Issue: June 2017
June 14, 2017
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Diaphyseal filling, eccentric reaming may improve results with modular revision stems

Issue: June 2017
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The benefits of using modular stems for revision hip arthroplasty include optimized diaphyseal and endosteal contact and the ability to select the size and design of the implant intraoperatively so that it helps improve torsional stability and minimizes axial migration. However, there remains a need for more predictable surgical outcomes when modular stems are used in revision procedures, according to an orthopedic surgeon at Hospital for Special Surgery.

“There are clear problems with modularity,” Jose A. Rodriguez, MD, said. “There is no question it is more complicated, there are modular junction fractures: There is sometimes a mismatch between the thin distal femur and large proximal segment of the modular implants, and the issue of cost is real and burdening us more every day.”

Jose A. Rodriguez, MD
Jose A. Rodriguez

However, Rodriguez noted, early modular stems had no reproducible trials, which were workable with a fluted conical stem because, should the stem happen to be set too high, it can readily be removed, the canal reamed deeper and the stem be reinserted. In addition, altering the proximal modular segment of the prosthesis to a longer option can help manage a distal stem that sits more deeply upon placement than was anticipated, he said.

Taper angle research

Rodriguez discussed research he and his colleagues did into the effect of prosthetic taper angle and spline geometry on the initial stability of a modular hip revision stem. They measured axial and torsional stability in a model that replicated 2-cm diaphyseal engagement and studied a broad and a narrow spline.

“These data demonstrate that with increasing taper angle there was increasing resistance to axial migration, most notably at the 3.5° mark. Moreover, there was a clear and consistent improvement in resistance to axial migration with the broad-splined design compared to the narrow-splined design,” Rodriguez said.

He said further testing that involved 3° and 5° taper angles showed “with a 5° taper angle, the length of the desired stem was going to be limited. Therefore, the conclusion of the study was a 3.5° taper angle and a flat design of the spline would be optimal to prevent distal migration.”

Larger stems mitigate fracture

Rodriguez discussed technical aspects of femoral stem modularity, such as undersizing the stem, which is associated with subsidence and migration.

It is important, he said, to optimize distal and endosteal contact. It can be achieved by progressively using precise, reamers in 1-mm increments.

“Therefore, when in doubt, you can always ream one size larger and that is a thought process that is deep in my head because [implant] fractures do occur,” Rodriguez said. “One size larger makes a big difference in preventing fractures.”

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He also encouraged the use of trials that reproduce final stem position and use of a special trochanteric reamer. The latter minimizes “the likelihood of trochanteric impingement getting in the way of proper distal implant contact and seating,” Rodriguez said.

When used properly, a power or hand reamer “gives you the exact position as to where the trial and the stem is going to fit, so that you can have that comfort and the predictability that what you plan preoperatively will match what you do intraoperatively,” he said – by Casey Tingle

Disclosure: Rodriguez reports he receives royalties from and is a consultant for ConforMIS, Exactech, Medacta International SA and Smith & Nephew.