Evaluation of the effect of joint constraints on the in situ force distribution in the anterior cruciate ligament

The function of the anterior cruciate ligament was investigated for different conditions of kinematic constraint placed on the intact knee using a six-degree-of-freedom robotic manipulator combined with a universal force-moment sensor. To do this, the in situ forces and force distribution within the porcine anterior cruciate ligament during anterior tibial loading up to 100 N were compared at 30, 60, and 90° of flexion under: (a) unconstrained, five-degree-of-freedom knee motion, and (b) constrained, one-degree-of-freedom motion (i.e., anterior translations only). The robotic/universal force-moment sensor testing system was used to both apply the specified external loading to the in tact joint and measure the resulting kinematics. After tests of the intact knee were completed, all soft tissues except the anterior cruciate ligament were removed, and these motions were reproduced such that the in situ force and force distribution could be determined. No significant differences in the magnitude of in situ forces in the anterior cruciate ligament were found between the unconstrained and constrained testing conditions. In contrast, the direction of in situ force changed significantly; the force vector in the unconstrained case was more parallel with the direction of the applied tibial load. In addition, the distribution of in situ force between the anteromedial and posterolateral bundles of the ligament was nearly equal for all flexion angles for the unconstrained case, whereas the anteromedial bundle carried higher forces than the posterolateral bundle at both 60 and 90° of flexion for the constrained case. This demonstrates that the constraint conditions placed on the joint have a significant effect on the apparent role of the anterior cruciate ligament. Specifically, constraining joint motion to one degree of freedom significantly alters both the direction and distribution of the in situ force in the ligament from that observed for unconstrained joint motion (five degrees of freedom). Furthermore, the changes observed in the distribution of force between the anteromedial and posterolateral bundles for different constraint conditions may help elucidate mechanisms of injury by providing new insight into the response of the anterior cruciate ligament to different types of external knee loading.     

Effect of combined axial compressive and anterior tibial loads on in situ forces in the anterior cruciate ligament: A porcine study

This study investigated the impact of a combination of axial compressive and anterior-posterior tibial loads on the in situ forces in the anterior cruciate ligament. An axial compressive load is believed to contribute to increased stability of the knee joint; however, its effect on in situ forces in the anterior cruciate ligament has not been clearly defined, to our knowledge. It was hypothesized that the application of an axial compressive load, when combined with an anterior tibial load, would result in larger in situ forces in the anterior cruciate ligament than those caused by an isolated anterior tibial load. With use of a porcine knee model, the results confirmed this hypothesis; the addition of a 200 N axial compressive load to a 100 N anterior tibial load increased knee stability by reducing anterior-posterior tibial translation and internal-external tibial rotation and also caused a significant increase in in situ forces in the anterior cruciate ligament (p < 0.05). Specifically, there was a 34% increase in the in situ force at 30° of flexion, a 68% increase at 60° of flexion, and an 84% increase at 90° of flexion compared with those for an isolated anterior tibial load of 100 N. Additionally, there was a statistically significant increase of the in situ forces in the anterior cruciate ligament at 60 and 90° as compared with those at 30°. These results suggest that axial compressive loads on the knee may play a role in injury of the anterior cruciate ligament when the knee is flexed.     

In situ forces in the posterolateral structures of the knee under posterior tibial loading in the intact and posterior cruciate ligament-deficient knee

