Anatomy of the posterior cruciate ligament and the meniscofemoral ligaments

This paper describes the anatomy of the posterior cruciate ligament (PCL) and the meniscofemoral ligaments (MFLs). The fibres of the PCL may be split into two functional bundles; the anterolateral bundle (ALB) and the posteromedial bundle (PMB), relating to their femoral attachments. The tibial attachment is relatively compact, with the ALB anterior to the PLB. These bundles are not isometric: the ALB is tightest in the mid-arc of knee flexion, the PMB is tight at both extension and deep flexion. At least one MFL is present in 93% of knees. On the femur, the anterior MFL attaches distal to the PCL, close to the articular cartilage; the posterior MFL attaches proximal to the PCL. They both attach distally to the posterior horn of the lateral meniscus. Their slanting orientation allows the MFLs to resist tibial posterior drawer.     

Attachments of separate small bundles of human posterior cruciate ligament: an anatomic study

Purpose

To date, there is no consensus on the detailed anatomy of the posterior cruciate ligament (PCL) and the most appropriate PCL reconstruction method. The purpose of this study is the detailed observation of separate small PCL bundles for better comprehension of detailed PCL anatomy to facilitate the design of anatomic PCL reconstruction.

Methods

Twelve cadaveric knees were used in this study. PCL was divided into anterolateral and posteromedial (PM) sections and then separated into about 20 small bundles of 1 mm in diameter while maintaining preservation of their attachment sites marked with fine coloured marker pens. The positional relationship between the femoral and tibial attachments of each small bundle was investigated.

Results

The small bundles of tibial and femoral attachments showed a topographical correlation. The anterior–posterior aligned bundles in the tibia corresponded to the bundles aligned in a low–high direction in flexion in the femur. The femoral attachment pattern was relatively similar in each specimen. However, the tibial attachment showed two patterns of small bundle footprint location: a parabolic type (6 of 12) and a transverse type (6 of 12). The posterior portion of the PM bundle was separately attached to the medial and lateral portions in the tibial attachment.

Conclusion

Small bundles of PCL showed a relatively layered arrangement between tibial and femoral attachments. Therefore, anatomic PCL reconstruction cannot be completed simply performed merely with straight bundles and circular bone tunnels.     

Double tunnel technique forreconstruction of the posterior cruciate ligament

Excellent biomechanical evidence supports the theory that the posterior cruciate ligament (PCL) acts primarily as two separate functional bundles, with the anterolateral or anterocentral portion of the ligament acting predominantly in flexion, and the posteromedial or posterior oblique portion of the ligament acting predominantly in extension. Because of the size of the sites of origin and insertion of the PCL, reconstruction of the ligament using a single graft necessitates that only one of these two bundles is reconstructed. Therefore, we prefer to reconstruct both bundles of the PCL, and pass two separate grafts through two separate femoral tunnels. The grafts are then passed through a single tibial tunnel, with the anterolateral/ anterocentral graft being tensioned at 90° of flexion, and the posteromedial/posterior oblique graft being tensioned at 30° of flexion. The improved reproduction of PCL anatomy and biomechanics should result in a decreased incidence of late laxity in the PCL-reconstructed knee.     

Patella-tibial transfixation for posterior cruciate ligament repair and reconstruction: a biomechanical analysis

The posterior cruciate ligament (PCL) restricts posterior translation of the tibia on the femur. Because flexion of the knee increases tension on the PCL, the knee is usually immobilized in extension after PCL repair or reconstruction. Patella-tibial transfixation (olecranization), however, has been proposed to reduce the tension on the PCL without requiring immobilization of the knee. The objective of this study was: (1) to evaluate the distribution of strain in the anterolateral and posterior oblique fiber bundles of the PCLs in eight cadaveric knees before and after olecranization and (2) to measure the patellofemoral contact pressures at various degrees of knee flexion. Olecranization significantly (P < 0.05) reduced the strain on the anterolateral fiber bundles of the PCL at 15°–45° of flexion. No significant strain reduction was observed in the posterior oblique fiber bundles. Patellofemoral contact pressures measured from digitized Fuji sensitive film indicated significantly increased contact pressures (P < 0.05) following olecranization from 0°–60° of knee flexion. Increased parapatellar soft tissue tightness limited knee flexion to 90° and patella lift-off occurred at 75°. Although olecranization of the patella does reduce strain on the intact PCL within a selected range of motion, the beneficial effect of allowing early motion may be negated by the potentially harmful effects imposed upon the patellofemoral articular cartilage by increased contact pressures. Received: 20 December 1996 Accepted: 24 June 1997     

