 2 separate implants were used for the medial and lateral condyles. This implant had high failure rates due to poor fixation of the tibial components.
In the geomedic knee, developed by Mark Coventry, MD, and colleagues at Mayo Clinic in the US, the single tibial component was more conformed to the femoral component. However, the design was too stiff and high failure rates were observed due to implant loosening.
The duocondylar knee, developed by Chitranjan S. Ranawat, MD; John N. Insall, MD; and Peter Walker, PhD, relied on native soft tissue balancing and preserved the cruciate ligaments. The tibial component was composed of 2 flat pads that were placed around the tibial eminence. After initial implantations, the surgeons noted several limitations: The tibial component was difficult to place around the tibial eminence; the flat pads of the tibial component did not contribute to native knee kinematics or stability; and preservation of the cruciate ligaments prevented deformity correction in severe cases.
They improved on their design with the total condylar knee, which greatly increased the longevity of the implants. This cruciate sacrificing (CS) design had an anterior femoral flange that improved patellar tracking and anterior and posterior lips on the tibial component that helped prevent dislocation. The main disadvantage was the limited femoral rollback and knee flexion.
The Insall–Burstein knee, developed bt Dr. Insall and Arnold H. Burstein, PhD, was introduced in 1978 and was touted as a cam-post design. Surgeons consider this to be the origin of most modern posterior stabilized (PS) knee implants. The femoral cam articulated with the post in knee flexion, improving femoral rollback and anterior and posterior translational stability of the knee. It also improved stair climbing by preventing posterior tibial subluxation. The main disadvantage of this design was a high rate of patellar dislocation. Newer implants have an asymmetric anterior flange, which is more prominent laterally to improve patellar tracking.
Native Knee Kinematics
An understanding of knee kinematics can help engineers and surgeons improve patient outcomes by restoring native anatomy. The posterior cruciate ligament (PCL) prevents posterior translation of the tibia. As the knee flexes, the contact point moves posteriorly with an intact tibia, known as femoral rollback. If the PCL is retained, a flatter polyethylene liner is needed to prevent impingement in deep knee flexion. If the PCL is sacrificed, a lip is necessary for anterior and posterior translational stability. Retention of the PCL with use of a lipped polyethylene liner can result in impingement and decreased knee flexion, a concept known as kinematic conflict.
It was initially thought that the PCL could better perform femoral rollback than the cam-post mechanism. However, fluoroscopic studies have shown that cruciate retaining (CR) implants do not replicate kinematics of the normal knee. [2,3] Improper PCL tensioning causes paradoxical roll forward of the femur and anterior translation of the contact area. This can lead to decreased knee flexion and reduced quadriceps efficiency. A tight PCL can cause accelerated polyethylene wear, whereas a loose PCL can lead to flexion instability and knee subluxation.
Although PS prostheses do not completely reproduce normal knee kinematics, reliable rollback does occur. The modern PS knee design is predicated on the “4-bar link” theory, which states that the knee’s flexion-extension axis of rotation is located at the intersection of the cruciate ligaments and moves posteriorly and inferiorly with increasing knee flexion. The cam-post mechanism of the PS design serves to replicate this phenomenon. Later biomechanical data demonstrated that the knee flexion-extension axis is more constant, however, approximately through the medial and lateral epicondyles. This is known as the “single radius” theory. 
However, PS implants do have issues. The most common complications include knee dislocation with the cam jumping the post, polyethylene wear at the cam/post interface, and patellar clunk syndrome. In addition, compared with CR implants, PS components require more bone from the femur, potentially increasing the risk of periprosthetic fracture.
Further data have demonstrated that native knee kinematics are a combination of the 2 theories: The medial compartment is relatively stable through flexion and extension, with evidence of femoral rollback in the lateral compartment. This phenomenon is known as “medial pivot,” and starting in the late 1990s, TKA implants were designed with this in mind. The medial pivot knee design was born.
Benefits of Medial Pivot Implants
The medial pivot knee design involves a CR femoral component and a highly congruent polyethylene liner. The medial compartment acts as ball and socket and pivot center, while the lateral compartment is less concave to allow the lateral femoral condyle to roll posteriorly during flexion. In addition, the anterior lip on the polyethylene liner functionally acts to replace the PCL by limiting excessive posterior translation of the tibia.
Fan et al  conducted a retrospective study of 5-year outcomes of knee replacements involving the medial pivot design (Advance medial-pivot knee system, MicroPort Orthopaedics; Arlington, Texas). The study involved 58 knees with mild to moderate genu vara (0° to 16°). The authors noted that postoperative range of motion improved on average by 12° (preoperative 103°, postoperative 115°) and that Knee Society scores improved significantly in the pain and function subsets. 
Mid-term follow-up studies also show beneficial outcomes with a medial pivot design (Advance medial-pivot knee system, MicroPort Orthopaedics). In a 10-year retrospective study of Japanese patients who underwent TKA with this design, the implant survival rate was 98.6% at final follow-up; no patients had aseptic loosening.  Japanese culture is a floor-based lifestyle that requires kneeling, sitting on the ground, and other deep flexion activities. The authors noted long-term stability of the medial pivot design in these patients, who place higher-than-average demand on their implants.
A French study analyzed the same implant and showed 95.9% survivorship at 10 years, with no aseptic loosening, in 74 knees.  Patients had improved long-term flexion from 98° preoperatively to 110° postoperatively. 
