Counteracting Knee Flexors Strength Could Effectively Prevent the Increase in Post-operative Posterior Tibial Slope in Patients Undergoing Open‑wedge High Tibial Osteotomy


 Background A difficult-solved problem is that the posterior tibial slope (PTS) increases after medial open-wedge high tibial osteotomy (OWHTO).Purpose To evaluate effects from knee flexors on changes in the posterior tibial slope after OWHTO and to determine whether counteracting the strength of knee flexors could prevent increase in post-operative PTS.Methods The study retrospectively analyzed 112 subjects (122 knees) [34 males, 78 females; mean age 59.1±6.6 (range 48–76) years; mean body mass index (BMI) 28.06±3.61 kg/m²] who underwent OWHTO. During fixing steel plates, 78 knees keeping suspended and extended to counteract the strength of knee flexors by gravity are comprised the no-flexor group and 44 knees in natural posture without counteracting knee flexors are comprised the flexor group. Patients were clinically assessed according to the Western Ontario and McMaster Universities (WOMAC) osteoarthritis index and a visual analogue pain scale (VAS). Radiological assessment was made according to the changes in the posterior tibial slope between preoperative, 1-day post-operative and final follow-up periods. Ultimately, difference in post-operative PTS changes between the two groups was statistically analyzed. The median follow-up period was 2.2 years (range 1.6–3.7 years).Results In the final follow-up period, significant improvements were observed in the clinical scores of the VAS and WOMAC in both groups (P < 0.001), and no difference was found between the two groups. Radiological assessment showed that the final follow-up PTS was significantly greater than preoperative PTS in the flexor group (t=-6.406, P < 0.001), but no significant increase was seen in the no-flexor group (P =0.482). Increase of PTS in the flexor group was significantly greater than that in the no-flexor group at 1-day postoperative (t=2.243, P=0.030) and final follow-up periods (t=6.501, P<0.001).Conclusion For OWHTO, knee flexors would aggravate the increase in post-operative PTS. Using gravity to counteract the strength of knee flexors when fixing steel plates could effectively prevent the increase in post-operative PTS.


Introduction
High tibial osteotomy (HTO) is widely accepted for medial compartment knee osteoarthritis in those young and active patients [1][2][3][4]. HTO achieves satisfactory clinical results by accurately correcting the force lines of lower limbs [5][6][7][8]. Good surgical effects could be guaranteed within 6-15 years after HTO which includes lateral closing-wedge high tibial osteotomy (CWHTO) and medial opening-wedge high tibial osteotomy (OWHTO) [9][10][11]. Because CWHTO needs to be accompanied by an additional osteotomy of the bula, with potential risk for peroneal nerve injury, OWHTO has gained popularity with its favorable clinical outcome. However, OWHTO is also associated with an unintentional increase in PTS which is di cult to avoid [12][13][14].The measurement of PTS has been widely studied for its causal relationship to tibial translation, knee joint stability, and anterior cruciate ligament injuries. Physiologic range of PTS is between 6°-10° in normal people [15]. Rodner et al observed that, increasing the PTS by an average of 5.5° was accompanied by redistribution of the location of intra-articular peak pressure, shifting it posteriorly by 24%. They suggested that inadvertent redistribution of contact pressure into this area may be a cause of pain and clinical failure after OWHTO[16].
Various methods have been tried to solve this tricky problem and a wider posterior opening gap than anterior has been proposed as a means of preventing from unintentional increase in PTS [4,13,17,18].
However, knee exors with strong strength, may be resistance to the posterior cortical gap opening.
Therefore, it is presumed that knee exors could change post-operative sagittal planes. To our knowledge, clinically, attitude towards the posterior knee exors is inconsistent, which determined by the evaluation of the knee mechanical environment. Some surgeons counteract the knee exors using gravity when xing steel plates, while others do not take it into consideration, suggesting that operation of OWHTO is still lack of standardization and optimization. However, there have been no previously published descriptions of the effect of knee exors on the tibial slope in the sagittal plane.
The purpose of this study was to investigate the effect of knee exors on the change in tibial slope in the sagittal plane. Our hypothesis was that no-counteracting knee exors will increase the posterior tibial slope after OWHTO.