Graft tension of the posterior cruciate ligament using a finite element model

Purpose

The aim of the study was to analyse the change in length and tension of the reconstructed single-bundle posterior cruciate ligament (PCL) with three different femoral tunnels at different knee flexion angles by use of three-dimensional finite element method.

Methods

The right knees of 12 male subjects were scanned with a high-resolution computed tomography scanner at four different knee flexion angles (0°, 45°, 90° and 135°). Three types of single-bundle PCL reconstruction were then conducted in a 90° flexion model: femoral tunnels were created in anterolateral (AL), central and posteromedial (PM) regions of the footprint. Length versus flexion curves and tension versus flexion curves were generated.

Results

Between 0° and 90° of knee flexion, changes in length and tension in the PM grafts were not significant. Whereas the lengths and tension of the AL and central grafts significantly increased in the same flexion range. The length and tension of the PM grafts at 135° of knee flexion were significantly higher than those at 90° of knee flexion, whereas the AL and the central grafts showed only slight length changes beyond 90° of flexion. However, the tension of the AL graft increased significantly beyond 90° of flexion.

Conclusions

Changes in the graft length, and tension were generally affected by different femoral tunnels and knee flexion angles. In groups with the AL and PM single-bundle reconstruction, the graft tension increased beyond 90° of knee flexion when the graft is tensioned at 90° of flexion. These data suggest that final fixation angle at 90° for the AL or PM graft would induce graft overtension in high knee flexion of 135°. Whereas central graft which is fixed in 90° of flexion is desirable in terms of prevention of graft overtension. Because the graft tension within it was relatively constant beyond 90° of flexion.     

3D kinematic in-vitro comparison of posterolateral corner reconstruction techniques in a combined injury model

With the variable injury pattern to the posterolateral structures (PLS) of the knee, a number of reconstructive procedures have been introduced. It was the aim of the present study to evaluate the resulting 3D kinematics following three different surgical techniques of reconstruction in a combined posterior cruciate ligament (PCL)/PLS injury model. In nine human cadaveric knees, 3D kinematics were recorded during the path of flexion–extension using a computer based custom made 6-degree-of-freedom (DOF) testing apparatus. Additional laxity tests were conducted at 30 and 90° of flexion. Testing was performed before and after cutting the PLS and PCL, followed by PCL reconstruction alone. Reconstructing the posterolateral corner, three surgical techniques were compared: (a) the posterolateral corner sling procedure (PLCS), (b) the biceps tenodesis (BT), and (c) a bone patellar-tendon bone (BTB) allograft reconstruction . Posterior as well as rotational laxity were significantly increased after PCL/PLS transection at 30 and 90° of flexion. Isolated PCL reconstruction resulted in a remaining external rotational deficiency for both tested flexion angles. Additional PLS reconstruction closely restored external rotation as well as posterior translation to intact values by all tested procedures. Compared to the intact knee, dynamic testing revealed a significant internal tibial rotation for (b) BT (mean=3.9°, p=0.043) and for (c) BTB allograft (mean=4.3°, p=0.012). (a) The PLCS demonstrated a tendency to internal tibial rotation between 0 and 60° of flexion (mean=2.2°, p=0.079). Varus/valgus rotation as well as anterior/posterior translation did not show significant differences for any of the tested techniques. The present study shows that despite satisfying results in static laxity testing, pathological 3D knee kinematics were not restored to normal, demonstrated by a nonphysiological internal tibial rotation during the path of flexion-extension.     

