I happen to be in the middle of a research project that address the role of
the fibula in human evolutionary history, so I can't resist putting my two
cents in on this posting.

The human fibula does not articulate with the femur, so it cannot
participate in rotational movements at the knee joint. (ASIDE: The
rotational movements at the elbow are termed supination and pronation. Many
kinesiologists uses these terms to describe accommodative movements in the
transverse tarsal and subtalar joint. Anatomically and morphologically the
use of these terms incorrect -- but I suppose that's the topic of another
posting).

The fibula does, however, contribute the formation of the talocrural joint
(the lateral malleolus), The actions of the fibula allow accommodations in
plantarflexion and dorsiflexion to keep the joint stable. In addition, the
fibula is an important anchor for a number of muscles that act upon the foot
and that help to maintain the foot arches (notably the fibularis (peroneus)
longus). Finally, the fibula aids in the transmission of body weight (10% to
20% depending upon who you ask), through the leg.

The tibiofibular articulations actually show some interesting qualities. The
distal joint is a syndesmosis. That is, the joint is bound only be
ligaments, which is usually the defining characteristic of an immovable
joint. Yet, this is where the accomodation to plantarflexion and
dorsiflexion occurs, so some degree of movement is necessary (in some
mammals, the tibia and fibula are fused at this location, and the more
proximal parts of the fibula have degenerated). The proximal tibiofibula
joint is a synovial joint. Synovial joints are the joints of movement, yet
it is not clear how, or if, the bone moves at this joint. The biceps femoris
muscle inserts onto the head of the fibula, which implies that it would
contribute to the movement of the fibula in relation to the tibia although I
cannot find any reports that this does in fact happen. In apes the proximal
fibula is very rounded and looks much like the head of the radius (although
developmentally the fibula is an analog of the ulna). The ape condition
implies that there is a great deal of movement here, but their distal
tibiofibular joint is extensive and thereby implies very little movement,
which appears to be an anatomical contradiction. Many old world monkeys
possess a very tenuous articulation between the tibia and fibula (the
proximal joint space can be hard to locate on dry tibias). These same
monkeys posses a unique muscle (fibulotibialis) that seems to be dedicated
the movement of the fibula -- but again I can find no reports in the
literature that document this function. So, we've got a lot of unusual and
unexplained characteristics in the fibula. I'm working on the answers, and
I'll let you know if I learn anything.

As to why we have a fibula in the first place, that's simply evolutionary
history. The basic tetrapod design stipulates the embryological development
of the limb (upper and lower) to follow the pattern: girdle -- single
proximal bone -- two distal bones -- three proximal carpals/tarsals -- four
distal carpals/tarsals -- five digital rays. You will see this basic pattern
in the embryological development of every fourlimbed vertebrate, the
differences among animal limbs comes latter. In amphibians and many reptiles
the fibula does articulate with the femur creating a joint situation that
permits pronation and supination of the leg (see comments above). More
derived animals (such as mammals) have moved the fibula away from the femur,
thereby creating a knee joint that is more useful in weight transmission at
the expense of mobility. So, its not a question of two bones being better
than one. Rather, it is that two bones were part of the original design, and
latter mammalian evolution reflect different types of modification of the
original body plan. Remember that evolutionary pressures would not remove
the bone unless (1) there was a benefit to do so and (2) there was a genetic
variant that allowed it. Neither of these conditions have existed in human
evolutionary history.

Thomas M. Greiner, Ph.D.
Assistant Professor
Department of Anatomy
New York Chiropractic College
Seneca Falls, NY 13148-0800

Phone: (315) 568-3183
Fax: (315) 568-3017
Email: tgreiner@nycc.edu


> -----Original Message-----
> From: ialdous [SMTP:ialdous@IS2.DAL.CA]
> Sent: Saturday, January 13, 2001 10:49 AM
> To: BIOMCH-L@NIC.SURFNET.NL
> Subject: Re: What does the fibula do?
>
> Hi Bimchers!
>
> I am puzzled as to why we have two bones in our lower leg. Is this for
> rotation similar to the radius-ulna design in the forearm or some other
> reason? We don't seem to be able to rotate the lower leg with the same
> range
> of motion as our forearm. What is the evolutionary history behind the two
> bone
> design in the lower leg?
> Are two bones really better than one?
> Your thoughts are welcomed and encouraged.
>
> Ian Aldous
> Dept. Biomedical Engineering
> Dalhousie University
> ialdous@is2.dal.ca
>
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