Anatomy of the cruciate ligaments and their function in extension and flexion of the human knee joint

The areas of the femoral origin of the cruciate ligaments have approximately the shape of sectors of ellipses, the one for the anterior ligament on the lateral condyle posteroproximally and the one for the posterior ligament on the medial condyle distally. By means of a new technique of dissection, combined with the use of X-rays, the change in distance between the origin and insertion and so the change of tension of single bundles of the ligaments could be analyzed. Only a rather thin bundle in each cruciate ligament is in constant tension: “guiding bundles.” The maximal diminution of distance between the origin and insertion for some bundles is 65%. In the anterior cruciate ligament the majority of fibers are taut in extreme extension: “limiting bundles.” The same is true in the posterior cruciate ligament in extreme flexion. There are also some fibers, especially in the posterior cruciate ligament, that are taut only in an intermediate position. The geometric analysis of the function of different groups of fibers was performed by a modification of Menschik's concept of a four-bar link.     

Structure and vascularization of the cruciate ligaments of the human knee joint

The structure and vascularization of the human anterior and posterior cruciate ligament were investigated by light microscopy, transmission electron microscopy, injection techniques and by immunohistochemistry. The major part of the anterior and posterior cruciate ligament is composed of bundles of type I collagen. Type III collagen-positive fibrils separate the bundles. The major cell type is the elongated fibroblast, lying solitarily between the parallel collagen fibrils. The histologic structure of the cruciate ligaments is not homogeneous. In both ligaments there is a zone where the tissue resembles fibrocartilage. In the anterior cruciate ligament the fibrocartilaginous zone is located 5–10 mm proximal of the tibial ligament insertion in the anterior portion of the ligament. In the posterior cruciate ligament the fibrocartilage is located in the central part of the middle third. Within those zones the cells are arranged in columns and the cell shape is round to ovoid. Transmission electron microscopy reveals typical features of chondrocytes. The chondrocytes are surrounded by a felt-like pericellular matrix, a high content of cellular organelles and short processes on the cell surface. The pericellular collagen is positive for type II collagen. The major blood supply of the cruciate ligaments arises from the middle geniculate artery. The distal part of both cruciate ligaments is vascularized by branches of the lateral and medial inferior geniculate artery. Both ligaments are surrounded by a synovial fold where the terminal branches of the middle and inferior arteries form a periligamentous network. From the synovial sheath blood vessels penetrate the ligament in a horizontal direction and anastomose with a longitudinally orientated intraligamentous vascular network. The density of blood vessels within the ligaments is not homogeneous. In the anterior cruciate ligament an avascular zone is located within the fibrocartilage of the anterior part where the ligament faces the anterior rim of the intercondylar fossa. The fibrocartilaginous zone of the middle third of the posterior cruciate ligament is also avascular. According to Pauwel’s theory of the ”causal histogenesis” (1960) the stimulus for the development of fibrocartilage within dense connective tissue is shearing and compressive stress. In the anterior cruciate ligament this biomechanical situation may occur when the ligament impinges on the anterior rim of the intercondylar fossa when the knee is fully extended. Compressive and shearing stress in the center of the middle third of the posterior cruciate ligament may result from twisting of the fiber bundles. Accepted: 12 March 1999     

An initial qualitative study of dual-energy CT in the knee ligaments

OBJECTIVE: To study the clinical application of dual-energy CT (DECT) in the knee ligaments. METHODS: Twelve cases (24 knees) were scanned using dual-energy CT for the knee. Two- and three-dimensional images were used for display in all cases by means of multi-planar reformation (MPR) and volume rendering technique (VRT). All images were ranked by two radiologists according to the grade of knee ligament displayed, the definition of edge and attachment points of the knee ligament. RESULTS: The partial ligaments of 24 knees, such as the patellar ligament, fibular collateral ligament, anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) were clearly displayed; the tibial collateral ligament was not satisfactorily displayed. The transversal ligaments, such as lateral patellar retinaculum and medial patellar retinaculum, and the posterior ligament, such as oblique popliteal ligament could not be shown clearly. CONCLUSION: The dual-energy CT is a new and valuable tool to qualitatively display the main ligaments of the knee.     

