Gregory J. Carlin

Quantitative Analysis of Human Cruciate Ligament Insertions

The objective of this study was to provide quantitative data on the insertion sites of the cruciate ligaments. In the first part of the study, we determined the shapes and sizes of the insertions of the anterior and posterior cruciate ligaments (ACL and PCL), and further compared these data with the midsubstance cross-sectional areas of the ligaments. The cross-sectional area of the ACL and PCL midsubstance of 5 human knees was measured using a laser micrometer system. The insertion sites of each ligament were then digitized and the 2-dimensional insertion site areas were determined. Relative to the ligament midsubstance, the PCL tibial and femoral insertions were approximately 3 times larger, whereas those of the ACL were over 3.5 times larger. In the second part of the study, the ACLs and PCLs of 10 knees were each divided into their 2 components and the areas of each insertion were determined. Each component was approximately 50% of the total ligament insertion area and no significant difference between the 2 could be shown.

The Human Posterior Cruciate Ligament Complex: An Interdisciplinary Study Ligament Morphology and Biomechanical Evaluation

To study the structural and functional properties of the human posterior cruciate ligament complex, we meas ured the cross-sectional shape and area of the anterior cruciate, posterior cruciate, and meniscofemoral liga ments in eight cadaveric knees. The posterior cruciate ligament increased in cross-sectional area from tibia to femur, and the anterior cruciate ligament area de creased from tibia to femur. The meniscofemoral liga ments did not change shape in their course from the lateral meniscus to their femoral insertions. The pos terior cruciate ligament cross-sectional area was ap proximately 50% and 20% greater than that of the an terior cruciate ligament at the femur and tibia, respectively. The meniscofemoral ligaments averaged approximately 22% of the entire cross-sectional area of the posterior cruciate ligament. The insertion sites of the anterior and posterior cruciate ligaments were evalu ated. The insertion sites of the anterior and posterior cruciate ligaments were 300% to 500% larger than the cross-section of their respective midsubstances. We determined, through transmission electron microscopy, fibril size within the anterior and posterior cruciate liga ment complex from the femur to the tibia. The posterior cruciate ligament becomes increasingly larger from the tibial to the femoral insertions, and the anterior cruciate ligament becomes smaller toward the femoral insertion. We evaluated the biomechanical properties of the femur-posterior cruciate ligament-tibia complex using 14 additional human cadaveric knees. The posterior cruciate ligament was divided into two functional com ponents : the anterolateral, which is taut in knee flexion, and the posteromedial, which is taut in knee extension. The anterolateral component had a significantly greater linear stiffness and ultimate load than both the postero medial component and meniscofemoral ligaments. The anterolateral component and the meniscofemoral liga ments displayed similar elastic moduli, which were both significantly greater than that of the posteromedial com ponent.

Determination of the In Situ Forces in the Human Posterior Cruciate Ligament Using Robotic Technology A Cadaveric Study

We examined the in situ forces in the posterior cruciate ligament as well as the force distribution between its anterolateral and posteromedial bundles. Using a robotic manipulator in conjunction with a universal force-moment sensor system, we applied posterior tibial loads from 22 to 110 N to the joint at 0° to 90° of knee flexion. The magnitude of the in situ force in the posterior cruciate ligament and its bundles was significantly affected by knee flexion angle and posterior tibial loading. In situ forces in the posterior cruciate ligament ranged from 6.1 6.0 N under a 22-N posterior tibial load at 0° of knee flexion to 112.3 28.5 N under a 110-N load at 90°. The force in the posteromedial bundle reached a maximum of 67.9 31.5 N at 90° of knee flexion, and the force in the anterolateral bundle reached a maximum of 47.8 23.0 N at 60° of knee flexion under a 110-N load. No significant differences existed between the in situ forces in the two bundles at any knee flexion angle. This study provides insight into the knee flexion angle at which each bundle of the posterior cruciate ligament experiences the highest in situ forces under posterior tibial loading. This information can help guide us in more accurate graft placement, fixation, and tensioning, and serve as an assessment of graft performance.

