Development of the human knee joint ligaments

The ligamentum mucosum: A new classification

The ligamentum mucosum (LM) is a ligamentous structure within the synovial layer of the knee joint capsule. For a long time LM was perceived as the vestigial remnant from the embryological development of the knee. However, last years have shown increased interest in this structure due to its potentially significant clinical role. Fifty-one, 12 females (mean age 83.1 ± 3.4 years) and 39 males (mean age 84.2 ± 6.8 years) fixed in 10% formalin were examined. Upon dissection, the following morphological features of the LM were assessed: the types of LM, morphometric measurement and histological analysis of each type. The LM was present in 66.7% of all examined specimens. Three different types were recognized: Type I (55.9%)-single band with attachment to the intercondylar notch, Type IIa-bifurcated ligament with attachment to the anterior cruciate ligament, Type IIb-bifurcated ligament with both attachments to the intercondylar notch, Type III-double ligament with two independent bands and attachments to the intercondylar notch and to the knee joint capsule. The LM is variable and probably evolutionary changes are the reason. In our study we propose the new clinically useful classification supported by its anatomical and histological characteristics. Type IIa seems to be the most important from the clinical point of view, as it may be responsible for clinical issues and should be paid attention while diagnosing patients suffering from anterior cruciate ligament torn or anterior knee pain.

Major change in morphology of the talofibular ligaments during fetal development and growth

BACKGROUND AND PURPOSE

Ankle sprain is often attributed to damage of the anterior and posterior talofibular ligaments (ATFL, PTFL). We compared the morphology of these ligaments in fetuses of different gestational ages (GAs) with the horizontal configuration in adults.

MATERIALS AND METHODS

Histological sections of unilateral ankles were examined in 22 fetuses, 10 at GA of 9-12 weeks and 12 at GA of 26-39 weeks.

RESULTS

At a GA of 9 to 10 weeks, the ATFL and PTFL consisted of horizontally running straight fibers. The initial ATFL appeared as a thickening of the capsule of the talocrural joint, although the initial PTFL was distant from this joint. Until a GA of 12 weeks, the talus and fibula were separated by an expanding joint cavity. Thus, the initial horizontal ligaments were "pulled" in a distal direction. The distal parts of the ligaments consisted of thin collagenous fibers that had an irregular array, whereas the short proximal parts had thick fibers and a horizontal array. In near-term fetuses, the ligaments contained no horizontal fibers. The ATFL had a wavy course around the thick synovial fold, and was exposed to the joint cavity along the entire course; the distal part was thinner than the proximal part. The PTFL was bulky and consisted of fibers with an irregular array. Therefore, the morphology in a near-term fetus was quite different from that in adults.

CONCLUSION

The horizontal and straight composite ankle fibers in adults apparently result from postnatal reconstruction, depending on mechanical demand.

Significance of the broad non-bony attachments of the anterior cruciate ligament on the tibial side

Knowledge of the anatomy of the anterior cruciate ligament (ACL) is important to understand the function and pathology of the knee joint. However, on the tibial side of ACL, its structural relationships with the articular cartilage and lateral meniscus remain unclear. Furthermore, conventional research methods are limited to analyzing the bone attachments. We provide a comprehensive, three-dimensional anatomical description of the tibial side of the ACL that questions the principle that “a ligament is necessarily a structure connecting a bone to another bone.” In our study, 11 knees from 6 cadavers were used for macroscopic anatomical examinations, serial-section histological analyses, and three-dimensional reconstructions. The attachments of the tibial side of ACL consisted of attachments to the bone (102.6 ± 27.5 mm²), articular cartilage (40.9 ± 13.6 mm²), and lateral meniscus (6.5 ± 4.6 mm²), suggesting that the ACL has close structural relationships with the articular cartilage and lateral meniscus. Our study demonstrates that the tibial side of the ACL is not attached to the bone surface only and provides new perspectives on ligamentous attachments. Considering its attachment to the articular cartilage would enable more accurate functional evaluations of the mechanical tensioning of the ACL.

Crossed doubled patellar tendon: A rare anatomical variant with potential clinical significance

The patellar tendon is an integral part of the knee extensor mechanism and has been historically described as a single tendon. A doubled patellar tendon is an exceedingly rare finding. We present a case of a crossed doubled patellar tendon in a 70-year-old male with a history of right knee pain, which to our knowledge has only been reported once before in the literature. The presence of a doubled patellar tendon has a potential influence on surgical planning and in the etiology of anterior knee pain.

Inverted deltoid posterior cruciate ligament femoral insertion accompanied with medial synovial fold: a case of a complex posterior cruciate ligament anatomical variation recalcitrant to conservative treatment

PURPOSE

The present study aims to report a symptomatic rare anatomical variation of the posterior cruciate ligament (PCL) that was encountered during arthroscopy.

