A laboratory exercise for Physiology and Comparative Anatomy teaching that leverages the power of 3D printing 

3D printing offers a novel avenue for hands-on science and classroom laboratory exercises because it allows students to actually feel 3D anatomy and physically measure attributes of models. Classical studies using authentic skeletal material are phenomenal, but it can be difficult to exclusively demonstrate a single physical attribute (e.g. jaw-closing out-lever) without confounding anatomical change. We have designed a laboratory exercise where students physically measure (using calipers) mechanical parameters of 3D-printed skulls (e.g. lever mechanics of the jaw) and anatomical determinants of muscle force (muscle belly volume) to calculate biting force in two distinct 3D printed models: a dog and a primate. Then, alternative models of muscle architecture are used whereby muscle fiber length either: 1) spans the entirety of MTU length; or 2) is the shortest possible length to create a contracting moment (i.e. is almost perpendicular to the MTU line of action). The third step in the exercise involves a digitally altered primate skull, whereby the out-lever (i.e. the rostrum and anterior teeth) has been digitally elongated to give the same mechanical advantage (in-lever to out-lever ratio) as the dog, but without altering the input muscle force, mechanical advantage, or adductor muscle volume. This exercise effectively leverages (pun completely intended) the utility of 3D printing to underscore specific anatomical characteristics (e.g. jaw output lever change) without confounding factors (e.g. cranial elongation).

Here's a link to our SICB 2018 Abstract: http://sicb.org/meetings/2018/schedule/abstractdetails.php?id=711

Activity

Problem

Species rarely differ in a single attribute. For example, when examining primate and pinniped skull anatomy (below left and below right, respectively) one of the biomechanically relevant metrics is the jaw-closing in-lever (distance between muscle attachment and jaw joint), and another is the jaw-closing out-lever (distance between jaw joint and biting tooth); the ratio of these two is especially relevant. However, in the skulls below these two metrics co-vary, so their ratio (the true metric of skeletal leverage) is equivalent, despite diversity in skeletal form. Independent variation in either of these metrics is rare.

Activity goals

1) To make specimens that students can examine that selectively vary only in a single skeletal attribute (e.g. skull leverage) without modifying others, such as muscle size/shape;

2) To have students physically measure biomechanically relevant metrics on these specimens and a variety of real skulls to compare the biomechanics of jaws across species;

3) To practice mathematical modeling to estimate various attributes of a musculoskeletal system (e.g. range of motion, force, etc.) and define the range of those parameters given skeletal diversity;

4) To explicitly demonstrate the interconnectedness of muscles and skeletons in the jaw system of mammals.

Materials

3D printed skull and modified skull, various other real mammal skulls as available, calipers

Download our modified models (plus originals):

https://drive.google.com/open?id=1257mglhtpaDzhSHpyxUfv_bnVSJ5oluK

Free sources of 3D models:

http://morphosource.org/ (animal and human bone surface models)

http://www.digimorph.org/ (animal micro-CT data and some models)

http://morphomuseum.com/ (animal bone surface models)

http://www.virtualfossils.com/ (models of fossilized animals)

[More coming soon!]