Keith L. Markolf

Direct measurement of resultant forces in the anterior cruciate ligament. An in vitro study performed with a new experimental technique.

A new technique was used to measure the resultant forces in the anterior cruciate ligament during a series of loading experiments on seventeen fresh-frozen cadaver specimens. The base of the ligaments tibial attachment was mechanically isolated with a coring cutter, and a specially designed load-transducer was fixed to the bone-plug that contained the ligaments tibial insertion so that the resultant forces were directly measured by the load-cell. Although the magnitudes of values for forces varied considerably between specimens for a given test condition, the patterns of loading with respect to direction of loading and the angle of flexion of the knee were remarkably consistent. Passive extension of the knee generated forces in the ligament only during the last 10 degrees of extension; at 5 degrees of hyperextension, the forces ranged from fifty to 240 newtons (mean, 118 newtons). When a 200-newton pull of the quadriceps tendon was applied to extend a knee slowly against tibial resistance, however, the force in the ligament increased at all angles of flexion of the knee. Internal tibial torque always generated greater forces in the ligament than did external tibial torque; higher forces were recorded as the knee was extended. The greatest forces (133 to 370 newtons) were generated when ten newton-meters of internal tibial torque was applied to a hyperextended knee. Fifteen newton-meters of applied varus moment generated forces of ninety-four to 177 newtons at full extension; fifteen newton-meters of applied valgus moment generated a mean force of fifty-six newtons, which remained unchanged with flexion of the knee. The force during straight anterior translation of the tibia was approximately equal to the anterior force applied to the tibia. The application of 925 newtons of tibiofemoral contact force reduced the mean force in the ligament that was generated by 200 newtons of anterior pull on the tibia by 36 per cent at full extension and 46 per cent at 20 degrees of flexion.

The role of joint load in knee stability.

Fresh cadaver knees were tested at full extension and at 20 degrees of fiexion in specially designed fixtures that allowed tibiofemoral contact forces of as much as 925 newtons (207 pounds) to be applied to the knee while movement of the joint was not restricted. Force versus displacement responses for anterior-posterior and medial-lateral movement of the tibia as well as moment versus rotation responses for varus-valgus angulation and tibial torsion were recorded with and without load both before and after medial and lateral meniscectomy. When joint contact force was applied, the loading apparatus could be adjusted so that the knee was in equilibrium and there was no tendency for the tibia to flex, extend, or subluxate anteriorly or posteriorly with respect to the femur. This position of static equilibrium (which we have called the neutral position) was represented graphically as the origin of the response curves. The slope of the response curve as it passed through the neutral position (neutral stiffness) is an important descriptive parameter for small movements about this equilibrium position. Laxity, the displacement (or rotation) of the tibia with respect to the femur at specified force (or moment) levels, also is a quantity of clinical interest. While a tibiofemoral contact force was applied, the anterior-posterior , medial-lateral, varus-valgus, and torsional stiffnesses at the neutral position always increased, while the corresponding laxities always decreased. Similar increases in joint stability associated with increased joint load were observed after medial and lateral meniscectomy. While a tibiofemoral contact force was applied to the knee, the joint was protected against varus-valgus angulation. In this situation, varus-valgus angulation of the tibia could not occur until the condylar lift-off moment (the force applied to the tibia multiplied by its distance from the condylar pivot point) had overcome the protective joint-load moment (the resultant joint force multiplied by its distance to the condylar pivot). Once this protective joint-load moment was exceeded, condylar lift-off and ligament stretch occurred. Joint load, whether This work was supported by United States Public Health Service Grant AM 19043-03 from the National Institutes of Health. t University of California at Los Angeles, School of Medicine, Los Angeles. California 90024. generated by gravitational, dynamic, or muscular forces, is an important protective mechanism that avoids ligament strain. After medial and lateral meniscectomy in the fully extended, unloaded knee, anterior-posterior neutral stiffness and varus-valgus neutral stiffness were reduced significantly and varusvalgus laxity was increased significantly. When the knee was in 20 degrees of flexion, internal and external rotation of the tibia decreased anterior-posterior laxity and increased anterior-posterior neutral stiffness. Hyperextension of the knee increased neutral stiffness and decreased laxity during the varus-valgus test. CLINICAL RELEVANCE: Load on the knee has an important stabilizing effect on tibiofemoral motions because it limits displacements and rotations and thereby protects the ligaments from excessive strains produced by external forces and moments. Removal of the menisci often makes a knee looser in the unloaded condition, but does not affect the stability of the loaded knee. Knee stability is provided by the complex interactions of a multitude of factors, including ligament and other soft-tissue restraints, condylan geometry, active muscular control, and tibiofemoral contact forces at the joint interfaces generated during weight-bearing activities. In a prior in vitro study4, we quantified the effects of sectioning the ligament-supporting structures on the stiffness and laxity of the knee. These experiments were performed manually with no joint load. Subsequent in vivo measurements3 of parameters of knee stability agreed well with our earlier results in cadavera and in addition provided quantitative information related to the stabilizing effects of the knee muscles during maximum isometric contraction. A review of the literature cited in these two earlier works indicated that most in vitro and in vivo testing in the past was conducted with the knee in the unloaded state. It was our purpose in the present study to investigate the effects of tibiofemonal contact force on knee stability and to determine whether the menisci have a stabilizing function duning conditions of direct load-bearing.

