The Chiari pelvic osteotomy, described by Karl Chiari in Vienna in 1953, is a medial displacement osteotomy of the ilium performed just above the acetabulum. The distal fragment — bearing the acetabulum and femoral head — is shifted medially beneath the intact proximal iliac shelf, which acts as a new, expanded acetabular roof. The interposed hip capsule undergoes fibrocartilaginous metaplasia, creating a new weight-bearing neosurface.

The fundamental concept: unlike the Ganz periacetabular osteotomy (PAO), which reorients the acetabulum to restore coverage while preserving native hyaline cartilage congruence, the Chiari displaces the acetabulum medially. It is therefore a salvage procedure — used when the femoral head and acetabulum are incongruent and cannot be made to match by reorientation alone.

Mechanism of action (three inter-related effects):

• Increased superolateral coverage — the proximal iliac fragment extends over the femoral head, deepening the acetabulum
• Medialisation of the hip centre — shifting the joint medially reduces the body-weight moment arm and decreases joint reaction force
• Capsular fibrocartilage metaplasia — the hip capsule, compressed between the new iliac roof and the femoral head, differentiates into a fibrocartilaginous weight-bearing surface over 12-18 months

Indications:

• Hip dysplasia with incongruent joint (femoral head and acetabulum do not have matching geometry) — the primary indication
• Lateral subluxation of the femoral head with significant uncovering (lateral centre-edge angle less than 20 degrees)
• Age under 40-50 years (some authors extend to 55 in selected patients) with preserved or partially preserved cartilage
• Failed prior hip surgery (e.g., prior innominate osteotomy, shelf procedure) with residual subluxation
• Neuromuscular hip dysplasia (cerebral palsy, myelomeningocele) — where joint incongruence and subluxation are typical
• Adolescent dysplasia after triradiate cartilage closure, when PAO is not feasible due to incongruence
• As a salvage after failed PAO or when acetabular reorientation is anatomically impossible

Relative indications:

• Legg-Calve-Perthes disease sequelae with aspherical femoral head and incongruent acetabulum
• Developmental dysplasia of the hip (DDH) late presentation with established incongruence
• Combined with femoral varus derotation osteotomy for coxa valga and excessive anteversion

Supra-acetabular iliac anatomy:

• The ilium above the acetabulum is a broad, flat sheet of cancellous bone with thin cortices — ideal for osteotomy and rapid healing. The bone is thickest just above the acetabular roof (the supra-acetabular region) and thins towards the iliac crest.
• The greater sciatic notch is the posterior boundary of the ilium; the sciatic nerve, piriformis, and superior/inferior gluteal vessels pass through it. The notch is the posterior exit point of the Chiari osteotomy.
• The anterior inferior iliac spine (AIIS) — giving rise to the direct head of rectus femoris — is the anterior exit point. The osteotomy passes just above the AIIS, staying extra-articular.
• The hip joint capsule attaches to the acetabular rim (labrum) and the intertrochanteric line anteriorly. In the Chiari, the capsule is deliberately preserved intact between the osteotomised surfaces — it is the substrate for fibrocartilaginous metaplasia.
• The triradiate cartilage (in children) fuses at approximately 12-16 years; the Chiari is performed after fusion to avoid growth disturbance.

Neurovascular structures:

• Sciatic nerve — exits the pelvis through the greater sciatic notch, typically deep to the piriformis. It lies immediately posterior to the ilium at the level of the osteotomy exit. This is the most critical structure at risk.
• Superior gluteal nerve and artery — exit the greater sciatic notch above the piriformis and supply the gluteus medius, gluteus minimus, and tensor fasciae latae. They may be stretched or injured during medial displacement of the distal fragment.
• Femoral nerve — runs on the anterior surface of the iliacus, medial to the AIIS. At risk from anterior retraction during exposure.
• Obturator nerve and artery — pass through the obturator foramen on the inner table of the ilium. Excessive medial displacement of the distal fragment can compress the nerve against the inner pelvic wall.
• Internal iliac vessels — lie on the inner table of the ilium posteriorly; protected by the iliacus muscle but at theoretical risk from penetration of the inner cortex.

Biomechanical principles:

• Medialisation of the hip centre reduces the body-weight moment arm (the horizontal distance from the body's centre of gravity to the hip centre). By the Pauwels principle, this decreases the resultant joint reaction force.
• However, medialisation also shortens the abductor lever arm (the distance from the hip centre to the greater trochanter), which may weaken abductor efficiency and contribute to a Trendelenburg gait.
• The fibrocartilaginous neosurface that develops from capsular metaplasia is biomechanically inferior to hyaline cartilage — it has lower wear resistance and less capacity for load distribution. This is why the Chiari is a time-buying salvage, not a definitive cure.
• The proximal iliac shelf provides approximately 15-20 degrees of additional lateral coverage, depending on the degree of displacement.

