Description of the Devices, Methods, and Approaches Used to Measure Anterior Cruciate Ligament Biomechanics in Vivo

Henning et al¹ were the first researchers to measure the elongation behavior of the ACL in vivo. A hooked pin was attached to the partially disrupted ACL in two patients, and the peak displacement of the pin was measured during different rehabilitation activities. Absolute strain values were not reported. The displacement measurements for various activities were compared with that produced by a 350N, anteriorly directed shear load applied to the tibia during the Lachman test. Although this method has several obvious limitations, it was one of the first studies that measured the ACL in vivo.

Subsequent to this work, ACL strain measurements have been performed in vivo using both the Hall Effect Strain Transducer (HEST, MicroStrain, Williston, VT) and the Differential Variable Reluctance Transducer (DVRT, MicroStrain, Williston, VT) (Fig. 64-1). Both displacement transducers are small (4–5 mm in length), are highly compliant, have a similar barbed attachment technique, can be sterilized, and can be implanted arthroscopically to the anteromedial aspect of the ACL in vivo.^2,3 Although the devices have many similarities, the sensing technology is different, and the DVRT is now more frequently used than the HEST, mainly due to its improved accuracy, better precision, and lower profile.³

Fig. 64-1 A schematic drawing showing the differential variable reluctance transducer (DVRT) applied to the anteromedial portion of the anterior cruciate ligament (ACL).

The HEST is composed of an inner tube that slides with an outer tube. At the end of each tube are barbs that attach the sensor to the ligament. The inner tube houses a magnet, and the outer tube has a Hall effect magnetic sensor. As the length of the ligament changes, the magnet moves relative to the Hall effect sensor, and this produces the relative change in length between the two barbs. In comparison, the DVRT detects the movement of the two barbs attached to the ligament by measuring the differential change in reluctance produced by the position change of a magnetically permeable core within two small coil windings that are excited with an alternating current (AC) signal.²

The DVRT is currently the displacement transducer of choice.^2,3 The monotonic sensing range of a 5-mm DVRT is 1.75 mm, creating a linear sensing range of 35%. The displacement sensitivity is typically 2 V/mm, and the signal:noise ratio is 1000:1. The DVRT has 3.5 μm of nonlinearity, 1 μm of hysteresis, 1 μm nonrepeatability, 0.1 μm/°C temperature error coefficient, and 7 μm root mean square (RMS) error (or 0.1% strain). The DVRT is calibrated with a specially designed micrometer system (AutoCal, MicroStrain, Burlington, VT).^2,3

The displacement transducer is implanted into the knee joint through a lateral parapatellar arthroscopic portal (incision) of the joint capsule with the knee at approximately 90 degrees of flexion. The sensing axis of the device is aligned with the anteromedial fibers of the ACL. The two fixation barbs of the device are then pressed into the ligament. Repeated anteroposterior shear loading tests (Lachman) are performed at the beginning and end of a protocol to determine the reference for strain calculation and to serve as a “repeated normal” test to ensure that the transducer measurements are reproducible.^2,3

For calculations of ACL strain, it is important to determine a reference length (the length of the transducer when the ACL becomes taut in response to palpation).⁴ When a posteriorly directed shear load is applied to the tibia with the knee at 30 degrees of flexion, the ACL becomes unstrained and is unloaded in response to palpation. When an anteriorly directed shear load is applied to the tibia, the ACL becomes taut.^2–4 This slack–taut transition is identified from the applied anteroposterior loading versus DVRT output plot as the inflection point.⁴ For the anteromedial portion of the ACL, this slack–taut transition point can estimate the absolute reference within 0.7% strain.⁴ The wire connections for data acquisition and transducer removal are allowed to course through the lateral portal, and the function of the sensor through the desired range of motion is checked prior to closing the arthroscopic portals and applying sterile dressing such as Tegaderm.³

The DVRT has many advantageous characteristics for measuring ACL strain in vivo. It is relatively small (approximately 5 mm), is lightweight, and can be attached to the ACL arthroscopically. Ligaments have a strain distribution about their length and cross-section, and the DVRT allows accurate, reliable, and repeatable strain measurements of specific regions of a ligament. In addition, the calibration remains stable in environments that range between room temperature and body temperature, making it very practical. Over the years, the DVRT has been shown to be biotolerable and safe, without any adverse long-term reactions.^2,3