后交叉韧带止点形态学测量及其重建骨道直径探讨

目的:在膝关节尸体标本上测量后交叉韧带(PCL)止点的形态大小,探讨重建PCL时骨道直径的选择。方法:解剖20具新鲜冷冻的膝关节尸体标本,标记PCL股骨和胫骨止点边界和前外束(AL)与后内束(PM)的分界。确定PCL、AL和PM纵轴并测量其长度,经过纵轴中点做其垂线,沿垂线测量其止点宽度,该宽度则为骨道最大直径。测量值用均数(第25百分位数~第75百分位数)表示。结果:PCL、AL和PM胫骨止点的纵轴宽度分别为10.64(9.52~11.51)mm,6.54(5.50~7.58)mm和5.85(5.37~6.56)mm。股骨止点纵轴宽度分别为11.01(10.27~12.04)mm,9.56(8.91~10.52)mm和9.47(8.85~10.28)mm。结论:根据解剖重建和胫骨、股骨骨道一致的原则,PCL单束重建的骨道直径可从9.5 mm到11.5 mm,均值10.5 mm;双束重建AL骨道直径可从6.5 mm到8.5 mm,均值7.5 mm;PL骨道直径可从6.0 mm到7.5 mm,均值6.5mm。     

Lateral collateral ligament reconstruction using a semitendinosus graft

We evaluated 13 reconstructions of the lateral collateral ligament (LCL) of the knee associated with a reconstruction of the ACL (n=6) and PCL (n=7). In all cases the LCL was reconstructed using a semitendinosus tendon graft through a tunnel in the fibular head and fixed in anatomical insertion of LCL at lateral femoral condyle. Patients were evaluated using the IKDC form. Lateral joint opening and posterior tibial translation were studied by radiographic stress views. The follow-up average was of 60 months (38–93). In the ACL group five patient were symptoms free while one reported moderate pain after strenuous activity. The lateral stress radiographs showed lateral joint opening of 0–2 mm in five knees, and 3–5 mm in one. In the PCL group four patients were symptom free while three complained mild to moderate pain or swelling. Lateral joint opening was 0–2 mm in six knees and 3–5 in one. External rotation at 30° of flexion was 10° in one and between 0–5° in the remainder. Eleven of 13 patients returned to their preinjury level and two returned to one level lower. These findings indicate that the LCL can be successfully reconstructed with a free ST graft at the time of ACL or PCL reconstruction.     

Posterior cruciate ligament (PCL) reconstruction-an in vitro study of isometry

Isometric positioning of the posterior cruciate ligament (PCL) graft is important for successful reconstruction of the PCL-deficient knee. This study documents the relationship between graft placement and changes in intra-articular graft length during a passive range of motion of the knee. In eight cadaveric knees the PCL was identified and cut. The specimens were mounted in a stabilising rig. PCL reconstruction was performed using a 9-mm-thick synthetic cord passed through tunnels 10 mm in diameter. Three different femoral graft placement sites were evaluated: (1) in four specimens the tunnel was located around the femoral isometric point, (2) in two specimens the tunnel was positioned over the guide wire 5 mm anterior to the femoral isometric point, (3) in two specimens the tunnel was positioned over the guide wire 5 mm posterior to the isometric femoral point. In all knees only one tibial tunnel was created around the isometric tibial point. The location of the isometric points is described in part I of this study. The proximal end of the cord was fixed to the lateral aspect of the femur. Distally, the cord was attached to a measuring unit. The knees were flexed from 0° to 110°, and the changes in the graft distance between the femoral attachment sites were measured in 10° steps. Over the entire range of motion measured, the femoral tunnels positioned around the isometric point produced femorotibial distance changes of within 2 mm. The anteriorly and posteriorly placed tunnels produced considerable changes in femorotibial distance with knee flexion, e.g. about 8 mm at 110° of flexion.     