The Chiari osteotomy uses an anterior approach to the hip and ilium — there is no true internervous plane at the iliac wing. The exposure is through a muscle-splitting and subperiosteal dissection.

Standard approach (modified Smith-Petersen / ilioinguinal variation):

• Skin incision: a longitudinal or slightly curved incision along the iliac crest, extending from the posterior third of the crest to the AIIS region, or a bikini-type incision for cosmesis.
• Superficial dissection: the interval between the tensor fasciae latae (superior gluteal nerve) and the sartorius (femoral nerve) is developed distally. The lateral femoral cutaneous nerve is at risk in this interval and must be identified and protected.
• Deep dissection — outer table: the gluteus medius and minimus are elevated subperiosteally from the outer table of the ilium, exposing the supra-acetabular region down to the hip capsule. The capsule is identified but NOT opened.
• Deep dissection — inner table: the iliacus is elevated subperiosteally from the inner table of the ilium. This exposes the quadrilateral surface and the margin of the greater sciatic notch from the medial side.
• Exposure of the greater sciatic notch: both inner and outer table dissections converge on the greater sciatic notch. A Blount retractor or Hohmann spike is passed through the notch (subperiosteally, hugging bone) to protect the sciatic nerve and superior gluteal vessels. This step is critical — the retractor must be placed on bone, not in the soft tissues.

• Position: supine on a radiolucent table with a small sandbag under the ipsilateral buttock to internally rotate the femur and improve access to the posterior ilium. The leg is draped free to allow hip flexion, abduction, and rotation during the procedure.
• Anaesthesia: general or spinal anaesthesia. A lumbar plexus block (or iliac crest block) may supplement analgesia but should not obscure assessment of sciatic or femoral nerve function post-operatively.
• Fluoroscopy: essential. The image intensifier is positioned for AP and iliac oblique (Judet) views of the acetabulum. The AP view confirms the osteotomy level and displacement; the iliac oblique view confirms the posterior exit at the sciatic notch.
• Tourniquet: generally NOT used for pelvic osteotomies (the field is too proximal). Meticulous haemostasis with electrocautery and bone wax is essential.
• Cell saver or typed and cross-matched blood: the subperiosteal dissection of the iliac wing and the cancellous bone of the osteotomy can produce meaningful blood loss (typical estimated blood loss 300-800 mL). Blood should be available.
• Pre-operative planning: standing AP pelvis, false-profile (Lequesne), and Dunn/lateral radiographs. Three-view congruence assessment (neutral, abduction, adduction). CT scan with 3D reconstruction is increasingly used to plan the osteotomy level and angle. The lateral centre-edge angle of Wiberg, Tönnis angle, and anterior centre-edge angle are measured.

The Chiari osteotomy carries a significant complication profile that reflects the magnitude of the procedure and the proximity of critical neurovascular structures.

• Sciatic nerve injury — the most feared complication. Reported rates vary from approximately 1-5% for transient neuropraxia and less than 1% for permanent palsy. The nerve is at risk at the greater sciatic notch exit point of the osteotomy and from stretching during medial displacement. Protection with a subperiosteal retractor is mandatory.
• Femoral nerve injury — from anterior retraction during exposure of the AIIS region. Usually a neuropraxia that resolves.
• Obturator nerve injury — excessive medial displacement can compress the obturator nerve on the inner table. Presents with medial thigh sensory changes and adductor weakness.
• Superior gluteal nerve injury — the nerve exits the sciatic notch and runs along the outer table of the ilium to supply the gluteus medius and minimus. Elevation of the glutei and displacement of the fragment may stretch or injure it, contributing to abductor weakness and a Trendelenburg gait.
• Loss of abductor power — medial displacement of the hip centre shortens the abductor lever arm (the distance from the hip centre to the greater trochanter decreases). Even with intact nerve supply, mechanical efficiency is reduced. This is an inherent trade-off of the procedure.
• Limb shortening — the osteotomy shortens the limb by approximately 5-10 mm (the amount of medial displacement). If combined with a femoral varus osteotomy, further shortening may occur. A shoe raise may be needed.
• Incomplete or inadequate coverage — under-displacement leaves the femoral head uncovered. The procedure fails if the displacement is insufficient to create a functional shelf. Intra-operative fluoroscopy must confirm adequate coverage before fixation.
• Medial over-displacement — excessive medial shift causes impingement of the femoral head against the inner pelvic wall and may narrow the obturator foramen. It also makes future THR more difficult by altering the anatomy.
• Intra-articular extension — the most devastating technical error; if the osteotomy enters the joint, the articular surface is destroyed. Prevention requires careful identification of the acetabular rim and staying proximal to it.
• Progression of osteoarthritis — the fibrocartilaginous neosurface, while functional, is biomechanically inferior to hyaline cartilage. Long-term progression to osteoarthritis is common, particularly if pre-operative arthritis was already present (Tönnis grade 2). The Chiari buys time, but most patients will eventually require THR.
• Heterotopic ossification — reported in approximately 5-15% of cases; prophylaxis with NSAIDs or single-dose radiation therapy is used in high-risk patients.
• Nonunion or delayed union — rare because the osteotomy is through well-vascularised cancellous metaphyseal bone, but reported in approximately 1-3% of cases. Smoking increases risk.
• Hardware irritation — prominent screws on the iliac crest or wing may cause pain and require removal after union.
• Wound haematoma and infection — the extensive subperiosteal dissection and cancellous bone surfaces create a risk of haematoma formation. Deep infection is reported in approximately 1-2% of cases.

