Steep medial posterior tibial slope angle and ramp lesion are independent risk factors for an increase in quantitatively measured pivot shift in patients with ACL‐deficient knees

Abstract Background Delayed anterior cruciate ligament (ACL) reconstruction often causes residual anterolateral rotatory instability (ALRI) and consequent knee osteoarthritis, warranting the recommendation of early ACL reconstruction within 6 months after injury. Nonetheless, some cases show notable instability, even shortly after injury. The purpose of this study was to identify risk factors for an increase in quantitatively measured pivot shift in patients with ACL‐deficient knees within 6 months after injury. Methods Patients with primary ACL reconstruction within 6 months after injury and quantitative triaxial accelerometer measurements of preoperative pivot shift were included. Descriptive statistics were calculated for 11 independent variables (age, gender, time from injury to surgery, KT‐1000, knee extension angle, lateral and medial posterior tibial slope angle, medial and lateral meniscus tears, ramp lesion and Tegner active scale). A single regression analysis was performed on the 11 items and acceleration during the pivot shift, and a multiple regression analysis was performed for items with p value less than 0.1. Results Overall, 111 patients met the inclusion criteria. Single regression analysis showed that medial posterior tibial slope angle, medial meniscus tear and ramp lesion were significantly correlated with acceleration during the pivot shift (p < 0.001, p = 0.04 and p = 0.001). Multiple regression analysis identified medial posterior tibial slope angles and ramp lesions as independent factors for increased preoperative pivot shift (p = 0.005 and 0.01). Conclusion A steep medial posterior tibial slope angle and ramp lesion were independent risk factors for increased quantitatively measured pivot shift in patients with ACL‐deficient knees. Level of Evidence Level IV, case series study.


INTRODUCTION
Residual anterolateral rotatory instability (ALRI) persisting after anterior cruciate ligament (ACL) reconstruction, as evaluated through the pivot shift test, is a risk factor for knee osteoarthritis [8] and correlates with reduced patient satisfaction [11].ACL reconstruction is recommended within 6 months following injury, as prolonged periods between injury and reconstruction may lead to further damage to the meniscus and articular cartilage, potentially exacerbating ALRI [17].Nevertheless, even when ACL reconstruction surgery is performed within 6 months postinjury, some cases may still show significant postoperative ALRI.
A high degree of preoperative pivot shift has been reported to increase the risk of residual pivot shift after ACL reconstruction [9,27].Therefore, proper evaluation of the pivot shift phenomenon should be conducted both before and after ACL reconstruction.The assessment of the pivot shift phenomenon has traditionally relied on subjective methods [1,13,14,19]; however, the recent development of various measurement techniques utilizing electromagnetic waves and accelerometers has enabled quantitative assessment of the pivot shift phenomenon in ACL-injured knees [2,12,15], with the aim of providing an objective assessment of ALRI.
The kinematic rapid assessment (KiRA) triaxial accelerometer (OrthoKey) is a device capable of measuring tibial acceleration during the pivot shift [14,18].KiRA measurements in ACL-reconstructed knees have revealed a significant decrease in acceleration of the pivot shift compared to the preoperative level [18].However, no analysis has yet been conducted using the triaxial accelerometer to investigate the risk factors associated with the increased preoperative quantitative pivot shift phenomenon.
The aim of this study was to use the triaxial accelerometer to investigate the factors influencing the quantitative pivot shift phenomenon in ACL-injured knees within 6 months of injury, with the goal of identifying cases with a high risk of pivot shift even shortly after injury.Our hypothesis was that concomitant meniscal injury and tibial plateau anatomy would increase the risk of preoperative ALRI as assessed using the triaxial accelerometer.

Patients
This retrospective study was approved by the Institutional Review Board at Tokyo Medical and Dental University (research protocol identification number: M2000-1566).Between June 2014 and August 2023, a consecutive series of patients who had undergone primary double-bundle ACL reconstruction with an autologous semitendinosus tendon within 6 months after injury were included in this study.Nine surgeons in the same group operated on the patients with the same method.There are no age restrictions on ACLR.Patients with previous knee trauma or compound ligament injuries (with concomitant injuries of posterior cruciate ligament (PCL), medial collateral ligament and/or lateral collateral ligament), as well as patients with contralateral injuries, were excluded.Cases in which meniscus procedures were performed together with ACLR were included in the study, while osteotomy and cartilage surgery were excluded.Patients were certainly taken the plain radiograph of bilateral knees and magnetic resonance imaging just before surgeries.

