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Reconstruction of the anterior cruciate ligament B B Seedhom, PhD, BSc Rheumatology and Rehabilitation Research Unit, University of Leeds, Clarendon Road, Leds Ligaments are strong collagenous structures that act as constraints on joint motion, thus confining the articular surfaces to more or less the same paths. In so doing they prevent arbitrary apposition of these surfacesfrom occurring and resulting in abnormal stresses which may damage the joint surfaces. Ligaments rupture due to excessive loads, particularly those resulting from trauma occurring during sporting events or motor vehicle accidents. Knee and ankle joints have the highest frequency of ligamentous injuries. This paper is a brief review of the current approaches to the reconstruction of the knee ligaments with spec@ reference to the anterior cruciate ligament (ACL) being the most frequently reconstructed. This is not only because it is frequently injured but also because of the debilitating consequences of such an injury. Approaches ranging from the conservative to those that advocate the use of frank prosthetic replacement have been adopted by surgeons at both ends of the spectrum. Following a discussion of the rationale for reconstruction of the ACL, the mechanical and biological considerations of the reconstructive procedure are discussed. The diflerent methods of ACL reconstruction are reviewed. These include: (a) primary repair, (b) reconstruction with different tissues, including autogenous allografts and xenografts, (c) reconstruction employing diflerent synthetic devices. A brief discussion of the procedures used for reconstruction with direrent types of tissue and of the surviving examples of the synthetic devices will follow.

1 INTRODUCTION

1.1 Ligaments and their function Ligaments are band or cord-like structures of dense, highly oriented connective tissue which link bones in the vicinity of every synovial joint. They consist mainly of pure type I collagen, with fibres arranged along the length of the ligament, that is in the direction of the acting load. In the resting state the collagen fibre bundles have a wavy appearance; they are crimped. This feature explains the characteristic non-linear J-curve response of ligamentous structures to tensile load (Fig. 1). The function of ligaments is to maintain the kinematics of a joint. This is achieved by a combination of a passive mechanical role and a proprioceptive one. In respect of the first role, ligaments act as constraints on the movement of the bones in different directions and about different axes of rotation. In respect of the second, it has been shown that ligaments have a rich sensory innervation that allows them to act as the first link in the ‘kinetic chain’. Impulses arising in the ligaments are transmitted through the central nervous system and back to the effector muscles, allowing for maintenance of normal, smooth, co-ordinated motion of the joint. Abnormally strong impulses, such as those initiated when a ligament is overstretched, result in contraction of allied muscle groups, thereby protecting the ligaments and preventing further injury and subluxation of the knee. The subject is vast and the reader is referred to the excellent review of the subject by Barrack and Skinner (1). 1.2 Knee ligaments-functional anatomy

This part of the review concentrates on the reconstruction of knee ligaments with specific reference to the ACL. This is because the ACL is the most frequently

injured ligament in the knee, and consequently the most frequently subject to reconstruction with numerous procedures. Only a brief account is given here of the knee ligaments anatomy and their function. For more detail the reader is referred to anatomical textbooks as well as many specialist textbooks readily available.

Fig. 1 The crimped collagen structure of ligaments is responsible for its non-linear behaviour. At low loads (position a) the crimped collagen structure allows large extensions for small load increments. At high loads (position b) the crimp has disappeared,resulting in a stiffer structure, where similar increments in load

result in smaller extensions

The M S wus received on I! March 1992. H01392 0 IMechE 1992

0954-4119/92 $3.00 + .05

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Four major ligaments contribute to the stability of the knee. They can be split into two groups: the cruciates and the collateral ligaments. Other smaller ligaments exist in the knee, whose size (and even existence) varies from person to person. These have little independent relevance to the stability of the knee and can usually be considered as part of the capsule. The cruciate ligaments are attached in the intercondylar space of the joint (Fig. 2). The anterior cruciate ligament is attached to the anterior part of the intercondylar area of the tibia. It passes upwards, backwards and then fans out to its attachment on the posterior part of the lateral condyle on the femur. The locations of attachments of the cruciates on the tibia are also illustrated in Fig. 2, from which it can be seen that the anterior cruciate ligament is important in preventing anterior movement of the tibia relative to the femur. The opposite is true of the posterior cruciate ligament. It is attached to the posterior part of the tibia near the intercondylar area. It runs upwards and forwards to its insertion on the anterior part of the medial condyle of the femur. Viewed from the side (Fig. 3) the cruciate ligaments cross each other. In this position it can be clearly seen that the posterior cruciate ligament conFemur

Posterior cruciate

strains the movement of the tibia relative to the femur in the posterior direction. The cruciates have secondary restraining functions. As the tibia is rotated internally the cruciates twist around each other more and tighten, and thus have a restraining effect upon internal rotation of the tibia on the femur. They do not have any effect on the external rotation as it causes them to ‘untwist’and become lax. The collateral ligaments are attached to the medial and lateral sides of the knee respectively. The medial collateral ligament strengthens the medial aspect of the capsule. It is a broad, flat band nearer to the back than to the front of the joint. It runs from the medial femoral condyle to the 9edial condyle and surface of the shaft of the tibia. The lateral collateral ligament reinforces the lateral aspect of the capsule. It runs from the lateral femoral condyle to the head of the fibula. The ligament takes the form of a strong, round cord. The medial ligament is responsible for valgus stability and the lateral ligament for the varus stability. The medial collateral ligament is the stronger of the two and, therefore, has a greater stabilizing effect on the knee. Again, these ligaments have secondary stabilizing functions. The fibres in the ligaments and capsule lie at an angle to the joint line. This makes the collateral ligaments important contributors to rotational stability in the joint, the medial ligament being capable of resisting internal rotation of the tibia on the femur as well as external rotation, and the lateral ligament is sometimes capable of resisting external rotation. Table 1 summarizes the functions of the four main ligaments. 1.3 The cruciates, kinematics of the knee and its

geometry

Lateral collateral -Medial meniscus

---Patellar

ligament

Posterior cruciate ligament

,/

/

Lateral collateral ligament

\I

Lateral meniscus -

Anterior cruciate-”-ligament

cdial mcniscus

Fig. 2 Anatomy of the knee showing the four main ligaments and menisci

Although the tibia rotates about its long axis and also about a perpendicular axis in the antero-posterior direction, knee movement occurs largely in the sagittal plane. This justifies the frequently employed two-dimensional model of the knee considered in motion and force analysis. Strasser (2), Kapandji (3), Menschik (4) and O’Connor et al. (5) showed that the basic kinematic principle of knee motion can be represented by the mechanism of a crossed four-bar linkage. This is best illustrated in Fig. 3. The solid line superimposed on the diagram is a four-bar linkage comprising the anterior cruciate ligament (ab), the posterior cruciate ligament (cd), the femoral link (cb) joining the ligament attachments on the femur and the tibia1 link (ad) joining their attachments to the tibia. An excellent analysis of this model was carried out by O’Connor et al. (5), who demonstrated the validity of the four-bar linkagc mechanism. They constructed a computer model of the system, by which they deduced the shape of one articular surface from given parameters, such as the length of the cruciate ligaments and the shape of the complemen-

