This document provides an overview of the biomechanics of the knee complex. It describes the anatomy of the tibiofemoral and patellofemoral joints, including the femoral condyles, tibial plateaus, and alignment of the femur and tibia. It also discusses how weight bearing forces are distributed during static and dynamic activities, and how malalignment can increase stresses on the medial or lateral compartments.

1. Biomechanics
of the
Knee Complex : 1
DR. DIBYENDUNARAYAN BID [PT]
THE SARVAJANIK COLLEGE OF PHYSIOTHERAPY,
RAMPURA, SURAT

2. Introduction
 The knee complex is one of the most often injured
joints in the human body.
 The myriad of ligamentous attachments, along with
numerous muscles crossing the joint, provide insight
into the joint’s complexity.
 This anatomic complexity is necessary to allow for the
elaborate interplay between the joint’s mobility and
stability roles.

3.  The knee joint works in conjunction with the hip
joint and ankle to support the body’s weight during
static erect posture.
 Dynamically, the knee complex is responsible for
moving and supporting the body during a variety of
both routine and difficult activities.
 The fact that the knee must fulfill major stability as
well as major mobility roles is reflected in its
structure and function.

4.  The knee complex is composed of two distinct
articulations located within a single joint capsule:
the tibiofemoral joint and the patellofemoral joint.
 The tibiofemoral joint is the articulation between the
distal femur and the proximal tibia.
 The patellofemoral joint is the articulation between
the posterior patella and the femur.

5.  Although the patella enhances the tibiofemoral
mechanism, the characteristics, responses, and
problems of the patellofemoral joint are distinct
enough from the tibiofemoral joint to warrant
separate attention.
 The superior tibiofibular joint is not considered to be
a part of the knee complex because it is not
contained within the knee joint capsule and is
functionally related to the ankle joint.

6. Animated Knee Joint

7. Structure of the Tibiofemoral Joint
 The tibiofemoral, or knee, joint is a double condyloid
joint with three degrees of freedom of angular (rotatory)
motion.
 Flexion and extension occur in the sagittal plane around
a coronal axis through the epicondyles of the distal
femur,
 medial/lateral (internal/external) rotation occur in the
transverse plane about a longitudinal axis through the
lateral side of the medial tibial condyle, and
 abduction and adduction can occur in the frontal plane
around an anteroposterior axis.

8.  The double condyloid knee joint is defined by its
medial and lateral articular surfaces, also referred to
as the medial and lateral compartments of the knee.
 Careful examination of the articular surfaces and the
relationship of the surfaces to each other are
necessary for a full understanding of the knee joint’s
movements and of both the functions and
dysfunctions common to the joint.

9. Femur
 The proximal articular surface of the knee joint is
composed of the large medial and lateral condyles of
the distal femur.
 Because of the obliquity of the shaft of the femur, the
femoral condyles do not lie immediately below the
femoral head but are slightly medial to it (Fig. 11-1A).

10.  As a result, the lateral condyle lies more directly in
line with the shaft than does the medial condyle.
 The medial condyle therefore must extend further
distally, so that, despite the angulation of the femur’s
shaft, the distal end of the femur remains essentially
horizontal.

11.  In the sagittal plane, the condyles have a convex
shape, with a smaller radius of curvature posteriorly
(see Fig. 11-1B).
 Although the distal femur as a whole has very little
curvature in the frontal plane, both the medial and
lateral condyles individually exhibit a slight
convexity in the frontal plane.
 The lateral femoral condyle is shifted anteriorly in
relation to the medial femoral condyle.

12.  In addition, the articular surface of the lateral
condyle is shorter than the articular surface of the
medial condyle.
 When the femur is examined through an inferior
view (Fig. 11-2), the lateral condyle appears at first
glance to be longer.
 However, when the patellofemoral surface is
excluded, it can be seen that the lateral tibial surface
ends before the medial condyle.

13.  The two condyles are separated inferiorly by the
intercondylar notch through most of their length but
are joined anteriorly by an asymmetrical, shallow
groove called the patellar groove or surface that
engages the patella during early flexion.

