science

Goodsall's rule

Goodsall's rule is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Goodsall's rule rather than just read about it. In short: Goodsall's rule, developed in 1887, relates the external opening (in the perianal skin) of an anal fistula to its internal opening (in the anal canal). It states that if the perianal skin opening is posterior to the transverse anal line, the fistulous tract will open into the anal canal in the midline posteriorly, sometimes taking a curvilinear course.

Key takeaways

Goodsall's rule belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Goodsall's rule to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Goodsall's rule from memory before moving on to harder problems.

Reference excerpt

Goodsall's rule, developed in 1887, relates the external opening (in the perianal skin) of an anal fistula to its internal opening (in the anal canal). It states that if the perianal skin opening is posterior to the transverse anal line, the fistulous tract will open into the anal canal in the midline posteriorly, sometimes taking a curvilinear course. A perianal skin opening anterior to the transverse anal line (within 3 cm radius) is usually associated with a radial fistulous tract but if it is beyond the 3 cm radius it will take a curvilinear course and open in the midline posterioly. Or in more direct terms, it means that anterior-opening fistulas tend to follow a simple, direct course if it is within the 3 cm radius, beyond which it takes a curving path and opens in poster midline. The posterior-opening fistulas may follow a devious, curving path with some even being horseshoe-shaped before opening in the posterior midline. Fistulas can be described as anterior or posterior relating to a line drawn in the coronal plane across the anus, the so-called transverse anal line. Anterior fistulas will have a direct track into the anal canal. Posterior fistulas will have a curved track with their external opening lying in the posterior midline of the anal canal. An exception to the rule are anterior fistulas lying more than 3.75 cm from the anus, which may have a curved track (similar to posterior fistulas) that opens into the posterior midline of the anal canal. According to a study on patients undergoing operations for cryptoglandular anal fistulas, 92.9% of fistulas within 1.5 cm followed the Goodsall rule, whereas only 47.4% followed it beyond 3.6 cm. In situations where there are multiple anal fistulae, the course would be similar to that of posterior-opening fistulae because of branching and communication between these openings. The investigation of choice for anal fistulas is magnetic resonance imaging (MRI). Fistulograms can be used to demonstrate the track of the fistula. Goodsall's rule is named after David Henry Goodsall who described it in 1900.

References "Perianal sepsis". Surgical Tutor. Philip H Gordon (2002). Principles and Practice of Surgery for the Colon, Rectum and Anus. Informa Health Care. p. 245. ISBN 0-8247-4282-6.

Worked examples

Example 1 — a first encounter with Goodsall's rule

Start with the simplest possible case. Write down what Goodsall's rule claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Goodsall's rule before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Goodsall's rule ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Goodsall's rule

In research: Goodsall's rule appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Goodsall's rule in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Goodsall's rule is common in secondary-school and first-year university syllabi. It links to neighbouring topics Colorectal surgery, Rules, so understanding it makes those chapters shorter.
In everyday life: Look for Goodsall's rule outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

Ask Teacher Smith questions about this articleOpens your AI tutor with a question about “Goodsall's rule” →

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Goodsall's rule in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Goodsall's rule means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Goodsall's rule out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Goodsall's rule in simple terms?

Why does Goodsall's rule matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Goodsall's rule?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Goodsall's rule.