Bone cement

Bone cement is a two-component system of powder and liquid that was originally developed for use in dental surgery. Despite the seemingly simple composition of these two components, bone cement is a complex material system that fulfills numerous tasks on site after implantation. The main task of the cement is to fix the implant. Secondarily, it transfers the forces acting from the implant to the bone and vice versa. This ability of the bone cement determines the stability of the implant in the long term. The interlocking between bone and cement, which is determined, among other things, by the nature of the patient's spongial layer, is particularly crucial. Chemically, it is a polymeric methyl methacrylate ( PMMA ). Gentamicin is often added to the bone cement to prevent possible infections . In order to be able to show the cement radiologically during and after the surgical intervention, i.e. using an imaging method, it is mixed with contrast media . Here come z. B. substances such as zirconium dioxide or barium sulfate are used. By adding these substances, the cement can be made visible on X-rays or in computed tomography. When implanting endoprostheses after joint infections, bone cements can be produced with antibiotic mixtures specially tailored to the patient. In this case, PMMA bone cements also take on the function of a local antibiotic carrier (drug delivery system).

In general, bone cement is indispensable for anchoring artificial joints. The bone cement creates the highest possible primary stability between the surface of the prosthesis and the bone . The advantage of using bone cement in prosthetic surgery lies in the rapid remobilization of the patient; the inserted endoprosthesis is fully resilient after the operation. The disadvantage is that removing the bone cement can be difficult when changing the implant . However, this also applies to implants anchored without cement and well fused with the bone. With both types of anchoring, bone damage and even perioperative fractures occur when the implant is explanted. Therefore, cement-in-cement revisions are increasingly being practiced in order to keep the damage to the bone as low as possible.

background

Bone cements have been used very successfully for anchoring artificial joints (hip joints, knee joints, shoulder and elbow joints) for over half a century. In surgery today there are hardly any routine interventions that are more successful than anchoring artificial joints (so-called prostheses) with bone cement. The bone cement itself fills the space between the prosthesis and the bone and takes on the important role of an elastic zone. This is necessary because about 10–12 times the weight of the body is placed on the human hip and the bone cement must therefore buffer the forces acting on the hip so that the artificial implant is preserved for a long time. In the long-term observations of the Scandinavian endoprosthesis registers, differences between cemented and non-cemented anchors could e.g. B. the hip joint can be shown in terms of downtime and / or comorbidities. In Norway, for example, the survival rate of cemented prostheses is higher than that of uncemented. In Sweden, too, the percentage of interventions that did not require revision is greater with the cemented anchorage.

Chemically, bone cement is a two-component system of powder and liquid that contains PMMA. Bone cement is a glass-like, solid material that has been used in many ways as Plexiglas (= polymethyl methacrylate (PMMA) ). The material was first used clinically in the 1940s in plastic surgery to close gaps on the roof of the skull . Extensive clinical studies on the body compatibility of bone cements preceded this surgical field of application. Due to the optimal tissue compatibility of PMMA, bone cements were then used in the 1950s to anchor head prostheses .

Today several million such procedures are performed worldwide each year and bone cements are routinely used in more than half of them. Due to the comfortable and easy handling of bone cements in clinical practice, but especially because of the demonstrably long service life of cemented prostheses, bone cement is a reliable anchoring material. An endoprosthesis register was also introduced in Germany in 2011.

application

The mixed components cure in about 9–13 minutes, depending on the ambient temperature. As the polymerization increases, so does the temperature. This can increase to up to 70 ° C, which can damage the surrounding tissue. The body would begin to break down this damaged tissue. For a long time, the brief temperature spike during the curing process was seen as the main reason for septic loosening. The remedy here is an operative procedure that is as precise as possible, so that only very thin layers of cement have to be used, so that less heat is generated.

A more recent application of the bone cement is the stabilization of broken vertebrae in osteoporosis or metastasis of malignant diseases (see also vertebroplasty and kyphoplasty ).

At the Charité Berlin, medical technicians and doctors are currently working as part of a project funded by the Technologiestiftung Berlin and the EU ( EFRE ) on the implementation of the clinical application of a bone cement, which is also visible in a magnetic resonance tomograph , so that operations in which bone cement is used in the future , without (X-ray) radiation exposure for the patient, doctor and involved staff.

composition

Bone cements are provided as two-part materials. Bone cements consist of a powder and a liquid. These two components are mixed together to create a homogeneous dough. This cement mixture is then poured into the bone. The prosthesis is then carefully embedded in the cement mass. The viscosity of the cement paste then increases increasingly until the material hardens to form a solid matrix. This type of anchoring fixes the new artificial joint - the prosthesis - permanently.

It is known that energy is released in the form of heat during the hardening of the cement. This heat of polymerization reaches a temperature of approx. 42–46 ° C in the body. This means that the temperature development is below the critical range for body proteins. The reason for this low polymerization temperature in the body is the relatively thin cement layer, which should not exceed 5 mm, as well as the temperature dissipation via the large surface of the prosthesis and the blood flow.

The individual components of the bone cement are u. a. also known from the field of application of dental filling materials. Acrylate-based plastics are also used there. While the individual components as pharmaceutical auxiliaries and active ingredients are not always harmless per se, the substances used in bone cement are either converted or completely enclosed in the cement matrix during the polymerization phase from the increase in viscosity to hardening. From today's point of view, hardened bone cement can therefore be classified as harmless, which was already shown by the early studies on body tolerance from the 1950s.

