Selection of femoral prosthesis in total hip replacement for ankylosing spondylitis.

Objective: To investigate the implication of morphological changes of the proximal femur in femoral prosthesis selection in total hip replacement for treatment of ankylosing spondylitis (AS).

Methods: Twenty-four patients (36 hips) admitted for cementless total hip replacement (THR) because of AS were enrolled in this study along with the control group of 30 patients (45 hips) without AS. Measurement of the Singh index, canal flare index, morphological index of the cortex, cortical index and stem-canal fit was performed in the two groups on the basis of roentgenograms.

Results: The AS patients had more serious osteoporosis in their proximal femur. There were significant differences between the two groups in Singh index, canal flare index, morphological index of the cortex and stem-canal fit (P<0.05), but not in the cortical index (P>0.05). The AS patients were found to have severe osteoporosis in the upper femur and massive spongy bone loss, so that the morphology of the canal was almost orthostatic in the upper femur because of the thinned femoral cortex.

Conclusion: Cemented prosthesis is recommended for AS patients with severe osteoporosis, who should be treated with revised THR to ensure good fit of the prosthesis to the canal. Cementless prosthesis, however, can still achieve good therapeutic effect in AS patients without significant morphologic changes in the proximal femurs. Computer-aided design and manufacture of individualized prosthesis better suits the AS patients than the currently available prosthesis.