Anatomy and Kinematics of the Normal Knee
Kenneth A. Krackow, M.D., and David S. Hungerford, M.D.


This chapter presents aspects of the anatomy and kinematics of the knee as they specifically relate to total knee arthroplasty. It is not intended to be an encyclopedic presentation replacing various anatomy texts or other articles on these topics. In fact, it is suggested that this material be read while simultaneously reviewing a standard anatomy text or atlas (2, 8). The anatomic features are discussed as they relate to surgical exposure, and as they define certain features of rotational alignment. In addition, structures involved in soft tissue balancing when correcting varus or valgus deformity are discussed, and the implications of extraordinary bone cuts on subsequent ligament balance are addressed. Last, the fine details of normal joint contours are presented followed by descriptions of the complex kinematic patterns of the normal bone.


The satisfactory vascularity of the skin over the knee provides several options regarding initial incision. Planning must include consideration of prior incisions as well as the extent of exposure necessary for handling any unusual deformity. Since lymphatic drainage of the anterior aspect of the knee has been shown to proceed principally toward the medial aspect (4), more extreme median parapatellar incisions may be expected to interrupt more of the medial lymphatic drainage and, thereby, predispose to flap edema over the patella. In addition, one must note the expected locations of major cutaneous branches from the saphenous nerve as these course from medial to central over the anterior knee. With these points in mind, a straight anterior or gently curving median parapatellar incision is the incision of choice (Fig. 2.1).

Figure 2.1. The dark line running from medial to the quadriceps tendon, along the medial edge of the patella and down along the medial border of the patellar tendon represents the location of the authors' standard skin incision. The second (red) line running along the junction of the medial and central thirds of the patellar tendon, to the superior medial corner of the patella and distally, medial to the patella and patellar tendon, represents the capsular incision.

The deep capsular incision is also generally made along the median parapatellar plane (Fig. 2.1). Certain anatomic points are, however, important. The incision must extend into the region of the quadriceps tendon for adequate exposure. Careful dissection through the overlying deep fat and identification of the medial and lateral margins of the quadriceps tendon are necessary so that the incision can be accurately placed within this structure, thereby affording strong repair at the time of closure and avoiding inadvertent transection of this major tendon.

An alternate exposure proceeding along the inner medial edge of the vastus medialis has been described, which obviates incision into the substance of the quadriceps tendon (6). Although we have not used this exposure ourselves, when one considers the rich vascularity entering the superior medial aspect of the patella from the vastus medialis attachment, this different approach may be quite appropriate in certain cases of severe deformity where extensive lateral release and tibial tuberde osteotomy or transposition are predicted at the outset.

Mobilization of the capsule and exposure of the proximal tibia, in consideration of the level of the transverse tibial cut, necessitate detachment of soft tissue from the tibia for a short distance below the joint line as far medially, posteromedially, and laterally as possible. Detachment of soft tissue from the medial flare of the tibia is limited by the insertion of the superficial medial collateral ligament. As one passes farther posteromedially, close to the joint line, dense fibers of the semimembranosus tendon and posterior oblique ligament are encountered. As the lateral tibial flare is exposed, there is no similar concern for the lateral collateral ligament since it attaches to the fibular head. However, upper fibers of the iliotibial band are commonly encountered and some may need to be freed. Removal of a portion of the infrapatellar fat pad facilitates exposure of the lateral compartment. However, the inferior blood supply to the patelia, near the distal pole, should be preserved to the extent possible, particularly if lateral patellar release is anticipated.

Exposure at the lateral flare of the tibia must be done with some caution regarding the perforating anterior tibial artery approximately an inch distal to the usual area of dissection. Careless use of a knife or elevator at this point could produce troublesome bleeding into the anterior compartment.

Eversion of the patella, the next step in exposure, requires adequate proximal incision into the quadriceps tendon, and sometimes reflection of the superior medial corner of the insertion of the patellar tendon on the tibia. Osteophytes on the lateral femoral condyle and lateral patella, and possible contracture of the lateral parapatellar soft tissues may restrict patellar mobilization. More proximal incision into the quadriceps tendon and greater elevation of the patellar tendon from the tibial tuberosity, in order to evert the patella, will not be effective if the surgeon fails to address these problems of patellofemoral osteophytes and synovial and lateral retinacular contracture.