Femoral avulsion fracture of the posterior cruciate ligament in association with a rupture of the popliteal artery in a 9-year-old boy: a case report

Ruptures of the posterior cruciate ligament (PCL) and especially proximal bony avulsion fractures in children are very rare. This in combination with a rupture of the popliteal artery is extremely rare. Thus, an exact incidence is not available from the literature. Overall, these injuries are severe and often lead to chronic knee instability. We report a case of a 9-year-old boy who suffered a traumatic displacement of the left knee with a rupture of the popliteal artery. Prior to transfer to our department, he was treated by a saphenous vein bypass graft and by a transfixation of the knee using two oblique percutaneous pins. We performed magnetic resonance imaging (MRI) scan of the knee which revealed a femoral avulsion fracture of the PCL. Other ligaments and menisci were intact. A transosseous femoral fixation using non-absorbable stitches was carried out. A 1-year follow-up after surgery demonstrates intact peripheral perfusion and sensation, straight axes of both legs and a physiological gait. Minimal differences of the length and circumference of both legs could be measured. The posterior laxity (Lachman-test) was about 5/8 mm (right/left knee) and 2/5 mm (right/left knee) in 90° flexion. The range of motion (extension/flexion) was 5/0/140°–/5/100° (right–left knee). Intact cruciate ligaments were confirmed by MRI. Minimal experience exists in treatment of combined injuries to the PCL and the popliteal artery in children.U. Bosch and C. Krettek contributed equally to this work     

The effect of femoral attachment location on anterior cruciate ligament reconstruction: graft tension patterns and restoration of normal anterior–posterior laxity patterns

The issue of the best place to attach an anterior cruciate ligament graft to the femur is controversial, and different anatomic or isometric points have been recommended. It was hypothesised that one attachment site could be identified that would be best for restoring normal anterior–posterior laxity throughout the range of knee flexion. It was also hypothesised that these different attachment sites would cause different graft tension patterns during knee flexion. Using six cadaver knees, an isometric point was found 3 mm distal to the posterior edge of Blumensaats line, at the 10:30–11:00 oclock position in right knees, at the antero-proximal edge of the anatomic ACL attachment. Anterior–posterior laxity was measured at ±150 N draw force at 20–120° flexion with the knee intact and after anterior cruciate ligament transection. The graft was placed at the isometric point, and AP laxity was restored to normal at 20° flexion, then measured at other angles. Graft tension was measured throughout, and also during passive flexion–extension. This was repeated for four other graft positions around the isometric point in every knee. Laxity was restored best by grafts tensioned to a mean of 9±14 N, positioned isometrically and 3 mm posterior to the isometric point. Their tension remained low until terminal extension. Grafts 3 mm anterior to the isometric point caused significant overconstraint, and had higher tension beyond 80° knee flexion. Small changes in attachment site had large effects on laxity and tension patterns. These results support an isometric/posterior anatomic femoral graft attachment, which restored knee laxity to normal from 20 to 120° flexion and did not induce high graft tension as the knee flexed. Grafts attached to the roof of the intercondylar notch caused overconstraint and higher tension in the flexed knee.     

Fixation strength of three different graft types used in posterior cruciate ligament reconstruction

Surgical reconstruction is generally recommended for PCL and associated ligament injuries. A variety of graft choices exist for PCL reconstruction surgery. This study evaluated the initial fixation strength of three grafts using in PCL reconstruction in a porcine model. Twenty fresh porcine knees were harvested and randomly assigned to four groups: bone–patellar tendon–bone graft, quadruple tendons graft, Achilles tendon graft, and normal PCL. After reconstruction the knee was tested on an MTS testing machine by translating the tibia posteriorly until failure at 30° of flexion, neutral rotation, and anatomical vertical alignment. Biomechanical parameters including maximal failure load, stiffness, and failure modes, were analyzed and compared. In the maximal failure load, the four-strand tendon group was significantly greater than the other two grafts. However, it had greatest translation. There were no significant differences between the three grafts in stiffness. All three of these commonly used grafts had weaker initial fixation strength and stiffness than normal PCL. Graft failure occurred mainly at the tendon-bone junction and tendon-suture sites. The Patellar tendon group had significantly least translation during continuing loading.     

