Consistent fixation, bone preservation with modular femoral stems

Aseptic failures with Paprosky Type IIIB and Type IV fractures show need for more fixation options.

ORLANDO, Fla. — Modular femoral stems provide reliable fixation and preserve proximal bone stock in complex revisions for Paprosky Type II and Type IIIA defects. Type IIIB and Type IV defects, however, may require additional fixation options, a recent study shows.

Researchers found that 84% of non-cemented stems had bony in-growth and proximal bone preservation at mean 14-year follow-up. Of those patients, 48% had bone hypertrophy in the diaphysis. “If well fixed and well fitted, a wide variety of bony deficiencies can be addressed and this fixation can be maintained in the long run,” Joseph C. McCarthy, MD, of New England Baptist Hospital in Boston, said.

Surgical technique

The study involved 42 hips in 40 patients, including 23 men and 17 women, with an average age of 55 years. Five aseptic failures occurred in Paprosky Type IIIB and Type IV defect revisions, but researchers found no aseptic failures in Paprosky Type II or Type IIIA defects.

Researchers reviewed preoperative radiographs using Paprosky’s classification and reviewed postoperative radiographs for osteolysis, endosteal hypertrophy, cortical hypertrophy, distal pedestals, breakage and loosening.

About 60% of the revision cases had previously cemented femurs, 28% had failed cementless implants and six hips were girdlestone conversions. Researchers classified two-thirds of the patients as Paprosky Type III and Type IV. “We had a good number of [Type] IIIB and [Type] IV ... where there was extensive metaphyseal and diaphyseal disruption in a widened canal,” McCarthy said.

Surgeons performed hybrid fixation, which involved cementing the spout, but not the stem, to better reproduce leg length. “We were looking for fit and fixation distally, but not ingrowth distally on these people to try and preserve bone stock. The hope was that we’d have less thigh pain,” McCarthy said.

Surgeons performed a trochanteric slide in two-thirds of the patients and acetabular revisions in 80%. They used proximal femoral allografts for 28 patients, strut grafts for 18 and cemented sleeves in eight. “Most of these stems were long and bowed in order to go through the deficiency and the neck lengths were quite variable with 34% of them being calcar replacements,” McCarthy said. He made the comments at the 22nd Annual Current Concepts in Joint Replacement Winter 2005 Meeting.

Results

Researchers were initially concerned with component uncoupling and breakage, but found neither occurred in any of the cases at the 14-year follow-up. They found nonbridging pedestals in 22% of hips and calcar resorption in 28%. “The most common feature was endosteal hypertrophy, 44% ... which remained in continued fixation of the component throughout this 14-year period,” McCarthy said.

The five aseptic failures included one patient who had osteogenesis imperfecta and 351 lower extremity fractures, and two patients with periprosthetic fractures in the lysis. All had two previous revisions.

Surgeons revised the five aseptic hips with stable fixation and an additional modular component. Two became reinfected, despite an interim girdlestone. “Ironically only two of the eight Paprosky [Type] IV became loose and half of the [Type] IIIB [became loose],” McCarthy said.

Stem weaknesses

Surgeons at New England Baptist Hospital began using a distally split modular stem in 1988 because they found most deficiencies were in the upper region of the femur. “[The stem] allowed independent metaphyseal, diaphyseal sizing and stem-to-neck ratios and adjustable offset inversion, which we felt was advantageous with some of these patients with significant leg-length disruptions,” McCarthy said.

However, they found some weakness with the modular stem, because it turns with the component, which affected orientation and anteversion. Additionally, in six cases, surgeons had to cut the anterior spline to insert the stem, although it remained well fixed after that. Also, McCarthy noted: “The flutes had to be over-reamed so they don’t create linear splines distally or within the diaphysis, but ... this had excellent fixation.”

For more information:
McCarthy J. The role of modular femoral components: Proximal fixation. #55. Presented at 22nd Annual Current Concepts in Joint Replacement Winter 2005 Meeting. Dec. 14-17, 2005. Orlando, Fla.