Surprising Outcome From New Test of TKA Prosthetic Designs

How do standard condylar designs compare to guided motion designs when it comes to knee kinematics? A team from NYU Langone Health in New York has looked at this, publishing a study, “The effect of total knee geometries on kinematics: An experimental study using a crouching machine.” Their work appears in the December 2021 edition of the Journal of Orthopaedic Research.

Their conclusions? “Standard condylar designs, even with variations in sagittal radii, are unlikely to reproduce anatomic kinematics. On the other hand, designs with asymmetric constraint between medial and lateral sides, and other guiding features, are likely to be the way forward. The mechanical testing method could be further improved by superimposing shear forces and torques during the flexion–extension motion, to include more stressful in vivo functional conditions.”

According to the research team, there are three standard condylar designs:

• femoral sagittal radii with constant radius,
• J-curve, and
• G-curve (low and high constraint tibial surfaces).

As for the guided motion designs, the researchers considered a medial pivot and a design with asymmetric condylar shapes and guiding surfaces.

Working with a crouching machine they built (based upon the so-called Oxford rig), the authors simulated the action of the lower extremity during crouching between 0° and 125° flexion, applied quadriceps and hamstring loading, and collateral soft tissues.

Co-author Peter Walker, M.D., director of the Laboratory for Orthopedic Implant Design and professor in the Department of Mechanical and Aerospace Engineering at the NYU Tandon School of Engineering, explained the purpose of the study to OTW, “Artificial knees have been used now for about 50 years. There is high durability of the procedure, even at 20 years or more of follow-up.”

“The function is satisfactory in most patients, although in many cases, not up the level of being normal. Improvements could be achieved by better artificial knee designs, surgical techniques, and rehabilitation. Two artificial knee designs, which were invented by 1980, dominate the usage today, so it is timely to ask whether new types of design would further improve the clinical results. A combination of both laboratory testing and clinical evaluations is needed to answer the question.”

“To compare different artificial knees, we bult a test machine which simulated a crouching action,” added Dr. Walker, “representing getting up and down from a chair, or climbing stairs, difficult activities for many patients. Our premise was that the ideal motion for an artificial knee would be that of the normal knee, with both rollback and axial rotation during the flexion motion.”

“The two asymmetric designs showed posterior displacements during flexion, but less axial rotations than anatomic knees,” wrote the authors. “The quadriceps forces throughout flexion were very similar between all designs, reflecting similar lever arms…”

“We found that the standard types of designs, had only small rollbacks and rotations. Tweaking the designs with small changes in the radii made little difference to the motion. However, designs with specially shaped bearing surfaces did produce motion which more resembled that of normal knees. Surprisingly, all of the designs required about the same muscle forces.”

“Over a period of time, if many other studies, both in the laboratory and in clinical practice, show advantages of certain new types of artificial knee, there will be a gradual shift towards the usage of those new design types. In the case of the artificial knee, it is likely that there will be parallel shifts in surgical technique, such as the use of computer-assisted surgery, and functional monitoring systems for improved rehabilitation. The end result will be a steady improvement in functional results for the patient.”