The posterolateral structures of the knee consist of a complex anatomical architecture that includes several components with both static and dynamic functions. Injuries of the posterolateral structures occur frequently in conjunction with ruptures of the posterior cruciate ligament. To investigate the role of the posterolateral structures in maintaining posterior knee stability, we measured the in situ forces in the posterolateral structures and the distribution of force within the structures major components, i.e., the popliteus complex and the lateral collateral ligament, in response to a posterior tibial load. Eight cadaveric knees were tested. With use of a robotic/universal force-moment sensor testing system, a posterior tibial load of 110 N was applied to the knee, and the resulting five-degree-of-freedom kinematics were measured at flexion angles of 0, 30, 60, 75, and 90°. The knees were tested first in the intact state and then after the posterior cruciate ligament had been resected. These tests were also performed with an additional load of 44 N applied at the aponeurosis to simulate contraction of the popliteus muscle. In the intact knee, the in situ forces in the posterolateral structures were found to decrease with increasing knee flexion. After the posterior cruciate ligament was sectioned, these forces increased significantly at all angles of flexion. With no load applied to the popliteus muscle, the in situ forces in the popliteus complex were similar to those in the lateral collateral ligament. However, with a load of 44 N applied to the popliteus muscle, in situ forces in the popliteus complex were three to five, times higher than those in the lateral collateral ligament. These results reveal that in response to posterior tibial loads, the posterolateral structures play an important role at full extension in intact knees and at all angles of flexion in posterior cruciate ligament-deficient knees. The popliteus muscle appears to be a major stabilizer under this loading condition; thus, the inability to restore its function may be a cause of unsatisfactory results in reconstructive procedures of the posterolateral structures of the knee.     

In situ calibration of miniature sensors implanted into the anterior cruciate ligament. Part I: Strain measurements

The goals of this study were to (a) evaluate the differential variable reluctance transducer as an instrument for measuring tissue strain in the anteromedial band of the anterior crudciate ligament, (b) develop a series of calibration curves (for simple states of knee loading) from which resultant force in the ligament could be estimated from measured strain levels in the anteromedial band of the ligament, and (c) study the effects of knee flexion angle and mode of applied loading on ouput from the transducer. Thirteen fresh-frozen cadaveric knee specimens underwent mechanical isolation of a bone cap containing the tibial insertion of the anterior cruciate ligament and attachment of a load cell to measure resultant force in the ligament. The transducer (with barbed prongs) was inserted into the anteromedial band of the anterior cruciate ligament to record local elongation of the instrumented fibers as resultant force was generated in the ligament. A series of calibration curves (anteromedial bundle strain versus resultant force in the anterior cruciate ligament) were determined at selected knee flexion angles as external loads were applied to the knee. During passive knee extension, strain readings did not always follow the pattern of resultant force in the ligament; erratic strain readings were often measured beyond 20° of flexion, where the anteromedial band was slack. For anterior tibial loading, the anteromedial band was a more active contributor to resultant ligament force beyond 45° of flexion and was less active near full extension; mean resultant forces in the range of 150–200 N produced strain levels on the order of 3–4%. The anteromedial band was also active during application of internal tibial torque; mean resultant forces on the order of 180–220 N produced strains on the order of 2%. Resultant forces generated by varus moment were relatively low, and the anteromedial band was not always strained. Mean coefficients of variation for resultant force in the ligament (five repeated measurements) ranged between 0.038 and 0.111. Mean coefficients of variation for five repeated placements of the strain transducer in the same site ranged from 0.209 to 0.342. Insertion and removal of this transducer at the anteromedial band produced observable damage to the ligament. In our study, repeatable measurements were possible only if both prongs of the transducer were sutured to the ligament fibers.     

In vivo anterior cruciate ligament elongation in response to axial tibial loads

Background  The knowledge of in vivo anterior cruciate ligament (ACL) deformation is fundamental for understanding ACL injury mechanisms and for improving surgical reconstruction of the injured ACL. This study investigated the relative elongation of the ACL when the knee is subject to no load (<10 N) and then to full body weight (axial tibial load) at various flexion angles using a combined dual fluoroscopic and magnetic resonance imaging (MRI) technique. Methods  Nine healthy subjects were scanned with MRI and imaged when one knee was subject to no load and then to full body weight using a dual fluoroscopic system (0°–45° flexion angles). The ACL was analyzed using three models: a single central bundle; an anteromedial and posterolateral (double functional) bundle; and multiple (eight) surface fiber bundles. Results  The anteromedial bundle had a peak relative elongation of 4.4% ± 3.4% at 30° and that of the posterolateral bundle was 5.9% ± 3.4% at 15°. The ACL surface fiber bundles at the posterior portion of the ACL were shorter in length than those at the anterior portion. However, the peak relative elongation of one posterolateral fiber bundle reached more than 13% whereas one anteromedial fiber bundle reached a peak relative elongation of only about 3% at 30° of flexion by increasing the axial tibial load from no load to full body weight. Conclusions  The data quantitatively demonstrated that under external loading the ACL experiences nonhomogeneous elongation, with the posterior fiber bundles stretching more than the anterior fiber bundles.     

