Microstructural and Mechanical Properties of the Posterior Cruciate Ligament: A Comparison of the Anterolateral and Posteromedial Bundles.

BACKGROUND: The microstructural organization (collagen fiber alignment) of the posterior cruciate ligament (PCL), which likely corresponds with its functional properties, has only been described qualitatively in the literature, to our knowledge. The goal of this study was to quantify the tensile mechanical and microstructural properties of the PCL and compare these qualities between the anterolateral and posteromedial bundles. METHODS: Twenty-two knee specimens from 13 donors (8 male and 5 female; mean age [and standard deviation] at the time of death, 43.0 ± 4.1 years; mean body mass index, 30.0 ± 6.7 kg/m2) were dissected to isolate the PCL, and each bundle was split into 3 regions. Mechanical testing of each regional sample consisted of preconditioning followed by a ramp-and-hold stress-relaxation test and a quasi-static ramp-to-failure test. Microstructural analysis was performed with use of a high-resolution, division-of-focal-plane polarization camera to evaluate the average direction of collagen orientation and the degree to which the collagen fibers were aligned in that direction. Results were compared between the anterolateral and posteromedial bundles and across the regions of each bundle. RESULTS: The anterolateral and posteromedial bundles demonstrated largely equivalent mechanical and microstructural properties. Elastic moduli in the toe and linear regions were not different; however, the posteromedial bundle did show significantly more stress relaxation (p = 0.004). There were also few differences in microstructural properties between bundles, which again were seen only in stress relaxation. Comparing regions within each bundle, several mechanical and microstructural parameters showed significant relationships across the posteromedial bundle, following a gradient of decreasing strength and alignment from anterior to posterior. CONCLUSIONS: The PCL has relatively homogenous microstructural and mechanical properties, with few differences between the anterolateral and posteromedial bundles. This finding suggests that distinct functions of the PCL bundles result primarily from size and anatomical location rather than from differences in these properties. CLINICAL RELEVANCE: These properties of the PCL can be used to assess the utility of graft choices and operative techniques for PCL reconstruction and may partly explain limited differences in the outcomes of single-bundle compared with double-bundle reconstruction techniques for the PCL.

The medial synovial fold of the posterior cruciate ligament: cadaveric investigation together with magnetic resonance imaging and histology.

OBJECTIVE: The purposes of our study were to analyze magnetic resonance imaging (MRI) and cadaveric findings concerning the medial synovial fold of the posterior cruciate ligament (PCL) and to classify the types of fold according to anatomic location. METHODS: Two musculoskeletal radiologists reviewed MR images of 17 cadaveric knees to classify the types of medial fold of the PCL by consensus. The MRI types were divided into 3 groups. In type A, there was no definitive medial fold; and in type B, inferior-short type, there was a small protrusion of the medial border. Type C, inferior-long type, had a long enough fold to exceed the imaginary line, which is connecting between the medial tibial condyle and posterolateral aspect of the medial femoral condyle. Correlations were sought between the findings derived from the MRI studies and cadaveric dissections. Histologic analyses of the medial fold were also performed. RESULTS: On MRI, the most common type of medial fold was type B (76.4%), followed by type C (11.8%) and type A (11.8%). In the cadaveric investigation, the medial folds of both types B and C were found to project into the medial femorotibial joint. Moreover, there was also a protruding medial fold at the superior aspect of the PCL in the A. Histologic examination of the medial folds revealed collagenous tissue surrounded by synovial cells. CONCLUSIONS: Medial folds of the PCL are normal synovial structures that can be seen by MRI and in cadaveric studies in a large proportion of the population.

Tendons and ligaments are anatomically distinct but overlap in molecular and morphological features--a comparative study in an ovine model.

Tendons and ligaments are similar in composition but differ in function. Simple anatomical definitions do not reflect the fact individual tendons and ligaments have unique properties due to their adaptation to a specific role. The patellar tendon is a structure of particular clinical interest. A null hypothesis was declared stating that the patellar tendon is not significantly different in terms of matrix composition and collagen fibril diameter to other tendons. The lateral and medial collateral ligaments (LCL, MCL), anterior and posterior cruciate ligaments (ACL, PCL), together with the long digital extensor, superficial digital extensor, and patellar tendons (LDET, SDFT, PT) were harvested from three cadaveric ovine hindlimbs. The extracellular matrix was assessed in terms of water, collagen, and total sulphated glycosaminoglycan (GAG) content. The organization of the collagen component was determined by an ultrastructural analysis of collagen fibril diameter distributions, together with values for the collagen fibril index (CFI) and mass-average diameter (MAD). There were significant differences between ligaments and tendons. The PT had a bimodal collagen fibril diameter distribution with CFI 72.9%, MAD 202 nm, water content 53.1%, GAG content 2.3 microg/mg, and collagen content 73.7%, which was not significantly different from the other tendons. The results of this study support the null hypothesis suggesting that the patellar tendon is similar to other tendons and demonstrate that tendons have different characteristics to ligaments.