CASE PRESENTATION

A 34-year-old female suffered from dull anterior pain in the right knee, along with stiffness and the presence of an audible click and occasionally locking during deep knee flexion. Physical examination revealed only slight pain during single-leg squatting and mild knee effusion with painful limitation of the last degrees of flexion. Following unsuccessful conservative treatment, knee arthroscopy was performed in which the PCL was found to be hypertrophic, having a broad femoral insertion that almost completely occupied the intercondylar notch and impinged the anterior cruciate ligament. Moreover, the PCL presented a large medial synovial fold that formed a plica inserting to the medial meniscus's posterior horn. Ligamentoplasty was performed by excising one-third of the PCL lateral portion. The PCL medial synovial fold and the plica attaching to the medial meniscus were resected. The patient was allowed to return to full activity when her symptoms resolved, and the knee function was restored, at 5 weeks post-operatively.

CONCLUSION

The current study presented a rare and complex anatomical variation of the PCL that was symptomatic and recalcitrant to conservative treatment. Magnetic resonance imaging (MRI) can reveal the variant morphology of the PCL, and arthroscopy provides the definite treatment. This case report may be useful for orthopaedic surgeons and radiologists to consider anatomical PCL variations during differential diagnosis in patients with non-specific clinical presentation and findings.

Prospects of circadian clock in joint cartilage development

Altering the food intake, exercise, and sleep patterns have a great influence on the homeostasis of the biological clock. This leads to accelerated aging of the articular cartilage, susceptibility to arthropathy and other aspects. Deficiency or overexpression of certain circadian clock-related genes accelerates the cartilage deterioration and leads to phenotypic variation in different joints. The process of joint cartilage development includes the formation of joint site, interzone, joint cavitation, epiphyseal ossification center, and cartilage maturation. The mechanism by which, biological clock regulates the cell-cycle, growth, metabolism, and other biological processes of chondrocytes is poorly understood. Here, we summarized the interaction between biological clock proteins and developmental pathways in chondrogenesis and provided the evidence from other tissues that further predicts the molecular patterns of these protein-protein networks in activation, proliferation, and differentiation. The purpose of this review is to gain deeper understanding of the evolution of cartilage and its irreversibility seen in damage and aging.

Bilateral congenital absence of the anterior cruciate ligament associated with bilateral knee and hip osteoarthritis: Case report

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Reports of the progression of congenital absence of cruciate ligament were very rare in old age.

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Authors reported a case of congenital absence of cruciate ligament with severe knee instability, knee and hip osteoarthritis.

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Congenital absence of cruciate ligament may progress to osteoarthritis, so early detection and proper management is needed.

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Total knee arthroplasty is useful for the treatment of arthritis and instability following congenital absence of cruciate ligament.

Introduction

Congenital absence of the cruciate ligaments is very rare condition. Its association with congenital abnormalities of the spine, knee and hip, has been rarely reported. Most of the case reports were mostly reported before adolescence, so reports of the progression of this congenital disease at old age were extremely rare.

Presentation of case

A 65-year-old woman had a 7-year history of both knee and hip pain. On physical examination, valgus and varus instability on both knees and painful limitation of motion in both hip joints were observed. Radiography and magnetic resonance imaging showed the absence of the anterior cruciate ligament with osteoarthritis on both knees and hip osteoarthritis. Total knee and hip arthroplasties were performed on both knee joints and total hip arthroplasty. The pain and instability of both knees and hips were lost for follow up of more than a year, and the patient is doing well in daily life.

Discussion

Congenital absence of cruciate ligament may progress to osteoarthritis, do early detection and proper management is needed. In the treatment of older aged patients. Reports of treatment for this disorder are also very rare in older age. In old age, arthroplasty can be a useful treatment when the severe osteoarthritis is accompanied by instability.

Conclusion

Since the congenital absence of the ACL in old age could lead to hip and knee osteoarthritis, periodic follow-up is necessary and arthroplasty can be a useful treatment when the severe osteoarthritis is accompanied by instability.

Classification of Cruciate Ligament Dysplasia and the Severity of Congenital Fibular Deficiency

BACKGROUND

Dysplasia of the cruciate ligaments has been found in many patients with congenital fibular deficiency. A recent classification system has shown that radiographic tibial spine changes can predict the hypoplasia and aplasia of the cruciate ligaments. We used this radiographic classification to determine the frequency of these abnormalities and how they correlate with the severity of fibular deficiency and lateral femoral condylar hypoplasia.

METHODS

Using a hospital database search for fibular deficiency, 99 patients ≥6 years with unilateral fibular deficiency were identified. Existing radiographs of both knees were available for 75 patients and reviewed for the tibial spine changes and Achterman and Kalamchi classification of the fibular deficiency. Measurements of femoral condyle heights in 74 of 75 patients were recorded before any surgery to the distal femoral physis to assess lateral femoral condylar hypoplasia.

