Pauwels Classification — Femoral Neck (Shear Angle)

Overview — Femoral Neck Fractures & the Pauwels Angle

Femoral neck fractures are the most clinically significant fractures in orthopaedic practice — they predominantly occur in elderly osteoporotic patients from low-energy falls, carry a 30-day mortality of approximately 5–10% and 1-year mortality of 25–35%, and their management represents one of the highest volumes of surgical work in orthopaedics worldwide. The Pauwels classification (1935), developed by Friedrich Pauwels, classifies femoral neck fractures by the angle that the fracture line makes with the horizontal — the `Pauwels angle`. This angle is a direct indicator of the ratio of shear forces to compressive forces acting at the fracture site, and therefore predicts the mechanical stability of the fracture and the risk of non-union or implant failure. While the Garden classification (grading displacement) is more commonly used clinically, the Pauwels system remains fundamental for understanding femoral neck fracture biomechanics.

Anatomy: the femoral neck connects the femoral head to the femoral shaft at an angle of ~130° (neck-shaft angle) and approximately 10–15° of anteversion; the blood supply to the femoral head enters via the retinacular vessels (medial femoral circumflex artery branches — superior and inferior retinacular vessels) running along the posterior neck beneath the capsule; intracapsular femoral neck fractures disrupt these vessels and create AVN risk; extracapsular fractures (intertrochanteric) do NOT disrupt this supply
Why the Pauwels angle matters biomechanically: the angle that the fracture line makes with the horizontal determines what proportion of the axial load through the hip is converted into compressive force at the fracture (compressive forces promote healing) vs shear force (shear forces tend to displace the fracture fragments relative to each other, preventing healing and causing non-union); a horizontal fracture line (low Pauwels angle, Type I) generates predominantly compressive forces across the fracture — favouring union; a vertical fracture line (high Pauwels angle, Type III) generates predominantly shear forces — the fracture is inherently unstable and prone to displacement and non-union

Pauwels Classification

Pauwels Type	Fracture Line Angle (to horizontal)	Force at Fracture Site	Stability	Non-Union Risk	Implications
Type I	<30° from horizontal — the fracture line is nearly horizontal (flat)	Predominantly COMPRESSIVE — the near-horizontal fracture line means that the body weight transmitted through the hip generates a compressive force along the fracture plane; compressive forces push the fragments together, creating stable contact	STABLE — the compressive forces at the fracture site tend to keep the fragments in contact; this geometry is the most favourable for bone healing	LOW — union rates with internal fixation are good for Type I fractures; the mechanical environment favours healing	Undisplaced Type I fractures can often be managed with internal fixation (3 cannulated screws) with a good prognosis for union; the horizontal fracture geometry means that correctly placed parallel cannulated screws maintain compression across the fracture
Type II	30–70° from horizontal — the fracture line is oblique (intermediate angle)	MIXED — a combination of compressive and shear forces; neither predominantly compressive nor predominantly shear; the intermediate angle means the mechanical environment is less favourable than Type I	MODERATELY STABLE — some shear component is present; fixation must resist both compression and shear; standard parallel cannulated screws may be supplemented with a valgus-producing osteotomy for very steep Type II fractures in young patients	MODERATE — non-union rates are higher than Type I but lower than Type III; appropriate fixation is important	Internal fixation (3 cannulated screws or sliding hip screw) is appropriate for undisplaced or reducible Type II fractures; the intermediate angle means that the choice of implant is important — a single sliding hip screw provides controlled dynamic compression along the neck axis; for younger patients with steep Type II, valgus-producing intertrochanteric osteotomy (converting the oblique fracture to a more horizontal orientation) was historically used to reduce shear stress
Type III	>70° from horizontal — the fracture line is nearly vertical (steep)	Predominantly SHEAR — the near-vertical fracture line means that the body weight generates a predominantly SHEAR force at the fracture plane; shear forces tend to displace one fragment relative to the other (sliding), preventing the fracture faces from staying in contact	UNSTABLE — the shear forces tend to displace the fragments; even with fixation, the screw purchase must resist shear rather than working with compressive forces; standard screws placed parallel to the neck axis actually experience shear stress at a vertical fracture, making them prone to `backing out` or fatigue failure	HIGH — the worst prognosis of the three types; non-union and implant failure are the most common complications; even with apparent anatomical reduction and fixation, the shear forces at the fracture site may cause progressive displacement	For young patients with displaced Type III fractures — valgus-producing intertrochanteric osteotomy (Pauwels` original description) converts the steep vertical fracture into a more horizontal plane, reducing shear stress and converting it to compression; this was Pauwels` original proposed treatment; in modern practice, arthroplasty is considered for elderly patients; for young patients, aggressive ORIF is attempted despite the high non-union risk

Garden Classification — Displacement (More Commonly Used Clinically)

While the Pauwels classification describes the fracture line angle (mechanical stability), the Garden classification (1961) grades displacement of the femoral head and is the more commonly used clinical classification for femoral neck fractures, directly guiding the decision between internal fixation and arthroplasty.

