Primary Knee Symposium: Biomechanical Considerations in the Knee

PS vs. CR vs. CS

Why I Prefer PCR TKR – Kenneth Krackow, MD

Dr. Krackow acknowledged that use of a particular design was more a matter of "religion" and that most surgeons tend to stick with the type of knee they trained on. Literature does not seem to be in favor of one type over the other, with the exception of TKR after a patellectomy. Dr. Krackow outlined three indications for when he would chose a PS design. PSlide A In a series of about 100 of his knee patients (Duracon PS), he had two peg subluxers and one peg fracture in a very active patient.

He disagreed that deformity management is easier with a PS design. He prefers a CR design in those cases, giving his reasons:

• There is added residual instability in the less than perfect balance that may result in a severe deformity
• There is a good secondary stabilization effect.

He does not believe resecting the PCL helps in flexion contracture. He is unconvinced that PS knees give varus-valgus stability, so he does not feel it provides stability in cases of great deformity unless the deformity is residual posterior tibial subluxation (very rare in his experience). He also finds it more difficult to prepare the box (which he conceded may be due to inexperience with the device).

Why I Prefer PS TKR – Thomas Schmalzried, MD

Dr. Schmalzried argued in favor of a PS knee, outlining the reasons he chooses PS. PSlide B He opined that all TKRs are ACL deficient. He said you need posterior-tibio contact if you want deep flexion and PS designs can give you that. And although there have been excellent results with many CR, PS, and PCR designs, he feels there may be more rapid poly wear with some CR knees. Higher conformity and lower contact stresses can lead to lower wear. His position here therefore was that what was really needed was better poly.

He presented that there is generally more controlled roll-back and more consistent flexion ROM with PS designs. He did however offer caution in attempting to achieve high flexion. PSlide C

Post wear/damage unique to some PS designs may include:

• Abrasive wear of post from cement
• Extrusion of cement into open box during implantation

CS Bearing Insert, The Right Choice For TKR – Kirby Hitt, MD

Dr. Hitt argued that the clinical results and loosening rates are essentially equal in PS and CR knees. He feels that technique issues should guide usage and that alignment and balancing are more important than the insert choice. He reviewed what a CS insert is PSlide D and discussed the potential advantages PSlide E including the potential for greater anterior constraint without sacrificing internal/external rotation. In his mind, there are two main reasons for why to use a CS insert: the potential for paradoxical anterior translation in PCR knees, and the questionable integrity of the PCL when retained.

He cautioned that a CS insert is not:

• A substitute for meticulous flexion and extension balancing
• A substitute for absent PCL function

He concluded by reviewing his indications for a CS insert. PSlide F