Study groups
From June 2017 to August 2019, 112 patients (122 knees) underwent OWHTO. Patients whose strength of knee exors ( Fig. 1) was counteracted when xing steel plates comprised the no-exor group and patients without counteracting the strength of knee exors were comprised the exor group.
The inclusion criteria were (1) clinically diagnosed medial compartment knee osteoarthritis; (2) with healthy knee ligaments and underwent OWHTO; (3) The minimum follow-up period was 2 years. The exclusion criteria (1) fracture of hinge occurred after operation in the follow-up periods; (2) loosening or breakage of steel plate; (3) infection (4) lost to follow-up.
Ultimately, the study enrolled 122 knees [34 males, 78 females; mean subject age 59.1 ± 6.6 (range 48-76) years; mean body mass index (BMI) 28.06 ± 3.61kg/m²]. Of the knees, 78 comprised the no-exor group and 44 comprised the exor group. 7 subjects were excluded because of fracture of hinge or lost to follow-up.
OWHTO was performed by one experienced surgeon. Accurate measurements on radiographs of limbs varus deformities were performed for adequate preoperative planning and a weight-bearing line ratio of 62% on the radiograph was anticipated [16]. Under general anesthesia, a thigh tourniquet was applied while the patient was in the supine position. Open-wedge biplanar osteotomy was performed in all patients using the Lobenhoffer surgical technique [19]. About 5cm incision was made longitudinally at the 4-5-cm medial portion of the anterior ridge of the tibia. The super cial medial collateral ligament was released subperiosteally from the medial proximal tibia and posterior edge of tibia was full exposed. Horizontal osteotomy line was designed from the end of the goose foot towards to bular head and ascending osteotomy line was designed at the medial edge of tibial tubercle. Target angle between the two osteotomy lines is 110°. TOMOFIX was used for the xing and other detailed surgical techniques have been described elsewhere [19]. TOMOFIX steel plates are designed to be xed using 8 screws ( Fig. 2). First, locking screws A, B, C, D were screwed in. Second, a tension screw would be screwed in nail hole 1. This step could x the position of steel plates and determine the result of osteotomy because that the xed part of steel plate has crossed the osteotomy gap. Third, 3 cortical screws were screwed in other nail holes in order. Finally, tension screwed in nail hole 1 was changed to a cortical screw. Therefore, xing steel plate position mentioned in this study was determined by the second step.
All patients were divided into two groups based on whether knee exors were counteracted. Knees in noexor group were suspended and extended to counteract the effect of knee exors by gravity when steel plates were xed. The key technique is placing a sterile cloth ball under the ipsilateral ankle to raise the ankle (Fig. 3). Knees in exor group were in a natural position when xing the steel plate with the ankle was placed directly on the operating table.

Postoperative treatment and follow-up
In the absence of major contraindications, all patients receive treatment for the prevention of deep vein thrombosis and infection in the early post-operative stage and quadriceps exercises and ankle pump exercises were started to exercise the isometric contraction of the quadriceps and gastrocnemius. Reexamination of X-ray was examined at 1-day post-operation. Standard anteroposterior (AP) and lateral radiographs of the knee and weight-bearing full-leg AP radiographs were performed in all patients at 1month, 2-month, 3-month, 6-month, 1-year and most recent post-operative follow-up visits. The lower limbs were allowed to bear weight normally at least 12 weeks after operation.

Basic clinical information
Age, sex, body mass index of all patients were recorded. Preoperative and most recent postoperative follow-up visual analogue scale/score (VAS) were used to evaluate the pain symptom relief. Preoperative and most recent postoperative follow-up Western Ontario and McMaster Universities (WOMAC) osteoarthritis index was used for the evaluation of the improvement of knee joint function.

Radiographic Measurement
The posterior tibial slope (PTS) is the most concerned index in this study and is measured using lateral radiographs with a digital image viewer. Preoperative, 1-day post-operative and most recent postoperative follow-up lateral knee radiographs were measured for each patient.
PTS was de ned as the narrow angle between the proximal tibial anatomic axis and the line tangent to the tibial plateau and was measured referring to the measurement method of Omer et al [20] (Fig. 4). The tibial diaphyseal line was centered through the tibial shaft using digitally generated circles. Two circles were digitally drawn on each radiograph with diameters equal to the width of the tibial shaft which were 15 cm distal to the tibial plateau and 5 cm distal to the tibial tubercle, respectively. Each circle was sized and positioned so that the anterior and posterior borders of the tibia were tangent to its circumference. A line parallel to the tibial axis was drawn through the centers of these two circles to ensure correct positioning within the center of the tibial shaft (Fig. 4)