The limitations of the DVRT must be appreciated. First, although the DVRT is small, the anatomy of the femoral intercondylar notch, combined with the constraints produced by the arthroscopic portals, constrains placement of the sensor to the anteromedial portion of the ACL in humans.^2,3 Although the current ACL reconstruction techniques aim to reproduce the function of the anteromedial bundle, recent reports suggest that it may be important to replicate the function of both bundles of the ACL to better restore rotational and anteroposterior limits of motion of the knee.^5,6 Second, impingement of the device against the roof of the femoral intercondylar notch does not allow measurement of ACL strain when the knee is in extension or hyperextension. Therefore it is difficult to study activities such as gait and landing from a jump.

An in vitro technique measuring both strain and resultant force in the entire ACL was developed by Markolf et al⁷ and is useful in interpreting the DVRT data. The technique involves mechanically isolating the bone insertion of the ACL and attaching a load cell to the bone–ligament complex. Throughout the procedure the anatomical origin and insertion are maintained in space.⁷ Loads and torques can be applied to the knee, and forces, stresses, and strains can be directly measured.^8–12 Markolf et al^13,14 tested the DVRT and the ACL mechanical isolation technique in the same experiment, creating calibration curves to estimate resultant forces in the ACL from strain measurements made in vivo. In so doing, all the data from the prior DVRT measurements can be related to resultant force measurements for common activities when the forces and moments produced across the knee in vivo are replicated in vitro.^2,3,15–17

Recently, noninvasive imaging techniques have been introduced for measuring the in vivo kinematics of the tibia relative to the femur, and these data have been used to estimate ACL biomechanics.^5,18,19 Sheehan and Rebmann¹⁹ used a cine–phase contrast magnetic resonance imaging (MRI) technique to evaluate the orientation of the attachment sites of the ACL during non–weight-bearing flexion, whereas Li et al^5,18 used a combined imaging and three-dimensional (3D) computer-modeling technique to evaluate the orientation of the attachment sites of the ACL during weight-bearing flexion of the knee (one-legged lunge). Although these new, MRI-based, noninvasive techniques have apparent limitations, they have opened a new era for measuring the in vivo kinematics of the knee.

For the cine–phase contrast MRI technique, the cine MRI produced the anatomical images during periodic motion, and phase contrast MRI measured the 3D velocities in the imaging plane.¹⁹ The ACL strain was calculated by combining the velocity and anatomical data obtained from the cine–phase contrast MR images. The insertions of the ACL were identified, and the lengths of the anterior and posterior regions of the ACL were calculated for a selection of different knee flexion angles. When compared with DVRT measurements, the cine–phase contrast MRI method revealed a similar strain pattern of the anterior region of the ACL during active extension of the knee. However, for the cine–phase contrast MRI method, the strain values were more than three times greater, approaching the failure strains of the ACL, and thus this approach may overestimate the ACL strain values.¹⁹

For the technique that combined imaging and 3D computer modeling, MR images were first taken of human subjects to construct a 3D model for each knee.^5,18 After modeling, each subject performed a lunge, and two orthogonal fluoroscopic images were taken at four selected flexion angles to re-create the in vivo knee positions. These orthogonal images and the 3D knee model were then manually matched to reproduce the kinematics of the knee. The tibial and femoral insertion sites were identified to investigate the ACL attachment site’s biomechanics. The position of knee at full extension was used as reference. During the one-legged lunge, Li et al¹⁸ demonstrated that the anteromedial bundle of the ACL decreased in length by 7% when the knee moved from extension to flexion. These results are in agreement with those measured with the DVRT.

Review of Studies That Have Characterized Anterior Cruciate Ligament Strain Behavior During Rehabilitation Exercises

In vivo ACL strain measurements of patient volunteers with normal ACLs have been carried out to describe the strain behavior of the ACL during commonly prescribed rehabilitation exercises and have been used to establish clinical criteria for ACL reconstruction. These studies also serve as a basis for development of rehabilitation programs that do not jeopardize the survival of the ACL graft but still allow exercises for optimal recovery of muscle strength and range of motion following ACL reconstruction. Rank comparison of peak ACL strain values produced during common rehabilitation activities are summarized in Table 64-1.

Table 64-1 Rank Comparison of Average Peak Anterior Cruciate Ligament Strain Values Measured During Various Rehabilitation Activities