Biplanar PSI Slope-reducing MOWHTO With Tibial Tuberosity Serving as Hinge Axis: Cadaveric Study

Introduction:

High tibial osteotomy (HTO) is a widely performed procedure in case of varus knee with medial compartment osteoarthritis (OA), with medial-opening wedge version performed more often than lateral-closing wedge one. While in most cases the goal of medial-opening wedge HTO (MOWHTO) is isolated correction of coronal limb alignment, unintentional increase of posterior tibial slope (PTS) after MOWHTO can occur, however its extent is usually limited. Meta-analysis by Nha et al. published in 2016 and summarizing 27 studies including 1260 MOWHTO procedures, reported mean increase of PTS by 2.02° (95% CI, 2.66° to 1.38°; P = .005). One of factors limiting the extent of PTS change both in above described unintentional circumstances and in desired biplanar corrections, can be the fact that in most cases lateral cortex of the tibia is not transected. Therefore, increase of PTS after MOWHTO was proposed to be caused by the unique anatomical characteristics of the proximal tibia such as non-perpendicular angle between anteromedial and lateral cortex. However, this phenomenon has its limitations and in cases when greater correction of PTS is required, transection of lateral tibial cortex may be necessary, as is routinely performed in osteotomies targeted at isolated PTS correction. In such cases, posterior cortex is remained intact to serve as hinge axis.

On the other hand, in some cases significant biplanar correction of both coronal and sagittal knee alignment is desired. However, transecting all three tibial cortexes increases technical difficulty of the procedure and may increase time to union and the risk of non-union. Therefore, optimal placement of hinge axis to achieve higher accuracy of desired biplanar correction is intensively studied and discussed. In general, anterolateral placement of hinge axis is established to be necessary in order to achieve both valgus correction and PTS decrease. Such biplanar correction may be desired i.e. in anterior cruciate ligament (ACL) revision cases with varus alignment, especially in cases with associated posterolateral corner (PLC) injury, as varus alignment was shown to increase stress both in ACL and PLC grafts. What is more, Won et al. have shown that as much as 19% of ACL revision cases presented with radiographic OA of Kellgren-Lawrence 2 or higher at the medial tibiofemoral joint, highlighting the potential indication for biplanar MOWHTO. Another issue faced during MOWHTO is its influence on patellar height. Multiple studies suggested the possibility of iatrogenic lowering of the patella or the patella baja after MOWHTO. However, as early as in 1979 Goutallier et al. reported that more anterior placement of hinge axis may diminish the impact of HTO on patellar height, which remains in agreement with the proposed anterolateral placement of hinge axis. Up to date, most of the studies assessing impact of hinge axis localization on biplanar correction are based on 3D models and simulations instead of cadavers of real-life procedures, with inherent limitations of that. What is more, accurate and reproducible placement of hinge axis remains technically challenging. One of the ideas developed to improve accuracy of hinge axis placement and accuracy of biplanar correction are Patient-Specific Instruments (PSI), with good results reported by multiple authors.

Therefore, the aim of this cadaveric study was to assess accuracy of biplanar correction with precisely planned increasing various coronal and sagittal amounts of correction utilizing novel, biplanar, medial-opening wedge, posterior tibial slope-reducing high tibial osteotomy aided by PSI, with tibial tuberosity serving as hinge axis. The primary hypothesis of this study was that: 1) There will be no significant differences between planned and achieved biplanar corrections neither in coronal nor sagittal planes. Secondary hypotheses were as follows: 2) Arhtrex PEEKPower HTO plates a) provide good intraoperative stabilization, b) regardless of the change of plate location there will be enough space to preserve soft tissues and no need for new plate design. c) Material used for plates construction will allow for precise evaluation in Computed tomography (CT); 3) No intraoperative fractures will occur; 4) No significant change of patellar height will occur.

