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

Henning et al. were the first to make in vivo measurements of ACL elongation behavior. In two patients, a hooked pin was attached to the partially disrupted ACL, and the peak displacement of the pin was measured during different rehabilitation activities. Absolute strain values were not reported. Displacement measurements for various activities were compared with those produced by a 350N, anteriorly directed shear load applied to the tibia during a Lachman test. Although this method had 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 (MicroStrain, Williston, Vermont) and the Differential Variable Reluctance Transducer (DVRT, MicroStrain, Williston, Vermont) ( Table 108.1 ). 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. Although the devices have many similarities, the sensing technology is different, and the DVRT is the displacement transducer of choice due to its improved accuracy, better precision, and lower profile.

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 signal. 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-to-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 error (or 0.1% strain). The DVRT is calibrated with a specially designed micrometer system (AutoCal, MicroStrain, Burlington, Vermont) and 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. Two fixation barbs on the device are then pressed into the ligament. 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. In order to determine the reference for strain calculation and to ensure that the transducer measurements are reproducible, repeated anteroposterior shear loading tests (Lachman) are performed at the beginning and end of a protocol.

When making 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 or applied load). A posteriorly directed shear load applied to the tibia with the knee at 20 degrees of flexion causes the ACL to become unstrained and unloaded. In contrast, when an anteriorly directed shear load is applied to the tibia, the ACL becomes taut. This slack–taut transition can be identified as the inflection point on a plot of applied anteroposterior loading of the knee versus DVRT output. For the anteromedial portion of the ACL, this slack–taut transition point can estimate the absolute reference within 0.7% strain.

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. However, the DVRT does have its limitations. Although the DVRT sensor is small, its placement is limited to the anteromedial portion of the ACL, due to the anatomy of the femoral intercondylar notch and the constraints produced by the arthroscopic portals. While 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. Additionally, the device can become impinged against the roof of the femoral intercondylar notch when the knee is moved into hyperextension. This impingement can act as a confounding factor when trying to obtain accurate measurements of ACL strain. Therefore certain activities that involve this motion (such as landing from a jump) may be difficult to study, and have only been pursued within very small cohorts.

Markolf et al. developed an in vitro technique measuring both strain and resultant force in the entire ACL that was found to be useful when interpreting DVRT data. This 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. Force and strain placed on the ACL are measured directly via the application of loads and torques about the knee. Markolf et al. 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 doing so, all data from 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.

Noninvasive magnetic resonance imaging (MRI) techniques have been utilized to measure in vivo kinematics of the tibiofemoral joint; these data have been used to estimate ACL biomechanics. Sheehan and Rebmann used a cine–phase contrast MRI technique to evaluate the orientation of the attachment sites of the ACL during non-weight-bearing flexion. Cine MRI was used to produce anatomical images during periodic motion, and phase contrast MRI was used to measure the three-dimensional (3D) velocities in the imaging plane. ACL strains were calculated by combining the velocity and anatomical data obtained from the cine–phase contrast magnetic resonance (MR) images. ACL insertions 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 within the anterior region of the ACL during active extension of the knee. However, this approach may have overestimated ACL strain, as measured strain values were more than 3 times greater than DVRT values, and were near the ACL’s failure strains. In a different MRI-based study, Li et al. used a combination of imaging and 3D computer-modeling to evaluate the orientation of the attachment sites of the ACL during weight-bearing flexion of the knee (one-legged lunge). First, MR images were obtained for each subject in order to construct a 3D model for each knee. 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. 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 a reference. During the one-legged lunge, Li et al. demonstrated that the anteromedial bundle of the ACL decreased in length by 7% when the knee moved from extension to flexion. These results were in agreement with values measured with the DVRT.

