title: Trade-off between stress shielding and initial stability on an anatomical cementless stem shortening: in-vitro biomechanical study.
abstract: Shortened cementless femoral stems have become popular with the advent of minimally invasive total hip arthroplasty (THA). Successful THA requires initial stem stability and prevention of stress shielding-mediated bone loss, although the effect of stem shortening is controversial. Here we experimentally examined whether stem shortening affects stress shielding and initial stability. Anatomical stems (length, 120 mm) were cut to an 80 mm or 50 mm length. Ten tri-axial strain gauges measured the cortical strain on each stem-implanted femur to evaluate stress shielding. Two transducers measured axial relative displacement and rotation under single-leg stance loading. The 50 mm stem increased the equivalent strains with respect to the original stem in the proximal calcar region (31.0% relative to intact strain), proximal medial region (63.1%), and proximal lateral region (53.9%). In contrast, axial displacement and rotation increased with a decreasing stem length. However, the axial displacement of the 50 mm stem was below a critical value of 150 µm for bone ingrowth. Our findings indicate that, with regard to a reduction in stem length, there is a tradeoff between stress shielding and initial stability. Shortening the stem up to 50 mm can promote proximal load transfer, but bone loss would be inevitable, even with sufficient initial stability for long-term fixation.
authors.affiliation: Organization for Promotion of Tenure Track, University of Miyazaki, 1-1 Gakuen-kibanadai-nishi, Miyazaki 889-2192, Japan. Electronic address: g.yamako@cc.miyazaki-u.ac.jp.
authors.forename: Go
authors.initials: G
authors.lastname: Yamako
authors.affiliation: Department of Medicine of Sensory and Motor Organs, Division of Orthopedic Surgery, Faculty of Medicine, University of Miyazaki, Miyazaki 889-1692, Japan.
authors.forename: Etsuo
authors.initials: E
authors.lastname: Chosa
authors.affiliation: Department of Medicine of Sensory and Motor Organs, Division of Orthopedic Surgery, Faculty of Medicine, University of Miyazaki, Miyazaki 889-1692, Japan.
authors.forename: Koji
authors.initials: K
authors.lastname: Totoribe
authors.affiliation: Department of Medicine of Sensory and Motor Organs, Division of Orthopedic Surgery, Faculty of Medicine, University of Miyazaki, Miyazaki 889-1692, Japan.
authors.forename: Shinji
authors.initials: S
authors.lastname: Watanabe
authors.affiliation: Department of Medicine of Sensory and Motor Organs, Division of Orthopedic Surgery, Faculty of Medicine, University of Miyazaki, Miyazaki 889-1692, Japan.
authors.forename: Takero
authors.initials: T
authors.lastname: Sakamoto
mesh_terms: D019644:Arthroplasty, Replacement, Hip; D001696:Biomechanical Phenomena; D019544:Equipment Failure Analysis; D005269:Femur / Q000503:physiopathology* / Q000601:surgery; D006622:Hip Prosthesis; D006801:Humans; D008422:Materials Testing; D011474:Prosthesis Design
keywords: Anatomical stem; Initial stability; Short stem; Stress shielding; Total hip arthroplasty