Table 1 The functions of the four main ligaments Ligament

Primary restraint to Secondary restraint to

Part H : Journal of Engineering in Medicine

Anterior cruciate

Posterior cruciate

Medial collateral

Lateral collateral

Anterior drawer Internal rotation Varus bending

Posterior drawer Internal rotation Valgus bending

Valgus bending Internal rotation Anterior drawer

Varus bending External rotation Posterior drawer @ IMechE 1992

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Recently, Biden et al. (12) investigated knee ligament forces during walking and from their work they reported maximum ACL forces ranging from 0.7 to 1.7 times body weight. These results are not realistic in that they approach the maximum loads recorded at rupture when subjecting a cadaveric ACL to tensile loads. 1.5 Mechanical properties

Anterior cruciate ligament Posterior cruciate ligament

Fig. 3 The basic kinematic principle of knee motion is represented by a crossed four-bar linkage. The cruciates are two of the links; the femur and tibia are the other two

tary articulating surfaces. The deduced shapes compared closely with the normal anatomical shapes of the corresponding parts. The concept of this close relationship between the length of ligaments, the positions of their attachments to bone, movement and geometry of the articulating surfaces was commented upon earlier by Fick (6). 1.4 Forces in the ACL

Surprisingly few data exist in the literature that relate in vivo estimates of forces acting in knee ligaments. Morrison (7-9) estimated the maximum forces in the ACL and other knee structures for a number of typical daily activities. Forces in the ACL were estimated to be 196 N for level walking, 72 N for ascending stairs, 93 N for descending stairs, 67 N for ascending a ramp and 445 N for descending a ramp (9). On the other hand, Grood and Noyes (10) used two different methods for making estimates of the ACL forces. The first was based on a factor of safety of five for the structures of the joint. Having measured the strength of the anterior cruciate ligament in the laboratory from cadavers of young and old humans, they estimated values of 200-400 N for young humans and 80-160 N for older humans. The factor of safety was an assumption on the authors' part, which they deemed reasonable. They also approximated the in uiuo forces by measuring the forces necessary to elongate the ACL by 10 per cent. These estimates yielded 249 N for young individuals and 160 N for older humans. The figure for elongation was selected by the authors because Wang et al. (11) measured comparable elongation in vitro for intact knees. @ IMechE 1992

Laboratory measurements on various ligaments have been carried out to determine their mechanical properties. Where human ligaments are concerned, more attention has been given to knee ligaments. Noyes and Grood (13) found that the anterior cruciate ligament (ACL) had a maximum tensile strength of 1725 (f269) N and a stiffness of 182 ( L 33) N/mm in young humans (16-26 years). Both the strength and stiffness were found to be lower for older humans. This has been confirmed by the work of Rauch et al. (14) and Hollis et al. (15). One major difficulty with ligament strength measurements is that of holding them satisfactorily in the gripping jaws of a testing machine. Not only are the structures of the ligaments complex, but so also are their attachments to bone. Some researchers have resorted to separate and test individual bundles of a ligament (16), but the majority of researchers have resorted to testing the whole structure of a ligament with both of its bony attachments intact. This approach presents its difficulties, since the results obtained are test-condition related. Using this method Woo et al. (17) showed a considerable difference in the strength of the ACL when the joint was loaded along the tibia1 axis and when the load was applied along the ligament axis with the knee held at 90" flexion. 2 INCIDENCE OF ACL INJURY

Ruptured ACL seems to have been observed more frequently in recent times, and this may be due to two reasons. The first is the greater interest in sport, combined with greater and more intense competitiveness among sporting individuals, resulting in more injuries. The second is the increased skill in clinical examination and available technical aids to diagnosis. Although there are no official statistics available on the number of injuries to the ACL, in 1982 the United States declared such knee injuries a national health problem! An estimated 200000 knee injuries occur in the United States each year of which 120000 receive surgery. The worldwide figures are likely to be much higher. 2.1 Is there a case for reconstruction of every

incompetent ACL? Many surgeons still maintain a conservative approach; their view is that a patient with a ruptured ACL should not be operated on unless the situation deteriorates and functional instability is experienced. However, from a mechanical viewpoint, a case could be made for the reconstruction of a ruptured ACL whether or not the patient is athletic. The main argument for this view is the vulnerability of the knee joint without the ACL to further injury to other structures such as the menisci and to consequent degenerative osteoarthrotic changes. Proc Instn Mech Engrs Vo1206

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This view may be easily corroborated on surveying the literature of the natural history of the ACL-deficient knee (1S20) which gives evidence that untreated ACL injury causes the joint to progress on a course of further injury and degenerative changes, although there may be disagreement on the percentage of knees that follow this course. Experiments on animals’ knees have also shown that induced ACL tears caused significant degenerative changes. Bohr (21) demonstrated the development of osteoarthrosis with signs of cartilage degeneration followed by subchondral changes in rabbits’ knees after resection of the ACL. Pond and Nuki (22) showed the same in dogs.

disappointing. Hence, the greater demands placed on designer, device and surgeon !

3.2 Mechanical considerations The constraints within which the designer has to work are many. There are important requirements for longterm success of a cruciate reconstruction procedure. All of these requirements are essential, and failure of the implant is certain if any of them is ignored. The stage at which failure occurs depends upon which design requirement has been ignored, since their individual influences come into play at different stages. 3.2,I Placement of the implant The natural ACL is a complex structure and, on the whole, present substitutes do not aim at anatomical reconstruction of the natural ligament. A pragmatic approach to the problem of placement of the implant is to attach it to the femur and tibia at locations that will remain at the same distance during extension and flexion of the knee. This is generally referred to as isometric placement. Viewing the knee in full extension and with the axis of the femur vertical, the attachment location on the femur may be described as the most posterior and superior position on the lateral femoral condyle, within the area of attachment of the natural ligament. It is the more critical of the two attachments of the implant. The position of the tibia1 attachment is much less critical in that the change in length of the implant is small whether it is attached at the most anterior or most posterior positions within the area of attachment of the natural ligament (23). Figures 4 and 5 show the change in length of the ACL when placed at different sites within the original attachment regions on both femur and tibia. To achieve this isometric placement is of prime importance as it means that the implant will be just taut during passive movement of the joint and will not be

