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THE EFFECT OF POSTERIOR TIBIAL SLOPE ON CORONAL ALIGNMENT IN TOTAL KNEE ARTHROPLASTY

Coronal alignment is an important factor in long-term survival of TKA. Many implant systems are available and most aim to produce a posterior slope on the tibial component to reproduce the 7⁰ seen in the normal tibia. We hypothesized that resecting the tibial plateau with a posterior slope can introduce error in coronal plane alignment in TKA.

We used a standard saw-bones model in conjunction with a computer navigation system that is available for use in TKA (Stryker Orthopaedics). The normal protocol for preliminary referencing was followed; care was taken to identify tibial landmarks (tibial plateau reference point, true sagittal plane and transmalleolar axis). We then used a standard extramedullary alignment jig (Scorpio TKR System, Stryker Orthopaedics) with cutting blocks designed to give 0, 3, 5 and 7 degrees of posterior slope and varied the position of the alignment jig.

Variations included:

1. Medial rotation of the cutting block,
   
2. Medialisation of the plateau reference point,
   
3. Mediolateral translation of the distal jig, and
   
4. External rotation of the distal jig.
   

In all experiments, there was a greater deviation from ideal coronal alignment as the slope on the tibial cut was increased. The greatest influence was with external rotation of the distal part of the jig, which produced 3⁰ of varus at only 15⁰ of external rotation with a 7⁰ slope. Medialisation of the proximal reference point worsened this to 4.5⁰ of varus.

We have quantified the degree of coronal malalignment that can occur for different posterior slopes during tibial resection for TKA. We recommend either using a minimal slope or navigation to ensure correct implant positioning.

Correspondence should be addressed to Major M Butler RAMC, Princess Elizabeth Orthopaedic Centre, Royal Devon and Exeter Hospital, Exeter, Devon.

Correspondence should be addressed to Major M Butler, CSOS, Institute of Naval Medicine, Crescent Road, Alverstoke, Hants PO12 2D