Aspects of the use of bone cement

The so-called bone cement syndrome is described in the literature. For a long time it was believed that incompletely converted monomer released from bone cement was the cause of circulatory reactions and embolism . However, it turned out that this monomer (residual monomer) has been shown to be metabolized via the respiratory chain and split into carbon dioxide and water and excreted. When artificial joints are anchored, embolisms can always arise if material is introduced into the previously cleared femur. The intramedullary pressure increase then occurs.

In the case of proven allergies to components of the bone cement, based on the current state of knowledge, no bone cement should be used to anchor the prosthesis. As an alternative, the anchoring can be done without cement - cement-free implantation.

Revisions

Revision is the replacement of a prosthesis. I.e. the prosthesis that has already been implanted in the body is removed again and replaced by another prosthesis. Compared to the first operations, revisions are often more complex (operations) and more difficult, since healthy bone substance is naturally lost with each revision. Revision operations are also associated with higher costs in order to produce a satisfactory surgical result. The primary goal is therefore always to avoid revisions through good surgery and the use of products with good (long-term) results.

Today, revisions cannot always be avoided. Revisions can have different causes: A distinction is made between septic and aseptic revisions. If the implant has to be changed without an infection being proven - i.e. aseptically - then the cement is not necessarily completely removed today. However, if the implant is septically loosened, the cement must be removed radically. According to current knowledge, cement removal is easier to accomplish than removing a well-ingrown cement-free prosthesis from the bone bed. Ultimately, it is important for the stability of the revised prosthesis to detect any loosening of the primary implant at an early stage in order to preserve as much healthy bone as possible.

A prosthesis fixed with bone cement ensures very high primary stability combined with rapid remobilization of the patient. The cemented prosthesis can be fully loaded immediately after the operation. A necessary rehabilitation is felt to be comparatively problem-free by patients who have been fitted with a cemented prosthesis. A load on the joints is possible again shortly after the operation, the use of walking aids to relieve pressure is necessary for reasons of safety within a reasonable period of time.

The bone cement proves to be particularly advantageous because the powder component has targeted active ingredients such. B. antibiotics can be added. After the new joint has been implanted, these active ingredients are released locally, i.e. in the immediate vicinity of the new prosthesis, and have been shown to reduce the risk of infections. I.e. Bacteria are fought effectively on the spot - namely in the open wound, without unnecessarily burdening the body with high antibiotic levels. This makes bone cement a modern drug delivery system - a local agent that is used directly in the surgical field. The decisive factor here is not how much active ingredient is contained in the cement matrix, but how much of the active ingredient used is actually released locally. Too much active ingredient in the bone cement would even be disadvantageous, since the mechanical stability of the fixed prosthesis is weakened by high amounts of active ingredient in the cement. The local levels of active ingredient in industrially manufactured bone cements that are built up when using active ingredient-containing bone cements are harmless (provided there is no intolerance) and are well below the clinical routine doses for systemic single injections.

Anaesthesiological aspects

From an anesthesiological point of view, it should be noted that when the bone cement is introduced, a drop in blood pressure and a drop in oxygen saturation can occur. There are various theories as to how these phenomena come about. A direct vasodilating (vasodilating) effect of the cement is discussed. The decrease in oxygen saturation can be explained by microembolisms, but it could also be the result of a redistribution of the pulmonary blood flow , which increases the shunt volume, i.e. the proportion of blood that leaves the lungs without contact with oxygen. It is noteworthy that this phenomenon only occurs when the cement has polymerized (shortly before the end of hardening). This leads to a thermodynamic (exothermic) reaction in which hot gases are generated during the polymerization. These then enter the bloodstream and are exhaled through the lungs. This leads to dangerous vascular dilatation in the small circulation, which can ultimately result in acute right heart failure.

swell

• Properties of bone cement ( PDF )
• Drug Commission of the German Medical Association: "UAW - Learning from mistakes" Cardiopulmonary incidents when using bone cement . Dtsch Arztebl 2008; 105 (50): A-2721 ( pdf )

Web links

• Bone cement for operations in open high-field MRI ( PDF )

See also

• Dental cement - a natural part of the teeth supporting structure

Individual evidence

1. ^ SJ Breusch, K.-D. Kühn: bone cements based on polymethyl methacrylate . In: Orthopedics . tape 32 , no. 1 , ISSN  0085-4530 , p. 41–50 , doi : 10.1007 / s00132-002-0411-0 ( springer.com [accessed February 17, 2018]).
2. ^ SJ Breusch, K.-D. Kühn: bone cements based on polymethyl methacrylate. In: The orthopedist. 32, 2003, pp. 41-50, doi : 10.1007 / s00132-002-0411-0 .
3. ↑ bone cement . In: Academic dictionaries and encyclopedias . ( deacademic.com [accessed February 17, 2018]).
4. ↑ AB Imhoff: shoulder / elbow / shock wave / hip . Springer-Verlag, 2013, ISBN 978-3-642-58706-1 ( google.de [accessed on February 26, 2018]).
5. ↑ Startside. Retrieved February 26, 2018 (Norwegian Bokmål). 
6. ↑ Swedish Annual Hip Report 2016. Retrieved February 26, 2018 . 
7. ↑ EPRD Deutsche Endoprothesenregister gGmbH: Home | EPRD. Retrieved February 5, 2018 . 
8. ^ MKD Nicholas, MGJ Waters, KM Holford, G. Adusei: Analysis of rheological properties of bone cements . In: Journal of Materials Science: Materials in Medicine . tape 18 , no. 7 , ISSN  0957-4530 , p. 1407–1412 , doi : 10.1007 / s10856-007-0125-2 ( springer.com [accessed February 5, 2018]).

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