With the patella everted and the knee flexed, several anatomic features relating to alignment can be seen. Upon viewing the distal femur end on, the lateral trochlear facet is seen to be more prominent anteriorly than the medial one. Posteriorly, the femoral condyles appear "level" in the normal knee and actually define neutral rotation of the femur (Fig. 2.2) .

Figure 2.2. View of the distal femur. The posterior aspects of the femoral condyles are "level" and define neutral rotation. The lateral trochlear facet is more prominent than the medial one.

Even in the diseased knee, preservation of the posterior aspects of the femoral condyles is the rule rather than the exception. They generally survive as reliable indicators for rotation of the femoral component.

Medial-lateral position of the knee may be expressed on the femoral side in relation to the patellar tracking mechanism, and one can define the mid point of the knee by following the patellar groove into the intercondylar notch. The relative prominence of the medial femoral epicondyle and the tissue covering the lateral aspect of the lateral femoral condyle, i.e. fat pad, other peripatellar tissue, etc., can combine to create an optical illusion that the center of the knee is more medial than it is in fact (Fig. 2.3).

Figure 2.3. lntraoperatively, infrapatellar soft dying the lateral femoral condyle may impression that the center of the knee is farther medial than it actually is.

From the tibial side, the tuberosity for the patellar tendon lies slightly lateral to the mid line. This is true in flexion and is even more pronounced in extension. In our experience, the medial-lateral position of the tibial tuberde is an unreliable landmark for definition of tibial rotation.

Figure 2.4. View of the ankle from below demonstrating orientation of malleoli in neutral rotation. A line connecting the center of each malleolus forms an angle of approximately 800 with the coronal or frontal plane.

While definitive rotational alignment of the tibia may be determined by the relative position of the ankle malleoli (Fig. 2.4), the positions of the posterior margins of the medial and lateral jibial plateaus are helpful in assessing rotation of the tibia. The posterior extent of each tibial plateau is approximately equal on a normal specimen; therefore, rotational alignment of a tibial component is facilitated by making use of this fact (Fig. 2.5). The presence of posterior osteophytes possibly remaining after performing the transverse tibial cut, and sometimes the more posterior projection of the medial tibial plateau, will occasionally alter the reliability of establishmg rotational alignment solely on the basis of the posterior margins of the tibial plateaus. Therefore, although the Universal Total Knee Instrumentation System utilizes the posterior margins of the tibial plateaus, it does not depend solely upon these, as will be fully developed in Chapter 3.

Figure 2.5. View of proximal tibial and fibula from above. This standard instructional skeleton shows the approximately equal posterior extent of both tibial plateaus.



Collateral and posterior cruciate ligament positions greatly affect the stability characteristics after total knee arthroplasty and place certain restrictions on the surgeon when bone resection and soft tissue releases are performed.

Figure 2.6. Medial view of the femur and tibia. Ligament attachment positions are demarcated. The origin of the medial collateral ligament from the medial femoral epicondyle certainly limits the extent of femoral resection on this side of the joint.


Figure 2.7. Lateral view of the knee. Ligament attachment positions are demarcated. The origin of the lateral collateral ligament from the lateral fern oral epicondyle certainly limits the extent of fern oral resection on this side of the joint.

The medial and collateral ligaments attach to the femur at their respective epicondyles (Figs. 2.6 and 2.7). They are covered by capsule and synovium and are not directly apparent by visual inspection. They are also difficult to identify by palpation with the knee in flexion as the ligaments are relatively relaxed and soft. These femoral attachments limit the level to which the distal femur may be resected. Furthermore, location and orientation of the collaterals make them vulnerable to injury from the saw when making the distal femoral, pos. tenor femoral, and proximal tibial cuts unless they are carefully protected. In addition, the femoral attachment of the posterior cruciate ligament is vulnerable to injury while removing the posterior aspect of the medial femoral condyle.

Figure 2.8. Posterior view of the knee showing ligament attachments. The normal insertion of the posterior cruciate ligament lies below the joint line and usually permits removal of the proximal aspect of the tibia without damage to the ligament.