Age-related changes in human PCLs: a light and electron microscopic study.

No studies have previously compared ultrastructural differences in human posterior cruciate ligaments in different age groups, but such differences may be of importance in the reconstruction of this ligament. We used light and transmission electron microscopy to compare ultrastructural differences in the posterior cruciate ligaments of 36 patients by age and sex. We found no differences between the sexes or in the groups aged between 30 and 59 years. However, collagen fibers with variable diameters were observed in those aged 10-19 years, with many oxytalan fibers between the collagen fibers. We also found a decrease in collagen fiber diameter and increase in collagen fibril concentration with aging, with a maximum increase in collagen fibril concentration in those aged 60-69 years.

Type II collagen deposition in cruciate ligament precedes osteoarthritis in the guinea pig knee.

OBJECTIVE: To examine the collagens in cruciate ligaments of young Dunkin-Hartley guinea pigs, to determine whether a change in specific collagen types is an early feature of the spontaneous osteoarthritis (OA), which consistently develops in the medial compartment of the knee in this strain. DESIGN: Collagen types I, II, III, IX, and XI were detected by immunofluorescence microscopy in the anterior and posterior cruciate ligaments of animals at 3, 4-5 and 12 weeks of age. Type II collagen in PCL was further analysed by confocal microscopy or biochemical assay after cyanogen bromide digestion, SDS-PAGE and immunoblotting. Interfibrillar proteoglycans were visualized by transmission electron microscopy. RESULTS: Collagen types I and III formed the bulk of fibrous mid-ligament tissue in all animals. Typical cartilage collagens, types II, IX and XI, were identified by immunolabeling where ligaments attached to tibial bone. Type II collagen, normally restricted to the fibrocartilage attachment sites, was also found at separate foci in anterior fiber bundles of the posterior cruciate ligament in 12-week-old animals. Biochemical data confirmed these observations which, together with electron microscopy showing large atypical proteoglycan structures, suggested the deposition of fibrocartilage within the fibrous mid-ligament. CONCLUSIONS: Cruciate ligaments, especially posterior cruciate ligament in Dunkin-Hartley guinea pigs synthesize cartilage-like matrix in mid-ligament prior to the appearance of classical signs of OA.

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.

Changes in the distribution of fibrillar collagens in the collateral and cruciate ligaments of the rabbit knee joint during fetal and postnatal development.

The collateral ligaments can be clearly distinguished in the 25-day fetal rabbit knee joint. Type I and V collagens are present in the extracellular matrix between the cells of the lateral and medial collateral ligaments and this distribution persists until the rabbit is skeletally mature. From 8 months onwards type III collagen is also present, particularly around the cells. Type I collagen mRNA is expressed by the cells from the 25-day fetal to 8-month-old adult ligament. The ligament sheath is composed of types III and V collagens. The cruciate ligaments are present between the femur and tibia in the 20-day fetus. The matrix is composed of types I and V collagens from the 25-day fetus until at 12- to 14-weeks postnatal, type III collagen appears in the pericellular regions together with type V. At 8 months and 2 years the amount of type III collagen has increased. All the cells express the mRNA for type I collagen at 12- to 14-weeks, but only isolated cells express this mRNA at 8 months. Thus, both the collateral and cruciate ligaments undergo changes in their complement of collagens during postnatal development and ageing. The implications of these complex interactions of different types of collagen are discussed in relation to healing and the surgical replacement of torn ligaments by tendons.

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 measured the cross-sectional shape and area of the anterior cruciate, posterior cruciate, and meniscofemoral ligaments in eight cadaveric knees. The posterior cruciate ligament increased in cross-sectional area from tibia to femur, and the anterior cruciate ligament area decreased from tibia to femur. The meniscofemoral ligaments did not change shape in their course from the lateral meniscus to their femoral insertions. The posterior cruciate ligament cross-sectional area was approximately 50% and 20% greater than that of the anterior 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 evaluated. 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 ligament 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 components: 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 posteromedial component and meniscofemoral ligaments. The anterolateral component and the meniscofemoral ligaments displayed similar elastic moduli, which were both significantly greater than that of the posteromedial component.

Collagen fibril diameter distribution in patellar tendon autografts after posterior cruciate ligament reconstruction in sheep: changes over time.

The alterations in collagen fibril diameter distribution, mean fibril diameter and the area occupied by collagen after posterior cruciate ligament reconstruction using a patellar tendon autograft were estimated 2, 6, 16, 26, 52 and 104 wk postoperatively. Patellar tendons and posterior cruciate ligaments from unoperated animals were used as control tissues. Collagen fibrils were divided into histograms according to their diameter in order to analyse distribution maxima. There was a significant decrease in mean fibril diameter of the grafts in comparison with the control tissues. At 104 wk it was only about 51% of that for control posterior cruciate ligaments. The total area occupied by collagen was significantly reduced at 6 wk postoperatively and was about 57% in comparison with normal posterior cruciate ligaments. A considerable increase of small diameter collagen fibrils together with a loss of large fibrils was responsible for these results. There was no evidence of reestablishment of large diameter fibrils, which are normally found in tendon and ligaments, up to 2 y after transplantation. The total area covered by collagen was still reduced at this stage although the number of fibrils had increased.