Statistical analysis
All statistical evaluations were performed using PASW Statistics (25.0, SPSS, Chicago, IL, USA). The Kolmogorov-Smirnov normality test was used before the statistical analyses to determine whether to use a parametric test. Continuous variables conforming to the normal distribution were expressed as the mean and standard deviation. Two independent sample t-test was performed to compare radiological measurements between the no-exor and exor groups and paired t-test was performed to analyze the changes of PTS after operation. Categorical variables were expressed as frequency (%) and the groups were compared with the chi-square test. Linear correlation analysis was used to analyze the correlation between the changes in 1-day post-operative or most recent follow-up PTS and preoperative PTS in the two groups respectively. Intra-and inter-class correlation coe cients (ICCs) with 95% con dence intervals (CIs) were used to assess intra-and inter-rater variability. ICC > 0.75 was considered to represent excellent agreement. A P-value < 0.05 was considered statistically signi cant.

Basic information
The median follow-up period was 2.2 years with range of 1.6-3.7 years. Good to excellent intra-and interobserver variability was achieved for all measurements with an inter-rater ICC between 0.803 and 0.916 and an intra-rater ICC between 0.909 and 0.976.

Differences in clinical information between two groups
T-test and Chi-square test show that clinical information, regarding patient age, sex, BMI, follow-up time, in the two groups has no statistically signi cant difference (all P > 0.05).

Preoperative and postoperative VAS
Two independent sample t-test and paired t-test show that, pain is signi cantly relieved after operation with a signi cantly decreased VAS in both groups (t = 9.988, P < 0.05; t = 45.951, P < 0.05). There is no signi cant difference in the preoperative or most recent follow-up VAS between the two groups (P > 0.05). (Table 2) Preoperative and postoperative WOMAC Two independent sample t-test and paired t-test show that, function of knee joint is obviously improved after operation with a signi cant decreased WOMAC in both groups (t = 14.110, P < 0.05; t = 42.530, P < 0.05). There is no signi cant difference in the preoperative or most recent follow-up WOMAC between the two groups (P > 0.05). (Table 3) Postoperative change in PTS Paired t-test shows that no signi cant difference is found in 1-day post-operative PTS between the two groups (P > 0.05). At the most recent follow-up period, PTS in the exor-group is signi cantly greater than preoperative PTS (t=-6.406, P < 0.05) but no difference is seen in the no-exor group (P > 0.05). Two independent sample t-test shows that no signi cant difference is found in 1-day post-operative PTS between the two groups but at the most recent follow-up period, PTS in exor group is signi cantly greater than that in no-exor group with statistical signi cance (t = 2.888, P = 0.006). Compared with the no-exor group, changes in 1-day post-operative and most recent follow-up PTS of exor group are all signi cantly greater and the differences are of statistical signi cance (t = 2.243, P = 0.030; t = 6.501, P < 0.001). Linear correlation analysis shows that, no statistical correlation is found between the changes in 1-day post-operative (r = 0.084, P = 0.806; r=-0.251, P = 0.123) or most recent follow-up PTS (r = 0.247, P = 0.465; r=-0.152, P = 0.355) and preoperative PTS in both groups. (Table 4)