Distribution of in situ forces in the anterior cruciate ligament in response to rotatory loads.

The anterior cruciate ligament (ACL) can be anatomically divided into anteromedial (AM) and posterolateral (PL) bundles. Current ACL reconstruction techniques focus primarily on reproducing the AM bundle, but are insufficient in response to rotatory loads. The objective of this study was to determine the distribution of in situ force between the two bundles when the knee is subjected to anterior tibial and rotatory loads. Ten cadaveric knees (50+/-10 years) were tested using a robotic/universal force-moment sensor (UFS) testing system. Two external loading conditions were applied: a 134 N anterior tibial load at full knee extension and 15 degrees, 30 degrees, 60 degrees, and 90 degrees of flexion and a combined rotatory load of 10 Nm valgus and 5 Nm internal tibial torque at 15 degrees and 30 degrees of flexion. The resulting 6 degrees of freedom kinematics of the knee and the in situ forces in the ACL and its two bundles were determined. Under an anterior tibial load, the in situ force in the PL bundle was the highest at full extension (67+/-30 N) and decreased with increasing flexion. The in situ force in the AM bundle was lower than in the PL bundle at full extension, but increased with increasing flexion, reaching a maximum (90+/-17 N) at 60 degrees of flexion and then decreasing at 90 degrees. Under a combined rotatory load, the in situ force of the PL bundle was higher at 15 degrees (21+/-11 N) and lower at 30 degrees of flexion (14+/-6 N). The in situ force in the AM bundle was similar at 15 degrees and 30 degrees of knee flexion (30+/-15 vs. 35+/-16 N, respectively). Comparing these two external loading conditions demonstrated the importance of the PL bundle, especially when the knee is near full extension. These findings provide a better understanding of the function of the two bundles of the ACL and could serve as a basis for future considerations of surgical reconstruction in the replacement of the ACL.     

Combined knee loading states that generate high anterior cruciate ligament forces

Injuries to the anterior cruciate ligament frequently occur under combined mechanisms of loading. This in vitro study was designed to measure levels of ligament force under dual combinations of individual loading states and to determine which combinations generated high force. Resultant force was recorded as the knee was extended passively from 90° of flexion to 5° of hyperextension under constant tibial loadings. The individual loading states were 100 N of anterior tibial force, 10 Nm of varus and valgus moment, and 10 Nm of internal and external tibial torque. Straight anterior tibial force was the most direct loading, mechanism; the mean ligament force was approximately equal to applied anterior tibial force near 30° of flexion and to 150% of applied tibial force at full extension. The addition of internal tibial torque to a knee loaded by anterior tibial force produced dramatic increases of force at full extension and hyperextension. This loading combination produced the highest ligament forces recorded in the study and is the most dangerous in terms of potential injury to the ligament. In direct contrast, the addition of external tibial torque to a knee loaded by anterior tibial force decreased the force dramatically for flexed positions of the knee; at close to 90° of flexion, the anterior cruciate ligament became completely unloaded. The addition of varus moment to a knee loaded by anterior tibial force increased the force in extension and hyperextension, whereas the addition of valgus moment increased the force at flexed positions. These states of combined loading also could present an increased risk for injury. Internal tibial torque is an important loading mechanism of the anterior cruciate ligament for an extended knee. The overall risk of injury to the ligament from varus or valgus moment applied in combination with internal tibial torque is similar to the risk from internal tibial torque alone. External tibial torque was a relatively unimportant mechanism for generating anterior cruciate ligament force.     