踝关节内侧韧带的应用解剖学研究

目的：探讨踝关节内侧韧带的组成及其各部的附着、走行及毗邻关系。方法：取30例40％甲醛溶液防腐保存的正常成人尸体标本及4侧临床截肢的新鲜成人正常踝关节标本。仔细解剖出踝关节内侧韧带各组成部分的起止点及毗邻关系,然后测量各部的长、宽及厚的数值。结果：踝关节内侧副韧带由深浅两层组成。浅层包括胫舟韧带、胫韧带及胫跟韧带,其中胫韧带和胫跟韧带是恒定的;深层由胫距前、后韧带组成,其中胫距后韧带是恒定的。测量结果显示：胫舟韧带最长,（29．2±3．54）mm;胫距后韧带最宽、最厚,分别是（12．5±3．51）mm和（10．1±2．34）mm。结论：本研究为踝关节内侧韧带损伤的修复重建提供了重要的解剖学基础。     

MRI探究交叉韧带松紧度与髌股关节面损伤相关性的临床应用

目的　探究交叉韧带松紧度与髌股关节面损伤程度的相关性,分析个体交叉韧带长度的解剖差异对髌股关节面损伤的影响,为预防和诊治髌股关节面疾病提供新的依据。 方法　随机收集南方医科大学珠江医院2016年10月至2017年12月110例膝关节MRI资料,磁共振矢状位上测量前、后交叉韧带长度A与B,测量前、后交叉韧带起止点距离La与Lb,计算交叉韧带松紧度M值;根据矢状位与冠状位上评判髌股关节面损伤程度0、I、II、III与IV级并分别设5个组。通过SPSS20.0统计学软件,将所得数据进行One-Way ANOVA方差分析与Spearman相关性分析,探究交叉韧带松紧度与髌股关节面损伤程度的相关性。 结果　One-Way ANOVA方差分析得出各组M值两两之间均存在显著性差异（P<0.05）,Spearman相关性分析显示M值与髌股关节损伤程度间存在正相关性。 结论　交叉韧带松紧度与髌股关节面损伤存在相关性,随着M值越大,髌股关节面损伤程度越严重。     

人工韧带修复膝关节交叉韧带损伤

背景：以往治疗膝关节交叉韧带损伤的主要手段是移植重建,最常用的移植材料为自体的骨髌腱骨、半腱肌腱和股薄肌腱。但由于此类移植物存在取材区并发症及韧带化过程中的各种问题,近年来人工韧带的研究受到重视。 目的：认识膝关节交叉韧带的结构及血供特点,以及膝关节交叉韧带损伤后人工韧带重建治疗机制与临床应用特点。 方法：①分析膝关节前、后交叉韧带的组织结构,功能学特点以及血供差异。②分析膝关节前、后交叉韧带损伤的类型及生物力学机制。③分析修复膝关节交叉韧带损伤的材料学分类及特点。④分析人工韧带修复后影响关节稳定性的因素。 结果与结论：修复膝关节前、后交叉韧带损伤时,应首先考虑到前、后交叉韧带的功能及血供情况,选择合适的重建物,使重建时过程简化,操作简单,重建材料的组织相容性较好,达到修复后的解剖与功能的双重建。     

The change in length of the medial and lateral collateral ligaments during in vivo knee flexion

The collateral ligaments of the knee are important in maintaining knee stability. However, little data has been reported on the in vivo function of the collateral ligaments. The objective of this study was to investigate the change in length of different fiber bundles of the medial collateral ligament (MCL), deep fibers of the MCL (DMCL) and the lateral collateral ligament (LCL) during in vivo knee flexion. The knees of five healthy subjects were scanned using magnetic resonance imaging. These images were used to create three-dimensional models of the tibia and femur, including the insertions of the collateral ligaments. The MCL, DMCL, and LCL were each divided into three equal portions: an anterior bundle, a middle bundle and a posterior bundle. Next, the subjects were imaged from two orthogonal directions using fluoroscopy while performing a quasi-static lunge for 0 degree to 90 degrees of flexion. The models and fluoroscopic images were then used to reproduce the in vivo motion of the knee. From these models, the length of each bundle of each ligament was measured as a function of flexion. The length of the anterior bundle of the MCL did not change significantly with flexion. The length of the posterior bundle of the MCL consistently decreased with flexion (p less than 0.05). The changes in deformation of the DMCL and LCL as a function of flexion were similar to each other. The length of the anterior bundles increased with flexion and the length of the posterior bundles decreased with flexion. These data indicate that the collateral ligaments do not elongate uniformly as the knee is flexed, with different bundles becoming taut and slack. These data may help to provide a better understanding of the in vivo function of the collateral ligaments and be used to improve surgical reconstruction of the collateral ligaments. Furthermore, the data suggest that the different roles of various portions of the collateral ligaments along the flexion path should be considered before releasing the collateral ligaments during knee arthroplasty.     