Discussion
The most important nding of this study is that, knee exors result in a signi cantly greater increase in the PTS and counteracting the knee exors by gravity with knee joints staying suspended and extended when steel plates are xed is effective in solution of this problem.
Based on a systematic review of open-wedge HTO studies, attitudes towards counteracting knee exors by gravity or not when steel plates were xed are inconsistent. However, despite the procedure is of great signi cance for preventing unexpected increase in PTS, previous studies have not focused on it and osteotomy procedure is still not completely standardized and consistent. Thus, the purpose of this study was to evaluate the effects of counteracting knee exors on PTS after OWHTO and to make the osteotomy technique more standardized.
Increase in PTS could result in the tibial forward movement relative to femur. Studies have shown that 1°i ncrease in PTS was accompanied by 1.45 ° increase in the loss of knee extension [15]. Pressure distribution in the knee compartments rearranged when PTS increases by 5.5 °: peak pressure point moves backward 24% of the knee compartment, which may be an important cause of pain or surgical failure after OWHTO [16]. In addition, tibial forward movement resulted from the increase of PTS results in increased tension of the anterior cruciate ligament (ACL) which would eventually aggravate the degeneration of ACL [21]. Some other studies have even con rmed that the increase of PTS is a risk for postoperative anterior cruciate ligament injury, which would result in knee instability and accelerate knee degeneration [22]. Many studies reported that, increase in post-operative PTS would increase the amount of osteotomy on anterior tibial plateau during the secondary total knee arthroplasty (TKA) [9,[23][24][25]. It has also been reported that increase in PTS would be accompanied by the contracture of the patellar tendon and the secondary TKA would be more di cult to perform [26][27][28][29]. Previous studies have made several recommendations to avoid an increase in the PTS after OWHTO, including making a complete posterior osteotomy, maintaining an optimal gap ratio, maintaining an optimal hinge position and steel plate position and so on.
Lee et al. suggested that the gap ratio should be kept the anterior opening gap approximately 67% of the posterior [13] and Noyes et al. found that the optimal ratio to maintain the normal sagittal tibial slope was 50% [4,17,18]. Dong et al. reported that autologous tricortical iliac bone graft could be effect in promising the optimal gap ratio [30]. In other words, despite the results they reported are different, they all aim to keep anterior opening gap narrower enough to the posterior.
Previous studies reported that hinge position was signi cantly correlated with post-operative PTS. Joon et al. found that a lateral hinge instead of a posterolateral hinge contributes to a complete opening of the posterior opening gap [16]. And according to Marti et al, increase in PTS after osteotomy will occur if the posterior gap is incompletely opened or the posterior soft tissue is not fully released [3]. Ho-Seung et al.
suggested that standard height of hinge should be adopted to prevent the increase of PTS. Horizontal osteotomy line should be designed from 3 cm below the medial edge of tibial plateau towards to the bular head. Lower hinge position could result in a signi cant increase in post-operative PTS and an increased risk of hinge fracture [31]. Therefore, keeping the posterior gap opened enough is a key surgical step which should be promised. However, knee exors with strong strength are resistance to the posterior gap opening. Although distraction forceps are used to open the osteotomy gap during the operation, it only promises an appropriate post-operative force line on the coronal plane, rather than a promise of optimal sagittal tibial plateau slope. In this study, using gravity to counteract the strength of knee exors is recommended, which can keep the entire posterior osteotomy gap evenly opened. In this proposal, a suspended and extended posture of the knee with placing a sterile cloth ball under the ipsilateral ankle to raise the ankle is adopted when steel plates are xed. The advantage of our operation is that it is simple and easy to perform and is effective in preventing the increase in post-operative PTS. In particular, this operation is not standardly or consistently performed during surgery, therefore, one of the main purposes of this study is to suggest this procedure should be performed in OWHTO standardly and consistently.
There are several limitations in this study. First, there were only a limited number of subjects. Other factors such as the corrective angle were not considered. However, study limitations were minimized by selecting the optimal corrective angle and surgical protocols based on previously studied outcomes for OWHTO. Second, weight of lower limbs has not been measured. We only compared the different results of anti-knee exor and non-anti-knee exor. Fortunately, we have come to a very important and practical conclusion. Third, although the follow-up period in all patients is more than 1 year, the follow-up time is still not long enough, which requires a continued follow-up investigation.

Conclution
Open-wedge high tibial osteotomy using gravity to counteract the strength of knee exors when steel plates are xed should be suggested because this suspended and extended posture could effectively prevent increase in post-operative PTS. With regard to clinical relevance, counteracting the strength of knee exors may be considered critical when performing OWHTO for a satisfactory outcome with standardization and consistency.

Declarations
Funding This study was funded by National Natural Science Foundation of China (NO. 31802022).

Con icts of interest
The authors declare that they have no con ict of interest.

Availability of data and material
All data and materials comply with current standards.

Consent to publish
Neither the article nor portions of it have been previously published elsewhere; the manuscript is not under consideration for publication in another journal and will not be submitted elsewhere; all authors consent to the publication of the manuscript in Journal of Orthopaedic Surgery And Research.

Author contributions
All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Wenru Ma, Zengshuai Han and Shengnan Sun. The rst draft of the manuscript was written by Wenru Ma and the necessary corrections and additions to the manuscript was made by Zengshuai Han and Shengnan Sun. Jinli Chen gave theoretical guidance. Tengbo Yu and Yi Zhang were responsible for the overall direction and review. All authors commented on previous versions of the manuscript. All authors read and approved the nal manuscript.
Knee exors.   Line b is parallel to the tibial axis and line a is parallel to the tibial plateau. The narrow angle A between line a and b is PTS.