Healing and repair of ligament injuries in the knee

Although methods of treating ligamentous injuries have continually improved, many questions remain about enhancing the rate, quality, and completeness of ligament healing. It is known that the ability of a torn ligament to heal depends on a variety of factors, including anatomic location, presence of associated injuries, and selected treatment modality. A grade III injury of the medial collateral ligament (MCL) of the knee usually heals spontaneously. Surgical repair followed by immobilization of an isolated MCL tear does not enhance the healing process. In contrast, tears of the anterior cruciate ligament (ACL) and the posterior cruciate ligament often require surgical reconstruction. The MCL component of a combined ACL-MCL injury has a worse prognosis than an isolated MCL injury. The results of animal studies suggest that nonoperative treatment of an MCL injury is effective if combined with operative reconstruction of the ACL. Experimentation using animal models has helped to define the effects of ligament location, associated injuries, intrinsic factors, surgical repair, reconstruction, and exercise on ligament healing. New techniques utilizing growth factors and cell and gene therapies may offer the potential to enhance the rate and quality of healing of ligaments of the knee, as well as other ligaments in the body.

The effect of knee flexion angle and application of an anterior tibial load at the time of graft fixation on the biomechanics of a posterior cruciate ligament-reconstructed knee

Ten knees were studied using a robotic testing system under a 134-N posterior tibial load at five flexion angles. Three knee positions were used to study the effect of flexion angle at the time of graft fixation (full extension, 60 degrees, and 90 degrees) and two were used to study the effect of anterior tibial load (60 degrees and 90 degrees). Knee kinematics and in situ forces were determined for the intact ligament and the graft for each reconstruction. Graft fixation at full extension significantly decreased posterior tibial translation compared with the intact knee by up to 2.9 +/- 2.9 mm at 30 degrees, while in situ forces in the graft were up to 18 +/- 35 N greater than for the intact ligament. Conversely, posterior tibial translation for graft fixation at 90 degrees was significantly greater than that of the intact knee by up to 2.2 +/- 1.1 mm at all flexion angles; in situ forces decreased as much as 33 +/- 30 N. When an anterior tibial load was applied before graft fixation at 90 degrees of flexion, posterior tibial translation did not differ from the intact knee from 30 degrees to 120 degrees, while the in situ force in the graft did not differ from the intact ligament at full extension, 60 degrees, and 120 degrees of flexion. These data suggest that graft fixation at full extension may overconstrain the knee and elevate in situ graft forces. Conversely, fixation with the knee in flexion and an anterior tibial load best restored intact knee biomechanics.

Effects of sectioning the posterolateral structures on knee kinematics and in situ forces in the posterior cruciate ligament

The objective of this study was to determine the effects of sectioning the posterolateral structures (PLS) on knee kinematics and in situ forces in the posterior cruciate ligament (PCL) in response to external and simulated muscle loads. Ten human cadaveric knees were tested using a robotic/universal force-moment sensor testing system. The knees were subjected to three loading conditions: (a) 134-N posterior tibial load, (b) 5-Nm external tibial torque, and (c) isolated hamstring load (40 N biceps/40 N semimembranosus). The knee kinematics and in situ forces in the PCL for the intact and PLS-deficient knee conditions were determined at full extension, 30 degrees, 60 degrees, 90 degrees, and 120 degrees of knee flexion. Under posterior tibial loading posterior tibial translation with PLS deficiency increased significantly at all flexion angles by 5.5+/-1.5 mm to 0.8+/-1.2 mm at full extension and 90 degrees, respectively. The corresponding in situ forces in the PCL increased by 17-19 N at full extension and 30 degrees of knee flexion. Under the external tibial torque, external tibial rotation increased significantly with PLS deficiency by 15.1+/-1.6 degrees at 30 degrees of flexion to 7.7+/-3.5 degrees at 90 degrees, with the in situ forces in the PCL increasing by 15-90 N. The largest increase occurred at 60 degrees to 120 degrees of knee flexion, representing forces two to six times of those in the intact knee. Under the simulated hamstring load, posterior tibial translation and external tibial and varus rotations also increased significantly at all knee flexion angles with PLS deficiency, but this was not so for the in situ forces in the PCL. Our data suggest that injuries to the PLS put the PCL and other soft tissue structures at increased risk of injury due to increased knee motion and the elevated in situ forces in the PCL.

Biomechanical analysis of a double-bundle posterior cruciate ligament reconstruction

The objective of this study was to experimentally evaluate a single-bundle versus a double-bundle posterior cruciate ligament reconstruction by comparing the resulting knee biomechanics with those of the intact knee. Ten human cadaveric knees were tested using a robotic/universal force-moment sensor testing system. The knees were subjected to a 134-N posterior tibial load at five flexion angles. Three knee conditions were tested: 1) intact knee, 2) single-bundle reconstruction, and 3) double-bundle reconstruction. Posterior tibial translation of the intact knee ranged from 4.9 +/- 2.7 mm at 90 degrees to 7.2 +/- 1.5 mm at full extension. After the single-bundle reconstruction, posterior tibial translation increased to 7.3 +/- 3.9 mm and 9.2 +/- 2.8 mm at 90 degrees and full extension, respectively, while the corresponding in situ forces in the graft were up to 44 +/- 19 N lower than those in the intact ligament. Conversely, with double-bundle reconstruction, the posterior tibial translation did not differ significantly from the intact knee at any flexion angle tested. This reconstruction also restored in situ forces more closely than did the single-bundle reconstruction. These data suggest that a double-bundle posterior cruciate ligament reconstruction can more closely restore the biomechanics of the intact knee than can the single-bundle reconstruction throughout the range of knee flexion.