3 BIOLOGICAL AND MECHANICAL CONSIDERATIONS IN THE DESIGN OF PROCEDURES AND DEVICES

3.1 The patient group While it may be argued that similar design principles are employed whether it is the design of a prosthetic ligament or a prosthetic knee, the designer of the former must give special thought to patients in need of ligament reconstruction. The patient group receiving prosthetic ligaments differ greatly from those receiving artificial knee or hip joints. The latter tend to be older and their conditions of deformity, pain and instability have developed over many years. For these patients a knee or hip replacement that removes pain and restores a modicum of function results in a welcome and even dramatic change in their lives. A reduced range of flexion may not affect their ability to cope with their daily needs. On the other hand, the ligament replacement patient group are mostly young and athletic individuals who have become incapacitated suddenly. Unlike the older group, the memory of what it was like to be fit and competitive has not been dimmed. To them a mediocre result is extremely unsatisfactory and greatly

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Fig. 4 Change in length of the ACL with knee flexion angle, for five different attachment sites on the femur within the natural region of attachment Part H : Journal of Engineering in Medicine

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‘i

Position 1 Anterior

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Knee angle of flexion _______

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deg

31 Fig. 5 The site of placement of the ACL on the tibia is not as critical as that on the femur. Both the anterior and posterior sites cause small changes in the ACL length as the knee is moved through 135” flexion

subjected to additional tensile loads (due to misplacement), besides those that it will experience during activities. Gross deviation from isometric placement will result in immediate disasterous postoperative results. The patient will have either a greatly reduced range of flexion or else considerable slackness in the implant which renders it almost redundant. Of the requirements for a successful reconstruction of the ACL correct placement of the implant is the least tolerant to ‘challenge’. 3.2.2 Anchoring technique Failure of the implant may occur in the immediate postoperative term if its anchorage to the bone does not have sufficient strength. The methods currently used are either pure mechanical anchorage methods or a combination of mechanical and biological. Mechanical methods rely on screws, toggles and bollards or staples (even Superglue has been used!) as well as a different variety of other more complex devices comprising various combinations of plates, screws and washers. The advantages claimed for pure mechanical fixation are a reduced period of immobilization and a quicker return to sporting activities. The disadvantages are that most of the mechanical methods do not afford large areas for transmitting the load acting on the anchoring component, and so they may fail due to high stress. Moreover, the strength of a mechanical anchorage does not increase with time; it is more likely to reduce due to fatigue because of cyclic load application. On the other hand, a well-conceived biological anchorage is likely to increase in strength with time, and can provide a large area for distributing the load acting along the implant. The disadvantage of a pure biological anchorage, however, is that it requires a finite time to be established. Therefore, the use of a pure biological anchorage requires immobilization of the patient for a period to protect the implant from slippage, in the immediate postoperative period, and the consequent return of joint laxity. However, immobilization is undesirable as muscles atrophy and the articular cartilage deteriorates ; this is not acceptable to young athletes. Recently additional mechanical fixation has been used in devices that would rely eventually on pure biological anchorage. Such early mechanical fixation is used to prevent excessive loads from acting on the biological anchorage until it has been established, and further, to prevent the Q IMechE 1992

implant from becoming loose, due to slippage, thus causing return of laxity and other undesirable biological consequences which are of particular relevance in the case of tissue ingrowth promotion devices.

3.2.3 Implant strength This should be seriously considered whether the implant is a true replacement or one that promotes ingrowth. It is of importance in the latter case since the implant will act initially as the primary load-bearing component. This will continue until fibrous tissue invading it has matured, become well aligned and developed sufficient strength to share the physiological tensile load acting on the implant.

3.2.4 Implant stiflness This property is defined as the rate of change of load with extension and thus it has the unit load/extension. In other words, it is the slope of the load-extension graph for an implant that possesses linear properties. Since most of the implants exhibit a non-linear behaviour the stiffness of any implant changes with the load applied. It is thus represented by the slope of the tangent to the graph (Fig. 6). There is a suggestion that this property of the implant should be matched to that of the natural ligament. This is of particular importance in the tissue ingrowth promotion type of implant, which acts as a scaffold to be initially invaded by tissue as part of the body’s natural reaction to foreign bodies. Once the implant is covered with tissue, the latter will remodel, if subjected to tensile strains, into wellorganized collagenous tissue. If too stiff an implant is used, the fibrous tissue will not mature into a wellaligned collagenous structure. This is because the implant to which the fibrous tissue adheres would be ‘inextensible’. As a result the ingrown tissue would be stress shielded and so would not experience the tensile strains essential for its alignment and maturation.

3.2.5 Longevity This is a requirement for success of the ligament reconstruction in the long term. The implant is normally given to the young and active individuals. It will be subjected to about 2 million tensile load cycles per annum Proc Instn Mech Engrs Vol 206

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Extension

Fig. 6 (a) If the ligament exhibits linear response to load, its stiffness is constant and is calculated as the slope of the load-extension line (b) As most ligamentous grafts possess non-linear properties the stiffness is determined as the slope of the tangent to the loadextension graph. The stiffness increases with load

in the knee of an individual who may not necessarily be active in sport and most likely to 4 million cycles per annum or more in a top-class runner.

attached to the external surface of the condyle with the surgeon’s method of choice. This route is referred to as the ‘over the top’ route.

3.2.6 Summary

4.1.2 Extra-articular reconstruction

The importance of each design requirement and the consequence of not fulfilling it have been discussed separately. The interaction of the consequences of two or more unfulfilled requirements is complex. To illustrate this, consider the likely course of implants with varying stiffnesses and degrees of misplacement. It can be easily demonstrated that a slight misplacement of a very stiff implant (for example a carbon fibre ligament firmly anchored to bone at both ends) would result in almost immediate failure of the implant or of its anchorage, whichever is weaker. On the other hand, a slight misplacement of an extensible implant will most likely result in this implant stretching a little and developing a tensile load during passive motion of the joint; this small extension would recover, as is likely to occur in the elastic region.

This attempts to reproduce the function of the ACL (namely constraining the anterior movement of the tibia with respect to the femur), with a link between the femur and tibia attached at external sites on the bones. The sites of attachments are normally on the lateral side of the knee.

