As society ages, cases of osteoporosis and bone fragility fractures have been on the increase in recent years. Each year, 150,000 cases of femoral neck bone fracture occur, necessitating 250,000,000,000 yen in medical costs. A considerable number of these are bone fragility fractures that are not caused by a fall. While bone fragility fractures do not initially involve dislocation (meaning that the bone fractured bone shifts or turns, as a result of which the dislocated region visibly appears to change shape), dislocation proceeds gradually, ultimately leading to highly invasive and expensive surgical procedures.
The typical surgical procedures employed for a bone fracture site are a locking plate procedure in which a metal plate is positioned against the bone fracture region, and the plate fastened to the bone with screws; and an intramedullary rod fastening procedure in which a long rod made of metal is implanted into the medullary cavity at the center of the bone from the end of the bone, and the rod fastened with screws. However, when the aforedescribed surgical procedures are performed on patients with osteoporosis or a bone fragility fracture, because the strength of the bone is less than the strength of the metal, various problems such as loosening and re-dislocation have occurred.
Another known surgical procedure for bone fracture sites is a method in which the bone marrow is scraped out, a structural framework furnished with a sheathing formed from a bioabsorbable material or the like is inserted therein, and once inserted the structural framework is expanded and packed with bone cement (see Patent Document 1).
More recently, balloon kyphoplasty, which is a technique used for patients whose pain caused by a spinal bone fracture due to osteoporosis does not improve, became covered by insurance starting in January of 2011, and has come to be commonly used in Japan. This method is noted for being a relatively non-invasive treatment method, and involves (1) insertion, from the patient's back, a small instrument with an attached balloon into a bone fracture region of the spine, (2) expanding the balloon within the vertebra to restore the broken bone to its shape prior to bone fracture, (3) then withdrawing the balloon to form a cavity inside the vertebra, and (4) packing bone cement into the cavity so formed to fill the cavity, by a surgery which can be completed in about one hour.
However, problems encountered with balloon kyphoplasty include (1) by first inserting a balloon made of silicone, and using the balloon to squeeze and crush the substantia spongiosa, which is the spongy bone inside the marrow, to create a wall and ensure a space, it is possible to reliably inject bone cement into the target region while preventing leakage, but since the hard bone (cortical bone) surrounding the bone fracture site is crushed, leaking from this area cannot be prevented; (2) the bone cement employed in balloon kyphoplasty is a not a calcium bone cement, instead employing a resin known as methyl methacrylate. While this resin has excellent strength and can reliably increase bone strength quickly, the material is unable to directly bond with bone, and during curing emits heat to around 90 degrees, thus affecting surrounding tissue; and (3) as the cured methyl methacrylate is too rigid, when used in patients who have brittle bones to begin with, bone fractures are sometimes induced in regions of bone that have weaker strength than the cured methyl methacrylate and that surround the site where it is used.
Furthermore, if the bone cement at a packing site should leak out from the bone and flow into blood vessels, there is a risk of causing a pulmonary embolism. To solve this problem, during packing of bone cement into a bone defect region such as a bone fracture site or the like, it is known to encapsulate the bone cement paste in a bioabsorbable material comprising one or more materials selected from fibrin sheets, collagen sheets, and homopolymers or copolymers of poly(lactic acid) and Poly(glycolic acid), in order to prevent the bone cement from leaking, and to prevent delayed curing time (see Patent Document 2).
Also known is a flocculant three-dimensional structure formed from a controlled release system of a chemical composition for effectively eliciting bone reconstruction capability, and, as a material for providing good fitting to the affected part, a siloxane-containing substance having as the principal component a synthetic polymer such as poly(lactic acid) (PLA), a copolymer of poly(lactic acid) and poly(glycolic acid) (PGA), polyethylene glycol (PEG), polycaprolactone (PCL), or a copolymer of PLA, PGA, PEG, and PCL, or a natural polymer such as fibrin, collagen, alginic acid, hyaluronic acid, chitin, chitosan, or the like (see Patent Document 3).
However, during packing of bone cement into a bone fracture site, it is necessary to induce the tacky bone cement paste to penetrate into every corner of the irregularly shaped medullary cavity, leaving no gaps. The sheath of the aforedescribed Patent Document 1 is manufactured from materials such as collagen, polyester fibers, poly(lactic acid), and the like, and lacks pliability, making close adhesion to bone a problem.
The bioabsorbable material indicated in the aforedescribed Patent Document 2, and the three-dimensional structure indicated in the aforedescribed Patent Document 3, lack excellent elongation, and therefore cannot withstand the pressure produced during injection of the bone cement paste, thus presenting the problem of a risk of the bone cement leaking out from the three-dimensional structure and, as a result, flowing into blood vessels, as well as difficulty in packing the viscous bone cement paste into every corner of the bone.
Additionally, the structural backbone indicated in the aforedescribed Patent Document 1, and most other such intramedullary rods designed to reinforce a bone fracture site from inside the bone, are produced from metal, and if a re-fracture occurs subsequent to the surgical procedure, the metal can break loose from the bone at the bone fracture site, with a risk of injury to biological tissue.
Meanwhile, in the case of packing only an osteoconductive calcium phosphate-based bone cement without the use of an intramedullary rod for surgery at a bone fracture site, the bone cement in the cured state lacks elasticity, posing a risk of breaking due to stress created by bending, pulling, compression, and the like. For example, when used in a (spinal) bone fracture of a patient who has suffered a compression bone fracture of the spine due to osteoporosis, the cured calcium phosphate-based bone cement does not adequately maintain mechanical strength for extended periods under conditions of being subjected to relatively high pressure, and it was difficult to consistently maintain the desired shape. Therefore, a filler material in which fibers having crimp are dispersed in a calcium phosphate-based bone cement is known (see Patent Document 4), but this material is not fully satisfactory in terms of bending and tensile strength.