Protrusio is a central acetabular defect resulting from migration of the femoral head medial to Kohlers line. Protrusio can develop in association with a number of conditions that affect the metabolism or mechanical behavior of the periacetabular bone. Total hip arthroplasty (THA) for acetabular protrusio requires reconstruction of the contained cavitary defect. The acetabular rim is generally intact. The defect can be effectively treated with a lateralized cementless porous-coated component supported on the peripheral acetabular bony rim. Morsellized autograft is used to fill the medial defect. However, if the rim is inadequate to provide mechanical support for a cementless cup due to bone loss or osteoporosis, cementless cup reconstruction can result in medial migration. If rim support is inadequate, a reconstruction cage is a more appropriate option to gain fixation to the pelvis above and below the acetabulum. Total hip arthroplasty for protrusio can also affect both offset and leg length, so careful preoperative templating is important to help determine implant position. Results of THA with medial acetabular bone grafting for protrusio acetabuli have been favorable, with success rates similar to conventional THA.
Acetabular protrusio is a cavitary deformity in which the femoral head has migrated medial to Kohlers line (on an anteroposterior pelvis radiograph, a line from the lateral border of the sciatic notch to the medial border of the obturator foramen) (Figure 1).1 Protrusio can develop in association with a number of etiologies, including osteogenesis imperfecta, osteomalacia, Pagets disease, bone tumors, rheumatoid arthritis, ankylosing spondylitis, and following prior trauma.2 If arthritic symptoms warrant treatment with total hip arthroplasty (THA), acetabular reconstruction options include use of a medialized shell with a lateralized liner or lateralized shell to restore the anatomic center of rotation of the hip.
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Figure 1: AP pelvic radiograph demonstrating a mild protrusio deformity (A). Kohlers line is illustrated from the lateral border of the sciatic notch to the medial border of the obturator foramen. The medial border of the acetabulum is medial to Kohlers line in protrusio (B).
Cementless Acetabular Cup Reconstruction
Stability of press-fit acetabular components relies on the mechanical support of the peripheral acetabular bony rim.3,4 To support a lateralized cementless cup in protrusio deformity, the peripheral acetabular rim is reamed with a slightly oversized reamer (Figure 2). Screw fixation is also required. However, the peripheral rim provides primary mechanical support of the cup to prevent medial migration into the acetabular defect.5 Morsellized bone from the acetabular reamings or femoral head can be used to fill the medial defect.6 Morsellized bone graft generally heals well, but does not provide mechanical support since the cup is primarily supported by the peripheral bony rim.
In post-traumatic deformity, the inferior pelvis is typically displaced medially. To position the cup to its anatomic location, it should be placed laterally to restore the prefracture lateral offset of the acetabulum. The relative position of the floor of the acetabulum can be determined by the location of the teardrop (Figure 3). However, if the inferior pelvis is displaced medially, the floor of the acetabulum will be relatively medial to its prefracture location. For this reason, the acetabular component should be placed in its prefracture anatomic position.
If the peripheral bony rim is deficient or osteoporotic, cementless cup reconstruction can be associated with medial and superior implant migration. In a study of 19 revision THAs performed for acetabular protrusio deformity, average preoperative cup position was 10.5 mm medial to Kohlers line, and postoperative position was 6 mm lateral to Kohlers line.7 At 3-year follow-up, no revisions were required, but 2 cups migrated 5 mm, both in elderly women (85 and 87 years), suggesting that a reconstructive cage may be more appropriate for treatment of protrusio deformity in association with osteoporosis (Figure 4).
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Figure 4: AP pelvic radiograph illustrating a large protrusio deformity that developed as a result of Charcot arthropathy in a 80-year-old man who had undergone multiple spine surgeries (A). The bone was osteoporotic and the peripheral rim insufficient to support a rim-fit cementless cup. Total hip arthroplasty was performed using a reinforcement cage and morsellized medial bone autograft obtained from the femoral head. Two years after THA, the bone graft appears healed and the hip is functioning well (B).
Effect on Leg Length and Offset
Lateralization of the acetabular component may create an undesirable increase in offset. Protrusio deformity, particularly in rheumatoid arthritis, can also be associated with a varus proximal femoral deformity. This requires a low femoral neck cut to avoid an increase in leg length after THA. Preoperative templating in protrusio deformity is important to determine the effect of acetabular component lateralization and femoral component placement on leg length and offset.
- Gates HS III, Poletti SC, Callaghan JJ, McCollum DE. Radiographic measurements in protrusio acetabuli. J Arthroplasty. 1989; 4(4):347-351.
- McBride MT, Muldoon MP, Santore RF, Trousdale RT, Wenger DR. Protrusio acetabuli: diagnosis and treatment. J Am Acad Orthop Surg. 2001; 9(2):79-88.
- Kroeber M, Ries MD, Suzuki Y, Renowitzky G, Ashford F, Lotz J. Impact biomechanics and pelvic deformation during insertion of press-fit acetabular cups. J Arthroplasty. 2002; 17(3):349-354.
- Ries MD, Harbaugh M, Shea J, Lambert R. Effect of cementless acetabular cup geometry on strain distribution and press-fit stability. J Arthroplasty. 1997; 12(2):207-212.
- Krushell RJ, Fingeroth RJ, Gelling B. Primary total hip arthroplasty using a dual-geometry cup to treat protrusio acetabuli. J Arthroplasty. 2008; 23(8):1128-1131.
- Mullaji AB, Marawar SV. Primary total hip arthroplasty in protrusio acetabuli using impacted morsellized bone grafting and cementless cups: a medium-term radiographic review. J Arthroplasty. 2007; 22(8):1143-1149.
- Hansen E, Ries MD. Revision total hip arthroplasty for large medial (protrusio) defects with a rim-fit cementless acetabular component. J Arthroplasty. 2006; 21(1):72-79.
Dr Ries is from the University of California, San Francisco, California.
Dr Ries is a consultant for Smith & Nephew.
Presented at Current Concepts in Joint Replacement 2008 Winter Meeting; December 10-13, 2008; Orlando, Florida.
Correspondence should be addressed to: Michael D. Ries, MD, University of California, San Francisco, 500 Parnassus Ave (MU 3220-W), San Francisco, CA 94143.