4 REVIEW OF ACL RECONSTRUCTION

4.1 Types of reconstruction and routes of the implant

4.1.1 Intra-articular reconstruction This reconstruction aims at restoring the link between the femur and the tibia within the knee articulation itself, that is within the intra-condylar space of the femur. The implant may be routed through one of many paths between its regions of anchorage to the femur and tibia. In some cases the implant is passed within two through tunnels, one in the femur and the other in the tibia. This route gives concern to some surgeons in that there is some likelihood of the implant abrading against the edge of the femoral tunnel and so some of these surgeons choose to route the implant in a manner that avoids passing it within a through femoral tunnel. Instead, it is passed first through the capsule at the posterior of the lateral femoral condyle and is then Part H: Journal of Engineering in Medicine

4.1.3 Approach combining both the intra- and extra-articular reconstructions In this approach both the intra-articular and extraarticular links are used simultaneously. The rationale behind this approach is that the intra-articular link alone may not be sufficient in joints, the lateral structures of which have been excessively stretched, and the constraint on internal rotation of the tibia is thereby greatly reduced. It is deemed by some surgeons that an extra-articular lateral reinforcement is needed in combination with the intra-articular link. This additional link provides a constraint on the internal rotation of the tibia. Other surgeons advocate that the intra-articular link alone is sufficient to restore the knee instability since, after an adequate intra-articular reconstruction, the stretched lateral structures recover and provide a measure of constraint on the internal rotation of the tibia. Figure 7 summarizes schematically the three basic reconstructions discussed above. 4.2 Materials used for the reconstruction substitute

4.2.1 Natural tissues The structures surrounding the knee joint have been a source of grafts for ACL reconstruction. Figure 8 illustrates the various tissues that have been used either as free grafts on being harvested and secured at both ends, Q TMechE 1992

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(a) Extra-articular

(b) Intra-articular

(c) Cumbined intra-articular reconstruction with lateral extra-articular reinforcement

Fig. 7 Three types of reconstructive procedures adopted for the ACL

or redirected and reattached. The tissues used as substitutes for the ACL are the iliotibial band, the semitendionosus tendon, the gracilis tendon and the patellar ligament. When these have been harvested from the same patient they are termed autografts or homografts. If they are harvested from cadavers they are referred to as allografts. The advantages of using homografts are that they are biocompatible (having been harvested from the patient). Also they do not need any treatment to sterilize them prior to their implantation. The disadvantage is that harvesting of tissue from the patient such as the patellar ligament is tantamount to ‘robbing Peter to pay Paul’,

crnitendinosus

since the extensor mechanism is thereby weakened. Other disadvantages will be discussed later. Allografts (tissues harvested from cadavers) obviate the need to sacrifice any of the patient’s tissues, and it is possible to be ‘generous’ with the size of the graft when harvesting it. A serious disadvantage is the hazard of various infections. For this reason, very drastic sterilization processes such as large doses of irradiation might be necessary, but this might greatly alter the properties of the graft.

4.2.2 Synthetic materials

Synthetic materials used so far have included polyethylene, polypropylene in its filamentous form, polyethyleneterephthelate (known in the United States as Dacron and in Europe by many other names, including Terylene), carbon fibre, both in its pure filamentous form as well as the glycolic acid coated version, and polytetrafluoroethylene (PTFE). The advantage of using synthetic materials is that they obviate the need for sacrificing any autogenous tissue. Also it is possible to provide a substitute of standard dimensions and of sufficient strength. The disadvantages of using substitutes made of synthetic materials are that they present greater problems in their anchorage to the bones. They abrade and so deteriorate in strength. Further, the particulate matter generated by abrasion may cause synovitis, as well as inflammation of the lymph nodes, should the particulate matter produced be of such a size to allow its migration to the nodes.

4.3 Methods of reconstruction of the ACL 4.3.1 Reconstruction with autogenous tissue

Fig. 8 .The tissues surrounding the knee joint have provided grafts for ACL reconstruction. Four types are illustrated, the bone-patellar ligament-bone graft being the most popular @ IMechE 1992

The idea behind reconstruction with autogenous tissue is that when the graft is placed in a synovial fluid environment and acted upon by appropriate forces, it may undergo morPhological changes Over time and take on the histological appearance of the normal anterior cruciate ligament, as indeed was found by Amiel et al. (24) and Arnoczky et al. (25). Proc Instn Mech Engrs Vol206

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On surveying the literature, which is vast, and the numerous reconstructive procedures and their variations, it is noteworthy that: 1. The choice of substitute tissue for reconstruction has changed over the decades, progressing towards the use of stronger tissue, the iliotibial band being the weakest used at the beginning and the patellar ligament being the strongest and the most commonly used at present. The semitendinosus is still in use, but no more alone as a single strand. Instead, it is used in a looped configuration or in conjunction with the gracilis tendon, and recently both grafts have been double looped to obtain an even stronger graft! This combination of four tendon strands requires large tunnels to be made in the bones (about 12 mm in diameter) which must represent some insult to the joint structures. Further, it complicates the technique, especially the provision of an effective fixation of this composite graft to the bone. The short- and medium-term results of the four-strand technique are still awaited. 2. All free graft autogenous tissue substitutes, which represent the most commonly used form, necrose before revascularization and remodelling, and lose much of their original strength. Kennedy et al. (26) demonstrated a loss of 55 per cent of the strength of the semitendinosus when implanted in the rabbit and Clancy et al. (27) demonstrated that the patellar ligament graft reduced in strength to 52 per cent of the strength of the normal anterior cruciate ligament within one year of ACL reconstruction in rhesus monkeys. Thus, despite the quest for a stronger ligament progressing from the iliotibial band to onethird of the patellar ligament, this does not appear to be an adequate measure; the device frequently stretches and is, unfortunately, most vulnerable for 6 months postoperatively. This is a stage at which most athletes would be most anxious to return to their sporting activities rather than to exercise more restraint on their activities. 3. To compensate for the anticipated loss in strength of autogenous tissues, surgeons have recently turned to allografts where a much larger graft might be harvested. Allografts, which are already necrotic materials, undergo revascularization and modelling processes and they also lose strength. Vasseur et al. (28) showed that in dogs the allografts possessed only 14 per cent of the strength of the contra lateral ACL at 9 months after surgery. 4. ACL reconstruction with autogenous tissue is still the most popular method, and the patellar ligament is used in more than 70 per cent of the procedures. This is despite the hazard involved in harvesting a sound graft and the complications with this procedure, which are rarely reported. Such complications include fracture of the patella (29) and rupture of the remaining portion of the ligament (30). 5. The use of the meniscus for an ACL substitute is a drastic measure: the meniscus has been conclusively shown to be a load-bearing structure in the knee, carrying much in excess of 50 per cent of the load action during ordinary ambulatory activities (31-34). Further, the deleterious consequences of meniscectomy to the joint are well documented. These Part H : Journal of Engineering in Medicine

include: wear of the articular cartilage evidenced by narrowing of the joint space, the formation of osteophytes and other degenerative changes (35-38). It is rather surprising, if not unfortunate, that the meniscus has been considered at all as a substitute for the ACL, let alone used in three different series of patients. 6. The results obtained over the past decades with various tissue, whether autogenous or allografts, seem to have left many surgeons dissatisfied in the long term. This is evidenced by the continuous search for different and stronger substitutes and various modifications and refinements to techniques and rehabilitation regimes. (The latter has evolved with complete contrast; until the past five years immobilization in a plaster cast was the norm. It is now the least practised in the majority of rehabilitation regimes. The emphasis is on early mobilization and the use of continuous passive motion within hours of completion of surgery.) 7. Finally, it must be pointed out that the dissatisfaction with the results with autogenous tissue has diverted many surgeons to look for a prosthetic solution to the problem of complex ligament reconstruction of the knee. There are injury cases where the damage is extensive so that not only the anterior cruciate has been damaged but also the posterior cruciate and medial collateral ligament have been ruptured. Autogenous tissues fail to offer a solution to this problem; there is not enough donor tissue to reconstruct such global destruction to the ligaments of the knee.

