High variability of proximal tibial asymmetry and slope: a CT data analysis of 15,807 osteoarthritic knees before TKA.

PURPOSE: As the correct rotational and sagittal alignment of the tibial tray are of key importance for optimal total knee arthroplasty (TKA) function, the objective of this study was to determine these individual variations in the proximal tibial geometry in terms of posterior tibial slope (PTS) and tibial surface asymmetry by analysing a large dataset of computer tomography (CT) information. METHODS: A retrospective two-part review was performed on 15,807 datasets that were generated during the design phase for a customized TKA implant. First, 15807 CAD (computer-aided-design) models derived from CT data were used to conduct the analysis on the variation of the PTS. Second, the axial cut of each proximal tibia in a consecutively selected subset of 2202 datasets was used to measure the tibial asymmetry. RESULTS: The majority (65.5%) of tibiae had a posterior slope between 5° and 10°, while 26.5% of knees had a slope > 10°. The asymmetry measured as offset between the lateral and medial posterior boundaries was highly variable, with overall an increasing proportion of patients with high asymmetry with increasing tibial ML width. Only 14% of tibiae exhibited symmetric (< 2 mm offset) lateral and medial plateaus, and 22% had an offset > 5 mm. CONCLUSION: This study from an extraordinary large data base reveals that tibial posterior slope and asymmetry of the tibial profile vary largely between patients receiving TKA with increasing tibial asymmetry with ML width. CT scans might help to preoperatively better select the best fitting TKA, otherwise surgeons intraoperatively will often have to deal with compromises regarding fitting, sizing and rotational issues. LEVEL OF EVIDENCE: Retrospective case series, Level IV.

Evaluation of implant fit and frontal plane alignment after bi-compartmental knee arthroplasty using patient-specific instruments and implants.

INTRODUCTION: The goals of successful bi-compartmental knee arthroplasty are to achieve correct fit and positioning of the implant, while appropriately correcting the mechanical alignment of the leg after surgery. As these requirements are not always reliably fulfilled using off-the-shelf implant systems, newer approaches for bi-compartmental resurfacing have been explored. MATERIAL AND METHODS: In this article we report the radiographic results of 30 patients with anteromedial osteoarthritis (OA) who were treated with a novel patient-specific fixed-bearing bi-compartmental knee resurfacing system using custom-made implants and instruments. Utilizing standardized pre- and postoperative radiographic analyses (based on anterior-posterior and lateral, anterior-posterior weight-bearing full-length radiographs, patella skyline views and preoperative computed tomography (CT) scanning) implant fit and positioning as well as correction of the mechanical axis (hip-knee-ankle angle, HKA) were determined. RESULTS: On average, HKA was corrected from 173.4 ±3.47° preoperatively to 179.4 ±2.85° postoperatively. The coronal femoro-tibial angle was corrected on average 5.61°. The preoperative tibial slope measured on lateral views was 6.38 ±2.4°, while the average slope in the CT-based planning protocol (iView) was 6.14 ±2.40°. Postoperative lateral tibial slope was determined to be 5.77 ±1.97°. The thickness of the posterior femoral cuts was measured intraoperatively and, in all cases, corresponded well to the targeted thickness of the cuts provided by the iView. The joint line was preserved in all cases and the average Insall-Salvati index was 1.078 ±0.11 pre- and 1.072 ±0.11 postoperatively. The fit of the implant components measured by over- or underhang was excellent throughout (< 1.01 mm). CONCLUSIONS: Custom-made bicompartmental knee arthroplasty can ensure optimized fitting and positioning of the implant with restoration of the leg axis. These implants could be considered as an alternative primary solution for knee surgeons treating bi-compartmental disease.