Prospective 5-year study with 96 short curved Fitmore\u2122 hip stems shows a high incidence of cortical hypertrophy with no clinical relevance

Caroline Thalmann,Patricia Kempter,Karl Stoffel,Arno Frigg,Thea Ziswiler

doi:10.1186/s13018-019-1174-1

Abstract

BackgroundAn increased occurrence of cortical hypertrophy (CH) was observed 1–2 years after implanting short curved Fitmore hip stems. There are no published data about either the clinical relevance or the progression of CH over the long term.MethodsNinety-six primary total hip arthroplasties were performed between 2008 and 2010 using the Fitmore hip stem. Clinical and radiological parameters were recorded preoperatively and at 1, 2, 3, and 5 year follow-up.ResultsCH appeared mainly on antero-posterior radiographs in Gruen Zones 2, 3, 5, and 6. After 1 year, the diameter was 10 ± 2 mm and remained constant thereafter. The CH rate after 1 year was 69% and after 5 years 71%. Subsidence after 1 year was 1.6 ± 1.55 mm and 1.93 ± 1.72 mm after 5 years. Cortical thinning was 46% after 1 year and 56% after 5 years, mainly in Gruen Zones 7 and 8. In the first year radiolucencies were found in 51% in all Gruen Zones, and in 20% after 5 years. Patient, implant, and surgical factors did not correlate with radiological outcomes except that larger stems had more CH. After 5 years, the Harris Hip Score had improved from 59 to 94 and the Oxford Hip Score from 22 to 41. Radiographic parameters, notably CH, were not associated with clinical outcomes except that cortical thinning correlated with lower outcome scores.ConclusionsCH correlated neither with clinical outcome nor with patient, surgical or implant factors, except for a positive correlation with stem size. The Fitmore hip stems settled within the first year to a stable fixation and then remained almost unchanged. However, cortical thinning is common in Gruen Zone 7 and 8 meaning that there is stress-shielding.

Highlights

An increased occurrence of cortical hypertrophy (CH) was observed 1–2 years after implanting short curved Fitmore hip stems
In the AP view, cortical thinning predominantly occurred in Gruen Zone 7 with 43%, but it was detectable in Gruen Zones 1, 2, 6 (Table 4, Fig. 1)
Clinical relevance We found that CH affects neither the clinical outcome nor thigh pain, which is in line with other findings [27, 29, 31,32,33]

Summary

Introduction

An increased occurrence of cortical hypertrophy (CH) was observed 1–2 years after implanting short curved Fitmore hip stems. Cortical hypertrophy (CH) is one of several observed bone remodeling mechanisms after total hip arthroplasty (THA). It is a thickening of cortical bone in an adaptive response to altered external mechanical loads, which can cause internal stress and proximal cortical thinning in the femur [1, 2]. To reduce proximal stress-shielding and to facilitate minimally invasive approaches, short curved stems with proximal fixation were developed [10, 11]. These stems are easier to implant through small incisions than. Major changes in femoral stem design were necessary that carried the risk of possible upcoming adverse effects like distal CH [12, 13]

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