膝关节后交叉韧带及板股韧带临床解剖学研究

目的:探讨后交叉韧带分束情况及板股韧带的作用,为临床重建后交叉韧带提供解剖学基础。方法:30例成人膝关节标本,在无载荷与后抽屉试验两种条件下分别观察其后交叉韧带纤维束及板股韧带的紧张-松弛模式,根据纤维束紧张与松弛情况对后交叉韧带分束,确定各束中最能代表该束功能的纤维束,即"功能束"及其止点位置,明确板股韧带的作用。结果:后交叉韧带中未见等长纤维束。无载荷条件下屈伸膝关节,后交叉韧带中各纤维束处于较松弛状态(过伸与过屈位除外);后抽屉试验条件下,不同纤维束紧张,维持胫骨后向稳定性。后交叉韧带可以恒定地分为前外侧束与后内侧束,两束作用不同。前外侧束与后内侧束中均存在"功能束",两者的"功能束"联合作用可基本维持膝关节活动范围内胫骨的后向稳定性,"功能束"止点与两束止点中心不吻合。前、后板股韧带的出现率分别为3%与90%,二者横截面积均较小,前者的紧张-松弛模式类似于后交叉韧带前缘纤维,后者则与后内侧束相一致,二者均有维持胫骨后向稳定性的作用,后者还有在最大屈膝位防止后交叉韧带与股骨髁间窝后缘发生撞击的作用。结论:后交叉韧带是不等长的复杂的纤维结合体,其双束重建应根据"功能束"止点位置进行骨道定位,板股韧带在有条件时应予以保留。     

Comparing in vivo kinematics of unicondylar and bi-unicondylar knee replacements

Preserving both cruciate ligaments in unicondylar knee arthroplasty likely provides more normal knee mechanics and contributes to enhanced patient function. It follows that preserving both cruciate ligaments with total knee arthroplasty should provide functional benefit compared to arthroplasty sacrificing one or both cruciates. The purpose of this study was to compare knee kinematics in patients with optimally functioning cruciate-preserving medial unicondylar and bi-unicondylar arthroplasty to determine if knee motions differed. Eight consenting patients with seven medial unicondylar and five bi-unicondylar arthroplasties were studied using lateral fluoroscopy during treadmill gait, stair stepping, and maximum flexion activities. Patient-specific geometric models based on CT and CAD data were used for shape matching to determine the three-dimensional knee kinematics. Tibiofemoral contact locations were computed for the replaced compartments. Maximum flexion in kneeling was 135°±14° for unicondylar knees and 123°±14° for bi-unicondylar knees (p=0.22). For 0°–30° flexion during the stair activity, the medial condyle translated posterior 3.5±2.5 mm in unicondylar knees and 4.7±1.9 mm in bi-unicondylar knees (p>0.05). Lateral posterior translation was 5.0±2.3 mm in bi-unicondylar knees for 0°–30° flexion. From heel-strike to mid-stance phase, there was little tibial rotation, but unicondylar knees showed 1.5±1.6 mm posterior translation of the medial condyle, while bi-unicondylar knees showed 5.1±2.2 mm (p<<0.05). The bi-unicondylar knees showed 3.8±3.4 mm posterior lateral condylar translation. Preserving both cruciate ligaments in knee arthroplasty appears to maintain some basic features of normal knee kinematics. Knees with bi-unicondylar arthroplasty showed kinematics closer to motions observed in total knee arthroplasty, slightly less weight-bearing flexion, and greater dynamic laxity in gait than unicondylar knees. Despite kinematic differences, knees with unicondylar and bi-unicondylar arthroplasty can provide excellent functional outcomes in appropriately selected patients.     