A Functional Comparison of Animal Anterior Cruciate Ligament Models to the Human Anterior Cruciate Ligament

Many investigators have used animal models to clarify the role of the human anterior cruciate ligament (ACL). Because none of these models are anatomically and biomechanically identical to the human ACL, there exists a need for an objective comparison of these models. To do this, we used a universal force-moment sensor to measure and compare the in situ forces, including magnitude and direction, of the ACL and the anteromedial (AM) and posterolateral (PL) bundles of human, pig, goat, and sheep knees. An Instron was used to apply 50 and 100 N anterior tibial loads at 90° of knee flexion, while a universal force-moment sensor was used to measure the forces applied by the ACL to the tibia, the in situ force of the ACL. We found significant differences between the magnitude of force experienced by the goat and sheep ACL and AM and PL bundles when compared with the human ACL and AM and PL bundles. Also, the direction of the in situ force in the ACL and AM bundles of the goat and sheep were different from the human. The pig knee differed from the human only in the magnitude and direction of the in situ force in the PL bundle in response under anterior tibial loading. A tally of the significant differences between the animal models and the human knees indicates that goat and sheep knees may have limitations in modeling the human ACL, while the pig knee may be the preferred model for experimental studies.

The Effects of a Popliteus Muscle Load on In Situ Forces in the Posterior Cruciate Ligament and on Knee Kinematics A Human Cadaveric Study

To investigate the effect of simulated contraction of the popliteus muscle on the in situ forces in the posterior cruciate ligament and on changes in knee kinematics, we studied 10 human cadaveric knees (donor age, 58 to 89 years) using a robotic manipulator/universal force moment sensor system. Under a 110-N posterior tibial load (simulated posterior drawer test), the kinematics of the intact knee and the in situ forces in the ligament were determined. The test was repeated with the addition of a 44-N load to the popliteus muscle. The posterior cruciate ligament was then sectioned and the knee was subjected to the same tests. The additional popliteus muscle load significantly reduced the in situ forces in the ligament by 9% to 36% at 90° and 30° of flexion, respectively. No significant effects on posterior tibial translation of the intact knee were found. However, in the ligament-deficient knee, posterior tibial translation was reduced by up to 36% of the translation caused by ligament transection. A coupled internal tibial rotation of 2° to 4° at 60° to 90° of knee flexion was observed in both the intact and ligament-deficient knees when the popliteus muscle load was added. Our results indicate that the popliteus muscle shares the function of the posterior cruciate ligament in resisting posterior tibial loads and can contribute to knee stability when the ligament is absent.

Anatomical and biomechanical characteristics of human meniscofemoral ligaments

The meniscofemoral ligaments (MFL) of 26 human cadaver knees were studied to determine their structural importance. The incidence of at least one MFL in each of the specimens studied was 100%, and 46% of the specimens had both MFL ligaments (Humphry and Wrisberg). Another 23% had a single Humphry ligament, and the remaining 31% had a single Wrisberg ligament. A laser micrometer system was used to measure cross-sectional shape and area. The average cross-sectional areas of the Humphry and Wrisberg ligaments were 7.8±4.7 mm2 and 6.7±4.1 mm2, respectively. In specimens with both a Humphry and Wrisberg ligament, the larger ligament area was on average 100% greater than the smaller ligament area. The average ratios of the cross sectional areas of Wrisberg and Humphry to that for the PCL within the same knee were 12.0%±7.7% and 11.9%±5.7%, respectively. The structural properties of the MFL bone-ligament-meniscus complex and the mechanical properties of the MFL midsubstance were determined by uniaxial tensile testing. The average stiffness, ultimate load, and energy absorbed at failure were, respectively, 49.0±18.4 N/mm, 297.4±141.4 N and 1125.4±735.8 N/mm. The tangent modulus between 4% and 7% strain was 355.1±234.0 MPa. Our findings suggest that the MFL is a significant biomechanical structure in the knee because of its size, stiffness, and strength.

Quantitative analysis of collagen fibrils of human cruciate and meniscofemoral ligaments

The ultrastructural anatomy of collagen fibril diameters in the cruciate and meniscofemoral ligaments, from four young human cadaver knees (mean age, 20 years, range, 17–22 years) was studied using transmission electron microscopy. Samples were harvested from the proximal, middle, and distal regions of the anterior and posterior cruciate ligaments, and the meniscofemoral ligament. Photomicrographs were taken and assessed quantitatively using image analysis software to determine the collagen fibril diameters and eccentricities, and the percentage of total cross sectional area occupied by collagen. The collagen fibril diameter for the anterior cruciate ligament was found to be largest in the distal region but it decreased as it moved proximally. The posterior cruciate ligament had an opposite trend because it decreased from the proximal to the distal region. For the meniscofemoral ligament, the fibrils of the middle region were larger than those of the proximal and distal regions. The percentage of total cross sectional area occupied by collagen, however, did not vary significantly between regions. Fibril eccentricity also varied little between ligament or location. The variability observed in fibril diameters may account for the different mechanical properties of the ligaments.