Since tibial attachment of the posterior cruciate ligament lies just below the normal joint line at the back of the tibia, it is generally possible to make an adequate transverse tibial cut completely across the top of the tibia without damaging this ligament (Fig. 2.8). However, if a relatively deep tibial cut is necessary due to differential wear in one compartment, provision for protection of the posterior cruciate ligament and moderate valgus deformity, the surgeon at its tibia I attachment will be necessary.


In order to perform lateral soft tissue release comfortably and adequately in situations of mild and moderate valgus deformity, the surgeon must be familiar with the locations of structures on the lateral and posterolateral aspects of the knee (7) (Figs. 2.9 and 2.10). The iliotibial blends proximally, posteriorly with the intermuscular septum and courses superficially to Gerdy's tubercle. The deeper lateral collateral ligament takes origin from the lateral femoral epicondyle and runs deep to the iliotibial band inserting on the proximal fibula. The deeper popliteus tendon and the intervening inferior lateral geniculate artery ought to be avoided, if possible, when resecting the lateral meniscus and debriding the posterolateral joint space. The locations of the posterolateral capsule, arcuate ligament complex, biceps femoris tendon and finally the lateral head of the gastrocnemius are important in cases of extensive lateral release. The course of the peroneal nerve relative to these structures must be appreciated to avoid not only direct damage, but also the possibility of stretch injury following extensive release.

Figure 2.9.
Lateral aspect of the knee. (Reprinted with permission from J. R. Seebacher et al.: Journal of Bane and Joint Surgery, 64A:536-541, 1 9B2 (7).)

Figure 2.10. Anatomy of the lateral side of the knee shown in transverse section, viewed from above. (Reprinted with permission from J. R. Seebacher et al.: Journal of Bone and Joint Surgery, 64A:536-541, 1982 (7).

On the medial side of the knee, the pes anserinus insertion overlaps the tibial attachment of the superficial medial collateral ligament, but does not obscure the tibial attachment of the deep medial collateral ligament (Fig. 2.11). When severe deformity requires exposure of this region, the upper border of the pes tendons must be identified followed by their retraction or reflection.

Figure 2.11. Medial aspect of the tibial side of the knee joint. The relationship of the pes tendons to the insertion of the superficial medial collateral ligament is shown.

While the normal attachment points of the major knee ligaments are important to the surgeon as routine exposure and bone cuts are made, understanding the effects of repositioning these ligaments when dealing with cases of more severe deformity is even more complex. Many of the specific problems encountered are discussed in the sections of Chapter 11 dealing with the surgical management of severe fixed deformity. For the purposes of understanding the relevant chapter, a few principles are sufficient.

Although the kinematics of the knee involve motion patterns comprised of axial internal and external rotation and slight varus-valgus movement during flexion and extension, the obvious predominant motion at the knee is flexion-extension. Furthermore, this flexion-extension or hinge type of motion occurs about centers of rotation located above the joint line on the femoral side. For this reason, the behavior of the ligaments in providing useful stability throughout a wide range of motion cycle is highly dependent upon the femoral attachment points of the ligaments and the relationships of these points to the instant centers of flexion-extension rotation. Femoral bone cuts which alter the relative positions of the ligamentous attachments on the femur with respect to the prosthetic joint line will simultaneously change the normal relationship of these ligament attachments to the instant centers of rotation. As a result, desired ligament balance throughout the motion cycle can be lost. While tibia! bone cuts may move the prosthetic joint line and the tibial attachments of the ligament insertions to new positions, the ligament relationship to the centers of rotation is less disturbed and the potential for disruption of overall ligament balance is consequently lost.

The goal of total knee arthroplasty is to reestablish the joint line in its normal position vis-a-vis the femoral and tibial attachments of the ligaments which are being preserved. Only then can these ligaments be expected to function in a balanced way through the full range of movement. The surgical techniques developed in Chapters 3 and 11 are designed, in most instances, to reestablish the joint line in its normal position. When this is impossible in cases of severe deformity, the consequences of moving the joint line in any given direction are addressed. It is critically important that surgeons doing total knee replacement with relatively nonconstrained prostheses comprehend the effect of individual component position on soft tissue balance throughout the full range of movement.



