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Modular hip implants are a boon to revision surgery because surgeons can replace the femoral head and liner while keeping well-fixed components in place. However, the interface between the head and trunnion is susceptible to corrosion, which increases the risk of implant loosening and fracture.
Ceramic heads have been shown to mitigate corrosion, but surface defects on a fixed metal stem can cause early failure. Therefore, manufacturers advise using a titanium sleeve over the metal stem to protect the head. How this sleeve affects the strength of the taper–head connection has not been investigated.
Miriam Chaudhary, PhD, research fellow, Sourabh Boruah, PhD, research fellow, and Kartik M. Varadarajan, PhD, principal investigator, all from the Technology Implementation Research Center in the Department of Orthopaedics at Massachusetts General Hospital, and Orhun K. Muratoglu, PhD, of the Harris Orthopedics Laboratory, recently conducted experiments that address that question. In the Journal of Orthopaedic Research, they report that when ceramic heads are used in revision surgery, sleeve size has a notable effect on fixation strength.
The researchers constructed an apparatus to investigate the assembly of a revision ceramic head, titanium sleeve and the trunnion portion of a titanium stem with a single hammer blow. The hammer was fixed to a lever arm and dropped onto a standard surgical impactor, equipped with a force sensor, that came down on the head. Additional sensors tracked the seating displacement of the head relative to the trunnion.
Two heads of different diameters (28 mm and 36 mm) and two sleeves of different offset lengths (short, −3mm and extra-long, +7 mm), mated with 12/14 trunnions, were tested at two impact forces (2 kN and 6 kN). After impact, each sample was transferred to a hydraulic testing machine that measured the force required to pull the head off the trunnion.
The researchers found that extra-long sleeves increased seating displacement by 31% and reduced pull-off forces by 15%. The impact force and sleeve offset length interacted to affect pull-off force. They found that extra-long sleeves reduced pull-off force by 4% for specimens assembled at 2 kN but by 27% for those assembled at 6 kN. Head diameter did not significantly affect seating displacement or pull-off force.
When a ceramic head is used in revision hip arthroplasty, the availability of sleeves of different sizes permits better control of neck length. However, the sleeve offset length has a notable effect on fixation strength. Surgeons should consider how their choice of sleeve size, combined with the amount of impact force, will affect the stability of the implant.
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Journal of Orthopaedic Research
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