In-situ force in the medial and lateral structures of intact and ACL-deficient knees

The anterior cruciate ligament (ACL) is the major contributor to limit excessive anterior tibial translation (ATT) when the knee is subjected to an anterior tibial load. However, the importance of the medial and lateral structures of the knee can also play a significant role in resisting anterior tibial loads, especially in the event of an ACL injury. Therefore, the objective of this study was to determine quantitatively the increase in the in-situ forces in the medial collateral ligament (MCL) and posterolateral structures (PLS) of the knee associated with ACL deficiency. Eight fresh-frozen cadaveric human knees were subjected to a 134-N anterior tibial load at full extension and at 15°, 30°, 60°, and 90° of knee flexion. The resulting 5 degrees of freedom kinematics were measured for the intact and the ACL-deficient knees. A robotic/universal force-moment sensor testing system was used for this purpose, as well as to determine the in-situ force in the MCL and PLS in the intact and ACL-deficient knees. For the intact knee, the in-situ forces in both the MCL and PLS were less than 20 N for all five flexion angles tested. But in the ACL-deficient knee, the in-situ forces in the MCL and PLS, respectively, were approximately two and five times as large as those in the intact knee (P < 0.05). The results of this study demonstrate that, although both the MCL and PLS play only a minor role in resisting anterior tibial loads in the intact knee, they become significant after ACL injury. Received: December 3, 1999 / Accepted: July 19, 2000     

Tension patterns of the anteromedial and posterolateral grafts in a double‐bundle anterior cruciate ligament reconstruction

The two functional bundles of the anterior cruciate ligament (ACL), namely, the anteromedial (AM) and posterolateral (PL) bundles, must work in concert to control displacement of the tibia relative to the femur for complex motions. Thus, the replacement graft(s) for a torn ACL should possess similar tension patterns. The objective of the study was to examine whether a double‐bundle ACL reconstruction with the semitendinosus‐gracilis autografts could replicate the tension patterns of those for the intact ACL under controlled in vitro loading conditions. By means of a robotic/universal force moment sensor (UFS) testing system, the in situ force vectors (both magnitude and direction) for the AM and PL bundles of the ACL, as well as their respective replacement grafts, were determined and compared on nine human cadaveric knees. It was found that double‐bundle ACL reconstruction could closely replicate the in situ force vectors. Under a 134‐N anterior tibial load, the resultant force vectors for the intact ACL and the reconstructed ACL had a difference of 5 to 11 N (p > 0.05) in magnitude and 1 to 13° (p > 0.05) in direction. Whereas, under combined rotatory loads of 10‐N‐m valgus and 5‐N‐m internal tibial torques, the corresponding differences were 10 to 16 N and 4° to 11°, respectively. Again, there were no statistically significant differences except at 30° of flexion where the force vector for the AM graft had a 15° (p < 0.05) lower elevation angle than did the AM bundle. © 2008 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 27: 879–884, 2009     

Determination of a zero strain reference for the anteromedial band of the anterior cruciate ligament

The objective of this study was to verify a method previously used to determine a reference length for calculations of anterior cruciate ligament strain. In nine knee specimens, an arthroscopic force probe and a Hall effect transducer were placed in the anteromedial band of the ligament. Anteroposterior-directed shear loads then were applied to the knee joint with the knee flexed to 30°. From the sigmoidal curve for shear load versus displacement of the anterior cruciate ligament midsubstance, the length of the transducer at the inflection point was determined graphically by two independent examiners. Previous studies suggested that the inflection point corresponds to the slack-taut transition of the anteromedial band. The force probe was used to determine the actual length of the transducer when the anteromedial band became load bearing. No significant differences were found between the reference lengths determined by the inflection point method and the force probe. The force probe demonstrated that the anterior cruciate ligament became load bearing when an anterior shear load of 8.8 N was applied to the tibia with the knee at 30° of flexion. Furthermore, multiple cycles of anteroposterior shear loading did not influence these values. The force probe verified that the inflection method provides a reasonable estimate of the absolute strain reference (within 0.7% strain).     

Strain in the anteromedial bundle of the anterior cruciate ligament under combination loading.