4.3.2 Reconstruction of the ACL with synthetic devices It is of interest to note that a silver wire loop was used to replace the torn ACL in a football player (39). Another case is reported of silk sutures having been used to reconstruct a torn cruciate ligament (40). However, these should be regarded as curiosities. The following review of devices used for the reconstruction of the ACL in humans will concentrate on the types described earlier, which have been widely used in the last two decades or so. (a) Frank replacement. By frank replacement is meant a ‘stand-alone’ device that is attached to the bones with mechanical means and does not rely upon any biological tissue ingrowth to strengthen the device or its anchorage to the bone. Early attempts at frank replacement of the ACL in humans were to use a polyethylene prosthetic implant (41). This device was made of medical grade ultra high molecular weight polyethylene. The device was 6.35 mm in diameter and 178 mm in length. It had a central section 35-40 mm in length with a diameter of 4.76 mm. The fixation of this device into the bone was by interference fit of a threaded metallic section within the tubular ends of the implant, which were placed in bony tunnels. Grood and Noyes (10) and later Chen and Black (42) concluded on laboratory evaluation of the implant that it was unsatisfactory as an ACL replacement. The implant had inadequate mechanical properties. Its creep especially was excessive under small loads @ IMechE 1992

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after relatively few cyclical load applications. In a clinical study of 38 patients with an average follow-up of 12-18 months, 30 patients had pain, 7 had instability, 17 had greater than 5 mm of anterior displacement of the tibia and 6 implants ruptured (43). Fox (44) reported 50 per cent of the patients required revision or removal of the prosthesis within one year of implantation. This device was a clinical failure. The other serious attempt at frank replacement was that in which the polytetrafluoroethylene (PTFE) prosthesis was used. This prosthesis, known as the Gore-tex prosthetic ACL, is made from expanded PTFE filaments which are plaited into a three-bundle braid (Fig. 9). The bundle of fibres is fused at both ends into solid rods with formed eyelets which are secured to bone with cortical screws intended for providing the initial fixation of the device, but permanent fixation was expected to be provided by tissue ingrowth among and into the strands of the implant in the intraosseous segments. The strength of this implant was much greater than that of the natural ligament. Laboratory tests indicated that the maximum tensile load was in excess of 4500 N. The implant was fixed in a bony tunnel within the tibia, passed through the capsule at the posterior of the lateral femorai condyle (the ‘over the top’ route), then through another oblique tunnel in the femur, where the ligament was finally fixed superior to the exit hole with another cortical bone screw. In clinical use of this ligament, abrasion and rupture of the implant were experienced. Wear debris caused effusion and synovitis in many patients. There were many such failures at a short follow-up of the implant (45).

(b) The stent device. The description of ‘stent’ is given to a device that is used in combination with autogenous tissue in a reconstruction procedure. The reason for this was the belief that autogenous tissue used in anterior cruciate reconstruction was subject to stretch and rupture in the early postoperative period. The device is attached to the graft along its length but only one of its ends to the bone. The earliest use of a stent, or temporary internal splint, augmenting medial and lateral extra-articular procedures by providing stability until healing took place, was reported by James et al. (46). The device used was the Proplast prosthetic ACL, which was quite complex. It consisted of a core of polyarimide fibre and fluorinated ethylene propylene polymer with a coating of a porous low modulus composite of polytetrafluoroethylene and vitrious carbon fibre! The prosthesis had a maximum tensile load of just under 1580 N. A report by the same authors on a short series of 15 patients with disabling knee instability showed only a 50 per cent satisfactory result. The other stent device reported is the Kennedy ligament augmentation device (LAD), which was first reported by Kennedy in 1983 (47). The device is a 6 mm

Fig. 9 The Gore-tex ligament is made of expanded PTFE filaments, in the form of a plait. The filaments at both ends are fused into a single rod with a formed eyelet which is attached to the bone using a cortical bone screw @ IMechE 1992

flat diamond braid polypropylene structure (Fig. 10). The goals of this device were: (i) to improve the initial strength of the biological graft in order to decrease the risk of the graft disruption when tensioning it and evaluating joint laxity after graft fixation and decrease the risk of graft elongation or disruption during the initial healing stage, while initiating a programme of early joint mobilization, (ii) to protect the biological graft from elongation or disruption during the period of graft remodelling and increasing limb activity and (iii) to share ligament load with the biological graft to enhance the ligament remodelling process.

A critical aspect of this synthetic augmentation of the biological graft was the recognition that long-term remodelling of the biological graft is dependent upon the tissue carrying a portion of the load. Biological tissues remodel according to the loads they carry and, therefore, the biological graft must be subjected to appropriate loading in order to develop a strong biological reconstruction (48). Thus, it was essential to the remodelling process that load sharing occurred between the augmentation device and the biological graft. In order to ensure load sharing, the augmentation device was coupled to the biological tissue with sutures, but the augmentation device was attached to the bone at one end only (Fig. 11). A mathematical model considered by Daniel and Van Kampen (49) demonstrated that the load transmitted from the femur to the tibia was distributed between the augmentation device and the biological graft by sutures along the length of the construction. Daniel et al. (50) reported the results obtained with his device from a multi-centre trial. The number of centres involved was 17 and the initial number of patients 157. Comparisons were made at 24 months postoperatively between only 75 of the total number of patients who received the augmented ACL reconstruction and 29 patients in whom the non-augmented procedure was carried out. The former group had

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Fig. 10 The Kennedy ligament augmentation device (LAD) is made of a flat diamond braid polypropylene structure

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Biological graft

Fig. 11 The Kennedy LAD is attached to the bone at one end and to the graft with sutures. The lower figure is the mechanical model adopted (49), in which the relative stiffnesses of the graft, the ligament augmentation device and the suture connection are given by K,, K , and K , respectively Proc Instn Mech Engrs Vol 206

