MRCS Part B Revision · Anatomy
Intercostals — MRCS Part B Anatomy
By Dr Richard Miller, MBChB FRCS · Reviewed
Intercostals is a surgical anatomy station. Three of the seventeen examined stations in the MRCS Part B OSCE are surgical anatomy. The anatomy stations put a prosected specimen, a model or an image in front of you and ask you to identify structures and say what they do, what they relate to and what happens when they are injured.
What you need to know for the Intercostals station
An intercostal space dissected in layers, which is the anatomy behind every chest drain and thoracotomy.
The three layers
External intercostal, with fibres running downwards and forwards (the direction of hands in pockets), from the tubercle of the rib to the costochondral junction, where it continues as the anterior intercostal membrane. Internal intercostal, with fibres running downwards and backwards, from the sternum to the angle of the rib, continuing behind as the posterior intercostal membrane. Innermost intercostal, deepest and incomplete, with the same fibre direction as the internal layer, separated from it by the neurovascular bundle. The two inner layers are at right angles to the outer, which lets the space resist both elevation and depression of the ribs.
Subdivisions of the innermost layer
Three muscles in the same plane: the innermost intercostals laterally, the subcostales posteriorly, bridging two or three ribs near the angles, and transversus thoracis (sternocostalis) anteriorly, running from the back of the sternum to the costal cartilages, with the internal thoracic vessels lying on it.
The neurovascular bundle
It runs between the internal and innermost layers, in the costal groove along the lower border of the rib, in the order vein, artery, nerve from above down. The posterior intercostal arteries come from the aorta (the first two from the costocervical trunk) and anastomose with the anterior intercostals from the internal thoracic artery. The nerves are the anterior rami of T1–T11; each gives a lateral cutaneous branch halfway round and ends as an anterior cutaneous branch. They supply the intercostal muscles of their own space, the parietal pleura and the skin, and T7–T11 continue into the abdominal wall.
Placing a chest drain
Go through the space just above the lower rib, on its upper border, to stay away from the bundle under the rib above. Use the safe triangle — between the anterior border of latissimus dorsi, the lateral border of pectoralis major and a line at the level of the nipple, with the apex in the axilla — which keeps the drain above the diaphragm and away from the internal thoracic artery, the long thoracic nerve and breast tissue. Blunt dissection over the rib, never a trocar. A collateral branch does run along the upper border of the rib below, so the lower part of the space is not entirely empty, but it is the safer side.
Function and innervation
The external intercostals and the interchondral part of the internal intercostals lift the ribs in inspiration; the rest of the internal and the innermost layers pull them down in forced expiration. All of them stiffen the chest wall so it does not sink in when the diaphragm descends. Each layer is supplied by the intercostal nerve of its own space.
Images from this station
What are you asked at the Intercostals station?
The station runs to 12 questions over nine minutes. These are the questions as they are put to you; the model answers are in the question bank.
- Describe the muscles found in the intercostal space
- Identify structures 1-4
- Identify structures 5-7
- Identify structure 8
- Identify structures 9 and 10
- Identify structures 11-12
- Between which intercostal muscle layers is the neurovascular bundle located?
- Why must care be taken when inserting a chest drain between ribs?
- What is the role of the intercostal muscles?
- What is the innervation of the internal and external intercostal muscles?
- How do the fibres of the internal and external intercostal muscles differ?
- How can the innermost intercostal muscle layer be sub-divided?
How is the anatomy station marked in MRCS Part B?
An anatomy station is marked out of 20, and all 20 marks go to clinical knowledge and its application. No communication, technical-skill or professionalism marks are available in an anatomy bay, which is unusual: in most other stations more than half the marks sit outside your factual knowledge.
FAQ
What does the Intercostals station ask?
It opens with "Describe the muscles found in the intercostal space" and runs to 12 questions over nine minutes. An anatomy station is marked out of 20, and all 20 marks go to clinical knowledge and its application. No communication, technical-skill or professionalism marks are available in an anatomy bay, which is unusual: in most other stations more than half the marks sit outside your factual knowledge.
How many anatomy stations are there in MRCS Part B?
Three of the seventeen examined stations are surgical anatomy, and two more are surgical pathology. Together they make up the five Anatomy and surgical pathology stations of the Applied Knowledge component.
How are the anatomy stations marked?
Each station is marked out of 20 and all 20 marks are awarded for clinical knowledge and its application. You also receive a separate global rating of pass, borderline or fail for the station.
Are the specimens real?
Yes. Anatomy bays use prosected cadaveric specimens, models and images. Candidates report being shown photographs of specimens as well as the specimens themselves, so practise on real dissection photography rather than diagrams alone.
What happens if I get an early question wrong?
The intercollegiate guidance says that if a wrong answer would affect your performance on later questions at the same station, the examiner gives you the correct answer before moving on. One mistake does not have to cost you the whole bay.
How many stations are in the MRCS Part B OSCE?
Seventeen examined stations of nine minutes each, with a minute to read the task before each one. Two preparation stations and at least one rest station bring the circuit to about twenty, and the exam takes about three and a half hours.
What is the pass mark for MRCS Part B?
There is no published pass mark. The cut score is set separately for Applied Knowledge and Applied Skills, for each circuit, by borderline regression. Published pass rates across the 2024/25 diets ranged from 51% to 66%.
Can I fail a station and still pass?
Yes. There is no rule about how many stations you may fail: the cut score applies to your total mark in each component, so a weak station costs the marks you lost on it and strong stations elsewhere can make them back. Applied Knowledge and Applied Skills are passed separately and must both be passed at the same sitting, so a strong anatomy performance cannot rescue a weak communication one.
Dr Richard Miller, MBChB FRCS
Station summaries are reviewed against the current intercollegiate MRCS syllabus and the published marking blueprint. Guidance changes between diets: check the royal colleges' own pages before relying on a date, a fee or a threshold.
Practise this station
The question bank carries the model answer to every question above, with the rest of the anatomy stations.
More anatomy stations