Organization of popliteus muscle for its function

By Rajendra Prasad R

Abstract

The lateral meniscus is freely mobile. It moves with Femur because of the ligaments

of Humphry and Wrisburg. As a result, the distortion of lateral meniscus is very

minimal. Yet there is an increased tendency for the posterior arch of the meniscus to

be pulled inwards by the above ligaments. The crushing by the rotating femoral

condyle is prevented by popliteus which retracts the posterior arch of lateral meniscus

backwards and downwards. ( 4, 7) The arcuate ligament helps the popliteal muscle to

function steadily. The ligaments of Humphry and Wrisburg, by their negative action

on popliteus produce a balance of power. This maintains the lateral meniscus in

constant relationship to the rotating lateral condyle of femur (5). This is very essential

for sudden or slow or variable conjunct rotation of femur. Absence of Rotation of

knee joint in fruit bat is confirmatory evidence that popliteus and lateral meniscus are

intimately related in their functions (2).

Key wordsLateral meniscus, conjunct rotation, tendon of popliteus, quadrilateral aponeurosis,

arcuate ligament, menisco-femoral ligament

Introduction

Conjunct Rotation of knee joint occurs in vertical axis and in menisco-tibial

compartment of knee. It is part of locking and unlocking mechanism of femur during

weight bearing. Screw home or locking of knee occurs at the end of knee extension.

During this movement the femoral condyles role and glide medially on articular facets

on tibia. The rotating condyles push the anterior horn of the lateral meniscus

anteriorly during locking. It is essential to untwist or unlock the articular surfaces of

the knee joint at the start of flexion. During this untwisting femoral condyle glide and

role laterally pushing the posterior mobile part of lateral meniscus medially(8).

1. Head, Senior lecturer, Department of Human biology, faculty of healthcare

sciences, Eastern university of Sri Lanka

Contraction of popliteus not only brings lateral rotation of femur and, at the same time

pulls the lateral meniscus posteriorly and prevents the lateral meniscus from crushing.

On the other hand the medial meniscus is relatively immobile and is fixed on the tibial

articular facet by its broad horns. It is attached by its whole length to the firm

coronary ligament (true capsule of tibia). Further it is also strongly attached to part of

the medial ligament of knee. For lateral meniscus, the coronary ligament is loose and

is not attached to fibular collateral ligament and the entire circumference of lateral

meniscus is freely mobile. This shows the effect of rotation of the knee joint on the

two menisci is dissimilar(9).

Medial meniscus is distorted during rotation as it is relatively immobile. This leads to

injury during sudden rotation of femar as there are no muscular control mechanisms.

The lateral meniscus is more circular and less widely fixed to tibia by its narrow

horns. Its posterior horn is attached by slips to lateral femoral condyle. The

mechanism of popliteus muscle prevents its medial mobility during conjunct rotation

of knee. It is thus relatively immune from injury than medial meniscus (3,4,7,9). To

understand this fact the anatomical basis of the popliteal muscle mechanism on lateral

meniscus has to be mentioned in detail.

a) tendon of popliteus muscle

Fig. 1

The tendon of popliteus originates from the anterior end of the popliteal groove. It is round, oblique, intracapsular and intervenes between lateral meniscus and fibular collateral ligament. It is surrounded by synovial sheath from the lower compartment of knee. As the tendon pierces the posterior capsule and comes under arcuate ligament, the synovial sheath herniates as bursa deep to lateral part of muscle and is not found beneath the medial part of the muscle (3, 4, 8). It is said earlier, that the tendon lies free inside the capsule and bevel the posterior edge of tibia by its movement and the bursa minimize the repeated rubbing injury to tendon. In reality, tendon does not lie free in knee joint. Instead, it is firmly attached along its lateral surface to the capsule which is reinforced by the lateral limb of arcuate ligament of knee. Medial surface of tendon projects as a firm ridge inside the joint cavity and is related to synovial membrane above and below the lateral lemniscuses. Usually there is no attachment of the tendon to the lateral lemniscus but is grooved obliquely. Some anastomists have noted the attachment of the tendon to lateral meniscus by some fibrousbands(1,6,7).

Upper half of popliteus attached to lateral meniscus

Ligaments of Humphry & Wrisburg tightly cover the posterior cruciate ligament

Fig. 2

b). Attachment of popliteus tendon to muscle fibres

The extra capsular tendon expands and attached to 50% of the muscle fibres of popliteus. In other words, the femoral tendon has 50% of muscle fibres only. Rest of the muscle fibers (50%) is inserted into a quadrilateral aponeurosis. It is seen medial to the femoral tendon and attached to the posterior arch of lateral meniscus. This aponeurosis may be cut during routine dissection leaving an upper free border to muscle. Deep to this quadrilateral aponeurosis lies the lax coronary ligament or lower joint capsule which is lined internally by synovial membrane. At times the quadrilateral aponeurosis arises across the width of the muscle. Here all the deep fibres of popliteus muscle are attached. The superficial fibers of the muscle converge and get attached to femoral tendon. In any case some of the posterior superficial fibers of the muscle are continued upwards covering the lateral meniscus and blend with the lateral part of the femoral capsule in strengthening it while contraction of the muscle.(4,7)

The articular bones and action of popliteus The tendon of popliteus orginate from the anterior aspect of the popliteal groove of the lateral condyle of femur. The posterior part of the popliteal groove is occupied by tendon in full flexion of knee joint. The oblique tendon as it comes out of the capsule of the joint lies in between tibial condyloma head of fibula. In fact if grooves the posterolateral border of tibia above the styloid process of fibula (7,10).