Strain within the anteromedial bundle (AMB) of the anterior cruciate ligament (ACL) was measured in 13 human knee specimens in order to determine the combination of external loads most likely to cause injury. Using a load application system that allowed 5 df with the flexion angle being fixed, pure loads of anterior/posterior force, medial/lateral force, varus/valgus torque, and internal/external axial torque were applied at three flexion angles: 0 degrees, 15 degrees, 30 degrees. Combined loads were applied in pairs at two flexion angles: 0 degrees and 30 degrees. Liquid mercury strain gauges were used to measure strain in the ACL. Anterior tibial force was the primary determinant of strain in the anteromedial bundle. This strain was significantly larger at 30 degrees flexion than at 0 degrees. The strain sensitivity of the AMB to medial force was approximately one-half that to pure anterior force. The effect of anterior and medial forces was additive when applied in combination. Neither pure axial torque nor pure varus/valgus torque was observed to strain significantly the AMB at any of the flexion angles investigated. However, valgus torque in combination with anterior force resulted in a significantly larger strain than pure anterior force. Internal axial torque in combination with anterior force also resulted in a larger strain than pure anterior force.     

Arthroscopic reconstruction of the anterior cruciate ligament using double anteromedial and posterolateral bundles

We propose a method for repairing the anterior cruciate ligament which takes advantage of the multifascular nature of the ligament to achieve better physiological anteroposterior and rotational stability compared with conventional methods. Arthroscopic reconstruction of the anteromedial and posterolateral bundles of the ligament closely reproduces normal anatomy. We have used this technique in 92 patients with anterior cruciate ligament laxity and present here the mid-term results. The hamstring tendons (gracilis and semitendinosus) are harvested carefully to obtain good quality grafts. Arthroscopic preparation of the notch allows careful cleaning of the axial aspect of the lateral condyle; it is crucial to well visualize the region over the top and delimit the 9 h-12 h zone for the right knee or the 12-15 h zone for the left knee. The femoral end of the anteromedial tunnel lies close to the floor of the intercondylar notch, 5 to 10 mm in front of the posterior border of the lateral condyle, at 13 h for the left knee and 11 h for the right knee. The femoral end of the posterolateral tunnel lies more anteriorly, at 14 h for the left knee and 10 h for the right knee. The tibial end of the posterolateral tunnel faces the anterolateral spike of the tibia. The tibial end of the anteromedial tunnel lies in front of the apex of the two tibial spikes half way between the anteromedial spike and the anterolateral spike, 8 mm in front of the protrusion of the posteriolateral pin. The posterolateral graft is run through the femoral and tibial tunnels first. A cortical fixation is used for the femoral end. The femoral end of the anteromedial graft is then fixed in the same way. The tibial fixation begins with the posterolateral graft with the knee close to full extension. The anteromedial graft is fixed with the knee in 90 degrees flexion. Thirty patients were reviewed at least six months after the procedure. Mean age was 28.2 years. Mean overall IKDC score was 86% (36% A and 50% B). Gain in laxity was significant: 6.53 preoperatively and 2.1 postoperatively. Most of the patients (86.6%) were able to resume their former occupation 2 months after the procedure. The different components of the anterior cruciate ligament and their respective functions have been the object of several studies. The anteromedial bundle maintains joint stability during extension and anteroposterior stability during flexion. The posterolateral bundle contributes to the action of the anteromedial bundle with an additional effect due to its position: rotational stability during flexion. In light of the multifascicular nature of the anterior cruciate ligament and the residual rotational laxity observed after conventional repair, our proposed method provides a more anatomic reconstruction which achieves better correction of anteroposterior and rotational stability. This technique should be validated with comparative trials against currently employed methods.     