Data collection
The demographic and image-measurement data were collected using the following methods.Data on (1) age (1-year increments), (2) gender (male or female) and (3) time from injury to surgery (0.5-month increments) were obtained during the initial interview.Preoperative anterior knee laxity was measured in 0.5 mm increments under anaesthesia in both injured and uninjured knees by five knee surgeons with more than 15 years of experience in the same group using a (4) KT-1000 arthrometer (MEDmetric) [27].The differences in laxity between the injured and uninjured knees were scored.(5) The knee extension angle was measured in 1°i ncrements using the preoperative lateral view of the plain radiograph of the uninjured knee [27].(6) The lateral posterior tibial slope angle and (7) medial posterior tibial slope angle were measured with MR images in 0.1°increments according to the three-step method [6].The central sagittal slice was first defined by the intercondylar eminence and then by the anterior and posterior tibial cortices that appeared concave.The tibial attachment of the PCL was defined and determined as the tibial longitudinal axis, and the angle between a line perpendicular to the tibial longitudinal axis and the tangent lines to the medial and lateral tibial plateaus was measured.Angle measurements were taken by an orthopaedic doctor (R. Y.).At least one month later, angles were measured again by the same assessor in 10 random cases, whereby the intraclass correlation coefficient (ICC) was measured.The intra-ICC of the angle measurements was 0.97.( 8) Medial meniscus tears, especially (9) ramp lesions and (10) lateral meniscus tears were diagnosed by senior knee surgeons using arthroscopy.(11) The Tegner activity scale was used as an indicator of the activity level prior to injury; this is a measure of specific exercise activity and is given a score of 10 for athletes who exercise at the competitive level and 0 for those who have no exercise habits at the daily living level [26].

Evaluation of ALRI
(A) Pivot shift based on seven-grade subjective evaluation ALRI was assessed using a pivot shift test performed with the patients under anaesthesia.Two experienced orthopaedic surgeons each performed an assessment before surgery and reached a consensus on the grade in every case.In the test, the modified International Knee Documentation Committee (IKDC) criteria [12] (Grade 0 = negative; Grade 1 = subtle glide, but not negative; Grade 2 = glide; Grade 3 = between Grades 2 and 4; Grade 4 = clunk; Grade 5 = between Grades 4 and 6; Grade 6 = gross) were used as the grading system to evaluate the pivot shift phenomenon.A previous report using an analysis of interobserver reliability showed that the inter-ICC for the sevengrade pivot shift test was 0.778 (95% confidence interval: 0.437-0.912)[18].

(B) Pivot shift based on the triaxial accelerometer
Quantitative assessment of the pivot shift phenomenon was performed by the same surgeons before surgery using the KiRA triaxial accelerometer (Figure 1).The intra-ICC of the KiRA is reported as 0.88-0.89[21].A built-in three-axis acceleration sensor was attached to the proximal lateral tibia, and the movement of the tibia during the pivot shift test was analysed through a wireless signal connected to a tablet terminal.The difference (range = a max − a min ) between the maximum (a max ) and minimum (a min ) values of acceleration (m/s 2 ) in the pivot shift test measured by triaxial acceleration was evaluated as the size of subluxation (Figure 2).Each pivot shift test was conducted five times, and the acceleration was measured for every test.Maximum and minimum values were excluded, and the three remaining data were averaged and used for the analysis.

Data analysis
The descriptive statistics, such as mean, standard deviation and minimum-maximum values, were first calculated for the 10 independent variables (age, gender, time from injury to surgery, KT-1000, knee extension angle, lateral and medial posterior tibial slope angle, medial and lateral meniscus tear and medial meniscus ramp lesion) and the Tegner active scale score was expressed as median and minimummaximum values.A single regression analysis was then performed on these 11 demographic and clinical data items as independent variables using the pivot shift phenomenon (triaxial accelerometer readings) as a dependent variable.A multivariate linear regression analysis was then performed on variables for which the p value was less than 0.1.
The sample size calculation was performed using G*Power (version 3.1.9.6) [5].Assuming a multiple regression model with four explanatory variables for which we want to estimate the partial regression coefficients, and given an effect size of f 2 = 0.15, when testing at a significance level of 5% and a power of 80%, the required sample size was calculated as a total of 85 cases.The data were analysed using Prism (version 9.5.0;GraphPad Software, LLC).In all analyses, a p value less than 0.05 was considered statistically significant.

Correlation of the subjective and quantitative pivot shift phenomenon
The pivot shift test subjective grade and acceleration measured with the triaxial accelerometer during the pivot shift under anaesthesia showed a weak positive correlation, with a correlation coefficient of 0.28 (p = 0.003) (Figure 3).
for an increase in pivot shift acceleration, as measured using the triaxial accelerometer (Table 4).

