Dynamics of femoral bone remodelling in well fixed total hip arthroplasty. A 20-year follow-up of 20 hips Justinas Stucinskas 1,2,3 , Martin Clauss 1, Sarunas Tarasevicius 2, Hans Wingstrand 3, Thomas Ilchmann 1 Department of Orthopaedics, Kantonsspital Liestal, Liestal - Switzerland Department of Orthopaedics, Lithuanian University of Health Sciences, Kaunas - Lithuania 3 Department of Orthopaedics, Lund University and Lund University Hospital, Lund - Sweden 1 2
The aim of our study was to investigate the dynamics of cortical thinning around well fixed cemented Muller straight stems without osteolysis at different time periods during long term follow-up. We investigated patients operated on for osteoarthritis with a cemented Muller straight stem, all with more than 15 years follow-up and no radiological signs of osteolysis. Cortical thinning in 20 THA hips (19 patients) followed for a mean of 20 (16 to 22) years was measured medially and laterally at six levels from the first postoperative, five, 10 years and the last follow-up x-rays. Sixty percent of observed cortical thinning occurred during the first five postoperative years, which was more evident proximally. We conclude that significantly greater cortical bone loss occurs around cemented Muller straight stems during the first five years, than is seen subsequently. We feel that this is a non-pathological process mainly related to the Muller straight stem. Keywords: Arthroplasty, Remodelling, Cemented, Cortical thinning Accepted: July 25, 2013
INTRODUCTION With increasing life expectancy of arthroplasty patients, the knowledge of long term femoral bone changes in total hip arthroplasty (THA) is of importance. Femoral bone remodelling after THA is a well-known phenomenon; however there is a lack of knowledge related to the dynamics of changes in THA femoral bone and its relation to age and type of fixation. Cortical femoral bone changes, mainly related to ageing, also occur in healthy/non-operated femurs (1-3). Age-related loss of cortical thickness in the femur continues after THA (3, 4). In THA femora the implant has an additional effect on bone remodellingl. Stress shielding is recognised as a factor affecting periprosthetic bone loss/ remodelling (5). These cortical bone changes can occur in stable implants (6) and can be misinterpreted as osteolysis or loosening. It is therefore of importance to identify the aetiology of cortical bone changes to avoid unnecessary revision surgery.
We found no reports on direct measurements of long-term cortical dynamic changes in THA. There are reports on bone mineral density (BMD) analysis with dual energy x-ray absorptiometry (DEXA), describing early postoperative changes and stabilisation within two years after THA (7-10). Drawbacks of DEXA studies include the variety of preoperative diagnoses, types of implants and fixation methods, which may all affect the prosthetic femoral bone behaviour. The aim of our study was to investigate the dynamics of changes in cortical bone thickness on patients operated for osteoarthritis with cemented Muller straight stems without osteolysis, followed for a minimum of 15 years.
MATERIALS AND METHODS Between July 1984 and June 1987, 161 (165 hips) consecutive patients underwent a total hip arthroplasty (THA) with a single type of cemented Muller straight stem (CoNiCr).
Dynamics of femoral bone remodelling after cemented total hip arthroplasty
Out of those we included 20 THA hips (19 patients) for the analysis. The inclusion criteria were: surgery performed for osteoarthritis (OA); non-revised femoral components without any radiological signs of osteolysis in any area of the femoral component during follow-up, and a minimum of 15 years radiological follow-up. Osteolysis was defined as a newly developed endosteal bone loss greater than 3 mm with either scalloping or bead-shaped lucency at the cement-bone-interface (11). From the initial 165 THA hips we excluded 66 patients deceased before 15 years follow-up, 32 hips operated for other reasons than OA, 32 hips with osteolysis, and three cases with x-rays not covering full length of the stem. Five stem revisions were excluded: three of those were revised because of aseptic loosening from five to seven years after primary THA, one because of recurrent dislocations and one because of periprosthetic fracture of the femur. Seven cases were excluded because of incomplete follow-up x-ray series. All patients were operated in supine position via a transgluteal approach. The components were fixed with Sulfix-6® bone cement (Zimmer® former Sulzer®, Winterthur, Switzerland) and a second generation cementing technique (distal plug, cement syringe, no vacuum-mixing, no jet lavage and no proximal sealing). Femoral components were combined with a 32 mm diameter head (12 CoCr, 8 Ceramic). The acetabular components were: cemented polyethylene-cups (Zimmer® Winterthur, Switzerland) in 17 cases, 7 of them with additional (armament) screws, and the remaining three were acetabular reinforcement rings (MullerRing, Zimmer® Switzerland) combined with a polyethylene-cup. Regular follow-up was scheduled prospectively after three months, one, two, five, 10 and 15 years postoperatively and included a standardised clinical and radiological examination. For clinical follow-up the Harris Hip Score (HHS) (12) was assessed. Radiological follow-up included a standardised anterior-posterior (AP) radiograph of the pelvis with the x-ray beam centred on the symphysis obtained in supine position with flexed knees hanging from the x-ray table to obtain standardised femoral rotation. For the radiographic assessment all radiographs were digitalised using the DICOM software (AGFA WEB 1000, V4.01; AGFA Company, Dubendorf, Switzerland). Radiographic measurements were performed using the ImageAccess 4 software (Release 133; Imagic Company, Glattbrugg, Switzerland). Measurements were calibrated to the true 32 mm femoral head size. 150
The thicknesses of the femoral cortices for evaluation of the dynamics of changes were measured on the first postoperative, two years, five years, 10 years and the last follow-up x-rays medially and laterally at six levels (12 measurement locations). All measurements were done by one of the authors (JS) blinded to the clinical data. For reliability analysis measurements were repeated in a blinded fashion on two separate occasions with three weeks interval between measurements. An intraclass correlation coefficient of 0.976 for intra-observer agreement was achieved. The levels were defined as follows: Level 1 is the line at the distal end of the minor trochanter perpendicular to the axis of the femur. Level 5 is parallel to Level 1 but at the tip of the prosthesis. Level 2, 3 and 4 are dividing the segment between Level 1 and Level 5 in 4 equidistance segments. Level 6 is distal to and equidistant from Level 5 (Fig. 1). The periosteal bone widths were measured and the mean annual changes (mm/year) in cortical thicknesses were calculated at the described levels. Collection of patient data was performed in accordance with national and institutional regulations.
Statistics To determine our data distribution, a Shapiro-Wilk normality test was conducted. If data was not normally distributed, it was presented as median and interquartile range. If distribution was normal data was presented as mean and standard deviation. Measurements were compared using nonparametric (Wilcoxon and Friedman) test for the related samples and Mann-Whitney U test for independent samples. A p-value of
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