Collagen fibril organization in the patellar tendon autograft after posterior cruciate ligament reconstruction. A quantitative evaluation in a sheep model.

We replaced the posterior cruciate ligament in 30 skeletally mature sheep with a patellar tendon autograft using the central third of the ipsilateral patellar tendon. The healing autograft was compared with the contralateral posterior cruciate ligament and the patellar tendons and posterior cruciate ligaments of nonoperated animals. The collagen fibril diameters were analyzed using transmission electron photomicrographs of fibril cross sections taken at six periods during the 2 years after surgery. The patellar tendon and posterior cruciate ligament were characterized by a broad, nongaussian distribution of collagen fibril diameters. The autografts shifted to a unimodal distribution by an increase of small-diameter collagen fibrils. The frequency of small-diameter fibrils measuring up to 100 nm was 99% after 2 years. At that time, these small-diameter fibrils represented 91.6% of the area covered by collagen fibrils. The mean diameter of the collagen fibrils in the autografts significantly decreased to 45% of the controls at Week 26 and remained at this level until the end of this study. The percentage of area covered by collagen fibrils per 1 micron 2 was 78% of the controls 2 years post-operatively. This study suggests that the patellar tendon autograft could not reproduce the collagen fibril organization of the posterior cruciate ligament. This may be a biologic factor responsible for inconsistent results in posterior cruciate ligament replacement.

The mechanical properties of the two bundles of the human posterior cruciate ligament.

Successful reconstruction of ligaments requires knowledge of the properties of the intact ligament. This study examined the strength of the human posterior cruciate ligament (PCL), treating it as two separate fibre bundles. It was hypothesized (i) that the mechanical and material properties of the anterolateral (aPC) and the posteromedial (pPC) bundles of the PCL were significantly different and (ii) that previous studies have underestimated the strength of the whole PCL. The properties of the two bundles were measured in 10 donors (53-98 yr). The mechanical and material properties of the two bundles were found to be significantly different, the aPC was six times as strong as the pPC. The aPC had a mean strength of 1.6 kN. Allowing for age effects this study suggests that the strength of the PCL in young active people is 4 kN, which is higher than that suggested by previous studies. Because of the difference in the strengths of the two bundles, we conclude that the aPC is primarily responsible for the stabilising effect of the PCL. We therefore recommend that PCL reconstructions should be centered on the middle of the aPC bundle.

Structure and function of matrix components in the cruciate ligaments. An immunohistochemical, electron-microscopic, and immunoelectron-microscopic study.

In the present study, the matrix components of 100 cruciate ligaments were analyzed by conventional electron microscopy, immunohistology, morphometry, and immunoelectron microscopy. The anterior (ACL) and the posterior (PCL) cruciate ligaments contained collagen types III, IV, and VI. Several structural glycoproteins, like fibronectin, laminin, entactin, tenascin, and undulin were detected using monoclonal antibodies. Whereas laminin and entactin were higher concentrated in the PCL, type VI collagen was more frequently found in the ACL. The ACL had a critical nourishment in its distal and middle thirds. In all ligament parts the PCL revealed a better vascular supply with strong correlation to type IV collagen expression. The normal matrix of the cruciate ligaments represented a complicated regulatory network of proteins, glycoproteins, elastic systems, and glycosaminoglycans with multiple functional interactions.

[Differences in condition between the anterior and posterior cruciate ligament in rheumatoid arthritis]. / Unterschiede im Erhaltungszustand zwischen vorderem und hinterem Kreuzband bei rheumatoider Arthritis.

In rheumatoid arthritis (RA) we examined the cruciate ligaments of 16 patients by the use of light- and transmission electron microscopy. Considerable differences between the anterior (ACL) and the posterior cruciate ligament (PCL) were observed. The ACL showed dramatic ultrastructural changes, against that the PCL was compact and well conserved. The preservation of PCL in joint surgery can morphologically be recommended. Perhaps the greater inflammatory process on the surface of ACL is responsible for the observed differences.

[Comparative microstructural studies on collagen and elastic fiber systems of the cruciate ligaments]. / Vergleichende mikrostrukturelle Untersuchungen an kollagenen und elastischen Fasersystemen der Kreuzbänder.

The aim of the present study was to compare the collagenous fibre systems of the anterior (ACL) with the posterior cruciate ligament (PCL). 22 cruciate ligaments were examined by the combined use of light-, transmission- and scanning electron microscopy. Characteristic interligamental differences were established. ACL: more anchoring type VI collagen, thinner but more branched fibrils, complex collagen networks, much more elastic elements. PCL: more type IV collagen, many intrinsic vessels, thicker fibrils, parallel fibre arrangement. The results indicate that the ACL is exposed at the first line to rotational forces, the PCL probably more to tensional forces. The collagen fibre arrangement of the cruciate ligaments is to take into consideration by the surgeon.