Many features of the femoral condyles and appearances of the distal femur have been described. The medial and lateral femoral condyles are inclined asymmetrical]y and are of different medial-lateral dimension. The medial Femoral condyle is thinner and more acutely inclined (Fig. 2.12).

Figure 2.12. View of the distal femur. This view demonstrates the slightly thinner medial femoral condyle which also has more acute inclination than the lateral. The anterior prominence of the trochlear flare on the lateral aspect demonstrates an obvious, increased anterior/posterior dimension for the lateral femoral condyle. (Reprinted with permission from I. A. Kapandji: The Physiology of the Joints, Vol. Il, Churchill LivingstoneNew York, 1970(5).)

Figure 2.13. (A) Diagrammatic sagittal section of the medial femoral condyle showing the radii of curvature. Anterior-posterior dimension is smaller than that shown in B, which represents the lateral femoral condyle. (B) Lateral femoral condyle showing radii and centers of curvature and longer anterior-posterior dimension. The overall forms of each femoral condyle is similar; however, the radii of curvature and general dimensions are dissimilar. (Reprinted with permission from I. A. Kapandji: The Physiology of the Joints, VoL II, Churchill Livingstone, New York, 1970(5).)


Sagittal sections defining profiles of the two condyles show the lateral one to be larger in the anterior-posterior direction (Fig. 2.13.). These diagrams show the loci of the centers of curvature and indicate approximate values for the radii of curvature. A similar, general pattern for the medial and lateral femoral condyles is seen, but the actual numbers are significantly different.


A coronal section through the upper tibia shown in Figure 2.14 demonstrates concave contours to both tibial plateaus. There is also a concave contour to the medial tibial spine while the lateral tibial spine is convex. Sagittal sections on the right show major medial-lateral differences in the tibial plateaus (Fig. 2.15). In this projection, the medial tibial plateau is concave while the lateral is convex.


Figure 2.14. Coronal section through the upper tibia. The contours of each tibia plateau, medial and lateral, are concave. The contour to the medial tibial spine is concave, while that of the lateral tibia spine is convex. (Reprinted with per-mission from I. A, Kapandji: The Physiology of the Joints, Vol. II, Churchill Livingstone, New York, 1970(5).)

Figure 2.15. (A)This sagittal section of the medial tibia plateau demonstrates a concave configuration. (B) This sagittal section of the lateral tibia plateau demonstrates a relatively convex contour. (Reprinted with permission from I. A. Kapandli: The Physiology of the Joints, Von (I, Churchill Living stone, New York, 1970 (5).)

The medial compartment geometry appears to provide more stability with respect to rotation than that of the lateral. This becomes an important point when considering the kinematics of the knee.

There is a general posterior or downward slope of the tibial plateau when viewed from the side and it is seen that this is more pronounced in the lateral compartment (Figs. 2.6, 2.7, and 2.16). One significance of this is that when the transverse tibial cut for total knee arthroplasty is made perpendicular to the sagittal plane of the tibial shaft, more bone is resected from the anterior aspect of the tibia than from the posterior portion.

Figure 2.16. When viewed from the side, the tibial plateau area is seen to slope downward posteriorly. The standard transverse tibial cut will, therefore, remove more bone from the anterior aspect of the proximal tibia than from the posterior portion.


The articular surface of the patella is roughly oval in shape and divided principally into lateral and medial facets by a vertical ridge. The lateral borders of the femoral condyles in flexion, facet is frequently of greater area while the medial facet is highly variable. The medial facet is further subdivided by a small cartilaginous ridge near the medial border of the patella, defining the odd facet (Fig. 2.17).

Figure 2.17. This view of the articular surface of the patella shows the complicated surface contours. There is a median ridge dividing the patella principally into medial and lateral facets.

Articulation of the patella with the trochlear groove of the femur initially, and later with the curs with variation of contact area on the patella. Furthermore, the patella undergoes a rotation in the sagittal plane, a rocking type of motion, as the knee flexes and extends, thereby keeping the vector of patellofemoral joint reaction force perpendicular to the contact area (Fig. 2.18).

Figure 2.18. This diagram of the patellofemoral relationship, from the lateral view, demonstrates the vector of patellofemoral joint reaction force which remains perpendicular to the contact area.