DISCUSSION
The most important finding of this study was that a steep medial posterior tibial slope angle and the presence of ramp lesion were risk factors associated with increased pivot shift phenomenon in ACL-deficient knees within 6 months after injury, as assessed using the triaxial accelerometer.Furthermore, only a weak correlation was observed between the acceleration measured by the triaxial accelerometer and the subjective grade of the pivot shift phenomenon, indicating that a more quantitative measurement of pivot shift provided a more precise assessment of the risk factors leading to an increased pivot shift.A significant association has previously been reported between ALRI and MM ramp lesions in ACLdeficient knees [3,25].A cadaveric study demonstrated that ramp lesions increased forces on the ACL and that lesions of the meniscotibial ligament increased rotational instability of the knee joint [20].A biomechanical study revealed that ramp lesions in ACL-injured knees induced a pivot shift phenomenon and that repairing these lesions during ACL reconstruction could effectively reduce knee instability [3].In the present study, the multivariate analysis identified ramp lesions as an independent risk factor for greater triaxial acceleration, suggesting that ramp lesions be treated during ACL reconstruction, as recommended in previous reports [22].
The findings of the present study also identified a steep medial posterior tibial slope angle as an independent risk factor for increasing the pivot shift phenomenon.The posterior tibial slope angle has an association with the stability of the knee joint [4,23], as an increase in the posterior tibial tilt angle has a clear association between knee joint instability and increased loading on the MM, particularly at the posterior root of the MM [23].Increases in the medial posterior tibial tilt angle are associated with a higher incidence of ramp lesions associated with ACL tears [7,10].Several reports have suggested that reducing the posterior slope of the tibia is effective in improving instability in ACL-deficient knees [16,28].Proper preoperative evaluation of the pivot shift phenomenon is therefore crucial in knees with a significant posterior slope for optimal management Several risk factors for a large pivot shift phenomenon have been reported using a manual pivot shift test.For example, Kamada et al. performed a manual pivot shift test under anaesthesia during implant removal after double-bundle ACLR and reported that age at surgery <20 years, preoperative high-grade pivot shift, and hyperextended knee were high-risk factors for residual postoperative pivot shift [9].Song et al. performed a manual pivot shift test on 90 knees with ACL reconstruction within 3 weeks of injury and identified pivoting sports at the time of injury, increased posterior-inferior tibial slope, anterolateral capsular ligament disruptions and combined lateral meniscal lesions as preoperative risk factors that influenced the postoperative persistence of a large pivot shift phenomenon [24].In these reports, the pivot shift tests were performed manually, with the poor reproducibility of the results cited as a limitation.By contrast, in the current study, the quantitative measurements of the pivot shift using the highly reproducible triaxial accelerometer had an ICC of 0.88-0.89.Therefore, the current identification of an independent risk associated with a steep medial posterior tibial slope angle and ramp lesion for a large preoperative pivot shift can lead to more reliable outcomes.
The main limitation of this study is that it was conducted retrospectively.Furthermore, the pivot shift tests were not performed blindly, and the technique was not necessarily consistent, as it was conducted by multiple examiners.However, efforts were made to ensure consistency in technique by attempting a more rigorous evaluation of pivot shift using the modified IKDC criteria.Additionally, to minimize variability among evaluators, each test was performed five times, with the highest and lowest values excluded and the remaining three values averaged.Finally, the present study did not consider detailed information on meniscus injuries and their treatment.The results of this study did not show that anything related to the meniscus was a risk, but it could have been the risk by analysing the detailed meniscus injury type.
The clinical relevance of this study is that risk factors for ALRI within 6 months of ACL injury were obtained using a triaxial accelerometer.The usage of a triaxial accelerometer for a more reproducible pivotshift assessment might help surgeons to better identify potential adverse biomechanical consequences of MM ramp lesions and increased tibial slope.This could lead to performing additional anterolateral structure augmentation and/or anterior closing-wedge osteotomy in combination with ACL reconstruction.
In conclusion, steep medial posterior tibial slope angles and ramp lesions were independent risk factors for increases in quantitatively measured pivot shifts in patients with ACL-deficient knees.
AUTHOR CONTRIBUTIONS Ryu Yoshida managed data, performed statistical analysis, participated in study design and wrote the manuscript.Hideyuki Koga contributed to the acquisition of data and participated in the study design.Tomomasa Nakamura, Yusuke Nakagawa, Mai Katakura, Masaki Amemiya, Takashi Hoshino, Aritoshi Yoshihara, Shoichi Hasegawa, Yasumasa Tokumoto and Ichiro Sekiya acquisition of data.Nobutake Ozeki contributed to the acquisition of data, participated in study design, interpreted results, edited the manuscript, and had full access to all of the data in the study and final approved manuscript.All authors read, approved the final manuscript and take responsibility for the integrity of the data and the accuracy of the data analysis.

F
I G U R E 1 Evaluation of the pivot shift with a triaxial accelerometer.White arrow: a sensor (placed between the lateral aspect of the tibia and a band).F I G U R E 2 A representative wave during the pivot-shift test obtained by KiRA.The difference between the maximum value (a max ) and minimum value (a min ) of the acceleration was calculated (a range = a max − a min ).
TA B L E 2 Pivot shift based on seventh-grade evaluation.Multivariate analysis of candidate risk factors.: MM, medial meniscus; R 2 , adjusted R 2 ; β, standardized beta coefficients.
F I G U R E 3 Evaluation under anaesthesia.Correlations between the pivot shift test subjective grading and acceleration in anterior cruciate ligament-injured knees during the pivot shift test.TA B L E 3 Univariate analysis of candidate risk factors.Abbreviations