Pauwels Angle Calculator (Femoral Neck)
Pauwels Angle Calculator (Femoral Neck)
The inclination of a femoral neck fracture line, with Pauwels’ three types — and the published verdict that the reference horizontal “can only be arbitrarily assigned” and the types predict neither non-union nor avascular necrosis.
Pauwels angle: fracture line inclination
Arctangent from the reference horizontalAn AP hip radiograph: the fracture line across the femoral neck rises 22 mm over a horizontal run of 30 mm, measured against a perpendicular to the femoral anatomic axis
An arctangent from a reference that has to be chosen
— “the degree of inclination of the fracture line measured from the horizontal on an AP radiograph”
- the rise is on TOP
- because the angle is measured away from a horizontal reference. Transposing the pair gives the complement: a type II fracture at 36 degrees reads 54 degrees transposed and becomes a type III, with no error anywhere. The proof asserts the angle rises with the rise and falls with the run, separately, away from the point where the two are equal and every transposed version returns 45
- which horizontal, and why it is the central weakness
- Nandi’s review states it plainly: a reference horizontal “can only be arbitrarily assigned”. The film edge, the inter-teardrop line, the pelvic brim and a perpendicular to the femoral anatomic axis are all in use and none is the definition. Nandi proposes the perpendicular to the femoral anatomic axis as the objective choice. A Pauwels angle quoted without its reference line is not reproducible
- a translation problem, not only a measurement one
- “Errors in interpretation of Pauwels’ original manuscript, written in German, have caused confusion”. That is the review’s own explanation for why renderings of the boundaries differ, and it is why this page prints two published renderings side by side instead of one
- what the types were supposed to mean
- compression across a horizontal fracture promoting union, shear and varus force across a vertical one resisting it. The mechanics are not in dispute; what is in dispute is whether the three categories predict anything. Two cited series found no relationship with non-union and none with avascular necrosis
- what this page is not
- not a displacement classification. Garden’s system describes how displaced a femoral neck fracture is and this angle describes how oblique it is; they are orthogonal descriptions of one injury. The Garden interpreter on this site carries the displacement categories with their reported avascular necrosis rates and kappa values, and none of that is restated here. No part of the AO/OTA alphanumeric scheme appears anywhere on this site: that compendium’s publisher states it cannot be used commercially without permission
- derivation
- Pauwels F, 1935. The original was not reachable; every boundary and every quotation here is from Nandi S, “Revisiting Pauwels’ classification of femoral neck fractures”, World J Orthop 2021;12(11):811-815, read in full, with Wikipedia’s differing table printed alongside
Worked example
An AP hip radiograph: the fracture line across the femoral neck rises 22 mm over a horizontal run of 30 mm, measured against a perpendicular to the femoral anatomic axis
Pauwels angle = arctan(22 / 30) = arctan(0.7333) = 36.3 degrees, inside Nandi's type II range of 30 to 50 degrees
Transpose the two distances: arctan(30 / 22) = 53.7 degrees, which is type III territory. The two sum to exactly 90. A swap here changes the type, not just the number, and it throws no error — which is why the direction of each argument is asserted separately and away from rise equal to run, where both orderings give 45
A transverse fracture: 6 mm over 34 mm gives arctan(6 / 34) = 10.0 degrees, type I. A vertical one: 40 mm over 20 mm gives arctan(40 / 20) = 63.4 degrees, type III
Land exactly on a boundary. A rise of 30 mm over a run equal to 30 divided by tan(30 degrees), or 51.96 mm, gives 30.0 degrees — type I on Nandi's lower range and type II on his upper one, because the two touch, and no type at all on Wikipedia's table. This page reads 30.0 as type II and says so
Now the part that matters more than the arithmetic. Change the reference horizontal by 5 degrees — which is less than the difference between the film edge and a perpendicular to the femoral axis on a rotated hip — and a 36.3-degree type II becomes a 31.3 or a 41.3, still type II here but a type I at the lower end of the range. Nandi's review states that the reference "can only be arbitrarily assigned"