The cartilaginous and osseous geometry of the femoral trochlear groove

Photography was used to study the geometry of the cartilaginous and osseous contours of the distal femur and the orientation of the trochlear groove in 9 fresh-frozen and 24 embalmed knees. The sulcus angle (146.1°±5.5°) decreased from 0° to 50° of femoral flexion then increased afterwards. The maximum slope of the lateral femoral condyle (20.2°±5.2°) also decreased with flexion. Both the sulcus angle (p =0.0007) and maximum slope (p =0.0001) were larger at 0° than they were for 60° cartilaginous surfaces. The lateral femoral condylar height decreased, whilst the medial femoral condylar height increased as the flexion increased. The femoral groove was midway between the two femoral epicondyles (49.5±3.9%), but deviated laterally as the flexion angle increased. The groove axis deviated distally and laterally from the femoral anatomical axis for both cartilaginous and bony surfaces, and the angle between the groove and anatomical axes was similar for both cartilaginous (19.1°) and osseous (16.8°) surfaces. Articular cartilage is not well represented on radiography yet it had a significant effect on the distal femoral geometry, and should be taken into account when evaluating the patellofemoral joint.     

Retropatellar forces after rupture of the PCL and patello-tibial transfixation: An in vitro study

Compared to injuries of the other knee ligaments, a rupture of the posterior cruciate ligament (PCL) is relatively rare. Treatment may be conservative or operative. A rupture that has been operated on temporarily can be stabilised using a Grammont patello-tibial transfixation (olecranisation). Flexion and extension between 30° and 60° are allowed. The advantage of this method is that it avoids complete immobilisation of the joint and also the reduction of pull on the PCL. However, patients treated with this method show long-term osteoarthritis of the retropatellar joint area. Our study aimed to show the distribution of forces at the dorsal patellar surface in the following: (1) knee with intact ligament; (2) knee with PCL rupture; (3) knee with PCL rupture plus olecranisation. Fourteen fresh knee specimens were investigated in a Plitz/Wirth knee kinemator. The femur was fixed while the tibia was flexed between 5° and 120°. Pull was placed on the patella and on the dorsal side of the tibia with weights over the tendons of the quadriceps and the roots of the ischio-crural muscles. With the aid of a special measurement device in the patella, the medially laterally, proximally and distally acting forces in a movement cycle could be measured as well as the total retropatellar force in the above experimental setups. We found that, dependent on load and flexion angle in each preparation, (1) the total retropatellar force and some of the force components were greater when the PCL was ruptured than in the intact specimen. (2) with olecranisation the total force and the medial and lateral components were less than in the unrestricted patella with ruptured PCL, (3) the proximal area of the patella with olecranisation was less loaded than in the knee with either intact or defective PCL, and (4) the distal area of the patella with olecranisation was more loaded than in the knee joint with either intact or defective PCL.Conclusions To avoid destruction caused by overloading, especially in the distal area of the patella, the duration of patello-tibial transfixation should be carefully considered. Patients should be kept moving, at the same time being warned about excessive loading.     

Measurements of bone mineral density of the proximal femur by two commercially available dual energy X-ray absorptiometric systems

Two dual energy X-ray absorptiometric (DXA) instruments have recently become commercially available for local bone densitometry: the QDR-1000 (Hologic Inc.) and the DPX (Lunar Radiation Corp.). We report the precision, influence of femoral rotation, correlation and agreement of bone mineral measurements of the proximal femur by these two instruments. In vitro (femur phantom) short-term precision was 1.1%–3.5%, and the long-term precision was 1.2%–3.8%. In vivo (groups of 10 premenopausal and 10 postmenopausal women) short-term precision of duplicate measurements was 1.6%–4.7%, and long-term precision was 1.9%–5.5%. Overall, the precision for Ward's triangle was over 3% and that for the femoral neck and trochanter, 2%–3%. Rotation of a femur phantom produced a statistically significant change in the bone mineral density (BMD) of the femoral neck. Within a clinically relevant range of femoral rotation (20° inward rotation ±5°) the coefficient of variation (CV%) increased by a mean factor of 1.1–1.4. Although the correlation (r < 0.9) between BMD measurements of the proximal femur by the DPX and QDR-1000 in 30 postmenopausal women was high, there was lack of agreement between the two instruments. We found no statistically significant differences between the right and left femur in 30 postmenopausal women. A bilateral femur scan took a mean total time of about 22 min. We conclude that with the introduction of DXA instruments, the precision of bone mineral measurements of the proximal femur has improved. However, for comparability between commercially available DXA instruments, it might be advantageous if units were standardized. Offprint requests to: O.L. Svendsen     