RESULTS

Twenty-two patients had hypoplasia of the lateral tibial spine+normal medial spine, 29 had absence of the lateral tibial spine+hypoplastic medial spine, and 11 had absence of both tibial spines. Five tibial spines were normal and 8 were unclassifiable. The severity of the tibial spine dysplasia, particularly absence of the lateral tibial spine, correlated with the severity of the fibular deficiency. (P<0.0001) The mean lateral femoral condylar hypoplasia, measured by involved: uninvolved lateral condyle heights, was 0.85±0.11. Those with some preservation of the lateral tibial spine had less lateral femoral condylar hypoplasia (P=0.0009). This lateral femoral condylar hypoplasia was positively associated with the severity of the fibular absence (P=0.039) and foot ray deficiency (P=0.036).

CONCLUSIONS

The severity of cruciate ligament dysplasia in fibular deficiency is directly correlated with the severity of fibular absence, lateral femoral condylar hypoplasia, and the absence of foot rays. This suggests that the embryological factors involved have a complex interplay for all of these clinical findings.

LEVEL OF EVIDENCE

Level III.

Bilateral absence of the cruciate ligaments with meniscal dysplasia: Unexpected diagnosis in a child with juvenile idiopathic arthritis

Bilateral agenesis of the cruciate ligaments is a rare congenital anomaly. We report a unique case of a young girl who had congenital short femur and diagnosed with polyarticular juvenile idiopathic arthritis (JIA) and later discovered to have congenital absence of both anterior and posterior cruciate ligaments and meniscal dysplasia in both the knees when MRI was performed at 11 years of age. The MRI was performed to evaluate knee laxity and persistent symptoms despite medical management and multiple steroid injections for arthritis treatment. This patient is one of the youngest with congenital absence of both the cruciate ligaments to be treated with ACL reconstruction. We highlight the unique radiographic imaging manifestations of congenital cruciate ligament agenesis and emphasize the role of MRI to confirm and depict additional intraarticular abnormalities.

The Unexplored Role of Intra-articular Adipose Tissue in the Homeostasis and Pathology of Articular Joints

Intra-articular adipose tissue deposits known as articular fat pads (AFPs) are described to exist within synovial joints. Their assumed role in normal joint biomechanics is increasingly objectivized by means of advanced methods of functional imaging. AFPs possess structural similarity with body subcutaneous white adipose tissue (WAT), however, seems to be regulated by independent metabolic loops. AFP dimension are conserved during extreme WAT states: obesity, metabolic syndrome, lipodystrophy, and cachexia. Hoffa fat pad (HFP) in the knee is increasingly recognized as a major player in pathological joint states such as anterior knee pain and osteoarthritis. HFP contains numerous population of mesenchymal and endothelial progenitors; however, the possible role of mature adipocytes in the maintenance of stem cell niche is unknown. We propose that AFP is an active component of the joint organ with multifunctional roles in the maintenance of joint homeostasis. Endowed with a rich network of sensitive nervous fibbers, AFPs may act as a proprioceptive organ. Adipokines and growth factors released by AFP-resident mature adipocytes could participate in the maintenance of progenitor stem cell niche as well as in local immune regulation. AFP metabolism may be locally controlled, correlated with but independent of WAT homeostasis. The identification of AFP role in normal joint turnover and its possible implication in pathological states could deliver diagnostic and therapeutic targets. Drug and/or cell therapies that restore AFP structure and function could become the next step in the design of disease modifying therapies for disabling joint conditions such as osteoarthritis and inflammatory arthritis.

Editorial Commentary: The Ribbon Theory. Another Quantum Leap? The Anterior Cruciate Ligament Is Twisted and in Fact a Flat Structure. Or not?

Is the double bundle structure of the anterior cruciate ligament (ACL) a myth? Derotation untwists the ACL and results in a flat ribbon structure. However, other researchers dispute this theory and revealed 2 distinct bundles separated by a synovial septum. The answer is simple: we do not know and the evidence is conflicting.

Meniscus, articular cartilage and nucleus pulposus: a comparative review of cartilage-like tissues in anatomy, development and function

The degradation of cartilage in the human body is impacted by aging, disease, genetic predisposition and continued insults resulting from daily activity. The burden of cartilage defects (osteoarthritis, rheumatoid arthritis, intervertebral disc damage, knee replacement surgeries, etc.) is daunting in light of substantial economic and social stresses. This review strives to broaden the scope of regenerative medicine and tissue engineering approaches used for cartilage repair by comparing and contrasting the anatomical and functional nature of the meniscus, articular cartilage (AC) and nucleus pulposus (NP). Many review papers have provided detailed evaluations of these cartilages and cartilage-like tissues individually but none have comprehensively examined the parallels and inconsistencies in signaling, genetic expression and extracellular matrix composition between tissues. For the first time, this review outlines the importance of understanding these three tissues as unique entities, providing a comparative analysis of anatomy, ultrastructure, biochemistry and function for each tissue. This novel approach highlights the similarities and differences between tissues, progressing research toward an understanding of what defines each tissue as distinctive. The goal of this paper is to provide researchers with the fundamental knowledge to correctly engineer the meniscus, AC and NP without inadvertently developing the wrong tissue function or biochemistry.

