CLINICAL SIGNIFICANCE OF BACKSIDE POLYETHYLENE WEAR OF A MODULAR INGROWTH BASEPLATE IN TKA 5–10YR FOLLOW-UP

Abstract

BACKGROUND: Clinical results at 10–14yrs of the NKI noncemented TKA(AAOS 2005, Plaster) showed no failure of ingrowth and no tibial osteolysis. A similar 10yr study of the NKII series indicated a higher revision rate based solely on tibial osteolysis at the tip of the medial screw, creating a 9% partial revision rate (poly-exchange and grafting of cyst) of NKII at 10yrs. There is no failure of the actual ingrowth bond.

METHODS: Between Dec 1995–01, 1410 pts were implanted with NKIIs (Zimmer). Prospective data was collected at preop, surgery & intervals up to 5–10yrs. Classification of osteolytic cysts is defined: Grade 1=cyst of 1cm; Grade 2=cyst > 1cm without cortial erosion; Grade 3A=cyst < 1cm with cortical erosion; Grade 3B=cyst 1cm with cortical erosion & Grade 4=stress fracture or perosteal reaction indicative of a stress fracture. Specimens from revisions have been evaluated by 2 independent labs. Inserts were replaced using a highly crosslinked poly.

RESULTS: 445 pts were seen for clinical and xray f/u, 65 expired. Osteolysis occurred in 36% most of which were early stages. Revision were required in 9%. Exams of the poly show wear with football shaped extrusions into the screw holes & abrasive scores consistent with micromotion. Microscopic analysis show macrophages with inclusions suggestive of poly debris but there is a paucity of actual poly seen.

DISCUSSION: Osteolytic cysts are occurring in the NKII & not in NKI. It is not known if these are design issues or poly “improvement” issues. One postulation is smaller particles but increasing numbers has led to a decrease in the bone tolerance. Early f/u of highly cross-linked poly shows marked improvement and no cysts.

Generation of the tibial cyst is multifactorial. Implantation techniques can increase poly imminence impingement by the femoral notch thus increasing forces on poly during gait. Lowering the notch on the NKII improves patella contact but can lead to impingement if the surgeon flexes the femur or places too much slope on the tibia. Hydraulic action is also postulated, joint fluid is being forced down the screw channel carrying with it small poly particles. The tight peripheral tolerances of the insert into the baseplate may create such a tight fit that the insert acts like a piston with each step. Lab testing is currently being done to test this hypothesis and evaluate micromotion and wear patterns. We are recommending polyexchanges for stg 3 & 4 pts to the highly crosslinked poly as well as curetting and grafting all cysts. Early results show cyst resolution. Stress fractures may require long stem revision baseplates. Stg 1 & 2 can be followed with serial xrays