The effect of release of the popliteus and quadriceps force on rotation of the knee

The current study was done to determine whether an isolated, partial, or complete injury to the popliteus at the femur increases rotational knee laxity. The other aim was to determine how quadriceps loading affects internal and external rotation. Ten cadaver knee specimens with an intact posterolateral complex were held in a biomechanical testing rig at 0 degrees, 30 degrees, 60 degrees, and 90 degrees flexion. Movement of the tibia relative to the femur was measured while internal and external moments of 3 N-m were applied about the long axis of the tibia. Laxity was assessed for an intact specimen, and with partial and complete detachment of the popliteus femoral insertion. In five of the 10 specimens laxity additionally was assessed with sufficient quadriceps loading to resist 100 N vertical force at the hip. The results showed that partial and total release of the popliteus increased external laxity of the knee by as much as 6.6 degrees (90 degrees flexion) and by as much as 3.5 degrees (90 degrees flexion). Quadriceps loading reduced internal and external knee laxity significantly. Injury of the popliteus at the femoral insertion may be associated with increased rotational laxity of the knee. An increase in quadriceps force may be necessary to control increased external rotation of the tibia.     

Eine neuartige Knieendoprothese mit physiologischer Gelenkform

The natural tibiofemoral joint (TFJ) functions according to a roll-glide mechanism. In the stance phase (0–20° flexion), the femur rolls backwards over the tibia plateau, while further flexion causes increased gliding. This kinematics is based on the principle of a quadruple joint. The four morphological axes of rotation are the midpoints of the curvatures of the medial and lateral femoral condyles and the medial and lateral tibia plateau. In addition, the medial and lateral compartments are shifted a few millimetres in a sagittal direction, the medial tibia plateau being concave and the lateral plateau convex. In most knee arthroplasties, these factors are not taken into account; instead they are equipped with symmetrical medial and lateral joint surfaces. Thereby, the midpoints of the curvatures of the sagittal contours of the lateral and medial joint surfaces, on the femoral as well as on the tibial sides, create a common axis of rotation which does not allow a physiological roll-glide mechanism. The goal of this study was therefore to report on the biomechanical basis of the natural knee and to describe the development of a novel knee endoprosthesis based on a mathematical model. The design of the structurally new knee joint endoprosthesis has, on the lateral side, a convex shape of the tibial joint surface in a sagittal cross section. Furthermore, from a mathematical point of view, this knee endoprosthesis possesses essential kinematic and static properties similar to those of a physiological TFJ. Within the framework of the authorization tests, the endoprosthesis was examined according to ISO/WC 14243 in a knee simulator. The abrasion rates were, thereby, lower than or at least as good as those for conventional endoprostheses. The presented data demonstrate a novel concept in knee arthroplasty, which still has to be clinically confirmed by long term results.     