Evolution of the patellar sesamoid bone in mammals

The patella is a sesamoid bone located in the major extensor tendon of the knee joint, in the hindlimb of many tetrapods. Although numerous aspects of knee morphology are ancient and conserved among most tetrapods, the evolutionary occurrence of an ossified patella is highly variable. Among extant (crown clade) groups it is found in most birds, most lizards, the monotreme mammals and almost all placental mammals, but it is absent in most marsupial mammals as well as many reptiles. Here, we integrate data from the literature and first-hand studies of fossil and recent skeletal remains to reconstruct the evolution of the mammalian patella. We infer that bony patellae most likely evolved between four and six times in crown group Mammalia: in monotremes, in the extinct multituberculates, in one or more stem-mammal genera outside of therian or eutherian mammals and up to three times in therian mammals. Furthermore, an ossified patella was lost several times in mammals, not including those with absent hindlimbs: once or more in marsupials (with some re-acquisition) and at least once in bats. Our inferences about patellar evolution in mammals are reciprocally informed by the existence of several human genetic conditions in which the patella is either absent or severely reduced. Clearly, development of the patella is under close genomic control, although its responsiveness to its mechanical environment is also important (and perhaps variable among taxa). Where a bony patella is present it plays an important role in hindlimb function, especially in resisting gravity by providing an enhanced lever system for the knee joint. Yet the evolutionary origins, persistence and modifications of a patella in diverse groups with widely varying habits and habitats-from digging to running to aquatic, small or large body sizes, bipeds or quadrupeds-remain complex and perplexing, impeding a conclusive synthesis of form, function, development and genetics across mammalian evolution. This meta-analysis takes an initial step toward such a synthesis by collating available data and elucidating areas of promising future inquiry.

Spatial Change of Cruciate Ligaments in Rat Embryo Knee Joint by Three-Dimensional Reconstruction

This study aimed to analyze the spatial developmental changes of rat cruciate ligaments by three-dimensional (3D) reconstruction using episcopic fluorescence image capture (EFIC). Cruciate ligaments of Wister rat embryos between embryonic day (E) 16 and E20 were analyzed. Samples were sectioned and visualized using EFIC. 3D reconstructions were generated using Amira software. The length of the cruciate ligaments, distances between attachment points to femur and tibia, angles of the cruciate ligaments and the cross angle of the cruciate ligaments were measured. The shape of cruciate ligaments was clearly visible at E17. The lengths of the anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) increased gradually from E17 to E19 and drastically at E20. Distances between attachment points to the femur and tibia gradually increased. The ACL angle and PCL angle gradually decreased. The cross angle of the cruciate ligaments changed in three planes. The primordium of the 3D structure of rat cruciate ligaments was constructed from the early stage, with the completion of the development of the structures occurring just before birth.

The embryogenesis of the equine femorotibial joint: The equine interzone

REASONS FOR PERFORMING STUDY

Articular cartilage regeneration is the focus and goal of considerable research effort. Since articular chondrocytes descend from a distinct cohort of progenitor cells located in embryonic nascent joints (interzones), establishing the timing of equine interzone formation is an essential first step towards understanding equine joint and articular cartilage development.

OBJECTIVES

To establish the time frame during which the equine femorotibial interzone forms.

STUDY DESIGN

Descriptive anatomical study.

METHODS

Equine embryos were harvested at 37 (E37), 40, 42, 45, 50 and 65 days' gestation. The femorotibial interzone was examined using high-resolution episcopic microscopy of E37, E42, E45, E50 and E65. Additional histology and collagen-II-immunohistochemistry were performed on E42.

RESULTS

At E37, the femorotibial interzone is first visible as a uniform layer, while at E42 the interzone is fully formed and consists of 3 morphologically distinct layers. The first evidence of cavitation was seen at E45. At E50, the cruciate ligaments were well formed and by E65, joint formation appeared complete.

CONCLUSIONS

The embryogenesis of the equine femorotibial joint is similar to the developmental timeline of stage-matched human and murine embryos. Further studies looking at interzone formation on a cellular and molecular level may further our understanding of the intricate developmental patterns and pathways of articular cartilage development.