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significantly better results. However, it is important to note that the study was not a randomized prospective and controlled study. The controls were an historic group, of a much lower number than the study group, of whom only 48 per cent were included in the final comparison. It is also interesting that, when compared, the patients in the study group had a slightly larger range of motion than patients in the control group. It is impossible to demonstrate how this difference can be related to the device, and most likely it is either a chance finding or due to a better developed surgical technique in the study group. (c) The tissue ingrowth promotion device. The concept of utilizing a synthetic device for the reconstruction of the ACL, which acts as a scaffold and encourages the ingrowth of new collagenous tissue, was proposed by Jenkins et al. (51). This technique provides only temporary mechanical integrity until the new tissue can assume the mechanical function. In the initial work, Jenkins et al. (51) experimented on sheep, where they excised the Achilles tendon and replaced it with double rows of multi-filament carbon fibre. This led to an anatomical structure, similar to the original, being formed. Microscopic examination of samples taken from the newly induced Achilles tendon indicated that there was a very low foreign body response to the presence of the carbon fibre, but a massive and organized fibroblastic response with new collagen laid down along the lines formed by the individual carbon fibre filaments. The euphoria accompanying this success led Jenkins (52) to extend the technique to the reconstruction of the cruciate ligaments. To achieve this, tows of carbon fibre were simply passed through bony tunnels and initially knotted to prevent slippage of the implant. Carbon fibre is brittle and, with fragmentation of the fibres, particles of carbon were noted to migrate to regional lymph nodes. To reduce carbon particle migration and improve handling properties, carbon fibre implants have been coated with polylactic acid polymer (53). Theoretically the co-polymer is resorbed shortly after implantation and the carbon undergoes gradual mechanical degradation over a long period. The load is initially taken up by the prosthesis and gradually transferred to the newly formed collagen. In a multi-centre study, Weiss et al. (54) used a prosthesis which was woven through a strip of distally detached ITB which was passed through the posterior capsule of the knee and then through a tibia1 tunnel. Fixation to the tibia was accomplished with a carbon polylactic acid fastner. Clinical improvement was reported in most patients who underwent ACL reconstruction using carbon fibre. In another application, Strum and Larson (55) found no apparent benefit one year postoperatively. When presented to the FDA, carbon fibre devices were not approved for use in the reconstruction of ligaments. Currently, the use of this material has diminished. The other device of the scaffold type which is currently in extensive use is the Leeds-Keio ligament (56, 57). This ligament is a hybrid-type implant. It is not a prosthesis in a strict sense, although initially the implant carries all of the tensile load, until the ingrown tissue invading the device matures and becomes wellPart H : Journal of Engineering in Medicine

aligned collagen capable of sharing the load with the implant. Neither is the implant of the stent type in that it does not biodegrade and entirely bow away to the ingrown tissue. Tests in the laboratory predict that it would retain a considerable fraction of its initial __ ~ ligament _ strength _ _ for a long period. The Leeds-Keio is made of polyester-polyethyleneterephthylate-a material that has been in use in the human body in the form of arterial grafts and heart valves for more than 25 years. The implant itself is in the form of a tubular mesh, loosely woven and thus with numerous holes (Fig. 12). The method adopted for anchorage to the bone is unique to this implant. It relies on and benefits from both bone and tissue ingrowth. The anchorage is achieved by trapping the implant between two freshly cut bony surfaces so that as they heal they unite through the holes in the implant and thus secure it by interlocking bone trabeculae and connective tissue. For the reconstruction of the ACL, the details of both procedure and implant design had to be worked out specifically. The procedure of implanting this device involved harvesting two bone plugs 9.5 mm in diameter and 20-25 mm long from the lateral femoral condyle and medial side of the tibia. The tunnels from which the bone plugs were harvested were extended by coaxial tunnels of smaller diameters which emerged at the intercondylar locations of attachment of the implant (Fig. 13). On placing the implant within the bony tunnels, the bone plugs were replaced in their respective original positions and the ligament placed under tension. Since anchorage with this method requires time to be established and because of the departure of most surgeons from slow rehabilitation regimes, additional mechanical fixation is used immediately postoperatively. With early mobilization the implant is kept taut and prevented from slipping from around the bone plugs, thus becoming loose, with the consequent return of laxity to the joint. The Leeds-Keio ligament was shown to induce tissue within the intra-articular link. In an early series of tests in immature pigs, it was shown in some of the animals that the composite consisting of the man-made fibres and ingrown tissue was stronger than the implant itself, as tested in the laboratory (56, 57). To date the load sharing of the ingrown tissue has yet to be reproduced ~

Fig. 12 The Leeds-Keio ligament is an open-weave structure made of polyester filaments. It is tubular in the main section but has pouches to receive bone plugs and at both ends are densely woven for ease of implantation. The cord attached at one end is for drawing the device within bony tunnels Q IMechE 1992

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RECONSTRUCTION OF THE ANTERIOR CRUCIATE LIGAMENT Artificial ligament filament

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Femur bone

Connective ingrowth

Fig. 13 The Leeds-Keio ligament is anchored to the bone using bone plugs harvested from the same sites into which they are eventually

replaced within the implant. Bone and tissue ingrowth anchor the device to femur and tibia. Tissue ingrowth occurs around the implant forming a new ligament in another animal model. In humans, in a study by Fujikawa et al. (58) the healing of the ACLs reconstructed with the Leeds-Keio ligament was observed by arthroscopy in 42 knees, and biopsies were taken from 19 knees at intervals from 3 to 24 months postoperatively. By 3 months the implant was covered with immature new tissue and a dense vascular network formed on its surface. At 12 months a new ligament had developed and matured, resembling the natural one in most cases. The small bulk of the Leeds-Keio ligament has the advantage of allowing much space for the invading tissue to grow into the implant around its monofilaments. It also allows the tissue to grow around the implant in the tunnels of bone where the ligament is attached to both femur and tibia. This provides protection for the implant from abrasion against sharp, bony spicules. Another advantage of the implant is its length, which allows it to be used for complex reconstruction procedures so that not only intra-articular reconstruction of the ACL is possible but also procedures involving the anterior cruciate ligament (ACL), posterior cruciate ligament (PCL) and medial collateral ligament (MCL) as well as lateral structure. Over 22000 units of this ligament have been implanted in patients worldwide (except for the United States) since February 1982. Few complications have been reported to date, although a substantial number of ruptured or loose ligaments have been observed in a small series (20 patients) reported by Macnicol and Penny (59). 5 FUTURE OF ARTIFICIAL LIGAMENTS