Significance of changes in the reference position for measurements of tibial translation and diagnosis of cruciate ligament deficiency

Measurements of tibial translation in response to an external load are used in clinical and laboratory settings to diagnose and characterize knee-ligament injuries. Before these measurements can be quantified, a reference position of the knee must be established (defined as the position of the knee with no external forces or moments applied). The objective of this study was to determine the effects of cruciate ligament deficiency on this reference position and on subsequent measurements of tibial translation and, in so doing, to establish a standard of kinematic measurement for future biomechanical studies. Thirty-six human cadaveric knees were studied with a robotic/universal force-moment sensor testing system. The reference positions of the intact and posterior cruciate ligament-deficient knees of 18 specimens were determined at full extension and at 30, 60, 90, and 120 degrees of flexion, and the remaining five-degree-of-freedom knee motion was unrestricted. Subsequently, under a 134-N anterior-posterior load, the resulting knee kinematics were measured with respect to the reference positions of the intact and posterior cruciate ligament-deficient knees. With posterior cruciate ligament deficiency, the reference position of the knee moved significantly in the posterior direction, reaching a maximal shift of 9.3 +/- 3.8 mm at 90 degrees of flexion. For the posterior cruciate ligament-deficient knee, posterior tibial translation ranged from 13.0 +/- 3.4 to 17.7 +/- 3.6 mm at 30 and 90 degrees, respectively, when measured with respect to the reference positions of the intact knee. When measured with respect to the reference positions of the posterior cruciate ligament-deficient knee, these values were significantly lower, ranging from 11.7 +/- 4.3 mm at 30 degrees of knee flexion to 8.4 +/- 4.8 mm at 90 degrees. A similar protocol was performed to study the effects of anterior cruciate ligament deficiency on 18 additional knees. With anterior cruciate ligament deficiency, only a very small anterior shift in the reference position was observed. Overall, this shift did not significantly affect measurements of tibial translation in the anterior cruciate ligament-deficient knee. Thus, when the tibial translation in the posterior cruciate ligament-injured knee is measured when the reference position of the intact knee is not available, errors can occur and the measurement may not completely reflect the significance of posterior cruciate ligament deficiency. However, there should be less corresponding error when measuring the tibial translation of the anterior cruciate ligament-injured knee because the shift in reference position with anterior cruciate ligament deficiency is too small to be significant. We therefore recommend that in the clinical setting, where the reference position of the knee changes with injury, comparison of total anterior-posterior translation with that of the uninjured knee can be a more reproducible and accurate measurement for assessing cruciate-ligament injury, especially in posterior cruciate ligament-injured knees. Similarly, in biomechanical testing where tibial translations are often reported for the ligament-deficient and reconstructed knees, a fixed reference position should be chosen when measuring knee kinematics. If such a standard is set, measurements of knee kinematics will more accurately reflect the altered condition of the knee and allow valid comparisons between studies.

Biomechanical analysis of a posterior cruciate ligament reconstruction. Deficiency of the posterolateral structures as a cause of graft failure

We hypothesized that posterior cruciate ligament reconstructions are often compromised by associated injuries to the posterolateral structures. Therefore, we evaluated a posterior cruciate ligament reconstruction in isolated and combined injury models using a robotic/universal force-moment sensor testing system. The resulting knee kinematics and the in situ forces in the native and reconstructed posterior cruciate ligament were determined under four external loading conditions. In the isolated injury model, reconstruction reduced posterior tibial translation to within 1.5+/-1.3 to 2.4+/-1.4 mm of the intact knee at 30 degrees and 90 degrees under a 134-N posterior tibial load. In the combined injury model, deficiency of the posterolateral structures increased posterior tibial translation of the reconstructed knee by 6.0+/-2.7 mm at 30 degrees and 4.6+/-1.5 mm at 90 degrees of flexion. External rotation increased up to 14 degrees while varus rotation increased up to 7 degrees. In situ forces in the posterior cruciate ligament graft also increased significantly (by 22% to 150%) for all loading conditions. Our results demonstrate that a graft that restores knee kinematics for an isolated posterior cruciate ligament deficiency is rendered ineffective and may be overloaded if the posterolateral structures are deficient. Therefore, surgical reconstruction of both structures is recommended in the setting of a combined injury.

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.

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.

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 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 degrees. 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.

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 degrees and 30 degrees 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 degrees to 4 degrees at 60 degrees to 90 degrees 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.