Infrapatellar fat pad pressure and volume changes of the anterior compartment during knee motion: possible clinical consequences to the anterior knee pain syndrome

This biomechanical study was performed to measure tissue pressure in the infrapatellar fat pad and the volume changes of the anterior knee compartment during knee flexion–extension motion. Knee motion from 120° of flexion to full extension was simulated on ten fresh frozen human knee specimens (six from males, four from females, average age 44 years) using a hydraulic kinematic simulator (30, 40, and 50 Nm extension moment). Infrapatellar tissue pressure was measured using a closed cell sensor. Infrapatellar volume change in the anterior knee compartment was evaluated subsequent to removal of the fat pad using a water-filled bladder. We found a significant increase of the infrapatellar tissue pressure during knee flexion, at flexion angles of <20° and >100°. The average tissue pressure ranged from 343 (±223) mbar at 0° to 60 (±64) mbar at 60° of flexion. The smallest volume in the anterior knee compartment was measured at full extension and 120° of flexion, whereas the maximum volume was observed at 50° of flexion. In conclusion, the data suggest a biomechanical function of the infrapatellar fat pad at flexion angles of <20° and >100°, which suggests a role of the infrapatellar fat pad in stabilizing the patella in the extremes of knee motion.     

Effect of knee flexion on the in situ force distribution in the human anterior cruciate ligament

This study was conducted to evaluate the effect of applied load on the magnitude, direction, and point of tibial intersection of the in situ forces of the anteromedial (AM) and posterolateral (PL) bands of the human anterior cruciate ligament (ACL) at 30° and 90° of knee flexion. An Instron was used to apply a 100 N anterior shear force to 11 human cadaver knees, 6 at 30° of knee flexion and 5 at 90° of knee flexion. A Universal Force Sensor (UFS) recorded the resultant 6 degree-of freedom (DOF) forces/moments. Each specimen then underwent serial removal of the AM and PL bands. With the knee limited to 1 DOF (anteroposterior), tests were performed before and after each structure was removed. Because the path was identical in each test, the principle of superposition was applied. Thus, the difference between the resultant forces could be attributed to the force carried by the structure just removed. The magnitudes of force in the ACL at 30° and 90° of knee flexion were 114.1±7.4 N and 90.8±8.3 N, respectively (P<0.05). At 30°, the AM and PL bundles carried 95% and 4% of the total ACL force, respectively. At 90°, the AM and PL bands carried 85% and 13%, respectively (P<0.05). The direction of the in situ force in the whole ACL as well as its two bands correlated with the anatomic orientation of the ligament. The resultant total ACL force intersected the tibial plateau at the posterolateral aspect of the AM band's insertion at 30° of knee flexion, while at 90°, the force intersection moved posteriorly to the AM/PL border. This research provides new insight into the fundamental force relationships of the ACL and its bundles. In response to an anterior tibial shear force, the AM band of the ACL was the predominant load carrier at both 30° and 90° of knee flexion. However, contrary to carlier reports, the in situ force carried in the PL band increased as knee flexion increased. Further, the tibial intersection of the resultant ACL force moved laterally with knee flexion. These findings confirm the dynamic structure of the ACL that in itself has no isometricity and may also indicate that there is no ideal location in which to position a replacement graft. The use of this methodology with more physiologically unconstrained motion should lead to more definitive clinical conclusions.