Abstract
Background: Coronal alignment is important in long-term survival of TKA. Many systems are available; most aim to produce a posterior slope on the tibial component in order to reproduce the 70 seen in the normal tibia. Some are designed to produce a bone cut with 70 of slope whereas others combine the slope of the bone cut with an in-built slope on the polyethylene insert. We have investigated the theory that resecting the tibial plateau with a posterior slope can introduce error in coronal plane alignment in TKA.
Methods: 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 extra-medullary 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:
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Medial rotation of the cutting block
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Medialisation of the plateau reference point
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Medio-lateral translation of the distal jig 4. External rotation of the distal jig
Results: 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 from 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.
Conclusions: 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: Tim Wilton, BASK, c/o BOA, The Royal College of Surgeons, 35–43 Lincoln’s Inn Fields, London WC2A 3PE.