Review of Studies that have Characterized Anterior Cruciate Ligament Strain Behavior During Rehabilitation Exercises

In vivo measurements of ACL strain have been made of patients who have normal ACLs in an effort to describe the strain behavior of the ACL during commonly prescribed rehabilitation exercises. Most of the strain transducer measurements have been performed under local (intra-articular) anesthesia, in order to allow patients to have full muscle control. Typically study participants have been candidates for arthroscopic partial meniscectomy or diagnostic arthroscopy without known ligament trauma. Preoperatively, these patients have had normal gait, normal range of knee motion, and normal ligament function, as documented by clinical examination and arthroscopic visualization. ACL strain measurements during passive flexion–extension motion of the tibiofemoral joint (e.g., movement of the leg by an examiner without muscle contraction) have revealed that the ACL is unstrained between 11.0 and 11.5 degrees of flexion and becomes increasingly strained as the knee is moved into terminal extension, with peak ACL strain occurring in hyperextension. These data have been used to develop rehabilitation programs that do not jeopardize the integrity of the ACL graft, but still allow 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 108.1 .

The following general conclusions can be made regarding the effect of loads externally applied to the knee on ACL strain values: The ACL is a primary restraint to anterior displacement of the tibia relative to the femur when the knee is near extension, and it also restrains internal (but not external) axial rotation of the tibia. Although cadaver studies have revealed that the ACL serves as an important secondary restraint to applied varus-valgus moments, in vivo measurements have revealed that ACL strain values are not increased when varus and valgus moments are applied to the knee at 20 degrees of flexion. In addition, in vivo measurements have shown different ACL strain values during non-weight-bearing versus weight-bearing conditions. For example, transitioning from non-weight-bearing to weight-bearing conditions increases ACL strain values when varus-valgus moments and external torque are applied to the knee. When anteriorly directed shear loads are applied to the tibia, ACL strain values are higher during weight-bearing conditions in comparison to strain values in the non-weight-bearing condition. These findings demonstrate that weight bearing challenges the ACL and knee, and increases the magnitude of ACL strain values in comparison to the non-weight-bearing condition.

When compared with the fully relaxed condition, extension torque produced by isometric quadriceps muscle contraction has been shown to strain the ACL near extension of the knee, but not beyond 60 degrees of flexion. Isometric hamstring contraction, on the other hand, has not been shown to produce ACL strain at any knee flexion angles. When compared with the relaxed condition, combined contraction of quadriceps and hamstring muscles has been shown to produce a significant increase in strain at 15 degrees of knee flexion, but not at 30, 60, or 90 degrees of knee flexion. Isometric gastrocnemius muscle contraction has been shown to strain the ACL when the knee is near extension (at 5 and 15 degrees of flexion), and when gastrocnemius muscle contraction was combined with quadriceps or hamstring muscle contraction, the strain was increased in comparison with isolated contractions of these muscles.

It has been common practice to consider rehabilitation programs as comprising open and closed kinetic chain exercises. Closed kinetic chain exercises, such as squats, are performed with the distal segment fixed against resistance, whereas during an open kinetic chain exercise, such as knee extension, the distal segment (the foot) is not fixed. Compressive loading of the tibiofemoral joint produced during closed kinetic chain exercise has been thought to protect the injured ACL or healing ACL graft because of the increased joint stiffness and decreased anterior displacement of the tibia relative to the femur. In addition, co-contraction of the hamstring muscles during closed kinetic chain exercises has been considered to protect the injured knee from excessive ACL strains.

Active extension–flexion motion of the knee (an open kinetic chain exercise) between the limits of 10 and 90 degrees produces peak ACL strains near extension, and these values gradually decrease with increasing knee flexion. Beyond 35 degrees of knee flexion, the ACL becomes unstrained. Application of weight during this exercise (applied to increase extension torque about the knee) produces significant increases of ACL strain values at 10 and 20 degrees of flexion and shifts the strained–unstrained transition to 45 degrees of knee flexion. A subsequent follow-up study confirmed that the peak ACL strain values increased when knee extension torque increased. It was also shown that application of compressive loading, such as that produced by body weight, did not reduce peak ACL strains during extension exercises. Application of flexion torque during flexor exercise produced significant decreases of ACL strain values; however, when compressive loading was added, such a decrease was not observed.