Surgical treatment of a rare isolated bilateral agenesis of anterior and posterior cruciate ligaments

The isolated bilateral agenesis of both cruciate ligaments is a rare congenital disorder. A 17-year-old male came to our attention due to an alteration in gait pattern, pain, and tendency to walk on the forefoot with his knee flexed. The patient did not recall previous injuries. Upon physical examination anterior and posterior chronic instability were observed. Radiographic examination of both knees showed hypoplasia of the tibial eminence, a hypoplastic lateral femoral condyle, and a narrow intercondylar notch. MRI brought to light a bilateral agenesis of both posterior cruciate ligaments. Arthroscopic evaluation confirmed bilateral isolated agenesis of both cruciate ligaments. We recommended a rehabilitation program to prepare the patient for the arthroscopic construction of both cruciate ligaments.

The fibular collateral ligament of the knee: a detailed review

The fibular collateral ligament (FCL) is one of the larger ligaments of the knee. The FCL, along with the popliteus tendon, arcuate popliteal ligament, and joint capsule, make up the posterolateral corner of the knee. Recently, there has there been an increased awareness and research on the structures of the posterolateral corner of the knee, particularly the FCL. Studying the detailed structure of the FCL may provide a better understanding that can lead to better diagnosis and treatments following injury. Therefore, this article reviews the FCL, which appears to be the primary restraint to varus rotation but is poorly oriented to resist external rotation of the knee.

Toward regeneration of articular cartilage

Articular cartilage is classified as permanent hyaline cartilage and has significant differences in structure, extracelluar matrix components, gene expression profile, and mechanical property from transient hyaline cartilage found in the epiphyseal growth plate. In the process of synovial joint development, articular cartilage originates from the interzone, developing at the edge of the cartilaginous anlagen, and establishes zonal structure over time and supports smooth movement of the synovial joint through life. The cascade actions of key regulators, such as Wnts, GDF5, Erg, and PTHLH, coordinate sequential steps of articular cartilage formation. Articular chondrocytes are restrictedly controlled not to differentiate into a hypertrophic stage by autocrine and paracrine factors and extracellular matrix microenvironment, but retain potential to undergo hypertrophy. The basal calcified zone of articular cartilage is connected with subchondral bone, but not invaded by blood vessels nor replaced by bone, which is highly contrasted with the growth plate. Articular cartilage has limited regenerative capacity, but likely possesses and potentially uses intrinsic stem cell source in the superficial layer, Ranvier's groove, the intra-articular tissues such as synovium and fat pad, and marrow below the subchondral bone. Considering the biological views on articular cartilage, several important points are raised for regeneration of articular cartilage. We should evaluate the nature of regenerated cartilage as permanent hyaline cartilage and not just hyaline cartilage. We should study how a hypertrophic phenotype of transplanted cells can be lastingly suppressed in regenerating tissue. Furthermore, we should develop the methods and reagents to activate recruitment of intrinsic stem/progenitor cells into the damaged site.

The role of hind limb tendons in gibbon locomotion: springs or strings?

Tendon properties have an important effect on the mechanical behaviour of muscles, with compliant tendons allowing near-isometric muscle contraction and facilitating elastic energy storage and recoil. Stiff tendons, in contrast, facilitate rapid force transfer and precise positional control. In humans, the long Achilles tendon contributes to the mechanical efficiency of running via elastic energy storage and recovery, and its presence has been linked to the evolution of habitual bipedalism. Gibbons also possess relatively long hind limb tendons; however, their role is as yet unknown. Based on their large dimensions, and inferring from the situation in humans, we hypothesize that the tendons in the gibbon hind limb will facilitate elastic energy storage and recoil during hind-limb-powered locomotion. To investigate this, we determined the material properties of the gibbon Achilles and patellar tendon in vitro and linked this with available kinematic and kinetic data to evaluate their role in leaping and bipedalism. Tensile tests were conducted on tendon samples using a material testing machine and the load-displacement data were used to calculate stiffness, Young's modulus and hysteresis. In addition, the average stress-in-life and energy absorption capacity of both tendons were estimated. We found a functional difference between the gibbon Achilles and patellar tendon, with the Achilles tendon being more suitable for elastic energy storage and release. The patellar tendon, in contrast, has a relatively high hysteresis, making it less suitable to act as elastic spring. This suggests that the gibbon Achilles tendon might fulfil a similar function as in humans, contributing to reducing the locomotor cost of bipedalism by acting as elastic spring, while the high stiffness of the patellar tendon might favour fast force transfer upon recoil and, possibly, enhance leaping performance.

Tensile properties, collagen content, and crosslinks in connective tissues of the immature knee joint

BACKGROUND

The major connective tissues of the knee joint act in concert during locomotion to provide joint stability, smooth articulation, shock absorption, and distribution of mechanical stresses. These functions are largely conferred by the intrinsic material properties of the tissues, which are in turn determined by biochemical composition. A thorough understanding of the structure-function relationships of the connective tissues of the knee joint is needed to provide design parameters for efforts in tissue engineering.