Batra et al  conducted a randomized controlled trial comparing PS and medial pivot implant designs in 53 patients who underwent bilateral knee replacements (1 PS, 1 medial pivot), all performed by a single surgeon. The average follow-up was 4 years. The Knee Society Score was, on average, 3 points higher and statistically significant in medial pivtor knees at 3 and 6 months and at final follow-up at 4 years.  The Oxford Knee Score, range of motion, and radiographic parameters assessing implant position were not significantly different between the 2 groups. 
In another comparison, Samy et al  demonstrated similar outcomes in range of motion and patient satisfaction with a PS knee (Persona PS, Zimmer-Biomet; Warsaw, Indiana) and a medial pivot knee (Evolution medial-pivot knee system, MicroPort Orthopaedics), all implanted by a single surgeon. The authors found no difference in range of motion at 1 year between the 2 groups. However, medial pivot knees scored higher in all activities, particularly deep knee flexion activities, in the patient-reported Forgotten Joint Score questionnaire, which measures awareness of the TKA implant with daily activities.
PS Implants Have Equivalent Outcomes
In a randomized controlled trial of patients undergoing staged bilateral TKA, Prichett et al  used a medial pivot implant in one knee and either a CR implant or a PS implant in the contralateral knee. Patients preferred the medial pivot knee: 79% versus the CR knee and 77% versus the PS knee.  Interestingly, there was no significant difference in preference for patients with a medial pivot knee on one side and a bicruciate retaining knee on the contralateral side. Despite preference for the medial piviot knee overall, there were no significant differences in postoperative range of motion, pain, or Knee Society Score. 
Choi et al  compared 52 mobile bearing knees with 49 medial pivot knees and also found no significant differences in patient-reported outcome measures. However, there was an increased rate of flexion contracture in the medial pivot group, which was independently associated with patient dissatisfaction. Patients who did not have flexion contracture had similar levels of satisfaction regardless of group. 
A prospective trial of 50 patients randomized to either a PS or a medial pivot knee was performed by Edelstein et al.  Although patients in the PS group reported lower satisfaction with activities designated to be “weight-bearing in flexion,” there were overall no differences between groups in any patient-reported outcome measure, as well as no differences in revision or complication rates. Of note, patients with more than 10° of valgus deformity were excluded from analysis.
A systematic review and meta-analysis of 13 level I-III studies by Tso et al  found overall similar outcomes between the medial pivot knee, referred to as “medial stabilized,” and non-medial stabilized designs. Of the outcome measures included in the meta-analysis, no statistically significant differences were seen in the Knee Society Score or WOMAC score. Nor were there significant differences in the Oxford Knee Score and Forgotten Joint Score that, while favoring the medial pivot design, exceeded the known minimal clinically important difference.  Most other studies included in the systematic review, but excluded from the meta-analysis, did not favor one design over another.
A more recent randomized controlled trial by Chang et al  compared 43 medial pivot knees with 45 PS knees. The primary outcome measure was range of motion, while the secondary outcome measures were various patient-reported outcomes (WOMAC, Knee Society Score, Oxford Knee Score) and radiographic parameters. There were no significant differences in any measured primary or secondary outcome.
Other authors have sought to determine whether knees display medial pivot kinematics in the operating room without the use of specific medial pivot knee designs. Intraoperative pressure sensor technology was used to divide patients into those displaying what was described as “medial pivot kinematics” versus those who did not display such behavior while going through a passive range of motion.  The authors discovered that approximately 50% of knees with a CR/CS implant and 28% of knees with a PS implant demonstrated medial pivot kinematics using sensor data, but there was no difference in patient-reported outcome measure between patients who displayed medial pivot kinematics and those who did not. 
Long-Term Survivorship Data from National Registries
Although medial pivot knee designs have been shown to perform at least as well as modern PS and CR designs, survivorship data are beginning to emerge from registry data, mainly in the United Kingdom and Europe.
Cassar-Gheiti et al  compared data from the UK National Joint Register (UK NJR), the Australian Orthopedic Association National Joint Replacement Registry, and the Dutch Arthroplasty Register and found:
- 15-year survivorship of 95.4% for the UK NJR
- 15-year survivorship of 91.9% for the Australian registry
- 10-year survivorship of 90.2% for the Dutch registry
These data were comparable to survivorships of PS and CR knees in the same registries. The etiologies for revision are generally not reported in national registers.
The American Joint Replacement Registry has not yet released data regarding survivorship of medial pivot knees.
The objective goal for TKA is to obtain a painless knee joint and restore native knee kinematics. There continues to be a high rate of patient dissatisfaction after conventional TKA. An improvement on the standard design, the medial pivot knee offers increased stability with more-normal knee kinematics. It is also associated with higher patient satisfaction scores and range of motion equivalent to that of PS and CR knees.
Additional long-term data are needed to determine whether the medial pivot design is a suitable replacement for more traditional PS and CR knee designs.
Originally published as Ortiz D III, Shah AK, Slover J, Schwarzkopf K. Is Medial Pivot Design a Major Advancement in Total Knee Arthroplasty? ICJR.net. April 15, 2021. https://icjr.net/articles/is-medial-pivot-design-a-major-advancement-in-total-knee-arthroplasty. Accessed November 17, 2021.
Dionisio Ortiz III, MD, and Akash K. Shah, MD, are NYU/ISK Adult Reconstruction Fellows at NYU Langone Health, New York, New York. James Slover, MD, MS, and Ran Schwarzkopf, MD, MSc, are from the Department of Orthopedic Surgery, Division of Adult Reconstructive Surgery, at NYU Langone Health, New York, New York.
Disclosures: The authors have no disclosures relevant to this article.
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