How the knee moves

The arc of flexion used in almost all the activities of everyday life extends from about 20°±10° to 110°/120°. During this arc, the human knee corresponds to the quadrupedal mammalian knee. Both the femoral surfaces are circular with a similar radius and rotate around their geometrical centres as the knee flexes. The medial femoral condyle does not move antero-posteriorly with flexion, i.e. stability depends on the medial side of the knee. In contrast, the lateral femoral condyle is antero-posteriorly mobile and as it moves it carries the meniscus with it. This AP movement results in longitudinal tibial rotation which is facultative rather than obligatory: if posterior motion occurs, the femur rotates externally around a medial axis with flexion whereas if no AP motion occurs, the knee can flex as would a uniaxial hinge. Rotation first appears in arboreal quadrupeds (apes) and may be becoming vestigial in Man. The axis of longitudinal rotation during flexion, parallel to the tibia and perpendicular to the flexion axis, approximately intersects the latter in the centre of the medial femoral condylar sphere. Varus/valgus rotation, around an AP axis which also passes through the centre of the femoral sphere, permits the lateral femoral condyle to lift away from the tibia because the lateral collateral ligament (LCL) is slack at 90° in mid external/internal rotation. Thus, in the arc ‘20’–120° the medial femoral condyle resembles the femoral head: it is spherical, it does not translate during flexion and all three axes of rotation intersect at its centre. At 90°, forced longitudinal rotation does result in AP movement of the medial condyle and on the lateral side in a reciprocal translation which is almost sufficient to abolish the translation accompanying flexion. This movement occurs around a vertical axis which is slightly lateral to that representing longitudinal rotation with flexion.The arc from 10° to full extension is accompanied by the so-called ‘locking’ and ‘screw-home’. It appears to be a feature of bipedal terrestrial gait with an erect stance, i.e. human gait. Although the arc exists, it is rarely used fully in everyday life. The motion is complex and involves asymmetrical articular surfaces other than those used from 20° to 120°. On the medial side, the femur ‘rocks’ forward onto the upward-sloping anterior surface of the tibia and then rotates into extension around an anterior, larger radiused circular surface. On the lateral side, the femur rolls down onto the anterior horn. The result is ‘lift-off’ of the posterior facets used in the arc ‘20’–120° and progressive tightening of the structures attached posteriorly to the femur, in particular the ACL. This ligament, as it tightens, may move the lateral femoral condyle anteriorly so that extension is accompanied by about 5° of obligatory femoral internal rotation. Flexion and longitudinal rotation occur by rotation around, and translation along, a 20° oblique screw axis penetrating medially the epicondyle and, laterally, the region of the tibio-femoral contact surface.From 120° to full flexion, the motion is passive rather than active. Both femoral condyles move backwards and both lose contact with the tibia. Thus, the tibio-femoral joint is strictly speaking subluxed. Medially, the femoral condyle rolls up onto the posterior horn. Laterally, the femoral condyle rolls backwards and downwards, finally to lie posterior to the tibia, resting on the posterior horn.Although the motion of the knee is complex, it can be (and has been) imaged by MRI in the unloaded cadaveric knee, the unloaded living knee and the loaded living knee. The keys to its understanding are to divide flexion into three arcs and to appreciate that in the functional active arc (‘20’–120°) the medial femoral condyle, like the femoral head, is spherical, that it does not translate and that it rotates around three axes which intersect at its centre. By contrast, the lateral femoral condyle rolls and slides antero-posteriorly on the tibia to result in longitudinal rotation (a possibly vestigial movement in Man) around a medial axis.     

Comparison of posterior-stabilized,cruciate-retaining,and medial-stabilized knee implant motion during gait

Accurate knowledge of knee joint motion is needed to evaluate the effects of implant design on functional performance and component wear. We conducted a randomized controlled trial to measure and compare 6-degree-of-freedom (6-DOF) kinematics and femoral condylar motion of posterior-stabilized (PS), cruciate-retaining (CR), and medial-stabilized (MS) knee implant designs for one cycle of walking. A mobile biplane X-ray imaging system was used to accurately measure 6-DOF tibiofemoral motion as patients implanted with PS (n = 23), CR (n = 25), or MS (n = 26) knees walked over ground at their self-selected speeds. Knee flexion angle did not differ significantly between the three designs. Relative movements of the femoral and tibial components were generally similar for PS and CR with significant differences observed only for anterior tibial drawer. Knee kinematic profiles measured for MS were appreciably different: external rotation and abduction of the tibia were increased while peak-to-peak anterior drawer was significantly reduced for MS compared with PS and CR. Anterior-posterior drawer and medial-lateral shift of the tibia were strongly coupled to internal-external rotation for MS, as was anterior-posterior translation of the contact center in the lateral compartment. MS exhibited the least amount of paradoxical anterior translation of the femur relative to the tibia during knee flexion. The joint center of rotation in the transverse plane was located in the lateral compartment for PS and CR and in the medial compartment for MS. Substantial differences were evident in 6-DOF knee kinematics between the healthy knee and all three prosthetic designs. Overall, knee kinematic profiles observed for MS resemble those of the healthy joint more closely than PS and CR.     