The medial condylar articular surface of femur is longer and slightly curved antero posteriorly than that of lateral condyle and also curved more in its anterior part . During extension the articular area on the lateral femoral codyle is used up earlier.The extra articular surface on the medial condyle allow the femoral rotation on tibia.The medial codyle rotates in a circle (30 degree) whose center is in the middle part of lateral condyle. This rotation is passive and around the taughten anterior cruciate ligament .thus the articulating surface become incongruent and the knee is loehet asImportance of popliteus muscle action in the movement of condyles of knee joint

Aponeurosis of popliteus divided to show posterior part of coronary ligament

Popliteus tendon removed to show popliteus bursa

The movement of flexion and extension at knee joint occurs above the meniscus. The action of muscle to the above movement is negligible. Important action of popliteus on the knee joint is that of rotation of femur on standing position.

Surface is longer longer than lateral and the anterior surface is curved. This allows passive radial rotation of the medial femoral condyle (300) in the terminal stage of extension. It is the ‘Locked’ or hyper extended position of knee joint and as a result the lower limb is converted to a rigid pillar.

The movement of unlocking of knee joint is very essential for flexion to start. This is brought by the femoral tendon of popliteus and is the only muscle to bring lateral rotation of femur. It rotates the lateral condyle and draws it posteriorly on the lateral facet on tibia. This movement tends to flatten the posterior arch of lateral meniscus and liable it to be crushed between femur and tibia. Meniscal attachment of popliteus muscle fibre pulls the posterior margin of lateral meniscus backwards and downwards, over the latral tibial facet and draws it out of harm.

On inspection, the posterior margin of the medial condyle of tibia is shaped in a right angled manner and the articular facet is confined to tibial condyle. The posterior margin of the lateral condyle is rounded and sloping and the articular surface is continued over it. And this can not be produced by the narrow popliteal tendon.As described earlier (IIa), the popliteal tendon is not unsteady in the capsule. Instead it is firmly attached to lateral aspect of capsule and is mobile in a steady manner. This grooving the postero lateral part of the menisci. It si clearly observed the tendon of popliteus never comes in to contact with posterior border of lateral tibial condyle in any of its movement. Instead it produces an oblique groove over the postero lateral border of tibia above the styloid process of fibula.

Further it is also observed that the lateral rotating lateral femoral condyle glides posteriorly but never slip behind the posterior border of the external condyle of tibia in a normal steady knee. It is evident that the rounded bevel of the posterior border is caused by the posterior arch of lateral meniscus which is pulled posteriorly and downwards, over the posterior border, with every contraction of the popliteus muscle.

c). Attachment of popliteus muscle fibers to arcuate ligament.

The stem of the arcuate ligament is attached to styloid process of the head of fibula. Its lateral limb is dense, wide, strong and is attached to tendon of popliteus and lateral condyle of femur beneath the fibular collateral ligament. The medial limb is weak, narrow and curves over popliteus muscle and gives attachment to lateral meniscus. The fibers of this limb of arcuate ligament reinforce the capsule and get attached to posterior intercondylar part of tibia. Some superficial fibers from lateral part of popliteal muscle are attached to the lower curved border of the medial limb of arcuate ligament. The tendon popliteus emerge under the border of arcuate ligament. To put it

simply, the arcuate ligament is attached to lateral meniscus and part of popliteus is attached to it. Functionally arcuate ligament brings (2, 5)

(a) Steadying of femoral tendon of popliteus via its lateral limb.(b) Maintenance of the crescentric border of the medial limb via the attachment of

the superficial fibers from the lateral part of popliteus muscle. Muscle fiber contraction pulls the arcuate band downwards. This can be shown by pulling physically the superficial fibers of the muscle downwards. In other words, the crescentic aperture for the tendon, which is supported by synovial sheath, is prevented from collapsing throughout muscle contractions.

(c) By its attachment to lateral meniscus with the superficial fibers of popliteal muscle prevent abrupt downward and backward movement of the lateral meniscus. Instead, make the movement regular and continuous.