The structural components of the intervertebral disc. A study of their contributions to the ability of the disc to withstand compressive forces.

To determine the contributions of the various parts of the intervertebral disc to its ability to withstand compressive forces, we did a series of static tests (load versus displacement, creep, and load relaxation) on fresh autopsy specimens. The following preparations were tested: intact disc, disc injected with saline, disc punctured through anular wall, disc with nucleus removed, and anulus alone. About the same compressive stiffness was shown by all specimens (after initial readjustment of height and bulging in some preparations), indicating that the anulus is much more important for the compressive behavior of the disc than has been hitherto believed.

Deformation of the thoracolumbar intervertebral joints in response to external loads: a biomechanical study using autopsy material.

Static loading tests on fresh human spinal segments obtained at autopsy were performed to measure the deformation of thoracic and lumbar intervertebral joints in response to lateral bending moment, flexion moment, extension moment, torsional moment, anteroposterior and mediolateral shear force, axia

Direct in vitro measurement of forces in the cruciate ligaments. Part I: The effect of multiplane loading in the intact knee.

Specially designed load-transducers that measured the resultant forces exerted by the posterior and anterior cruciate ligaments on their respective femoral and tibial insertions were applied to eighteen fresh-frozen cadaveric knees for a series of controlled loading experiments. The mean force in the posterior cruciate ligament at 5 degrees of forced hyperextension of the knee was 23 per cent of the mean force in the anterior cruciate ligament. When the knee was hyperflexed by application of 10.0 newton-meters of bending moment to the tibia, the mean force in the posterior cruciate ligament was 55 per cent of that in the anterior cruciate ligament. Quadriceps tendon pull increased the force in the posterior cruciate ligament in twelve of the fourteen specimens to which it had been applied, at 80 and 90 degrees of flexion only. The force generated in the posterior cruciate ligament by applied internal tibial torque was greatest when the knee was in 90 degrees of flexion; the force in the anterior cruciate ligament was greatest when the knee was fully extended. External tibial torque generated force in the posterior cruciate ligament in only eight specimens, and only at 80 and 90 degrees of flexion. The levels of force that were generated in the posterior cruciate ligament by applied varus and valgus bending moment were greatest at 90 degrees of flexion of the knee; the levels of force in the anterior cruciate ligament were greatest with the knee in full extension. With the knee flexed 90 degrees and the tibia in neutral rotation, fifty newtons of applied posterior tibial force increased the mean force in the posterior cruciate ligament by 58.4 newtons; at full extension, no increase in the force in the ligament was recorded, indicating that tensed capsular structures were absorbing the applied load. When the tibia was internally or externally rotated by applied tibial torque, the increases in the force in the ligament from applied posterior tibial force were sharply diminished.