The role of the posterolateral and cruciate ligaments in the stability of the human knee. A biomechanical study

Injury to the posterolateral structures of the knee, including the popliteus tendon and arcuate complex, frequently results in poorly understood patterns of instability. To evaluate the static function of these tissues, we used a mechanical testing apparatus that allowed five degrees of freedom to test seventeen specimens from human cadavera at angles of flexion that ranged from zero to 90 degrees. Selective section of the lateral collateral ligament, popliteus-arcuate (deep) ligament complex, anterior cruciate ligament, and posterior cruciate ligament was performed. At all angles of flexion, the lateral collateral ligament and deep ligament complex functioned together as the principal structures preventing varus rotation and external rotation of the tibia, while the posterior cruciate ligament was the principal structure preventing posterior translation. However, at angles of flexion of 30 degrees or less, the amount of posterior translation after section of only the lateral collateral ligament and the deep structures was similar to that noted after isolated section of the posterior cruciate ligament. Isolated section of the posterior cruciate ligament did not affect varus or external rotation of the tibia at any position of flexion of the knee. When the posterior cruciate ligament was sectioned after the lateral collateral ligament and deep ligament complex had been cut, a large increase in posterior translation and varus rotation resulted at all angles of flexion. In addition, at angles of flexion of more than 30 degrees, external rotation of the tibia also increased. The application of internal tibial torque resulted in no increase in tibial rotation after isolated section of the anterior cruciate ligament or combined section of the lateral collateral ligament and deep ligament complex. However, combined section of all three structures increased internal rotation at 30 and 60 degrees of flexion. The increases in external rotation that were produced by section of the lateral collateral ligament and deep ligament complex were not changed by the addition of the section of the anterior cruciate ligament.     

In vivo kinematic analysis of cruciate-retaining total knee arthroplasty during weight-bearing and non-weight-bearing deep knee bending

The purpose of the present study was to evaluate the in vivo kinematics of the posterior cruciate ligament-retaining total knee arthroplasty during weight-bearing and non-weight-bearing deep knee bending and compare these 2 different conditions. We evaluated the in vivo kinematics of the knee using fluoroscopy and femorotibial translation relative to the tibia tray by 2-dimensional/3-dimensional registration. In the weight-bearing state, the femoral component showed central pivot and bicondylar posterior rollback pattern. During non-weight-bearing, the movement anteriorly occurred on both the medial and lateral side during early flexion, whereas bicondylar femoral component rollback occurred after that. During non-weight-bearing, both the medial and lateral condyle significantly moved anteriorly compared with the weight-bearing state during early flexion. However, bicondylar femoral rollback occurred under both these conditions.     

Limits of movement in the human knee. Effect of sectioning the posterior cruciate ligament and posterolateral structures

We applied specific forces and moments to the knees of fifteen whole lower limbs of cadavera and measured, with a six degrees-of-freedom electrogoniometer, the position of the tibia at which the ligaments and the geometry of the joint limited motion. The limits were determined for anterior and posterior tibial translation, internal and external rotation, and varus and valgus angulation from zero to 90 degrees of flexion. The limits were measured in the intact knee and then the changes that occurred with removal of the posterior cruciate ligament, the lateral collateral ligament, the popliteus tendon at its femoral attachment, and the arcuate complex were measured. The cutting order was varied, allowing us to determine the changes in the limits that occurred when each structure was cut alone and the amount of motion of the joint that was required for each structure to become taut and to limit additional motion when the other supporting structures had been removed. Removal of only the posterior cruciate ligament increased the limit for posterior tibial translation, with no change in the limits for tibial rotation or varus and valgus angulation. The additional posterior translation was least at full extension and increased progressively, reaching 11.4 millimeters at 90 degrees of flexion. The progressive increase in posterior translation with flexion was apparently due to slackening of the posterior portion of the capsule, as the translation nearly doubled when the posterolateral structures subsequently were removed. Removal of only the posterolateral extra-articular restraints increased the amount of external rotation and varus angulation. The average increase in external rotation depended on the angle of flexion; it was greatest at 30 degrees of flexion and decreased with additional flexion. At 90 degrees of flexion, the intact posterior cruciate ligament limited the increase in external rotation to only 5.3 degrees, less than one-half of the 13.0-degree increase that occurred at 30 degrees of flexion. Subsequent removal of the posterior cruciate ligament markedly increased external rotation at 90 degrees of flexion, resulting in a total increase of 20.9 degrees. The limit for varus angulation was normal as long as the lateral collateral ligament was intact. When the lateral collateral ligament was cut, the limit increased 4.5 degrees (approximately 4.5 millimeters of additional joint opening) when the knee was partially flexed (to 15 degrees).(ABSTRACT TRUNCATED AT 400 WORDS)     