Double-bundle PCL and Posterolateral Corner Reconstruction Components are Codominant

A more complete biomechanical understanding of a combined posterior cruciate ligament and posterolateral corner knee reconstruction may help surgeons develop uniformly accepted clinical surgical techniques that restore normal anatomy and protect the knee from premature arthritic changes. We identified the in situ force patterns of the individual components of a combined double-bundle posterior cruciate ligament and posterolateral corner knee reconstruction. We tested 10 human cadaveric knees using a robotic testing system by sequentially cutting and reconstructing the posterior cruciate ligament and posterolateral corner. The knees were subjected to a 134-N posterior tibial load and 5-Nm external tibial torque. The posterior cruciate ligament was reconstructed with a double-bundle technique. The posterolateral corner reconstruction included reattaching the popliteus tendon to its femoral origin and reconstructing the popliteofibular ligament. The in situ forces in the anterolateral bundle were greater in the posterolateral corner-deficient state than in the posterolateral corner-reconstructed state at 30° under the posterior tibial load and at 90° under the external tibial torque. We observed no differences in the in situ forces between the anterolateral and posteromedial bundles under any loading condition. The popliteus tendon and popliteofibular ligament had similar in situ forces at all flexion angles. The data suggest the two bundles protect each other by functioning in a load-sharing, codominant fashion, with no component dominating at any flexion angle. We believe the findings support reconstructing both posterior cruciate ligament bundles and both posterolateral corner components. One or more of the authors (CDH) have received funding from the Aircast Foundation, Pittsburgh, PA. Each author certifies that his or her institution either has waived or does not require approval for the human protocol for this investigation and that all investigations were conducted in conformity with ethical principles of research.     

Importance of the medial meniscus in the anterior cruciate ligament-deficient knee.

The incidence of meniscal tears in the chronically anterior cruciate ligament-deficient knee is increased, particularly in the medial meniscus because it performs an important function in limiting knee motion. We evaluated the role of the medial meniscus in stabilizing the anterior cruciate ligament-deficient knee and hypothesized that the resultant force in the meniscus is significantly elevated in the anterior cruciate ligament-deficient knee. To test this hypothesis, we employed a robotic/universal force-moment sensor testing system to determine the increase in the resultant force in the human medial meniscus in response to an anterior tibial load following transection of the anterior cruciate ligament. We also measured changes in the kinematics of the knee in multiple degrees of freedom following medial meniscectomy in the anterior cruciate ligament-deficient knee. In response to a 134-N anterior tibial load, the resultant force in the medial meniscus of the anterior cruciate ligament-deficient knee increased significantly compared with that in the meniscus of the intact knee; it increased by a minimum of 10.1 N (52%) at full knee extension to a maximum of 50.2 N (197%) at 60 degrees of flexion. Medial meniscectomy in the anterior cruciate ligament-deficient knee also caused a significant increase in anterior tibial translation in response to the anterior tibial load, ranging from an increase of 2.2 mm at full knee extension to 5.8 mm at 60 degrees of flexion. Conversely, coupled internal tibial rotation in response to the load decreased significantly, ranging from a decrease of 2.5 degrees at 15 degrees of knee flexion to 4.7 degrees at 60 degrees of flexion. Our data confirm the hypothesis that the resultant force in the medial meniscus is significantly greater in the anterior cruciate ligament-deficient knee than in the intact knee when the knee is subjected to anterior tibial loads. This indicates that the demand on the medial meniscus in resisting anterior tibial loads is increased in the anterior cruciate ligament-deficient knee compared with in the intact knee, suggesting a mechanism for the increased incidence of medial meniscal tears observed in chronically anterior cruciate ligament-deficient patients. The large changes in kinematics due to medial meniscectomy in the anterior cruciate ligament-deficient knee confirm the important role of the medial meniscus in controlling knee stability. These findings suggest that the reduction of resultant force in the meniscus may be a further motive for reconstructing the anterior cruciate ligament, with the goal of preserving meniscal integrity.     

