Unstable versus stable uncemented femoral stems: a radiological study of periprosthetic bone changes in two types of uncemented stems with different concepts of fixation.
BACKGROUND
Uncemented stems in total hip arthroplasty (THA) are used increasingly often because they are believed to offer a reliable long-term fixation. However, periprosthetic bone remodelling has been a worrying issue. A proximal demineralization has been noted in femurs with well-fixed stems, and it has been explained as by-passing of mechanical forces along the fixed implant (stress-shielding). Aseptic loosening has been a major problem in several uncemented series with earlier designs. The objective for this study was to investigate how the host bone adapts to a loose stem compared with a well-fixed stem after a long time.


METHODS
An investigation with dual-energy X-ray absorptiometry (DEXA), scintimetry and radiological assessment was carried out in 20 patients 8 years after a THA for arthrosis with two different uncemented stems. Ten patients received a stem coated with polytetrafluoroethylene (Anaform); all prostheses showed migration and were considered unstable. Ten patients received a hydroxyapatite-coated stem (Bi-Metric); no prosthesis migrated.


RESULTS
Different remodelling patterns were seen. In the unstable group, the periprosthetic bone mineral density (BMD) was significantly reduced along the entire stem, while in the stable group only proximal bone loss was seen. The scintigraphic uptake was increased under the stem tip in both groups, and among unstable stems uptake was also increased in the calcar region.


CONCLUSIONS
The assessment of periprosthetic bone remodelling after uncemented THA with long-term observation shows a different host-bone response in stable versus unstable femoral implants. Prior to a femoral revision, measurement of the BMD could be beneficial; it may guide the surgeon when deciding which surgical technique to use.
Bodén
Henrik
H
Division of Orthopaedics, Karolinska Institutet at Danderyd Hospital, 18288, Danderyd, Stockholm, Sweden. henrik.boden@ort.ds.sll.se
Adolphson
Per
P
Oberg
Michael
M
D015502:Absorptiometry, Photon; D000328:Adult; D000368:Aged; D019644:Arthroplasty, Replacement, Hip / Q000009:adverse effects* / Q000379:methods; D001843:Bone Cements; D015519:Bone Density; D016723:Bone Remodeling / Q000502:physiology*; D015331:Cohort Studies; D005260:Female; D005270:Femur Head / Q000000981:diagnostic imaging* / Q000503:physiopathology; D005500:Follow-Up Studies; D006622:Hip Prosthesis; D006801:Humans; D007593:Joint Instability / Q000517:prevention & control; D008297:Male; D008875:Middle Aged; D010024:Osteoporosis / Q000000981:diagnostic imaging*; D011336:Probability; D011474:Prosthesis Design; D011475:Prosthesis Failure; D011877:Radionuclide Imaging; D012189:Retrospective Studies; D018570:Risk Assessment; D018709:Statistics, Nonparametric
D001843:Bone Cements