固定平台后稳定型假体全膝关节置换术后的运动学研究

 目的 探讨固定平台后稳定型假体全膝关节置换(total knee arthroplasty,TKA)术后膝关节在负重屈膝下蹲时的运动学特征。方法 选取10名健康志愿者和10例固定平台后稳定型假体TKA术后患者。制作骨骼及膝关节假体三维模型,在持续X线透视下完成负重下蹲动作,膝关节屈曲度每增加15°截取一幅图像。通过荧光透视分析技术完成三维模型与二维图像的匹配,再现股骨与胫骨在屈膝过程中的空间位置,通过连续的图像分析比较正常与固定平台后稳定型假体TKA术后膝关节在负重下蹲时股骨内、外髁前后移动及胫骨内外旋转幅度。结果 负重下蹲时,正常膝关节平均屈曲136°,股骨内、外髁分别后移(7.3±1.2) mm和(19.3±3.1) mm,胫骨平均内旋23.8°±3.4°;TKA术后膝关节平均屈曲125°,股骨内、外髁分别后移(1.4±1.6) mm和(6.4±1.7) mm,胫骨平均内旋8.5°±3.4°。结论 固定平台后稳定型假体TKA术后膝关节运动与正常膝关节相似,均表现出股骨内、外髁后移及胫骨内旋运动,但幅度小于正常膝关节,且在屈膝过程中存在股骨矛盾性前移及胫骨外旋现象。     

Variations in medial‐lateral hamstring force and force ratio influence tibiofemoral kinematics

A change in hamstring strength and activation is typically seen after injuries or invasive surgeries such as anterior cruciate reconstruction or total knee replacement. While many studies have investigated the influence of isometric increases in hamstring load on knee joint kinematics, few have quantified the change in kinematics due to a variation in medial to lateral hamstring force ratio. This study examined the changes in knee joint kinematics on eight cadaveric knees during an open‐chain deep knee bend for six different loading configurations: five loaded hamstring configurations that varied the ratio of a total load of 175 N between the semimembranosus and biceps femoris and one with no loads on the hamstring. The anterior–posterior translation of the medial and lateral femoral condyles’ lowest points along proximal‐distal axis of the tibia, the axial rotation of the tibia, and the quadriceps load were measured at each flexion angle. Unloading the hamstring shifted the medial and lateral lowest points posteriorly and increased tibial internal rotation. The influence of unloading hamstrings on quadriceps load was small in early flexion and increased with knee flexion. The loading configuration with the highest lateral hamstrings force resulted in the most posterior translation of the medial lowest point, most anterior translation of the lateral lowest point, and the highest tibial external rotation of the five loading configurations. As the medial hamstring force ratio increased, the medial lowest point shifted anteriorly, the lateral lowest point shifted posteriorly, and the tibia rotated more internally. The results of this study, demonstrate that variation in medial‐lateral hamstrings force and force ratio influence tibiofemoral transverse kinematics and quadriceps loads required to extend the knee. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1707–1715, 2016.     

Anatomy of the posterolateral aspect of the goat knee.

The purpose of this study was to determine the anatomy of the posterolateral aspect of the goat knee for future in vivo studies using a goat model to examine the natural history of posterolateral knee injuries. Twelve non-paired, fresh-frozen, adult goat knees were dissected to determine the anatomy of the posterolateral corner. The main posterolateral structures identified in the goat knee were the lateral collateral ligament, the popliteus muscle and tendon, popliteomeniscal fascicles, and the lateral gastrocnemius muscle. The lateral collateral ligament was extra-articular and coursed from its proximal attachment, located posterior and proximal to the lateral epicondyle, to its distal attachment on the lateral aspect of the fused proximal tibiofibula. The popliteus muscle attached to the medial edge of the posterodistal tibia, traveled anterolaterally, became intra-articular at its musculotendinous junction, and attached to the lateral femur just distal to the lateral epicondyle. Distinct popliteomeniscal fascicles attached the lateral meniscus to the popliteus muscle and tendon, and a fascial attachment from the musculotendinous junction of the popliteus muscle coursed to the lateral tibial plateau. This study provided information on the structures present in the posterolateral aspect of the goat knee and enhanced our understanding of their relationships to analogous structures in the human knee. This information is important to enable future development of potential models of the natural history of posterolateral knee injuries and also to test surgical techniques and the in vivo effects of these injuries on cruciate ligament reconstructions.     