METHODOLOGY/PRINCIPAL FINDINGS

The objective of this study was to perform a comprehensive characterization of the tensile properties, collagen content, and pyridinoline crosslink abundance of condylar cartilage, patellar cartilage, medial and lateral menisci, cranial and caudal cruciate ligaments (analogous to anterior and posterior cruciate ligaments in humans, respectively), medial and lateral collateral ligaments, and patellar ligament from immature bovine calves. Tensile stiffness and strength were greatest in the menisci and patellar ligament, and lowest in the hyaline cartilages and cruciate ligaments; these tensile results reflected trends in collagen content. Pyridinoline crosslinks were found in every tissue despite the relative immaturity of the joints, and significant differences were observed among tissues. Notably, for the cruciate ligaments and patellar ligament, crosslink density appeared more important in determining tensile stiffness than collagen content.

CONCLUSIONS/SIGNIFICANCE

To our knowledge, this study is the first to examine tensile properties, collagen content, and pyridinoline crosslink abundance in a direct head-to-head comparison among all of the major connective tissues of the knee. This is also the first study to report results for pyridinoline crosslink density that suggest its preferential role over collagen in determining tensile stiffness for certain tissues.

Isolation and characterization of human anterior cruciate ligament-derived vascular stem cells

The anterior cruciate ligament (ACL) usually fails to heal after rupture mainly due to the inability of the cells within the ACL tissue to establish an adequate healing process, making graft reconstruction surgery a necessity. However, some reports have shown that there is a healing potential of ACL with primary suture repair. Although some reports showed the existence of mesenchymal stem cell-like cells in human ACL tissues, their origin still remains unclear. Recently, blood vessels have been reported to represent a rich supply of stem/progenitor cells with a characteristic expression of CD34 and CD146. In this study, we attempted to validate the hypothesis that CD34- and CD146-expressing vascular cells exist in hACL tissues, have a potential for multi-lineage differentiation, and are recruited to the rupture site to participate in the intrinsic healing of injured ACL. Immunohistochemistry and flow cytometry analysis of hACL tissues demonstrated that it contains significantly more CD34 and CD146-positive cells in the ACL ruptured site compared with the noninjured midsubstance. CD34+CD45- cells isolated from ACL ruptured site showed higher expansionary potentials than CD146+CD45- and CD34-CD146-CD45- cells, and displayed higher differentiation potentials into osteogenic, adipogenic, and angiogenic lineages than the other cell populations. Immunohistochemistry of fetal and adult hACL tissues demonstrated a higher number of CD34 and CD146-positive cells in the ACL septum region compared with the midsubstance. In conclusion, our findings suggest that the ACL septum region contains a population of vascular-derived stem cells that may contribute to ligament regeneration and repair at the site of rupture.

Anatomic single- and double-bundle anterior cruciate ligament reconstruction, part 1: Basic science

Anterior cruciate ligament reconstruction is a frequently performed orthopaedic procedure. Although short-term results are generally good, long-term outcomes are less favorable. Thus, there is renewed interest in improving surgical techniques. Recent studies of anterior cruciate ligament anatomy and function have characterized the 2-bundle structure of the native ligament. During non-weightbearing conditions, the anteromedial (AM) and posterolateral (PL) bundles display reciprocal tension patterns. However, during weightbearing, both the AM and PL bundles are maximally elongated at low flexion angles and shorten significantly with increasing knee flexion. Conventional single-bundle reconstruction techniques often result in nonanatomic tunnel placement, with a tibial PL to a femoral "high AM" tunnel position. In vitro studies have demonstrated that these nonanatomic single-bundle reconstructions cannot completely restore normal anterior-posterior or rotatory laxity. Cadaveric studies suggest that anatomic single-bundle and anatomic double-bundle reconstruction may better restore knee stability. Although many cadaver studies suggest that double-bundle reconstruction techniques result in superior stability when compared with single-bundle techniques, others failed to demonstrate a clear benefit of this more complex procedure. Cadaver studies generally do not apply physiologically relevant loads and provide only a "time-zero" assessment that ignores effects of healing and remodeling after anterior cruciate ligament reconstruction. In vivo, dynamic studies offer the most comprehensive assessment of knee function after injury or reconstruction, as they can evaluate dynamic stability during functional joint loading. Studies of knee kinematics during activities such as gait and running suggest that nonanatomic single-bundle anterior cruciate ligament reconstruction fails to restore preinjury knee function under functional loading conditions. Similar studies of more anatomic single- and double-bundle surgical approaches are in progress, and preliminary results suggest that these anatomic techniques may be more effective for restoring preinjury knee function. However, more extensive, well-designed studies of both kinematics and long-term outcomes are warranted to characterize the potential benefits of more anatomic reconstruction techniques for improving long-term outcomes after anterior cruciate ligament reconstruction.