In-situ forces in the human posterior cruciate ligament in response to posterior tibial loading

Although some investigators have referred to the human posterior cruciate ligament (PCL) as the center of the knee, it has received less attention than the more frequently injured anterior cruciate ligament (ACL) and medial collateral ligament (MCL). Therefore, our understanding of the function of the PCL is limited. Our laboratory has developed a method of measuring thein-situ forces in a ligament without contacting that ligament by using a universal force-moment sensor (UFS). In this study, we attached a USF to the tibia and measuredin-situ forces of the human PCL as a function of knee flexion in response to tibial loading. At a 50-N posterior tibial load, the force in the PCL increased from 25±11 N (mean±SD) at 30° of knee flexion to 48±12 N at 90° of knee flexion. At 100 N, the corresponding increases were to 50±17 N and 95±17 N, respectively. Of note, at 30° knee flexion, approximately 45% of the resistance to posterior tibial loading was caused by contact between the tibia and the femoral condyles, whereas, at 90° of knee flexion, no resistance was caused by such contact. For direction of thein-situ force, the elevation angle from the tibial plateau was greater at 30° of knee flexion than at 90° of knee flexion. The data gathered on the magnitude and direction of thein-situ force of the PCL should help in our understanding of the dependence of knee flexion angle of the forces within the PCL.

Assessment of Posterior Cruciate Ligament Graft Performance Using Robotic Technology

We used the information on in situ forces provided by robotics to compare two methods of posterior cruciate ligament graft fixation. Twenty porcine knees were studied using robotic technology to determine and re peat intact, deficient, and reconstructed knee motion under 110 N of posterior tibial loading at 30°, 60°, and 90° of knee flexion. Reconstruction was performed using a bone-patellar tendon-bone graft with the distal end of the graft placed in the posterolateral aspect of the posterior cruciate ligament tibial insertion. Speci mens were separated into two groups based on the femoral fixation site: the proximal or anterior aspect of the femoral insertion. Repetition of knee motion al lowed measurement of the force in the intact posterior cruciate ligament and graft using the principle of su perposition. The forces in the graft and the intact liga ment provided additional information to evaluate graft performance. Force in the intact posterior cruciate lig ament was significantly greater at 90° than at 30° and 60° of knee flexion. The forces in both graft types were significantly lower than those of the posterior cruciate ligament, but the force in the anteriorly placed graft was significantly greater at 90° than at 30° and 60° of knee flexion, similar to the intact posterior cruciate ligament. Thus, the anteriorly placed graft had a more physiologic increase in tension with knee flexion, when the joint provided less restraint.

Quantitative anthropometry of the subatlantal cervical longitudinal ligaments.

Study Design. The quantitative anthropometry of the cervical longitudinal ligaments was determined in 20 human cadaveric subatlantal cervical spines at the limits of flexion and extension. Objectives. To provide measurements of cervical anterior and posterior longitudinal ligament lengths, widths, and cross‐sectional areas at segmental levels. Summary of Background Data. Although mathematical models of the cervical spine require specific data to predict kinematics, the anthropometry of the cervical spine has not been examined in detail. The dimensional changes of ligaments in physiologic motion are not well characterized. Methods. Segmental lengths and widths of the cervical longitudinal ligaments were measured in sagittal plane flexion and extension, using a three‐dimensional electromagnetic digitizer. The cross‐sectional areas of the ligaments at resting length were measured with a laser micrometer system. Comparisons between anterior and posterior location and among segmental levels were made. Several ligaments were examined histologically to determine the insertion sites and, thus, to define the segmental length. Results. The anterior longitudinal ligaments were shorter in flexion than in extension. In extension, they were longer than the posterior longitudinal ligaments in flexion. The resting isolated ligaments were longer than the longest in situ lengths at several vertebral levels. The anterior longitudinal ligaments were wider at the disc than at the body. The cross‐sectional area at C2‐C3 was smaller than at subaxial levels. The longitudinal ligaments were observed to insert along the entire underlying vertebral body. Conclusions. The quantitative anthropometry of the cervical longitudinal ligaments is important in the development of accurate mathematical models of the cervical spine. The in situ ligaments may not be under tension in the physiologic range of motion.