The role of the popliteofibular ligament and the tendon of popliteus in providing stability in the human knee

Techniques for the selective cutting of ligaments in cadaver knees defined the static contributions of the posterolateral structures to external rotation, varus rotation and posterior tibial translation from 0 degrees to 120 degrees of flexion under defined loading conditions. Sectioning of the popliteofibular ligament (PFL) (group 1) produced no significant changes in the limits of the knee movement studied. Sectioning of the PFL and the popliteus tendon (femoral attachment, group 2) produced an increase of only 5 degrees to 6 degrees in external rotation from flexion of 30 degrees to 120 degrees (p < 0.001). Even when other ligaments were sectioned first (group 3), the maximum effect of the PFL was negligible. Our findings show that the popliteus muscle-tendon-ligament complex, lateral collateral ligament, and posterolateral capsular structures function as a unit. No individual structure alone is the primary restraint for the movements studied. Operative reconstruction should address all of the posterolateral structures, since restoration of only a portion may result in residual instability.     

Experimental studies of the kinematics of the stable and unstable human knee joint

In an experimental study the three dimensional kinematic behaviour of fourty knee joint specimen was investigated. We found a continous passive tibial rotation during flexion, so the terminus "screw home mechanism" should no longer be used. There was also an automatic adduction and medialisation movement of the tibia. We calculated the roll-gliding ratio separately for the medial and the lateral side. Only the lateral femoral condyle displayed increasing gliding with pure rolling near extension and pure gliding near flexion. We found little differences concerning qualitative characteristics but not in the amount of passive motions.     

Motion of the femoral condyles in flexion and extension during a continuous lunge

Numerous studies have reported on in‐vivo posterior femoral condyle translations during various activities of the knee. However, no data has been reported on the knee motion during a continuous flexion‐extension cycle. Further, few studies have investigated the gender variations on the knee kinematics. This study quantitatively determined femoral condylar motion of 10 male and 10 female knees during a continuous weightbearing flexion‐extension cycle using two‐dimensional to three‐dimensional fluoroscopic tracking technique. The knees were CT‐scanned to create three‐dimensional models of the tibia and femur. Continuous images of each subject were taken using a single‐fluoroscopic imaging system. The knee kinematics were measured along the motion path using geometric center axis of the femur. The results indicated that statistical differences between the flexion and extension motions were only found in internal‐external tibial rotation and lateral femoral condylar motion at the middle range of flexion angles. At low flexion angles, male knees have greater external tibial rotation and more posteriorly positioned medial femoral condyle than females. The knee did not show a specific pivoting type of rotation with flexion. Axial rotation center varied from lateral to medial compartments of the knee. These data could provide useful information for understanding physiological motion of normal knees. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 33:591–597, 2015.     

Three‐dimensional patellar motion at the natural knee during passive flexion/extension. An in vitro study

Patellar maltracking may result in many patellofemoral joint (PFJ) disorders in the natural and replaced knee. The literature providing quantitative reference for normal PFJ kinematics according to which patellar maltracking could be identified is still limited. The aim of this study was to measure in vitro accurately all six‐degrees‐of‐freedom of patellar motion with respect to the femur and tibia on 20 normal specimens. A state‐of‐the‐art knee navigation system, suitably adapted for this study aim, was used. Anatomical reference frames were defined for the femur, tibia, and patella according to international recommendations. PFJ flexion, tilt, rotation, and translations were calculated in addition to standard tibiofemoral joint (TFJ) kinematics. All motion patterns were found to be generally repeatable intra‐/interspecimens. PFJ flexion was 62% of the corresponding TFJ flexion range; tilt and translations along femoral mediolateral and tibial proximodistal axes during TFJ flexion were found with medial, lateral, and distal trends and within 12°, 6 and 9 mm, respectively. No clear pattern for PFJ rotation was observed. These results concur with comparable reports from the literature and contribute to the controversial knowledge on normal PFJ kinematics. Their consistence provides fundamental information to understand orthopedic treatment of the knee and for possible relevant measurements intraoperatively. © 2009 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 27:1426–1431, 2009     