Relationship between human femorotibial joint configuration and the morphometry of the anterior cruciate ligament

INTRODUCTION

Individual variations in the anatomy of the knee joint have been suggested to affect the ability to functionally compensate for ACL insufficiency or to put an individual at an increased risk of ACL injury. These variations include the posterior tibial slope, the concavity of the medial tibial plateau, the convexity of the lateral tibial plateau, and the configuration of the femoral condyles.

METHOD

This anatomical study investigates if there is a correlation between the individual surface geometry of the femorotibial joint and the morphometry of the ACL. These data were assumed to provide evidence whether or not the functional stability of an ACL-insufficient knee may be derived from its radiographic surface geometry. Standardised measurement techniques were used to analyse the surface geometry of 68 human cadaver knees. Data were correlated with the cross-sectional area, the area of insertion and position of the footprint of the ACL and its functional bundles.

RESULTS

Analysis revealed that there was a significant, but weak correlation between the femoral and tibial area of ACL insertion and the depth of the medial and lateral femoral condyle. No correlation was found between the surface geometry of the femorotibial joint and the cross-sectional area of the ACL. The results of this anatomical study suggest that the relationship between the joint surfaces and the morphometry of the ACL primarily is a function of size of the knee joint.

CONCLUSIONS

Based on our results, there is no evidence that the stability of the knee can be derived from its radiographic surface geometry.

The morphometry of patella and patellar ligament during the fetal period

This study aims to determine the development and morphology of the patella and patellar tendon and to obtain morphometric data about these structures during the fetal period. One hundred five human fetuses (55 males and 50 females) aged 9-40 weeks were used in this study. Fetuses were divided into four groups between gestational weeks; Group I (9-12 weeks), Group II (13-25 weeks), Group III (26-37 weeks), and Group IV (38-40 weeks). The patella and patellar ligament were exposed via anatomical dissection; the dimensions (length, width, thickness) and the width of the lateral and medial articular surfaces of the patella, and the length and width of the patellar ligament, were measured using a Vernier's caliper. No significant differences were observed between genders or sides for any of the parameters (P > 0.05), and a significant correlation was found between gestational age and all parameters (P < 0.001). All parameters of the patella and patellar ligament were found to be different statistically between trimesters (P < 0.05). This study reveals the development, morphological changes, and the morphometric measurements of the patella and patellar ligament during the fetal period. We hope that the present results can be useful for future studies.

Current knowledge in the anatomy of the human anterior cruciate ligament

The anterior cruciate ligament (ACL) is one of the most frequently studied structures of the musculoskeletal system and continues to stimulate debate and challenges among researchers and surgeons. The ultimate goal of anatomic reconstruction surgery is to restore the native anatomy as much as possible. However, this requires thorough knowledge of its anatomy. The aim of this article is to review the current knowledge of the anatomy of ACL along with its macrostructural and ultrastructural properties.

The fetal anterior cruciate ligament: an anatomic and histologic study

PURPOSE

The purposes of this study were to better understand the fetal development of the anterior cruciate ligament (ACL); to identify the gross anatomy of the ACL; to perform a complete histologic evaluation of the ligament, particularly with respect to the distinction between bundles; and to evaluate ACL length, diameter, cellularity, vascularity, and insertion sites.

METHODS

By use of 40 intact knee joints of human fetuses, the gross anatomy of the ACL was inspected under a stereomicroscope (n = 40). The histologic evaluation was performed on the sagittal (n = 20) and transverse (n = 10) sections.

RESULTS

The gross observations revealed the presence of 2 distinct bundles: anteromedial (AM) and posterolateral (PL). The femoral origin of each ACL bundle was located in the posterior aspect of the medial surface of the lateral femoral condyle. The footprint of the tibial insertion was ovoid, with the AM bundle located anterior and medial to the PL bundle. The mean length of the ACL was 3.7 mm, the mean width was 1.1 mm, and the mean thickness was 0.9 mm. There was high cellularity, with approximately 5,600 cells/mm2, and intense vascularity. The AM and PL bundles were divided by a well-defined septum. The femoral origin had less dense connective tissue compared with the tibial insertion.

CONCLUSIONS

From the time of fetal development, the ACL is composed of 2 bundles, AM and PL. The gross morphology of the ACL in fetuses is similar to that reported in adults; the histology is diverse in cellularity and vascularity.

CLINICAL RELEVANCE

This study provides useful information about the anatomy and histology of the fetal ACL.

