Proximal high tibial osteotomy (HTO) is the most widely accepted treatment for physically active patients with initial stages of medial femorotibial compartment degeneration and varus alignment of the limb [1]. Due to its particular advantages, opening-wedge high tibial osteotomy (OWHTO) has become more widely used in recent years than lateral closing-wedge osteotomies. Among these advantages, the lack of a need to perform an osteotomy of the fibular head with the subsequent risk of neurovascular injury [2] and less surgical exposure without muscle detachment have been cited [3]. In addition, the effect of OWHTO on the tibial metaphysis comparatively facilitates the performance of future knee arthroplasty [4].
However, OWHTO is not without potential disadvantages, including an increase in the posterior tibial slope (PTS) and a decrease in the patellar height (PH) [5].
Recently, the effects of the PTS on the biomechanics and stability of the knee [6, 7] have been described, as well as the tension exerted on both native and reconstructed cruciate ligaments [8,9,10]. Clinical and biomechanical studies have reported that the tension on the native or reconstructed anterior cruciate ligament (ACL) increases significantly after minimal changes in the PTS of only 2°, increasing the risk of failure [11, 12]. It is believed that the triangular cross-section of the tibia at this level as well as the way the osteotomy gap is produced highly influence the increase in the PTS [13, 14]. However, the inclination of the osteotomy plane has not been investigated.
The objective of this study was to evaluate the effect on the PTS of modifying the distance from the joint line where the osteotomy begins in the medial cortex of the tibia and of varying the placement of the opening wedge from posterior to anterior.
An increase in the PTS was hypothesized to be observed both when distalizing the point where the osteotomy begins and when anteriorizing the tibial opening wedge.
Materials and methods
For this analysis, 12 cadaveric human knees were used, stored at a temperature of − 18 °C, and thawed at room temperature for 24 to 36 hours prior to the procedure, during which they were wrapped in gauze moistened with saline solution. Each knee included the distal two-thirds of the femur and the proximal two-thirds of the tibia and fibula. The knees were macroscopically evaluated and showed no signs of having undergone surgery. In addition, prior to use, the absence of local radiographical bone lesions or any significant bone malformation that might affect the anatomy was confirmed.
The knees were randomly divided into two groups based on the distance from the start of the osteotomy to the joint line: 3 and 4 cm (cm). All surgical procedures were performed by the same team of two senior orthopaedic surgeons.
The study was approved by the clinical research ethics committee of our institution (protocol number PR260/21-CSA PR22/2021).
Radiographic technique
Prior to performing the osteotomy, the radiographic procedure was standardized to achieve greater reproducibility of the technique and to ensure that no magnification errors would influence the measurements [15]. Initially, each knee was placed extended in the anteroposterior (AP) plane, and the image was focused on the centre of the tibial shaft. One-third of the fibular head was covered by the lateral tibial plateau to ensure a correct AP view. This AP view was used to measure the width (mediolateral) of the tibial epiphysis by creating a parallel line that passed 5 mm distal to the joint space. In addition, the mediolateral size of the tibia was measured 3 and 4 cm from the joint line using a line parallel to that described above.
Subsequently, a strict lateral knee X-ray, where both femoral condyle contours perfectly overlapped, was performed at 30° of flexion. This projection was used to measure the PTS.
Radiological PTS evaluation
The medial and lateral tibial plateaus were initially identified to establish the PTS. The mean between the inclinations of both plateaus was used as previously published [16]. In measuring both tibial plateaus, the existence of osteophytes was excluded. In the same lateral view of the knee, the longitudinal component of the PTS was established by following the proximal anatomical axis of the tibia. For this analysis, two points were identified at 5 and 15 cm distal to the joint line. The midpoint of these levels, namely, the distance between the anterior and posterior cortex of the tibia, was identified. These midpoints were connected with a line that determined the longitudinal axis mentioned above and are shown in Fig. 1 [17, 18]. The same protocol was used for the radiographic technique and measurement of the PTS after osteotomy.
All radiographic measurements were performed by two orthopaedic surgeons using the picture archiving and communications system (PACS) (Centricity Enterprise Web V3.0; GE Healthcare).
Osteotomy technique
A 6- to 8-cm longitudinal skin incision was made in the anteromedial and proximal area of the leg midway between the anterior tuberosity and the posteromedial edge of the tibia. After dissecting the subcutaneous tissue, the tendons of the pes anserinus were incised longitudinally in a single plane at the level of the anterior margin of the medial collateral ligament. Both structures were carefully retracted posteriorly with the help of a Hohman retractor. The patellar tendon was identified and protected with a retractor. After the metaphyseal-diaphyseal transition zone of the tibia was identified, according to the randomization group (namely, 3 or 4 cm) (Fig. 2), a 2.4-mm Kirschner wire was introduced under fluoroscopic control (C-ARM Fluoroscopy 4400, FM Control, Alava, Spain) in the medial cortex aiming posterolaterally towards the proximal end of the fibular head. The tip of the wire was located 1.5 cm distal to the joint space and 1 cm medial to the lateral cortex. Following wire insertion, the osteotomy was performed, and special attention was paid to maintaining perpendicularity with respect to the sagittal long axis of the bone. Initially, the procedure was started with a saw and continued with a calibrated osteotome, with particular focus on completion of the osteotomy in the posterolateral region of the tibia and preserving one cm of bone in the lateral tibial cortex, which acts as a hinge [14, 19]. Subsequently, a lamina spreader was used to verify a sufficient opening to progressively place a 10° opening wedge (Newclip Technics, Haute-Goulaine, France).
After the anteromedial, medial and posteromedial (AM, M and PM, respectively) thirds of the medial cortex of the tibia were identified, the same 10° opening wedge was placed in each third (Fig. 3), and radiographic projections were taken as explained previously (Fig. 4).
Internal fixation was not performed with plates since the objective of the study was to evaluate the possible variations in the tibial slope and not the stability provided by an implant.
Statistical analysis
A descriptive analysis of the study variables was performed. Radiographic parameters were measured twice by two independent orthopaedic surgeons at 6-week intervals. The inter- and intrarater reliability for the radiographic measurements was assessed by calculating the intraclass correlation coefficients (ICCs). The ICC values were interpreted as follows: ICC < 0.40, poor agreement; 0.4 > ICC < 0.75, fair to good agreement; and ICC > 0.75, excellent agreement [20, 21].
The chi-square or Fisher’s exact test was used to compare categorical variables, and the t test was used for continuous variables. Pearson’s correlation coefficient was calculated to compare the PTS measurements at the different sites where the opening was performed and the starting point of the osteotomy.
The sample size was calculated a priori. With six subjects per group (n = 12), a statistical power of 80% was obtained to detect a difference ≥ 2° between the different tests [22], accepting an alpha risk of 0.05 and a beta risk of 0.2 in a bilateral comparison.
Because this is a cadaveric study, a loss to follow-up rate of 0% was estimated. All data were analysed with SPSS Statistics (v 21; IBM), with statistical significance was established for p < 0.05.