Two published renderings of the same three types
| Type | Nandi 2021, reviewing Pauwels | Wikipedia’s table |
|---|---|---|
| I | “from 0 degrees to 30 degrees” | “<30 degrees” |
| II | “inclination of 30 degrees to 50 degrees” | “>30 degrees / <50 degrees” |
| III | “inclination of 50 degrees and greater” | “>50 degrees” |
| What happens at exactly 30 or exactly 50 degrees | Belongs to BOTH adjacent types, because the ranges touch | Belongs to NEITHER type: both boundary values are left unassigned |
What the classification was found to predict
| Question | What the review reports | Cohort |
|---|---|---|
| Does it predict non-union? | “Parker and Dynan found no relationship between Pauwels fracture type and rate of non-union” | Not stated in the review, and therefore not stated here |
| Does it predict avascular necrosis? | “Calandruccio and Anderson did not observe a higher rate of avascular necrosis with increasing Pauwels inclination angle” | Not stated in the review |
| Can two readers assign the same type? | “Several studies have found inter- and intra-observer reliability of the Pauwels classification to be worse” than that of the AO and Garden classifications | No kappa values and no cohort sizes are given in the review, so none is quoted |
| Is the reference line well defined? | A reference horizontal “must be reliably established to measure fracture inclination but can only be arbitrarily assigned”. The review proposes “the line perpendicular to the anatomic axis of the femur as an objective reference horizontal” | A methodological argument rather than a cohort finding |
| Overall | The classification “cannot be directly applied in its entirety” in modern practice, “given poor reliability, dated treatment recommendations, and unreliable prognostic value” | The review’s own conclusion |
An angle, three types, and an arbitrary horizontal
The Pauwels angle is “the degree of inclination of the fracture line measured from the horizontal on an AP radiograph” of a femoral neck fracture. Pauwels divided it into three types in 1935 on mechanical reasoning: compression predominates across a horizontal fracture and promotes union, while shear and varus force increase as the line becomes more vertical. A reader with the two ends of the fracture line marked has a rise and a run rather than an angle, which is what this page takes; because the reference is horizontal, the rise is the numerator.
The boundaries are 30 and 50 degrees, and two published renderings of them disagree about the boundaries themselves. Nandi’s 2021 review gives type I as 0 to 30 degrees, type II as 30 to 50 and type III as 50 and greater, so each boundary value belongs to two types. Wikipedia’s table gives under 30, over 30 and under 50, and over 50, which assigns exactly 30 and exactly 50 to no type. This page gives a boundary value to the higher type and says so on the page, because a silent choice at a threshold is how one patient in a hundred moves category without anyone noticing.
The harder problem is the reference line. Nandi’s review states that a reference horizontal “must be reliably established to measure fracture inclination but can only be arbitrarily assigned”, and proposes the perpendicular to the femoral anatomic axis as the objective choice. The film edge, the inter-teardrop line and the pelvic brim are all used in practice, and a rotated or abducted hip moves the fracture line relative to all of them. Five degrees of reference-line disagreement is enough to move a fracture near either boundary into the next type.
An angle is only as good as the two lines it is drawn between. Every threshold on this page assumes the measurement convention stated above, and a different landmark gives a different number from the same hip, heel, knee or spine — not a more or less accurate one. Agreement between observers is part of what a measured angle means, and for several of these angles the published spread between experienced readers is wider than the interval between the thresholds they are read against. Each page here gives the reliability figures with the number of observers and the number of radiographs they came from. This page renders no clinical or surgical decision; it computes the published quantity and states what the literature reports for it, with every threshold and performance figure given with the cohort it came from. A measurement is not a diagnosis, and a figure from a published series is a property of that series rather than a fact about one patient.
Frequently asked questions
What are the Pauwels types?
