Objective

It is reported that 20% to 30% of patients are not happy after a total knee replacement (TKR). As a result, medial pivot designs have been introduced, these implants are designed to improve the kinematics of the normal knee by restricting the paradoxical anterior movement of the femoral condyles and improving natural rollback. It is unclear whether these newer designs of TKR prostheses improve the clinical results.

Method

A randomized control clinical trial (RCT) was conducted comparing three different prosthetic designs. The prostheses consisted of the U2 Cruciate Retaining (CR) knee (United Orthopaedic Corporation), a conventional fixed-bearing prosthesis with symmetric non-anatomic tibial plateaus. The Journey II CR (Smith and Nephew) with an anatomic coronal joint line as well as asymmetric tibial plateaus to increase femoral roll back and the Persona MC CR (Zimmer Biomet) a medial pivot design. Both the modern designs have asymmetric anatomic tibial plateaus.
The trial was done in a single institution and the surgery was performed by the same surgeon using the same surgical technique with a similar post-operative regimen. The type of prosthesis was randomly assigned using a computer program. Preoperative both the Knee Injury and Osteoarthritis Outcome Score (KOOS) and UCLA activity score were done. The scores were repeated at 6- and 12-months post-operative and a forgotten knee score (FJS-12) was added to the scores at 12 months.
174 patients, 59 males, and 115 females with a mean age of 68.3 years received primary unilateral cruciate-retaining (CR) TKR’s between June 2018 and April 2019.

Results

The average KOOS outcome score improved significantly from 43 preoperative to 79 at 6 months and 86 at 12 months.
At 12 months follow up the total (p = 0.049) score as well as the subscores of symptoms (p = 0.03), and life-quality (p = 0.02) of KOOS were significantly higher for U2 than Journey II. The difference in median of life-quality KOOS between the U2 and Journey II was greater than the minimum clinical differences (MCID), indicating that the difference may be clinically relevant when the outliers are disregarded. No significant difference was observed between the two modern designs (Journey II vs Persona) and the Persona MC and the conventional prosthesis (Persona MC vs U2)
There was no significant difference in the UCLA activity score between the three prostheses. It improved, on average, from 3.9 preoperative to 5.6 at 12 months post-operative.
The FJS- 12 also did not show any significant difference with an average score of 86 at 12 months.

Discussion

Notwithstanding claims that the modern medial pivot designs improve knee kinematics, we were unable to find them superior in clinical scores at 12 months post knee replacement. The one medial pivot design (Journey II) had two significantly lower subscores in the KOOS category.
More factors than knee prosthetic design may play a role in unsatisfactory results.