Direct in vitro measurement of forces in the cruciate ligaments. Part II: The effect of section of the posterolateral structures.

Specially designed load-transducers were applied to eight fresh-frozen cadaveric knee specimens in order to measure resultant forces in both cruciate ligaments as the knees were subjected to straight varus-valgus bending moment and to tibial torque (with and without a superimposed posterior tibial force). The forces in the ligaments and tibial rotation were recorded at seven angles of flexion of the knee, between 0 and 90 degrees, before and after section of the posterolateral structures. Ligamentous section increased angulation of the tibia when varus moment was applied to the knee; the large increases in lateral opening of the knee joint were accompanied by increases in the force in the anterior cruciate ligament at all angles of flexion and increases in the force in the posterior cruciate ligament between 45 and 90 degrees of flexion. When valgus moment was applied, there were no significant changes in valgus angulation or the resultant force in either cruciate ligament after ligamentous section. Ligamentous section increased rotation of the tibia when external torque was applied to the knee. The increased external rotation was accompanied by decreases in the force in the anterior cruciate ligament between 0 and 20 degrees of flexion of the knee and increases in the force in the posterior cruciate ligament between 45 and 90 degrees of flexion. In the studies involving applied internal tibial torque, after ligamentous section, rotation of the tibia increased slightly between 60 and 90 degrees of flexion. The force in the anterior cruciate ligament increased between 0 and 20 degrees of flexion, while the force in the posterior cruciate ligament was unaffected.(ABSTRACT TRUNCATED AT 250 WORDS)

Biomechanical Consequences of Replacement of the Anterior Cruciate Ligament with a Patellar Ligament Allograft. Part II: Forces in the Graft Compared with Forces in the Intact Ligament*

Seventeen fresh-frozen knee specimens from cadavera were instrumented with a load-cell attached to a mechanically isolated cylinder of subchondral bone containing the tibial insertion of the anterior cruciate ligament. The forces in the intact anterior cruciate ligament were recorded as the knee was passively extended from 90 degrees of flexion to 5 degrees of hyperextension without and with several constant tibial loads: 100 newtons of anterior tibial force, ten newton-meters of internal and external tibial torque, and ten newton-meters of varus and valgus moment. The anterior cruciate ligament was resected, and a bone-patellar ligament-bone graft was inserted. The knee was flexed to 30 degrees, and the graft was pre-tensioned to restore normal anterior-posterior laxity. The knee-loading experiments were repeated at this level of pre-tension (laxity-matched pre-tension) and at a level that was forty-five newtons greater than the laxity-matched pre-tension (over-tension). During passive extension of the knee, the forces in the graft were always greater than the corresponding forces in the intact anterior cruciate ligament. Over-tensioning of the graft increased the forces in the graft at all angles of flexion. At full extension, the mean force in the anterior cruciate ligament was fifty-six newtons; the mean force in the graft at laxity-matched pre-tension was 168 newtons, and it was 286 newtons in the over-tensioned graft. Greater pre-tensioning may be required when the knee demonstrates apparent tightening of the graft in flexion. The mean forces in the graft generated during all constant loading tests were greater than those for the intact anterior cruciate ligament over the range of flexion. When the graft was over-tensioned, the forces generated by the anterior tibial force and by varus and valgus moment increased but those generated by internal and external tibial torque did not. There was no significant change in the mean tibial rotation as a function of the angle of flexion of the knee after insertion of the graft; normal tibial rotation of the knee during passive extension (the so-called screw home mechanism) was eliminated. CLINICAL RELEVANCE: When a patellar ligament allograft was pre-tensioned to restore normal anterior-posterior laxity, the forces in the graft were markedly greater than those in the intact anterior cruciate ligament. Thus, the penalty of increased forces in the graft must be accepted if anterior-posterior laxity is to be restored. Of particular concern are the large forces in the graft generated by loading states, such as external tibial torque and varus moment, which normally generate minimum force in the intact anterior cruciate ligament. In terms of force magnitude, internal torque applied to an extended knee is likely to be the most dangerous loading state for a patient who has a patellar ligament graft. There is a current trend toward early postoperative mobilization and intensive rehabilitation after substitution of the anterior cruciate ligament with a graft. Although this approach results in an excellent range of motion, the surgeon should be aware that a return to full activity could produce forces in the graft that are many times greater than those in the intact anterior cruciate ligament. For this reason, early return to full activity may not be indicated until full biological maturation of the graft.