Effect of Form on Functon of the Knee

Although the principal motion of the knee is clearly flexion and extension, it is neither pure flexion nor simple flexion. In fact, the combinations of possible movements of this complicated joint have lead to considerable controversy concerning which movements actually do occur with the knee under conditions of functional loading. The remainder of this chapter will deal with the impact of anatomy on knee motion while the following chapter will deal with the dynamic aspects of rotational stability under load as experimentally determined.

Normal knee flexion is from 0° to 135° or 140° for most occidentals, being primarily limited by intervening soft tissue of the calf and posterior thigh. For heavily muscled or obese individuals this limitation can be in the 120°-125° range. During the initial degrees of flexion, the femoral condyles roll posteriorly bn the tibial plateaus. Gradually, pure rolling motion converts to sliding motion as the posterior margins of the tibial plateaus are approached. Since the medial tibial plateau is convex and the medial meniscus is less mobile than the lateral, rolling motion is converted to sliding motion earlier on the medial side than on the lateral meniscus. This differential produces an automatic internal rotation on of the tibia on the femur as flexion proceeds and conversely an external rotation in the last degrees of extension. This tatter motion has been referred to as the "screw-home" mechanism and wag well described by Sir John Goodsir in 1851 (1).

Although the points of attachment of the posterior cruciate ligament remain essentially equidistant during this movement, this is not true for the medial and lateral collateral ligaments. Because of the decreasing radii of curvature of the condyles, and the posterior slope of the tibial plateaus, the femoral and tibial points of attachment of the respective collateral ligaments begin to approximate one an6ther with flexion, thereby causing some relaxation of the collateral ligaments in flexion. Furthermore, because of the medial and lateral compartment asymmetry, the potential for this approximation and hence soft tissue relaxation is greater on the lateral than the medial side. These differential alterations in the ligament attachment separation with flexion are additional factors permitting greater translation of the lateral tibial plateau with respect to the femur and, as a result, the passive internal, screw-home rotation with flex-ion. In addition, this relaxation relates to the potential for additional tibial rotation discussed below. It should be understood that this relaxation is assured because the femoral point of attachment occurs within the concavity of the line joining the centers of the radii of curvature. If the attachment occurred outside this concavity, especially if it were more anterior on the femoral condyle, no relaxation and possibly even a stretching of the ligaments would occur. Moreover, the functional position of this ligament attachment can be altered by changing the radius of curvature of the condyles either by prosthetic design or prosthetic component placement. The posterior slope of the tibia enhances the approximation of these points of attachment and is also important for maintaining the posterior cruciate at constant length, but it is the femoral points of attachment relative to the concavity of the line connecting the instant centers which is capital.

Because of the posterior roll back and the collateral ligament relaxation, the second kind of rotational movement, elective rotation, is made possible. Elective rotation is rotational movement which may occur in response to muscle movements or externally applied rotational loads. Unlike automatic rotation which must occur and does occur with each flexion and extension as a result of passive factors, operative at the knee, elective or active rotation may or may not occur.

The actual movement of the condyles in relation to the tibial plateau during active rotation depends on the degree of flexion of the knee.

The medial condyle will have rolled back to the posterior margin of the tibial plateau by 20-25° of flexion, whereas the lateral condyle will not reach the posterior margin of the plateau until after 400 of flexion.

Figure 2.19. Line drawings show the femur above the tibial plateaus. The tibial plateaus are shaded. The greater prominence of the lateral trochlear facet on the femur indicates that these figures represent the view from above opening down oh a right knee. The figure to the far left shows the relationship of the femoral condyles and tibial plateaus with the tibia in the internal rotation. The central figure shows the bones in neutral rotation and the figure on the right shows the tibia in external rotation. The relative displacement of the lateral plateau in external rotation is much greater than the displacement of the medial plateau with internal rotation.

From its posterior position the medial condyle rotates only slightly posteriorly on the tibial plateau with external tibial rotation, whereas the lateral condyle is free to rotate anteriorly all the way to the anterior margin of the lateral tibial plateau (Fig. 2.19). With internal tibial rotation the opposite movements occur. However, because the lateral tibial plateau is both longer and the lateral ligament more lax, external rotation of the tibia on the femur is both freer and greater in degrees than internal rotation.

