Open wedge high tibial osteotomy does not decrease patellar height relative to femur: A three-dimensional computer model analysis

Abstract

Background

Patellar height, which decreases after open wedge high tibial osteotomy (OWHTO), has conventionally been assessed by tibial references using lateral radiographs of the knee; however, changes in the proximal tibia shape after OWHTO may affect this method. We aimed to evaluate the changes in patellar height position relative to the transepicondylar axis of the femur after OWHTO using in vivo three-dimensional (3D) computer models.

Methods

Fourteen patients who underwent 3D magnetic resonance imaging (MRI) at 30° and 50° knee flexion before OWHTO and after hardware removal were included. 3D computer models of the knee were created from the MRI scans and superimposed over the images taken in each position using voxel-based registration. For patellar height evaluation, a patellar reference point was established at each flexion angle and the femoral condylar planes (FCP) were set, including the transepicondylar axis. The patellar center angle was defined as the angle between an FCP that included the top of the intercondylar notch and an FCP that included the patellar reference point. The patellar center angle was evaluated at 30° and 50° knee flexion before and after OWHTO.

Results

The patellar center angle at 30° and 50° knee flexion did not significantly decrease after OWHTO, whereas the Caton-Deschamps index and Blackburne-Peel index based on tibia-referenced measurements significantly decreased postoperatively.

Conclusion

Patellar height position relative to the femur in the 3D computer model did not decrease after OWHTO, whereas tibia-referenced conventional radiographic measurements significantly decreased. When evaluating patellar height, characteristics of each parameter should be considered.

Introduction

Open wedge high tibial osteotomy (OWHTO) is a successful treatment option for medial unicompartmental arthritis of the knee. However, concerns about the negative effects of OWHTO on the patellofemoral joint have been growing over the past decade. Many clinical studies have revealed a significant decrease in the patellar height measured by radiographic evaluation, including the Caton-Deschamps index (CDI) and the Blackburne-Peel index (BPI), after OWHTO [[1], [2], [3], [4], [5]]. Several clinical studies have reported worsening of cartilage damage in the PF joint after OWHTO [[6], [7], [8], [9]]. Therefore, some researchers recommend performing other procedures that do not induce patellar infera, including closed wedge high tibial osteotomy (CWHTO) or distal tuberosity osteotomy in OWHTO, in patients with patella infera or cartilage degeneration in the patellofemoral joint [10,11].

The problem with assessment methods of patellar height is that post- OWHTO, it is dependent on the index used. There are two types of radiographic methods to measure patellar height: those that relate the patellar position to the tibia (tibia-referenced measurements), including the CDI and the BPI, and those that relate it to the femur (femur-referenced measurement) [12]. Theoretically, changes in the anatomical location of the proximal tibia after high tibial osteotomy (HTO) affect methods based on tibial references and result in inconsistent pre- and postoperative patellar height values. Gaasbeek et al. stated that the BPI and the CDI do not accurately measure the alteration of patellar height after HTO because they are dependent on the tibial inclination and the anteroposterior translation of the proximal tibia [13]. Recently, Ihle et al. proposed a simple method for patellar height measurement based on femur as reference, using anteroposterior radiographs and this new method revealed no change in patellar height following OWHTO [14]. However, it would be unreasonable to compare the results based on tibia-referenced measurements with those based on femur-referenced measurement, since the two methods differ in the reference point used, knee flexion angle (30° flexion or full extension), and patient position (supine or standing position). In addition, the measurement baseline, including TEA and femoral joint surface, used for femur-referenced methods, also change as the lower limb alignment changes after OWHTO. The evaluation of patellar height after OWHTO is controversial.

Recent technological advances in three-dimensional (3D) visualization have enabled the recognition and quantitative evaluation of human knees with high accuracy and reproducibility [[15], [16], [17], [18], [19]]. Yamada et al. reported a new measuring system for the evaluation of patellar height based on femoral references using in vivo 3D computer models [16]. This method would allow for the assessment of subtle changes in patellar position relative to the femur after OWHTO. The purpose of the present study was to evaluate the changes in patellar height based on femur-referenced measurements after OWHTO using in vivo 3D computer models, and compare the results with those of the conventional methods based on tibia-referenced measurements. We hypothesized that the relative patellar height position to the femur does not change after OWHTO when using femur-referenced measurements.