Garden Grade	Displacement	AVN Risk	Management (Elderly)	Management (Young)
Grade I — Incomplete/Impacted	Incomplete or valgus-impacted fracture; the trabeculae of the femoral head are aligned in the same direction as the acetabular trabeculae; the neck is in valgus	LOW (~10%)	Internal fixation (3 cannulated screws)	Internal fixation with screws
Grade II — Complete, undisplaced	Complete fracture with NO displacement; the femoral head trabeculae remain aligned; the fracture line is complete but the fragments are not displaced	LOW-MODERATE (~15–20%)	Internal fixation (3 cannulated screws)	Internal fixation with screws
Grade III — Displaced, partial	Complete fracture with partial displacement; the femoral head trabeculae are malaligned with the acetabular trabeculae; the femoral head is rotated but maintains some contact with the neck	HIGH (~30–35%)	Hemiarthroplasty (unipolar or bipolar) in fit elderly; THA in active elderly; fixation in elderly is not recommended (high non-union + AVN rate)	URGENT ORIF (within 6–12 hours reduces AVN risk); 3 cannulated screws or sliding hip screw; accept higher non-union/AVN risk in young patient rather than arthroplasty
Grade IV — Completely displaced	Complete fracture with full displacement; the femoral head is completely separated and displaced; the femoral head trabeculae are realigned with the acetabular trabeculae (paradoxically — the head re-rotates within the acetabulum to align with it)	VERY HIGH (~50%)	Hemiarthroplasty or THA — arthroplasty is the definitive treatment for displaced femoral neck fractures in elderly patients; internal fixation for displaced femoral neck fractures in elderly patients has a very high failure rate (non-union + AVN + reoperation) and is no longer recommended	URGENT ORIF + anatomical reduction

Exam Pearls

Pauwels classification: angle of fracture line from horizontal; Type I (<30° — predominantly compressive — stable — low non-union); Type II (30–70° — mixed — moderately stable); Type III (>70° — predominantly shear — unstable — high non-union risk)
Biomechanical principle: horizontal fracture = compressive force at fracture site = good for healing; vertical fracture = shear force = bad for healing; the more vertical the fracture, the higher the shear component and the worse the prognosis
Pauwels` original treatment for Type III: valgus-producing intertrochanteric osteotomy — converts the vertical fracture into a more horizontal plane, reducing shear stress and converting to compression; largely superseded by arthroplasty in elderly but the biomechanical principle is still exam-relevant
Garden classification: I (impacted/incomplete — low AVN); II (undisplaced complete — low AVN); III (partial displacement — high AVN); IV (complete displacement — very high AVN); Grades I and II = internal fixation; Grades III and IV = arthroplasty in elderly
Displaced femoral neck fracture in elderly: arthroplasty is the standard (hemiarthroplasty or THA); internal fixation has a ~30–50% failure rate (non-union + AVN + reoperation) for displaced fractures in elderly; arthroplasty provides immediate definitive treatment and avoids reoperation
Undisplaced femoral neck fracture (Garden I/II): internal fixation with 3 parallel cannulated screws is the standard; the `inverted triangle` configuration (2 inferior screws + 1 superior screw) provides the best resistance to varus collapse; the screws are placed parallel to the neck axis; a calcar screw (inferior screw along the medial neck — the `calcar screw` position) prevents varus collapse
Hemiarthroplasty vs THA for displaced femoral neck fractures: hemiarthroplasty (unipolar or bipolar) = single femoral component + femoral head prosthesis replacing the femoral head; THA = femoral component + acetabular component replacing both sides; THA provides better long-term function and lower revision rates but higher dislocation risk; THA is recommended for active elderly patients (>60 years) with pre-existing acetabular OA or high functional demand; hemiarthroplasty for less active, less fit elderly patients or those with cognitive impairment (lower early dislocation risk)

Pauwels Classification — Femoral Neck (Shear Angle)

References

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