Novel method to evaluate femoral head penetration in the polyethylene liner after total hip arthroplasty: three-dimensional evaluation on standing radiographs.
BACKGROUND
After total hip arthroplasty, the femoral head penetrates the liner because of polyethylene wear and the so-called bedding-in phenomenon. The head moves three-dimensionally, but few reports have evaluated such movement radiographically. Moreover, even fewer reports have evaluated it in a standing position, although the load stress influences the movement. The purpose of the present study was to evaluate the head movement in the liner three-dimensionally using standing images.


METHODS
The distance between the centers of the metal shells and femoral heads were measured in 29 unilateral hips implanted with porous-coated acetabular components. Standing anteroposterior and lateral radiographs obtained approximately 2 years after operation were photographed by a digital camera, and the data were used. The measured distances divided by the time after operation and enlargement ratio of the radiographs gave linear penetration rates in the frontal and sagittal planes, and three-dimensional linear penetration rates were calculated from the penetration vectors. Frontally evaluated and three-dimensionally calculated penetration rates were compared, and the correlation of age and other factors with the penetration rate was evaluated.


RESULTS
Linear penetration rates were 0.14 +/- 0.11 mm/year frontally, 0.18 +/- 0.08 mm/year sagittally, and 0.21 +/- 0.10 mm/year three-dimensionally. The mean three-dimensional penetration rate was 42% greater than the frontal rate. An inverse correlation was observed between age and the sagittal penetration rate (r = -0.621), and between age and the three-dimensional penetration rate (r = -0.388). The sagittal and three-dimensional penetration rates were significantly higher (P = 0.0008 and 0.0010, respectively) in patients aged <75 years compared to those aged > or = 75 years.


CONCLUSIONS
The average three-dimensional penetration rate during the first 2 years after operation was markedly greater than the frontal two-dimensional penetration rate that was conventionally applied for postoperative evaluation. Three-dimensional evaluation using both frontal and sagittal radiographs is necessary to avoid undervaluation of the head movement after total hip arthroplasty.
Department of Orthopaedic Surgery, Tokyo Metropolitan Geriatric Hospital, 35-2 Sakae-cho, Itabashi-ku, Tokyo, 173-0015, Japan.
Hiroshi
H
Hamaji
Seizo
S
Yamamoto
Hideaki
H
Ishibashi
Yorito
Y
Anamizu
D000368:Aged; D000369:Aged, 80 and over; D019644:Arthroplasty, Replacement, Hip; D005260:Female; D005270:Femur Head / Q000000981:diagnostic imaging*; D006622:Hip Prosthesis; D006801:Humans; D021621:Imaging, Three-Dimensional; D008297:Male; D008875:Middle Aged; D020959:Polyethylene; D011187:Posture; D011475:Prosthesis Failure; D011859:Radiography
D020959:Polyethylene