膝关节交叉韧带和侧副韧带的断面解剖学研究及其意义

目的研究膝关节交叉韧带和侧副韧带的断面形态特征和变化规律,为诊断膝部韧带病变提供更为详尽的形态学资料。方法利用27例正常成人膝关节标本制作连续断面,其中矢状断面9例,冠状断面12例,横断面6例。通过横、矢、冠状断面标本,观测膝关节韧带的断面形态特征及定量测量。结果矢状面上测量前、后交叉韧带长度分别为(29.66±4.21)mm、(40.26±6.81)mm,厚度分别为(10.03±1.97)mm、(11.24±3.50)mm。冠状面上前、后交叉韧带长径分别为(15.18±3.25)mm、(18.79±3.35)mm,短径分别为(6.37±1.32)mm、(8.03±1.46)mm;胫、腓侧副韧带长度分别为(102.85±19.64)mm、(45.52±14.91)mm,厚度分别为(2.63±0.72)mm、(3.43±1.04)mm。髁间隆起的横断面上胫、腓侧副韧带长径分别为(21.98±11.95)mm、(5.25±1.93)mm,短径分别为(2.03±0.59)mm、(2.87±0.64)mm。结论 (1)观测交叉韧带最好的断面是膝关节正中矢状面,其次是正中旁开1个矢状断面。除厚度外,在矢状面上前后交叉韧带长度、股、胫骨附着区宽度均有明显差异。(2)胫、腓侧副韧带在连续的冠状断面及横断面上均可显示,以冠状断面配合横断面相对为佳。     

半月板和交叉韧带矢状断层研究

目的　观察膝关节矢状断层的半月板和交叉韧带的形态、可出现的层面情况并测量有关数据 ,为影像学诊断提供解剖学依据。方法　 11例 (2 2侧 )经福尔马林液防腐的膝关节标本按MRI检查姿势标线 ,冷冻后用断层带锯作厚 5mm的矢状断层 ,获得 314断层 (观察 5 84层面 ) ,对各断层面的半月板和交叉韧带进行观察 ,并用电子游标卡尺进行测量。结果　半月板后角高度明显高于前角 (P <0 0 1) ,内侧半月板后角宽度约是前角的 2倍 ,而外侧半月板宽度前、后角基本一致 ,外侧半月板体部宽于内侧且更靠近中轴 ,盘状半月板主要发生于外侧 ;正中矢状面显示交叉韧带全长的出现率为 82 % ,其全长的最好层面为正中矢状面和正中旁 1cm层面 ;前交叉韧带与矢状面的夹角较后交叉韧带小 ;前、后交叉韧带在股骨和胫骨的附着点宽约 12mm ,各附着点宽差异无显著性 (P >0 0 5 )。结论　膝关节矢状断层形态学研究提示半月板的后角容易损伤 ;外侧半月板后角明显较内侧窄而高 ,此为临床所见外侧半月板较内侧半月板更易损伤的形态学基础。     

基于MRI对交叉韧带松紧度与半月板损伤的相关性研究

目的　探究交叉韧带的松紧度与半月板损伤的相关性,从解剖学上探索半月板损伤的影响因素,为预测和诊断半月板损伤提供新的依据。 方法　收集南方医科大学珠江医院2014年1月~2016年12月260例患者单膝关节伸直位的MRI资料。在各膝MRI相同矢状面分别测量前、后交叉韧带的长度a和p,及其起止点的距离la和lp。计算出交叉韧带松紧度系数R=(a+p)/(la+lp)。用t检验比较各组患者间的差异性,用Spearman 相关分析分别探讨松紧度系数与半月板损伤的相关性。 结果　 t检验：无论是否存在骨关节炎,半月板损伤组和无半月板损伤组的R值均存在显著差异性（P <0.05）。Spearman相关性分析：无骨关节炎患者中rs=0.620,R值与半月板损伤存在较强的正相关性;有骨关节炎患者中rs =0.313,R值与半月板损伤存在弱正相关性。 结论　交叉韧带松紧度与半月板损伤存在一定程度的相关性,特别在膝关节无骨关节炎的年轻患者中,韧带越松弛,半月板损伤越容易发生。但在膝关节有关节炎的老年患者中,交叉韧带的松紧程度并非半月板损伤的主要因素。     

