CAN MIGRATION OF THE PRESS-FIT REVISION FEMORAL STRAIGHT STEM (PFM-R) BE AVOIDED? SEARCH FOR PREDICTIVE FACTORS IN A RETROSPECTIVE SERIES OF 53 CASES

Abstract

Purpose of the study: The revision modular femoral prosthesis (PFM-R) (Centerpulse) uses a cone-shaped straight femoral stem with winglets for press-fit revision fixation in the femoral shaft. The risk of secondary migration further into the shaft is well known. The purpose of this paper was to assess the degree of PFM-R stem migration and to search for predictive factors in order to better ascertain the limits of this type of revision anchorage in the femoral shaft.

Material and methods: Fifty-three files (48 patients) were reviewed retrospectively at minimum none months follow-up. Mean patient age was 56.6 years. Revision was required for loosening (SOFCOT classification): grade 1 (n=5), grade 2 (n=8), grade 3 (n=17), grade 4 (n=10). For 13 hips, the implant to be replaced was not cemented (monobloc stem, infection), or a first-intention PFM-R.

Results: Mean migration was 4.1 mm (0–17 mm), less than 5 mm in 73.5% of cases. There was no correlation between migration and SOFCOT grade. Conversely, there was a significant difference in migration between the grade 1 and grade 3B and between Paprosky grade 1 and 4 (p=0.05). The degree of migration was correlated with the length of the persistent isthma (p< 0.0001), with the morphology of the isthma [conic/inverted conic (p< 0.01), conic/cylindric (p< 0.051)], with the cortical index (p=0.06), the length of the anchor wings (p=0.051), but not with the length of the femorotomy bridge. The length of the wing anchorage was correlated with the length of the persistent isthma (p=0.002) and with the morphology of the isthma [(conic/inverted conic p=0.02), cylindric/inverted conic (p=0.02)], but did not increase significantly with bridging length. There was a trend towards migration in osteoporotic bone (p=0.07).

Discussion and conclusion: Use of a straight stem for anchorage in the femur is associated with secondary migration which depends on the quality of the bone in the anchorage zone and the extent of the press-fit. The persistence of a cone-shaped or cylindrical isthma measuring greater than 4 cm associated with a cortical index greater than 45% and a long wing anchorage can provide excellent primary stability. It would be useless to use long stems since they do not increase the quality of anchorage. The absence of an isthma and the presence of osteoporosis are limitations for this concept.