The general feeling at present is that stand-alone prostheses with pure mechanical fixation are out of favour due to repeated failures of the implant, either due to abrasion and rupture of the implant or to failure of its @ IMechE 1992

mechanical fixation. Autogenous tissues themselves leave much to be desired as they necrose and lose strength at about 6 months postoperatively, and with the increasing interest in early rehabilitation they become vulnerable to stretching and return of laxity. There is also a limited quantity of such autogenous tissue that can be harvested without endangering the stability of the joint and/or the integrity of its musculature. Hence the reason for surgeons turning to allografts. These again are subjected to the same process of losing strength before they are revascularized and remodelled. This is despite the greater strength that an allograft may have due to the liberality exercised in the harvesting process. One of the problems that has yet to be resolved is the initial fixation of the tissue, be it autogenous or allograft, to the bone. So far there is no satisfactory mechanical fixation device that will provide good initial anchorage of the grafts to the bone. Many surgeons favour autogenous tissue augmented with synthetic materials, and one of the most extensively used devices for augmentation is the Kennedy LAD. Despite its claimed benefits the Kennedy LAD has proved to be safe and with no more complications than those experienced with autogenous tissue used by themselves. Devices used for the promotion of tissue ingrowth would perhaps be more attractive and more extensively used if it was possible to prove the efficacy of the composite comprising natural tissue and the synthetic material forming the neoligament. At best this might be possible to prove in animals, but not in humans. In the latter model, what can be shown is the abundance of ingrown collagenous tissue well aligned in the direction of load application. The most interesting step forward would be an advance on the tissue ingrowth promotion device with one that is in itself biodegradable. Thus, once the neoligament has been fully formed and matured it could be Proc Instn Mech Engrs Vol 206

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the sole load-bearing link between the two bones. This requires the use of asynthetic material that biodegrades over a much longer period than the currently available materials. REFERENCES 1 Barrack, R. L. and Skinner, H. B. The sensory function of knee ligaments. In Knee Iiyamenls: structure, function, injury and repuir (Ed. D. Daniel), 1990, pp. 95-1 14 (Raven Press). 2 Strasser, H. Lehrbuch der muskel und gelenkmechunik, Vol. 111, 1917 (Springer, Berlin). 3 Kapandji, I. The physiology uf the joints, Vol. 2, 1970 (Churchill Livingstone, Edinburgh). 4 Menschik, A. Mechanik des kniegelenks, Part 1. Z. Orthop., 1974, 113,481-495. 5 O’Connor, J., Shercliff, T. and Goodfellow, J. Mechanics of the knee in the sagittal plane-mechanical interactions between the muscles and ligaments. In Surgery and arthroscopy ofthe knee (Eds W. Muller and W. Hackenhruh), 1988, pp. 12-30 (Springer-Verlag, Darl i- I UCIIIII,.

6 Fick, R. Anatomie und mechanic der gelewke unter berucksichtigung der bewengenden mulken. In Handbuck der anatomie des rnunschne (Ed. K. von Berdeleken), Vol. 11, Part 111, 1911 (Fisher, Jena). 7 Morrison, J. B. Bioengineering analysis of force actions transmitted by the knee joint. J . Biomech. Engng, 1968,3, 164-170. 8 Morrison, J. B. Function of the knee joint in various activities. J. Biomech. Engng, 1969,4,573-580. 9 Morrison, J. B. The mechanics of the knee joint in relation to normal walking. J . Biomechanics, 19743, 51-61 10 Grood, E. S. and Noyes, F. R. Cruciate ligament prosthesis: strength, creep and fatigue properties. J . Bone J t Surg., 1976, SSA, 1083-1088. 11 Wang, C.-J., Walker, P. S. and Wolf, B. The effects of flexion and rotation on the length patterns of the ligaments of the knee. 1. Biomechanics, 1973,6,587-596. 12 Biden, E., O’Connor, J. and Collins, J. J. Gait analysis. In Knee ligaments: structure, function, injury, and repair (Ed. D. Daniel), 1990, pp. 291-311 (Raven Press). 13 Noyes, F. R. and Grood, E. S. The strength of the anterior cruciate ligament. Age and species-related changes. J . Bone J t Surg., 1976, 58A, 1074-1082. 14 Rauch, G., Allzeit, B. and Gotzen, L. Biomechanical studies on the tensile strength of the anterior cruciate ligament with special reference to age-dependence. Unfallchirugie, 1988,91,437-443. 15 Hollis, J. M., Marcin, J. P., Horibe, S. and Woo, SL-Y. Load determination in ACL fiber bundles under knee loading. Trans. Orthop. Res. Soc., 1988, 13, 58. 16 Butler, D . L., Kay, M. D. and Stouf€er, D. C. Comparison of material properties of fascicle-bone units from patellar tendon and knee ligaments. 1.Biomechanics, 1986,19,425-432. 17 Woo, SL-Y., Hollis, J. M, ROW, R. D., Gomez, M. A., Inone, M., Kleiner, J. B. and Akeson, W. H. Effects of knee flexion on the structural properties of the rabbit femur-anterior cruciate ligament-tibia complex (FATC). J . Biomechanics, 1987, 20(6), 557563. IS Feagin, J. A. The syndrome of the torn anterior cruciate ligament. O r t h p . Clin.North Am., 1979,10,81-90. 19 Fetto, J. F. and Marshall, J. L. The natural history and diagnosis of anterior cruciate ligament insufficiency. Clin.Orthop., 1980, 147, 29-37. 20 Odensten, M., Lysholm, J. and Gillquist, J. The course of partial anterior cruciate ligament rupture. Am. J . Sports Med., 1985, 13, 183-186. 21 Bobr, H. Experimental osteoarthntis in the rabbit knee joint. Acta Orthop. Scund., 1976,47, 558-565. 22 Pond, M. J. and Nuki, G. Experimentally induced osteoarthritis in the dog. Ann. Rheum. Dis., 1973,32,387-388. 23 Ball, W. G. and Gibson, D. N. Investigation into the positioning of cruciate ligament prostheses. Final year report, Department of Mechanical Engineering, University of Leeds, 1985 (unpublished). 24 Amiel, D., Kleiner, J. B., Row, R. D., Hanvood, F. L. and Akeson, W. H. The phenomenon of ‘ligamentization’: anterior cruciate ligament reconstruction with autogenous patellar tendon. J . Orthop. Res., 1986,4, 162-172. 25 Amnoczky, S. P., Tarvin, G. B. and Marshall, J. L. Anterior cruciate ligament replacement using patellar tendon. J . Bone Jt Surg., 1982,64A, 217-224. Part H: Journal of Engineering in Medicine