Material and Methods:

Preoperative measurements:

Six fresh-frozen cadaveric lower limbs with 2/3 distal of femoral shaft and full-length leg with ankle and feet will be included in the study. CT scans encompassing full cadaveric specimen will be performed in full extension of the knee, or in maximal extension possible in given limb. Both coronal and sagittal anatomical axis will be assessed on CT scan. Femoral anatomical axis will be defined as a line connecting center of the marrow cavity at the most proximal level of femoral shaft available and the center of the knee at the level of the line tangent to the distal ends of femoral condyles. Tibial anatomical axis will be defined as a line connecting the center of the tibial articular surface in the knee and the center of the talus, similarly to the methodology of Wu Chi-Chuan. Medial proximal tibia angle (MPTA) will be defined as the angle between tibial anatomical axis and the medial tibial plateau joint line. Anatomical femorotibial angle (aFTA) will be defined as the angle between tibial anatomical axis and femoral anatomical axis. PTS will be assessed on CT scans using the methodology provided by Meier et al. and Calek et al. Due to the fact that mean intraindividual difference between PTS measured on medial tibial condyle (medial tibial posterior slope, MTPS) and lateral tibial condyle (lateral tibial posterior slope, LTPS) was reported to differ as much as 2.9 ° (range 0.0°-10.8°) or 2.6° (range 0.0°-9.5°), MTPS and LTPS will be measured separately. They will be defined as an angles between the plane perpendicular to the mechanical axis of the tibia (established using the ankle center and tibial spine) and the line tangent to the most prominent aspects of the anterior and posterior cortices of the medial and lateral compartments, respectively.

Patellar height will be assessed utilizing Insall-Salvati Index (ISI), Blackburne-Peel Index (BPI) and Caton-Deschamps Index (CDI). Two independent observers will perform the measurements separately, each of them will perform the measurements two times. Intra- and inter-reliabilities will be calculated.

CT scans will be performed on Siemens Somatom Go Top tomograph with 140kV voltage, slice thickness 0,6mm, increment 0,4mm. Two scans will be performed during each time, with tin filter and without it. Images of better quality will be chosen for measurements Surgical procedure: All specimens will undergo novel, biplanar, medial-opening wedge, posterior tibial slope-reducing high tibial osteotomy aided by PSI, with tibial tuberosity serving as hinge axis. The following corrections will be performed: group A, 3 specimens, in all cases PTS decrease by 6° and valgus correction of anatomical axis by 6°, 9° and 12°; and group B, 3 specimens, in all cases PTS decrease by 10° and valgus correction of anatomical axis by 6°, 9° and 12°. Randomization of specimens will be performed, irrespective of their native alignment, as the primary aim of this study is to assess the accuracy of planned biplanar correction. PSI will be created for every specimen by the engineer with years of practice in designing orthopaedic PSI (author J.P.). The following computer programs will be used: for creating 3D models out of DICOM files - 3D Slicer 4.11.20210226 (Brigham and Women's Hospital (BWH) \& 3D Slicer contributors, 2021); for designing 3D PSI surgical guides -SolidWorks 2016 (Dassault Systèmes SolidWorks Corporation, 2016). After completion of the design, PSI surgical guides will be 3D printed in two versions: trial, not destined for medical usage - with the 3D printer Stratasys Dimension 1200es (Stratasys Ltd), from material ABS (HMF Chemical); final, destined for medical usage - with the 3D printer EOS Formiga P110 (EOS GmbH), from material PA2200 Balance 1.0 (EOS GmbH). Obtained 3D PSI surgical guides are compliant with the requirements set out in Directive 93/42/EEC concerning medical devices as well as PN-EN ISO 15223-1, EN 1041 +A1:2013, EN ISO 14971, PN-EN ISO 17664:2005 (EN ISO 17664:2004) and PN-EN ISO 10993-1:2010 (EN ISO 10993-1:2009+AC:2010).

Postoperative measurements: MPTS, LPTS, coronal alignment and patellar height will be measured postoperatively utilizing the same methodology as described preoperatively. CT scans will be performed in the same angle of knee extension as preoperatively. Postoperative CT scans will be also assessed for the presence of fractures, in accordance with higher detection rate than when utilizing plain X-ray, as reported by Sang-June Lee. Once again, two observers will perform the measurements/ assessments separately, each of them will perform the measurements two times and they will be blinded as to desired amount of correction in the given extremity. Intra- and inter-reliabilities will be calculated. Statistical analysis will be performed in Statistica 13.3 software (StatSoft/ TIBCO Software Inc, 2017).