Retain or sacrifice the posterior cruciate ligament in total knee arthroplasty? A histopathological study of the cruciate ligament in osteoarthritic and rheumatoid disease

BACKGROUND: The decision whether to retain or resect the posterior cruciate ligament in total knee arthroplasty is at present determined clinically by preoperative radiological variables focusing upon the amount of joint destruction, and subsequent soft tissue contractures. However, these variables give only indirect information on the histological integrity and proprioceptive properties of the posterior cruciate ligament. METHODS: Twenty posterior cruciate ligaments, obtained during total knee arthroplasty, were evaluated histologically to study the relation between the degree of preoperative radiological joint destruction, structural integrity of the posterior cruciate ligament and the neurological integrity of the targeted tissue. Eleven patients had osteoarthritis and nine patients rheumatoid arthritis. Haematoxylin and eosin, Alcian blue (mucoid degeneration), elastica von Gieson, Gomori (elastic fibres and collagen), and immunohistochemical staining for neural structures were used. RESULTS: In all but one of the posterior cruciate ligaments, morphologically intact neural tissue was present in the peritendineum of the ligaments. Structural integrity of the collagen framework was present in only seven posterior cruciate ligaments. These cases all had grade three or four radiological joint destruction. In 13 of the specimens a certain degree of mucoid degeneration of collagen was present. All patients with grade five radiological knee joint destruction displayed mucoid degeneration and irregularity of the posterior cruciate ligament fibres. CONCLUSION: Because of the extensive architectural and probably functional damage of the posterior cruciate ligament in patients who have grade five radiological knee joint destruction, retention of the posterior cruciate ligament in knee prosthesis should not be advocated.     

Erratum to "The change in length of the medial and lateral collateral ligaments during in vivo knee flexion"

The collateral ligaments of the knee are important in maintaining knee stability. However, little data has been reported on the in vivo function of the collateral ligaments. The objective of this study was to investigate the change in length of different fiber bundles of the medial collateral ligament (MCL), deep fibers of the MCL (DMCL) and the lateral collateral ligament (LCL) during in vivo knee flexion. The knees of five healthy subjects were scanned using magnetic resonance imaging. These images were used to create three-dimensional models of the tibia and femur, including the insertions of the collateral ligaments. The MCL, DMCL, and LCL were each divided into three equal portions: an anterior bundle, a middle bundle and a posterior bundle. Next, the subjects were imaged from two orthogonal directions using fluoroscopy while performing a quasi-static lunge from 0 degrees to 90 degrees of flexion. The models and fluoroscopic images were then used to reproduce the in vivo motion of the knee. From these models, the length of each bundle of each ligament was measured as a function of flexion. The length of the anterior bundle of the MCL did not change significantly with flexion. The length of the posterior bundle of the MCL consistently decreased with flexion (p < 0.05). The change in length of the DMCL with flexion was similar to the trend observed for the MCL. The length of the anterior bundle of the LCL increased with flexion and the length of the posterior bundle decreased with flexion. These data indicate that the collateral ligaments do not elongate uniformly as the knee is flexed, with different bundles becoming taut and slack. These data may help to provide a better understanding of the in vivo function of the collateral ligaments and be used to improve surgical reconstructions of the collateral ligaments. Furthermore, the data suggest that the different roles of various portions of the collateral ligaments along the flexion path should be considered before releasing the collateral ligaments during knee arthroplasty.     

An in vitro biomechanical comparison of anterior cruciate ligament reconstruction: single bundle versus anatomical double bundle techniques