关节镜下半腱肌腱和股薄肌腱双隧道重建前十字韧带

目的探讨关节镜下联合应用半腱肌腱和股薄肌腱重建前十字韧带(anteriorcruciateligament,ACL)的方法及疗效。方法回顾自1998年4月～2000年5月在关节镜下联合应用半腱肌腱和股薄肌腱重建ACL的患者12例。于ACL前内侧束和后外侧束的附着部分别钻直径4.5mm的隧道,用半腱肌腱重建前内侧束,股薄肌腱重建后外侧束,保留半腱肌腱和股薄肌腱的附着点,在股骨隧道外口将半腱肌腱和股薄肌腱打结固定,不行内固定。所有患者术前及术后18个月行膝关节屈曲30°、60°、90°前抽屉试验,Lysholm评分方法评定膝关节功能。结果术后随访18～43个月,平均26个月。术前所有患者前抽屉试验均为阳性,术后9例阴性,2例屈膝30°位阳性,1例屈膝30°、60°位阳性。术前Lysholm评分为40～58分,平均50.5分,手术后18个月为62～92分,平均85分,总优良率为91.7%。结论应用半腱肌腱和股薄肌腱联合重建ACL,术后膝关节动态稳定性好,疗效满意。     

Comparison of the ACL and ACL graft forces before and after ACL reconstruction an in-vitro robotic investigation

Background?Long-term follow-up studies have indi-cated that there is an increased incidence of arthrosis following anterior cruciate ligament (ACL) reconstruc-tion, suggesting that the reconstruction may not repro-duce intact ACL biomechanics. We studied not only the magnitude but also the orientation of the ACL and ACL graft forces

Methods?10 knee specimens were tested on a robotic testing system with the ACL intact, deficient, and recon-structed (using a bone-patella tendon-bone graft). The magnitude and orientation of the ACL and ACL graft forces were determined under an anterior tibial load of 130?N at full extension, and 15, 30, 60, and 90° of flexion. Orientation was described using elevation angle (the angle formed with the tibial plateau in the sagit-tal plane) and deviation angle (the angle formed with respect to the anteroposterior direction in the transverse plane)

Results?ACL reconstruction restored anterior tibial translation to within 2.6?mm of that of the intact knee under the 130-N anterior load. Average internal tibial rotation was reduced after ACL reconstruction at all flexion angles. The force vector of the ACL graft was significantly different from the ACL force vector. The average values of the elevation and deviation angles of the ACL graft forces were higher than that of the intact ACL at all flexion angles

Interpretation?Contemporary single bundle ACL reconstruction restores anterior tibial translation under anterior tibial load with different forces (both magni-tude and orientation) in the graft compared to the intact ACL. Such graft function might alter knee kinematics in other degrees of freedom and could overly constrain the tibial rotation. An anatomic ACL reconstruction should reproduce the magnitude and orientation of the intact ACL force vector, so that the 6-degrees-of-freedom knee kinematics and joint reaction forces can be restored.     

Determination of the in situ loads on the human anterior cruciate ligament

A noncontact, kinematic method was used to determine the lengths and in situ loads borne by portions of the human anterior cruciate ligament (ACL) by the combination of kinematic data from the intact knee and load-length curves of the isolated ACL. Specimens from knees of cadavers of young people were tested in passive flexion and extension as well as with 100 N of anterior tibial drawer at 0, 30, 45, and 90° of flexion. The results showed that the in situ load on the whole ACL (as much as 129 N) can exceed the magnitude of the applied anterior tibial drawer. The load distribution within the ligament changes with flexion of the knee. The anterior and posterior portions share the anterior drawer force equally toward full extension. However, at flexion >45°, the anterior portion supports 90–95% of the load. This information is important for the determination of the function of the entire ACL and of its subportions in response to external loading of the intact knee. In particular, the preferential loading found for one of the portions of the ACL demonstrates that successful operative reconstruction of this ligament may not be achieved simply by reproduction of its gross anatomy; consideration of the role of the ligament in the overall kinematics of the knee is necessary.     

Knee stability and graft function following anterior cruciate ligament reconstruction: Comparison between 11 o'clock and 10 o'clock femoral tunnel placement

