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Friday, March 17, 2000

Should the tibial stem be moved on the tray?

Researchers who developed scientific exhibit 36 on "Laterally Eccentric Tibial Stem Position in Total Knee Arthroplasty, asked the question, "should the stem be moved 3 mm medial on the tray?"

The central stem on a tibial tray in primary and revision total knee arthroplasty gives stability of tray fixation, they said. The stem with extensions will impinge on the lateral cortex of the tibia require greater proximal bone resection if maximal surface coverage by the tray is to be maintained.

The researchers concluded from their study that the central tibial stem usually will not cause lateral cortex impingement unless further proximal resection, stem extensions or inadvertent varus position of the tray is required. Since the resected surface is the reference for the stem preparation, the tip of the stem will follow the lateral arc of the medial cortex at each resection level. Placement of the stem 3 mm medial of center will avoid most problems and support the area of the tray where more force passes in a total knee.

The study was done in three stages. In stage I, a solid three-dimensional digital model of a tibia from CAT scan data was taken. Three levels of resection were done at 10 mm intervals. A size three PFC (DePuy) modular tray was placed on the first two resections and a size 2.5 on the last one.

The researchers measured the eccentricity of the central stem from the midline with the standard stem, 30 mm, and 60 mm stem extensions. In stage II, they reviewed X-rays of 30 uncomplicated primary and 30 uncomplicated revision total knee replacements with the PFC system and measured the millimeters of eccentricity with the X-Caliper (Eisenlohr) measuring device at the stem tip. In stage III, they took three standard saw bones and introduced and removed the intramedullary (IM) guide rod. They resected at 3 levels of 8-10 mm each while using an extramedullary guide.

At the proximal level, they introduced the stem punch through the standard tray template in the proper position for tray coverage. At each two subsequent resections, they measured the position of the IM guide hole and the stem punch hole with reference to the preparation using the new tray at optimal orientation.

The computer simulation found the stem to be eccentric at its tip by at least 3 mm to the lateral in the primary resection. With each resection the stem got closer to the lateral cortex. All of the stem extensions did the same thing. The Stage II postoperative measurements averaged 3 mm eccentric to the lateral cortex.

The saw bone preparation demonstrated progressive lateral placement of the stem with each distal resection level if the tray was oriented for proper rotation and coverage. The intramedullary stem for the resection guide was never at the proper position with reference to the tray preparation.

The researchers, are Wayne M. Goldstein, MD, chief, division of orthopaedic surgery at Lutheran General Hospital, Park Ridge, Ill, and assistant professor, orthopaedic surgery at Northwestern University; Thomas F. Gleason, MD, clinical assistant professor of orthopaedics at the University of Chicago; S. David Stulberg, MD, professor of clinical orthopaedics at Northwestern University; Jill J. Branson, BSN; and Kimberly Berland, CST.

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2000 Academy News March 17 Index C

Last modified 24/February/2000 by IS