INTRODUCTION: Anterior cruciate ligament ruptures are frequent, especially in sports. Surgical reconstruction with autologous grafts is widely employed in the international literature. Controversies remain with respect to technique variations as continuous research for improvement takes place. One of these variations is the anatomical double bundle technique, which is performed instead of the conventional single bundle technique. More recently, there has been a tendency towards positioning the two bundles through double bone tunnels in the femur and tibia (anatomical reconstruction). OBJECTIVES: To compare, through biomechanical tests, the practice of anatomical double bundle anterior cruciate ligament reconstruction with a patellar graft to conventional single bundle reconstruction with the same amount of patellar graft in a paired experimental cadaver study. METHODS: Nine pairs of male cadaver knees ranging in age from 44 to 63 years were randomized into two groups: group A (single bundle) and group B (anatomical reconstruction). Each knee was biomechanically tested under three conditions: intact anterior cruciate ligament, reconstructed anterior cruciate ligament, and injured anterior cruciate ligament. Maximum anterior dislocation, rigidity, and passive internal tibia rotation were recorded with knees submitted to a 100 N horizontal anterior dislocation force applied to the tibia with the knees at 30, 60 and 90 degrees of flexion. RESULTS: There were no differences between the two techniques for any of the measurements by ANOVA tests. CONCLUSION: The technique of anatomical double bundle reconstruction of the anterior cruciate ligament with bone-patellar tendon-bone graft has a similar biomechanical behavior with regard to anterior tibial dislocation, rigidity, and passive internal tibial rotation.     

The menisco-femoral ligaments

The menisco-femoral ligaments were studied in 60 knees from 30 dissecting room cadavers. The anterior horns of the menisci were attached to the intercondylar area of the femur by discrete antero-medial or antero-lateral menisco-femoral ligaments, separate from the anterior cruciate ligament, in 15% of knees for each meniscus, more frequently than previously appreciated; these anterior horn ligaments may exacerbate a meniscal tear. The posterior horn of the lateral meniscus was connected with the intercondylar area of the femur in 100% of knees. In 93% of knees a ligament ran behind the posterior cruciate ligament while in 33% of knees a ligament ran in front of the posterior eruciate ligament. We propose renaming these the pre-cruciate postero-lateral menisco-femoral ligament and post-cruciate postero-lateral menisco-femoral ligament, respectively, to avoid confusion with the ligaments of the anterior horns. The menisco-femoral ligaments may function in controlling movement of the menisci, especially during rotation of the knee. The posterior horn of the medial meniscus has no direct femoral attachment and this may be a factor in the increased risk of injury to this meniscus. © 1995 WiIey-Liss, Inc.     

足韧带的解剖学研究及其临床意义

目的:研究足韧带的解剖学特点,探讨其临床意义。方法:30侧成人足标本解剖观测各韧带起至、走行和比邻,分析其作用。结果:距舟背侧韧带分为内、外两束。楔舟背侧韧带分为内、中、外、斜4束。足横弓和纵弓交汇处为足底最凹点,足底长短韧带、腓骨长肌腱、胫骨后肌腱为"外三角",楔舟足底韧带、楔骰足底韧带、跟舟足底韧带为"内三角",维系该凹点。跟舟足底韧带承托距骨头部,是将踝压力分向第1跖骨头和足跟的首要韧带。各楔骨、楔骰骨间韧矢状面上都位于关节前侧,不规则形,质地强韧,维系足横弓。结论:足部韧带分为足背、足底和骨间3个系统,结构复杂,其功能及其在创伤外科中的意义有待深入研究。     

Fiber orientation of posterior cruciate ligament: An experimental morphological and functional study,part 2

The posterior cruciate ligament (PCL) can be anatomically divided into three bundles: anterolateral, posteromedial, and posterior oblique. The changes in distance between the femoral and tibial attachment sites of these three bundles were measured in 10 human knee specimens with intact ligamental structures. The femoral to tibial distance (and thus the length) of the posterior oblique bundle remained nearly the same throughout flexion between 0° and 90°. The femoral to tibial distance of the anterolateral and the posteromedial bundles distinctly changed throughout the same range of motion. For a truly functional replacement of the PCL, correct isometric placement of the transplant is especially important. Based on the results of the present study, an isometric reconstruction of the PCL is achieved by positioning the graft within the original attachment site of the posterior oblique bundle. © 1995 WiIey-Liss, Inc.     

3．OTMRI 3D扫描对膝横韧带的观察

目的研究3．OTMRI3D扫描对膝横韧带的显示价值。方法回顾性分析利用3．0TMRI进行膝关节3D扫描的137例患者资料,共152个膝关节．除外严重外伤、肿瘤、关节感染及大量关节积液患者,共90人、100个膝关节纳入研究,平均年龄42岁．男性42位,女性48位。通过横断面及曲面重建观察并测算膝横韧带的出现率、长度、宽度、N-L分型、并测量其后缘与前交叉韧带胫骨附着点前缘的距离。结果共观察到膝横韧带54例,出现率54％;平均长度36．99mm（范围18．8-48．8mm）,平均宽度1．4mm（范围约0．80-2．53 mm）,N-L分型中,Ⅰ型、Ⅱ型、Ⅲ型所占比例分别为59．3％、25．9％、14．8％;其后缘与前交叉韧带胫骨附着点前缘的距离5．25mm（范围1．4-8．6mm）。结论3.0T MRI 3D扫描可从不同方位显示膝横韧带并对其解剖特点进行细致观察。     