Fig. 3

d). Ligaments of Humphry, wrisburg and popliteus muscle

The posterior horn of lateral meniscus is narrow, small and is attached to tibia. It gives two strong fibrous slips, one in front and other behind the posterior cruciate ligament. They are called anterior menisco-femoral ligament of Humphry and posterior menisco-femoral ligament of wrisburg. These ligaments embrace the posterior cruciate ligament tightly. At the beginning of unlocking the lateral rotation of lateral femoral condyle tenses the above ligaments and draws the mobile posterior arch of lateral meniscus medially. This movement tends to flatten the posterior arch and is liable to get crushed. The popliteal mechanism draws it out of harm way. When the lateral condyles move anteriorly. (Locking) relaxation of the ligaments of Humphry and wrisburg and popliteus muscle occurs and the mobile posterior arch glide outwards resume its resting position. Thus there are two opposing mechanisms, one is formed by menisco femoral ligaments and the other the popliteus mechanism, are involved in the mobility of the posterior arch of lateral meniscus. Balance of

Attachment of popliteus fibers to the upper capsule

Popliteus tendon fused to lateral limb of arcuate ligament

Popliteus fibers & meniscus attached to medial limb of arcuate ligament

power of these opposing mechanisms is very essential for sudden or slow or variable conjunct rotation of knee. (4, 2, 5)

e). Functional importance of lateral meniscus and popliteal muscle

The medial medial meniscus is relatively immobile as it is firmly fixed to tibia and capsule of knee joint. During lateral rotation of femur, its horns move with tibia and the rest with femur. Distortion of it is obvious and prone to injury to a higher degree.

On the other hand, lateral meniscus is circular and fixed by narrow horns to tibia. It is loosely fixed to the capsule of knee joint. Further the posterior horn is attached by two strong menisco femoral ligaments to lateral femoral condyle. The horns of the lateral meniscus moves with femur during lateral rotation of femur than tibia. The distortion is minimum but, the highly mobile posterior arch may be pulled inwards during lateral rotation of femur. The popliteal mechanism has become very essential to pull the meniscus posteriorly and prone to injury is very minimum. When it is injured, there may be seldom injury to capsule and ligaments. Fault in the popliteal mechanism may be assumed. Further, locking of the knee joint occurs more often with medial meniscus injury as the torn part get impacted between femur and tibia. In the case of lateral meniscus injury, the torn segment is still controlled by the regulated popliteal mechanism.(4,2)

f). Evolutionary importance of lateral meniscus and popliteus mechanisms

In Pteropus Aegepticus, the fruit Bat, the knee has no menisci and there is no rotation. There is also limitation of flexion and extension and the animal is unable to walk on the ground. In the knee of Tortise the medial meniscus is absent. As the medial condyle is round and the lateral meniscus is well developed. Again the flexion and extension at knee joint is of limited value. The rotation of knee joint is spinning around a vertical axis, passing through the centre of condyle. These facts form a confirmatory evidence for the functioning of lateral meniscus and popliteal mechanism. Further in man, a small part of the posterior horn of lateral meniscus is attached to tibial condyle but by strong ligaments of Henry and Wrisburg to the medial femoral condyle. In most mammals, where sudden rotatory function at knee is an essential necessity, the posterior horn of the lateral meniscus is not attached to tibia. But, by stronger ligaments of Henry and Wrisburg. This clearly reveals the fact that rotating femur appears to be an integral part of the mobility of lateral meniscus and its controller. The popliteal mechanism (2, 5)

III. Conclusion

Popliteal muscle consists two parts. One part is attached to lateral condyle of femur by a tendon and this rotates the femur laterally in unlocking. Rest of the muscle is attached to posterior arch of lateral meniscus. This part of the muscle simultaneously retracts the posterior arch of lateral meniscus. There by prevent it from crushing injury by lateral condyle. The arcuate ligament helps the popliteus muscle to function steadily. The ligaments of Humphry and Wrisburg by its negative action to popliteus action maintain the lateral meniscus in a constant relationship to the rotating lateral condyle of femur. As a result of controlled mobility of popliteus, the lateral meniscus is neither distorted nor even pulled medially by laterally rotating femur. In other words, popliteus functions not merely as a muscle of lateral rotation to femur but essentially as a controller of the position the lateral meniscus. (2)

References

1. Alfred J Tria, Christopher D. Johnson, M.D., and Joseph P Zawadsky. The journal of Bone and Joint surgery, The popliteus Tendon

2. BARNET CH. Structure and functions fibro cartilages within vertebrate joints (1954 July). Journal of anatomy, 88(pt 3), pp 363-368.

3. Kulkarni N.V, Clinical anatomy for students, 1st edition (2007) Jaypee

brothers , New Delhi4. McMinn R. M. A. Last Regional & applied anatomy 8th edition.

5. Last RL. The popliteal muscle and lateral meniscus (1950 Feb) Journal of Bone and Joint surgery, Vol 32 B, No.1, pp 93-100.

6. Last.RL. Related articles and popliteus tendon (available at www.jbjs.org.uk)

7. William PL, Bannister LH, Berry MM, etal. Grays anatomy, 38 th British edition, Churchill Livingstone, London

8. Locking and unlocking mechanism (available at www.sportsresource.org/Documents/screw % 20 home% 20 mechanism.doc)

9. R. Kanagasuntheram, P. Sivanandasingham, A. Krishnamurti, Textbook of ANATOMY

10. Available @ www.wheelessonline.com/ortho/popliteus muscle (to fill)