Shape Memory Implants
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We will aim to develop implants made of a Ni-Ti shape memory alloy which can be applied for the treatment of midface fractures, such as isolated orbital floor fractures. These can then be implanted in a compressed form and unfold automatically in the body. With the help of newly developed application instruments, the implants can be applied along transnasal and transantral approaches into the maxillary sinus.
Adaptable Orthopedic Shape Memory Implants - ScienceDirect
Our objective is to evaluate the operation process and the functionality of these implants, already in a pre-investigation by an experienced surgeon on a phantom. The functionality of the surgical procedure and an implant prototype were both evaluated with the help of a realistic phantom. The minimally invasive application was carried out using the transnasal and transantral approach. In addition, VBS could reduce the volume of bone cement needed and thus reduce its side effects.
Another study by Ghofrani [ 17 ] showed that titanium mesh implants without cement can effectively achieve endplate reduction, but also can acquire enough biomechanical spinal stability to be effective. Compared to other currently available metal materials that are vertebral body reduction implants, Nickel-Tianium Ni-Ti shape memory alloys have unique temperature and shape memory characteristics.
The design of the implant allows increases in contact surface area between the leaflets of the implant and the vertebral endplate, and in supportive force of the implant in vertebral fracture reduction by increasing the stretching force of the memory alloys when they are extended. In addition, the symmetrical design of the implant improved manipulation efficiency during the procedure. Although their wide application in orthopedics, however, the implantation of Ni-Ti shape memory alloys in the treatment of human adult vertebral body compression fractures have not been studied experimentally.
In this study, we designed a Ni-Ti shape memory alloy of vertebral body reduction implant which substitutes dilated balloon in PKP to ensure the spinal stability for effective endplate reduction. The implant was placed in the human adult fresh-frozen fractured spine specimen. Endplate reduction and ultimate load of the experimental vertebra were measured.
The experiment demonstrates that, without using bone cement, the Ni-Ti shape memory alloy can achieve similar therapeutic effects in the treatment of human vertebral body compression fractures in comparison with conventional percutaneous kyphoplasty.
The vertebral reduction implant in collapsed status at lower temperature can be installed into a fractured vertebral body through the pedicle. By heating to the characteristic transformation temperature of the Ni-Ti SMA and returning the implant to its stretched status, fractured vertebral body reduction can be achieved without using PMMA.
Therefore, a prototype implant was designed and tested in this study. The implant was made of Ni-Ti shape memory alloys material in a lantern shape with six evenly placed leaflets around the core stem and with both ends closed. One end of the implant stem was connected to the implant delivery system. The total length of the implant was The diameter of the stem at both ends was 5.
Each leaflet was Representative pictures Left and computer-aided designs Right of Ni-Ti shape memory alloys of vertebral body reduction implant.
Ten fresh frozen human cadaveric spines from T6 to L5 were included in this study. Average death age of the specimen donors was The donors included 3 males and 7 females.
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The study was approved by the regional ethics committee, Ruijin Hospital, Shanghai Jiaotong University School of Medicine, Shanghai, China and written consent had been obtained. Plain X-ray was performed prior to the study to exclude the pathological changes of the specimens from deformity, previous surgery, excessive osteophytes, or any previous fracture.
The specimens were denuded of soft tissues fat and muscle and disarticulated.
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Each specimen consists of three vertebral bodies with intact ligaments, joint capsules, discs, and bone structures. Then, each spinal specimen was separated into three independent units: Therefore, a total of 30 vertebral units were harvested for this study. The cranial and the caudal vertebrae of each unit were embedded in clinical bone cement by an in-house designed circular tube fixture, which was to be mounted into the flanges of the spinal biomechanical testing system. Thus, the compression fracture only happens in the middle vertebral body Instron , Norwood, MA.
Concept of patient-specific shape memory implants for the treatment of orbital floor fractures.
Thirty isolated 3-vertebra specimen units were randomly assigned to three groups according to the three different repair techniques: Compression fractures were developed in 24 specimens from the PKP and Ni-Ti groups but not in the 6 control group specimens. The height loss was later verified in plain X-ray or CT reconstruction films. After the specimens were thawed, three different treatment procedures were performed by one author C.
No repairing procedures were applied to the 6 specimens in the control group. Vertebral height was measured and peak load was tested. Percutaneous kyphoplasty PKP group: The unipedicular kyphoplasty was performed according to a previously described technique under biplanar fluoroscopic control [ 3 ].
A deflated balloon tamp was inserted into the fractured vertebral body by cannulating one of the pedicles. The cannula was positioned at a relatively large convergent angle so that the tamp could be placed as close to the center of the vertebral body as possible. Balloon inflation was carefully controlled until it reached the subchondral plate, the lateral borders, or the anterior cortex. Upon removal of the tamp, polymethylmethacrylate cement was injected to an average volume of 3. Injection stopped when cement leakage occurred. Percutaneous Ni-Ti shape memory alloys implant group: The Ni-Ti implant was soaked in an ice-water mixture to preserve its original collapsed form before the procedure.
An implant of appropriate size, which was determined by measurement of the vertebra after X-ray before the procedure, was inserted into the fractured vertebral body using procedures similar to those described in the above kyphoplasty group. The reduction of the vertebra with the expanded lantern-shape implant was confirmed with plain X-ray radiography. If vertebral endplate reduction was not satisfactory due to inadequate implant expansion, the actuator rod of the implant could be rotated allowing further expansion and higher supporting force to vertebral endplates.
After the treatment of the vertebral body, each unit was mounted on an electromechanical testing system Instron , Norwood, MA.
A time-cycling vertical preload of N was applied to avoid creep and fatigue damage to the bone. The load-deflection curve was recorded with a data acquisition rate of Hz, from which the ultimate load value was calculated. The vertical height of all middle vertebrae in each unit was measured before and after the procedure using lateral radiographs of the spine. Particular attention was directed to the middle height. In order to adapt the implant to the actual healing situation, nickel-titanium NiTi shape memory alloy SMA based implants that exhibit a variable middle piece were developed.
Due to contactless induction heating, the transition temperature of the SMA is reached and triggers the one-way shape memory effect SME. This leads to a slight modification of the second moment of inertia and to an adaption of the bending stiffness of the implant, respectively. Thereby, two approaches, increasing the bending stiffness and decreasing the bending stiffness, have been realized.
Currently, the implant design is being adapted in order to meet the requirements for human applications.