The intact, injured and repaired knee: in-vitro experimental biomechanics and level walking]

In vitro simulation of knee joint movement during walking on flat ground was developed in this experimental study. Such movements are of interest for surgical and rehabilitation medicine in order to improve knowledge in the field of dynamic behavior of the knee joint during the entire range of motion: spatial kinematics and strains and stresses on the different components, in the case of intact knee, as well as of the operated one. As internal strains and stresses could not be measured directly, the aim of this experimental study was also to simulate such factors in a model using experimental input data. A simulator of joint flexion was built: it was composed of a hydraulic universal testing machine which allowed the main flexion-extension of the knee joint and an additional hydraulic device to impose the quadriceps extension force which represented the muscular action, synchronized with the imposed flexion-extension. The anatomical automatic passive rotation and valgus-varus motion were freely allowed, and these were measured during joint motion. In addition the lateral ligaments were fitted with strain sensors in order to measure their time-dependent behavior. The imposed flexion motion and quadriceps force were also measured to verify that they were accurately synchronized. The time-dependent values of force and flexion were taken from the literature. The analysis gave the mean result of eight reliable knee joint determinations: first of all intact, then after removing the anteroexternal cruciate ligaments, and finally after joint replacement by total knee prosthesis. One main conclusion was the comparison between automatic rotations, which decreased significantly after prosthesis surgery.(ABSTRACT TRUNCATED AT 250 WORDS)     

Kinematics of medial osteoarthritic knees before and after posterior cruciate ligament retaining total knee arthroplasty

Total knee arthroplasty (TKA) is a widely accepted surgical procedure for the treatment of patients with end‐stage osteoarthritis (OA). However, the function of the knee is not always fully recovered after TKA. We used a dual fluoroscopic imaging system to evaluate the in vivo kinematics of the knee with medial compartment OA before and after a posterior cruciate ligament‐retaining TKA (PCR‐TKA) during weight‐bearing knee flexion, and compared the results to those of normal knees. The OA knees displayed similar internal/external tibial rotation to normal knees. However, the OA knees had less overall posterior femoral translation relative to the tibia between 0° and 105° flexion and more varus knee rotation between 0° and 45° flexion, than in the normal knees. Additionally, in the OA knees the femur was located more medially than in the normal knees, particularly between 30° and 60° flexion. After PCR‐TKA, the knee kinematics were not restored to normal. The overall internal tibial rotation and posterior femoral translation between 0° and 105° knee flexion were dramatically reduced. Additionally, PCR‐TKA introduced an abnormal anterior femoral translation during early knee flexion, and the femur was located lateral to the tibia throughout weight‐bearing flexion. The data help understand the biomechanical functions of the knee with medial compartment OA before and after contemporary PCR‐TKA. They may also be useful for improvement of future prostheses designs and surgical techniques in treatment of knees with end‐stage OA. © 2010 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 29:40–46, 2011     

Three dimensional kinematics of the knee joint

The study deals with 3 d-Kinematics and stability patterns according to a knee joint testing machine. During flexion the lateral femoral condyle displays near extension pure rolling, near flexion pure gliding, on the medial side this ratio is vice versa. 41 knee joint specimen provided internal-external transverse rotation during there whole range of motion. Additional varus rotation and medial translation occurred. In osteoarthritis the movement patterns were completely changed.     

Tibiofemoral movement 2: the loaded and unloaded living knee studied by MRI

In 13 unloaded living knees we confirmed the findings previously obtained in the unloaded cadaver knee during flexion and external rotation/internal rotation using MRI. In seven loaded living knees with the subjects squatting, the relative tibiofemoral movements were similar to those in the unloaded knee except that the medial femoral condyle tended to move about 4 mm forwards with flexion. Four of the seven loaded knees were studied during flexion in external and internal rotation. As predicted, flexion (squatting) with the tibia in external rotation suppressed the internal rotation of the tibia which had been observed during unloaded flexion.     

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.