Purpose: To study how well an anterior cruciate ligament (ACL) graft fixed at the 10 and 11 o'clock positions can restore knee function in response to both externally applied anterior tibial and combined rotatory loads by comparing the biomechanical results with each other and with the intact knee. Type of Study: Biomechanical experiment using human cadaveric specimens. Methods: Ten human cadaveric knees (age, 41±13 years) were reconstructed by placing a bone–patellar tendon–bone graft at the 10 and 11 o'clock positions, in a randomized order, and then tested using a robotic/universal force-moment sensor testing system. Two external loading conditions were applied: (1) 134 N anterior tibial load with the knee at full extension, 15°, 30°, 60°, and 90° of flexion, and (2) a combined rotatory load of 10 N-m valgus and 5 N-m internal tibial torque with the knee at 15° and 30° of flexion. The resulting kinematics of the reconstructed knee and in situ forces in the ACL graft were determined for each femoral tunnel position. Results: In response to a 134-N anterior tibial load, anterior tibial translation (ATT) for both femoral tunnel positions was not significantly different from the intact knee except at 90° of knee flexion as well as at 60° of knee flexion for the 10 o'clock position. There was no significant difference in the ATT between the 10 and 11 o'clock positions, except at 90° of knee flexion. Under a combined rotatory load, however, the coupled ATT for the 11 o'clock position was approximately 130% of that for the intact knee at 15° and 30° of flexion. For the 10 o'clock position, the coupled ATT was not significantly different from the intact knee at 15° of flexion and approximately 120% of that for the intact knee at 30° of flexion. Coupled ATT for the 10 o'clock position was significantly smaller than for the 11 o'clock position at 15° and 30° of flexion. The in situ force in the ACL graft was also significantly higher for the 10 o'clock position than the 11 o'clock position at 30° of flexion in response to the same loading condition (70 ± 18 N v 60 ± 15 N, respectively). Conclusions: The 10 o'clock position more effectively resists rotatory loads when compared with the 11 o'clock position as evidenced by smaller ATT and higher in situ force in the graft. Despite the fact that ACL grafts placed at the 10 or 11 o'clock positions are equally effective under an anterior tibial load, neither femoral tunnel position was able to fully restore knee stability to the level of the intact knee.Arthroscopy: The Journal of Arthroscopic and Related Surgery, Vol 19, No 3 (March), 2003: pp 297–304     

In situ forces in the human posterior cruciate ligament in response to muscle loads: a cadaveric study.

The objectives of this study were to determine the effects of hamstrings and quadriceps muscle loads on knee kinematics and in situ forces in the posterior cruciate ligament of the knee and to evaluate how the effects of these muscle loads change with knee flexion. Nine human cadaveric knees were studied with a robotic manipulator/universal force-moment sensor testing system. The knees were subjected to an isolated hamstrings load (40 N to both the biceps and the semimembranosus), a combined hamstrings and quadriceps load (the hamstrings load and a 200-N quadriceps load), and an isolated quadriceps load of 200 N. Each load was applied with the knee at full extension and at 30, 60, 90, and 120 degrees of flexion. Without muscle loads, in situ forces in the posterior cruciate ligament were small, ranging from 6+/-5 N at 30 degrees of flexion to 15+/-3 N at 90 degrees. Under an isolated hamstrings load, the in situ force in the posterior cruciate ligament increased significantly throughout all angles of knee flexion, from 13+/-6 N at full extension to 86+/-19 N at 90 degrees. A posterior tibial translation ranging from 1.3+/-0.6 to 2.5+/-0.5 mm was also observed from full extension to 30 degrees of flexion under the hamstrings load. With a combined hamstrings and quadriceps load, tibial translation was 2.2+/-0.7 mm posteriorly at 120 degrees of flexion ut was as high as 4.6+/-1.7 mm anteriorly at 30 degrees. The in situ force in the posterior cruciate ligament decreased significantly under this loading condition compared with under an isolated hamstrings load, ranging from 6+/-7 to 58+/-13 N from 30 to 120 degrees of flexion. With an isolated quadriceps load of 200 N, the in situ forces in the posterior cruciate ligament ranged from 4+/-3 N at 60 degrees of flexion to 34+/-12 N at 120 degrees. Our findings support the notion that, compared with an isolated hamstrings load, combined hamstrings and quadriceps loads significantly reduce the in situ force in the posterior cruciate ligament. These data are in direct contrast to those for the anterior cruciate ligament. Furthermore, we have demonstrated that the effects of muscle loads depend significantly on the angle of knee flexion.