A BONE INGROWTH CHAMBER

Abstract: The disclosure relates to an apparatus for studying tissue ingrowth in an implant under different experimental conditions, in particular in response to locally administered test substances. The apparatus is of a bio-compatible material, preferably pure titanium, and is intended to be implanted in living bone tissue, primarily for animal-experimental activities. The apparatus consists of an outer portion (1) with a central recess in which an inner portion (2) is inserted. In the assembled state, the two portions (1, 2) form at least two bone ingrowth through channels (10, 11, 12, 13; 17, 18, 19) which are exposed when the inner portion (2) is dismounted, for comparative examination of the bone ingrowth promoting/retarding properties of test substances disposed in one or more of the channels.

Description: TECHNICAL FIELD The present invention relates to an apparatus for studying the tissue ingrowth in an implant under different experimental conditions. The apparatus is primarily intended for use in animal experimental activities, but a certain clinical, experimental use may also occur. The apparatus is of a bio-compatible material, preferably titanium, and suitable for implantation in living bone tissue. BACKGROUND ART It is previously known in this art how artificial implants may be anchored directly in bone tissue. In order to avoid the risk of loosening or detachment, every attempt is made to achieve a direct contact, i e an exact adaptation between the implant and the surrounding bone tissue, so-called osseointegration. Such an exact adaptation may be achieved by sophisticated operational technique and by a suitable design of the implant. The osseointegration principle developed by professor Brånemark et al has successfully been used clinically for 20 years for maxillary-anchored dental bridges and is described in, for example: P-I Brånemark et al, "Osseointegrated titanium fixtures in the treatment of edentulousness". Biomaterials, 1983. Vol 4, January; and Rickard Skalak, "Biomechancial considerations in osseointegrated prostheses". The Journal of Prosthetic Dentistry, June 1983, Volume 49, Number 6. The principle is based on the fact that the implant is of pure titanium, at least in the interface zone between living tissue and implant. Swedish patent specification 79.02035-0 also discloses and describes the importance of the surface structure of the titanium for a powerful connection between the living bone tissue and the implant. By a special so-called microporous surface structure of the implant, the preconditions are further improved for a more or less permanent anchorage of the implant in the tissue. To be able to assess the preconditions and potential for different implants to form a permanent anchorage with the tissue, there is a need to be able to study in greater detail and under standardized conditions the bone ingrowth of the implant, and also those phases which precede the bone formation proper. Thus, it is previously known in this art how a special examination chamber may be operated into the bone tissue of a living animal, the chamber being designed such that samples of newly-formed bone tissue may be harvested from the chamber at regular intervals, and be examine. Hence, the chamber makes possible a quantification of bone ingrowth/implant incorporation under different experimental conditions without the need of sacrificing the animal. Such a test chamber, "The Harvest Chamber", is described in T, Albrektsson, M. Jacobsson and P. Kälebo, "The Harvest Chamber - A newly developed Implant for Analysis of Bone Remodelling in situ", Biomaterials and Biomechanics 1983, pp. 283-288. The harvest chamber is constructed from an outer portion with a central recess in which an inner, removable portion is inserted. The two portions together form a bone ingrowth through channel which is intended to be exposed when the inner portion is removed in order, by such a manner, to make possible examination of tissue which has grown into the channel. With the assistance of the harvest chamber, the bone ingrowth may be studied under different, pathological/experimental conditions and be compared with bone formation under normal conditions. For comparative tests, one examination chamber is normally inserted surgically into each extremity, for example the tibia, of the experimental animal. The one chamber acts as a test chamber and is surgically placed in that bone tissue which is exposed to, or will be exposed to, a potentially bone-stimulating/bone-retarding effect of some type, for example a locally administered test substance, while the second chamber constitutes a control reference chamber and is surgically inserted into bone tissue which is not under such influences. Evaluation of the results is affected in that bone ingrowth in the test chamber inserted in the one extremity is compared with bone ingrwoth in the control reference chamber in the other extremity. Suitably, data-based microradiography is employed for quantification. While the difference between the bone tissues, including their growth capacity, is slight under normal conditions, see for example the reference "The Harvest Chamber...", Fig 2, differences in the bone tissue between the two extremities may, in certain cases, derange the result of the evaluation. OBJECT OF THE PRESENT INVENTION The object of the present invention is, therefore, to realize an apparatus which permits comparison of bone ingrowth under as similar conditions as possible. To this end, the invention is based on the fact that one and the same test chamber may be employed for simultaneous comparative tests. According to the the invention the apparatus conists of a cylindrical outer portion provided with an external thread and a central recess and a removable inner portion inserted into the recess and being lockable to the outer portion. Said portions are together forming at the bottom of the apparatus at least two separate bone ingrowth channels which are exposed when the inner portion is removed and wherein one or more of said channels have been prepared with a test substance. In a first preferred embodiment of the present invention, the bone ingrowth channels consist of two discrete through channels, one of the channels having been prepared with the test substance in question. In a second preferred embodiment of the present invention, the channels adopt a Y-shaped pattern, in which case comparison can be made between test substances disposed at the prong ends of the Y. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS The nature of the present invention and its aspects will be more readily understood from the following brief description of the accompanying Drawings, and discussion relating thereto. In the accompanying Drawings: Fig. 1 illustrates the first embodiment of the present invention with two discrete through channels; and Fig. 2 illustrates the second embodiment of the present invention in which the channels form a Y-shaped pattern. DESCRIPTION OF PREFERRED EMBODIMENTS As was mentioned by way of introduction, the apparatus, hereinafter designated the implant, is based on the previously mentioned so-called Harvest Chamber. Thus, the implant consists of an outer cylindrical portion 1 of pure titanium and in which an inner, cylindrical portion 2 is inserted so that it rests with its lower end surface 3 on the bottom 4 of the outer portion. The outer cylindrical portion is provided with an external thread 5 for the careful threading in place of the implant in a bore in the bone tissue. Advantageously, use is made of the atraumatic insertion technique of the titanium implant which is previously known from Professor Brånemark's maxillary-anchored dental bridges and which constitutes a precondition for osseointegration. The inner, central portion 2 is locked to the outer portion by means of two locking screws 6 and 7 which are screwed in place in two threaded bottom holes 8 and 9 in the base plate 4. On its inner surface, the base plate had two grooves 10, 11 which cooperate with two corresponding grooves 12, 13 in the lower end surface of the central portion so that two through channels are formed for tissue growth when both of the portions 1 and 2 are assembled. Instead of causing the bone growth channels to be formed by cooperating grooves or grooved recesses in both the base plate and the central portion, the channels may be formed by grooves provided solely in the central portion, while the inner defining surface of the base plate is planar. In such instance, the grooves in the central recess may suitably be made somewhat deeper. Once the implant has been surgically inserted in the bone tissue, for example in the tibia of a rabbit, a bone growth and healing process commences and the channels will duly be filled with newly-formed bone tissue. The bone formation process and the ingrown bone tissue may be studied at regular intervals in that the central portion of the implant is removed. Before the implantation, one of the channels is prepared with that test substance whose properties are to be examined. The test substance may be deposited in the channel by different means, for example the channel may be doped with the test substance in question, the walls of the channel may be coated with the test substance etc. The implant is then inserted in, for example, the tibial metaphysis 14 of an experimental animal, it being ensured that the bone ingrowth channels are localized in the cortical bone. After the operation, the implant is covered with the soft tissue, skin and fascia which is sutured. After a bone ingrowth period of, for example, four weeks, the soft tissue is reopened, the locking screws are loosened and the central portion is removed. The newly-formed bone tissue in both channels, in the form of cylindrical rods 15, 16, are also removed. Evaluation of the results is effected in that the bone sample from the test substance channel is compared with the bone sample from the reference control channel. For quantification, use is made of data-based microradiography which is per se known and does not form part of the present invention, see for example the reference "The Harvest Chamber..." above. Fig. 2 illustrates an alternative embodiment of the present invention with three bone ingrowth channels 17, 18 and 19 which together form a Y. This configuration is particularly suited when different test substances are deposited through millipore filters 20. In such instance, the ends of each prong of the Y are closed by the millipore filters with the different substances which are to be tested. The implant is inserted in the same manner as that described above for the twin-channel chamber. After a suitable bone-ingrowth period, the implant is exposed, the central portion dismounted, the bone sample 21 is removed and the bone ingrowth in the three channels may be compared. The present invention should not be considered as restricted to that described above and shown on the Drawings, many modifications being conceivable without departing from the scope of the appended Claims.

Claims: An apparatus for studying the tissue ingrowth in an implant under different experimental conditions, the apparatus being of a bio-compatible material and suitable for implantation in living bone tissue, the apparatus consisting of a cylindrical outer portion (1) provided with an external thread (5) and a central recess and a removable inner portion (2) inserted into the recess and being lockable to the outer portion, said portions together forming at the bottom of the apparatus at least two separate bone ingrowth channels (10, 11,12,13; 17,18,19) which are exposed when the inner portion (2) is removed, and wherein one or more of said channels have been prepared with a test substance. An apparatus as claimed in claim 1 wherein said bone ingrowth channels consist of two discrete through channels (10, 12; 11,13). An apparatus as claimed in claim 1 wherein the bone-ingrowth channels (17,18,19) form a Y-shaped pattern, for comparison between three different test substances disposed at the ends of each prong of the Y. An apparatus as claimed in claim 3 wherein the ends of the Y are closed by millipore filters (20).

Inventors: ALBREKTSSON, THOMAS
Applicants: NOBELPHARMA AB
Application year: 1987
Main classification: A61B10
IPC codes: A61F2, A61B10