16. Tibia
 The asymmetrical medial and lateral tibial condyles
or plateaus constitute the distal articular surface of
the knee joint (Fig. 11-3A).
 The medial tibial plateau is longer in the
anteroposterior direction than is the lateral plateau;
however, the lateral tibial articular cartilage is
thicker than the articular cartilage on the medial
side.

18.  The proximal tibia is larger than the shaft and,
consequently, overhangs the shaft posteriorly (see
Fig. 11-3B).
 Accompanying this posterior overhang, the tibial
plateau slopes posteriorly approximately 7° to 10°.

19.  The medial and lateral tibial condyles are separated
by a roughened area and two bony spines called the
intercondylar tubercles (Fig. 11-4).
 These tubercles become lodged in the intercondylar
notch of the femur during knee extension.

21.  The tibial plateaus are predominantly flat, with a
slight convexity at the anterior and posterior
margins,
which suggests that the bony architecture of the
tibial plateaus does not match up well with the
convexity of the femoral condyle.
 Because of this lack of bony stability, accessory joint
structures (menisci) are necessary to improve joint
congruency.

22. Tibiofemoral Alignment
and Weight-Bearing Forces
 The anatomic (longitudinal) axis of the femur, as
already noted, is oblique, directed inferiorly and
medially from its proximal to distal end.
 The anatomic axis of the tibia is directed almost
vertically.

23.  Consequently, the femoral and tibial longitudinal
axes normally form an angle medially at the knee
joint of 180° to 185°;
that is, the femur is angled up to 5° off vertical,
creating a slight physiologic (normal) valgus angle at
the knee (Fig. 11-5).

24.  If the medial tibiofemoral angle is greater than 185,
an abnormal condition called genu valgum (“knock
knees”) exists.
 If the medial tibiofemoral angle is 175° or less, the
resulting abnormality is called genu varum (“bow
legs”).
 Each condition alters the compressive and tensile
stresses on the medial and lateral compartments of
the knee joint.

25.  An alternative method of measuring tibiofemoral
alignment is performed by drawing a line from the
center of the femoral head to the center of the head
of the talus (see Fig. 11-5).
 This line represents the mechanical axis, or weight
bearing line, of the lower extremity, and in a
normally aligned knee, it will pass through the center
of the joint between the intercondylar tubercles.

27.  The weight-bearing line can be used as a
simplification of the ground reaction force as it
travels up the lower extremity.
 In bilateral stance, the weight-bearing stresses on
the knee joint are, therefore, equally distributed
between the medial and lateral condyles (or medial
and lateral compartments).

28.  However, once unilateral stance is adopted or
dynamic forces are applied to the joint,
compartmental loading is altered.
 In the case of unilateral stance (e.g., during the
stance phase of gait), the weight-bearing line must
shift medially across the knee to account for the now
smaller base of support below the center of mass
(Fig. 11-6A).

30.  This shift increases the compressive forces on the
medial compartment (see Fig. 11-6B).
 Abnormal compartmental loading may be also be
caused by frontal plane malalignment (genu varum
or genu valgum).
 Genu valgum, for instance, shifts the weight-bearing
line onto the lateral compartment, increasing the
lateral compressive force while increasing the tensile
forces on the medial structures (Fig. 11-7A).

32.  whereas the tensile stresses are increased laterally
(see Fig. 11-7B).
 The presence of genu valgum or genu varum creates
a constant overload of the lateral or medial articular
cartilage, respectively, which may result in damage
to the cartilage and the development of frontal plane
laxity.
 Genu varum, for instance, may con-tribute to the
progression of medial compartment knee

33.  In the case of genu varum,
the weight-bearing line is shifted medially,
increasing the compressive force on the medial
condyle,
causes osteoarthritis and lead to excessive medial
joint laxity as the medial capsular ligament’s
attachment sites are gradually approximated
through the erosion of the medial compartment’s
articular cartilage.

34. End of Part - 1