Topographical histology of the posterolateral corner of the knee, with special reference to laminar configurations around the popliteus tendon: a study of elderly Japanese and late-stage fetuses

We investigated the histology of laminar configurations at the posterolateral corners of 40 knees (18 late-stage fetuses, 22 elderly persons). In the fetuses, the deeply located popliteal fascia and the superficially located inferior geniculate vascular sheath were evident in a space between the popliteus tendon (PT) and the biceps femoris and gastrocnemius. Along, and deep to, the popliteal fascia, the popliteus muscle parenchyma developed into both the PT and another dense connective tissue mass. The PT attached to the fibula (i.e., the fetal popliteofibular ligament) and to the base of the lateral meniscus (i.e., the fetal popliteomeniscal fasciculus). This laminar configuration was essentially maintained in the elderly. The fetal dense connective tissue of the popliteus origin seemed to correspond to the adult arcuate ligament. However, because a connective tissue complex (including the PT and the arcuate and popliteofibular ligaments) was often well developed, the complex involved the inferior geniculate vascular sheath and popliteal fascia. A spectrum of variations was found in how thickly the complex developed. Thus, clear separation of the PT from the surrounding tissues often became difficult. Notably, the connective tissue complex could be peeled in different manners, depending on the site and the individual. Therefore, macroscopic variation in the posterolateral corner, including the apparent absence of some ligaments, seemed to be related to the thickness and number of layers developed by the connective tissue complex, possibly due to an adaptation to the individual's mechanical environment during growth and aging.

The proximal attachments of the popliteus muscle: a quantitative study and clinical significance

Controversies about the existence of accessory proximal popliteus muscle attachments can be found in the literature. The aim of this study was to verify the occurrence and width of popliteus attachments on the articular and periarticular structures of the knee joint. The relation of these attachments to tibiofemoral cartilage and meniscus degeneration was also investigated. Forty-two anatomical specimens were dissected. The incidence of accessory proximal attachments was determined and their width measured using a caliper. The fibular attachment of the popliteus was observed in 98% of cases; its mean width was 11 (SD 3) mm. At least one attachment on the lateral meniscus was found in 95% of the specimens, with a mean width of 6 (SD 2) mm. Three types of meniscal attachments of the popliteus could be identified. The severity of meniscus and tibiofemoral cartilage alterations was significantly related to the number of meniscopopliteal fascicles: more severe alterations were seen in knees with fewer meniscopopliteal fascicles. Popliteus attachments on the posterior knee joint capsule (57%), arcuate (90%) and oblique popliteal (79%) ligaments were also observed in most specimens. Popliteus muscle relationships with the posterior cruciate (5%) and meniscofemoral (33%) ligament were less common. In conclusion, accessory popliteus attachments on the fibula, lateral meniscus and arcuate popliteal ligament can be considered constant characteristics. The results of this study suggest a role of the popliteus in the protection of knee menisci and tibiofemoral cartilage.

Morphological and biochemical re-evaluation of the process of cavitation in the rat knee joint: cellular and cell strata alterations in the interzone

To assess the contribution of apoptosis to the mechanism of synovial joint cavitation, and to clarify morphological cellular changes during cavitation, we investigated the development of the rat knee joint by light and electron microscopy, TUNEL methods, and electrophoresis of DNA fragments. Although cavitation occurred within the interzone, which consists of 2 outer and a middle layer termed the intermediate zone, no morphological or biochemical signs of cell death, in particular apoptosis, were seen in the interzone at any embryonic stage. Microscopic and ultrastructural alterations affecting cell differentiation were clearly observed in the interzone, i.e. mesenchymal cells gradually showed elongation, cytoplasmic vacuolation and pyknosis in the intermediate zone where the elongated cells were arranged in parallel in some strata. Some of these cells were further flattened into spindle cells and the number of strata decreased to 2. The rest of the cells were incorporated secondarily into the outer layers, becoming chondroblasts. Collagen fibrils were arranged in a network structure in the outer layers, which obviously differed from the directional pattern parallel to the long axis of elongated cells in the intermediate zone. In addition, the density of collagen fibrils was higher in the outer layers than in the intermediate zone. During cavitation, the initial separation was detected between the elongated cells in the intermediate zone in paraffin sections at E16.5 and the spindle cells in epoxy sections at E18.5. The spindle cells lining the cavity, namely, the surfaces of the epiphysis and meniscus, finally became chondrocytes. The diminution of proteoglycans and collagen fibrils and the synthesis of hyaluronan in the extracellular matrix are now generally believed to be parts of the mechanism for cavitation based on the concept of 'loss of cohesion'. The microscopic and ultrastructural alterations in the interzone seemed to reflect differences in the arrangement and density of collagen fibrils and the developmental condition of the extracellular matrix between layers. Also it did not seem likely that these alterations inhibit the synthesis of hyaluronan at the presumptive joint line because this synthesis takes place at the plasma membrane. Separation between spindle cells should therefore represent the mechanism for developmentally programmed cavitation. Reorganization of the extracellular matrix is probably necessary for the cellular metamorphoses in the interzone involved in the process of cavitation.