计算机辅助技术在骨关节疾病中的应用

背景：以往采用二维图像资料如X射线片、CT、MRI扫描等进行骨关节病手术设计,在反映骨关节病变严重程度、病变位置和畸形情况等方面不全面、欠准确,而且缺少直观性。 目的：研究计算机辅助设计和快速成型技术辅助骨关节伤病手术治疗的新方法。 方法：按反求工程的基本原理,采用医学CT/MRI扫描获取106例骨伤病患者骨骼二维图像资料,采用计算机辅助三维重建建立骨、关节解剖模型,将骨、关节解剖模型输入CAD软件进行精确分析,进一步采用快速成型技术制作骨关节原型进行实物原型分析,然后将骨关节解剖模型输入计算机进行外科手术过程设计、预演,选择合适的内固定材料,计算机辅助设计、快速成型技术制作外科手术辅助模板、个性化植入物等,最后精确实施骨关节外科手术。 结果与结论：31例骨关节畸形患者术后恢复良好的解剖外形和功能;17例前后交叉韧带损伤患者术后膝关节功能良好;31例骨折患者术后3~6个月获得骨折愈合;7例个性化假体和8例内固定重建肿瘤切除后骨缺损患者,随访期间未发现内固定器械断裂、假体松动和肿瘤复发;12例髋臼发育不良患者术后恢复正常髋臼包容和良好髋关节功能。提示计算机辅助技术可应用于骨关节畸形精确数字化矫形,设计个性化假体,辅助前后交叉韧带重建,疑难假体置换,辅助特殊疑难骨折、关节内骨折、陈旧性骨折复位、固定,骨肿瘤的个性化切除设计、结构与功能重建。     

保留前后交叉韧带人工全膝关节假体的胫骨近端解剖测量

背景：目前使用的保留前后叉韧带的假体仍存在诸多问题。改良和设计保留前后交叉韧带膝关节假体相关的形态测量,特别是胫骨近端未曾见到过详细的报道和研究。 目的：通过对正常膝关节三维数字化模型进行相关的胫骨近端形态学的分析,从而为改良和设计保留前后交叉韧带人工全膝关节假体提供参考。 方法：40例正常志愿者进行膝关节的CT及MRI扫描,然后将膝关节CT、MRI断层影像数据导入Mimics 10.01进行图像分割与三维重建,重建出完整的膝关节三维实体数字化模型后再导入Geomagic Studio 11,在膝关节模型上测量前后交叉韧带在胫骨近端解剖参数,模拟截骨后测量胫骨近端截骨面的解剖参数。 结果与结论：前后交叉韧带总的宽度为(14.94±2.56) mm,性别间比较提示男性需适当增加横向的截骨宽度以避免损伤前后交叉韧带。胫骨髁间棘顶部的长度为(8.02±1.03) mm,底部的长度为(15.19± 1.71) mm,截骨后高度(9.13±0.88) mm,而且此梯形结构位于前后径48%-82%。胫骨截骨面内侧的内外径短于外侧的内外径,内侧前后径大于外侧的前后径,因而保留前后交叉韧带的假体更需要非对称性的设计。     

The restraining function of the cruciate ligaments on hyperextension and hyperflexion of the human knee joint

Each of the cruciate ligaments contains functionally different fiber groups; one fiber bundle is always taut; numerous others are taut in intermediate or extreme positions. The bulk of the fibers of the anterior cruciate ligament (ACL) is taut in maximal extension, while that of the posterior cruciate ligament (PCL) is taut in the intermediate positions and in maximal flexion. Fibers taut in extreme positions serve as restraints: during hyperextension, the ACL restrains forward migration of its tibial attachment, while the PCL interacts with other structures to prevent posterior opening of the joint. The inverse situation occurs in hyperflexion. Cruciate fibers are dissimilar in length and angular arrangement so that, when movements are restrained, they lengthen to different extents. To define this phenomenon in quantitative terms, the term isokolyons was coined for lines from which fibers showing identical elongation in percentage on exposure to a force take their origin.