Malrotation of the Tibial Component in TKR:The Impact of Implant Design and Surgical Experience

Introduction

Malpositioning of the tibial component is a common error in TKR. In theory, placement of the tibial tray could be improved by optimization of its design to more closely match anatomic features of the proximal tibia with the motion axis of the knee joint. However, the inherent variability of tibial anatomy and the size increments required for a non-custom implant system may lead to minimal benefit, despite the increased cost and size of inventory.

This study was undertaken to test the hypotheses:

1. 1.

   That correct placement of the tibial component is influenced by the design of the implant.

2. 2.

   The operative experience of the surgeon influences the likelihood of correct placement of contemporary designs of tibial trays.

Materials and Methods

CAD models were generated of all sizes of 7 widely used designs of tibial trays, including symmetric (4) and asymmetric (3) designs. Solid models of 10 tibias were selected from a large anatomic collection and verified to ensure that they encompassed the anatomic range of shapes and sizes of Caucasian tibias. Each computer model was resected perpendicular to the canal axis with a posterior slope of 5 degrees at a depth of 5 mm distal to the medial plateau. Fifteen joint surgeons and fourteen experienced trainees individually determined the ideal size and placement of each tray on each resected tibia, corresponding to a total of 2030 implantations. For each implantation we calculated: (i) the rotational alignment of the tray; (ii) its coverage of the resected bony surface, and (iii) the extent of any overhang of the tray beyond the cortical boundary. Differences in the parameters defining the implantations of the surgeons and trainees were evaluated statistically.

Results

On average, the tibial tray was placed in 5.5 ± 3.1° of external rotation. The overall incidence of internal rotation was only 4.8%: 10.5% of trainee cases vs. 0.7% of surgeon cases (p < 0.0001). The incidence of internal rotation varied significantly with implant design, ranging from 1.7% to 6.2%. Bony coverage averaged 76.0 ± 4.5%, and was less than 70% in 8.6% of cases. Tibial coverage also varied significantly between designs (73.2 ± 4.3% to 79.2 ± 3.8%; p < .0001). Clinically significant cortical overhang (>1 mm), primarily in the posterior-lateral region, was present in 12.1% of cases, and varied by design, as expressed by the area of the tray overhanging the cortical boundary (min: 2.3 ± 6.7 mm²; max: 4.7 ± 7.9 mm²; p < .0001). The surgeons and the trainees also differed in terms of the incidence of sub-optimal tibial coverage (10.0% vs. 14.4%, p < 0.001), and cortical overhang (7.4% vs. 9.7%, p < 0.001).

Discussion

1. 1.

   Malrotation, bony coverage and cortical overhang are all strongly influenced by the design of the tibial tray selected and the experience of the surgeon.

2. 2.

   Compared to trainees, experienced surgeons tend to position tibial trays in more external rotation, and with less concern for reduced bony coverage and cortical overhang than trainees.

3. 3.

   This study supports the hypothesis that improvements in the outcome and reliability of TKR may be achieved through attention to implant design.