Nandi’s review states the original boundaries as type I from 0 to 30 degrees of fracture line inclination, type II from 30 to 50 degrees, and type III at 50 degrees and greater. Wikipedia’s table renders them as under 30, over 30 and under 50, and over 50, which leaves the boundary values unassigned. This page assigns a boundary value to the higher type.
What is the angle measured against?
The horizontal, which is the weakness of the measurement rather than a detail. Nandi’s review states that a reference horizontal “can only be arbitrarily assigned” and proposes the line perpendicular to the anatomic axis of the femur as an objective reference. Whichever you use, record it with the angle.
Does a high Pauwels angle predict non-union?
The published evidence in the review read for this page says no. It reports that Parker and Dynan found no relationship between Pauwels fracture type and the rate of non-union, and that Calandruccio and Anderson did not observe a higher rate of avascular necrosis with increasing inclination. The review gives no cohort size for either study.
How does this relate to the Garden classification?
They describe different things about the same injury. Garden describes displacement, the Pauwels angle describes obliquity, and neither substitutes for the other. The Garden interpreter on this site carries the displacement categories with their reported avascular necrosis and non-union rates and the published kappa values for assigning them.
Why is there no AO/OTA code on this page?
Because the AO/OTA Fracture and Dislocation Compendium is copyright Lippincott Williams & Wilkins and the Orthopaedic Trauma Association’s own page states it cannot be used commercially or for-profit without permission of the publisher. No part of that alphanumeric scheme is reproduced anywhere on this site.
Related calculators
References
- Nandi S. Revisiting Pauwels’ classification of femoral neck fractures. World J Orthop. 2021;12(11):811–815. The quantity: “the degree of inclination of the fracture line measured from the horizontal on an AP radiograph”. The types: “Type I, with fracture line inclination from 0° to 30°; Type II, with inclination of 30° to 50°; Type III, with inclination of 50° and greater.” On the reference line: “Errors in interpretation of Pauwels’ original manuscript, written in German, have caused confusion”; “a reference horizontal must be reliably established to measure fracture inclination but can only be arbitrarily assigned”; and the review proposes “the line perpendicular to the anatomic axis of the femur as an objective reference horizontal”. On the mechanics: “Compressive forces predominate across horizontally oriented fractures with a low degree of inclination” while “Shear stresses and varus forces increase along more vertically oriented fractures”. On whether it works: “Several studies have found inter- and intra-observer reliability of the Pauwels classification to be worse” than the AO and Garden classifications; “Parker and Dynan found no relationship between Pauwels fracture type and rate of non-union”; “Calandruccio and Anderson did not observe a higher rate of avascular necrosis with increasing Pauwels inclination angle”. It concludes the classification “cannot be directly applied in its entirety”, “given poor reliability, dated treatment recommendations, and unreliable prognostic value”. No kappa value and no cohort size is reported in it, so none is quoted here.
- Wikipedia. Pauwel’s angle. A THIRD rendering of the boundaries, different again from Nandi’s: its table gives Type I as “<30°”, Type II as “>30°/<50°” and Type III as “>50°”, which leaves exactly 30° and exactly 50° assigned to no type at all. Its definition, attributed there to Orthobullets, is “the angle between the line of a fracture of the neck of the femur and the horizontal” “as seen on an anterio-posterior radiograph”. The fetched page is about 1,500 characters including its reference list, and it carries no discussion of the reference line or of reliability; its reference list names a 2014 Bone and Joint item titled “Misinterpretation of Pauwel’s Classification”, which was not read and is not relied on here. This page assigns a boundary value to the HIGHER type, which matches Nandi and not Wikipedia, and says so.
- This site’s own Garden classification of femoral neck fracture interpreter, read in the data directory on 10 Oct 2026. The Garden classification describes the DISPLACEMENT of a femoral neck fracture and the Pauwels angle describes its OBLIQUITY: they are orthogonal descriptions of one injury and neither substitutes for the other. That record carries the displacement categories, the avascular necrosis and non-union rates reported for each, and the published kappa values for assigning them, so none of that is restated here.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/