Cyclic Loading of Posterior Cruciate Ligament Replacements Fixed with Tibial Tunnel and Tibial Inlay Methods

Background: The optimal method of replacement of the posterior cruciate ligament with a bone-patellar tendon-bone graft is not known. The purpose of this study was to compare the mechanical responses to cyclic loading tests of bone-patellar tendon-bone allograft replacements fixed to the tibia with one of two methods: a tibial tunnel or a tibial inlay technique. Methods: The proximal ends of sixty-two posterior cruciate graft replacements, thirty-one fixed with the tibial tunnel technique and thirty-one fixed with the tibial inlay technique in cadaver knees, were subjected to 2000 cycles of tensile force of 50 to 300 N with the angle of pull at 45° to the tibial plateau. The central 10 mm of the medial and lateral halves of previously fresh-frozen bone-patellar tendon-bone preparations from cadaver knees were used as the grafts. Two pairs of tibiae were used for testing; the two types of fixation and the medial and lateral halves of the patellar tendons were distributed between the tibial pairs. Graft thickness was measured at the point of highest anticipated tissue deformation and at two additional locations at distances from these points. The total change in graft length after cyclic loading at an applied force level of 200 N was recorded. Elongation of the graft during loading cycles between 20 and 200 N of applied tensile force was also measured. A repeated-measures analysis of variance was used to compare all measurements between the inlay and tunnel techniques, and between the medial and lateral halves of the graft used for the inlay method. Results: Ten of the thirty-one grafts that had been passed through a tibial tunnel failed at the acute angle before 2000 cycles of testing could be completed; all thirty-one grafts that had been fixed to the tibia with use of the inlay method survived the testing intact. Evaluation of the twenty-one graft pairs that survived testing after both fixation techniques revealed that the grafts that had been fixed with the inlay method had significantly less thinning at all three measurement sites at the completion of testing; the mean reduction of thickness was 40.6% (at the acute angle) in the grafts fixed with the tunnel method and 12.5% (adjacent to the bone block) in those fixed with the inlay method. After 2000 cycles, the mean lengths of the grafts fixed with the inlay and tunnel methods increased 5.9 and 9.8 mm, respectively; 38% of this increase occurred during the first six loading cycles. After both methods of fixation, the mean graft elongation during a loading cycle decreased approximately 50% from cycle 1 to cycle 2000, resulting in an effectively stiffer graft construct. There was no significant difference in any measured parameter between medial and lateral graft halves. Conclusions: These tests showed that the inlay technique of posterior cruciate ligament replacement was superior to the tunnel technique with respect to graft failure, graft thinning, and permanent increase in graft length. Clinical Relevance: Grafts replacing the posterior cruciate ligament are subjected to repetitive mechanical loading, and our results demonstrated that, with either the tunnel or the inlay fixation technique, the graft undergoes thinning and permanent length changes at the load levels used in these tests. These permanent length changes could be reduced substantially if the graft were cyclically preconditioned in situ before final pretensioning and fixation. The marked thinning of graft tissue at the acute angle and the permanent length changes of the tunnel grafts that did not fail may explain the increased posterior laxity observed in many patients who have undergone posterior cruciate replacement with use of the tunnel technique. The inlay technique of fixation significantly reduced these degradative effects. Regardless of the type of fixation to the tibia, there appears to be no advantage to using either the medial or the lateral half of a bone-patellar tendon-bone allograft preparation.

