In the years after the historical articles of Slocum and Larson and Galway and MacIntosh, several biomechanical studies quantified the anterior-posterior translation of the tibia [15, 28, 66]. These studies described the ATT in the ACL-intact and ACL-deficient knees and found significant results between both. Jakob and colleagues described the subjective grades of the pivot shift [41]. The authors assessed the ATT with anesthesia by forcing the tibia in maximal subluxation under anesthesia and estimating the anterior-posterior translation by hand. They measured both the medial and lateral compartment and found the medial compartment was a better predictor for pivot shift grading. ATT can be measured either by measuring the ATT of both compartments (so-called coupled ATT or cATT) [37] or by measuring the lateral compartment only.

Bedi and colleagues used a mechanized pivot shifter to assess the correlation between different pivot shift grades and the ATT of both the medial and lateral compartment [10]. They performed the pivot shift in 77 cadavers in where they dissected the ACL or a combination of the ACL with the medial collateral ligament, medial meniscus, lateral meniscus, or a combination of these structures. This resulted in different pivot shift grades in the cadavers. A continuous passive motion (CPM) machine was used to mechanize the pivot shift test and measured the ATT in both compartments with tracking of reference points in a three-dimensional motion path system. Contrary to the findings of Jakob and co-workers [41], they found that lateral ATT was a better predictor for a positive pivot shift (grade 0 vs. grade 1) compared with medial ATT. They found there was a threshold of 6 mm of lateral ATT for a positive pivot shift, whereas there was no significant difference between grade 0 and grade 1 in the medial compartment. Knees with a grade 0 pivot shift had an average (± SD) lateral ATT of −2.1 mm (±8.1 mm), with grade 1 an average 11.1 mm (±2.2 mm) and with grade 2 correlated with 19.6 mm (±2 mm) (Fig. 40.2). Therefore, measuring lateral ATT seems useful in the quantification of the pivot shift test.

Hoshino and colleagues assessed the lateral anterior femoral translation of the reduction phase in clinical patients, which is similar to a posterior tibial translation since these landmarks are relative to each other [35]. Under anesthesia, they performed a manual pivot shift test, and the movement of the lateral compartment was captured with skin markers and a standard digital camera and was analyzed with a two-dimensional software system. They found in 5 ACL-deficient patients with a subjective pivot shift grade 1 an anterior femoral translation in the reduction phase of 3.7 ± 2.1 mm. These authors then developed an application for a computer tablet (e.g., iPad) and performed a new study where they examined 20 patients [34]. The patients were scheduled for isolated ACL reconstruction and the pivot shift was grade 1 in 10 patients and grade 2 in 10 patients. With the same method, they measured the lateral anterior femoral translation in the reduction phase and found a significant difference between grade 1 (2.7 mm ± 0.6 mm) and grade 2 (3.6 mm ± 1.2 mm). These studies showed that measuring the lateral ATT and posterior translation in the reduction phase are both reliable in quantifying the pivot shift.

The role of ITR in quantifying the pivot shift has also been extensively assessed [14, 21, 23, 40, 60, 62]. Bull and colleagues showed a strong correlation between the pivot shift and ITR [14]. They described that the tibia during the pivot shift test between 0° and 25° flexion showed progressive ITR, and this was suddenly reversed during the reduction phase starting at 36° (±9°) with external tibial rotation. After ACL reconstruction, this external rotation in the reduction phase was significantly reduced. Colombet and colleagues found similar results [17] when they performed manual pivot shift tests in four cadavers and measured the ITR in ACL-intact and ACL-deficient knees. They found that internal rotation was larger in the positive pivot shift (≥ grade 1) compared to the negative pivot shift (27° ± 2° vs. 20° ± 2°). Lane and colleagues found an in vivo significant correlation (R ² = 0.77) between pivot shift grades and tibial rotation in the reduction phase [54]. Although some studies showed a high ICC for measuring ITR [62, 84], several authors state that there is a lack of validated devices for assessing rotatory knee laxity, and these techniques are considered complex and invasive and are therefore clinically not applicable [2, 3, 5, 21]. Diermann and colleagues performed a simulated pivot shift test at different flexion angles and compared ACL-intact, ACL-deficient, and ACL-reconstructed knees [21]. They did not find significant differences in ITR between the different knees, but they did find significant differences in ATT of the lateral compartment when comparing these knees. They suggested using lateral compartment ATT rather than internal rotation to assess the integrity of the ACL. Hoshino and colleagues also found that tibial rotation is not reliable in differentiating a negative from a positive pivot shift and recommended using lateral ATT or acceleration [36]. Moreover, some systematic reviews of rotational laxity are not convinced of clinical application of measuring internal tibial rotation and state that simple, accurate, and noninvasive devices are needed [2, 3, 5]. It seems there are differences in ITR between different pivot shift grades, but these are not easy measurable with the available devices.

