PROTRUSIO ACETABULI (OTTO PELVIS)

The German pathologist Otto first described Protrusio acetabuli, (also known as ‘arthrokatadysis’,) in 1824. Hence also known as Otto Pelvis

Aetiology:

Idiopathic, or primary protrusio acetabuli: no causative factors are found in this group
· Secondary protrusio acetabuli: The causes are as follows

Infectious
ü Gonococcus
ü Echinococcus
ü Staphylococcus
ü Streptococcus
ü Mycobacterium tuberculosis

Neoplastic
ü Hemangioma
ü Metastatic carcinoma (breast, prostate most common)
ü Neurofibromatosis
ü Radiation-induced osteonecrosis

Inflammatory
ü Rheumatoid arthritis
ü Ankylosing spondylitis
ü Juvenile rheumatoid arthritis
ü Psoriatic arthritis
ü Acute idiopathic chondrolysis
ü Reiter’s syndrome
ü Osteolysis following hip Replacement

Metabolic
ü Paget’s disease
ü Osteogenesis imperfecta
ü Ochronosis
ü Acrodysostosis
ü Osteomalacia (very high incidence –50%)
ü Hyperparathyroidism

Traumatic
ü Sequalae of acetabular fracture
ü Surgical error during hip Replacement

Genetic
ü Trichorhinophalangeal syndrome
ü Stickler syndrome
ü Trisomy 18
ü Ehler-Danlos syndrome
ü Marfan’s syndrome
ü Sickle cell disease

Pathomechanics
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The typical orientation for the joint reaction force is 69 degrees from horizontal during the stance phase of gait.
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McCollum et al.. found that protrusio acetabuli occurs at 65 degrees from the horizontal and concluded that the axis of migration was nearly the same as that of the joint-reaction force during stance(1)
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Eppinger’s theory: the condition is secondary to a chondrodystrophy wherein the three plates of the triradiate cartilage remain unfused, allowing protrusion of the femoral head medially into the pelvis
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Inflammatory causes lead to destruction and weakening of the bone surrounding the hip with resultant migration along the joint-reaction vector

The deformity may progress until the greater trochanter impinges on the side of the pelvis

An associated varus deformity of the femoral neck is often seen

CF:
-Idiopathic protrusio presents in early adolescence, hence should be kept in the differential diagnosis of hip pain in a teenager
-Common in younger women
-Present with pain and stiffness, rarely with knee pain
-Arising from a seated position is a frequent cause of exacerbation

X-Rays:
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-The Wiberg’s center-edge angle over 40 degrees is diagnostic of protrusio acetabuli
-Normally on an AP radiograph the medial wall of the acetabulum lies 2 mm lateral to the ilioischial line in a male and 1 mm medial to this line in a female.
-If the medial wall of the acetabulum protruded medial to the ilioischial line (Kohler’s line) by 3 mm in males or 6 mm in females it favours the diagnosis of protrusio.
-May be graded as mild (1 to 5 mm), moderate (6 to 15 mm), or severe (>15 mm), with reference from the ilioischial line.

Treatment:
· In skeletally immature patients, with an open triradiate cartilage: surgical closure of the triradiate cartilage is done. A valgus intertrochanteric osteotomy may be combined

· In adolescent or skeletally mature patients: Valgus intertrochanteric osteotomy (VITO). This lateralises the mechanical axis of the limb. Soft tissue releases of the psoas tendon may be done.

· VITO procedure should not be performed on patients who are over age 40 years or whom have significant degenerative changes visualized on plain radiographs.

Older Adult Patients:
· Total hip Arthroplasty is the treatment of choice

The principles of THA in protrusio acetabuli are:
-Restoration of hip center at its anatomical location for proper joint biomechanics
-The intact peripheral rim of the acetabulum should be used to support the acetabular component
-Cavitary and segmental defects in the medial wall must be reconstructed with bone grafting

Surgical pearls:
-The sciatic nerve will lie near the joint compared to normal patients, and should be routinely identified and protected.
-Trochanteric osteotomy may be required for exposure
-When dislocation is difficult, removal of portion of the posterior acetabular wall maybe required.
-In severe cases, the head is incarcerated into the acetabulum, such cases require osteotomy of the neck at the desired level and removal of the head is facilitated by a corkscrew or rarely as piecemeal.
-The medial wall of the acetabulum is thin or may be partly membranous, and it should not be perforated.
-Medial reaming should be avoided
-Only peripheral reaming should be done to make the acetabular dome to converge
-A protrusio cup is available which avoids medial bone grafting
-If the pelvis is osteoporotic it is better to fix an antiprotrusio cage rather than attempting to press-fit the acetabular component and causing a fracture.


Ref:
1. McCollum DE, Nunley JA, Harrelson JM: Bone-grafting in total hip replacement for acetabular protrusion. J Bone Joint Surg Am 1980;62:1065-1073.
2. Ranawat CS, Zahn MG: Role of bone grafting in correction of protrusion acetabuli by total hip arthroplasty. J Arthroplasty 1986;1:131-137.
3. Van de Velde S, Fillman R, Yandow S. Protrusio acetabuli in Marfan syndrome: indication for surgery in skeletally immature Marfan patients. J Pediatr Orthop. 2005; 25:603-6.
4. Van De Velde S, Fillman R, Yandow S: The aetiology of protrusio acetabuli: literature review from 1824 to 2006. Acta Orthop Belg 2002; 72:524.
6. Dunlop CC, Jones CW, Maffulli N: Protrusio acetabuli. Bull Hosp Jt Dis 2005; 62:105