Closed kinetic chain squatting exercises are commonly prescribed to improve muscle strength after ACL reconstruction. Because of the compressive joint load and co-contraction of muscles spanning the knee, advocates of closed kinetic chain exercise consider it to be safer than active flexion–extension exercises. It has been demonstrated that squatting and active flexion–extension exercises produce similar strain patterns (strain is greatest near full extension and gradually decreases as the knee moves into flexion) and maximum strain values, indicating that compressive joint force does not necessarily protect a healing ACL graft. It has to be emphasized, however, that in contrast with active extension of the knee, increasing resistance during squatting to the limit of 134N did not significantly increase ACL strain values. Heijne et al. measured the strain behavior of the ACL during four different closed kinetic chain exercises: (1) step-up, (2) step-down, (3) lunge, and (4) one-legged sit to stand. They found that the strain produced during these four exercises was not significantly different at all knee positions (knee flexion angles of 30, 50, and 70 degrees). The largest strain values were measured when the knee was near extension, and the strain values decreased significantly as the knee was flexed.

Stair climbing is largely a closed kinetic chain exercise, and this step-up exercise has been shown to reduce anterior translation of the tibia with respect to the femur and is therefore commonly a part of the rehabilitation protocol following ACL reconstruction. In vivo measurements during stationary stair-climbing exercises have demonstrated that ACL strain is increased when the knee moves from a flexed to an extended position, and the average strain values were moderate when compared with other commonly prescribed rehabilitation activities tested with the same technique. However, the strain values were highly variable, with peak values ranging as much as 7%. These strain magnitudes may produce detrimental effects to the healing graft, and therefore caution should be exercised when making any recommendations for stationary stair climbing following ACL reconstruction.

Most clinicians have considered bicycling to be a relatively safe rehabilitation exercise with many therapeutic qualities, and therefore it is commonly recommended for rehabilitation following ACL injury or reconstruction. In vivo ACL strain measurements during stationary bicycling also support this observation. Stationary biking was performed at six different riding conditions (three power levels and two cadences). Power levels of 75 Watts (W), 125 W, and 175 W simulated downhill, level, and uphill riding conditions, respectively. Results from this study revealed that with this selection of power and cadence levels, stationary bicycling produces relatively low peak strain values (mean 1.7%) when compared with other rehabilitation activities commonly prescribed after ACL injury or reconstruction. For these reasons stationary bicycling is considered to be an exercise that does not excessively strain the graft and is safe to employ early during ACL rehabilitation. However, the safety and efficacy of bicycling, or of any rehabilitation exercise for that matter, following ACL reconstruction can only be determined via clinical studies.

Though the importance of rehabilitation following ACL reconstruction is widely appreciated, there is little consensus as to which restrictions and exercises should be utilized, as well as how these choices influence the long-term outcome and healing response of the graft and knee. Previously mentioned studies characterizing the behavior of the ACL during different activities have been used to design accelerated and nonaccelerated rehabilitation programs that gradually increase the strain experienced in the graft. The effects of these programs have been studied via a prospective, randomized, double-blinded clinical trial. The accelerated program (19 weeks) included exercise that produced high graft strain values early after the reconstruction and permitted immediate full range of motion, weight bearing as tolerated, quadriceps activity with the knee near extension, and return to unrestricted activity within 6 months of reconstruction. In contrast, these same activities were prescribed over a delayed time interval within the nonaccelerated program (32 weeks), and the graft was therefore not strained as vigorously during healing. At 2-year follow-up, both rehabilitation programs produced the same increase of anterior knee laxity and the same effect with regard to clinical assessment, patient satisfaction, functional performance, and the biomarkers of articular cartilage metabolism.