More recently authors have quantified the pivot shift by measuring the acceleration of the tibia in the reduction phase (Fig. 40.3). Several studies have described the acceleration and used acceleration to differentiate between different pivot shift grades. Hoshino and colleagues assessed the acceleration of the reduction phase in 30 clinical patients under anesthesia [37]. They measured the acceleration with an electromagnetic system in a manually performed pivot shift. In the negative pivot shift, the acceleration was −795 mm²/s, while in grade 1, 2, and 3 pivot shifts, the accelerations were, respectively, −1247 mm²/s, −2381 mm²/s, and −2735 mm²/s. These accelerations were significantly different from negative pivot shift, and acceleration of tibial reduction was larger in correlation with higher clinical grading (p < 0.01). Other authors also assessed the relationship between acceleration and pivot shift grades and reported a strong correlation between acceleration and different pivot shift grades [3, 11, 50, 51, 54, 59–61, 65]. Labbe and colleagues assessed different components of pivot shift grades (tibial translation, rotation and acceleration) in 127 in vivo pivot shifts [52]. They found that acceleration of tibial translation better distinguished the different pivot shift grades than tibial translation and tibial rotation. Ahlden and colleagues compared the relationship of acceleration and ATT with clinical pivot shift grades using electromagnetic sensors and skin sensors [1]. With both methods, the authors found a stronger correlation between acceleration and clinical pivot shift grades than ATT with clinical grades. Contrary, Lopomo and colleagues showed in a linear regression that rotational laxity before and after reconstruction is better evaluated by measuring lateral ATT compared with measuring acceleration [58]. However, there are many studies in the literature that support using both lateral ATT and acceleration in quantifying the pivot shift and different pivot shift grades. Because several studies question the use of tibial rotation in quantifying the pivot shift, development of simple devices is necessary before reliable and clinically applicable ITR measurements can be performed.

Many biomechanical studies used the six-degree-of-freedom robot with universal force sensor to quantify the pivot shift with a simulated pivot shift test [22, 29, 44, 45, 48, 49, 59, 67, 97, 99]. Matsumoto and colleagues assessed the role of several structures in the knee on the pivot shift [67], and they found that solitary ACL injury, solitary lateral structure injury, and combined ACL and lateral structure injury could cause a positive pivot shift. With this study, they objectively confirmed the results of other studies that used the manual pivot shift [24, 38, 63]. Kanamori and colleagues further quantified the ATT in the ACL-intact and ACL-deficient knee with the simulated pivot shift [44]. They reported that different ATT between ACL-intact and ACL-deficient knees was seen between 0° and 30° of flexion. In a follow-up study, they assessed the role of internal torque and valgus torque on the ATT of the pivot shift [45]. They found that a 10 Nm valgus torque gave the largest difference in ATT between the ACL-intact and ACL-deficient knee, which is confirmed by others [29]. The largest difference in ATT between the ACL-intact and ACL-deficient knee was seen with a 1.6 Nm rotational torque and a 10 Nm valgus torque. The authors therefore suggested that optimal performance of the pivot shift requires a moderate amount of valgus force with only a small internal rotation torque. Engebretsen and colleagues performed a simulated pivot shift test and compared the applying of different torques and forces. The authors confirmed that coupled internal rotation and valgus torques best recreated the lateral ATT that occurs in the simulated pivot shift test [22].

Many studies used the manual pivot shift to quantify the pivot shift. One of the great advantages of the manual pivot shift is the fact that no equipment or additional personal is necessary and results are immediately obtained [12]. However, one major disadvantage is the difference in technique among examiners. In a survey among 33 orthopedic surgeons, several differences in technique and reporting outcomes were detected [51]. More specific, 60 % of the surgeons performed flexion from extended position, whereas 40 % performed extension from flexed position, and 70 % applied internal rotation during the pivot shift test, while 30 % applied external rotation. In addition, the authors compared the quantified pivot shift between five examiners with an electromagnetic device. They found no differences in ATT and acceleration between the examiners but did find differences in ITR and when the pivot shift was applied in the contralateral knee [51]. Because of different surgeon preferences in the pivot shift technique, a standardized pivot shift test is suggested [33, 71]. In the first step, the leg is in extension with slight hip abduction and the leg is internally rotated. In the second step, gentle valgus stress is applied and the leg is brought into flexion. The internal rotation is maintained until 20° of flexion and is then released when the knee is further flexed. Between 20° and 40° of flexion, the reduction phase occurs, and posterior translation of the tibia along with external rotation can be measured. Hoshino and colleagues [33] compared the inter-rater variability among 12 orthopedic surgeons and found a smaller variability in the standardized technique compared to their preferred technique. However, it remains difficult to quantify the pivot shift with a manual examination since educational differences and preferences in technique exist among orthopedic surgeons.

The senior author (AP) and colleagues used a mechanized pivot shifter in order to quantify the pivot shift test in human cadavers. A whole hip-to-toe cadaver is mounted to a setup, and a CPM machine is secured to the table. The foot is placed in a holder and fixed in an internal rotation moment at the knee. A valgus moment is applied with a three-degree-of-freedom arm, and the leg is moved from extension through the flexion arch (Fig. 40.7). The mechanized pivot shift test was compared to the manual pivot shift test and was found to be more accurate in measuring ATT than the manual performed pivot shift test (0.92 vs. 0.76, respectively) [72]. A second-generation mechanized tester was developed which was more accurate than the first generation (0.99 vs. 0.92, respectively) [16]. With this mechanized pivot shift test, the examiners quantified the role of the primary and secondary stabilizers on in ATT and ITR [10, 68, 70, 86, 94]. They were also able to assess the influence of different tunnel positions on knee stability [9, 93] and compared different reconstruction techniques in knee stability with the meniscus [73] and without the meniscus [69] in situ.

The mechanized pivot shift is shown to have high ICC and is able to distinguish different pivot shift grades as earlier discussed. However, the setup of this method is invasive since Steinmann pins are drilled in the distal femur and proximal tibia. Therefore, this method is not applicable in the clinical setting. As previously discussed, some studies have examined the application of noninvasive reflective markers on Steinmann pins [83] and found a high ICC for internal rotation. Further research is needed to show reliability in ATT and acceleration. Furthermore, it would be of value to further develop the mechanized pivot shifter to a device that can be easily used in the clinical setting.