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Evaluation of a Femoral Stem with Reduced Stiffness

A Randomized Study with Use of Radiostereometry and Bone Densitometry

Investigation performed at Sahlgren University Hospital, Göteborg, Sweden, and Buskerud Hospital, Drammen, Norway

Johan Kärrholm, MD, PhD
Jonas Thanner, MD, PhD
Henrik Malchau, MD, PhD
Peter Herberts, MD, PhD
Department of Orthopaedics, Institute of Surgical Science, Sahlgren University Hospital, SE 413 45 Göteborg, Sweden. E-mail address for J. Kärrholm: eva.fermen{at}orthop.gu.se

Christian Anderberg, MD
Department of Orthopaedics, Länssjukhuset, SE 301 85 Halmstad, Sweden

Finnur Snorrason, MD, PhD
Norvald Langeland, MD, PhD
Buskerud Hospital, Ortopediska Kliniken, Dronninggaten 28, N 3004 Drammen, Norway

In support of their research or preparation of this manuscript, one or more of the authors received grants or outside funding from IngaBritt and Arne Lundbergs Foundation, Neubergh Research Foundation, Zimmer USA, Göteborgs Läkaresällskap, and Hallands Läns Landstigs Research Foundation. None of the authors received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.

Background: Femoral stems with reduced stiffness were introduced in total hip arthroplasty to facilitate proximal load transfer and thereby reduce periprosthetic bone loss. Poor fixation and unacceptably high revision rates turned out to be a major problem with these prostheses. The purpose of the present study was to evaluate the early performance of a low-modulus stem (one that is less stiff) with the potential to address the problem of fixation by the use of a surface coating. The coating consisted of a titanium porous mesh proximally covered with a mixture of hydroxyapatite and tricalcium phosphate.

Methods: Thirty-nine men and twenty-six women (sixty-eight hips) with noninflammatory arthritis were studied. The patients were randomized to receive either a porous-coated composite stem with reduced stiffness (Epoch) or a stiff stem with a porous coating (Anatomic). Both stems were inserted without cement, had a ceramic coating on the proximal two-thirds, and were supplied with tantalum beads. At the time of the operation, tantalum markers were also placed into the proximal part of the femur. The patients were followed for up to two years after the operation and were evaluated with use of repeated radiostereometric measurements, measurements of bone-mineral density, conventional radiography, and Harris hip scores.

Results: Both stems showed optimum fixation with median subsidence and stem rotations that were close to zero. Repeated measurements of bone-mineral density revealed early loss of bone mineral in all Gruen regions in both treatment groups. The loss of bone mineral around the Epoch stems was significantly reduced at two years in Gruen regions 1, 2, 6, and 7 (p < 0.0005 to 0.04). Measurements on postoperative radiographs showed no difference in stem positioning or fill, but the Epoch stems had significantly more endocortical contact on both the anteroposterior (p < 0.0005) and the lateral radiograph (p = 0.02).

At two years postoperatively, the Epoch stems had fewer sclerotic lines surrounding the stem (p £ 0.002) and less sclerosis at the tip of the prosthesis (p = 0.001) compared with the Anatomic stems. The clinical results in terms of the Harris hip score, which was determined in all hips, and pain or discomfort, which was evaluated in thirty-seven hips treated at the same hospital, were not found to be different, with the numbers available.

Conclusions: Contrary to previous studies of other designs with reduced stiffness, the Epoch stem achieved excellent primary fixation. Despite this rigid fixation, the proximal loss of bone-mineral density was less than that associated with the stem with a stiffer design. These results should encourage additional long-term studies with a larger patient population.

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