Effects of Applied Quadriceps and Hamstrings Muscle Loads on Forces in the Anterior and Posterior Cruciate Ligaments

Background Muscle contraction can subject healing knee ligament grafts to high loads. Purpose To directly measure the effects of quadriceps and hamstrings muscle loads on forces in the anterior cruciate ligaments and posterior cruciate ligaments. Study Design Controlled laboratory study. Methods Thirteen cadaveric knee specimens had load cells installed to record resultant forces in both anterior and posterior cruciate ligaments under 5 loading conditions. Cruciate force measurements were repeated with a 100-N load applied to the quadriceps tendon and again with a combined 50-N biceps load and 50-N semimembranosus-semitendinosus load. Results Applied quadriceps loads resulted in mean changes in anterior cruciate ligament and posterior cruciate ligament forces that were less than 20 N for all loading conditions. Hamstrings load significantly increased mean posterior cruciate ligament force between 30° and 105° of flexion with 100 N of applied posterior tibial force. Conclusions At the muscle force levels used in this study, the hamstrings were more effective than the quadriceps in altering cruciate force levels, especially near 90° of flexion, where they have an excellent mechanical advantage for controlling anterior-posterior tibial translation. Clinical Relevance Isolated hamstrings activity generally had little or no effect on anterior cruciate ligament forces but significantly increased forces in the posterior cruciate ligament beyond approximately 30° of flexion.

Effects of femoral tunnel placement on knee laxity and forces in an anterior cruciate ligament graft.

The purpose of this study was to measure the effects of variation in placement of the femoral tunnel upon knee laxity, graft pretension required to restore normal anterior–posterior (AP) laxity and graft forces following anterior cruciate ligament (ACL) reconstruction. Two variants in tunnel position were studied: (1) AP position along the medial border of the lateral femoral condyle (at a standard 11 oclock notch orientation) and (2) orientation along the arc of the femoral notch (oclock position) at a fixed distance of 6–7 mm anterior to the posterior wall. AP laxity and forces in the native ACL were measured in fresh frozen cadaveric knee specimens during passive knee flexion‐extension under the following modes of tibial loading: no external tibial force, anterior tibial force, varus–valgus moment, and internal–external tibial torque. One group (15 specimens) was used to determine effects of AP tunnel placement, while a second group (14 specimens) was used to study variations in oclock position of the femoral tunnel within the femoral notch. A bone‐patellar tendon‐bone graft was placed into a femoral tunnel centered at a point 6–7 mm anterior to the posterior wall at the 11 oclock position in the femoral notch. A graft pretension was determined such that AP laxity of the knee at 30 deg of flexion was restored to within 1 mm of normal; this was termed the laxity match pretension. All tests were repeated with a graft in the standard 11 oclock tunnel, and then with a graft in tunnels placed at other selected positions. Varying placement of the femoral tunnel 1 h clockwise or counterclockwise from the 11 oclock position did not significantly affect any biomechanical parameter measured in this study, nor did placing the graft 2.5 mm posteriorly within the standard 11 oclock femoral tunnel. Placing the graft in a tunnel 5.0 mm anterior to the standard 11 oclock tunnel increased the mean laxity match pretension by 16.8 N (62%) and produced a knee which was on average 1.7 mm more lax than normal at 10 deg of flexion and 4.2 mm less lax at 90 deg. During passive knee flexion‐extension testing, mean graft forces with the 5.0 mm anterior tunnel were significantly higher than corresponding means with the standard 11 oclock tunnel between 40 and 90 deg of flexion for all modes of constant tibial loading. These results indicate that AP positioning of the femoral tunnel at the 11 oclock position is more critical than oclock positioning in terms of restoring normal levels of graft force and knee laxity profiles at the time of ACL reconstruction.