26 Kennedy, J. C., Roth. J. H.. Mendenhall, H. V. and Sanford, J. B. Intra-a;ticular replacement in the anterior cruciate ligamentdeficient knee. Am. J . Sports Med., 1980,8, 1-8. 27 Clancy, W. G., Narechania, R. G., Rosenberg, T. D., Gmeiner, J. G., Wisnefske, D . D. and Lange, T. A. Anterior and posterior cruciate ligament reconstruction in Rhesus monkeys. J. Bone Jt Surg., 1981,63A,1270-1284. 28 Vasseur* *’ B’9 Rodrig’, J. J.y s.9 G. and Sharkey, B. S. Replacement of the anterior cruciate ligament with a bone-ligament-bone anterior cruciate ligament allograft in dogs. Clin. Orthop., 1987, 219,268-277. 29 McCaroll, J. R. Fracture of the patella during a golf swing following reconstruction of the anterior cruciate ligament. Am. J . Sports Med., 1983, 11, 26-27. 30 Bonamol J‘ J.9 Krinick9 R* M’ and A* A’ Rupture Of the patellar ligament after use of its central third for anterior cruciate reconstruction. J . Bone J t Surg., 1984,66A, 1294-1297. 31 Seedhom, B. B7 . Dowson, D. and Wright, v . The loadbearing function of the menisci: a preliminary study. In The knee joint (Eds 0. S. Ingwersen, B. Van Linge, Th. J. 6 . Van Rens, G. E. Rosingh, B. E. E. M. J. Veraart and D. Le Vay). Proceedings of the International Congress, Rotterdam, September 1973 (Excerpta Medica, Amsterdam, and Elsevier, New York). 32 Seedhom, B. B. and Hargreaves, D. J. Transmission of the load in the knee joint with special reference to the role of the menisci. Part 11: experimental results, discussion and conclusions. Engng Med., 1979,8,220-228. 33 Shrive, N., O’Connor, J. and Goodfellow, J. Load bearing in the human knee. Clin. Orthop., 1977,131,279-287. 34 Walker, P. S. and Erkman, M. J. The role of the menisci in force transmission across the knee. Clin. Orthop. Related Res., 1975, 109. 184-192. 35 Fairbank, T. J. Knee changes after meniscectomy. J . Bone Jt Surg., 1948,30B,666-670. 36 Jackson, J. P. Degenerative changes in the knee after meniscectomy. Br. Med. J., 1968,2, 525-527. 37 Murray-Leslie, C. F., Lintott, D. J. and Wright, V. The knees and ankles in sport and veteran military parachutists. Ann. Rheum. Dis., 1977,36, 327-331. 38 Saugman-Jensen, J. Meniscus injuries of the knee joint, 1963 (Knaeets Minisklaesioner, Copenhagen). 39 Corner, E. M. Notes of a case illustrative of an artificial anterior cruciate ligament, demonstrating the action of that ligament. Proc. R. Soc. Med., 1914,7,120-121. 40 Smith, A. The diagnosis and treatment of injuries of the cruciate ligaments. Br. J . Surg., 1918,6, 176-189. 41 Kennedy, J. C. Experience with polypropylene ligament. Presented at the Canadian Orthopaedic Association Meeting, Ottawa, Canada, June 1975. 42 Chen, E. H. and Black, J. Materials design analysis of the prosthetic anterior cruciate ligament. J . Biomed. Muter. Res., 1980, 14, 567-586. 43 FDA Orthopaedic Device Classification Panel. Initial survey of Richards Polyflex cruciate ligament prosthesis. Transcript, Washington, D.C., November 1978. 44 Fox, J. Report on the clinical results of Polyflex (TM) ligament replacement. Prepared for the FDA Orthopaedic Panel and presented 15 April 1977. 45 Rosenberg, T. Salt Lake City knee and sports medicine experience with Gore-tex anterior cruciate ligament prosthesis: brief summary of preliminary results. Presented at Sixth International Symposium on Advances in cruciate ligament reconstruction of the knee: uutogenous us. prosthetic, Los Angela, 3-5 March 1989. 46 James, S. L., Woods, G. W., Homsy, C. A., Prewitt, J. M. and Slocnm, D. B. Cruciate ligament stents in reconstruction of the unstable knee. Clin. Orthop., 1979, 143,90-96. 47 Kennedy, J. C. Application of prosthetics to anterior cruciate ligament reconstruction and repair. Clin. Orthop., 1983, 172, 125-128. 48 Akeson, W. H., Frank, C. B., Amiel, D. and Wood, S. Ligament biology and biomechanics. In AAOS Symposium on Sports medicine: the knee edit 6 (Ed. G . Fineman), 1985 (CV Mosby, St Louis, Mo.). 49 Daniel, D. M. and Van Kampen, C. L. Synthetic augmentation of biological anterior cruciate ligament substitution. In Prosthetic ligument reconstruction o f f h e knee (Eds M. J. Friedman and R. D. Ferkel), 1988, pp. 65-70 (W.B. Saunders Company). 50 Daniel, D. M., Woodward, E. P., Lossee, G. M. and Stone, M. L. The Marshall/Macintosh anterior cruciate ligament reconstruction with the Kennedy ligament augmentation device: report of the ’

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United States clinical trials. In Prosthetic ligament reconstruction ofthe knee (Eds M. J. Friedman and R. I>. Ferkel), 1988, pp. 71-78 (W. B. Saunders Company). Jenkins, D. H. R., Forester, 0. W, McKibbins, B. and Ralis, 2. A. Induction of tendon and ligament formation by carbon implants. J. Bone Jt Surg., 1977, SB,53-57. Jenkins, D. H. R The repair of cruciate ligaments with flexible carbon fibre. J. Bone Jt Surg., 1978,60B, 520-522. Alexander, H., Parsons, J. R, Smith, G., Fong, R., Mylod, A. and Weiss, A. B. Anterior cruciate ligament replacement with filamentous carbon. Trans. Orthop. Res. Soc., 1982,7,45. Weis, A. B., Blazina, M. E., Goldstein, A. R. and Alexander, H. Ligament replacement with an absorbable copolymer carbon fiber scaffold--early clinical experience. Clin. Orthop. Related Res., 1985, I%, 77-85.

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55 Strum, G. M. and Larson, R. L. Clinical experience and early results of carbon fiber augmentation of anterior cruciate reconstruction of the knee. Clin.Orthop., 1985,196, 124-138. 56 Seedhorn, B. B., Fujikawa, K. and Atkinson, P. J. The Leeds-Keio artificial ligament for replacing the cruciates. In Engineering and clinical aspects of endoprosthetic fixation, 1984, pp. 99-109 (Mechanical Engineering Publications, London). 57 Seedhorn, B. B. The Leeds-Keio ligament: biomechanics. In Prosthetic ligament reconstruction of the knee (Eds M. J. Friedman and R. D. Ferkel), 1988, pp. 18-131 (W.B. Saunders Company). 58 Fujikawa, K., Iseki, F. and Seedhorn, B. B. Arthroscopy after anterior cruciate reconstruction with the Leeds-Keio ligament. J. Bone Jt Surg., 1989,71B, 565-570. 59 Maenicol, M. F. and Penny, I. D. Proceedings of the British Orthopaedic Research Society. J. Bone Jt Surg., 1990,72B, 167.

Proc Instn Mecb Engrs Vol 206

Reconstruction of the anterior cruciate ligament.

Ligaments are strong collagenous structures that act as constraints on joint motion, thus confining the articular surfaces to more or less the same pa...
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