Bone Compaction Enhances Fixation of Weightbearing Titanium Implants

Bone Compaction Enhances Fixation of Weightbearing Titanium Implants
February 2005
By: Kold, S??ren MD, PHD*ÇƒÜ; Rahbek, Ole MD, PHD*ÇƒÜ; Vestermark, Marianne MD*ÇƒÜ; Overgaard, S??ren MD, DMSCÇƒ?; S??balle, Kjeld MD, DMSC*ÇƒÜ - From the *Orthopaedic Research Laboratory, Department of Orthopaedics, Aarhus University Hospital, Aarhus; the ÇƒÜInstitute of Experimental Clinical Research, Aarhus University Hospital; and the Çƒ?Department of Orthopaedics, Odense University Hospital, Odense, Denmark.
LLWOnline / The International Journal of Oral Implantology
Abstract
Implant stability is crucial for implant survival. A new surgical technique, compaction, has increased in vitro implant stability and in vivo fixation of nonweightbearing implants. However, the in vivo effects of compaction on weightbearing implants are unknown. As implants inserted clinically are weightbearing, the effects of compaction on weightbearing implants were examined. The hypothesis was that compaction would increase implant fixation compared with conventional drilling. Porous-coated titanium implants were inserted bilaterally into the weightbearing portion of the femoral condyles of dogs. In each dog, one knee had the implant cavity prepared with drilling, and the other knee was prepared with compaction. Eight dogs were euthanized after 2 weeks, and eight dogs were euthanized after 4 weeks. Femoral condyles from an additional eight dogs represented Time 0. Compacted specimens had higher bone-implant contact and periimplant bone density at 0 and 2 weeks, but not at 4 weeks. A biphasic response of compaction was found with a pushout test, as compaction increased ultimate shear strength and energy absorption at 0 and 4 weeks, but not at 2 weeks. This biphasic response indicates that compaction enhances implant fixation by mechanical and biological mechanisms. Therefore, compaction might have potential value in total joint replacement in the future.
Materials and Methods
Implants were tested to failure by pushout testing on an Instron Universal test machine (Instron Ltd, Buckinghamshire, UK). The specimens were placed on a metal platform with a central circular opening supporting the bone 700 ¨µm from the bone-implant interface as recommended by Dhert et al.12 A pushout direction equal to the load transfer direction was chosen. A preload of 2 N was applied to define the contact position for the start of the test. A displacement rate of 5mm/minute was used, and load displacement curves were obtained on a personal computer. Ultimate shear strength (MPa) was determined from the maximum force applied until failure of the bone-implant interface. Apparent shear stiffness (MPa/mm) was obtained from the slope of the straight part of the load displacement curve. Energy absorption (kJ/m2) was calculated as the area under the load displacement curve until failure. All pushout parameters were normalized by the surface area of the implant specimen tested.
Results
All dogs were fully weightbearing by 3 days after surgery. At necropsy, no clinical signs of infection were present. One dog from the 4-week observation period was excluded because of intraoperative breakage of a drill, resulting in misplacement of the implant.3 All specimens failed at the bone-implant interface during pushout testing.
Histologically, compacted specimens had a high presence of nonvital bone in the bone implant interface at 2 weeks. This nonvital bone consisted of small bony chips or larger pieces of bone with cracking of the lamellar structure and loss of osteocytes. A high resorptive activity with resorptive lacunae in the surfaces of the nonvital bone was found. The main contribution to bony tissues found in the interface for drilled specimens at 2 weeks came from vital bone. Fibrous tissues were not found in the bone-implant interface for compacted or drilled specimens after 2 weeks. The bone in the interface at 4 weeks mainly consisted of vital bone for drilled and compacted specimens, and resorptive surfaces were evenly present in compacted and drilled specimens. Two of seven drilled specimens were partially surrounded by a fibrous membrane after 4 weeks compared with no fibrous membranes seen in compacted specimens.
Discussion
To our knowledge this is the first study to evaluate the effects of compaction using weightbearing implants in vivo. The model we used mimicked the clinical cementless joint replacement with weightbearing implants inserted intraarticularly allowing access of joint fluid to the bone-implant interface. Primary cementless femoral stem fixation relies on cancellous bone ingrowth in the proximal region, as the majority of bone initially in contact with the implant is cancellous bone.22 Therefore, the implants were inserted into cancellous bone in canines, which is similar to human cancellous bone.2 Using conventional bone preparation techniques, it is evident that initial direct apposition between femoral components and bone often is limited to relatively small areas.22,33,36 It has been speculated that the gaps between implant and bone are created as the broach tears out chunks of cancellous bone thereby removing it.33 In this study, we compared compaction with drilling as in an earlier nonweightbearing experimental study.17 Drilling technique was used for practical reasons because of the size and anatomic configuration of the canine femoral condyle in which broaching is difficult to do. However, we wished to investigate the mechanical and the biological effects of bone compaction. Bone compaction is a bone-saving procedure with an element of in situ autografting compared with a bone-removing technique. Even though broaching and drilling are different modes of bone preparation, they represent bone-removing techniques, and therefore we found that the choice of drilling technique in the control group was justified. Therefore, the results of our study represent the use of compaction in a clinically relevant, but experimental canine model in which the remodeling rate of bone is 2-3 times higher than in healthy humans.27 The nonsignificant differences between compacted and drilled specimens should be interpreted with caution because of low power of the statistical test (small samples of seven or eight pairs at each time).
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