There is no single universal guideline specifically for the Chiari osteotomy; it is increasingly regarded as a historical or specialist salvage technique in the era of PAO. However, its role is recognised in several international contexts:

Key guidance principles:

• AAOS (US): the AAOS clinical practice guideline for hip dysplasia in the young adult recognises pelvic osteotomy (including Chiari) as a treatment option when conservative measures fail. PAO is the first-line recommendation for congruent dysplasia; the Chiari is mentioned as an alternative for incongruent joints.
• BOA/BOAST (UK): the British Orthopaedic Association standards for hip preservation surgery recommend that pelvic osteotomies be performed in specialist centres with expertise in hip dysplasia management. PAO is the preferred reorientation procedure; Chiari is reserved for salvage situations.
• EFORT/European consensus: European centres, particularly in Austria, Germany, and Japan, have the largest published experience with Chiari osteotomy. The technique is accepted as a valid salvage when PAO is not feasible.

Global epidemiology of DDH:

• Developmental dysplasia of the hip has an incidence of approximately 1-3% in neonatal screening programmes worldwide, with geographic and ethnic variation (higher in populations practising swaddling with hip extension and adduction).
• Despite universal neonatal screening in many countries, late-presenting DDH and adolescent/young adult dysplasia remain common, particularly in regions without screening programmes.
• The pool of patients who may benefit from a salvage osteotomy (Chiari) persists globally, particularly in low- and middle-income countries where late presentation is more common.

Registry evidence:

• National joint registries (NJR — UK, AOANJRR — Australia, SHAR — Swedish) do not specifically track Chiari osteotomy as a separate procedure, but they record conversion to THR after prior pelvic osteotomy. Registry data suggest that prior pelvic osteotomy (of any type) increases the complexity of THR conversion but does not compromise long-term implant survivorship when performed by experienced surgeons.
• The Swedish Hip Arthroplasty Register has published data showing that THR after prior Chiari osteotomy has a slightly higher early complication rate (dislocation, nerve injury) compared with primary THR for primary OA, but revision rates at 10 years are comparable.

Global practice variation:

• Japanese and Austrian centres have the largest historical experience with Chiari osteotomy, reflecting the influence of Chiari's Vienna school and the high incidence of DDH in Japan.
• In North America and the UK, PAO has largely superseded Chiari for primary dysplasia treatment since the 1990s; the Chiari is now reserved for salvage cases.
• In low- and middle-income settings, the Chiari remains a valuable option because it does not require specialised implants (screws alone suffice) and provides durable results without the need for arthroplasty in young patients who perform manual labour.
• The trend globally is toward PAO for congruent dysplasia and Chiari (or hip replacement) for incongruent joints, with the Chiari occupying a narrowing niche as a salvage procedure in younger patients not yet ready for THR.

Conversion to THR after Chiari:

• Conversion is more complex than primary THR due to altered acetabular anatomy (medialised socket, expanded roof, fibrocartilaginous neosurface, potential hardware).
• The medialised hip centre may require medial placement of the acetabular component, and bone defects in the supra-acetabular region may need bone grafting or augments.
• Despite technical challenges, long-term THR survivorship after prior Chiari is generally comparable to primary THR when performed by experienced surgeons.