It should also be noted that, because of the posterior downward slope of the plateaus, the lateral condyle is moving up as well as anteriorly with external tibial rotation. This movement produces a varus shift of the axial alignment of the knee. Internal rotation has the opposite effect, producing a valgus shift. Although the importance of this shift is not known, it does occur, and to ignore it in carrying out dynamic tests on the knee in a loading apparatus will produce spurious results.

Kinematics of the Patellofemoral Joint

The role of the patellofemoral joint in stabilizing the knee has been known for decades, but unfortunately the importance of its anatomy in carrying out that role has not been fully appreciated in prosthetic design. With the knee in full extension and the quadriceps contracted, the patella is actually out of contact with the trochlear groove. Depending on the length of the patellar tendon, stable contact begins between 5° and 20° of flexion. The contact begins at the inferior margin of the patella and moves proximally as flexion proceeds. The contact area extends from the medial margin of the medial facet to the lateral margin of the lateral facet as a broad band of contact moving from distal to proximal (Fig. 2.20).

Figure 2.20. Patellofemoral contact prints. (Reprinted with permission from R. P. Ficat and D S Hungerford: Disorders of the Patello-Femoral Joint, Williams & Wilkins, Baltimore, 1977.)

The shape of the patella is best known as it is seen in the skyline view, i.e. in the transverse plane. In this plane it is perfectly congruent with the trochlea which assures its medial-lateral stability (Fig. 2.21). However, longitudinal sectioning shows that it is almost a flat surface which means that it is perfectly unconstrained in the longitudinal plane as far as its anatomical form is concerned (Fig. 2.22). It is, therefore, the length of the patellar tendon and the angle between the patellar tendon and the quadriceps tendon which determine the area of the patella which will be loaded. The patellar form, therefore, provides stability against lateral subluxation but does not impede the patella from rocking around its transverse axis to the point at which the resultant of the patellofemoral joint reaction force will be perpendicular to the contact surface, as it must always be (Fig. 2.18).


Figure 2.21. Transverse section through the patellofemoral joint as seen radiographically. This view demonstrates good congruity between the medial surfaces of the articulation as welt as the lateral surfaces.

Figure 2.22. (A) Sagittal section through a human patella and femur at the patellofemoral joint. The surfaces are unapproximated in this view. (B) The patella is shown applied to the femoral surface. Compression of the opposing articular cartilaginous surfaces yields an extended region of contact, a line in this plane rather than a point.


This chapter has described, first, the anatomy of the knee as it relates to standard surgical exposure, followed by consideration of landmarks for estimating rotational alignment. Second, it has examined the structures approached for soft tissue release and considered the limitations presented by the capsular ligamentous structures when addressing bone resections for correction of fixed deformity. Last, it has examined the contours of the knee joint including the patellofemoral articulation and described the normal motion patterns, emphasizing the automatic external rotation of the tibia with knee extension, known as the "screw-home" mechanism, and the additional "active" rotation capable as a result of specific muscle forces or other torques applied externally.


  1. Goodsir S: Anatomy and mechanism of the knee joint. In Turner The Anatomical Memoirs of John Goodsir. Edinburgh, Adams and Charles Black, 1868.
  2. Grant JCB: Grant's Atlas of Anatomy. Baltimore, Williams & Wilkins, 1972.
  3. Gray H: Anatomy of the Human Body; Ed. 29, ed. CM Goss. Philadelphia, Lea & Febiger, 1973.
  4. Hastings DE, Hewitson WA: Double hemiarthroplasty of the knee in rheumatoid arthritis J Bone Joint Surg 55B:112-118, 1973.
  5. Kapandji IA: The Physiology of the Joints, Vol 1I New York, Churchill Livingstone, 1970.
  6. Manning JO, Walter RS: Quadriceps Reflecting Approach for Total Knee Arthroplasty. Poster Exhibit, American Academy of Orthopaedic Surgeons, Las Vegas, Nev., March 1981.
  7. Seebacher JR, Inglis AE, Marshall JL, et al: The structure of the posterolateral aspect of the knee. J Bone Joint Surg 64A:536-541, 1982.